for Maintenance Optimisation - folk.ntnu.no · Keywords: Wind turbine, reliability, availability, monitoring, development, operating results, external conditions I. INTRODUCTION ...

ABSTRACT Offshore wind energy use is commonly suggested to

play an important role in future electricity supply. However, long-term experience with thousands of onshore wind turbines explicitly hint on possible barriers for a save, efficient, economic and user friendly supply relying on offshore wind energy. A national German programme shall on the one hand support the wind energy branch improving technology and O&M procedures towards higher reliability and availability, and on the other hand monitor the development in terms of techniques, electricity yields and cost. The scientific programme ‘Offshore~WMEP’ aims to help in answering essential questions concerning offshore wind energy. One of the most important issues of this monitoring project, in close collaboration with operators, manufactures, suppliers and scientists, is to generate a data pool that enables topic-specific evaluations for all participants. In the following the actual challenge of convincing numerous players with diverse points of interests to collaborate in gathering data and providing it for a jointly utilized database is shown.

Keywords: Wind turbine, reliability, availability,

monitoring, development, operating results, external conditions

I. INTRODUCTION 20 years ago the German government made

renewable energies a priority. Especially wind energy became a leading role and experienced an enormous upturn due to the Electricity Feed-in Act. The wind energy utilization is on its way to become the most important renewable energy source. With the “Scientific Measurement and Evaluation Programme“ (WMEP) [1], included in the German subsidy measure “250 MW Wind” and funded by the federal government, the deployment of this technology was monitored over a period of more than fifteen years. The resulting data base of this programme contains a quantity of detailed

information about reliability and availability of wind turbines (WTs) and subassemblies and provides the most comprehensive study of long-term behaviour of WTs worldwide. It provides the opportunity to gain basic insights into wind energy and to address larger political questions.

Today, the offshore wind energy faces similar challenges as the wind energy on land at the beginning of the WMEP. It can not be considered as assured that the wind energy use up to 100 kilometres off the coast in water depths up to 40 metres fulfils the technical and economic hopes. Several years after the erection of first offshore wind farms decision-makers in politics and in the energy and finance sector will need to rely on detailed information for defining the future of offshore wind energy in Germany. For further development, data and insights on technology and cost developments must be available. Risks in the range of installation, logistic, operation, and grid integration of large offshore wind farms should be minimized by means of verifiable analyses and results.

Additionally, a trend to decreasing reliability of large WTs with high rated power and more complex technical concept can be recognized [2]. The availability of different offshore wind farms in UK, Denmark, or Netherlands does not reach in the least the availability onshore of about 97%. Obviously, in terms of reliability and availability the optimisation of maintenance processes and component design is urgent. To accomplish these challenges, reliability and availability of WTs for offshore use has to improve and hints at the feasibility and economic efficiency of offshore wind energy are required. A common data pool enables future operators of offshore wind farms to make statistically reliable predictions concerning the success of operational and system concepts. As a result of a common data base weak points can be identified, components can be qualified in cooperation with manufactures and suppliers and statements about the probability of failure behaviour can be made. Using these findings, maintenance efforts can be reduced while reliability and availability improves.

for Maintenance Optimisation

S. Faulstich, P. Lyding, B. Hahn, S. Lopez Fraunhofer Institute for Wind Energy and Energy System Technology - IWES, Kassel, Germany E-mail: [email protected], Tel.: +49 561-7294-253, www.iwes.fraunhofer.de

Offshore~WMEP – Monitoring offshore wind energy use

II. OFFSHORE~WMEP The Offshore~WMEP (www.offshore-wmep.de) is part

of the research initiative ‘Research at alpha ventus’ (RAVE) funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU). On the one hand fundamental questions concerning the use of offshore wind energy shall be answered by a general monitoring, and on the other hand operating experience shall be collected and analysed systematically in collaboration with operators and manufacturers.

After a certain time of operation investors, grid operators, banks, insurance companies, and politicians will need reasonable data and findings to decide on the next investments or to provide economy with adequate frame conditions respectively. Gathering data will generate a large database which will contribute to political decision-making processes and facilitate further technological progress. The generation of a common database will, due to its size, enable statistically reliable predictions concerning the success of operational

concepts. Furthermore, based on anonymous benchmarking and weakness analyses, operators and manufacturers have the opportunity to test and, if necessary, to optimise the performance of their offshore wind farms.

