HIGH LEVEL REVIEW HELIDECK AND ACCOMMODATION Helideck and accommodation facilities on offshore platforms for wind farms. TenneT.

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1 HIGH LEVEL REVIEW HELIDECK AND ACCOMMODATION Helideck and accommodation facilities on offshore platforms for wind farms TenneT Public version Report No.: NLLD-R1, Rev. A-Public Date: 9 June 2015

2 Project name: High level review helideck and accommodation DNV GL Energy Report title: Helideck and accommodation facilities on offshore platforms for wind farms Customer: TenneT, Utrechtseweg 310, 6812 Arnhem Contact person: Harry van der Heijden Date of issue: 9 June 2015 Project No.: Report No.: NLLD-R1, Rev. A-Public R&S/RES Utrechtseweg 310, 6812 AR Arnhem The Netherlands Tel: Applicable contract(s) governing the provision of this Report: Objective: High level review to assess the advantages and disadvantages of a helideck and accommodation facilities on an offshore substation platform Prepared by: Verified by: Approved by: Eeke Mast Senior Consultant Robin Redfern Project Engineer Hans Cleijne Head of Section Frenando Sevilla Offshore wind engineer [Name] [title] Erika Echavarria Engineer [Name] [title] Copyright DNV GL All rights reserved. This publication or parts thereof may not be copied, reproduced or transmitted in any form, or by any means, whether digitally or otherwise without the prior written consent of DNV GL. DNV GL and the Horizon Graphic are trademarks of DNV GL AS. The content of this publication shall be kept confidential by the customer, unless otherwise agreed in writing. Reference to part of this publication which may lead to misinterpretation is prohibited. DNV GL Distribution: Unrestricted distribution (internal and external) Unrestricted distribution within DNV GL Limited distribution within DNV GL after 3 years No distribution (confidential) Secret Keywords: Offshore platform, helideck, helipad, accommodation, offshore wind Rev. No. Date Reason for Issue Prepared by Verified by Approved by A report after mid-term memo - FINAL Erika Echavarria, Fernando Sevilla, Robin Redfern, Eeke Mast, Robin Redfern, Eeke Mast Hans Cleijne DNV GL Report No NLLD-R1, Rev. A-Public Page i

3 Table of contents 1 EXECUTIVE SUMMARY Background Substation Maintenance Requirements Helideck considerations Accommodation considerations Conclusions and recommendations 6 2 INTRODUCTION Background Aim and approach 8 3 CURRENT APPLICATION ON OFFSHORE PLATFORMS Inventory of offshore platforms in North-West Europe Trends from the platform inventory 11 4 QUALITATIVE DESCRIPTION OF THE MAINTENANCE NEEDS Preventive and corrective maintenance of offshore substations Major Component Replacement Availability at Offshore Substations 14 5 REVIEW OF ACCESS SYSTEMS Introduction Transferring Systems for personnel and small components Onshore-based Marine Access Offshore-based Marine Access Other vessels 22 6 HELIDECK ON AN OFFSHORE PLATFORM Helicopters at offshore wind projects Regulations and requirements Helidecks and winching platforms Summary of Helicopter Logistics Estimations workability helicopter versus vessel 30 7 OFFSHORE ACCOMMODATION FACILITIES Offshore accommodation for offshore wind Regulations and requirements Summary of Offshore Accommodation 38 8 CONCLUSIONS AND RECOMMENDATIONS Background and Overview of Existing Projects Substation Maintenance Requirements Helidecks and Heli-hoist Platforms Offshore Accommodation 42 9 REFERENCES DNV GL Report No NLLD-R1, Rev. A-Public Page ii

4 1 EXECUTIVE SUMMARY 1.1 Background The Dutch government has stated its intention to start five tender rounds in five wind farm zones in They have also appointed TenneT as the offshore TSO. TenneT will be responsible for installing the offshore high voltage stations to export the generated electricity to shore. TenneT intends to construct 5 identical offshore substation platforms, each with up to 700MW of capacity up to 38km from the nearest coast. These substations are substantially larger in power capacity than any existing offshore (AC) substations. This study is a high level, qualitative review to assess the advantages and disadvantages of a helideck and accommodation facilities on an offshore substation platform by identifying possible barriers and/or opportunities of such facilities. This study is a qualitative review, not a quantitative review: it will not include an economic comparison of the perceived impacts of different access methodologies. The aim is to help TenneT conclude whether quantitative investigation is likely to be worthwhile. 1.2 Substation Maintenance Requirements DNV GL estimates an average of 10 to 30 days of scheduled maintenance per year to be required for a substation of this scale. Considering these relatively low scheduled maintenance requirements and that the shortest maintenance interval required is expected to be monthly or more, it is clear that the addition of a helideck or accommodation facilities to the offshore substations would not provide significant benefits to the operation of the substation platforms themselves. Assuming good levels of redundancy are implemented within the configuration of the substation power and SCADA systems, most failures will not incur production losses and therefore the repair or replacement of such components can often be carried out as scheduled maintenance or scheduled access to the substation, reducing or even eliminating the benefit provided by quick transfer of technicians by helicopter. A major failure on the offshore substation, as occurred at the Nysted Offshore substation in 2007, will require specialist technicians, vessels and replacement components and therefore is constrained much more by mobilisation of the necessary resources than rapid deployment of technicians from offshore based accommodation or by helicopter transfer. 1.3 Helideck considerations Trends from the platform and helideck inventory There are no clear trends for the number of projects featuring helidecks or heli-hoist platforms with distance from shore. However, there is a strong trend with respect to project capacity, with all projects greater than 400MW and the majority of projects above 300MW featuring either a helideck or a heli-hoist platform on the associated offshore substation(s). Currently, helicopters are in regular use for turbine O&M purposes at the Horns Rev Project in Denmark, Alpha Ventus, Global Tech 1 and Borkum Phase 1 (when commissioned) in Germany as well as Greater Gabbard in the UK. Additionally, contracts are in place at a number of other projects for provision of a helicopter for emergency search and rescue services. It is not clear whether projects which feature helidecks utilise these for maintenance of the offshore substation, although such use is anticipated.

