Logistic & Service Optimization for O&M of Offshore Wind Farms

Transcription

1 Logistic & Service Optimization for O&M of Offshore Wind Farms Model Development & Output Analysis Faculty of Aerospace Engineering

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3 Logistic & Service Optimization for O&M of Offshore Wind Farms Model Development & Output Analysis For the degree of Master of Science in Sustainable Energy Technology at Delft University of Technology March 25, 2014 Faculty of Applied Sciences (TNW) Delft University of Technology

4 The work in this thesis was supported by Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). Their cooperation is hereby gratefully acknowledged. All rights reserved. Copyright c Aerospace Engineering

5 Delft University of Technology Department of Aerospace Engineering The undersigned hereby certify that they have read and recommend to the Faculty of Applied Sciences (TNW) for acceptance a thesis entitled Logistic & Service Optimization for O&M of Offshore Wind Farms by in partial fulfillment of the requirements for the degree of Master of Science Sustainable Energy Technology Dated: March 25, 2014 Supervisor(s): Prof.dr. G.J.W.van Bussel Dipl.-Ing.,M.Sc. K. Rafik Reader(s): Dr.ir. W.A.A.M. Bierbooms Dr. H. Peng

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7 Abstract The offshore wind industry is growing fast with an average annual market growth rate of almost 7% in the next five years. Service of wind turbines has proven to be expensive and difficult, especially offshore. A well-coordinated support organisation, optimized logistic and maintenance strategies are required to effectively reduce the costs associated with wind farm support. The thesis work focuses on developing a stochastic time based Logistic and Service model to analyse wind farm maintenance and logistic support organisation. The primary objective is to obtain the most cost-effective strategy, besides improving the availability of the wind farm. Within the scope of project, the wind turbine technology and the causes of its failures are reviewed. From the sub-assembly components, a list of critical spare parts is short-listed. As part of the data analysis and field studies, reliability patterns of the wind turbines are obtained from the Fraunhofer IWES WMEP database. The spare part support organisation is based on two running wind farms, namely Nysted and OWEZ. The weather information provided by the FINO 1 and FINO 2 MET masts are employed to estimate the accessibility to the wind farm. Further, based on the classification of the maintenance type of spare parts, the logistic and service strategies are implemented. The optimization of logistic and service model has been done separately, where the best possible values of ordering parameters and service aspects like finding the right access vessel strategy, the crew strength, the shift patterns, the feasibility of an offshore accommodation or renting of huge ships like mother vessel are explored. To verify the working of the model, sensitivity analysis, comparison studies and extreme value testing are performed. The model is able to predict a suitable strategy for a given wind farm, which is shown by implementing the model for a planned wind farm. With the developed O&M model, accurate inventory stocks, downtime, availability, service and logistic parameters and hence the inventory and maintenance cost is obtained. The results from the reference farms are encouraging as different strategies are compared for a cost-effective solution.

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9 Table of Contents Acknowledgments Acronyms Definitions ix xi xiii 1 Introduction Offshore Wind Energy Offshore Operation and Maintenance Fraunhofer MAS Project Economic Understanding Research Objectives Research Aim Research Questions Approach & Methodology Research Approach Literature Studies Field Studies & Data Processing Theoretical Logistic & Service Model MATLAB Modelling Verification & Analysis Outline of the Thesis Report Wind Turbine Technology Wind Energy Industry Wind Farms Wind Turbine Sub assemblies- Functionality & Failure Characteristics Cost Significant items within a wind turbine The Effect of Logistic & Service efficiency on WT availability

10 iv Table of Contents 3 Theoretical Background Maintenance Activities Multi-Echelon system optimization Spare Part Demand Different Ordering Policies Single-Item vs. Multi-Item Inventory Optimization MTTR Modelling and Estimation Economic Parameters Logistic and Service Models: Existing Models & Theory Logistic Model Global Assumptions of the O&M model O&M Model Framework Logistic Model Scope, Delimitations & Characteristics Scope of the Logistic Model Delimitations of the Logistic Model Characteristics of the Logistic Model Logistic Model Overview Logistic Inputs Single-Item Logistic Evaluation Operation: Spare Part Demand Estimation Operation: Stock Handling Operation: Inventory Parameter Computation Operation: Inventory Cost Single-Item Logistic Optimization Service Model Classes for type of Maintenance Service Model Scope & Assumptions Scope of service aspects Assumptions of the service model Service Model Overview Service Inputs Service Simulation Block Operation: Weather Time Series Operation: Accessibility Vector Operation: Corrective Replacement Operation: Mean Time to Repair Modelling Operation: Mean Logistic Delay Time & Access Delay Estimation Operation: Mean Waiting Time Evaluation Operation: Scheduled Maintenance Service Outputs

