main author:

Andrew R. Henderson, Delft University of Technology, The Netherlands; A.Henderson@CiTG.TUDelft.nl, please link to www.offshorewindenergy.org

acknowledgements to:

Colin Morgan, Garrad Hassan & Partners Ltd, United Kingdom,

Bernie Smith, John Brown Hydrocarbons Ltd, United Kingdom,

Hans C. Sørensen, Energi & Miljoe Undersoegelser (EMU), Denmark,

Rebecca Barthelmie, Risø National Laboratory, Denmark,

Bart Boesmans, Tractebel Energy Engineering, Belgium,

Abstract

After several decades of theoretical developments, desk studies, experimental wind turbines and prototype windfarms, the first large-scale commercial developments of offshore windfarms are now being built. To support this, a number of partners from a wide range of fields have produced a state-of-the-art review of the current situation, with this article drawing heavily on the information gathered. The complete report is also available online at http://www.offshorewindenergy.org

Introduction

Offshore wind farms promise to become an important source of energy in the near future: it is expected that by the end of this decade, wind parks with a total capacity of thousands of megawatts will be installed in shallow seas across the world. It is likely that the initial developments will continue to be in European but both North America and Japan are also now showing interest and developing plans for very large scale offshore windfarms. These developments will be equivalent to several large traditional coal-fired power stations. Plans are currently advancing for such large-scale wind parks in Swedish, Danish, German, Dutch, Belgian, British, Irish as well as US and Canadian waters and the first such parks are currently being constructed at Horns Rev, off Denmark's western coast and Rødsand, in the Danish part of the Baltic coast.

Onshore wind energy has grown enormously over the last decade to the point where it generates more than 10% of all electricity in certain regions (such as Denmark, Schleswig-Holstein in Germany and Gotland in Sweden). However, this expansion has not been without problems and the resistance to windfarm developments experienced in Britain since the mid 1990s, is now present in other countries to a lesser or greater extent. One solution, of avoiding land-use disputes and to reduce the noise and visual impacts, is to move the developments offshore, which also has a number of other advantages:

• availability of large continuous areas, suitable for major projects,
• higher wind speeds, which generally increase with distance from the shore (Britain is an exception to this as the speed-up factor over hills means that the best wind resources are where the turbines are also most visible),
• less turbulence, which allows the turbines to harvest the energy more effectively and reduces the fatigue loads on the turbine,
• lower wind-shear (i.e. the boundary layer of slower moving wind close to the surface is thinner), thus allowing the use of shorter towers.

• the more expensive marine foundations,
• the more expensive integration in to the electrical network and in some cases a necessary increase in the capacity of weak coastal grids,
• the more expensive installation procedures and restricted access during construction due to weather conditions,
• limited access for O & M during operation which results in an additional penalty of reduced turbine availability and hence reduced output.

The total wind power resources available offshore are vast and will certainly be able to supply a significant proportion of our electricity needs in an economic manner. Earlier studies by Garrad Hassan / Germanischer Lloyd [11] and EWEA / Forum for the Future [10] concluded that a large proportion of Europe's power could be supplied from offshore wind turbines.

A BRIEF HISTORY

The first wind turbine to generate electricity was a traditional wooden windmill converted by Poul la Cour in Denmark over 100 years ago. In the early part of the 20th century, there were further experimental machines but serious developments only began with the two oil shocks in the 1970s, when governments around the world reacted by directing R&D money to alternative fuel sources. The early '80s saw major developments in California and the construction of the famous fields of hundreds of small turbines and by the end of that decade there were 15,000 turbines with a total generating capacity of 1,500 MW in that state [1]. The stabilising of the oil price in the '80s and resulting reductions in the state subsidies for windpower, meant that purchases from the crucial American market dried up and many wind turbine companies withdrew from the field or went bankrupt. An exception was in Denmark, where government support meant that the knowledge base was not dissolved and the companies there were able to quickly respond when wind energy's fortunes revived once more in the early '90s, to the point where they and their partners continue to have a strong position in the market today. It should be pointed out that the foundations of renewable energy's fortunes are today based on the solid necessity of alleviating climate change and increased energy autonomy rather than the fickle nature of oil prices.