5 1.3.2 Helicopter logistics for offshore wind farms The distances from shore of the 5 proposed TenneT substations are well within the operational range of commonly used twin-engine helicopters, such as the Airbus EC135, provided the helicopter landing site is not located far inland. The substantial size of offshore substations makes them well suited to helidecks or heli-hoist platforms. In most cases it is believed that a helideck on the offshore substation is largely intended to facilitate helicopter access to the turbines and to support emergency response procedures as opposed to solely providing access to the substation platform itself. Helicopter access to the turbines is performed by hoisting and for many aircraft, payload is limited for undertaking such heli-hoist operations due to a requirement to be able to maintain hover in the event of one engine failing. On this basis, some operators are known to land on the platform helideck to temporarily drop off technicians on the substation helideck prior to performing heli-hoist operations. Therefore, the greatest benefit of a helideck is likely to be in support of helicopter logistics at the wider wind farm project. The benefits of a heli-hoist platform will be largely limited to O&M of the substation platform itself.. Projects utilising helicopters for the O&M of wind turbines are understood to have found them to be cost effective. They provide fast response to small repairs or diagnosis works where large parts or tools are not required Helicopter requirements Recent incidents in the North Sea oil and gas industry have led to recent changes in regulation stated by the British CAA with respect to the sea states in which helicopters may be deployed. This has now limited the use of helicopters by the lesser of sea-state 6 or the certified ditching performance of the helicopter, understood to be the sea state in which the aircraft may remain floating upright in the water. Due to European cooperation concerning regulations and guidelines on aviation, the restrictions on offshore helicopter flights as stated by the CAA could be taken into account for the Dutch airspace as well. This would mean that, apart from the requirements of flying under the meteorological limitations imposed due to adherence to visual flight rules, restrictions could also be placed on the helicopterspecific sea state permitted for safe offshore flight as they are in place now in the UK. For example, the Airbus EC135 is understood to be limited to sea-state 4, comparable to the access limitations from specialist marine access systems such as the Ampelmann in conjunction with large vessels. This would greatly reduce the greatest benefit of helicopters to offshore wind projects: their insensitivity to sea states. From a regulatory perspective, the addition of a helideck is not restricted. The structure should be carefully located on the superstructure in the design phase. The implications of adding a helideck to a normally unmanned platform and the associated maintenance burden should be noted. According to helicopter landing areas regulation also adhered in the Netherlands (CAP 437, wherever practicable, helicopter hoisting should not be employed as the standard method for transfer of personnel, suggesting that a helideck is the only option if regular access by helicopter is to be adopted. Helidecks, and to some extent heli-hoist platforms, are aircraft specific and must be designed for the dimensions and loading requirements of the aircraft with which they shall be used. Helidecks (and helihoist platforms) require maintaining and certification with particular emphasis on fire fighting, visual aids and surface friction. This may require frequent maintenance visits to remove guano and check equipment. DNV GL Report No NLLD-R1, Rev. A-Public Page 4

