Concrete Towers for Onshore and Offshore Wind Farms

Concrete Towers for Onshore and Offshore Wind Farms CONCEPTUAL DESIGN STUDIES

PROFILES OF THE CONCRETE CENTRE AND GIFFORD The Concrete Centre is dedicated to heping architects, designers, engineers, constructors, deveopers and cients get the best out of concrete and hence improve their own efficiency. With a remit to technicay deveop and promote appropriate concrete soutions, the Centre provides a foca point for innovation, research, deveopment and technica exceence. This is achieved by deveoping design guidance on a wide range of topics such as structura design, fire, sustainabiity, acoustics, therma properties and durabiity, and providing a comprehensive education and training programme to provide in-depth knowedge and examination of concrete issues and deveopments. The use of concrete in the UK wind energy sector to date has been imited predominanty to onshore foundation appications. This contradicts experience from esewhere, where appication of concrete for pyons and offshore appications, such as foundations, is commonpace. As such, The Concrete Centre is committed to chaenging the UK wind industry s current thinking on wind tower design and to iustrate how the benefits of concrete construction can be reaised more fuy. For more information on the use of concrete in wind tower appications and the services that the Centre offers, incuding pubications, seminars and technica support and advice from our regiona teams, pease visit www.concretecentre.com Gifford is a eading engineering design consutancy with over 50 years experience of strategic panning, design deveopment and construction design of major projects for the buit and natura environment. Project experience covers: highways, rai and marine transport infrastructure; industria and commercia faciities incuding shipyards, and production pants; energy infrastructure for storage faciities, power stations and wind farms; buidings of a kinds; urban regeneration; environmenta management, mitigation and restoration; resources management incuding waste management. Founded on the basis of invention of new prestressing systems for concrete and on the innovative design of prestressed concrete structures, the firm has maintained a tradition of innovation in emerging materias, technoogies and fieds of engineering. These have incuded pre-fabricated buiding systems, hovercraft, wave energy systems, wind turbine bades, strengthening techniques for structures and ow energy buidings. Resources incude a network of UK and overseas offices with over 600 staff covering discipines and speciaisms reating to civi, structura, buiding and environmenta engineering and science, anaysis and modeing, project information production and project management.

Preface PAGE i PREFACE Considered opinion supporting concrete as an efficient high performance structura materia, in the rapidy growing UK programme for wind energy generation, recenty ed to the need for verification of this potentia. In 2003, The Concrete Centre commissioned Gifford to undertake conceptua design studies of concrete towers for wind energy converters. Studies were subsequenty undertaken in two stages during a 15 month period between eary 2004 and May 2005. As offshore sites were perceived to present the greatest opportunities for the UK, initia research focused on this fied of study. It is appreciated that, whie many common issues exist between onshore and offshore wind tower structures, there are aso many important differences that coud significanty infuence design concepts and feasibiity. This document draws together the resuts presented in the onshore and offshore research into one pubication. It presents ideas and issues on the design and depoyment of concrete towers and associated structures, and points to a rea opportunity for the substantia and economic use of concrete tower structures in future wind energy deveopments. We trust that it wi hep to raise awareness of this important potentia. Aan Bromage The Concrete Centre Aan Trickebank Gifford Prepared by: A H Trickebank P H Haberstadt BSc Eng(Hons) CEng FIStructE FICE FCIWEM MEng(Hons) ACGI Civi Eng Gifford and Partners Ltd Gifford and Partners Ltd B J Magee BEng Phd CEng MICE MICT The Concrete Centre With advice and assistance from: A Bromage BSc CEng MICE MCIM The Concrete Centre

Contents PAGE ii CONTENTS PROFILES OF THE CONCRETE CENTRE AND GIFFORD PREFACE 1 INTRODUCTION 1.1 Background 1.2 Looking to the future 1.3 The roe of concrete 1.4 Current appications of concrete in the wind energy sector 1.5 Background design considerations 1.6 Towards more competitive concrete design soutions 1.7 Aim of this document STUDY OF CONCRETE TOWERS FOR ONSHORE WIND FARMS 2 DESIGN PHILOSOPHY 2.1 Genera approach and configuration 2.2 Design for construction 2.3 Design concepts 3 DESIGN 3.1 Indicative designs 3.2 Foundations 3.3 Tower eements 3.3.1 Base zone 3.3.2 Midde zone 3.3.3 Upper zone 3.4 Prestressing of the tower 3.5 Thin waed she units 3.6 Joints 3.7 Construction sequence 3.8 In-situ sipform construction 3.9 Cranage and ifting 3.10 Structura anaysis 4 QUANTITIES AND COSTS 5 CONCLUSIONS 6 FIGURES

Contents PAGE iii STUDY OF CONCRETE TOWERS FOR OFFSHORE WIND FARMS 7 DESIGN PHILOSOPHY 7.1 Design drivers and approach 7.2 Design scenarios and criteria 8 DESIGN 8.1 Tower configuration 8.2 Pyon 8.3 Foundation and substructure 8.4 Structura design methodoogy 8.5 Commentary on ife cyce issues 8.6 Description of outine designs 8.6.1 Continuous taper design 8.6.2 Necked stem design 8.6.3 Concrete pyon with concrete gravity foundation 8.6.4 Concrete pyon on stee monopie 8.7 Pyon fabrication and erection 8.7.1 Precast concrete method 8.7.2 Sipforming the tower 8.7.3 Prestressing techniques and detais 8.7.4 Anciary detais 8.7.5 Formwork for precast units 8.7.6 Joint detais 8.8 Construction and instaation of the foundation and tower 8.8.1 Current instaation methods for stee towers 8.8.2 Instaation of gravity foundations 8.8.3 Aternative proposas for caisson and tower construction and instaation 8.9 Versatiity of the concrete tower concept 8.10 Standardisation of pyon geometry 8.11 Prototype and demonstration designs 9 QUANTITIES AND COSTS 10 COMPARISON OF STEEL AND CONCRETE TOWERS 10.1 Comparision of the dynamics of stee and concrete towers 10.2 Comparision tabe of offshore tower structures characteristics 11 CONCLUSIONS 12 FIGURES 13 GENERAL CONCLUSIONS 14 REFERENCES A1 APPENDIX A Embodied CO 2 estimates for wind towers

List of figures PAGE iv LIST OF FIGURES STUDY OF ONSHORE WIND TOWERS Figure No. 1.1 70m CONCRETE TOWER - Outine and indicative dimensions 1.2 100m CONCRETE TOWER - Outine and indicative dimensions 1.3 HYBRID TOWER - Arrangement with stee upper section 1.4 70m CONCRETE TOWER - Assemby from precast concrete units 1.5 100m CONCRETE TOWER - Assemby from precast concrete units 1.6 TOWER CONSTRUCTION SEQUENCE 1.7 ASSEMBLY OF RING UNITS INTO TOWER SECTIONS 1.8 TOWER SECTION JOINTS - Anchorage section and pain joints 1.9 RING UNITS - Reinforcement and segment joints 1.10 TENDON RESTRAINTS - Yokes to pain units STUDY OF OFFSHORE WIND TOWERS Figure No. 2.1 GENERAL STUDY SCENARIO 2.2 CONCRETE TOWER - Continuous taper design 2.3 CONCRETE TOWER - Necked stem design 2.4 CONCRETE FOUNDATION - Gravity base soution 2.5 CONCRETE PYLON - With stee monopie substructure 2.6 PRODUCTION PROCESS - Concrete pyon 2.7 TOWER SECTIONS - For offshore erection 2.8 TOWER SECTIONS - Showing buid-up from precast concrete ring units 2.9 PRECAST CONCRETE RING UNITS - Outine detais 2.10 JOINT BETWEEN TOWER SECTIONS - Interna prestress arrangement 2.11 COMPOSITE/HYBRID ARRANGEMENTS FOR WIND TOWER STRUCTURE - Fexibiity of soution 2.12 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION - Methods 1& 2 (Pyon construction offshore) 2.13 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION - Method 3 (Pyon construction inshore) 2.14 DYNAMIC RESPONSES OF VARIOUS TOWER DESIGNS

Introduction PAGE 1 1 INTRODUCTION 1.1 Background Harnessing wind energy is one of the most commerciay deveoped and rapidy growing renewabe energy technoogies, both in the UK and wordwide. In 1991, the first UK wind farm depoyed ten 400kW turbines. Just 14 years ater, turbines capabe of generating ten times that energy output are in use. Energy generation sophistication has evoved through achievements in engineering design, aerodynamics, advanced materias, contro systems and production engineering. Rotor diameters have grown from 30m to over 80m, with diameters of up to 120m now being triaed. Tower heights have risen from 40m to over 100m, and power outputs from 200kW up to 5MW. These dimensions and ratings are set to grow even further. Looking to the future, the UK is committed to working towards a 60% reduction in CO 2 emissions by 2050. A core part of achieving this aim is the deveopment of renewabe energy technoogies such as wind. The UK s wind resources are the best and most geographicay diverse in Europe, more than enough to meet the target of generating 10% of UK eectricity from renewabe energy sources by 2010, with an aspirationa target of 20% pus by 2020. The current UK consumption of eectricity suppied by the grid is about 35TWh (Terawatt hours) or 35,000GWh (1 Gigawatt hr = 10 6 KW hours). The target of 10% therefore represents deivery of 3.5TWh. In its recent report Offshore Wind at the Crossroads, the British Wind Energy Association (BWEA) concudes that, with modest additiona government support, offshore wind capacity coud rise to 8GW by 2015. By then, Rounds 1 & 2 woud be competed and a new Round 3 woud have started, taking instaed capacity to 10.5GW by 2017. Annua growth rates of instaed capacity coud be expected to be 1.2GW per year eary in the next decade. Other scenarios see this increasing to 2GW per year by 2015. The tabe beow sets out the effects of this scenario for offshore capacity, combined with current predictions for onshore capacity growth. Capacity utiisation factors for offshore wind of about 36% are assumed, currenty growing to 38% over time for new capacity. For onshore wind a utiisation factor remaining at about 30% is assumed. [The ink between rated capacity output is easiy cacuated, for exampe as foows: Current Onshore Rated capacity = 1.0GW Output = 1.0GW x0.30x365x24 = 2628 GWh] 2006 Capacity GW 2006 Annua Output TWh % of Tota UK Output 2024 Capacity GW 2024 Annua Output TWh % of Tota UK Output Offshore 0.2 0.7 0.3% 28.3 94.0 26.2% Onshore 1.0 2.6 0.7% 10.0 26.0 7.2% Tota 1.2 3.3 1.0% 38.3 120.0 33.4% At the time of writing this report, the rate of instaation of offshore capacity is beow expectation due to technica, cost and financia issues. It is beieved that these issues are ikey to be resoved during the next two years, and that actua instaation wi trend back towards target over the next four to five years. Given the benefits of construction, instaation cost and risk reduction, improved turbine reiabiity and suitabe adjustments to the financia support mechanisms, a of which can be reasonaby expected, there are good grounds for thinking that UK offshore and onshore wind can contribute some 33% of the required eectrica power output by 2024. 1.2 Looking to the future At present, most recenty instaed onshore UK wind towers typicay have a rotor diameter of 40m, tower height of 70m and power outputs between 1.0 and 2.75MW. The argest individua turbines currenty instaed offshore (at Arkow Bank) are rated at 3.6MW, with a rotor diameter of 111m and a hub height of 74m. However, even arger machines are ikey to be instaed over the next few years as various manufacturers reease new generation turbines in the output range of 4.5 to 5MW with machines of up to 7MW currenty under consideration. Athough these arger generating units are primariy aimed at the offshore industry, experience with some arger onshore prototypes demonstrates the potentia viabiity of onshore expoitation where the constraints of transportation of components can be removed or overcome. To date, a 4.5MW turbine has been instaed as a prototype on an onshore wind farm near Magdeburg, Germany. Onshore turbines of this size wi require bades in the region of 60m ong. They must be supported on taer, stronger towers that stand up to and beyond 100m ta. Onshore sites generay require taer towers than offshore sites for a given power output. The principa onshore factors incude a ower bade tip speed to imit noise generation, eading to reativey arger rotor diameters. The rotor is set at a greater height above ground to overcome greater surface friction effects on the wind speed profie. Onshore towers are currenty frequenty imited in height by oca government panning restrictions to around 100m at the tip of the rotor.

Introduction PAGE 2 In view of the imited number of avaiabe onshore sites with suitabe wind cimate, ocation and access, it wi become increasingy important to make best use of principa sites by harvesting the optimum proportion of resource avaiabe, using the best technoogies avaiabe. This wi put great emphasis on using towers in the range of 100m pus, exceeding existing heights currenty used on most UK onshore sites. In summary, the future trend for UK wind farms is that they are ikey to require taer towers, supporting higher powered, onger baded turbines, many of which may be ocated at remote or ess accessibe sites. 1.3 The roe of concrete The consequence of taer wind towers is the need to increase the structura strength and stiffness required to carry both increased turbine weight and bending forces under wind action on the rotors and the tower, and to avoid damaging resonance from excitation by forcing frequencies associated with the rotor and bades passing the tower. In turn this wi require arger cross sectiona diameters, which may introduce significant transportation probems, bearing in mind that 4.5m is the practica imit for the diameter of compete ring sections that can be transported aong the pubic highway. It is ceary shown in this report that concrete towers can accommodate these requirements and aso offer a range of associated benefits. Concrete is a versatie materia and can be used structuray in many different ways. Structura concrete may be reinforced (with stee bar or other suitabe materias), prestressed (with pre- or post-tensioned stee bars or strands, or other suitabe materias) or mass (with no reinforcement). Concrete used for structures shoud be regarded as a high performance materia. Its properties (particuary, but not ony, strength), can be tuned by design over a wide range from norma structura grade to very high performance grades. Mass concrete has a very ong history as a construction materia. In the modern era, reinforced concrete has been used for at east 100 years and prestressed concrete for over 70 years. Some key benefits of the materia are summarised here: Low maintenance Concrete is an inherenty durabe materia. When designed and constructed propery, concrete is capabe of maintaining its desired engineering properties under extreme exposure conditions. Cost-competitive and economica Concrete soutions can combine ow first cost with significanty enhanced ife cyce vaue. Concrete s constituent materias are reativey ow cost. The work processes for concrete production are not inherenty expensive and are routiney engineered and mechanised to a high eve for simiar production situations. The potentia requirement for the production of significant numbers of simiar structures with some commonaity of eements aso provides a major opportunity for a highy production-oriented design, thereby adding significanty to the potentia overa economy. For ta wind towers, in particuar those standing in excess of around 90m, concrete can deiver cost-effective, ongife soutions. Soutions with a practica design ife of 40 to 60 years pus are feasibe. This opens up the possibiity of significant ife cyce cost savings on towers and foundations if couped with a turbine re-fit phiosophy. It may be anticipated that improved technoogy turbines woud be instaed at each re-fit. Large diameter structures can easiy be constructed in concrete without disproportionate increases in cost. In addition to potentiay ower reative grid-connection and operationa costs, taer more durabe towers can generate increased eves of power and deiver ower payback times. Design and construction fexibiity Concrete s versatiity enabes design soutions with no restriction on height or size to meet chaenges infuenced by site conditions and accessibiity. Designs can be adapted to both in-situ and precast construction methods and offer a wide range of construction fexibiity to suit site conditions, avaiabiity of speciaised pant and abour and other oca or market circumstances. Mix design fexibiity Concrete is an adaptabe construction materia that can be finey tuned through aterations in mixture design to optimise key parameters such as strength, stiffness and density. Exceent dynamic performance Concrete has good materia damping properties. In particuar, prestressed concrete has a high fatigue resistance, providing more toerance and ess risk from dynamic faiure. By deivering improved eves of damping to vibrations and aso noise, concrete designs may pay a centra roe in gaining pubic acceptance in environmentay sensitive areas. Low environmenta impact Not ony is reinforced concrete 100% recycabe, but its embodied CO 2 and energy content can be much ower than that of other construction materias. For instance, for a typica 70m high onshore wind tower configuration, reative to tubuar stee, the embodied CO 2 content of a prestressed concrete design option is approximatey 64% ower. In addition, a concrete wind tower has the abiity to consume CO 2 from the atmosphere both during and after its service ife (see Appendix A for further detai).