In order to get a large statistical basis for evaluations and therefore results with strong validity, it is planned to include as many German wind farms as possible and even wind farms abroad. Leading operators have already shown interest to include data of their international wind farms. The incentive for the operators consists of a systematic preparation of operating and maintenance data that should indicate weak points of technology and non-effective maintenance processes. The participants of the programme provide a data pool held in trust by the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES, former ISET) with their data and get scientifically substantiated analyses of their offshore wind farm performance in return. The basic concept of the Offshore~WMEP is shown in figure 1. The main focus of this paper is on the general monitoring. Therefore, it is described more detailed in the following.

Figure 1: Concept of the Offshore~WMEP

A. Concept of data acquisition As mentioned above a large database for results with

strong validity is indispensable. To obtain a large statistical basis for analyses, information has to be stored in a standardized form [3, 4]. Even databases like WMEP reach their limits of statistical capacity due to the parameter diversity. Hence, a common and broad database as well as a standardized data structure is absolutely essential. Empirical experience with as many as possible WTs of similar design running under similar operational conditions should get evaluated jointly, to increase the statistical basis. Therefore, Offshore~WMEP collaborates with several partners in a project named ‘EVW - Erhoehung der Verfuegbarkeit von Windenergieanlagen’ (Improving reliability of WTs) [5], in which a standardized data structure was developed. This structure has been adapted for specific offshore conditions.

For an optimisation of reliability and availability a clear and unambiguous database is needed. The data structure consists basically of three parts: core data, working data, and result data, whereas the working data include information of all failures and damages. Wind turbine subassemblies are part of the core data. They have to get designated and structured by all players in a standardised way to correctly identify the subassembly affected. A ‘Reference Designation System for Power Plants’ (RDS-PP) [6], which is commonly used by operators of conventional plants, is going to be adopted by wind turbine operators.

Failures and damages are part of the working data. Appropriate data sets about failures and causes also include information about the current operational conditions at the very moment of occurrence. Thus, besides climatic and operational parameters also the status of operation has to get stored. Current work is aiming at adapting an existing designation system for failure attributes from German association VGB Power Tech [7] to the necessities of wind energy use. Conclusively, data have to be systematically collected, subassemblies consistently designated and operating conditions, failures and damages comparably described.

Figure 2: Concept of data acquisition

B. Concept of data transfer Because of the large amount of data to be processed

in future, a largely automated data transfer has to be realised. These challenges are solved with the help of so-called middleware. In the given case, Web services take the role of middleware. The transport protocol for this broadcast is the SOAP (Simple Object Access Protocol). This is based on well known standards such as XML (for the representation of the data) and Internet protocol IP for the transport layer TCP (TCP/IP) (to transfer the data). The contents of the SOAP document itself are subject of a separate working group within the ‘Foerdergesellschaft Windenergie und andere erneuerbare Energien’ (FGW), which specify the definition of the various attributes and information to be transmitted. The document to be sent is called “global service protocol” (GSP). The GSP is supposed to represent the common protocol standard for communication in the future maintenance of wind turbines.

C. Concept of confidentiality The Offshore~WMEP will collect, process, analyse,

and disseminate essential data and results respectively to the public, to the government, to operators, to manufacturers, etc. Wind turbine manufacturers and wind farm operators will consider at least parts of the information required as sensible and may remain reluctant to provide it. Thus, one of the most important issues of Offshore~WMEP first phase is to maintain credibility and trust with the participants and to develop a concept of confidentiality, which will allow even competing parties to participate and to support the project actively. The confidentiality of data and analyses is central to accomplishing the Offshore~WMEP. Therefore an essential characteristic of the project is that only anonymous and non-confidential results are made available to the public. To avoid unauthorized transfer and access from third parties, compliance with the confidential treatment of data and results are secured by contractual arrangements and technical measures.

data

anal

yses

public group results individual results

non-confidential confidential

data

anal

yses

public group results individual results

non-confidential confidential

Figure 3: Concept of confidentiality Participant specific analyses The common data pool enables future operators of

offshore wind farms statistically reliable predictions concerning the success of operational and system

concepts. Furthermore, based on anonymous benchmarking and weakness analyses, operators and manufacturers have the opportunity to test and, if necessary, to optimise the performance of their offshore wind farms. As a result of a common data base weak points can be identified, components can be qualified in cooperation with manufactures and suppliers and statements about the probability of failure behaviour can be made. Using these findings, maintenance efforts can be reduced while reliability and availability improves. Manufactures of offshore wind turbines, foundation structures, cables, etc. will receive an anonymous, cross-manufacture comparison of their technological concepts that serves as innovation and indicator of weak points. Basis for fair competition will be a jointly defined degree of transparency, which will furthermore accelerate the success of offshore wind energy.