6 For large substations, such as those proposed by TenneT, the additional cost of a helideck (anticipated to be in the region of 1M to 2M) is likely to be dwarfed by the overall cost of the platform and therefore may be justifiable on a percentage cost basis to retain future flexibility and improve the safety case. A heli-hoist platform is expected to be in the region of 200k - 500k. In all cases the use of a helicopter for O&M purposes at an offshore wind farm is heavily subject to the design risk assessment conducted by the developer of the project and associated advisors. 1.4 Accommodation considerations Accommodation inventory Only 3 offshore wind projects to date feature offshore accommodation and with the exception of Horns Rev II, these are located more than 70km from the coast. To date only Global Tech 1 is known to include permanent accommodation on the offshore substation. Thjs reinforces the assumption that distance from O&M port is a primary driver for offshore accommodation facilities Offshore Accommodation facilities requirements The reduced cost through combining the offshore substation and accommodation module on one platform is anticipated to be counteracted by the increased design challenges of ensuring safety of all personnel against fire and electrical faults, minimising long-term exposure to electromagnetic fields and ensuring access for maintenance to the heavy major electrical components Platform maintenance Accommodation for the maintenance of solely the substation is not justified due to the relatively minimal anticipated maintenance requirements. Therefore any offshore accommodation module would need to be primarily intended for use by the wind farm. The proposed TenneT platforms are located comparatively close to shore (<40km) and therefore, unless a suitable O&M port is much further away, there is no strong requirement for offshore accommodation in order to maintain the substation platform or adjacent wind farms. Offshore accommodation becomes more economically attractive with distance from O&M port and wind farm project size, with most projects further than about 30NM to 40NM (55km 75km) from port expected to be reliant upon offshore accommodation to avoid excessive travel times, low productivity due to sea sickness and fatigue and an on-site parts and consumables store Fixed versus floating accommodation Offshore accommodation can take two basic forms, either a fixed platform or floating accommodation with a variety of different vessels available. Crucially, fixed platform accommodation reduces transfer time and therefore also the likelihood of sea sickness, but it does not itself increase the sea states in which transfers to turbines can be achieved. For this reason, current industry trends suggest that the market is moving more towards floating accommodation configured to provide the dual purpose of accommodating technicians and providing direct, safe access to offshore structures in higher sea-states than could normally be achieved by traditional work boats. A further benefit of floating accommodation is likely to be the potential to operate at night, due to the intrinsically safer walk to work approach enabled by the use of a specialist access system such as the Ampelmann or similar systems. DNV GL Report No NLLD-R1, Rev. A-Public Page 5

7 1.5 Conclusions and recommendations Helideck For the maintenance of the platform itself, the helideck will have limited advantages, especially in the case of stricter sea state regulation for helicopters. The primary reason for considering a helideck on an offshore substation is therefore the support of O&M logistics at the wind farm. For instance technicians could be dropped off at the platform to reduce the payload and make hovering for a heli-hoist to the turbines possible. Therefore the case for installing a helideck is not clear-cut and is likely to be heavily driven by possibilities to use the helideck for the maintenance of the surrounding wind farms and detailed safety reviews. For large substations, such as those proposed by TenneT, the additional cost of a helipad is anticipated to be in the region of 1M to 2M and therefore may be justifiable on a percentage cost basis to retain future flexibility Accommodation facilities From the results of this review, DNV GL believes that the costs and other constraints associated with the installation of an accommodation platform, either on the same structure as the offshore substation or as an independent structure, are unlikely to justify the benefits at the proposed TenneT project sites. Instead, the adoption of a strategy where the majority of scheduled maintenance at all 5 proposed platforms is performed as part of an annual maintenance campaign, for which the chartering of an OSV or similar vessel providing access and accommodation will likely prove more cost effective. Alternatively a similar solution may be adopted at each platform independently by collaborating with the associated wind farm owner for the purposes of the annual scheduled maintenance campaign, nominally to be performed during summer months for improved access and minimal loss of production. DNV GL Report No NLLD-R1, Rev. A-Public Page 6

8 2 INTRODUCTION 2.1 Background The Dutch government has stated its intention to start five tender rounds in five wind farm zones in They have also stated their intention to appoint TenneT as the offshore TSO. TenneT will then be responsible for installing the offshore high voltage stations to export the generated electricity to shore. The schedule for the five tender rounds and the areas are summarised in Table 2-1 and graphically shown in Figure 2-1. For the transformer station for these wind farm zones, TenneT plans a standardised design for all five platforms. Each platform will have a capacity of 700 MW and the connecting inter-array cables will be 66 kv. Tender Year Wind farm zone Capacity [MW] Distance to Coast [km] Distance to port [km] 2015 Borssele Wind Farm Zone Borssele Wind Farm Zone South Holland coast Wind Farm Zone South Holland coast Wind Farm Zone North Holland coast Wind Farm Zone Table 2-1: Short description of the planned 5 tender rounds. Figure 2-1: Existing wind farms and assigned wind farm zones for the upcoming tenders. DNV GL Report No NLLD-R1, Rev. A-Public Page 7

9 2.2 Aim and approach This study is a high level review to assess the advantages and disadvantages of a helideck and accommodation facilities on an offshore substation platform. This high level review will assess the advantages and disadvantages by identifying possible barriers or opportunities for the application of a helideck or accommodation facilities on the platforms. This shall be performed under the following methodology: Reviewing existing and under-construction offshore substation platforms (Section 3); Describing the maintenance needs of an offshore platform (Section 4); Inventory of existing access systems (Section 5 Review of helicopter logistics, regulations and the associated use of helidecks and heli-hoist platforms at offshore wind projects (Section 6 ); Review of the logistics, regulations and requirements for offshore accommodation (Section 7); and Conclusions of the above findings for the proposed TenneT Platforms (Section 8). This qualitative assessment aims to help TenneT to make a more informed decision on whether inclusion of a helideck and/or accommodation on the platform could form an opportunity for TenneT. This study is a qualitative review, not a quantitative review: it will not include an economic comparison of the perceived impacts of different access methodologies. The aim is to help TenneT conclude whether quantitative investigation is likely to be worthwhile. DNV GL Report No NLLD-R1, Rev. A-Public Page 8