Introduction PAGE 3 1.4 Current appications of concrete in the wind energy sector The use of concrete in the wind energy sector has so far been predominanty in foundation appications, either to form gravity foundations or pie caps. There have been at east two major projects offshore, at Midde Grunden and Rodsand wind farms, which both use gravity foundations with ice cream cone stems to suit the particuar conditions of the Batic Sea. The associated Danish energy production and distribution company ENERGI E2 have decared significant cost savings through the use of concrete gravity foundations for offshore wind farms and reportedy intend to expoit this potentia for future sites such as London Array. In terms of pyon appications, concrete soutions are being expoited onshore by at east three turbine manufacturers: Enercon, GE Wind and Nordex. Enercon has progressed furthest with the deveopment of concrete pyons, with a prototype precast concrete soution now having moved into fu scae commercia production. Enercon now offers the option of a concrete tower soution for turbines with hub heights of 75m and above. For hub heights of up to 113m, reinforced precast concrete rings of around 3.8m high with wa thickness of around 350mm are used. The rings range from between 2.3m and 7.5m in diameter, with the arger ower rings spit in haf verticay to simpify transportation. Once assembed, the concrete rings are post-tensioned verticay. Enercon has recenty opened a dedicated pant to produce these precast concrete segments in Magdeburg, Germany. Enercon proposes an in-situ concrete soution for towers standing above 113m high, athough no information on this is avaiabe at the time of pubication. GE Wind is currenty investigating the viabiity of hybrid concrete and stee pyons. A 100m ta prototype constructed in Barrax, Spain, features in-situ, post-tensioned concrete rings for the bottom 70m and a stee tube for the top section. The concrete section, which uses diameters up to 12m, is post-tensioned verticay with the cabes running inside the tower, but externa to the concrete wa. Again, a conventionay reinforced section with wa thicknesses of 350mm is used. The stee section is spit into five sections, each weighing a maximum of 70 tonnes, with a maximum diameter of 5.7m. A joint venture between the two Dutch companies Meca and Hurks Beton has produced a design for a 100-120m concrete-stee hybrid tower which is the subject matter of severa written papers. The soution is simiar to the GE Wind hybrid tower, in that it uses post-tensioned concrete, with the tendons inside the tower but externa to the concrete section for the ower concrete section and a stee tube for the top section. However, the design makes use of ong and narrow precast concrete eements rather than in-situ rings. Again, these eements are conventionay reinforced, with wa thicknesses of 250-350mm. Ceary, Enercon beieves that the precast ring segment soution is economicay viabe for heavier turbines for arge towers. Indeed, the company has invested heaviy in a production pant and is producing a stock of segments. Meca and Hurks Beton aso beieve that a concrete/stee hybrid tower is economicay viabe, focusing on whoe-ife costing for a wind farm and suggesting that athough the initia investment in the hybrid tower wi be greater than a stee tower, the returns wi be greater because the tower is taer and generates more power. As of August 2006, Nordex is offering concrete/stee hybrid towers for hub heights of 120m. Previousy, it used soey stee towers but has recognised that concrete offers a reativey inexpensive aternative [8]. The soution comprises a 60m ength of moduar stee in three sections on top of a concrete tower produced in different engths to provide hub heights between 100m and 120m. This invoves the use of ocay suppied materias and ensures an optimum turbine height to make the most of prevaiing conditions. Nordex aso recognize that this approach offers ogistic advantages as the restrictive stee diameters normay appicabe during transportation do not appy. 1.5 Background design considerations Design concepts have been deveoped invoving the use of concrete for wind towers, with the aim of achieving cost competitive and practica soutions for UK conditions. During the evoution of these concepts, information has been pubished on a number of other European designs that are currenty avaiabe or under deveopment. In the structura concepts set out here there are, unsurprisingy, some features shared with these other designs. The process has been to deveop a synthesis of ideas and concepts arising from a fundamenta consideration of the requirements and opportunities by the study team. It is hoped that this wi point to some new directions and, at the very east, confirm and highight important existing soutions which are worthy of greater consideration and appication in UK practice. Design soutions for offshore and onshore wind pyons have many simiarities. Whist some of the design and production thinking and subsequent deveopment experience coud be interchangeabe, there are nevertheess some profound differences (see Section 7). One obvious difference is that offshore structures are subjected to additiona oadings from waves and currents and to generay more aggressive conditions. Furthermore the construction and operating regime offshore is consideraby more severe and hazardous. Speciaist heavy pant needed for work offshore is subject to serious and, to some extent unpredictabe, disruption from bad weather. Eary wind farms and turbine technoogies were deveoped in the more benign conditions prevaiing onshore. To date, concrete has been used much more widey in onshore wind energy structures than in offshore wind farms. Foundations for onshore wind towers are aready predominatey constructed using reinforced concrete, either as gravity bases or for

Introduction PAGE 4 caps over pies. Ony a imited number of wind pyons wordwide have so far been constructed using concrete, and these have been onshore. In the UK, exampes of any size are restricted to one or two arge towers for eary prototypes. The overwheming majority of wind towers constructed to date have been buit using tapered stee tubes formed from seam weded roed pates with fanged boted connections at the terminations. Taer towers were buit up from separate engths determined by transport and ifting constraints. For the arger towers speciaist transporters are required to carry the tower sections from the fabrication yard and maximize the ruing diameter of the tube within the highways oading gauge. This has generay imited maximum stee tower diameters to 4.5m. However, it is understood that work is underway to deveop segmented designs to overcome this imitation. This wi require the introduction of costy boted joints into the thickest and most heaviy oaded sections of the tower. Ta concrete towers and chimneys have a ong and successfu history and, whist there are good reasons why stee towers have been the dominant soution to date, issues associated with increasing height, diameter and oading tend to point towards the use of concrete tower designs providing aternative, and potentiay more competitive, soutions for future wind farms. It woud seem that the onshore industry is moving to a position where there coud be substantia progress on the widespread use of concrete towers. Since insitu concreting is currenty utiised for a onshore sites, ogisticay it woud not be a big step to use sipforming. This is a particuar in-situ technique which is entirey crane-independent. It can be used in conjunction with precast concrete or stee eements prefabricated off-site, to provide an efficient way of overcoming transportation imitations that woud otherwise arise with the very argest eements such as structura towers. Sipforming can be used to construct tapering towers of any height desired. The outcome is that the introduction of concrete-reated construction techniques into the pyon eement of towers, either in ower pyon sections or over their entire height, is a reativey simpe and ogica design step. 1.6 Towards more competitive concrete design soutions Concrete wind tower soutions must be cost competitive with aternative and existing design options. For typica wind tower heights of 60-80m constructed to date, it is difficut to achieve designs and construction approaches where the ower specific cost of concrete as a materia offsets the required increase in materia quantity and weight. Even by adopting radica design approaches to minimise the thickness, and therefore the weight, of concrete towers, this approach is sti ikey to be significanty heavier overa than a stee design. As construction of next-generation wind farms with towers up to and beyond 100m high comes into coser focus, and given an expanding programme of construction, a number of factors make concrete an attractive design option for deivering arge diameter pyons at acceptabe cost. An increase in tower weight may cause a number of significant effects. Beneficiay, the inherent weight and stiffness of concrete towers can offer improved fatigue and dynamic performance eves and may aso improve the efficiency of the foundation in resisting overturning forces. By carefu attention to the distribution of weight over the height of the tower, the potentiay adverse effects of this weight on the dynamic characteristics (natura frequency) of the tower can be minimised. The abiity to tune the stiffness reativey easiy by adjustment of the ower profie and thickness of the tower she can more than offset any disadvantages. Adversey, there may be a need to use arger, more powerfu pant both for transportation and for construction, both of which are ess readiy avaiabe and more expensive. Ideay the optimum size of cranage for erection shoud be determined by the nacee weight (typicay in the range of 70-150Te depending on the turbine rating). These issues may be more easiy resoved by using in-situ concrete techniques (incuding those mentioned above) or precast concrete soutions that end themseves to being spit into a greater number of segments, or combinations of the two. Whie an increased number of segments might resut in an increased number of transport movements and some increase in construction time, this approach may offer a cost-effective soution to site constraint chaenges. Fundamenta characteristics of concrete in its freshy produced state are its pasticity (moudabiity) and reative toerance of handing. These make it extremey adaptabe to a wide range of construction methods and to the production of compex or curved shapes. A variety of in-situ methods are avaiabe in addition to precast soutions. If hybrid concrete soutions (the combined use of precast and in-situ concrete) are considered, the adaptabiity of soutions increases even more. The characteristics of concrete as a high performance structura materia are of course we demonstrated in the huge inventory of major and compex structures around the word, both onshore and offshore. Given that the broad materia factors are favourabe to durabe and economic structures, there is sti ceary a need to consider carefuy a the detais affecting the key ife cyce stages, in order to ensure the reaisation of maximum economy and competitiveness with aternative materias.

Introduction PAGE 5 1.7 Aim of this document The aim of this study is to encourage the deveopment of concrete wind tower soutions and to iustrate how the benefits of concrete construction can be reaised more fuy by the wind industry. By focusing on key issues pertaining to wind tower fabrication, the intention of the document is not to propose definitive soutions, but rather to highight practica methods and technoogies that, through optimisation, coud ead to competitive soutions. Conceptua configurations are presented for both onshore and offshore faciities, aong with design phiosophies and construction methodoogies for concrete wind tower soutions. These concepts have been arrived at independenty by the authors of the report and, perhaps not surprisingy, there are some simiarities with the thinking of others working in this fied. Whoe ife issues are accounted for, incuding fabrication, transportation, instaation, maintenance, decommissioning, remova and disposa. Outine soutions for both concrete and stee are used for the assessment of their reative merits and viabiity.

Onshore concrete wind towers study PAGE 6 STUDY OF CONCRETE TOWERS FOR ONSHORE WIND FARMS The onshore study was undertaken subsequent to the work on offshore wind farms. Some of the concepts touched on in the offshore study have been deveoped in more detai. There is inevitaby some repetition between the studies, readers are advised to read both in order to get the fuest information from this report. 2 DESIGN PHILOSOPHY 2.1 Genera approach and configuration The overa structura form seected for this study is a tapered tube, which has become the predominant form for wind towers over the past decade. This might be more fuy characterised as a smoothy tapering, soid waed, circuar section tube which can provide the foowing properties easiy by virtue of its form: The weight and strength profies of the cross-section foow the genera pattern of that required by the function. Due to the taper, the weight per metre height reduces with increasing height, giving more favourabe dynamic characteristics, as does cross-sectiona strength, which matches the genera pattern of overturning and shear forces. The circuar form is efficient for a structure which can generay be oaded fuy from any compass direction. Good appearance (particuary with carefu attention to taper and proportions). Good aerodynamics. Provides shetered interna space for vertica access for maintenance and cabing. Minimises surface area and edges open to aggressive weathering and corrosion. Is very we suited to concrete technoogy. The choice of form may seem obvious but it is perhaps worth recaing that there is a ong history of ta attice towers for various purposes, incuding the traditiona agricutura wind pump. In recent years attice towers have been used in China for modern wind turbines. Some eary consideration has been given to attice or perforated tube structures in these studies as they might conceivaby offer some benefits, particuary in the upper midde and top zones of the tower. However these have not been expored further. Within the genera constraints of the seected tapered tubuar form, the key area of the design phiosophy is fexibiity of the soution. Each wind farm site is different and each project wi have its own set of constraints and probems. Fexibiity is thus needed at various eves of design, from overa configuration through to main strength and stiffness design, right down to key detais. At the goba structura eve, fexibiity can be provided by a framework of reated and aternative structura configurations. By fitting into the overa outine enveope of the tower, this wi aow individua project designs to be more cosey taiored to the circumstances of the project and thereby faciitate more cost-effective outcomes. As shown in Figures 1.1 to 1.4, the approach invoves notionay spitting the tower into three broad zones: Upper zone Midde zone Base zone These are chosen to mirror different zones of design and construction constraints over the height of the tower. The extent of the zones is not fixed, so that the design concept has a sufficient degree of fexibiity to cope with the changing constraints and design drivers of future wind farms. One or more construction types for each of the zones are considered (in the foowing sections). Typicay these are: The upper zone Precast concrete rings capped by a stee fabrication comprising the turbine yaw ring/nacee bearing patform within a tapered stee tubuar section The midde zone - Precast concrete segmented or monoithic rings The base zone - In-situ concrete or precast concrete segmented rings

Onshore concrete wind towers study PAGE 7 Aternativey, for in-situ sipform construction over the fu height of the tower, the zoning has ess apparent significance, but may sti provide a usefu way of characterising the different bends of design drivers over the height of the tower. This framework is intended to faciitate practica conceptua design strategies. It may aso point towards a structured approach to maintaining significant fexibiity through the project preparation and procurement process for specific wind farm deveopments. The aim is to aow major component parts of the design to be readiy adjusted during the project deveopment programme to meet the changing market conditions. These take account of such factors as avaiabiity of materias, construction pant, fabrication faciities, contractors preferences and risk perceptions, and possiby fine tuning to emerging oad information reating to adjustments in turbine specification. Given the major programme of wind farm deveopment projected for the next decade and the impied steep rise in demand for fabrication and instaation resources, fexibiity is a desirabe characteristic. 2.2 Design for construction This section might aso be caed design for production, where construction or production encompasses fabrication, assemby and erection. The concept impies a design approach that invoves significanty greater than norma attention being given to shaping the design of the fina product to take account of these aspects of the construction process. It aso impies that some design of the construction process itsef is aso invoved. It is meant to put emphasis on an approach that can achieve significant process improvement and overa economy, particuary where there are potentia economies of scae in terms of the numbers of units to be produced within particuar projects or in the market at arge. This opportunity is surey there, given that many currenty panned wind farms may contain 30 to 150 units and the tota future market impies the construction of many hundreds of units. It is perhaps hepfu to draw attention to this concept for the genera construction industry, where so many projects are essentiay prototypica, athough paradoxicay by the nature of its business the precast concrete industry is very used to the mass production of standardised components. In its broadest sense the concept invoves an interactive and chaenging approach to the vaues put on other design criteria. For instance, in reation to particuar functiona performance, it might invove critica examination of the trade-off of some oss of performance against production economy. Such an approach can usuay often ead to better outcomes a round, particuary if it is appied to a eves of the design, from the overa configuration of the tower and major component segments through to a the key detais. 2.3 Design concepts The various overa configurations and design concepts for towers and detais iustrated in this report have been deveoped in the ight of the above design phiosophy. They are discussed further in Section 3. Iustrations shown are indicative ony and woud require significant further engineering attention for reaisation. 3 DESIGN 3.1 Indicative designs Indicative designs and detais for onshore concrete towers are iustrated in Figures 1.1 to 1.10. The two configurations seected for this study as representative of current and near-future scenarios are: 70m high tower; 2MW Turbine; 39m bades/80m rotor Figure 1.1 100m high tower; 4.5MW Turbine; 58.5m bades/120m rotor Figure 1.2 The near-future scenario is intended to cover the trend towards taer towers and/or arger turbines aready we underway on some European onshore sites, where towers of 100m pus are considered to be cost-effective because of the better wind cimate and therefore productivity. Interestingy the transition in height between current and future scenarios appears to require a step-change for stee towers in fabrication method and technoogy, and approach to transportation. Overcoming these constraints impies some reative increase in the cost of stee towers against concrete towers and therefore appears to shift in favour of concrete towers. The figures indicate the type of concrete construction that is considered to be most appropriate for the different zones. These are: Upper and midde zone: Base zone: Foundation zone: Precast prestressed concrete In-situ concrete, precast concrete or hybrid of the two In-situ concrete