Currently there is a total installed capacity of approximately 24 GW on land and over the last few years the annual installation rate has passed 4GW/annum. The average rating of turbines being installed is now approaching 1MW per unit internationally, Figure 3. With the resulting economies of scale, wind energy now competes on price with the traditional generators, such as coal and nuclear, in areas of rich wind resources.
 
 

OFFSHORE WIND ENERGY TODAY

Although the challenges of building large offshore windfarms will be considerable, many of the problems relating to the turbine will have previously been faced on-land, and relating to the support structure, by the offshore and coastal engineering industries. The key point will be to know how to integrate these two technologies, as it is all too easy for each branch of engineering to underestimate the complexities in the other. In fact, the combination is not always equal to the sum of the parts, both in a beneficial and a detrimental sense, hence a cost-saving opportunities may be missed and unexpected problems may be encountered during construction and operation. Avoiding unnecessary costs is especially important now when offshore wind energy has the opportunity of also becoming competitive on price with traditional energy sources.
 
 

Offshore Wind Energy Technology

The wind turbines being used in current offshore projects tend to be machines designed for land-use but with modifications, such as a larger generator, a higher instrumentation specification and component redundancy, particularly of electrical systems, see Table I. If the market expands as expected, machines designed for optimised performance offshore will be developed and utilised but it is not certain how they will look. On one hand, the requirements from an offshore machine differ from those on land, however the requirement for high reliability would suggest the use of well-proven turbines. Modifications may include:

• larger machines, up to 5 MW or 10 MW,
• faster rotational speeds than on land, where noise restrictions generally mean that the turbine operates slightly below optimum speed,
• larger generators for a specific rotor size, to enable the additionally available energy to be efficiently harvested,
• high voltage generation, also possible in DC instead of AC,
• in the longer term, downwind machines with flexible blades or multiple rotors might become an option, but engineering effort will be needed to achieve the theoretical potential.

A selection of the main findings and conclusions from the main report with regard to offshore wind turbine technology are presented below with a focus on identifying expected technology trends.

Wind turbine size: Rotor diameter and power rating for offshore applications is continuously increasing, the relationship is shown in Figure 4. The largest commercial turbines currently available have a diameter range of 65 - 80 m and rated power output of 1.5 - 2.5 MW. Prototypes are under development with respective values of up to 120 m and 5 MW. It appears that the largest current machines (offered especially for offshore markets) exploit significantly higher tip speeds than onshore machines. An increase of between 10 % and 35 % is typical, resulting in tip speeds of up to 80 m/s, as detailed in Table I. Increased tip speeds result in lower torque, less mass and thus reduced costs of tower top systems.

Costs: Under conditions of true similarity in design style, state of technological progress and design specification, costs of large turbines might be expected to scale cubically with rotor diameter. Considering however historical data over the range of machine sizes, ongoing technology development results in a scaling closer to a square law than a cubic law. Price data of onshore machines show a gently rising cost/kW for rotor diameters of 40 m and greater, Figure 5, although this could partly reflect the fact that smaller turbines were generally introduced to the market longer ago and with the development costs previously having been recouped, the selling-price can be reduced. Although marinisation of onshore design generally adds 10% in costs, the currently available specific offshore machines are now essentially on lower cost curves than onshore predecessors.

Blade technology: The demand for high strength blades of low solidity in conjunction with diminishing carbon fibre costs may drive the industry in the direction of carbon epoxy. Carbon prices are falling and if it were used in significant quantities in blades for offshore machines, that could become by far the largest market for high quality carbon fibres, thus resulting in further cost reduction.

Gearbox: It is not clear whether the current gearbox concept (three stage units, input stage planetary, the two higher speed stages parallel with helical gears) will be applicable for larger, offshore turbines, since it is likely that for larger machines, i.e. > 3MW, an additional gearbox stage will be required, resulting in increased complexity and probability of failure. This may be an important driver towards direct drive systems.