10 Number of projects Number of projects Number of projects Number of projects 3 CURRENT APPLICATION ON OFFSHORE PLATFORMS 3.1 Inventory of offshore platforms in North-West Europe DNV GL has examined the current application of a helideck and accommodation facilities for wind farms in North-West Europe. This includes offshore wind farms in Belgium, Denmark, Germany, United Kingdom and The Netherlands. Only wind farms that are currently commissioned or under construction have been included. Note: All information in this table is based on public domain information and therefore may be subject to inaccuracies. Table 3-1 presents the results. Figure 3-1 to Figure 3-3 present the results graphically N/A Accommodation on platform Separate accomodation platform No accommodation >70 Distance to coast [km] Year Figure 3-1: Number of projects with accommodation, categorised in terms of distance to coast and five-year commissioning interval >70 Distance to coast [km] Year N/A Heli-hoist Heli deck No helideck or heli-hoist Figure 3-2: Number of projects with a heli-hoist or helideck, categorised in terms of distance to coast and five-year commissioning interval. DNV GL Report No NLLD-R1, Rev. A-Public Page 9

11 Name Country No of offshore Distance to Capacity Helideck or Accommodation Year substations coast [km] [MW] Heli-hoist facilities Blyth United Kingdom N/A N/A Horns Rev 1 Denmark Helideck No Nysted Denmark No No Scroby Sands United Kingdom N/A N/A North Hoyle United Kingdom N/A N/A Kentish Flats United Kingdom N/A N/A Barrow United Kingdom No No Burbo Bank United Kingdom N/A N/A OWEZ Netherlands N/A N/A Beatrice United Kingdom N/A N/A Prinses Amaliapark Netherlands No No ThorntonBank 1 Belgium N/A N/A Lynn and Inner Dowsing United Kingdom N/A N/A Rhyl Flats United Kingdom N/A N/A Belwind 1 Belgium No No Gunfleet Sands United Kingdom No No Rodsand II Denmark No No Robin Rigg United Kingdom No No Thanet United Kingdom No No Horns Rev 2 Denmark Helideck Separate (24) Alpha Ventus Germany Helideck No Baltic 1 Germany No No Walney 1 United Kingdom No No Ormonde United Kingdom No No Sherringham Shoal United Kingdom No No Walney 2 United Kingdom No No Teeside United Kingdom N/A N/A Lincs United Kingdom No No Anholt Denmark Helideck No London Array United Kingdom Heli-hoist No Thornton Bank 3 Belgium N/A N/A Thornton Bank 2 Belgium Helideck No Greater Gabbard United Kingdom Helideck No Bard Offshore 1 Germany Helideck No West of Duddon Sands United Kingdom Heli-hoist No Riffgat Germany Helideck No Northwind Belgium No No Belwind demo Belgium N/A N/A Meerwind Ost/Sud Germany Helideck No Nordsee Ost Germany Heli-hoist No Global Tech 1 Germany Helideck Yes (34) Westermost Rough United Kingdom Heli-hoist No Humber Gateway United Kingdom No No Gwynt Y Mor United Kingdom Heli-hoist No Luchterduinen Netherlands No No Butendiek Germany Helideck No Baltic 2 Germany No No Amrumbank West Germany Helideck No Borkum Riffgrund I Germany Helideck No Borkum Phase 1 Germany Helideck No Dan Tysk Germany Helideck Separate (50) Horns Rev III Denmark Helideck No Gemini Netherlands Helideck No Note: All the information in this table is ased on public domain information and therefore may be subject to inaccuracies. Table 3-1: List of offshore wind farm and helideck and accommodation facilities. DNV GL Report No NLLD-R1, Rev. A-Public Page 10

12 Number of projects N/A Heli-hoist Heli deck No helideck or heli-hoist Project capacity [MW] Figure 3-3: Number of projects with a heli-hoist or helideck, categorised in terms of project capacity intervals. 3.2 Trends from the platform inventory Accommodation The only substation platform identified with accommodation facilities on the same structure is at the Global Tech 1 project in the German North Sea. This wind farm is located more than 100 km off the coast, substantially further than the proposed TenneT platforms which are located less than 40 km from the nearest land. Two further wind farms, Horns Rev II and Dan Tysk, were identified with separate accommodation platforms located adjacent to the substation platform. Dan Tysk is another far-shore project, located approximately 70km from the nearest land and is due to be fully commissioned this year. The accommodation platform for Horns Rev II was the first accommodation platform for offshore wind. This project is considered something of an outlier from the perspective of accommodation facilities, since the wind farm is located only 32 km off the coast. It is connected to the transformer station with a walkway (see Figure 7-1). It is understood by DNV GL that access between the accommodation platform and the shore is conducted by helicopter or vessel, but vessels alone are used for the subsequent shuttling of technicians to the turbines, implying that transfers are still subject to metocean access constraints. Although the number of 3 offshore accommodation platforms is insufficient to draw firm conclusions, the trends in Figure 3-1 agree with DNV GL expectations, that accommodation platforms are mostly only justifiable at far-shore projects. Clearly Horns Rev II provides an interesting exception to this rule and it is not clear to DNV GL why the Dong Energy chose to adopt this strategy for the project. For this project, Ramboll [ 5] remarks that: On Horns Rev 2 the operation is carried out by two operators one for the substation and one for the wind turbines & accommodation. The accommodation platform was intended to save transport cost and time due to higher maintenance activity on the wind turbines than expected. On Horns Rev 2 the personal is shuttled between onshore and substation/wind turbines by boat which is the primary means of transportation. The maintenance activity has subsequently DNV GL Report No NLLD-R1, Rev. A-Public Page 11