Onshore concrete wind towers study PAGE 8 There is aso a strong case for in-situ construction of the whoe tower using the sipform method. Sipform is a continuous casting technique in which formwork is steadiy moved upwards on screw jacks at a rate which aows the concrete paced in the top of the form to set sufficienty to support itsef before emerging from the bottom of the rising form. The overa design approach is to achieve best overa economy by matching construction process carefuy to the particuar needs and characteristics of each zone. The taper profie of the tower is kept simpe. For the 70m tower a uniform taper has been considered; for the 100m tower a bi-inear taper, with a more marked taper in the base zone giving a better fit to the strength and stiffness requirements. It is possibe to achieve an even better fit by using a muti-inear taper, which does not necessariy introduce significanty greater production compexity if the taper within the height of the individua precast units is kept uniform. A zona definition of the tower structure is given earier in Section 2. From this perspective the idea of matching the type of construction to different zones points to the possibe use of other materias for certain zones, in conjunction with concrete for others. One such hybrid configuration is shown in Figure 1.3 where the combination incudes: Foundation, base and midde zones concrete Upper zone stee The rationae for this configuration might be that, in the circumstances where a fu concrete tower was competitive, it coud provide a cost-effective way of tuning the dynamics of the tower (where weight reduction at the upper eve is ikey to prove beneficia in this respect). It might aso provide a way of utiising avaiabe fabrication and erection resources to best effect, particuary for a major wind farm deveopment where the pressure on a particuar resource might become critica to construction programming. Additionay this configuration provides a possibe way of extending the height of present stee tower designs, without incurring the greater fabrication and transportation difficuties that are ikey to arise from an a-stee tower design. 3.2 Foundations Foundations for onshore wind towers wi generay invove the use of a significant amount of reinforced concrete. Typica foundation arrangements for different soi conditions are shown beow. SOIL CONDITIONS Good rock cose to ground surface Firm ground and underying sois Weak or oose sois to depth FOUNDATION TYPE Reinforced concrete (RC) base with rock anchors or bots a) RC gravity base acting as a spread footing b) RC base with pies/tension anchors RC base with pies Bases wi generay be circuar or poygona (hexagona/octagona) in pan shape. Pies may be stee or concrete. The stiffness and dynamic behaviour of the foundation system is ikey to be of concern, particuary in weak and oose sois. Indicative weights of tower and nacee in reation to the foundations are shown beow. WEIGHT in Tonnes CONCRETE TOWER STEEL TOWER 70m High 2MW 100m High 4.5MW 70m High 2MW 100m High 4.5MW Tower Head Mass (THM) 105 220 105 220 Tower stem 450 1050 135 240 Foundation 1400 3000 1500 3100 TOTAL WEIGHT 1955 4270 1740 3560 These figures are approximate and can be infuenced by many variabes between sites and systems. Foundation design is strongy driven by the arge overturning forces that are fundamenta to this type of Wind Energy Converter (WEC) system. In this respect the additiona dead weight of a concrete tower as compared to a stee tower is ikey to prove beneficia for foundation design in counteracting the vertica tension resutant.

Onshore concrete wind towers study PAGE 9 3.3 Tower eements The tower shaft may be spit into a number of different sections for the purposes of strength and stiffness design, fabrication and erection. The possibe types of construction featured in this study and their ikey zona appication have been indicated in Section 3.1. This shows how various combinations of in-situ, precast, and hybrid concrete/stee construction might be appied, thereby aowing optimisation for different physica and deveopment circumstances. The use of precast concrete units for a three zones of the main shaft is an important option. A possibe outine arrangement for such units is shown in Figures 1.4 and 1.5 for 70m and 100m high towers respectivey. The units may comprise a number of segments (2, 3 or more) to compete a ring, or whoe ring units (cast as one unit) according to the ring section diameter and to the weight of individua units, a to take account of ease of transportation and handing. The wa thickness of units/ring sections may be varied between different zones and possiby within the separate zones. Simiary, concrete strength may be varied to achieve an optimum baance between the various cost drivers and physica constraints. The construction sequence for an a-precast tower as shown in Figure 1.6 is described further in Section 3.7. 3.3.1 Base zone Generay in-situ concrete is required for foundation construction (Section 3.2). The construction of the base zone of the tower using in-situ concrete woud not require the depoyment of substantiay different resources, apart from the particuar formwork system for the base zone of the tower she. Wa thicknesses typicay in the range of 350-400mm pus woud be verticay prestressed. This wa thickness is needed for strength. Thickening the wa even further in this zone may provide an efficient way of tuning up the overa tower stiffness since, amongst other things, the additiona mass here has very itte effect on the natura frequency of the structure and can in fact provide a usefu added compressive oad on the foundation. Prestressing tendons coud be accommodated within the wa thickness if desired and as such coud be either bonded or unbonded. The overa construction operations have quite simiar ogistics and do not incur heavy additiona costs in this respect. Aternativey, the use of thick waed (300-400mm) precast concrete segments can provide a soution. These woud be conditioned in size for transport to the site, and might have a width of 3-3.5m and a ength (height when erected) of 12-18m. They woud probaby be best erected in-pace before joining with in-situ poured concrete stitches to form a monoithic ring she. Such joints coud be convenienty made as wide joints, with projecting reinforcement from the precast units interocking within the joints. In either case the base zone construction in concrete can easiy accommodate the oca detais (doorway etc.) for access to the tower. The design can be easiy adapted to provide the base support to an upper stee tower as a hybrid soution. A hybrid design may offer an immediatey attractive soution for those projects with towers above 80-90m height, which are aready essentiay committed to the use of stee towers. Such an approach coud avoid the step change in stee pate thickness required for the she in the base zone, and thereby avoid the additiona fabrication and transportation difficuties that may be experienced with the use of the heavy arge diameter stee tower shes for such towers. 3.3.2 Midde zone Weight minimisation of the midde zone upwards is key to achieving a cost-competitive soution for a fu-height concrete tower design. For a given outine profie of the tower, the direct variabes affecting the weight of the tower she are effective concrete density (i.e. the density of the concrete reinforcement composite) and wa thickness. For the present study the use of ightweight concrete (using specia manufactured aggregates) has not been considered in any depth. The unit weight (density) of concrete considered for structura design purposes is reativey independent of strength and for present purposes has been taken as 24 kn/m 3, which is typicay used for reinforced concrete with moderate eves of stee bar reinforcement. The wa thickness is taken to be uniform for individua units (except ocay at prestressing tendon anchorage points on certain units ony). Various drivers incude structura strength, stiffness, oca stabiity and durabiity requirements which define minimum concrete cover to stee reinforcements or otherwise unprotected stee prestressing ducts and strand. In the midde zone upwards the minimum wa thickness may be determined by the concrete cover to reinforcement requirement rather than by the necessary strength and stiffness. This becomes more critica as the design strength cass of the concrete is increased. It is considered that even by using ony a singe centray paced ayer (two-way grid) of norma stee rebar, but taking account of reaistic pacement toerance even with factory production, it wi not be practicabe to achieve sufficienty reiabe durabiity for a ong ife tower structure (50 years pus) in an aggressive externa environment with a wa thickness of ess than 150 to 175mm.

Onshore concrete wind towers study PAGE 10 Overcoming the restrictions imposed by the norma concrete cover requirement aows the use of thinner concrete was. This coud be important to the minimisation of the weight of the midde and upper zones of the tower and the cost of the tower overa. Various techniques are avaiabe which can overcome these restrictions. These are discussed further, aong with a possibe design soution, in Section 3.5. 3.3.3 Upper zone The design ogic of weight minimisation coud be even more reevant to the upper zone. It is envisaged that the thinnest practicabe she eements wi be used here, subject of course to the provision of sufficient resistance to the maximum cycic wind effects. For present purposes the minimum wa thickness has been set at 100mm. The top of the tower has to support the turbine nacee and yawing bearing. For present purposes it is considered that this connection wi be provided by means of a turbine spur. Notionay this wi be a two metre high section of stee she matching the out tower profie, within which there is a transfer patform supporting the yaw ring, and through which there is a centra access hatchway into the nacee. This fabrication woud be fixed to the head of the concrete tower by a ring of stressed bots. 3.4 Prestressing of the tower The concrete tower is designed as a verticay prestressed tapering tube. A circumferentiay uniform pattern of prestressing forces is to be appied at a magnitude which changes with height (in a stepped pattern) according to the forces that the tower has to resist. This wi provide a circumferentiay uniform vertica compressive stress sufficient to avoid vertica tensie stresses in the tube was under norma design oading. This vertica prestress is aso important to the shear strength of the tower tube. Prestressing provides an efficient and economic means of achieving good a round strength with exceent durabiity, fatigue and dynamic performance. It aso provides a straightforward and positive means of unifying the separate rings of a precast concrete soution so that they act as a monoithic tower structure. With carefu design and detaiing, prestressing can aso provide soutions to the probems of handing units for erection and maintaining adequate strength in the partiay competed tower during its construction. This can be further appreciated by reference to Section 3.7 and Figures 1.6 to 1.8. The choice of genera technique and configuration ies between bonded and unbonded tendons, and interna or externa pacement. In genera, unbonded externa tendons are favoured in this onshore study for their simpicity and ease of instaation athough bonded or unbonded interna tendons coud aso provide a satisfactory soution, particuary for the thicker sections of wa and especiay in the base zone. Externa prestressing invoves the pacement of the prestressing tendons (cabe or bars) outside the structura section which is to be prestressed, and therefore removes the need to accommodate the tendons and any ducts within the thickness of the she wa. The stressing force is imposed through an anchorage detai that transmits it back into the section. An anchorage is of course required at each end of the tendon. The tendon is not continuousy bonded to the section in question athough it may be restrained back to it at discrete points aong its ength. In the case of the tower, vertica prestressing of the tower she may be appied by a circuar array of tendons paced within the concrete tower she cose to its inside face. Cruciay the use of externa prestressing permits the use of thin-waed concrete sections for the tower, since wa thickness is not governed by the need to accommodate tendons internay. Another important advantage is that instaation of the tendons at the various stages of construction can be more easiy achieved. It might be argued that the externa prestressing tendons wi be more at risk from corrosion. There is ong experience of the use of externa tendons on major bridge structures. Whist care aways has to be taken to ensure the durabiity of highy stressed components, particuary in the anchorage areas, there is no reason to think that this represents an unusua or insovabe probem. The tendons woud be protected by factory instaed corrosion protection systems, incuding inhibitors, greasing and heavy pastic sheathing. Anchorage areas wi be accessibe for proper finishing, capping and inhibiting grease injection. Additionay, unike internay ocated prestressing strands, the externa tendons woud be visibe, easiy inspected and even repaceabe. Location within the interna tower shaft provides a shetered and reativey benign environment within which humidity and condensation coud be activey and economicay controed as a further precaution. There are further advantages from using externa unbonded prestressing. There wi be even higher fatigue toerance of dynamic oading because the cabes wi experience an overa averaged and therefore ower stress range during oad cycing, as compared with bonded cabes which are subjected to rather higher more ocaized stresses. There wi aso be ower friction osses during stressing. The eventua demoition of the structure wi be simpified, and the recovery of both the concrete and stee component materias for recycing wi be simper and cheaper. Outine detais for cabe ayout, anchorage and restraint are shown in Figures 1.6 and 1.8.

Onshore concrete wind towers study PAGE 11 It is considered that some restraint of the individua unbonded tendons is required, inking them back to the tower she. Figure 1.10 shows a simpe detai that woud provide such restraint. This restraint woud provide some additiona stabiity to the tower under accidenta overoad or high eve impact as might occur during erection. It woud aso act as a safety measure during construction by restraining any whipash movement shoud a tendon inadvertenty fai due to overstressing during the stressing operation. 3.5 Thin waed she units The importance of minimising wa thickness to the economy of the concrete tower design has been discussed in Sections 3.3.2-3. This is particuary important when cranage is needed to ift arge units into pace. For a vertica direct oad and /or overturning bending capacity, the tube wa thickness can be reduced by increasing the concrete strength cass (it is considered practicabe to increase this up to a compressive strength of 100N/mm 2 as required, and possiby beyond). However there are imitations to the minimum thickness of conventionay reinforced concrete sections due to the need to provide a minimum thickness of concrete cover as expained in 3.3.2 and possiby from the accommodation of prestressing tendons. An approach to overcoming the atter imitation has been set out in Section 3.4. The concrete cover restriction may be overcome by the use of non-corroding reinforcement. Some possibe techniques and ways that these might be used are expained beow: i) The use of non-corroding reinforcement woud remove the need for thick cover (50-75mm) required for externa or aggressive environments, and woud aow cover thickness to be reduced to dimensions necessary to aow good penetration and compaction of the concrete around the reinforcement, and provide good bond strength and transfer of forces from the reinforcement into the body of the concrete. By imiting rebar diameters to sma to medium range (10-25mm dia) cover dimensions coud be reduced to 25mm assuming that the durabiity issue had been overcome. Assuming the use of stee rebar, the options are epoxy coated, gavanized or stainess stee rebar. Both the coated bars (epoxy/gavanized) are susceptibe to mechanica damage or oca weakness of coating, and to consequentia anodic corrosion which coud imit their durabiity and reiabiity for ong ife structures. For the present study it is assumed that stainess stee rebar wi be used where stee reinforcement is required in thin wa situations, or in other detais where ow cover is desirabe or necessary for design reasons. It is generay accepted that stainess stee rebar can be used in conjunction and in contact with norma stee reinforcement within concrete members without any serious risk of cathodic/bi-metaic corrosion probems. It is important to note that stainess stee costs approximatey five times more, weight for weight, than norma high tensie stee rebar. It is therefore vita to imit the use of such reinforcement to a minimum in order to achieve the potentia economy of thinner waed sections. ii) Other techniques for achieving non-corroding reinforcement incude the use of non-ferrous reinforcement in an anaogous way to conventiona stee reinforcement or by using significant quantities of chopped fibre as part of the concrete matrix. The former approach has not been considered any further here as such reinforcements are not in widespread current use with concrete and do not appear to offer significant overa advantages at present. On the other hand the use of chopped fibre reinforcement concrete (FRC) is being widey used in various appications. Techniques and suppy arrangements are we estabished with a significant internationa research effort continuing to achieve improved structura properties and performance for FRC. Suitabe chopped fibre materias are certain poymers (poyesters/poypropyenes), stainess stee wire or arguaby even norma stee wire. Fibre reinforcement can give consideraby increased resistance to cracking and much greater toughness over pain concrete. At present FRC cannot be reied upon to provide reiabe tensie strength to resist major primary structura actions. The question arises as to what strength characteristics are needed for the concrete tower she? The tubuar form provides exceent vertica oad carrying capacity and there is no primary need for tensie reinforcement for this purpose. Overturning bending of the tower wi be resisted by the action of the prestressed concrete tube, as wi horizonta shear and torsion forces about the vertica axis. Bending in the wa she about the vertica axis and associated shear forces, and horizonta hoop forces, are generay ikey to be very sma from externay imposed forces. Such forces, to the extent that they are significant, are ikey to be the resut of strain induced oads from temperature, creep etc. It is considered that these secondary effects can be resisted safey by oca reinforcement and the provision of a eve of genera toughness and crack resistance. The effects of oadings from the turbine at the very top of the tower, particuary cycic oadings, may require wa thicknesses greater than the minimum achievabe. However moving down and away from the uppermost eve, the stress eves from these oads wi drop with the increasing section diameter, and may provide a zone of tower where these waed units provide an appropriate economica and usefu design soution. By combining the use of imited quantities of stainess stee reinforcement with the use of FRC it is considered that thinwaed she units with the necessary structura properties can be economicay constructed. Fibre reinforcement is proposed as the most suitabe materia for aternative concrete reinforcement. Fibre reinforcement is to be used throughout the precast ring segments, but with a the stee reinforcement omitted over the centra region