Variable speed: There is a tendency towards variable-speed designs. Wide range variable speed has a further advantage in the ability to avoid damaging resonances, important for offshore turbine structures, where the resonant frequencies have proved difficult to predict accurately, and may also change over the lifetime of the structure. It remains somewhat unclear whether power electronic converters can be made reliable enough at suitable cost.

Support structure: The current design philosophy for wind farms in water depths up to 20 m is based on the monopile, except in the shallowest waters (up to 5 m) where gravity base structures have been preferred, Figure 6. The installation methodology (driving, drilling or combination) will depend on soil properties and water depth. For deeper waters, tripod support structures are being considered but the optimum solution is not yet certain and may well be a concept currently being brought into the field by offshore and coastal engineering specialists. Floating support structures remain a challenge with regard to cost but this will need to be met if countries with less extensive regions of shallow water, such as Japan are also to exploit offshore wind energy to a significant extent [12].

Operation and Maintenance (O & M) of offshore wind farms is more difficult and expensive than for equivalent onshore wind farms. The current reliability and failure modes of commercial offshore wind turbines are such that a "no maintenance" strategy is not a viable option. Improved preventive and corrective maintenance schemes will become crucial for economic exploitation of offshore wind power. In particular improving the accessibility is a key factor in increasing availability [5]. A number of current projects are attempting to address the issue of improved access to offshore wind turbine installations: with most focusing on maintaining existing boat access methods with emphasis on addressing the issue of motion compensation or complete removal of the vessel from the water at the turbine location. Suggested improvements to the base of OWECS to facilitate safe personnel access include: fixed platforms, flexible gangways, friction posts against which the vessel maintains a forward thrust during transfer, vessel lifting facilities and winch / netting for personnel and equipment (eliminating the need for specialist lifting vessels for major component replacement).

The issue of availability should also be addressed through improvements in offshore wind turbine reliability. Unplanned maintenance levels can be reduced by increasing the reliability of the turbine. Particular emphasis is being placed on reliability issues from overall design improvements through to component level. Designing for reduced maintenance could drive wind turbine designs away from current onshore standards, such as towards two bladed configurations, direct drive technology or application of electrical actuators; however use of unproven technologies is intrinsically less reliable than proven systems.

Electrical systems: There are many areas where technical developments in electrical systems are expected which will improve the economics and reliability of offshore wind farms. Some of these will arrive because of developments in other industries and in onshore wind, but others are specific to offshore wind and are therefore more risky. Developments can be expected to take place within the wind turbine (such as generation at high voltage) and with in the wind farm electrical systems, regarding set-up of substations, use of HVDC technology and cable technology.
 
 

Grid Integration and Energy Supply

In the past, grid operators have had to deal with stochastic variation of the power demand only but wind and some other renewables introduce stochastic variation into the supply as well. Up to a fairly high penetration level into the national supply (in the order of 10%), the power can generally be absorbed without too much difficulty, as in principal, stochastic mismatch is the same problem, whichever side it originates from. Beyond that, non-dispatchable generators pay a price penalty, if additional conventional reserve power is required to ensure the balance between production and consumption. This can be reduced by using wind-forecasting to predict the output further in advance or by matching with dispatchable but limited sources of power, such as hydro-electric. The coming decade is likely to see the expansion of several previously non-conventional electricity generation technologies, including micro-generation [9], hence grid operators will have an incentive and pressure from several generator-technologies to react positively to these challenges. In addition, integrating large offshore windfarms into the electricity grid may pose problems if the coastal network is weak and more advanced power control systems may be used to conform with grid connection requirements. This may provide opportunities for the application of new technologies for electricity storage combined with more reliable models for predicting fluctuations from wind energy over longer periods.