11 Table of Contents v 6 Field Studies and Data Processing Spare Part Data Spare Part Selection Spare Part Price Spare Part Type Spare Part lead Times Wind Turbine Failure and Downtime Data Failure Data Processing Mean Time to Repair (MTTR) Data Processing Weather Time series Preparation Weather Time Series Processing Accessibility of Access Vessels Logistics and Services Data Reference wind farms and Base Scenario Characterization Economic Data Considerations Model Verification and Analysis Sensitivity Analysis Logistic Model: Sensitivity Analysis for Lead time to Local or Central Warehouse Service Model: Sensitivity Analysis for selection of Access Vessels Service Model: Sensitivity Analysis for Lead time of Contract-based Vessels Service Model: Sensitivity Analysis for Crew Strategy Working patterns Comparison Study of Logistic & Service Strategies Logistic Model: (R, Q) policy vs. (S 1, S) policy for Central Workshop Service Model: Offshore Accommodation Possibility Service Model: Crew Shift Working Strategies Service Model: Access Vessel Strategies performing Schedule Maintenance Extreme Value Testing Service Model: Distance from shore for Access Vessel Operation Service Model: Maximum Crew allowed on a Vessel Implementation of Model for Planned Wind Farm Conclusions 83 A Logistic & Access Vessel Delay Decision Flow 85 B GUI Implementation 87 B-1 Features of GUI:

12 vi Table of Contents C Spare Part Data 89 C-1 Maintenance & Repair Report (WMEP) C-2 Spare Part Data C-3 Failure Rate/Demand Rate for Corrective Replacements D MTTR Data Processing 93 D-1 Estimation of MTTR D-2 MTTR Values as Inputs E Support Organisation Information 97 E-1 Reference Wind Farms (Depot Locations) E-2 Reference Wind Farms (Central Workshop to Depot Distance Estimates) E-3 Access Vessels employed for Maintenance F Weather Time Series Preparation 101 F-1 Met Mast Data Sheet F-2 Weather Time Series Processing G Class of spares-type of Maintenance 103 H Event List 107 Bibliography 109

13 List of Figures 1-1 Fraunhofer IWES MAS Module [1] The working process Modern WT with Sub assembly description (adapted from [2]) Causes of Offshore Wind turbine failure in the Netherlands [3] Relative cost for the main components of an offshore wind turbine [4] Typical Layout of an Offshore wind farm with Support Organization [5] A typical layout of a Multi-Echelon System Universal Bathtub Curve Lognormal Distribution for estimating MTTR Model Framework implementing Logistics & Services for an offshore wind farm Simplified flowchart for the Logistic Model Flowchart of the Logistic Evaluation Block Simplified flowchart of the Service Model Flowchart of the Service Operation Block Location for three FINO Met Masts [ Sensitivity Analysis for change in Lead Time of the spare part at depot Sensitivity Analysis for change in Access Vessel Speed at (i) 50 km and (ii) 100 km distance from shore Sensitivity Analysis for change in Wind Speed Threshold Sensitivity Analysis for change in Lead Time of a Contract Vessel Sensitivity Analysis for change in Shift Working Hours

14 viii List of Figures 7-6 Sensitivity Analysis for change in Minimum feasible Hours Inventory Cost following a (S 1, S) policy with two separate Central Workshops Inventory Cost following a (R, Q) policy with a single Central Workshop Offshore Accommodation installed at Horns Rev Mother Vessel Sketch [6] Sensitivity with Distance Sensitivity Verification with the month of Scheduled Maintenance Sensitivity with Distance and Number of turbines (Case1) Sensitivity with Distance and Number of turbines (Case4) Gradient Analysis: Case 1 [L] and Case 4 [R] Comparison of Strategies for three planned wind farms A-1 Flowchart for estimating the MLDT and Access Vessel Delay B-1 GUI for Logistic & Service Model D-1 Sample Lognormal distribution of MTTR E-1 Egmond aan Zee (OWEZ) Wind Farm operated from the depot Ijmuiden (near Wijk aan Zee), Netherlands [7] E-2 Nysted (Rødsand) Wind Farm operated from the depot Port of Rodby/ Gedser (small ferry harbor), Denmark [7] E-3 Estimated distance between the Main Central Workshop of Vestas (Randers), Denmark and the local depot (IJmuiden), Netherlands- for OWEZ wind farm (Google Maps) E-4 Estimated distance between the Main Central Workshop of Siemens (Brande) and the local depot (Port of Gedser), Denmark - for Nysted Offshore wind farm (Google Maps) E-5 FOB Lady for Class B/Class E repair E-6 Crane Vessel for Class C repair E-7 Jack-up barge for Class D repair F-1 Wind Time Series before processing F-2 Wind Time Series after processing