Onshore concrete wind towers study PAGE 12 of the rings. The prestressed design of the concrete tower ensures that for utimate imit states, no tension occurs in the concrete. Consequenty the fibre reinforcement provides some genera toughness and resistance to secondary effects such as therma stresses. Bands of stainess stee reinforced concrete are ocated at the ends of each ring segment to provide extra toughness during ifting and handing and act as crack stops down the tower. Figures 1.7 1.10 show the arrangement for a typica ring segment. It must be emphasised that whie this approach uses existing concrete technoogy to its imits, in essence nothing new is being proposed. The materias are we proven for the functions they perform. However, this approach coud pay an important part in achieving a precast concrete soution that can be both cost competitive and abe to chaenge the current stee soution. 3.6 Joints These comments appy to the precast concrete soution or possiby to the joint between an in-situ concrete base and a precast midde/ upper section of tower. Simpicity of connection and accuracy (and trueness) of eve are the two key drivers for joint design. The simpicity of the connection wi keep the formwork costs and on-site construction time to a minimum. Accuracy of eve across the top of the joints is imperative in order to ensure verticaity of the fina compete tower. Two basic types of joint are envisaged. One wi be at a connection of the precast units where the prestressing strands terminate, the other wi be at an intermediate connection of the precast units where the prestressing tendons are possiby inked to the tower wa but not terminated. The joints wi be provided with a simpe mechanica connection such as stainess stee dowes to faciitate accurate engagement and assemby, to fix each unit s ocation immediatey after pacement and before overa prestress is appied. This takes account of the possibiity that prestressing may not be appied unti severa units have been stacked up. One possibe concept for achieving a sef-eveing joint invoves the use of three oca seating pads on each side of the joint to ensure verticaity to within a few miimetres. The seating pads are envisaged as thin projections above the genera joint surface that woud be ground to a precise eve to set the genera thickness of the joint. The thin joint wi be fied with epoxy resin. The method of appication of the resin woud need to be carefuy deveoped to be weather toerant and reiabe. It may invove such techniques as injection or pre-paced mortar/ resin pre-paced impregnated tapes. Some of the stainess stee dowes woud be made onger to act as ocators to faciitate rapid acquisition and correct orientation of the incoming unit, and achieve fast connection of the joints in the difficut conditions on site. It is important that simpe jointing and connection methods are devised to faciitate straightforward and rapid erection. Indicative detais are iustrated in Figures 1.7 1.10. These detais are based on the use as far as possibe of pain thin epoxy joints between units, with the units couped verticay both individuay and overa by prestressed boting and tendons. 3.7 Construction sequence A notiona construction sequence for a precast concrete tower is iustrated in Figure 1.6. The precast units wi be transported to site as whoe rings for smaer diameter sections and as ring segments for the arger diameter rings. Ring segments wi be joined into a compete ring on site. Ring sections wi be stacked up, either in pace for the owest heaviest rings, or into tower sections comprising three or four rings for ifting into pace on the tower. Some imited stressing together of tower sections prior to ifting into pace may be usefu to assist with handing, or aternativey a specia ifting crade may provide a quicker and more cost-effective soution. Newy erected sections of the tower woud be stressed either by bots/tendons connected to the underying section, or by fu ength tendons emanating from the base anchorage. This woud provide both the short term stabiity during construction and contribute to the permanent works strength. The aim woud be to achieve an efficient pattern of curtaiment of tendons and consequenty appied prestress which was we matched to the design enveope of externay appied force resutants. This approach to prestressing requires anchorage arrangements at a number of eves over the height of the tower. Figure 1.8 indicates a possibe outine detai invoving the provision of an annuar ring beam formed inside the tower she on seected units. The ring beam woud contain vertica duct hoes at reguar spacing around the perimeter set to aign with the panned tendon paths. This detai coud provide anchorage to ong tendons, short bots, or act as a oca duct for tendons continuing up the tower. The ring beam woud require reativey heavy oca reinforcement in accordance with norma practice for prestressing anchorages. This woud probaby be best provided by stainess stee bars in the form of weded cages, aowing cover to be minimised and thereby reducing the propensity for cracking and aso of course minimising the additiona weight of the ring beam. The number of tendons, curtaiment pattern, anchorage positioning, and so on are interinked with the requirements for erection and construction as we as those for the permanent design. Carefu design and panning at a eves of detai wi be an essentia part of producing an efficient and cost-competitive soution.

Onshore concrete wind towers study PAGE 13 3.8 In-situ sipform construction Precast concrete units offer a viabe design soution for the whoe tower, and aso for the midde and top zones in combination with an in-situ concrete, sipformed base zone. Aternativey, in-situ sipformed construction for the entire pyon height offers some potentia advantages, avoiding the need for transporting precast units to the wind farm site and the provision of heavy cranage. This coud be particuary usefu for sites in areas where access for very arge vehices is probematic. Whist in-situ operations ceary invove the transportation of basic materias to site, this can be undertaken off critica timeines and does not pose specia probems of vehice size or oad size. State-of-the-art mixing equipment such as voumetric truck mixers or mobie site pant can be easiy used to deiver high quaity concrete on site. Minimum wa thicknesses, achievabe without incurring specia difficuties, are in the range of 150 175mm. Beow this thickness, proneness to horizonta cracks due to sipform drag may become a probem. Certain minimum eves of stee reinforcement are aso required to provide the necessary eary support and strength. Thus the thin she concepts described in Section 3.5 do not appy to the same extent to in-situ sipforming. In genera this is not ikey to be a significant source of diseconomy, since cost and productivity considerations of heavy cranage do not appy to sipforming. With regard to productivity and programme time, sipforming, by its nature, is a continuous 24/7 operation with output rates of four to six metre heights per day routiney achievabe. 3.9 Cranage and ifting A current feature of wind farm construction is the depoyment of heavy ong jib cranes to hande and assembe the arge and heavy components or units. This may invove ifting from the transporter for preassembing of sub-units on the ground, and then ifting assembed units from the ground, or ifting direct from the transporter into pace on the foundation or party constructed tower. Such cranage is expensive, particuary if there are deays due to weather, suppy or assemby probems. Sipforming iustrates a technique which removes the need for cranage, at east for the tower construction itsef. At present it woud nevertheess be necessary to depoy a heavy crane for ifting the yaw ring/ nacee/ generator and the turbine rotor either competey separatey, or in a bunny ears configuration with the nacee. A fundamenta feature of sipformed tower construction is the use of the existing tower to provide the main oad path to ground for the raising of the new construction. The tower aso provides the main stabiizer/ atera strength for the man/ materias ift mast. This approach, using the party constructed tower as a major component of the temporary works, provides a possibe pathway to greater economy and construction simpicity for the future, whether for ifting precast concrete units, nacees or others. It woud be interesting to specuate on the form of devices using this approach. It is perhaps usefu to note that there are features of a concrete tower that woud seem to give greater scope and faciity for accommodating such devices, compared to a stee tower. The greater wa thickness of the concrete tower can more easiy accommodate the imposition of high oca whee or camping oads, whist the provision of embedded fixings for track-ways or masts are aso ess ikey to cause stress raiser or notching probems, which coud adversey affect fatigue strength and durabiity. 3.10 Structura anaysis The utimate strength of the tower was determined from the oading at the turbine s maximum operating wind speed. The extreme design bending moment at the base of the 70m ta tower is 70,000 knm and for the 100m ta tower is 210,000 knm both with a oad factor of 1.5. The dynamics of the tower were assessed by cacuating the natura frequency of the tower and comparing this to the 1P-range (singe bade passing frequency). The range for the first natura frequency of the towers was found to be 0.4Hz to 0.8Hz for the 70m high tower, and 0.3Hz to 0.7 Hz for the 100m high tower, encompassing the variabiity of soi stiffness, concrete stiffness and turbine mass ikey. This is simiar to other such towers for wind energy converters. The dynamic oad inputs into tower structures depend upon the characteristics of both the rotor/ generator characteristics, the tower/ foundation structura system characteristics and their interaction. Within the turbine system there are many variabes under the manufacturer s contro, incuding active contro/ management agorithms, which affect its behaviour of, and therefore inputs into, the overa structura system. At genera concept stage it is not possibe to take fu or precise account of a this compexity, athough a range of representative vaues for the major oad effects have been considered. It is beieved that prestressed concrete towers, particuary in the midde/ ower tower zones, wi be ess sensitive than stee towers in their critica design sizing to the variations between different turbines of simiar rating. Exporing and determining this sensitivity is an important part of the next stage of work in deveoping economic concrete designs.

Onshore concrete wind towers study PAGE 14 There is ampe scope to tune the towers dynamic performance both during design and during commissioning. During design the Young s moduus of the concrete, the foundation and in-situ base section geometry and mass, and the mass at the top of the tower can a be varied to produce the desired dynamic performance for the tower. Once constructed the mass at the top of the tower can be atered to tune the tower based on as-buit testing. This might be usefu during prototype deveopment. The use of mass damping at the top of the tower in the turbine woud aso aow the dynamic performance of the tower to be tuned. This abiity to tune the dynamic performance of concrete towers, incuding the abiity to tune the tower during operation, reduces the potentia risk of adverse dynamic behaviour of the tower. It ensures that the concrete tower soution can be designed to avoid interference with the 1P-range and can be tuned to individua site constraints. An assessment of the potentia fatigue damage to the tower design over a 25 year design ife was aso carried out, with concrete performing favouraby. 4 QUANTITIES AND COSTS The eary estimation of costs of innovative design soutions is often very difficut as, by the nature of the situation, itte direct data is avaiabe to the designer. It requires considerabe engineering design, construction and ogistics panning to get to a point where high eves of assurance can be given on cost estimates. In the case of wind energy much of the commercia and detaied construction cost information is hed by the wind turbine companies and to some extent the wind farm deveopers. It is understandaby regarded as inteectua property which is carefuy guarded and, if avaiabe at a, such information is usuay very genera with imited contextua supporting information. In this study we have based our judgement of costs on basic and comparative cost exercises using estimated quantities of the main constituent materias couped with unit rates adjusted for the adverse or advantageous factors that are judged appropriate. The costing exercise was undertaken on the two demonstration a-concrete design soutions (70m ta tower and 100m ta tower). Costs were cacuated based on a worked-up cost per tonne for the instaed tower (excuding foundations and turbine) of around 580/te to 600/te, and using the tower weights detaied in Section 3.2. Comparisons with estimated costs for equivaent 100m and 70m ta stee tower show that the demonstration concrete tower designs are at east cost competitive, with potentia cost savings of up to 30%. In this respect it is considered that sipformed towers provide the greatest potentia savings. These savings are postuated on the basis of a substantia design deveopment process couped with the construction and testing of some prototypica towers. They aso assume that reasonabe economies of scae are avaiabe from the size of the individua deveopment projects and from some continuity on key features of the design between projects. 5 ONSHORE STUDY CONCLUSIONS The contribution of wind energy to the UK s suppy of eectricity is panned to grow substantiay over the next decade, with the number and size of wind farm projects in process and being commissioned now starting to rise in ine with these pans. Therefore the demand for turbine towers is aso rising in both numbers and size. The wind turbine industry is ooking for better soutions for ta wind turbine towers of 100m or more in height. This is being driven partiay by the imminent reease of a new generation of arger wind turbines with onger bades, as we as the reduction in the avaiabiity of unexpoited favouraby windy sites which wi drive the need to expoit ess windy sites, with ess advantageous topography. 100m ta towers are at the imit of existing stee designs and the instaed fabrication technoogy needed to produce them. They aso present serious difficuties for transportation of the ower stee sections for the taer towers. The trends towards increased size of turbines (rating, weight), tower height and therefore diameter and weight, and to arger wind farms a provide more favourabe competitive opportunities for concrete towers. Concrete towers (100m pus height) have been used successfuy esewhere in Europe, particuary in Germany. These have featured both precast and in-situ sipformed designs, simiar in genera terms to some of the design options considered in this study. A the designs considered in this report invove the use of vertica prestressing. Various configurations, techniques and detais have been proposed, based upon the existing state of the art. In order to achieve sufficienty competitive fu height soutions which seriousy chaenge onshore stee tower designs in the UK, substantia design and deveopment effort to optimise concrete designs is needed.

Onshore concrete wind towers study PAGE 15 Key issues for attention are minimising materia content and structura weight, particuary in the midde and upper zones of the tower, whist achieving construction simpicity with short construction times. Concrete technoogy and the materia itsef offer great fexibiity for optimising the design soution for particuar circumstances. Usefu weight and cost savings in the midde and upper zones of the tower may be avaiabe from some deveopment in the use of fibre reinforcement, as suggested in the report. The use of in-situ sipform construction methods provide an efficient way of overcoming the difficuties of transporting arge tower rings/sections to site, and removes the need for the use of a arge crane for the tower erection. The deveopment of craneess methods for the erection of the turbine (rotor) nacee and rotor using ifting patforms cimbing up the tower itsef coud offer further important overa economies. The concrete tower she is we suited to supporting oca concentrated oads that woud be imposed by such devices. The design phiosophy presented draws on concrete s fexibiity and promotes an approach to a soution that can be adapted to suit the individua demands of each wind farm deveopment, and thereby provide a more cost-effective outcome. The use of precast concrete, in-situ concrete and stee in various composite or hybrid configurations is aowed for. Expoitation of the durabiity and onger fatigue ife of prestressed concrete towers coud provide greater sustainabiity and ong term economic benefits. With the tower service ife extended to between 40 and 60 years, one or more turbine re-fit cyces coud take pace with minima further expenditure on the tower, foundations and other infrastructure. Concrete soutions for a 70m and 100m ta tower are derived from the design phiosophy. They show cost competitiveness with ikey equivaent stee soutions. Costings indicate that there is a potentia for savings of up to 30% over stee towers for taer towers (70-120m pus) supporting arger turbines. 6 FIGURES - ONSHORE WIND TOWERS Figure No. 1.1 70m CONCRETE TOWER - Outine and indicative dimensions 1.2 100m CONCRETE TOWER - Outine and indicative dimensions 1.3 HYBRID TOWER - Arrangement with stee upper section 1.4 70m CONCRETE TOWER - Assemby from precast concrete units 1.5 100m CONCRETE TOWER - Assemby from precast concrete units 1.6 TOWER CONSTRUCTION SEQUENCE 1.7 ASSEMBLY OF RING UNITS INTO TOWER SECTIONS 1.8 TOWER SECTION JOINTS - Anchorage section and pain joints 1.9 RING UNITS - Reinforcement and segment joints 1.10 TENDON RESTRAINTS - Yokes to pain units