13 dropped due to higher experience and improved parts, therefor the need for the accommodation has diminished. It should be noted that emergency accommodation, or a refuge room, is required on all offshore substations and therefore such facilities have been specifically excluded from this analysis Helideck According to the Mijnbouwwet, offshore platforms in the Dutch EEZ in the Oil & Gas sector have to have a helideck, unless exemption has been granted by the Minister. However, these Oil & Gas platforms are quite different from the usually unmanned offshore wind transformer stations. When looking at the results of the inventory in Figure 3-2, the presence of an offshore helideck or helihoist platform appears to have a weak correlation with distance from coast. This is in line with expectations, since whilst proximity to shore is certainly a contributor, the preferences of the wind farm developer and the severity of the metocean climate, as well as national trends, appear to be stronger drivers. For example all German wind farms in the North Sea have a helideck or heli-hoist platform, whereas in the UK, only Greater Gabbard has a helideck. Four others have a heli-hoist platform. Figure 3-3, shows a stronger correlation with project capacity, with all offshore substations above 400MW featuring either helidecks or heli-hoist platforms, and the vast majority above 300MW as well. This is likely to be attributable to providing as much flexibility for future maintenance strategies and campaigns in these larger projects, particularly as the high overall cost of larger substation platforms will tend to dwarf the additional costs due to helidecks. DNV GL Report No NLLD-R1, Rev. A-Public Page 12

14 4 QUALITATIVE DESCRIPTION OF THE MAINTENANCE NEEDS 4.1 Preventive and corrective maintenance of offshore substations The preventive and corrective maintenance activities of an offshore substation can be divided into three main categories: Non-Intrusive scheduled maintenance: This maintenance category comprises any task which is pre-planned and which could be performed without affecting production of the wind farm. These scheduled works are often conducted on a seasonal basis, with the bulk of work being carried out in the summer to maximise the probability of access to the offshore platform. However, some minor tasks may be required on a more frequent basis throughout the year. Intrusive scheduled maintenance: This maintenance category comprises any task which is pre-planned and which requires the equipment to be temporarily stopped for maintenance work to be undertaken. If no redundancy is allowed, this activity will limit the production of the plant. For this reason, these scheduled works are often conducted on a seasonal basis, with the bulk of work being carried out in the summer to maximise the probability of access to the offshore platform and minimise lost production. Unscheduled maintenance (failure repairs): Any unplanned maintenance activities resulting from a failure of a system, sub-system, or component fall within this group. The level of corrective action, and the impact of the unscheduled maintenance upon the substation availability, depends on the severity of the failure and the extent of any redundancy. Minor equipment failures could be repaired without incurring production losses, while major failure events can have a greater impact on the availability of the project for long periods depending upon the location and equipment involved. TenneT has provided DNV GL with its estimations of scheduled maintenance needs for standard AC platforms on a Monthly, 3-Months and 6-Months frequencies bases. These estimations have been estimated from the maintenance needs of the Helwind Beta HVDC substation as provided by Siemens in its Material Handling-Component Inspection and Maintenance Overview report [ 1]. DNV GL has performed a brief review of these data by comparing the requirements estimated by TenneT against the Helwind Beta reported requirements (with HVDC-specific equipment removed) and against DNV GL s balance of plant maintenance database, as sourced from a variety of public domain sources (see Appendix). To perform this brief review, DNV GL has undertaken the following steps: 1. Quick scan and selection of the equipment requirements for AC substations from the Material Handling-Component Inspection and Maintenance Overview report prepared by Siemens [ 1]. 2. Classification of the maintenance requirements into the following categories: Mechanical, Piping, Electrical, Instrumentation, HVAC, Fire Protection Systems & Rescue, Structural, Architectural and Outfitting (as classified by TenneT in its assumptions). 3. Estimation of total hours per maintenance category of the envisaged maintenance requirements for the AC substation for the full lifetime of the project. 4. Comparison against TenneT s estimations and DNV GL s typical assumptions. DNV GL Report No NLLD-R1, Rev. A-Public Page 13

