OFFSHORE WIND ENERGY, THE RELIABILITY DILEMMA. Dr. G.J.W. van Bussel Section Wind Energy, Faculty Civil Engineering and Geosciences Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands Tel (31) 15 278 51 78 Fax (31) 278 53 47 e-mail: g.van.bussel@citg.tudelft.nl ABSTRACT: Future offshore wind farms will be of a much larger size than those currently in operation. But the reliability of the wind turbines used until now is not sufficient for such future large -scale wind farms at sites significantly farther away from shore. Hence the requirement, often posed by bankers, investors and insurance companies, to rely on proven technology is not a very sound proposition. Development of new, larger machines can lead to more profitable exploitation of the offshore wind potential. But this can only be achieved when scaling up the wind turbines to the 5 MW size more is not the only target of the offshore wind turbine manufacturer. The efforts to realize a significant increase in reliability of the wind turbine and the other components are at least of equal importance. Integrated design of the complete wind farm, covering all major design aspects simultaneously, and in their mutual dependence in alls phases of the design process will lead to the most economical solution. Keywords: Offshore wind farms, Reliability, Operation and Maintenance. 1. INTRODUCTION At present a limited number of wind turbines have been erected in an offshore environment. Offshore wind energy experience started with the location of a 220kW wind turbine just off the coast in Sweden. The projects that followed afterwards have all been realised at rather benign sites, with the exception of two 2 MW wind turbines located in the North Sea 2 km in front of Blyth Harbour (U.K). Future offshore wind farms will be realised both at inshore sites, mainly around Denmark, in the Baltic, at North Sea locations and at a number of sites around the UK and Ireland. All the offshore wind farms that are planned or that are currently being realised are larger than the largest farm realised so far, the 20 x 2 MW wind farm at Middelgrunden, just in front of Copenhagen. Increasing the size of offshore wind farms from the present 10 to 40 MW installed capacity per wind farm to values of 150 and probably as large as 500 MW is not a straightforward procedure. Of course it is fairly easy to double or triple the number of turbines in a wind farm, extending the present 10 to 20 up to maybe 40 or 50 and hence increase its capacity proportionally. But this does require an equivalent increase in the installation time of the wind farm. Assuming a typical value of 3 to 5 days per turbine this would already require more than half a year of carefully planned continuous offshore activity. Weather conditions need to be favourable for installation activities, and thus they are usually planned in the summer season. Evidently a significant amount of time can be gained by parallel operations. This is implemented in the realisation of the Horns Rev wind farm, consisting of 80 2 MW wind farms in front of the Danish North Sea coast near Esbjerg. But installing a larger number of turbines within one year will be virtually impossible. Thus realising a 500 MW wind farm using present state-of-the-art 2 to 2.5 MW wind turbines will take several years with inherent loss of investment capital. The challenge to explore the enormous wind potential of the North Sea drives manufacturers towards the development of multi megawatt wind turbines. At the moment there are at least 5 manufacturers active in the design and construction of wind turbines with an installed capacity of 4.5 Megawatt and above. These wind turbines will be equipped with rotors of around 120 m. diameter, and will become the largest mass produced revolving structures on earth. Wind plants with a capacity of several hundreds of megawatts can then be realised. Experience in the design and operation of wind turbines however is gained with sizes that are significantly smaller. Design tools have been validated for 500-600 kW machines, having a typical diameter of around 40 m. It is therefore very difficult to guarantee the reliability of the machines. But that is exactly what investors and insurance companies want: they require proven technology!! Is there a way out of this dilemma?? 2. RELIABILITY OF PRESENT WIND TURBINES Over the past years an important amount of information has become available regarding the operational behaviour of wind turbines. Especially the wind turbines in the 500 to 600 kW class have been the topic of surveys and monitoring. Public information regarding the maintenance demand of such machines can e.g. be found in references [1] to [3]. The 500 to 600 kW class is of special interest since this the first class that is massproduced. Thus a significant amount of identical machines could be monitored, and fairly reliable statistics can be developed. Unfortunately the above-mentioned sources do not always distinguish the different makes. Furthermore their system of classification and data ga thering differs, In figure 1 below the failure frequency information from the four different databases has been summarised and compared. Because of the different classifications used in the databases this can only be done over a