Onshore - figures PAGE 16 Figure 1.1 70m CONCRETE TOWER -

Onshore - figures PAGE 17 Figure 1.2 100m CONCRETE TOWER -

Onshore - figures PAGE 18 Figure 1.3 HYBRID TOWER -

Onshore - figures PAGE 19 Figure 1.4 70m CONCRETE TOWER -

Onshore - figures PAGE 20 Figure 1.5 100m CONCRETE TOWER -

Onshore - figures PAGE 21 Figure 1.6 TOWER CONSTRUCTION SEQUENCE

Onshore - figures PAGE 22 Figure 1.7 ASSEMBLY OF RING UNITS INTO TOWER SECTIONS

Onshore - figures PAGE 23 0 F 7 Figure 1.8 TOWER SECTION JOINTS -

Onshore - figures PAGE 24 Figure 1.9 RING UNITS -

Onshore - figures PAGE 25 Figure 1.10 TENDON RESTRAINTS -

Offshore concrete wind towers study PAGE 26 STUDY OF CONCRETE TOWERS FOR OFFSHORE WIND FARMS This study was undertaken before the work on onshore wind farms. Some of the concepts touched on in this section have aready been covered, particuary in reation to the design of a ighter weight pyon. Readers are advised to read both the onshore and offshore studies in order to get the fuest information from this report. 7 DESIGN PHILOSOPHY 7.1 Design drivers and approach This section focuses on producing a design soution for offshore wind farms. The design scenarios considered make some aowance for the arger turbines and taer towers that are being deveoped to meet current trends. The aim is to iustrate that the conceptua concrete designs proposed are adaptabe to both present and future industry needs. Design drivers incude: The size of turbines and rotors is increasing, demanding increased tower height, strength and stiffness. Offshore towers have to resist significant wave and wind oadings during storm conditions. Bade rotation eads to cycic oading of both the bades themseves and the overa tower structure. Wave oading on the tower pyon aso invoves a high number of cyces with some reguarity in frequency. Fatigue effects and tower resonance may both strongy affect design. Competitive and economic design requires simpicity of form and detai aied to a basicay competitive technoogy. Aggressive marine environments require durabe materias and detais. Offshore operations are difficut, expensive and weather prone. Carefu panning and design are required for ease and speed of erection and competion. Low maintenance and straightforward decommissioning are aso important. Limited avaiabiity of offshore heavy ifting pant and the imited offshore construction season needs to be considered in design. For greatest economy, arge scae production utiising production-engineered design is needed. Life cyce issues are especiay important for renewabe energy systems. During initia design stages, genera consideration has to be given to buidabiity - incuding fabrication, transportation and erection - based on current capabiities and practice. Reversa of the construction process to aow for eventua remova and disposa at the end of the tower s operationa ife must aso be considered. These issues have been given considerabe thought when deveoping the indicative designs. 7.2 Design scenarios and criteria A generic scenario has been seected as a basis for preparation of tria designs. This is shown in Figure 2.1 and is intended to provide a reasonaby representative but demanding test of both the present and future appicabiity of concrete designs for offshore appications. Tower hub heights of both 80m and 100m above owest astronomica tide (LAT) are considered in order to represent both common current practice and the taest offshore wind towers ikey to be constructed in the next 10 years. A water depth of 25m (from LAT) is chosen. This is based on an assessment of a UK Round 2 wind farm sites. Being at the deeper end of the range, 25m has been chosen to meet future as we as current demand. The scenario is based on the trend of increasing generator sizes, which demand arge rotor diameters and tower heights. It is anticipated that construction in water depths greater than those typica of the first generation offshore sites is ikey to become necessary over the next decade as the avaiabiity of further shaow water sites diminishes, putting more importance on harvesting energy from sites that are currenty ess economic to expoit.

Offshore concrete wind towers study PAGE 27 These further design criteria were appied for preiminary design purposes: A design ife for the structure of 20 years and a return period for extreme storm events of 50 years were chosen, conforming to current practice. The critica design condition is the extreme storm event. Wave oading, wind oading and turbine oads are incuded. Wave induced/tida current oads have not been considered since they are negigibe compared to the extreme wave oading. A 3MW turbine is seected as the reference design case, as it is in the upper range of turbine capacity currenty avaiabe. A commony used turbine mode has been considered, and the manufacturer s design oads have been used for the study. In accordance with the Intergovernmenta Pane on Cimate Change, expected sea eve rise is taken as 0.005m per year, which is negigibe for the purposes of this study. The effect of cimate change on storms and on the recurrence of extreme vaues of wind speeds or wave height has not been considered for present design purposes. An aowance of two metres for seabed eve changes (from scour, etc.) is incuded. Highest astronomica tide (HAT) is taken as 3.8m above mean water eve, with an aowance for storm surge of 1m on top of HAT. An extreme wave height of 12m and wave period of 10 seconds are seected. 8 DESIGN 8.1 Tower configuration For offshore situations, wind towers may be convenienty defined as having two distinct sections, namey: The pyon from the nacee to the eve of the highest wave crest The foundation and substructure everything beow the pyon Pyons experience substantia horizonta oads imposed from wind, through the operating turbine and directy on the tower stem. Additionay the substructure and foundations are subjected to arger horizonta forces from waves and possiby vesses. In comparison with structura forces imposed by turbine and pyon oads, the overturning moments imposed at foundation eve by wave, and vesse oading in deep water sites (20-30m) are virtuay an order of magnitude arger. For deep water sites, it is ikey that spread foundations gravity or pied caisson, or tripod rather than monopie, wi best provide the required dynamic and structura characteristics. Caisson structures can be designed to be sef-buoyant for tow-out and instaation and, therefore, minimisation of sef weight is not critica. Indeed, in such instances additiona mass may be advantageous for stabiity. Nevertheess, it is important to consider imiting the mass of the materia in the design for the sake of economy. On the other hand, construction of pyons onshore with subsequent instaation offshore might invove the use of cranage to ift arge pyon segments. Minimisation of the pyon sef weight wi be important using such methods. The distribution of mass over the height of a pyon is aso important as this is ikey to contribute significanty to its dynamic response. This may need to be tuned in reation to the overa dynamic characteristics of the foundation/pyon system. 8.2 Pyon A tapering hoow cyinder has been seected as the structura configuration of the main wind tower pyon. Recenty constructed wind energy towers, both onshore and offshore, have predominanty used this simpe and eegant form. It provides both an efficient structura member and an aestheticay peasing appearance and, as such, is an appropriate starting point for this design exercise. Variants on the tapering tube structure, or even perhaps some other form, might offer some potentia improvements in efficiency, particuary in terms of specific strength or stiffness (i.e. strength or stiffness per unit weight of structure).

Offshore concrete wind towers study PAGE 28 8.3 Foundation and substructure Ground conditions are ikey to vary consideraby between different sites. Other site conditions may aso vary, incuding water depths. In combination, the variations may demand a range of different foundation types and designs. To take some account of this, the study ooked at two different soutions for concrete pyon foundations: a concrete gravity foundation and a stee monopie with a grouted concrete stem. The concrete gravity foundation considered is a caisson fied with baast or rock. The caisson woud be circuar or poygona in pan. This is a tried and tested soution for underwater foundations which is potentiay simpe and for which there is a range of possibe instaation methods. Where surface sea bed strata have sufficient strength, caissons can transfer oads through direct bearing. Aternativey, caissons coud be pied for weaker underying ground conditions, to increase the strength and stiffness of the foundation as necessary. The caisson woud be set down on a prepared bed - usuay comprising a eveed stone banket. Typicay the caisson may have a down-standing skirt or cutting edge around its perimeter. This woud ensure a good horizonta key-in and provide some protection against bed scour and oca soi weakness. An outer annuus of anti-scour rock may aso be needed, depending upon the scour potentia from waves, currents and bed mobiity. It is remarked that scour protection is aso an important and critica issue for monopie foundations. An important aim is the achievement of maximum simpicity for gravity foundation designs and instaation which can be appied to as wide a range of site conditions as possibe. It wi therefore be necessary to devise soutions which can, to a satisfactory extent, cope with a range of sea bed sopes, with poorer near-surface ground bearing strengths, and which aow the instaation of scour bankets with minimum on-site preparation and reativey ow sensitivity to wave and tida conditions. Whist the initia focus for this study was the design of the tower superstructure, it is recognised that it is ony one component of an overa structura system comprising: Bearing strata (ground/sea bed) Formation and protection at interface Foundation and tower stem (substructure) Interface formed by transition piece providing adjustment and connection Pyon (tower superstructure) Interface providing yaw ring/bearing, connection and adjustment Rotor and nacee (carrying the hub and drive shaft, gearbox, eectrica generator, anciary equipment, contros etc.). The rotor and nacee have been umped together here for simpicity but for other purposes they might be better defined as a number of separate components The conceptua and detaied designs of these interfaces/joints are fundamenta to the success of the eventua system and need to be considered, to some extent, even at the initia feasibiity stage. It is amost certain that the potentia benefits from the use of concrete tower structures wi be most fuy reaised in conjunction with the use of concrete offshore foundations and substructures. 8.4 Structura design methodoogy Preiminary design has been based on a static anaysis of the proposed towers under the chosen design oads. The dynamic response of the towers has been anaysed to assess susceptibiity to resonant excitation by either the turbine bades (cycic variation in oading from bades passing the tower) or by consecutive waves hitting the tower. The tower s natura frequency has been cacuated and compared with the range of the turbine bade passing frequency and the wave periods for the arger waves. Natura frequency response of various types and dimensions of tower, both stee and concrete with different foundation conditions, has been estimated. The comparisons provide pointers to possibe design issues for overa tower systems in deep waters, particuary where underying soi properties do not provide sufficienty stiff support. It was identified that concrete tower pyon and foundation systems are ikey to offer advantages over current stee designs in this regard, particuary as tower heights increase. These comparisons are discussed further in Section 10.2.

Offshore concrete wind towers study PAGE 29 8.5 Commentary on ife cyce issues The economics of renewabe energy systems are dominated by initia capita, maintenance/operationa and decommissioning costs, whist fue is effectivey free. This can ead to quite different cost/benefit baances and, therefore, different ifecyce strategies for design and operation from fossi or other fue-driven systems. In order to achieve the potentia benefits of these different strategies, the design approaches and ife cyce criteria for offshore wind machines need to be considered on their own merits. This is now becoming more widey understood. Athough there is sti some debate about the actua overa benefit in terms of CO 2 reduction, taking into account the variabe nature of the output, there is a broad acceptance of the important environmenta benefits of offshore wind generation. For the purposes of this study it is satisfactory to consider potentia improvements on a comparative basis. An interesting potentia benefit from the use of a prestressed concrete design coud be the increased ife span of the tower structure. To fuy reaise this, the substructure woud have to have simiar durabiity. It is possibe to think of re-fitting the turbine after its design ife of 20 years with a next generation design over two, three or even four turbine ife cyces, thereby avoiding the financia and environmenta costs of reconstructing the tower and foundations. (Current thinking in Denmark in reation to stee towers is that they shoud be designed for a ife of 50 years). For the present exercise, the main interest ies in design concepts and approaches which can minimise production and instaation costs. However, even at this eary conceptua stage it is possibe to envisage detaied design arrangements that coud deiver ife cyce improvements in this way. Other sections of this study describe possibe production processes and design detais which are potentiay simpe to fabricate and assembe, have good durabiity and reiabiity, and aow reversa of the erection process for eventua decommissioning. 8.6 Brief description of the outine designs Figures 2.2-2.5 iustrate variants of the basic pyon design, aied to different substructure/foundation configurations. They provide for different site situations and a different baance between wa thickness and overa tube diameter, to achieve simiar strength and stiffness performance. It shoud be noted that no attempt has been made at this stage to identify an optimum configuration, which may in any case be strongy infuenced by the characteristics and constraints of specific sites. Variabes such as turbine oading and environmenta conditions are ikey to vary consideraby depending on the project. Consequenty the pyon soutions presented are considered to be a first design iteration. 8.6.1 Continuous taper design (Figure 2.2) Within the ower pyon sections, the pyon diameter is kept down by the use of a wa thickness of 400mm pus. This aows a smooth tapering profie to be maintained through the upper wave zone and down the ower stem without attracting excessive wave forces. For the arger water depths considered in this report, the strength and stiffness of the ower stem may be increased as necessary by faring out the stem diameter or buttressing. 8.6.2 Necked stem design (Figure 2.3) This aternative iustrates a different approach to maintaining the necessary pyon strength and stiffness. The maximum wa thickness in the pyon stem has been imited to 250mm and the pyon diameter increased accordingy. In order to avoid excessive wave oadings the tower has been necked through the upper part of the wave-affected zone. The approach to maintaining sufficient strength and stiffness in the ower stem mentioned in Section 8.6.1 can aso be appied here. 8.6.3 Concrete pyon with a concrete gravity foundation The options in Figures 2.2 and 2.3 incorporate the use of a concrete gravity foundation. This type of foundation achieves stabiity by virtue of its mass and effective weight. Since the horizonta component of the principa operationa and environmenta oads may be appied with generay simiar magnitude from any compass direction, a circuar or poygona pan shape is most efficient. Simpe gravity foundations transmit the vertica resutants of the appied oading (incuding sef weight) by direct bearing on the sea bed foundation. The strength and stabiity of the upper eves of the sea bed strata have a particuary important effect on the behaviour of the foundation, athough, as a rue of thumb, soi properties within a depth of 1.5 times the base diameter need to be considered.

Offshore concrete wind towers study PAGE 30 Acceptabe foundation diameters are considered on moderate strength sois. For situations where ground bearing strength is ow, it is common practice to use pies to support the base and thereby imit base diameter. This option is notionay avaiabe offshore, but the instaation of substantia numbers of integray connected pies for each foundation woud be impractica and uneconomic. Certain ground improvement techniques might be feasibe and coud widen the conditions for use of gravity foundations. This approach coud be particuary vauabe for deaing with margina variations in ground conditions within particuar wind farm sites. Vertica oad is necessary to provide acceptabe siding resistance and resutant bearing stresses, under the high horizonta and overturning forces.the additiona weight of the concrete tower is advantageous in this respect. The foundation needs to be massive, particuary bearing in mind the buoyant upift on submerged structures. It is economica to provide part of the foundation mass as baast, in the form of dense rock or sand (if suitaby contained). Gravity foundations are conventiona engineering soutions, both onshore and offshore, and have been used widey for towers and chimneys. The dominance of the horizonta oading component in wind energy structures (and the associated cycic fuctuations) is towards the extreme end of the spectrum. Experience of ong term foundation behaviour under these conditions is imited but this is true of any appicabe foundation type. The gravity foundation might take the form of a arge ceuar caisson with a base sab, either open topped or cosed with a top sab. The open ces faciitate the pacement of rock baast onto the foundation after instaation. A tria design for this type is shown in Figure 2.4. Design exercises have shown that this is probaby not the most economica type for the arger diameter bases under consideration. Aternativey the cosed ces provide the possibiity of buoyancy to assist fotation of the caisson for instaation. A cosed ceuar foundation can aso be permanenty baasted with sand, pumped into pace, which generay woud provide a cheaper soution than rock. Additionay, the ower tower stem may be buttressed against the caisson base to assist in the efficient transmission of oads from the tower into the foundation. An efficient version of the cosed ce concept can be achieved by faring the tower stem into a arge cone, with a base diameter approaching that of the required foundation base size. This gives a simpe tower she which is efficient in spreading the oads to the base sab, with a structuray efficient form for tower strength and stiffness. Additionay it can provide temporary buoyancy for the tow out condition, can be easiy water baasted as part of, or immediatey foowing instaation, and can be economicay baasted with sand for the permanent condition. Base diameter for such conditions woud be in the region of 30-40m, depending on the depth of water, site exposure and turbine rating. The presence of a foundation and tower structure projecting above the sea bed wi affect the fow patterns and bed stresses in the vicinity of the tower, due to waves and tida currents. Depending on the bed conditions, scour protection wi be needed. This may be provided at various eves of protection, but particuary by means of a perimeter skirt to the caisson penetrating into the bed, in conjunction with an annuar banket. This banket woud need to be fexibe and simpy instaed. It might simpy comprise suitaby graded stone or be a more connected arrangement of eements. Gravity bases have not to date been used extensivey for offshore wind towers. However concrete bases have been depoyed successfuy and economicay on two or three substantia Danish wind farms (incuding Middegrunden and Nysted). To date, there are no exampes in UK offshore wind farms. A key feature of their potentia ies in their simpicity. The opportunity exists for a reativey simpe and quick procedure for instaation, which coud reduce time on site and weather risk. The achievement and expoitation of this potentia simpicity is discussed in Section 8.8. Reaisation of the potentia simpicity of offshore instaation of the gravity foundations, couped with expoitation of the associated instaation benefit for the whoe tower, coud provide significant cost savings. The abiity of gravity foundation designs to cope satisfactoriy with the range of topography and bed conditions encountered in particuar sites coud have an important bearing on the number of sites for which this concept might be appied economicay. There is a variety of design configurations and techniques which can enhance versatiity. These questions can ony be answered with rea authority on the basis of further study. However there are good grounds to beieve at this stage that there is sufficient potentia benefit to justify those studies. 8.6.4 Concrete pyon on a stee monopie (Figure 2.5) Concrete pyons coud be founded on a stee monopie if desired and if foundation conditions aow. The design ife of monopies coud be increased, if required, by additiona corrosion protection; for instance by jacketing the most vunerabe upper section of the stem and/or with other measures such as cathodic protection. For arger wind machines and greater water depths, monopies are reaching economic and performance imits, and aternatives such as pied tripod foundations or gravity foundations may be technicay more suitabe and cheaper. Nevertheess, some arge wind farm sites may encompass a wide range of foundation conditions with water depths varying between, say, 5m and 30m and bed soi profies and characteristics aso vary significanty. In these circumstances, it may be necessary to adopt more than one foundation type. Versatiity and interchangeabiity of component designs coud be vauabe in such situations.