15 Based on the high level review described above, DNV GL considers TenneT s estimated maintenance requirements to be in line with good practice and with DNV GL s generic and typical assumptions. It must be noted that TenneT only supplied estimations for Monthly, 3-Monthly and 6-Monthly maintenance intervals, whereas the data provided by Siemens in [ 1] also gave maintenance requirements for additional frequencies, including yearly, 2-yearly etc. Therefore DNV GL considers that TenneT s estimates from the Siemens data are appropriate when combined with the effort included under these additional maintenance frequencies. DNV GL s database of maintenance requirements for offshore substations has been divided into the three main categories described above and is reproduced in Appendix A. From these assumptions, DNV GL estimates that a typical, large AC offshore substation with 2 or more offshore transformers will require on average 10 to 30 days per year of scheduled maintenance (including maintenance of the SCADA systems, communications systems and MV switchgear, typically operated and maintained by wind farm operators). All data in Appendix A are based on a variety of public domain sources as well as DNV GL experience and whilst these values are representative of a variety of projects, actual scheduled and unscheduled tasks should always be carried out in line with Original Equipment Manufacturer (OEM) guidelines. 4.2 Major Component Replacement Major components within offshore substations, such as transformers, reactors and gas insulated switchgear, are unlikely to fail once installed correctly. Nevertheless change-out procedures for these items are considered a high priority as failure of these components can result in significant lost power production capacity. Therefore, it is critical to mitigate this risk with suitable preparations and efficient procedures, including the careful location of any additional structures, such as helidecks or accommodation modules. Given that transformers and reactors are likely to be the most difficult major component to both procure and change-out and their relevance to transmit the energy of the wind farm, this sub-section discusses some important issues relating to transformer change-out strategy as well as serving as an indicative illustration of a major offshore substation component. Furthermore, other major offshore substation components will typically have relatively similar change-out procedures to the transformer (e.g. disconnecting, lifting, re-commissioning, etc.) albeit with lower weights to be lifted. Offshore substation transformers are substantially heavier than most of the major components, with indicative weights in the order of tonnes to lift for a high voltage AC transformer. Change-out of offshore substation transformers require a large crane vessel which lifts the failed component out of the roof of the substation and exchanges it for the replacement component. Substation design will have a significant impact on these operations. The time required for these large crane vessels to remain onsite may be relatively short, in the order of 1 8 days; however the total time for the repair can extend substantially beyond this due to all the preparation and re-commissioning works as well as a large dependence on vessel, parts and equipment lead times and weather delays. A well-documented transformer replacement at the Nysted Offshore Wind Project led to a 4.5 month outage and even this duration was the result of a fortuitous prompt availability of both a heavy-lift vessel and a spare transformer winding [ 2]. 4.3 Availability at Offshore Substations DNV GL has performed a variety of modelling studies in the past to estimate the average annual availability of balance of plant for wind farm projects. The studies suggest that the average annual DNV GL Report No NLLD-R1, Rev. A-Public Page 14

16 energy availability of a single HVAC offshore substation (assuming a 2 offshore transformers configuration to allow for some redundancy) range between 99.2% and 99.7%. As detailed in Appendix A, losses in production incurred by the offshore substation are mainly related with the required scheduled maintenance of the offshore transformers and with potential minor and major failures in the transformers and their cooling systems. Different sensitivity analyses performed by DNV GL have demonstrated that the availability of offshore substation platforms is heavily dependent on the following parameters: Redundancy levels: The main equipment which could incur production losses is the offshore transformer. Despite their generally good reliability, major incidents have occurred in the past, such as the Nysted Offshore Wind Farm transformer major failure in 2007 which caused a total project outage of approximately 4.5 months. Due to the significant cost impact that such outages could represent, redundancy of equipment and system configuration is of vital importance. Redundancy could be implemented by: utilisation of multiple transformers operating at 100% or less or their rated power, interconnection between offshore substations (when applicable) and installation of more than one export cable. It is important to note that failures of export cables are estimated to be the single greatest contributor to project unavailability after the wind turbines. However, this review is focused on the offshore substation components and therefore excludes all array and export cabling. Stocking strategy: Due to the highly customized nature of offshore transformers, the lead time required to source a spare could range from a few months to a year. For this reason, operators have started to assess the benefits of stocking a spare major transformer. Stocking of a transformers will reduce production losses substantially in the event of a failure and some assessments previously performed by DNV GL suggest that the potential production losses could justify the capital expenditure required for stocking one transformer. Scheduled intrusive maintenance of the offshore transformers: As this activity requires the shutdown of the transformer and hence generates losses in production, an efficient and well planned scheduled maintenance regime will help to minimise production losses. DNV GL Report No NLLD-R1, Rev. A-Public Page 15