limited number of component groups. Despite that a fairly good agreement can be seen in the failure levels. The most striking difference can be seen in the data from WindStats. There is a very significant difference between the registered failure frequency of the Danish and of the German wind turbines. But when the data is clustered on a more global level, as is done by the author in figure 1, one can conclude that the Danish results in WindStats seem to under predict the actual number of failures significantly. Yearly Wind Turbine Failure Frequencies 0.60 0.50 0.40 LWK MWEP WindStats DK WindStats GE 0.30 0.20 0.10 Bl ad es + Oth ers Ro tor + Ro tor Pi brak tch e m ec ha nis m Ge arb Ge ox ne rat or Ele Y ctr aws ics ys + in tem Co ve ntr rte ol r sy ste Hy m dr au + lic ins Sh tru Elec s m tr aft + B ent onic ati s ea rin on gs +B rak e 0.00 Figure 1: Monitored yearly failure frequencies from different public databases. LWK: data from SchleswigHolstein [1], MWEP: data from the German monitoring programme [2] and data from the WindStats journal [3] The general picture is that accumulated failure frequency is in the order of 2.5 events/year. Furthermore regular (scheduled) maintenance of these wind turbines takes place twice a year, which brings the total number of visits to at least 4 per year. 3. OFFSHORE AVAILAIBLITY LEVELS For a 150 unit wind farm consisting of 2 MW wind turbines at least 600 visits have to be paid each year to keep it in full operation. For land based wind turbines and wind farms this is not a problem. Modern wind turbines and wind farms reach availability levels of 98% or more, but once these wind turbines are placed offshore the restricted accessibility of the site can make things much worse. This can be seen from figure 2 In this figure the availability of an offshore wind farm is depicted as a function of the accessibility of the site. This is dependent upon the wave and wind conditions of the location of the offshore wind farm, but also upon the way in which access is obtained to the wind turbines. 100% Availability which makes it very difficult to compare and synthesize the different sources. The database developed by Eggersglüß, ref [1], turns out to be the most suitable for the characterisation of the reliability profile of the machines in this class. The main reasons are that a sufficient amount of turbines have been monitored, and that the database distinguishes the turbines of different makes. A small drawback is that the ages of the machines are averaged per type, which makes it more difficult to find possible ageing trends. 90% 70% Offshore designed heli 80% vessel Offshore adapted vessel 60% 50% 100 % (onshore) 80 % (near shore) 60 % (offshore) 40 % (remote offshore) Accessibility Tuno & Vindeby (DK inshore) Horns Rev (North Sea) Figure 2: Availability of an offshore wind farm as a function of the accessibility of the site. Results obtained from an expert system calculation, see ref []. As can be seen the calculated availability of the Danish Tunø Knob and Vindeby wind farms, both having an averaged accessibility of around 85% are still at the level of onshore wind farms. The large North Sea wind farm at Horns Rev, would however have an availability of around 90%, according to the calculations performed with the expert system, ref [4], which are on their turn based upon the Monte-Carlo O&M simulation tool, ref [5]. The estimated accessibility percentage for vessel access at Horns Rev is around 65%, and this is significantly lower than for the two other farms situated in the Danish inner seas. However when a maintenance strategy is adopted in which the maintenance crew can access the wind turbines though a helicopter, the availability goes up again, and the targeted level of 95% can be achieved. Visiting wind farms by helicopter is expensive and is certainly not a straightforward procedure. All the Horns Rev wind turbines had to be equipped with a platform on the nacelle, in order to enable maintenance crew to land down from a hovering helicopter. Again an adaptation of, what is in principle an onshore machine. Until now all wind farms have been built which adapted onshore wind turbines. Extra protective coatings, sealing of bearings and nacelles against salt spray etcetera have evidently been applied, but for future wind farms, certainly when they are realised in the harsh North Sea conditions, and further from shore, this is not sufficient. For such sites a significant step has to be taken in terms of reducing the maintenance demand of future offshore wind farm. There are two ways in which this can be achieved. At first the wind turbines have to be designed with more reliable and/or redundant components and systems together with a reduction of the regular (scheduled) maintenance interval to once a year. But it can also be achieved in the design of larger machines, hence aiming at fewer components per MW installed in the wind farm. The maintenance costs of series produced 2.5 MW machines will most probably be around three times higher than the O&M costs of a 500 kW machine in the onshore case. For offshore application the advantage will be larger, due to the higher costs for offshore working activities and transport. Hence operational costs, apart from investment costs require the use of the largest machines available for true offshore sites. 