Offshore concrete wind towers study PAGE 31 8.7 Pyon fabrication and erection A significant part of the capita cost of wind towers wi be determined by the offshore erection operation and the perceived eve of risk associated with it. In order to achieve the owest instaed cost, it is important to recognise and aow for this and a other key fabrication and instaation operations in the seection and deveopment of conceptua designs. It is important to deveop the engineering design to a high eve of detai, with fu regard to production engineering and the environmenta conditions that might be experienced during erection. Carefu attention must aso be paid to risk minimisation through design of the structure and erection methodoogies. It can be concuded right from the start that it is not feasibe to undertake extensive in-situ concrete construction work offshore on tower structures. The approach considered here reies on the use of precast concrete units and is broady anaogous to the current methods of offshore assemby of stee towers. The method proposed is not intended to be definitive, but provides a usefu basis for reaching initia concusions about feasibiity and cost. This is described beow and is iustrated in Figures 2.6 2.8. 8.7.1 Precast concrete method The pyon is envisaged as being erected offshore from three or four arge segments depending on the avaiabe ifting capacity pre-seected for the project in hand. Other things being equa, it is preferabe to use the owest number of segments possibe to achieve the required tower height in order to minimise the number of ifts. Transport to site may have a bearing on the decision. Each segment woud be pre-assembed from a number of she units as iustrated in Figure 2.6. A set of shes woud be cast and then assembed into a segment, with prestressing strand threaded down the vertica ducts cast into the wa of each she. The strands woud be prestressed and grouted to produce a monoithic segment ready for transport offshore. In prestressed concrete terminoogy, the segments woud be post-tensioned bonded structures. 8.7.2 Sipforming the tower Sipforming the tower is a continuous casting technique for buiding chimneys, sios and towers, which avoids the formation of joints and the use of heavy cranage. In the right conditions it can provide very economica structures, particuary where the specia rig fabrication costs can be defrayed over a number of simiar structures. Sipforming offshore is not considered to be economicay feasibe. However, if transport and instaation techniques can be devised that aow construction onshore, or inshore in shetered waters, then it coud provide an attractive option for tower (stem and pyon) construction. This is considered further in Section 8.8. 8.7.3 Prestressing techniques and detais The basic technique considered is the use of bonded (grouted) post-tensioned stee cabes, housed within gavanized stee or pastic ducts. This requires sufficient wa thickness to accommodate and give proper concrete cover to the duct, which may imit the minimum wa thickness. This may be an unwecome constraint in the midde and upper zones of the tower pyon. It aso invoves the grouting operation after prestressing, which, if it coud be eiminated, woud usefuy reduce the work operations offshore. Aternative approaches coud invove the use of unbonded tendons, aso in post-tensioning mode. These might be simpy substituted for a bonded system and instaed in the cast-in ducts with suitabe protective systems; for instance, the norma sheathed strand for unbonded appications pus a duct fi of corrosion inhibiting grease/wax. More radicay they coud be instaed as externa tendons on the inside face of the pyon she. They woud be restrained and inked back to the separate shes by suitabe brackets at each she joint to ensure composite action under abnorma horizonta or bucking oading. There woud need to be a robust and reiabe corrosion protection system in pace, but given the simpicity of the system this shoud be practicabe, particuary for the tower pyon. It woud be possibe to provide a further ine of defence by introducing some active contro of the tower s interna reative humidity. This might invove (and aso benefit) cimate contro of the nacee. Such approaches are being considered for stee towers and are thought to be reativey simpe and cost-effective. Potentia advantages of externa cabe systems woud be great simpicity, economy and speed of instaation, couped with inspectabiity, repaceabiity and ease of dismanting pyons at the end of their ife cyce.

Offshore concrete wind towers study PAGE 32 8.7.4 Anciary detais There woud of course be a number of detaied finishing operations; for exampe the instaation of a ight stee stair/ift tower. These woud incude interna anding patforms to assist in the offshore operations during pacement and joining of segments. This ight interna structure woud aso serve to provide access for subsequent inspection and maintenance of the turbine assemby and of course, the tower itsef. Simiary, certain other fixtures coud be made, such as the J tube/eectrica cabe conductor tubes (whether of externa or interna arrangement), the boat anding patform for maintenance and genera access. 8.7.5 Formwork for precast units Formwork to produce the set of tapering units needed for each tower represents a significant setup cost. Given sufficient initia investment in formwork and other production equipment and faciities, repeatabe accurate surfaces and dimensions can be achieved without a major cost premium. This investment needs to be defrayed for a significant voume of business (i.e. sufficient numbers of units to be produced on an essentiay continuing basis). For instance, given 50-100 reuses for formwork, fabricating high accuracy stee formwork to produce coned concrete shes/segments shoud be economic. (Conica stee formers may be produced by pate shaping and roer techniques simiar to those used for forming the structura shes of stee towers.) Such voumes are avaiabe within a singe project of the size of the arge Round 2 wind farms. 8.7.6 Joint detais The aim is to keep joint detais as simpe as possibe, both in form and in assemby. Joints between she units within segments are iustrated in Figure 2.9. These joints are based upon the use of a thin epoxy resin fier, with materia being paced before the positioning of the upper she. It is assumed for the moment that vertica assemby woud be most suitabe. Accurate aignment and fatness of she unit joint surfaces wi be required. It is envisaged that assemby wi take pace under controed and efficient conditions with an assemby frame providing access, weather protection and jigging. The segments wi be prestressed in this assemby position and the stee strand grouted into preformed ducts, with post-tensioning in the norma way. Joints between segments woud need to be made in the more exposed and difficut circumstances of the offshore site uness the erection/instaation approach discussed in Section 8.8.3 is adopted. It is important that the segment being erected can quicky be owered and accuratey paced into position and secured. This may invove detachabe temporary guides or fixings. The joint detai shown in Figure 2.10 is intended to aow for this. It may be possibe to use a number of ong prestressing bots to provide initia fixing, with a nomina prestress prior to seaing and grouting the thicker in-situ epoxy joint. Accurate vertica aignment of the upper unit coud possiby be achieved by pacement onto a imited number (say three) of sighty raised and accuratey pre-set bearing pads. These might be stainess stee pates or epoxy concrete. The joint woud be edge seaed and injected with epoxy grout. It woud be necessary to design this operation to be undertaken quicky and simpy and for it to cure sufficienty rapidy to aow eary stressing. During she and base segment assemby, a proprietary gasket or connector might be used to achieve aignment of the duct tubes and seaing against epoxy grout intrusion. The prestressed bots coud be eft unbonded, but woud be endcapped and the ducts fied with protective grease. This coud provide not ony good protection, but woud aow sampe bots to be inspected during their ifetime, aow any repacement if necessary and faciitate eventua decommissioning. There are, of course, other ways that joints coud be made and a more thorough study of joint design woud be an important part of the next stage of work for a precast method.

Offshore concrete wind towers study PAGE 33 8.8 Construction and instaation of the foundation and tower 8.8.1 Current instaation methods for stee towers For offshore wind farms the predominant structura configuration at present is stee tower/stee monopie foundation. The instaation of offshore wind towers currenty invoves at east two distinct major operations at the offshore site, namey: Foundation - substructure instaation Erection of the tower pyon, nacee and rotor Monopies comprise heavy, arge diameter, stee tubes driven to considerabe depth (typicay 25-30 metres pus) where conditions aow. Driving through gacia deposits (bouder cay, moraines etc) or weathered rock can be probematic and uneconomic. Where there is underying rock at shaow depths, the pie may need to be housed into sockets cut into the rock and grouted. Commony, an adjustment piece comprising a substantia ength of oversize tube is fitted over the driven pie. This tube provides the arrangement for correcting any deviation from verticaity in the monopie and has a connection fange at its top end for the connection of the tower above. After setting to the required vertica aignment, the annuar space between the adjustment tube and the pie is cement grouted to provide the required structura connection. Purpose-designed instaation pant has been commissioned in the past two years or so, incuding various jack-up patforms for monopie handing and driving, and jack-up ships for transporting and erecting pre-assembed pyons, nacees and rotors in one or severa ifts according to size. Tower instaation vesses are capabe of transporting severa (three or four) towers on one trip. In suitabe conditions, this pant can compete these operations efficienty within quite short periods on-station. It shoud be possibe to use the current speciaised pant for tower configurations using stee monopies and concrete pyons. Even so, erection cyces for a concrete pyon may be a itte more extended, due to the greater number of ifts per pyon dictated by the maximum crane ift capacity. With carefu panning and optimisation of the concrete pyon weight, this may not invove any serious increase in the on-site time and weather risk. 8.8.2 Instaation of gravity foundations There are many situations where gravity foundations might be used as an aternative to monopies. The use of concrete gravity foundations for offshore wind energy converters has so far been imited to two or three projects in the Batic Sea with reativey shaow water depths and where winter sea icing has been an issue. Typicay instaation has been undertaken efficienty using adapted fat top barges carrying the foundations as deck cargo, aong with separate foating cranage for pacement. Towers and turbines have been erected ater as a separate operation as is norma for monopie foundations. Gravity foundations (concrete caissons) for deeper water woud necessariy have to be quite arge, typicay with a diameter of 35-40m (being circuar, octagona or hexagona in pan). The instaation methods adopted to date woud invove the use of very heavy offshore cranage and invove a prohibitive cost. It is understood that the use of concrete gravity foundations for the Batic Sea projects has resuted in usefu cost savings. For simiar circumstances, it is possibe that the techniques used to date coud be further improved to good effect. However, they are unikey to be sufficienty economica for the greater proportion of future offshore wind sites where deeper water and more severe wave conditions wi be encountered, particuary in view of the arge foundation sizes and weights ikey to be associated with these conditions. In order to achieve the substantia cost and risk reductions that are needed to attract investment for the offshore wind programme, a radicay different approach that addresses the major cost and risk centres must be found. There is currenty no purpose-designed pant to hande such units athough, with some ingenuity, it shoud be possibe to create faciities and pant at a viabe cost. Caisson foundations in nearshore and offshore situations have a ong history in civi engineering practice and aso in the offshore oi industry over the past few decades. 8.8.3 Aternative proposas for caisson and tower construction and instaation As mentioned in Section 8.7.2 it shoud be possibe in principe to achieve a very efficient process with instaation work offshore imited essentiay to the pacement of a competed tower structure in one operation, thereby avoiding the potentia weather risk from extended erection activities on the tower superstructure. This woud invove assemby and erection of the foundation and tower structure inshore, foowed by a tow-out of the whoe unit as a singe entity. Siting a shore-based waterside production faciity where there is access to reasonaby shetered deep water, such as an estuary-ocated port or shipyard woud aow the foowing method to be used:

Offshore concrete wind towers study PAGE 34 1) Construct the caisson and tower stem on a waterside construction berth (with suitabe aunch arrangements) or in a wide dry dock. 2) Launch or foat-out caisson with suitabe temporary fotation attached for stabiity. 3) Sink onto prepared harbour bed pad in shetered ocation (water depth say minimum eight to ten metres). The construction of the tower onshore or in shetered inshore conditions opens up the possibiity of using two quite different methods of tower construction as aternatives: a) precast concrete units/ segments, b) sipforming (see Sections 8.7.1 and 8.7.2). 4a) Precast methods. Construction pads might be arranged aongside a quay with suitabe water depth (say 8-10m incuding neap tida range) or either side of a finger jetty projecting from the shoreine. The quay/jetty woud carry a traveing gantry crane of sufficient capacity to ift at east the argest singe precast units, (say 50 tonnes), or ideay the fuy fitted or substantiay fitted out nacee (100-250 tonnes pus depending on turbine type/rating). The capacity of the gantry crane coud be substantiay reduced to, say, 60 tonnes if the requirement was imited to ifting individua precast concrete tower units, eaving the turbine (rotor generator etc.) to be ifted by crane barge, either inshore or offshore, depending on the preferred methodoogy. Simiary, a smaer crane barge for ifting the precast units coud provide a more economic ifting faciity for situations where the number of tower units to be produced was insufficient to support the investment required for a more efficient faciity. 4b) Sipforming method. Cose proximity of construction pads to a quay or jetty woud be ess important as the tower sipform operator in the inshore pads coud be supported by a simpe foating barge or jack-up patform, to provide accommodation for personne, deck storage for materias deivered by barge and space for a sma concrete batching/mixing pant. The sipform rig woud probaby be attached to the base caisson onshore and sufficient height of tower stem/pyon woud be competed before aunch and movement to the pad. Once the caisson was setted down on the (temporary) construction pad, sipforming woud resume unti fu competion of the tower. The next stages of ifting the caisson/tower structure off the pad and transporting to site woud proceed simiary for each aternative. 5) A catamaran barge with an overa width of about 40m woud be moved to stradde the tower and ift the caisson and the whoe tower unit off the pad for transport offshore. The caisson/ower tower woud be connected to the barge using ifting tendons we distributed around the structure. Lifting might be achieved by, say, strand jacking or might be achieved or assisted by de-baasting the barge or the effect of the rising tide. 6) The design of the shape and sizing of the catamaran barge is itsef a matter of detaied engineering and nava architecture study. The outcome woud depend upon the range of tower unit sizes and site water depths to be catered for. The tower caisson configuration coud aso affect the ifting/carrying capacity needed, as some designs coud provide substantia buoyancy assistance, reducing the required capacity from, say, 4000-5000 tonnes to 2000-3000 tonnes. For instance, sufficient capacity for the atter case coud be provided by twin hus, each of 60m x 10m x 5m depth, operating at a maximum draft of, say, 3-3.5m. This spacing of the hus woud be determined by the tower stem/upper caisson shape and dimension. Aowing for buttressing or significant fare on the tower stem, a separation of some 20 pus metres may be required. The tower woud be ifted off the pad and moved to deeper water if necessary, where the draft to the underside of the caisson unit coud be adjusted for the tow offshore. A draft of 8-10m is expected to give good stabiity for open sea towing. 7) The whoe unit woud be towed by tugs to site to arrive during a predicted weather window, suitabe for owering or sinking the tower onto the bed (prepared as may be necessary). 8) Rock barges or a sand dredger woud rendezvous at the site to baast down the caisson once the catamaran barge had ceared the site, to give fu storm stabiity to the tower. This method coud be used for constructing and pacing a concrete caisson unit with stem, ready for in-situ erection of a stee or concrete tower, if circumstances favour such an approach. The faciity for constructing and aunching the substructure unit (caisson and stem) aone woud not need the cranage faciities for buiding the pyon. Figure 2.11 iustrates this atter situation, whist Figure 2.12 shows the in-shore construction and tow out of the whoe tower. The methods iustrated provide different methodoogies for offshore instaation of a pre-assembed foundation/ tower at varying degrees of competion. They incude the possibiity of achieving the utimate eve of prefabrication, by the inshore competion of a fuy fitted out and equipped caisson foundation/tower/turbine unit pre-tested and commissioned before tow out, requiring ony imited finishing operations offshore, incuding fina baasting, scour protection, and hauing in and connection of the eectrica cabing. This may seem too ambitious at this stage, but it nevertheess appears to provide a pathway to maximum economy for offshore wind farms, based upon horizonta axis wind turbines as conceived at present.