15 List of Tables 2-1 Wind Turbine Sub-assembly Inputs to the Logistic Model Service Model Inputs Outputs generated from the Service Model Accessibility of Access Vessels w.r.t two Met Masts Wind Farm Description: Reference Wind Farms Input Parameters for Validation of the model Comparison with or without an offshore accommodation based on the lifetime analysis of a WF Difference in Net Revenue for a WF with and without offshore accommodation Comparison of three different Crew Strategies Comparison for choosing overnight in large contract based vessels Summary of input parameters for the comparison of access strategy Simulation Inputs for the arbitrary wind farm (sensitivity analysis) Extreme value testing for distance from shore Extreme value testing for Crew Strength Input Parameters for planned WFs D-1 Raw MTTR values (in minutes) D-2 Frequency of MTTR Values F-1 Characteristics of MET masts H-1 Event List generated as an output from Service Model

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17 Acknowledgments The completion of the Master thesis would not have been possible without the able assistance of some people, whom I would like to thank. Firstly, I would like to thank my University supervisor Prof. dr. Gerard J.W. van Bussel.I consider myself fortunate to have you as my supervisor. Without your attention, enthusiasm and guidance, my work would not have been productive. Your approach towards problem solving and defining the project scope helped me to streamline the work and the report itself. Further, I am grateful to Fraunhofer IWES for providing me an opportunity to experience their research institute. Their cooperation and assistance during the project period is truly appreciable. But sincerely, I would like to thank my daily supervisor Mr. ir. Khalid Rafik, who was always available to help me out during the 6 months period of mine at IWES. I honestly appreciate your willingness to discuss even the smallest of problems with me during the course of this work. Moreover, a special thanks to all my colleagues at Fraunhofer for being generous enough to a non- German speaker. Also, I am grateful to Dr. Engin Topan and Dr. Hao Peng from TU Eindhoven for their participation and out of the way help for making me understand and implement inventory theory in offshore wind energy scenario. I want to express my gratitude to the rest of the examination committee. I appreciate their time devoted to reading and evaluating this document. My special gratitude to my fellow friends at TU Delft- Akshay Hattiangadi, Maneesh Kumar Verma, Nishant Narayan, Prakhar Kapoor and Sidharth Mahalingam for their enthusiasm and help during the testing periods of the project. Finally, I would like to thank my family for their selfless love and support in completion of the studies and the project in particular. Delft, University of Technology March 25, 2014

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19 Acronyms O&M M LDT MT T R MW T W F OW EZ F IN O W T GU I W M EP LCC LSC OP EX CAP EX M AS M T BF M DT M T BM LOP CBM CW M ET RIC SCADA Operation and Maintenance Mean Logistic Delay Time Mean Time To Repair Mean Waiting Time Wind Farm Offshore Wind Farm Egmond aan Zee Forschungsplattformen in Nord- und Ostsee Wind Turbine Graphical User Interface Scientific Monitoring and Evaluation Program Life Cycle Cost Life Support Cost Operational Expenditure Capital Expenditure Multi Agent System Mean Time Between Failures Mean Downtime Mean Time Between Maintenance Loss of Production Condition Based Maintenance Central workshop Multi Echelon Technique for Recoverable Items Supervisory Control and Data Acquisition

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21 Definitions Spare part is a replaceable unit for a technical system, e.g. a wind power system. Spare parts are used to repair a WT when any of its spare fails [8]. Spare part stock, or only stock, is the spare parts stored for future use. The facility where this is done is referred to as a depot (a warehouse). A depot can be located right next to a WT site or some distance away. For offshore wind farms, it is normally at the nearest harbour [8]. Spare part strategy is the result of a number of decision variables regarding initial spare part investment, reorder points and allocation between depots [8]. Support organisation is the complete arrangement that maintains a technical system, such as a wind farm, and provides it with personnel and equipment made available all the time. A support organisation has a certain structure, where depots, service stations and workshops, in some way are connected to each other [8]. Reliability of a sub-assembly is the probability that it will perform its required function under stated conditions for a specified period of time. The unreliability is related to the failure intensity function, λ(t) [9]. Availability is a fundamental measure of reliability. It combines both the outage time when an interruption has occurred and the frequency of interruption [10]. Failure is the inability of a sub-assembly to perform its required function under defined conditions; the item is then in a failed state, in contrast to an operational or working state [11].