Cross-border power transmission limitations prevent a geographical smoothing of the production/consumption imbalance. Solutions to deal with large imbalances are: Demand Side Management, increased flexibility and dispatching capability of conventional plants, the use of energy storage, application of wind power forecasting techniques and increasing the controllability of wind farm output. It is concluded that, although all options could eventually contribute to the solution (requiring much more RTD), the most promising immediate step is to increase the accuracy and reliability of wind power forecasting techniques.

The impact of large-scale offshore wind power on power systems performance (power quality) requires special attention since coastal connection points will often be relatively weak. Flicker, harmonics and interharmonics and static stability are not considered as limiting factors but dynamic grid stability may be a limiting factor, in particular in relation to wind farm correlated sudden shut-down of wind turbines. These problems may lead to modifications in wind turbine control philosophies for the high wind speed cut-out.

Large scale offshore wind power will further impose an increase of primary control (response time of the order of seconds) and secondary control (response time of the order of minutes) requirements of the conventional production components of the system; such requirements could also be imposed on large wind farms, although it remains unclear how such requirements could be efficiently implemented.

The connection technology between offshore wind farms and the grid is characterised by large power (> 100 MW) and potential long distances. HVDC links offer a possible lower-cost solution, which could also contribute to alleviate the power quality management problems mentioned above.

Access of large offshore wind farms to the grid must be in accordance with national grid codes. Current requirements imposed by national grid codes are in general not considered to be a limiting factor for the development of large-scale offshore wind power, although these requirements are not particularly suitable for non-predictable, highly variable energy sources. Project developers may have to take additional measures to comply with the grid codes, such as: use of variable speed wind turbines, special purpose remote control systems (with individual power set points for the wind turbines, etc). In the long term, HVDC transmission and/or on-site large storage facilities with controllable reactive power output, might present interesting opportunities allowing large scale offshore wind power plants to meet grid access requirements more easily to the point of possibly having a positive impact on grid stability.
 
 

Economics & Financing

Turning to the market developments in the energy industry, relevant for the development of offshore wind power, in a number of EU countries (such as Belgium, Denmark) minimum shares of renewable energy are required, either for utilities to sell, or consumers to buy. In other countries (Ireland, The Netherlands) green certificate markets have been established, see Table IV. Both systems are expected to support the demand for renewable energy in general and experience has to show which system has the strongest impact on RES development.

Economics: Offshore projects require initially higher investments than onshore due to turbine support structures and grid connection. The cost of grid connection to the shore is typically around 25% a much higher fraction than for connection of onshore projects. Other sources of additional cost include foundations (up to 30%), operation and maintenance (with expected lower availability) and marinisation of turbines, Figure 8. Investment costs have been reduced from about 2200 €/kW for the first Danish offshore wind farms to an estimated cost of 1650 €/kW for Horns Rev (equivalent to 4.9 €c/kWh). This compares with typical figures for onshore sites of investment 700-1000 €/kW and estimated energy cost of 3-8 €c/kWh for a mean wind speed of 5-10 m/s.

Projected costs are downwards as the industry determines less expensive methods for installation and maintenance using experience gained in the offshore industry and at the first offshore wind farms and contractors gain sufficient confidence to tender at lower levels, with larger project and turbine sizes also reducing costs per installed MW. Operation and maintenance charges are variable according to site but a rough estimate is 30 €/kW with 0.5 €c/kWh variable. A tentative conclusion is drawn that for good sites (not too deep water, benign wave climate, not too distant from shore, high enough windspeeds) large offshore wind farms could in the near future generate electricity at costs, which allow for true commercial exploitation.

Whether offshore wind power could be commercially viable depends on whether sufficient project income can be generated. This depends on whether the energy produced can be sold on the (then likely to be) fully liberalised market at a reasonable rate and how the environmental benefit is valued. There are a number of factors (such as use of forecasting techniques), which are of influence on energy sales in a liberalised market and it can be concluded that severe risks exist associated with market liberalisation where the environmental benefits are not adequately valued. This may jeopardise development at some sites. Despite the average cost of offshore wind energy being competitive with many traditional energy sources, projects may not be viable. This may leave Europe in the curious position of possessing an abundant environmentally friendly energy resource whose exploitation enjoys a high degree of public and governmental support but without the market framework, which can support its development.