Offshore concrete wind towers study PAGE 35 8.9 Versatiity of the concrete tower concept The design concepts may be appied in a wider way than simpy presenting concrete as the aternative soution for the entire wind tower. Concrete foundation eements and tower eements may be used separatey in combination with stee structures in a variety of configurations. Options incude: Concrete pyon with a concrete foundation. Concrete pyon grouted onto a stee monopie. Stee pyon boted to a concrete foundation. Concrete/ stee hybrid tower onto any of the above foundation types. Figure 2.13 iustrates the versatiity of the concept. Up to now, stee monopie foundations and towers have dominated the offshore scene. The projected major increase in the instaed capacity of offshore wind energy generation over the next decade or two provides an opportunity for a wider range of structura types and designs to be used to achieve optimum economy. The abiity to use hybrid materia combinations for foundation/tower structures, and even within the tower pyon itsef, provides significant fexibiity for design optimisation for particuar projects. It aows better design tuning for a wide spectrum of static and dynamic oading, for site circumstances (water depth; wave cimate; bed and foundation conditions), ogistics, and the prevaiing economics of construction, pant avaiabiity and materia suppy. Certain hybrid combinations, besides having their own intrinsic merits for particuar circumstances, may aso be used to provide a series of deveopment stages prior to the depoyment of the fuy assembed and outfitted concrete foundation/tower units proposed in Section 8.8. 8.10 Standardisation of pyon geometry The achievement of a high eve of re-use of specia production equipment is ikey to be a key part of cost competitiveness and reduction. In the case of the tower pyons constructed using the precast method, this woud concern the formwork/ mouding system, for instance, which woud necessariy be purpose-made and represent a significant initia investment. At the most basic eve the mouds woud have a fixed geometry with itte or no adjustment. These coud therefore ony produce a particuar tower profie (maximum/minimum outer diameter and taper). The production of a tapering tower barre (for chimneys and other towers) is routine in sipforming practice, with weproven arrangements for continuous hydrauicay-driven adjustment of the diameter of circuar section forms. Simiary adjustabe forms are possibe for the production of precast units. The baance between the compexity, and therefore the cost, of adjustment mechanisms and the benefits is a speciaist technica matter. However the economics wi need to be predicted from the costs of acquiring and operating the formwork system, its actua output as judged from its potentia output and ikey utiisation. It can be reasonaby supposed that some effort to define a standard profie or standard range of profies, taking account of these considerations, is ikey to be beneficia to promoting the wider use of concrete pyons. Given that considerabe finer tuning of the design can be achieved reativey easiy by adjustment of concrete strength, prestress eve, reinforcement and wa thickness, it may be possibe to define a imited set of standard profies that are suitabe for a wide range of tower heights and duties. 8.11 Prototype and demonstration designs A number of onshore concrete tower pyons of considerabe size (height 80-120m) have been constructed in Germany in the past five years, using both precast and in-situ sipform techniques. Concrete gravity foundations have been used on at east two or three Danish wind farm projects in the Batic Sea. No concrete wind tower pyons have been used offshore anywhere in the word as far as is known. However it is remarked that a substantia number of very arge concrete production patform structures have been depoyed in the North Sea for the ast two or three decades. These incude major sipformed concrete towers supporting the deck. It wi be amost certainy necessary to produce a number of prototype concrete structures at fu scae or near fu scae for deveopment purposes and for credibiity and confidence. This is ikey to be the case for the main eements - foundations and tower pyons - both separatey and in combination. For concrete tower pyons the easiest prototype deveopment route woud be their use in onshore wind farms. This shoud reduce deveopment costs significanty by offsetting the prototype costs against the market price of the conventiona soution.

Offshore concrete wind towers study PAGE 36 If a precast concrete soution were to be adopted, usefu guidance on various practica issues might be gained from experience in Germany, from discussions with speciaist formwork manufacturers and from speciaist fabricators producing conica and tapered stee towers. The aternative technique of in-situ sipform construction coud probaby be more easiy adopted for singe tower or a imited production run. The unit costs coud be significanty reduced for the atter case. 9 QUANTITIES AND COSTS The object of this study is to draw initia concusions about the feasibiity and cost of concrete towers for offshore wind energy converters. Reative costings are necessariy based upon educated judgement of the aowances to be made for both positive and negative uncertainties. Quantified estimates have been made upon simpe mode designs for the tower and foundations. Some aowance has been made for possibe improvement in the design. However, reaising the benefits of scae, possibe improvements from quantity production and design refinement are set against the uncertainties connected with such things as the cost of the particuar offshore operations required. At this eary stage, absoute cost evauations are ikey to be ess reiabe than comparative costings between concrete and stee soutions. Even more so, since there remains considerabe difficuty in obtaining actua cost information on components of wind energy systems, despite the now substantia experience of constructing offshore wind farms, as this is a cosey guarded commercia asset for the main providers. Various aternative mode designs have been seected for the mast. Tria designs for a concrete caisson to act as a gravity base foundation have aso been undertaken as a base point for costing assessments. Our costings of the pyon designs indicate that the instaed cost of concrete pyons coud be competitive with stee pyons, provided that the initia investment costs for formwork and unit production were spread over a sufficient production run of, say, 50 units. Simiary we concude that concrete caisson foundation units, apart from specia circumstances in shaow waters such as those in the Batic Sea, coud be competitive with aternative monopie, or more particuary, tripod foundations for deeper water sites to about 20-30m and for arger rated turbines, say 3.5MW pus. As for towers, the efficient production of the arger caisson units associated with these conditions is ikey to require the depoyment of purpose-designed aunching and transport arrangements. Production runs of 100 to 200 units woud probaby be required to defray the capita costs sufficienty. The possibiity exists for deeper water sites to construct a concrete gravity foundation and concrete pyon as an integrated unit in a shetered inshore ocation, fit it out competey and then tow it offshore using a catamaran barge stradde carrier, setting it onto a prepared bed in a singe operation. This integrated approach to instaation coud offer a route to substantia cost reduction if appied on a sufficienty arge scae as mentioned above. Our assessments suggest that savings of 20-30% coud be avaiabe in both the buid and instaation cost over current methods. The foundation/ tower buid costs and the associated offshore instaation, incuding the instaation of the turbine, represent about 40% of the tota instaed capita cost for a wind farm. The tabe iustrates the range of saving to the overa capita costs that is potentiay avaiabe from the integrated approach using concrete foundations and towers as described in this study. Item % item cost reduction % item cost to tota capita expenditure Construction cost 25-50 20 5-10 Instaation cost 25 20 5 Potentia reduction 10-15 % capita expenditure reduction If this assessed potentia can be shown to be achievabe for a sufficient number of wind farm sites by more detaied feasibiity studies, then this method and simiar approaches coud make a very substantia contribution to the cost and risk reduction needed to move forward new investment in UK offshore wind farms.

Offshore concrete wind towers study PAGE 37 10 COMPARISON OF STEEL AND CONCRETE TOWERS This section provides an overview and comparison of the features and reative advantages of stee and concrete towers. Athough this section was prepared specificay for offshore towers many of the points are reevant to onshore towers. 10.1 Comparison of the dynamics of stee and concrete towers Existing stee monopie wind towers have natura frequencies faing comfortaby in between the bade passing frequency and the wave excitation zone. The newer generation of taer stee towers wi have onger natura periods of osciation than currenty instaed designs which move their natura period into the wave excitation zone (assumed to be six seconds and above). Larger and heavier turbines wi aso give the towers onger natura periods, exacerbating the probem. A chart comparing the natura frequencies of various stee and concrete towers has been produced (Figure 2.14). With different foundation conditions, concrete towers on stee monopie foundations woud potentiay have a simiar probem to stee monopie towers. Concrete can be more easiy and cost-effectivey adapted to arger diameters to produce stiffer towers to combat this potentia probem. The stiffness of the overa tower structure depends upon the stiffness of the pyon, tower stem and foundation. Monopie foundations have reativey imited structura dimensions and a restricted interface with the supporting soi. This consideraby imits the stiffness achievabe, particuary on poorer sois. Gravity foundations invove more of the surrounding soi in resisting oading than the monopies. They therefore reduce the natura period of the concrete tower system compared to a simiar tower with a monopie foundation. This is advantageous for the taer wind towers and provides more scope for tuning the design to bring the natura period back in between the two excitation zones. Concrete used for either the pyon, foundation or both can therefore offer advantages when designing for the dynamic performance of the tower. Concrete aso has higher materia damping properties than stee. Prestressed concrete has a high fatigue resistance, providing more toerance and therefore ess risk from dynamic probems. 10.2 Comparison Tabe of Offshore Tower Structures Characteristics STEEL TOWER Onshore fabrication/construction We-estabished technoogy. Fabricate sections in factory under controed conditions. Prefabricate in arge sections, with weding at shore ocation. Larger diameters (5m+) and higher wa thicknesses (60mm+) required for deeper water and arger turbines become reativey more expensive to fabricate. CONCRETE TOWER Construct at factory faciity at suitabe coasta site. Concrete can achieve arger diameters without disproportionate increase in cost. Concrete can accommodate detaied section changes reativey easiy. Significant initia investment in formwork. Formwork can be reused over ong production runs giving ower unit costs. Pyon stages Precast method (construction offshore or inshore). Precast ring units assembed into arger segments for instaation offshore. Sipform method (inshore ony). Form tower on gravity caisson for tow out and instaation offshore. Gravity base stages Construct in dry dock or on/at top of sipway by shore. Food dry dock or aunch from sipway/ shipift. Compete top sections with base on seabed at quayside.

Offshore concrete wind towers study PAGE 38 Comparison Tabe of Offshore Tower Structures Characteristics (continued...) STEEL TOWER Transportation Larger diameters (4.3m+) and weight of sections become harder to transport by road from fabrication shop to coasta yard. Reativey ightweight overa (order of 500 tonnes overa for a 90m pyon). Can be transported to site on barge, severa at a time. Can be transported to site by foating out with seaed ends. Instaation Equipment Insta using a arge jack-up barge, shear-eg crane barge or speciaised jackup vesse (e.g. Mayfower). 300 tonnes or more can be ifted at present. Stages for stee monopie instaation Insta monopie and scour protection (if needed). Transition piece grouted to monopie head to form vertica connection. Stages for pyon instaation Insta first pyon section, boted to adjustment piece. Insta second pyon section. Insta nacee and rotor bades. Insta eectrica cabes test and commission. Features Reativey ight weight per unit ength. May require fewer ifts (two pyon sections). May therefore require shorter weather windows for instaation. Tried and tested technoogy. CONCRETE TOWER Pyon stages (if separatey transported to offshore site) Lift onto barge or aunch down sipway. Transport horizontay or possiby verticay to site on barge or foat out in segments with ends seaed. Gravity base stages Lift with specia catamaran barge and transport (part submerged) to site. Pyon erected inshore Buit onto gravity base. Whoe assemby transported and paced by specia barge. Equipment Insta using a arge jack-up barge, shear-eg crane barge or speciaised jackup vesse (e.g. Mayfower). 300 tonnes or more can be ifted at present. Stages for stee monopie foundation instaation Insta monopie and scour protection. Junction piece grouted to monopie head to form vertica connection. Stages for gravity base instaation Prepare seabed Lower or sink from submerged position hed by catamaran barge. Pace oose rock baast into caisson ces for ong term storm stabiity. Pace scour protection. Stages for pyon instaation (if undertaken offshore) Insta first pyon segment. Insta second pyon segment. Insta third pyon segment. Insta nacee and rotor bades. Prestress, test and commission. Features Greater weight or unit ength than stee pyon requires more segments and correspondingy more ifts for compete erection. Ony three segments are needed when using arge ifts of 300 tonnes. With carefu connection detaiing and erection panning increased disruption risk may not be significant. Aternative method Inshore construction of foundation tower unit inshore. Compete unit tow-out option simpifies offshore operations. Sinking and pacement of whoe unit essentiay simiar to gravity base operation above.

Offshore concrete wind towers study PAGE 39 Comparison Tabe of Offshore Tower Structures Characteristics (continued...) STEEL TOWER Performance Static oading: penty of reserve capacity in section. Dynamic oading and response: the natura periods of present designs (circa six seconds) ie between the bade passing period (one to two seconds) and the periods of extreme wave conditions (six seconds pus). Taer turbines need arger pyon and foundation diameters, providing sufficient stiffness to contro the dynamic response (i.e. to keep the natura base period of the pyon-foundation system beow the periods of extreme wave conditions). Larger diameters wi require arger wa thickness, making stee harder to procure and fabricate than hitherto, as we as heavier. Larger diameter eements wi be more difficut to insta. Connections (fanges etc.) ives are imited by fatigue. Maintenance Corrosion protection: high specification paint systems have about 15 to 20 years to first maintenance. Repainting may therefore be necessary if a design ife of more than 20 years is required, particuary in the intertida and spash zones. Cathodic protection beow mean sea eve wi aso require periodic maintenance. Maintenance is difficut in isoated offshore structures. Boted fanges particuary at ower eves are vunerabe to corrosion. CONCRETE TOWER If supported on a monopie foundation, require arge diameter for concrete soution to avoid wave period resonance. A spread foundation (gravity or pied) in contrast has potentia to give good dynamic response for a arge height tower. Low maintenance. Prestress can improve durabiity and provide a good fatigue performance. Concrete has higher materia damping than stee: potentiay more toerant of occasiona resonance and avoidance of ringing due to impact by breaking waves. Significant potentia for further vaue engineering: use of higher strength concrete; use of fibres to reduce vertica rebar and improve durabiity; use of carbon fibre reinforced poymer (CFRP) prestressed concrete. Highy durabe if good quaity construction. Very itte maintenance for structure. Upgrade Offshore wind energy converters have a typica design ife of about 20 years. It may be possibe to extend their ife if required, but contro of corrosion and of fatigue damage at connections coud present significant probems. If the foundations and pyon are sti serviceabe, then the turbine coud be repaced with a newer mode. Concrete can be very durabe (design ife 50 to 100 years), thus the reuse of tower by upgrading the turbine at 20 years is highy possibe. At this stage, the foundation and pyon, which may originay comprise some 40% of the overa cost of a new deveopment, is amost free. A prestressed concrete section coud be strengthened reativey easiy for potentiay higher oads, from newer arger turbines by providing some margin in strength of concrete seected, and by subsequenty increasing the vertica prestress force by instaing externa unbonded tendons couped to the inner face of the pyon was. Decommissioning Large diameter monopies cannot be extracted at present; therefore they wi have to be cut off at or beow seabed eve. Stee of tower can be brought onshore and recyced. Offshore assembed towers Pyon unboted in sections and returned to shore for disposa or recycing. Gravity foundation coud be debaasted and foated back to shore for decommissioning. Large diameter monopies if used cannot be extracted at present; therefore they wi have to be cut off at or beow seabed eve. Inshore constructed tower/ foundation units The whoe instaation can be reversed in effect, utiising equipment simiar to that for origina instaation.