17 5 REVIEW OF ACCESS SYSTEMS 5.1 Introduction The following sub-sections describe the access systems used to transport personnel and components to or from an offshore platform including a review of access systems used to transfer personnel or small parts and components between the vessel and the offshore platform, and vice versa. The last section focuses on access systems for change out of major components such as heavy lift vessels and cranes. The access methods, vessels, and concepts detailed in this section are in various stages of development. Despite a wide range of emerging approaches, most currently operational projects have adopted the standard access method of stepping from a marine vessel directly to the access ladder. However, despite the good safety record that has been maintained to date, it is evident that there is scope for improvement, both in terms of the accessibility of offshore structures and in the safety of the access methods adopted to achieve such access. The market is responding to the potential risks of current access methods and the reduction of revenue that results from poor accessibility, while the slow evolutionary improvement in work boat access is becoming interspersed with the adoption of more revolutionary solutions, as some projects start to embrace helicopters, offshore-based working, SWATH vessels, and sophisticated access systems. As projects are situated farther from port and in more onerous conditions, these trends are likely to continue, with developers seeking to identify approaches which best suit their projects in terms of both direct costs and project revenue. This section comprises publically available information from manufacturers websites and promotional material, as well as appropriate conference papers and DNV GL experience. Note that the capability of the vessels given in the following sections, stated in terms of significant wave height (Hs), is purely indicative based upon supplier information and, where available, industry experience. The limiting factor for access capability is primarily wave height, but factors such as current, wind speed, wave period, water depth, localized wave effects, wave direction, ice, and visibility are also important parameters. 5.2 Transferring Systems for personnel and small components Transferring Personnel There are a few options for transferring personnel to the offshore platform. These are: The step-over approach, with technicians stepping across from the bow of the vessel directly to the platform access ladder (boat landing). Heave-compensated gangway between the vessel and the platform or the boat landing. Some large vessels with dynamic positioning capabilities, such as floatels or motherships (see also Section 7.1.2) may use specialist gangways to dock directly with the platform at the base of the tower via a gate in the railings. Lifted from the vessel to the offshore platform in a basket-type arrangement. This typically requires a large vessel and crane arrangement rated for man-riding. Helicopter landing or hoisting personnel and small parts. For this a helideck or a hoisting platform is required. More information about transfer systems is given below in Section DNV GL Report No NLLD-R1, Rev. A-Public Page 16

18 5.2.2 Transferring components Davit cranes are the principal method for lifting smaller objects between a vessel and the platform and the weight limit of such lifts will depend upon both the safe working load of the davit and the current sea conditions. Typically, davit cranes are limited to somewhere between 100 2,500 kg depending upon the specifications of the particular crane adopted, but the true safe working load will also depend on the dynamic amplification factor applied to account for wave-induced motions and such as transverse and snatch loads. The maximum size of parts, tools and consumables that may be transported by workboats is usually governed more by the lifting capacity of the davit (or other crane) on the offshore structures than by the deck capacity of the work boat, although work boats are not suitable for very large components due to the limitations in deck strength and anchor points for sea lashings Transfer systems Table 5-1 shortly describes different systems commercially used or under development for transferring personnel and small components from a vessel to an offshore structure. Table 5-1: Summary of main transferring systems Type OAS (Offshore Access System) Deployable gangway Development Stage: Trailing on offshore wind farm / Deployed in offshore oil and gas. Description OAS was developed by Offshore Solutions, which operates as a subsidiary of Ampelmann Operations B.V. since November 22, The OAS system comprises a pedestal crane with a telescopic gangway of up to 21m long, fitted to the deck of a vessel. The gangway is hydraulically operated and heave compensated. It operates dynamically until the clamped or elephant s foot connection is made, at which point the control system is stopped and the structure is free to move passively in all six degrees of freedom as required to accommodate differences in motion between the vessel and offshore structure. Only then are transfers made. The company claims that OAS can connect in sea states significantly higher than 3m Hs when mounted mid-ships. Ampelmann Motion compensated gangway Development Stage: Commercially available (support of both installation and O&M activities in the offshore wind industry). The Ampelmann is an inverted Stewart Platform, an assembly of hydraulically- or electrically-actuated rams operating in six degrees of freedom. The Ampelmann is designed to be fixed to the deck of a large vessel (ideally more than 70m LOA). A control system monitors the real-time motion of various accelerometers positioned on the Ampelmann platform and vessel, and uses these measurements to compensate for the motion of the vessel and create a steady base for personnel and equipment transfer. The transfer is made across a telescopic gangway attached to the platform (up to 25m long). No connection is made between the gangway and offshore structure, relying solely on the heavecompensation and vessel DP systems to maintain minimal differential motion between the two. This has the added benefit of requiring little or no modification to foundation designs for the Ampelmann system to be used. Ampelmann claims that when installed on a 25m vessel, the system can fully compensate for the vessel motion in Hs of up to 1.5m; when installed on a larger 50m vessel the system can compensate in up to 2m Hs; and in larger vessels up to 2.5m. DNV GL Report No NLLD-R1, Rev. A-Public Page 17