Financing: From the current developments of demonstration offshore projects of various sizes, it would appear that sufficient equity capital is available for financing offshore wind farm projects. Some major oil & gas companies and utilities have announced projects, which could be financed by company equity. However it still remains to be determined under which conditions (due diligence, certification, insurance etc) bank loans will be granted for offshore wind farm projects. Only test and demonstration projects will provide information to allow an answer to this question. At least they will reduce the present uncertainties related to the cost of energy generated. Important support comes from a variety of national incentive mechanisms, such as investment subsidies, tax exemptions, fixed tariffs and green certificate schemes.
 
 

Resources

The resources available across the offshore regions of the world are vast, particularly in far northern and southern latitudes, and are in theory capable of supplying all electrical needs of the whole of the European continent for example (though currently at an uneconomical price). In practice, offshore wind energy could become a major source of energy for several countries at a competitive price in the medium term.

So far, all offshore windfarms have been built in seas off north European coasts, where there are large flat and shallow regions a short distance away from the coast and hence suitable for development. Other regions, such as the east and west North American coasts also have such regions but the continental shelf around much of the Mediterranean Sea and Japanese coasts, for example, falls off much faster leaving little space for bottom-mounted windfarm developments and hence limited prospects for offshore windenergy unless floating wind energy can overcome its current cost disadvantage [12].

Wind resource studies for offshore regions are based on monitoring data and modelling techniques. The issue of offshore wind resources is complicated by a number of factors. Low roughness gives low turbulence and wind shear but thermal effects are important, particularly in coastal regions: wind speed profiles deviate from logarithmic and thermal flows are generated, such as sea breezes and low level jets. The report discusses both offshore wind monitoring and state of the art modelling techniques and a major conclusion is that while current-modelling techniques can provide good representation of general resources, specific site resource estimation still requires on-site measurements.

The offshore wind potential is derived from the wind resource in combination with a number of local constraints, such as technology limits (such as water depth), economy, ecology and conflicts of interest with other users. The resulting wind potential is thus a function of constraints considered, the assumptions applied and the level of detail. Available studies of the offshore wind power potential in the EU were reviewed, though unfortunately, most studies have been performed on a national basis and a specific set of assumptions and hence can not easily be combined for the EU total. Notwithstanding this difficulty, the overall resource is estimated to be 140 GW, which is well in excess of the EU White paper target of 10 GW in 2010.

In the last decade of the 20^th century 80 MW of offshore wind power was installed in Europe. These wind farms have operated successfully and have proved that offshore wind energy is technically, economically and environmentally viable. Continued monitoring and detailed investigation of these wind farms will provide invaluable data for use in better evaluating and harnessing the offshore wind resource and for meeting the challenges of installing large wind farms.

The next generation of wind farms in the 100 MW range consisting of multi-megawatt turbines provide new challenges. Hub-heights are now beyond the height that measuring masts can be installed economically, wakes within such large farms are not well understood and the influence of upwind farms requires further research. Surprisingly, wind-turbine technology will be less -proven than was the case for the first offshore demonstration projects. Larger distances to the coast and deeper water give harsher conditions for the turbines and supporting structures. Access for maintenance is more difficult, combined with the demand for better availability. However, the physical and environmental challenges are within the grasp of the offshore and wind energy industries. A greater challenge is posed by market uncertainty, which has not been detailed in this report.

Activities and Prospects

To date, eight small and medium sized offshore windfarms have been built (the first offshore plant consisted of a single wind turbine and was abandoned after a fire) and the main details are summarised in Table II and Figure 10. The first and largest offshore windfarms are at Vindeby (1993) and Middelgrunden (40 MW) respectively, both being located in Denmark. This year, the first truly large-scale windfarm is being built at Horns Rev off the west coast of Denmark, a windfarm consisting of 80 turbines, each rated at 2 MW. This 160 MW capacity is likely to be surpassed in the next handful of years by many other projects across Northern Europe and possible elsewhere. As can be seen in Figure 10, all realised projects to-date have been within Europe, however there has been a recent resurgence of research and resource prediction activities in both Japan [18], and the United States [19].
 