Offshore concrete wind towers study PAGE 40 11 OFFSHORE STUDY CONCLUSIONS The main concusions are: Offshore wind farms are generay becoming arger, with a move to more units in deeper water, and depoying arger turbines. This trend points to the need for taer and stronger towers. An outine design of a prestressed concrete tower has been prepared for use offshore with the arger turbines ikey to be used in the Round 2 offshore programme. The design is considered to be feasibe and economic, athough it wi require further deveopment to achieve optima competitiveness. Concrete soutions offer advantages and disadvantages for the various scenarios for current offshore wind energy converters. The baance of advantage for taer offshore wind energy converter towers is shifting towards concrete design soutions. Concrete gravity foundations are competitive with stee aternatives (tripod system) in deeper water (particuary in the range of 15-30 metres), athough the actua advantage woud depend on the specific site conditions. An integrated design of concrete caisson and tower coud be assembed in shetered inshore waters, towed out as a competed unit and sunk at its offshore ocation. This approach coud reduce substantiay the number of critica offshore operations and thereby the associated weather risk. It coud provide a radica aternative to current offshore wind farm construction methods and potentiay coud provide significanty ower out-turn costs than other existing deeper-water soutions such as the stee tripod foundation. Concrete design soutions have a potentia for onger reiabe operationa ives (50-100 years) aowing reuse of the tower and foundation structure for ater re-fits of the turbine and mechanica/eectrica pant. They coud thereby give significanty improved onger term financia returns and substantia improvements in sustainabiity. There is a very arge programme of offshore wind farm deveopment panned for UK and European coasta waters over the next five years. There is ikey to be a continuing programme beyond that, with an increasing proportion of sites to be found in deeper waters further offshore. A this impies the need for a significanty increased industria capacity to achieve it. The introduction of concrete designs as an aternative to stee coud offer the important benefit of mobiising a wider sector of the industria and technoogica resources of the civi engineering industry. This woud tend to maintain or extend competition and provide more options for accommodating oca economic and empoyment circumstances. 12 FIGURES - OFFSHORE WIND TOWERS Figure No. 2.1 GENERAL STUDY SCENARIO 2.2 CONCRETE TOWER - Continuous taper design 2.3 CONCRETE TOWER - Necked stem design 2.4 CONCRETE FOUNDATION - Gravity base soution 2.5 CONCRETE PYLON - With stee monopie substructure 2.6 PRODUCTION PROCESS - Concrete pyon 2.7 TOWER SECTIONS - For offshore erection 2.8 TOWER SECTIONS - Showing buid-up from precast concrete ring units 2.9 PRECAST CONCRETE RING UNITS - Outine detais 2.10 JOINT BETWEEN TOWER SECTION - Interna prestress arrangement 2.11 COMPOSITE/HYBRID ARRANGEMENTS FOR WIND TOWER STRUCTURE - Fexibiity of soution 2.12 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION - Methods 1 & 2 (Pyon construction offshore) 2.13 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION - Method 3 (Pyon construction inshore) 2.14 DYNAMIC RESPONSES OF VARIOUS TOWER DESIGNS

Offshore - figures PAGE 41 Figure 2.1 GENERAL STUDY SCENARIO

Offshore - figures PAGE 42 Figure 2.2 CONCRETE TOWER -

Offshore - figures PAGE 43 Figure 2.3 CONCRETE TOWER -

Offshore - figures PAGE 44 Figure 2.4 CONCRETE FOUNDATION -

Offshore - figures PAGE 45 Figure 2.5 CONCRETE PYLON -

Offshore - figures PAGE 46 Figure 2.6 PRODUCTION PROCESS - Figure 2.7 TOWER SECTIONS -

Offshore - figures PAGE 47 Figure 2.8 TOWER SECTIONS -

Offshore - figures PAGE 48 For aternative soution using externa unbonded prestressing see onshore study figures Figure 2.9 PRECAST CONCRETE RING UNITS -

Offshore - figures PAGE 49 Figure 2.10 JOINT BETWEEN TOWER SECTION -

Offshore - figures PAGE 50 Figure 2.11 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION -

Offshore - figures PAGE 51 Figure 2.12 INSTALLATION OF CONCRETE TOWER AND GRAVITY FOUNDATION -

Offshore - figures PAGE 52 Figure 2.13 COMPOSITE/HYBRID ARRANGEMENTS FOR WIND TOWER STRUCTURE -

Offshore - figures PAGE 53 Comparison of the dynamic response of stee and concrete towers for different heights and foundation conditions. Seabed Conditions 9 8 Stiff Soi Average Soi Soft Soi Natura Period (secs) 7 6 5 4 3 Wave Excitation Zone 2 1 0 Bade Passing Excitation Zone a. Scroby Sands b. Arkow Bank c. Tria 1 100m x 6m Ø d. Tria 2 100m x 6m Ø Stee Tower Designs e. Tria 3 100m x 7.5m Ø f. Tria 4 100m x 7.5m Ø g. Gravity Necked h. Gravity Tappered j. Monopied Necked 1 k. Monopied Necked 2 Concrete Tower Designs. Monopied Tapered 1 m. Monopied Tapered 2 Type of Tower NOTES ON BAR CHART DIAGRAM Existing wind farms (Stee monopie soutions) a) Scroby Sands Pyon Height 60m b) Arkow Bank Pyon Height 70m Theoretica Larger Stee monopie soutions c) Tria 1 6m diameter stee monopie foundation 3.6MW GE Wind turbine 100m ta pyon d) Tria 2 6m diameter stee monopie foundation 3MW Vestas turbine 100m ta pyon e) Tria 3 7.5m diameter stee monopie foundation 3.6MW GE Wind turbine 100m ta pyon f) Tria 4 7.5m diameter stee monopie foundation 3MW GE Wind turbine 100m ta pyon Concrete Soutions g) Gravity Necked (fig 4) Mass gravity pad foundation Necked Pyon soution 100 m ta pyon h) Gravity Tapered (fig.3) Mass gravity pad foundation Concrete Soutions Continua tapered pyon soution 100 m ta pyon g) Gravity Necked (fig 4) j) Pied Mass Necked gravity1 pad foundation Necked 5m diameter Pyon soution stee monopie foundation 100 Necked m ta pyon Pyon soution 100 m ta pyon h) Gravity Tapered (fig.3) k) Pied Mass Necked gravity2 pad foundation Continua 7.5m diameter taperedstee pyonmonopie soutionfoundation 100 Necked m ta pyon Pyon soution 100 m ta pyon j) Pied Necked 1 ) Pied 5mTapered diameter1stee monopie foundation Necked 5m diameter Pyon soution stee monopie foundation 100 Continua m ta pyon tapered pyon soution 100 m ta pyon k) Pied Necked 2 m) Pied 7.5mTapered diameter 2 stee monopie foundation Necked 7.5m diameter Pyon soution stee monopie foundation 100 Continua m ta pyon tapered pyon soution 100 m ta pyon ) Pied Tapered 1 Figure 2.14 DYNAMIC RESPONSES OF VARIOUS TOWER DESIGNS

Genera concusions PAGE 54 13 GENERAL CONCLUSIONS Athough some arge concrete wind towers have been constructed in Germany and esewhere in Europe, they represent a sma fraction of the present tower popuation. This report points to the potentia for improved competitiveness of prestressed concrete towers, particuary for taer towers and arger rated turbines for both offshore and onshore wind farms. For onshore wind farms viabe designs for both precast and in-situ sipform construction methods are possibe. For offshore wind farms, precast concrete tower designs foowing current offshore erection methods are considered to be feasibe, and possiby competitive. The ower unit construction costs wi tend to be offset by possiby sighty onger erection cyces, and the baance of competitiveness coud be affected by pant avaiabiity and other market circumstances. Concrete gravity foundations coud provide significant benefits and cost savings given appropriate site circumstances, particuary for water depths in the range 15-30 metres. An innovative approach to construction of an integrated tower/ foundation structure at an inshore site is proposed, foowed by transportation and pacement offshore of the fuy outfitted unit by a purpose designed ift barge. This combines the most economic construction soutions and circumstances with minimum offshore work and risk. This scheme has the potentia to reduce the instaed cost of the tower structure by as much as 25%. Further concusions are: Concrete tower soutions are adaptabe and durabe. The high fatigue resistance of prestressed concrete, combined with the good materia damping properties of the concrete, can provide a soution which wi be potentiay ess susceptibe to and more toerant of occasiona resonance, and therefore have a reduced risk of dynamic probems. Given carefu design of key detais, the high potentia durabiity of concrete can be reaised. The tower wi need itte maintenance during a ong design ife. The use of prestressed concrete can further increase the tower s durabiity. Prestressed concrete towers can be adapted reativey easiy to cope with some future increase in oading associated with re-using the tower. For taer towers with high top head oading, concrete pyons can be cost-effectivey stiffened by enarging the diameter of the ower tower section above the wave affected zone. This wi reduce susceptibiity to resonant exposure to excitation by energetic waves. Concrete gravity foundations have the potentia to give taer towers an improved dynamic response over monopie foundations. Concrete soutions are heavier than equivaent stee soutions. Carefu seection of section size, concrete strength and the use of prestress can reduce the difference. A substantia initia investment in formwork and other equipment wi be needed for ow unit cost of tower pyons and rapid production. A arge enough base market wi be needed to fuy reaise competitive advantages. These investment costs wi be arger for offshore designs than for onshore designs. The potentiay onger service ife of concrete towers can be used to significant ong term advantage by the adoption of a turbine re-fit programme, invoving the repacement of the turbine after the expiry of its (20 year) ife. This approach coud have a profound impact on the sustainabiity of the wind energy programme as a whoe.

References and background reading PAGE 55 14 REFERENCES 1. BWEA Report, Offshore Wind at the Crossroads, British Wind Energy Association Offshore Wind Conference Apri 2006 2. Boye, G, UK Offshore Wind Potentia, refocus magazine, Esevier, Juy/August 2006 3. Per Vøund, Concrete is the future of offshore foundations, Internationa Conference and Exhibition - Copenhagen Offshore Wind 2005, ENERGI E2, 2005 4. Enercon GmbH, Tower Versions, 2004. Avaiabe from: www1.enercon.de 5. Seide, M, Experiences with two of the word s argest wind turbine towers. Proceedings of the 2003 European Wind Energy Conference and Exhibition, 16-19 June 2003 Madrid, GE Wind Energy GmbH, 2003 6. Brughuis, FJ, Advanced tower soutions for arge wind turbines and extreme tower heights. Proceedings of the 2003 European Wind Energy Conference and Exhibition, 16-19 June 2003 Madrid, Meca Appied Mechanics BV, 2003 7. Brughuis, FJ, 2004. Improved return on investment due to arger wind turbines. Proceedings of the 2004 European Wind Energy Conference and Exhibition, 22-25 November 2004 London. Netherands: Meca Appied Mechanics BV 8. See Nordex website: www.nordex-onine.com 9. See BWEA website: www.bwea.com Background reading BWEA Press Reease, Goba wind industry grows as positive assessment of grid integration reported from Germany, 2005. Avaiabe from: www.bwea.com/media/2005.htm BWEA Report, Onshore Wind: Powering Ahead, March 2006 The Concrete Centre, An estimate of the embodied CO 2 in stee and concrete wind turbine towers, 2005. Gifford, Review of the estimated embodied CO 2 in stee and concrete wind towers. (Report No. 12318/R02), 2005 Wind Power in the UK, Sustainabe Deveopment Commission, 2005. Avaiabe from: www.sd-commission.org.uk APPENDIX A Embodied CO 2 estimates for wind towers Tabe A1 provides comparisons of the typica quantities of CO 2 embodied in structura stee and reinforced concrete. Cacuated by independent industry experts, the vaues reported encompass average CO 2 eves associated with materia extraction, production and transportation. Based on these figures, the estimated tota mass of embodied CO 2 in a 70m wind tower constructed from stee and concrete is compared in Tabe A2. 70m was chosen as a height representative of typica onshore soutions and owing to the widespread avaiabiity of industry data on stee towers of this height. Recognising the indicative nature of these cacuations, it is cear that embodied eves of CO 2 in concrete wind towers can be significanty ower than for stee (64% reduction in Tabe A2). Concrete offers this improvement as, despite cement production s energy-intensive nature, it is a ocay sourced materia produced predominanty from materias with ow environmenta footprints. For further information on the cement and concrete industry s commitment to sustainabiity, pease refer to www.cementindustry.org.uk and www.sustainabeconcrete.org.uk

Appendix A PAGE 56 Additiona benefits of concrete to be considered in this regard incude: The embodied CO 2 vaues shown in Tabe A1 are for concrete containing Portand cement CEMI as the excusive binder materia. Using ground granuated bast furnace sag (GGBS) or fy ash as a repacement for cement can reduce the vaues in Tabe A1 by as much as 40% depending on the concrete mix design and the appication. This CO 2 reduction is in addition to performance benefits inherenty associated with these materias incuding improved concrete strength and durabiity performance. Concrete inherenty consumes CO 2 from the atmosphere it is exposed to, a process known as carbonation. Particuary for high strength concrete structures, carbonation eves during service ife wi be minima, but if recyced and crushed at the end of its service ife, CO 2 uptake is much more rapid. Based on a study by the British Cement Association of a types of concrete, on average concrete can consume 20% of the CO 2 emitted from its cement manufacture [A1]. Tabe A1 Structura materia Embodied mass of CO2 by mass of materia (kgco 2 /tonne) Reinforcement content none ow high average * Density taken from EN 1991-1-1:2002, Eurocode 1. Actions on structures. Genera actions. Densities, sef-weight, imposed oads for buidings. Tabe A2 Wind tower option Mass of construction materias (tonnes) Structura stee Reinforced concrete 70m stee tower * 125 ~ 242 70m concrete tower ** 2.0 550 88 Mass of embodied CO 2 (tonnes) * Estimate using avaiabe industry information [A4-A7] ** Estimated based on Gifford conceptua design (average reinforcement eve, stee contribution at joints) Information sources [A1] C Cear, T De Saues, Carbonation Scoping Study, British Cement Association, 2007, draft in progress [A2] The Concrete Centre study, Embodied CO 2 of various concrete mixes, due 2007 [A3] [A4] [A5] [A6] [A7] Amato A and Eaton K J., A Comparative Environmenta Life Cyce Assessment of Modern Office Buidings, The Stee Construction Institute, 1998 Northern power Systems NW 100/19 wind turbine (www.northernpower.com) Danish Wind Industry (www.newenergy.org.cn/engish/guide/towerm.htm) Crokwe, New South Waes 1998 (www.buecopestee.com) Cashindarroch deveopment (www.amec.com/wind)

PAGE 57 Concrete Wind Towers CONCRETE SOLUTIONS FOR OFFSHORE AND ONSHORE WIND FARMS Concrete Wind Towers concrete soutions for offshore and onshore wind farms This document demonstrates the key roe that concrete can pay in reaising cost efficient, sustainabe and constructibe energy converters, addressing the major issues reevant to any onshore or offshore wind deveopment. Concrete is uniquey adaptabe in terms of performance, design and constructibiity; making it the materia of choice for wind farm deveopers. To downoad or order your free hard copy, visit www.concretecentre.com/ pubications. If you have a genera enquiry reating to the design, use and performance of cement and concrete contact The Concrete Centre s nationa hepine. Advice is free and avaiabe Monday to Friday from 8am to 6pm. Ca 0700 4 500 500 or 0700 4 CONCRETE Emai hepine@concretecentre.com

www.concretecentre.com Free Nationa Hepine Avaiabe Monday to Friday from 8am to 6pm Ca 0700 4 CONCRETE 0700 4 500 500 A advice or information from The Concrete Centre is intended for those who wi evauate the significance and imitations of its contents and take responsibiity for its use and appication. No iabiity (incuding that for negigence) for any oss resuting from such advice or information is accepted. Readers shoud note that a the Concrete Centre pubications are subject to revision from time to time and shoud therefore ensure that they are in possession of the atest version.