19 Personal Transfer System (PTS) Type: Crew lifting system Development Stage: Final testing This system is being developed by Personnel Transfer System GmbH. The PTS comprises a remote controlled crane installed on each turbine (or substation) which lifts the technicians, one at a time, from the vessel to the working deck of the structure. It also compensates for the motion of the vessel, and the crane control system allows for an automatic transfer of personnel. According to the designers, the limit for safe operation is 500 kg in seastates up to 3 m Hs, and 800 kg in sea-states up to 1.5 m, allowing it to be used for transfer of large components to the turbine working deck. The PTS has passed the prototype phase and is understood to be ready for final testing offshore. During November/December 2007 and January 2008 PTS was tested at a harbour site in Hamburg, Germany. The system developer states that the system can be used in sea-states characterized by significant wave heights of up to 3 m if used in conjunction with a large vessel. FROG and OWAS systems Type: Personnel transfer pod Development Stage: In use in the oil and gas industry and under development for the offshore wind industry. FROG OWAS Maxccess Transfer System Type: Platform with motion compensation Development Stage: Commercially available This comprises a buoyant personnel transfer capsule which is transported using a standard deck crane on a larger vessel. It can transfer up to nine personnel, with light equipment and tools, per lift, and is designed to protect crews against any vertical and lateral impacts which might occur during transfer. These systems are used for vessel-to-vessel and vessel-to-installation transfers and have accrued significant use in the oil and gas industry. Reflex Marine is now proposing a smaller, lighter personnel transfer capsule that can be easily stowed on a workboat, combined with a specially-built turbine crane. This system, known as the Offshore Wind Access System (OWAS), is a simpler utilization of the FROG specifically for offshore wind purposes. An advantage of this system is that it can be used in conjunction with small work boats. Developed by UK-based OSBIT Power Limited, Maxccess has been chosen for use at the Sheringham Shoal Wind Farm on the UK East Coast, following a series of sea trials, including at Statoil s Hywind demonstration floating wind turbine in Norway. Maxccess is a device which may be mounted to the foredeck of most work boats. It clamps onto either of the vertical tubular spars of the boat landing and allows the vessel to roll, pitch and yaw freely, while preventing vertical and horizontal bow motion. The connection is created without the need for active compensation or complex control software. A small, stepped gangway then provides direct access to the ladder. The Windlift Type: Suspended Platform Development Stage: Prototype The Windlift system, developed by Fassmer, is a heightadjustable platform for access to offshore wind turbines from small, floating vessels. Personnel and equipment are transferred to the platform at vessel-deck level. The platform, which is fitted around the turbine foundation, is then hoisted to the working deck level, avoiding the need for technicians to climb external ladders. DNV GL Report No NLLD-R1, Rev. A-Public Page 18

20 Autobrow Type: Work Boat and bridge system Development Stage: Design Designed by Ad Hoc Marine Designs, developed by Otso Ltd, and supported by work boat supplier South Boats, the Autobrow is an actively compensated gangway system designed to be lightweight, reliable and flexible, which can be retrofitted to existing vessels with no requirement for vessel or boat landing modifications. Vertical active compensation from a hydraulic system is intended to remove the effects of heave and pitch while passive mechanisms are understood to allow the gangway to compensate for roll to a limited extent. BMT & Houlder Turbine Access System (TAS) Mark II Type: Bridge system Development Stage: Prototype This transfer system is a development of the award-winning TAS system, developed by Houlder with BMT Nigel Gee. The device can be fitted to small vessels without the need for dynamic positioning capabilities. TAS is understood to utilize a system of damped rollers and active compensation to reduce the differential motion between the vessel and offshore structure. In this manner, reliance on the standard friction grip between the vessel and boat landing is minimised. 5.3 Onshore-based Marine Access Vessel Access A wide range of conventional and specialist vessels are available to provide frequent personnel transportation and access to offshore wind farm developments from an onshore location (e.g. O&M port). These vessels vary in capacity, speed, and significant wave height (Hs) transferring capabilities and include: Quick response vessels (e.g. Rigid Inflatable Boats (RIB)); Work boats (traditional catamarans); and Small Water-plane Area Twin Hull vessels (SWATH vessels). A brief review of these vessels is provided in Table 5-2. Examples of onshore-based access vessels are given in Table 5-3 and Table 5-4. DNV GL Report No NLLD-R1, Rev. A-Public Page 19

21 Table 5-2: Summary of onshore-based marine vessels for offshore structure access Vessel Advantages Disadvantages Hs Limit (1) Quick Response Vessel (i.e. Rigid Inflatable Boats (RIB) Provide fast access to the site Widely available in the market More fuel efficient than most work boats Potential for use as daughter craft Unsuitable for transit over large distances Unsuitable for transit in onerous conditions Unsuitable for transferring spare components and consumables larger than ~50 kg ~0.75m- 1.25m Workboat (Aluminum or Composite Catamarans) Operational experience at most offshore wind projects to date Can lease vessel on long-term basis Widely available in the market Large work boats can accommodate lifting equipment Potential to accommodate some access systems May also operate from fixed offshore bases, floatels or motherships if these are fitted with boat landings. Personnel facilities and comfort make it unsuitable for journeys longer than ~2 hours m (1) Small Waterplane Area Twin Hull (SWATH) Vessel Vessels already in use for commercial and military applications, including at the Bard 1 offshore wind project. More stable vessel may facilitate personnel transfer in more onerous conditions Passenger comfort during transit improved compared to monohulls / catamarans Can accommodate medium-size spare parts Potential to accommodate some access systems Expensive Large vessel draft m (1) 1. Assuming standard turbine access, involving crew stepping from vessel bow to turbine ladder while vessel is driven against turbine, unless otherwise stated. These correspond to a DNV GL estimate based on manufacturer or supplier claims and/or operational experience. These limitations may vary with wave period, current, wind, or other access criteria. DNV GL Report No NLLD-R1, Rev. A-Public Page 20