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23 Chapter 1 Introduction The world needs a transition from its current unsustainable energy paradigm to a future powered entirely by renewable energy supply. The development of wind power has been steady in recent years, specifically in Europe, North America and developing countries like India and China. By end of the year 2012, the worldwide capacity by wind power generation reached 273 GW [12]. The current trend of the wind energy industry is to expand by developing more wind farms using turbines of high capacity ratings. Globally, very significant financial investments have been made in developing wind farms with a wide range of stakeholders. However, with this huge investment potential and significant increase in generation capacity comes an additional and often overlooked responsibility- the management of wind farms to ensure the lowest total Life Cycle Cost (LCC). Profit from a wind farm is the revenue generated by sale of electricity minus the investment cost and the operation and maintenance (O&M) expenditure [10], the latter being the focus of this thesis research. Thus, to increase the productivity and profitability of the existing wind farms and to ensure the lowest total LCC for successful future developments, will require maintenance strategies that are appropriate (technically feasible and economically viable) over the life-cycle of wind turbines. A well-coordinated support organization and optimized maintenance strategies are required to effectively reduce the costs associated with the WT support. This also includes handling and storage of spare parts, commissioning of access vessels at the right time, employing the appropriate transport and crew strategy, etc. These types of problems are common in most of the industries and so is applicable for WT industry as well. If these processes are re-optimized, there is substantial money to be saved over time. These circumstances have built the ground for this thesis research.

24 2 Introduction 1-1 Offshore Wind Energy Large scale wind power has recently grown offshore due to lack of space in densely populated areas, aesthetics and noise issues, social acceptance, as well as favourable wind resources. An offshore wind farm typically consists of large multi-megawatt wind turbines clustered together in an offshore location some kilometres far from the coast, feeding the high voltage grid on an onshore connection point through cables that are carefully buried in the seabed. Since early nineties these kinds of projects have been realized mainly in Denmark, UK and the Netherlands. These countries possess advanced wind energy know-how, offshore experience from oil and gas platforms, and favourable characteristics such as shallow sea-water, strong winds and reliable electrical grids. Nowadays offshore wind farms are gaining increasing interest as an alternative option for electricity generation and various projects are under development (in the Netherlands, Belgium, UK, Denmark, Germany, Spain, and USA) [13]. At the moment, 3.8 GW is the installed capacity from the offshore wind farms in European countries. However, more than 40 GW is expected to be in operation by 2020 [14]. The development of offshore sector is essential for the majority of the EU countries to meet their respective targets of renewable energy production. The offshore wind energy industry, which is globally at a nascent stage has to face lots of challenges and find solutions mainly because of special conditions encountered in the marine environment. Extra loads due to waves and currents, water depths, and soil properties of the seabed are just some of the other factors that have to be considered during the structural design of an offshore wind turbine. Moreover, the saline environment accelerates unfavourable processes such as corrosion and crack growth in the structure. For this, the offshore wind turbines require coatings and materials, which are usually more expensive than the ones used onshore. Additionally, some other vital technical and economic issues in which offshore wind energy differs from onshore is its technology, availability, energy production, capital cost, operation and maintenance activities, etc. [15]. When initially developed, offshore wind farms were not much different from onshore, since the same wind turbine technology with slight modifications was being directly applied to the new environment. In early wind farms, low water depths and small distance from shore were the reasons to justify this option. Recently with offshore wind farms being built further deep in the sea, the turbine developers accordingly plan to manufacture machines specially made for offshore purposes. 1-2 Offshore Operation and Maintenance Offshore wind operation and maintenance (O&M) resembles the oil industry to a certain extent. Though the experience of the oil industry is handy in terms of technical planning of O&M, the difference in the total project budget poses a challenge. The money invested for an individual project, the energy produced from it is quite less compared to oil industry and hence offshore wind industry has to develop maintenance schemes under those limited budget constraints. Also, the maintenance management for wind turbines (WT) aims on one hand at reducing the overall maintenance cost and on the other hand at improving the availability [1].

25 1-3 Fraunhofer MAS Project 3 Moreover, as compared to onshore wind farms, even small failures will have major impact on the overall availability of the wind farm. The contribution of the OPEX to the kwh price is approximately 25 to 30 % which is about 10-15% more than onshore. Equipment downtime costs include spare parts purchasing, repair labour, transportation, crane rentals and energy production [16]. Further, costs for corrective maintenance are a factor of two higher than that of preventive maintenance, whereas for onshore, there is not much difference [15]. These above mentioned points make the operation and maintenance of prime importance to the offshore wind farm with certain limits of availability. 1-3 Fraunhofer MAS Project Offshore wind industry is still in its learning process. The project owners take an assurance in the form of a contract that their farm will operate under certain high limits of availability. Hence, modern offshore wind turbine manufacturers are compelled to operate their wind turbines with an availability of nearly 95%. To achieve such levels, considerable amount of additional maintenance work and costs are necessary. There is a substantial scope for optimizing reliability and maintenance procedures. One of the possibilities is to systematically make use of available knowledge and past experience. The consideration of various parameters such as weather conditions, power prognostics, stock keeping, etc. is essential for optimal decisions. Such complex inter-related models require the use of sophisticated tools. Fraunhofer IWES is in process of making such a tool. It is a Multi-Agent-System (MAS), a new discipline in the world of Artificial Intelligence (AI) and Data Mining (DM). It enables to observe and deduce the hidden knowledge and logical dependencies of a great amount of data using appropriate algorithms. MAS consist of five closely interconnected modules- Failure-Rates Module, Weather Module, Production Module, Logistic & Service Module and finally the Cost Module. The same can be seen in the Figure 1-1. This separation provides the option of using different simulation methods as well as an easy extension [1]. Figure 1-1: Fraunhofer IWES MAS Module [1]