 

Electricity production has generally exceeded expectations and costs have steadily fallen to the point where offshore wind energy is competitive on price with many of the current onshore developments. Two years ago (2000), three windfarms were built, all using MW sized turbines for the first time in the offshore environment, and one, at Blyth, which is in a location facing one of the most hostile seas in Europe and is being accompanied by an extensive measurement programme. Last year (2001) a single windfarm was built in Sweden of 10MW capacity however the next years are likely to see a significant increase in both the size and the number of the offshore windfarms being built, a list is given in Table III and shown in Figure 12 and many countries have set ambitious targets, Table IV.
 
 

• In January 2002 the Danish Government announced a change in policy regarding the offshore wind farms at Gedser, Læsø Syd and Omø Stålgrunde. It is not clear at the time of writing to which extent the construction of these wind farms will be delayed.

Figure 12: Locations of Some of the Planned Windfarms

In Japan, studies have shown that the potential resource is significant however much of this is in waters too deep for the technologies being developed for the European projects [18]. The United States and Canada should also see several offshore wind energy developments within this decade, with the Massachusetts coast being a likely location [19].
 
 

However, if it were not for the decade-long research effort, it is unlikely that the development of these projects would be possible. The last few years has seen a number of intensive projects, including a selection of recent and ongoing research and demonstration projects listed in Table V. Further details are available from the CORDIS database at http://www.cordis.lu. Offshore wind energy has received prominent coverage at recent national and international conferences, such as the European Wind Energy (www.ewea.org) and the OWEMES [Offshore Wind Energy in the Mediterranean and Other European Seas; Gaetano.Gaudiosi@casaccia.enea.it] conferences and other important activities include an increasing number of electronic and traditional press journals specifically focused on wind or giving wind energy a prominent position and MSc courses in wind or renewable energy at several universities across the whole world.
 
 

Social Acceptance, Environmental Impact & Politics

Experiences from current offshore projects indicate that social acceptance is closely connected to environmental impacts. Public concern is specially related to the impacts on birds and the visual impact and although the impacts will change somewhat, when moving further offshore, it is crucial that aspects like bird migration paths and the visual impact of offshore wind turbines in an otherwise structureless landscape are taken seriously already from the planning phase. The Middelgrunden windfarm makes an interesting example: the initially proposed "efficient" rectangular layout was replaced by a gracefully curve following consultations, this has helped the windfarm to become a tourist attraction in Copenhagen and avoid excessive criticism in spite of the prominent visibility from many locations in the city, including the royal palace.

Furthermore, potential effects on fish, marine mammals, fauna and benthos need to be investigated, as the impacts from large scale offshore wind farms (e.g. sound emissions and electromagnetic fields from underwater cables) are currently relatively unknown. The effect from wind turbines on radar systems is also an important issue that is currently poorly understood, and must be dealt with both in generic studies and in site-specific pre-investigations (Environmental Impact Assessments). These investigations and the planning process prior to large scale offshore projects should be as open as possible and allow local involvement.

The role that public opinion plays is essential and has been an important difference between the countries where onshore wind energy has become widespread and where it has not become successful is in public support. Care should be taken to ensure that public opinion continues to support the advancement of offshore wind energy.

The objective of this part of the project was to consider:

• environmental impacts
• social acceptance
• conflicts of interest
• national planning rules throughout the EU

• human beings, fauna and flora,
• soil, water, air, climate and the landscape,
• material assets and the cultural heritage,
• the interaction between these factors mentioned.