4 THE ROUTE TOWARDS 500 MW OFFSHORE WIND POWER PLANTS What will be the most probable route towards future large-scale offshore wind farms? When this question is posed to different people in the wind energy business the answer will not be consistent. Within the wind turbine technology world there is a strong drive towards the development of very large machines. The tools and skills developed for the design of these machines have been validated with 500 to 600 kW machines from which experience has been gathered over a number of years. Experimental data from the (commercial) wind turbines in the 1 to 2 MW class becomes available at present. But designing machines of a scale five to ten times larger than the scale at which the tools have been validated implies a significant development risk. This is the primary reason for the investors, bankers, and insurance companies to demand “proven technology” for the wind turbines to be used in (future) offshore wind farms. Probably they are willing to accept the current largest land based machines (2.5 MW) as “proven technology” at the time the offshore projects of the size of a power plant (typically 500 MW) will be built. But it is very questionable that they will be willing to invest and/or insure a GigaEuro project incorporating new 5 MW dedicated offshore wind turbines. In the figure below this dilemma is visualised: 5 THE INTEGRAL DESIGN APPROACH FOR FUTURE OFFSHORE WIND POWER PLANTS Only when treated as one entire system a 500MW scale offshore wind power plant will provide a considerable amount of electric power produced in a reliable and costefficient way over its projected lifetime. Therefore a number of objectives for an optimum wind farm design can be stated: • optimum distribution of investment and operation and maintenance (O&M) costs over the entire windfarm and over its lifetime • design optimisation of sub-systems with respect to ‘global’ goals, e.g. minimum energy price, windfarm availability, overall structural dynamics, etc • sufficiently high reliability of the system as a whole and of essential sub-systems e.g. grid connection system, wind turbines, etc • adaptation to economy-of-scale: solutions developed for plants comprising about 100 units or more • a true symbioses of present experience from the wind energy with the offshore technology High investment costs for the fixed cost elements, i.e. support structures and grid connection, favour large, multimegawatt converter units and large windfarms. The cost breakdown between the major subsystems (wind turbine, support structure and grid connection) is nearly equally shared. Above that the operation and maintenance costs are an equally important part of the break down of the cost of energy of the electricity produced in an offshore wind power plant. Thus optimisation has to consider all of these elements simultaneously and contradictory goals have to be balanced with respect to production costs and revenue over the entire life. (onshore) Increased farm size 500 MW 50 MW grat es ed d ign road Tuno & Vindeby (DK inshore) 100 X 5 MW ts cos rt & effo ity &M il te O availab ergyy a r e n e mod high o st of risk est c her low ut hig b Inte 500 MW wind farm 2000 inshore nolo gy r oad O&M ive ens y exp it us & vailabil ergy io r labo lower a t of en k s is er co /low r high derate mo Pro 4- 40 MW wind farms 1- 2 MW units ch n te 200 X 2.5 MW ve Increased reliability 2010 offshore Figure 3: Development lines for a 500 MW offshore wind power plant Evidently the “proven technology” road incorporates less risk than a road along the development of very large machines. On the other hand the development of very large dedicated offshore wind turbines, i.e. wind turbines that have been designed specifically for operation and maintenance in the harsh marine environment, will eventually result into the lowest cost of energy of a 500 MW scale offshore wind power plant. However this can only be established if the development of such large machines takes place within the framework of an integral design approach of a large -scale offshore wind farm. So reliability as such is not a goal, it is a means to achieve a certain availability level necessary to comply with the primary goals of the wind farm. Without a thorough analysis it is difficult to determine the availability level that can be reached for a future offshore wind farm. The reason is that achieved availability just as strongly depends upon the reliability of the system as upon the maintenance strategy that is applied for the system. Key factors in such an analysis are: • the accessibility of the site, which is on its term determined by the weather conditions (wave height, wind speed and visibility) • the availability of lifting equipment • the ease of maintenance • the required maintenance and service level of the machines and its components Reliability and O&M strategy together determine the wind farm availability ands thus its energy yield. Whenever an optimal trade-off is achieved between reliability and O&M strategy for one site this needs certainly not to be optimal for another case. One of the complicating factors is that harsher weather conditions will simultaneously lead to decreased