26 4 Introduction This research focusses on Logistics & Services for which a probabilistic model is implemented in MATLAB. The topic of Logistic and Services is quite vast. Though certain information w.r.t inventory data, weather module and production module is not available in its current space, maximum aspects of logistics and services are modelled to estimate the most suitable strategy for a given WF. 1-4 Economic Understanding In Economic terms, the initial project investment is termed as CAPEX and the operation cost for maintaining that project is characterized as OPEX. The total project cost is the sum of CAPEX and OPEX besides the decommissioning cost at the end of the lifetime of WF. When estimating cost of a project over its lifetime, the same is designated as Life Cycle Cost (LCC). Considering wind energy market, the LCC is the summation of- Development and manufacturing cost, Life Support Cost (operating cost) and Phase-out cost. This research deals with only the Life Support Cost of an offshore wind energy system. Life Support Cost (LSC) can be further divided into two components- Maintenance or Services of the wind farm and Spare part logistics. As explained before in Section 1-3, this is a part of Fraunhofer MAS which is named as Services and Logistics Module. Some of the typical variable costs involved in Logistics and Services are explained in Section Research Objectives Research Aim The aim of the thesis is to model and analyse the wind farm maintenance and support organisation. Optimal ordering policy parameters and improved service strategies are aimed for, thereby achieving maximum cost efficiency Research Questions Some of the research questions that will be dealt are classified as follow: What are the common reasons for failure of different WT sub-assembly? What are the best policies for stock-keeping for the wind power systems modelled? What are the best access and crew strategies when implementing a replacement operation? Under this work, development of models and solutions for multi-echelon spare parts management in wind turbine industry will be performed. This includes demand estimation of the spare parts, computation of downtime and availability of the wind farm and finally estimating the overall life support costs.

27 1-6 Approach & Methodology Approach & Methodology Research Approach To fulfil the objective of this thesis there are three main components needed; understanding of failure and demand behaviour of spare parts, a support organisation model and an optimisation tool to realise the theoretical design. The research approach can be summarized with five method elements, shown in Figure 1-2. Figure 1-2: The working process Literature Studies Literature studies are concentrated on two different scientific areas, WT technology and optimization theory. Basic WT sub-assemblies are studied besides understanding the reason of its failure and what procedures are followed when a severe fault occurs. Also, critical spare parts and components of a WF are short-listed. Further, to help develop a basic logistic model, inventory theory applicable in general is understood. Ordering policies applicable to a multi-echelon support structure as in the WF industry is detailed. For implementing the service model, failure and spare part demand behaviour, MTTR modelling and maintenance activities are discussed. Also, already existing logistic and service models are summarized to finish off the literature studies Field Studies & Data Processing As mentioned before, the research is not focussed on any particular WF. However, two existing and running offshore WFs (Egmond aan Zee and Nysted) are referred for acquiring information regarding its logistic and service structure. Since, the two named WFs are demonstration projects of Netherlands and Denmark respectively, sufficient information is available for research in the field of wind energy. Further information on WT failure or spare part demand and mean time to repair are gathered from the Fraunhofer WMEP database. Lastly, the weather data is retrieved from two MET masts -FINO 1 and FINO 2, from which the necessary wind-wave time series are prepared. For each of the field studies performed, the raw data is processed in order to be in line with the stochastic model requirements.

28 6 Introduction Theoretical Logistic & Service Model Besides acquiring relevant and useful data, it is necessary to conceive an algorithm and a theoretical design pertaining to the scope and the assumptions defined for both the logistic and service model. The same is done before implementing it in a computational language like MATLAB MATLAB Modelling When modelling complex systems, it is inevitable to use a computer software. MATLAB, which is a standardized and accepted industry tool is used for solving the O&M problem. The objective of both the logistic and service model is to compare different strategies and provide the best results in terms of availability and life cycle support costs of the WF Verification & Analysis With the development of any new simulation model, need of its verification and validation is important. Both the logistic and service model are tested for different scenarios in terms of sensitivity analysis, comparison studies and extreme value testing. Finally, optimal results are generated and compared in response to different input strategies chosen by the user. 1-7 Outline of the Thesis Report Chapter 2 presents the WT sub-assembly and its failure characteristics. The important components of the WT nacelle are short-listed and the need for having an optimized support organisation is conceptualized. Chapter 3 gives a theoretical background pertaining to inventory theory, concepts related to reliability, spare part demand and maintenance and an overview of logistic and service modelling applicable to offshore wind. Further, Chapter 4 and Chapter 5 describe the logistic and service model respectively implemented in MATLAB. Both the models are explained in detail with necessary flowcharts and diagrams. Relevant examples are also provided for better understanding. Chapter 6 discusses mainly the field studies and the data processing required to be passed as input to the O&M model. The report proceeds to Chapter 7 where the validation of the model, testing of the extreme cases and implementation is performed. The report is completed with Chapter 8 providing the conclusions.