III. GENERAL MONITORING – RESEARCH TOPICS

The research contents that should be inspected within the scope of the Offshore~WMEP, regarding the particularities of offshore technology and future potentials for development include:

• location-specific offshore conditions (wind supply, wind profile, special meteorological situations, turbulence, state of the sea, ...)

• Installation procedure (planning, authorization, installation time, expense, costs, transport and construction techniques, ...)

• Energy loads (full capacity hours, annual amount, performance fluctuations, short-term deviations, outages due to weather, ...)

• WT reliability (frequency of WT failure , failure due to components, particular damage, reliability values, ...)

• Farm availability (effects of special meteorological situations, disruptions, non-accessibility, replacement part deficiencies, logistics problems, ...)

• Potentially different installation concepts • Advantages and disadvantages of different

maintenance concepts • Investment and operating costs

The “Wind Energy Report 2009 – Offshore”

(www.windmonitor.de) compiled as part of the research project Offshore~WMEP was recently published, in which some results are summarized in a compact form [8]. A short description of some exemplary results is given in the following.

A. General development While other European countries have already had

intensive experience in the sector of offshore wind energy, Germany is just at the beginning. The start was

delayed in Germany particularly due to the consideration of ecological requirements. The German offshore wind farm projects were primarily planned in about 15 m deep water about 10 km from the coast in order to not hinder the national park tidelands. Thus, the available locations for offshore wind energy in Germany are considerably different than the locations of the offshore projects already realized internationally (see figure 4).

Figure 4: Comparison of the locations planned in

Germany with the existing international locations Denmark is still leading the charge for offshore wind

energy. Not only did Denmark erect the first large commercial offshore wind farm, it also has the highest number of installed facilities (345 in the North and Baltic Seas). However, other nations are catching up and Great Britain took over the top position of installed power with over 890 MW. Other nations like Norway want to build more offshore wind farms in the coming years and with the start-up of alpha ventus last year, Germany has started its offshore wind energy production.

Figure 5: Installed wind power and offshore wind

turbines from different countries

In 2009, about 190 turbines were installed in the sea,

now more than 780 exist. A supply of about 370 MW was installed, which results in a total installed supply of about 1,800 MW.

B. External conditions Due to the higher average wind speeds on the sea, the

exploitation of energy offshore should be considerably higher than on land. Because of the higher dynamic requirements from combined loads through wind turbine operation, wind and waves with irregular sea swells, the design of the carrying structures and WT components must be adjusted in line with the special challenges. Furthermore, environmental factors such as water, salt, increased UV radiation and biological sea influences play an important role. Technical adjustments to WTs, intelligent logistics and maintenance concepts as well as permanently connecting maritime know-how with the wind energy technology will have a considerable influence on the development of wind energy production offshore.

In figure 6, the frequency of wind speeds of the two measuring stations FINO 1 and FINO 2 for 2008 are illustrated in a comparative manner.

Figure 6: Comparison of the frequency of wind speeds

on the wind readers of the location of FINO 1 and FINO 2

C. Full load hours For the evaluation and comparison of WT

performance capabilities, the annual energy delivery is usually related to the rated power of the WT. The number of so-called equivalent full load hours depends upon the voltage capability of the WT and the local conditions. With the WMEP analyses the variance of the local conditions could be shown for the older WTs. Particularly at coastal locations high values are reached; individual WTs there reached values of over 3000 hours per year. The WTs on land, in contrast, could only be operated because parts of the costs were covered by direct financial support measures. In figure 7, different values are illustrated for actual full load hours reached, for which the comparison of very different situations must be taken into consideration. The WMEP numbers come from 1500 older, smaller WTs, however, they have

been recorded over a long period of time. The average value for Germany includes all old and new WTs from the coast to the middle mountain ranges, while the data for the offshore wind farms come from a few farms and was determined for different periods of time. Despite all limitations to the comparison, the results of the offshore wind farms are still not very persuasive.

Figure 7: Full load hours of on shore compared to

offshore

D. Availability The objective of the maintenance is to reach a high

availability of the WT with the lowest possible costs. Modern WTs normally reach an availability of between 95 and 99 percent on land. For offshore wind energy production, due to the special location situation and the challenges connected with this (for example, loads, accessibility), mostly a considerably lower availability is feared. This fear is confirmed through the previously recorded results, as shown in figure 8, of already realized wind farms. The image shows the technical availability of different offshore wind farms, whereby the wind farms for this are sorted according to the respective point in time of the start-up and they are marked in color according to the size of the turbines used. Trends can also be recognized, both with regard to the age of the respective farm and the size of the turbines. While the availability of the older farms, which have comparatively small rated power and are relatively close to the coast, is comparable with the average onshore availability, the availability of newer farms has decreased strongly.