availability and to increased gross energy yield for a given level of reliability under a given O&M strategy. Therefore the operation and maintenance (O&M) aspects of the complete offshore windfarm has to be analysed for each specific site in a comprehensive way in order to determine the optimal reliability level of the wind turbine 6 IMPLEMETATION OF THE INTEGRAL DESIGN APPROACH What can be observed, when following the current developments in the wind turbine industry, is that the experience so far with offshore wind turbines indeed has lead to the awareness of the importance of reliability and maintainability of the design future offshore wind turbines. But as far as the merge with the knowledge and experience of the offshore technology is concerned there is still a long way to go. This can be seen in e.g. a separate design of the wind turbine and the supports structure, with only a fairly straightforward interface in the sense of common design specifications without regular feedback in the design process. Another example is the move of most manufacturers towards more complicated wind turbines: from constant speed stall controlled towards variable speed (individually) pitch controlled wind turbines with doubly fed generators and power electronics. Arguments in favour of such move can be found in terms of better controllability of the power, the reduction of dynamic loads and an increase in production. But for offshore applications robustness might be a better guideline in the design. An increase of a few percent in energy production onshore through such modifications might in an offshore situation lead to a significant reduction of availability (and thus production) due to the increased maintenance demand and the inability to perform repair actions in harsh weather conditions. Furthermore offshore O&M activities are far more expensive than equivalent onshore maintenance, thus influencing the cost of energy in a negative way. The results from a DOWEC concepts study (ref. [4]) show that indeed an advanced wind turbine design is not always an economic design for offshore application. In that case a robust, yet cheap design (two bladed stall controlled constant speed) turned out to give the highest yearly yield for remote offshore locations. A third example is the observation that many manufactures still approach the issue of more severe repair activities in an onshore fashion. Although internal cranes are added in the offshore designs, to enable the remove of heavier components from the nacelle, the handling of such components outside the nacelle is still a problem. Onshore the assistance of a hoisted component is usually not a problem, under certain restrictions with respect to maximum wind speed. But a similar procedure offshore is not possible, and the receiving ship or barge is moving as well. A true offshore solution needs to be found, maybe in the direction of a wind turbine design that enables an easy integral exchange operation using a stable, wave independent jack-up barge. 6 CONCLUSIONS Future offshore wind farms will be significantly larger than exiting and currently developed projects. With respect to the installed capacity the wind farms will become comparable to conventional power plants. Hence one can speak of the development of future offshore wind power plants. The reliability of present machines is sufficient for land based wind turbines, where access is in general not a problem. Offshore wind turbines however need to be designed with significant higher reliability specs. Furthermore the regular maintenance demand needs to be reduced from twice a year to at most once a year. This enables the planning of regular maintenance in the summer season. The use of proven technology is often heard as a requirement for the technology to be used offshore. Referring to the current maintenance demand however this is not a very clever approach. It will lead to offshore wind farms with high O&M expenses, and is hence not very economical. The current trend to develop very large machines for offshore applications is usually driven by the awareness that it needs to be done to obtain the most profitable investment cost for future offshore wind power plants. Within this process the urgency to focus as much on reliable and robust design is not always seen as of equal importance. 6 REFERENCES [1] W. Eggersglüß; Wind Energie IX-XI, PraxisErgebnisse 1995-2000, Landwirtschaftskammer Schleswig-Holstein, Osterrönfeld, Germany. [2] MWEP Jahresauswertung 1996-2000, ISET, Kassel, Germany, ISBN 3-9805896- 6-8/5-X/n-n. [3] WindStats Newsletters 1998 –2001, Vorlaget Vistoft, Denmark, ISSN 0903-5648. [4] G.J.W. van Bussel; The Development of an Expert System for the determination of Availability and O&M Costs for Offshore Wind Farms, Proceedings of the 1999 European Wind Energy Conference, Nice, France, March 1999, pp. 402-405 [5] G.J.W. van Bussel; Chr. Schöntag, Operation and Maintenance Aspects of Large Offshore Wind farms, Proceedings of the 1997 European Wind Energy Conference, Dublin, Ireland, October 1997, pp. 272279. [6] G.J.W. van Bussel, M.B. Zaaijer; DOWECconcepts study, reliability, availability and maintenance aspects, Proceedings of the 2001 European Wind Energy Conference, Copenhagen, Denmark, June 2001, pp. 557-560.