29 Chapter 2 Wind Turbine Technology This chapter reviews literature pertaining to Wind Energy Industry and causes of failures for various WT sub-assembly. The wind energy industry and the critical parts of the turbine are discussed in Section The Section reviews failure of horizontal axis wind turbines and, identifies some common causes of failure in wind turbines. A review of the cost-significant items within a wind turbine is presented in Section 2-2. In the concluding para of Chapter, in Section 2-3, the need of optimization of support organisation is discussed with an example. 2-1 Wind Energy Industry A modern wind turbine consists of two or three blades that are set in motion by the passing wind. This rotating motion is either transferred directly to a generator, or through a gearbox which increases the rotational speed into the generator. All the WTs in this study, a gearbox is used, which is the most common type for WT. Another fundamental difference in WT design is how the blades handle fluctuating winds. There are three types of regulation: stall, pitch and a combination of stall and pitch. With stall regulation, the blades are formed with an aerodynamic structure causing turbulence near the blade at high wind speeds. The turbulence decreases the lifting power of the blades and thereby limiting the rotational speed to acceptable levels. Compared to stall regulation, WTs using pitch regulation have rotational blades. During high speeds, the blades are rotated from the wind, letting more wind through, which decreases the lifting power, and thereby the rotational speed. The WT models included in our study all have pitch regulation [17] [18] Wind Farms Average capacity of today s offshore wind turbine is 3 MW. However, a Nuclear plant alone is capable of producing nearly MW of power. If we were to compare the nuclear plant to a fairly average 3 MW commercial wind turbine, it would take about wind turbines to equal the nuclear plant s capacity, provided the turbines run at their maximum

30 8 Wind Turbine Technology capacities. One WT looks very small in this context but if put together in large groups, i.e. wind farms, they are merged into a power plant of 100 MW or more. Therefore, the construction of the wind farms has led the wind power technology into a new era. When moving from these single scattered WTs to larger production facilities, the maintenance work is simplified. Instead of long travelling distances between a few WTs, service technicians are able to work at one location, performing daily maintenance work on a close range. This also leads to a greater knowledge of a certain WT type and quicker repairs, since a technician is available in the area. A wind farm comprises of number of WTs connected with electric cables, either located onshore or offshore. To minimize the effect of wake losses, the WTs are placed meters apart, depending on the rotor size [19]. Offshore wind farms were not really much different from onshore initially, since the same wind turbine technology with slight modifications was being directly applied to the new environment. Earlier in wind farms, low water depths and small distance from shore was the reason to justify this option. However nowadays with offshore wind farms being built further deep in the sea, the turbine developers accordingly plan to manufacture machines specially made for offshore purposes Wind Turbine Sub assemblies- Functionality & Failure Characteristics When analyzing logistics of wind energy system, it is relevant to identify the important subsystems a WT is composed of. In this section, such a literature study is performed. The critical components of a WT along with their reason of failure is mentioned. For the scope of this report, all these components have been referred to as spares. A WT is to a large extent built with standardised items used in many other industrial applications. Therefore there is an open market, especially for the majority of the mechanical items [8]. Figure 2-1 presents the most important components in a WT. Most of the parts or systems addressed in the picture below are explained in the following subsections [2]. Figure 2-1: Modern WT with Sub assembly description (adapted from [2])