• Collision of birds with turbines
• Ousting birds off their traditional feeding/roosting grounds
• Unknown effect of low frequency noise emissions on fish life and sea mammals
• Impacts on fish larvae
• Disturbances of seabed and fauna during construction and operation.

The effect on seals has also been examined, at both Tunø Knob and Bockstigen-Valor [17] and no impact found; hopefully a similar study at the Rødsand windfarm will concur with this. A potentially more serious problem is that fish have been known to lose consciousness from the pile-driving shock-waves; the effect is temporary and the fish do recover, however sensitive breeding periods should be avoided and the sight of stunned fish could lead to public relation problems. During operation, the noise from wind turbines should not be audible above ambient underwater noise beyond about 20m [15] / [16].

Public attitudes are in general positive but may turn negative with actual projects. This is based on two different issues:

• the perceived potential of ecological damage, in particular in relation to birds
• the perceived visual and noise impact, in particular in relation to the recreational use and value of the adjacent coast.

The main other conflicts of interest in developing offshore wind farms are with radar systems and marine traffic. Careful planning should resolve this conflict, as especially the potential effects on radar systems may become a barrier for future development of offshore wind energy projects. Regarding marine traffic, improved and suitable ship collision risk and damage consequence models should become available.

Since in most countries the political attitude towards offshore wind power is positive, national planning and regulation rules are being adapted for licensing offshore wind farms, both in and outside the 12 mi zones.

The direct employment effects of offshore wind power are estimated as 4-5 full time jobs/MW [3] with there being an estimated 9,000 and 30,000 people employed in Denmark [7] and Germany [8] respectively.

About the Project: Concerted Action on Offshore Wind Energy

The objectives of the project "Concerted Action on Offshore Wind Energy in Europe" [CA-OWEE] were to define the current state of the art of offshore wind energy in through gathering and evaluation of information from across many countries and to disseminate the resulting knowledge to all interested parties, in order to help stimulate the development of the industry. The project was funded by the European Commission and was completed at the end of last year (2001). The knowledge gathered is freely available through an internet site, http://www.offshorewindenergy.org and a printed report.

The project focused on the large scale exploitation of the offshore wind resource through the use of very large wind turbines with improved performance, reliability and reduced environmental impacts. The objective of the project was to collect the accumulated knowledge about offshore wind energy from all sources, evaluate and summarise this knowledge and distribute it to all who can benefit. In addition, a report has been produced for the Commission with recommendations for which actions are needed to ensure that the development of offshore wind energy continues to expand and becomes a major source of power in the coming century.

This paper summarises the conclusions of this project, covering the main technologies, activities, issues, challenges and recommendations for RTD support relating to the development of offshore wind energy thus giving a summary of the state of the art of this rapidly expanding field of engineering.

This project divided offshore wind energy into five clusters of subjects, reviewed the recent history and summarised the current state of affairs, relating to:

• offshore technology, of the wind turbines and the support structures,
• grid integration, energy supply and financing,
• resources and economics,
• activities and prospects,
• social acceptance, environmental impact & politics.

• Delft University of Technology, The Netherlands
• Garrad Hassan & Partners, United Kingdom
• John Brown Hydrocarbons Ltd. (formerly Kvaerner Oil & Gas), United Kingdom
• Energi & Miljoe Undersoegelser (EMU), Denmark
• Risø National Laboratory, Denmark
• Tractebel Energy Engineering, Belgium
• CIEMAT, Spain
• CRES, Greece
• Deutsches Windenergie-Institut (DEWI), Germany
• Germanischer Lloyd, Germany
• ECN, The Netherlands
• Espace Eolien Developpement (EED), France
• ENEA, Italy
• University College Cork, Ireland
• Vindkompaniet i Hemse AB, Sweden
• VTT, Finland
• Baltic Energy Conservation Agency (BAPE), Poland

The authors would like to acknowledge the contributions made by all partners in the CA-OWEE project to this paper.

The project Concerted Action on Offshore Wind Energy in Europe [CA-OWEE] was being funded by the European Commission under contract number NNE5-1999-00562.
 
 

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