Figure 8: Development over time of the availability

E. Accessibility The accessibility of a WT is an important aspect in

order to increase the availability and to reduce losses in product.

With onshore WT in Germany, the accessibility is not normally a problem. Exceptions can occur in cases of very strong winds, storms or snow. In contrast, the actual availability of offshore WTs decreases considerably through the factor of accessibility. Moreover, special equipment is required for the work. Larger repairs (replacing motors, rotor blades, etc.) require, for example, pontoon cranes, which can only be used when the weather is mild. If a helicopter is necessary in order to gain access, the wave height and the indirect wind speed determine the accessibility of an offshore location. Weather situations with a wave height of more than 1.5 m are commonly referred to as “weather days”, because the WT can no longer be reached without danger. The average number of such days is illustrated for different offshore wind farms.

Figure 9: Accessibility of different offshore wind farms Due to the problem of the limited accessibility, new

strategies must be used in order to reach high availability for the offshore WTs. Thus, both the existing access systems and the maintenance strategies must be optimised to enable the efficient use of offshore wind energy.

IV. CONCLUSIONS In fact, the first offshore projects do not meet all high

expectations, but it appears that the use of offshore wind energy can be controlled. It will be essential in future to identify offshore-specific problems and to provide appropriate solutions to ensure that the offshore wind energy overcomes the upcoming challenges and therefore undertakes their role in the energy supply.

The presented results show the necessity for considerable efforts regarding operating results (Adapted facilities, improved access systems, intelligent logistics and maintenance concepts, sustainable combination of maritime know-How and the wind energy technology ...) and the economic competitiveness (Optimisation of production and assembly, adapted

installation techniques, Grid integration concepts ...). However, the government action (renewable energy

law (EEG), infrastructure planning acceleration law and funding of research …) are fundamental for the success and the further extension of offshore wind energy use.

V. OUTLOOK Before the Offshore~WMEP moves on from their

current concept phase to the operating phase, stipulations have to be made to guarantee data’s confidentiality, which is held in trust by IWES. To jointly put the concept to practice in cooperation with the offshore wind farm operators is the major task for the near future. Therefore, IWES is in close contact with relevant companies and in advanced discussion.

ACKNOWLEDGEMENTS The projects WMEP, EVW and Offshore~WMEP are

funded by the German Federal Ministry of Environment, Nature Conservation and Nuclear Safety.

Besides, the partners ‘Ingenieurgesellschaft für Zuverlaessigkeit und Prozessanalyse’ (IZP), Dresden, and ‘Foerdergesellschaft Windenergie und andere erneuerbare Energien’ (FGW), Kiel, support by contributing to technical aspects as well as by implementing the results into guidelines and standards.

REFERENCES [1] Faulstich, S. et al.: ‘Windenergie Report Deutschland 2008’,

Institut für solare Energieversorgungstechnik (Hrsg.), Kassel, 2008

[2] Faulstich, S., Hahn, B.; ‘Comparison of different wind turbine concepts due to their effects on reliability’; UpWind Deliverable 7.3.2; 2009

[3] Faulstich, S. et al; ‘Suitable failure statistics as a key for improving availability’; Proceedings of the European Wind Energy Conference, EWEC 2009, Marseille

[4] Faulstich, S. et al; ‘Appropriate failure statistics and reliability characteristics‘; Proceedings of the German Wind Energy Conference, DEWEK 2008; Bremen

[5] Research project ‘Erhöhung der Verfügbarkeit von Windenergieanlagen’; www.evw-wind.de; funded by the German Federal for the Environment, Nature Conversation and Nuclear Safety

[6] VGB PowerTech (Hrsg.), Richtlinie ‘Referenz-kennzeichensystem für Kraftwerke RDS-PP – Anwendungserlaeuterungen für Windkraftwer-ke‘, VGB-B 116 D2, 2006

[7] VGB PowerTech (Hrsg.), Richtlinie ‘EMS - Er-eignis-Merkmal-Schluesselsystem‘, VGB-B 109, 2003

[8] Fraunhofer Institut for Wind Energy and Energy System Technology (Ed.); ‘Windenergie Report Deutschland 2009 – Offshore’, Kassel, 2009