31 2-1 Wind Energy Industry 9 Basically, there are 4 main reasons of failure of any equipment; human error 1, Acts-of-God 2, design faults and components related failure 3. The wind turbine has to be design tested as per the industry standards. However, these design tests cannot accurately predict all the actual environment factors which vary from site to site or all possible reasons that may occur during the operating life of the wind turbine. Thus to assess field failure characteristics of wind turbines, it is essential to understand the likely failure behaviour of the turbines when they are exposed to the natural environment. ECN did a study on the failure behaviour of the offshore wind turbines in the Netherlands. The results are presented in Figure 2-2. It is seen that, the blade failures, generator failures, and gearbox failures contribute together over 75% to the costs and the downtime. Figure 2-2: Causes of Offshore Wind turbine failure in the Netherlands [3] Hereby, different sub-assemblies are described with its functionality, critical components of the assembly, common reasons of failure, actions needed when a failure occurs, and related failure examples from real. (a) Blades Wind turbine blades are designed to harness power from wind and then transmit the rotational energy to the gearbox through a hub and main shaft. The blades are designed for optimized output and minimum noise and light reflection. The blades are made of fibre glass reinforced epoxy and carbon fibre [20]. Composite materials are often preferred because of its possibility of achieving high strength and stiffness-to weight ratio [17]. Composite material is also corrosion resistant and good electrical insulator which is an advantage in an offshore environment. As a part of annual servicing, the blades bearings are lubricated automatically from an electrically driven unit [20]. The core of a blade is the part that receives the main load. The most important design factor for the core is that it has to be light and flexible and still be able to handle heavy loads [17]. Cracks can occur on the surface of the blade due to the fatigue and lightning strikes. Ice build-up is also known to cause failure of fibreglass reinforced plastic (GRP) blades [3]. The cracks are not affecting the WT to function but they still need to be repaired so that they do not get worse. 1 Gap between what is done and what should have been done such as wrong installation of components 2 Refers to natural events where the occurrence cannot be reasonably foreseen or avoided e.g. lightening 3 Deterioration of equipment in its normal operating context such as fatigue, wear-out, etc.

32 10 Wind Turbine Technology When there are sights of cracks or other weakening in the supporting structure, the blade needs to be replaced immediately [8]. It is difficult to evaluate and repair a blade with a broken structure, hence completely new blade is mounted and the old one is discarded. E.g. at Tuno Knob, the blades were required to be replaced entirely after structure failures in the same [21]. (b) Hub The hub of a wind turbine connects the blades to the main-shaft, and transmits rotational force generated by the blades. Hubs are generally made from steel which can be welded or casted [17]. The topology of a wind turbine determines the specific type of hub design to be used on the wind turbine. The hub also consists of spinner and spinner bracket [22]. The blades, hubs and the fasteners are made of different materials. Thus, interactions between these three components in terms of stiffness during variable loading constitute huge operating problems. Modern wind turbine blades have threaded bushes glued into their roots, and are connected to the hub by using bolts [3]. (c) Pitch System For WTs with pitch regulation, the hub provides bearings for the blades allowing them to rotate relative to the hub. Within the hub the pitch system is performing the rotation of blades. Blades can be pitched individually or by a common, central pitch mechanism. Today most of WTs have an individual pitch system, which is either controlled electrically or hydraulically. An electrical pitch uses a slip ring to transfer electric power from the nacelle and out to the hub. If it is a hydraulic pitch system a rotating union is used to transfer the pressure. Some years ago, there were a lot of problems with leakages in rotating unions, but during the last couple of years, there have been paramount improvements in their reliability [17] [8]. The industry is split with about 45% electric and 55% for hydraulic controls. The advantage of the hydraulic control is that its power density is higher than electrical equipment and it needs fewer components, making it a simpler system [23]. For example in Vestas V90 turbines, changes of the blade pitch angle are made by hydraulic cylinders, which are able to rotate the blade by 95. Every single blade has its own hydraulic pitch cylinder [20]. It is important that the blades can be pitched even if the slip ring, rotating union or some cables or hoses fail. Therefore, batteries or hydraulic accumulators are installed in the hub as a backup system [24]. (d) Drive Train Mostly in all WTs, there is a main shaft connecting the rotor to the drive train. A shaft connection prevails between the rotor and the gearbox and further to the generator. They are used to transmit torque within the WT. Shafts are not only under stress from torque load, but also there is a bending load on the shaft. These loads are time-varying, so fatigue of the shafts is an important factor [8]. Bearings are closely connected with the shafts since they are carrying the weight of the rotating shafts. Bearings have an important function for the drive train, as well as for rotating the blades (pitch system) or the whole nacelle (yaw system) [17].

33 2-1 Wind Energy Industry 11 Poor lubrication, wear, pitting, deformation of outer race and rolling elements are the main reasons of the failures [3]. Problems can also occur with shafts, if they are often operating under critical speed. At some turning speeds, shafts have resonant frequencies, creating vibrations. Since they are heavy, replacement of bearings and the drive train are complicated procedures, where a large crane is needed to lift them up and down the nacelle [8]. (e) Gearbox The gearbox of a wind turbine increases the rotational speed of the main shaft from as low as revolutions per minute (RPM) to as high as 1500 RPM which is necessary to drive a generator of the wind turbine. It is one of the heaviest and most expensive components of a wind turbine. A three- stage planetary gearbox is usually utilized in wind turbines [17]. For e.g. in V90, the gear unit is a combination of a 2-stage planetary gear and a 1-stage helical gear [20]. Gearboxes are built up of shafts, gears, bearings and seals, mounted in a metal cover. The weight of the gearbox increases dramatically in relation to the rated power of the WT. The main load a gearbox has to handle is torque of the rotor. This load, as mentioned earlier, is sometimes constant and sometimes fluctuating. It also suffers loads from the generator when it is started. These loads mainly affect bearings, gear teeth and seals, causing them to fail [17] [3] [8]. To minimize fatigue of gearbox parts, a functional and efficient lubrication system is highly relevant. For e.g. in Vestas turbines, the oil is collected in a tank. From the collection tank, the oil is pumped to a heat exchanger and back to the gearbox. The pumps distribute the oil to the gear wheels and bearings [20]. It has been observed that the under-dimensioned gearboxes have had a large part in WT failures. The reason for under-dimensioned gearboxes can be that the manufacturers do not fully understand the operating conditions. For e.g. OWEZ offshore wind farm in Netherlands had a complete gearbox change programme for all turbines [25]. Another problem with the gearbox is that even if it is only a small component breaking, the whole system needs to be cleaned out and thoroughly tested. With the gearboxes, the replacements are mostly pro-active [26]. Faults with gearboxes are primarily discovered within the first two years of operation-as in the case of OWEZ and Nysted Offshore wind farms. If a gearbox last during the first two years, it is likely that it will last for good period of time [25] [27]. (f) Generator The generator within a wind turbine converts the mechanical rotational energy from the gearbox into electrical energy. The generator is slightly different from other generating units connected to the electric grid because it works with a power source (the wind turbine rotor) that supplies fluctuating mechanical power (torque) [3]. The most common type of generator today is the induction generator, also referred to as an asynchronous generator. This type of generator is used in modern type of WTs with variable speed [17]. Variable speed allows varying the rotor speed within a wide speed range. This reduces power fluctuations in the power grid system as well as minimizes the loads on vital parts of the turbine [20]. A generator needs to be protected from water, dust and other foreign particles. There

34 12 Wind Turbine Technology are two common types of protection, totally enclosed fan cooled (TEFC) or an open drip protection. There are only a few components in a generator that are exposed to electrical or physical stress. The windings in the rotor and stator are sensitive to high currents leading to increased temperatures that are wearing the windings and can lead to a failure. These windings can be replaced but the generator has to be taken out from the nacelle in turn to make this type of repair possible. The generator bearings and different fans are subject to an almost constant mechanical wear and have to be exchanged from time to time [17] [8]. Also it is observed that that at power frequencies, SCIG is inherently stable, but when connected to a weak grid with an unbalanced three-phase load, overheating and torque pulsations may occur [20]. (g) Mechanical Brakes Mechanical brakes in a WT have two functions. Usually they are used as parking brakes, when power production is down, but occasionally they are used for emergency braking also. The brake system consists of a brake disc, brake pads and callipers [22]. A mechanical brake can be located somewhere along the drive train. There are two main types of brakes, disc brakes and clutch brakes. Disc brakes need a hydraulic pressure, supplied from a hydraulic pump or accumulator, to operate. Springs are often used to activate clutch brakes, using hydraulic or pneumatic pressure to release it [17] [8]. Excessive wear on brake linings which happens mainly due to emergency breaking causes brake failure or even fire [3]. (h) Yaw System The yaw system is used to set the nacelle and rotor in an effective position against the wind. A rotating nacelle requires a yaw bearing supporting the load of the nacelle. The circumference of the bearing has gear teeth which are connected to a yaw gear. The yaw gears are driven by electrical motors (called yaw motors), shifting the speed of the pinion conducted to the bearing s tethering [17] [8]. In Vestas V90, four electrical gears with motor brakes rotate the nacelle [20]. The major causes of failure of a yaw system include bearing failures, pinion and bull gear teeth pitting, yaw brake failure, pinion and bull gear teeth wear-out [3]. To limit the wear on the yaw system, yaw breaks are installed to hold the nacelle in place when the WT is not running [17]. (i) Hydraulics The hydraulic system operates the mechanical braking system, the pitching system and the yaw control system. It also operates the on-board cranes and locking systems for canopies and spinners in larger wind turbines. Main components of the hydraulic system include pumps, drives, oil tanks, filters and pressure valves. The hydraulic system contains hydraulic oil which is put under pressure to move pistons in hydraulic cylinders. This system ensures that pressure is established when the wind turbine starts and it releases the pressure, when the turbine stops. The pump builds up the pressure which is controlled by a pressure sensitive valve to ensure safe attainment of the required pressure level. For effective operation, a reserve pressure steel tank is often included in the system [3].