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Energy 41 (2012) 298e312

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Review

The current state of offshore wind energy technology development

Xiaojing Sun a,b, Diangui Huang a,b,*, Guoqing Wu c

a Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, Chinab Shanghai Key Laboratory of Mechanics in Energy and Environment Engineering, Shanghai 200072, Chinac School of Mechanical Engineering, Nantong University, Nantong 226019, China

a r t i c l e i n f o

Article history:Received 30 September 2011Received in revised form13 February 2012Accepted 24 February 2012Available online 30 March 2012

Keywords:OverviewOffshore wind energyCurrent status of developmentRecent technological progress

* Corresponding author. Shanghai Institute ofMechanics, Shanghai University, Shanghai 200072, Ch

E-mail addresses: dghuang@staff.shu.edu.cn, dghu

0360-5442/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.energy.2012.02.054

a b s t r a c t

Wind power has been the fastest growing form of renewable energy for the last few years. According toIntergovernmental Panel on Climate Change (IPCC) report, 80% of the world’s energy supply could comefrom renewable sources by 2050 and wind energy will play a major role in electricity generation in 2050.In the growing market for wind energy and the limited available space onshore, the development ofoffshore wind farms become more and more important. With a rapid development of technology, theoffshore wind power projects have become a trend in many countries like Europe now. Therefore, thispaper aims to provide a brief overview of the current development status of offshore wind power indifferent countries and also explore the technical, economic and environmental issues around itsdevelopment. Without doubt, offshore wind will lead technology advances in the wind sector in a nearfuture as it seeks to exploit resources further offshore.

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1. Introduction

Excessive use of fossil fuels has caused climate changewhich hasbecome obvious in the last few decades and threatened humansecurity and development. Nowadays, serious energy crisis andenvironmental pollution have forced people and governmentsthroughout the world to look for sustainable alternative sources ofenergy. As a result, wind power as a type of abundant, clean,renewable energy sources has received considerable attentionworldwide and its development is growing at an unprecedentedrate in recent years. In fact, global wind power installations havereached 194390 MWat the end of 2010, 2% of global energy supplyand according to the Global Wind Energy Council (GWEC) report,global wind energy capacity will grow by 160% over the next fiveyears, resulting in the accumulated capacity reaching 409 GW in2014 [1].

The onshore wind farm development is usually restricted byland availability. Problems such as wind turbine noise and theirvisual impact on the natural environment are the main reasons forpeople to refuse to accept the building of onshore wind turbinesclose to residential areas. In contrast, although offshore windturbines operate in the same manner as onshore wind turbines,installation at sea has a number of advantages: there is a lot more

Applied Mathematics andina. Tel.: þ86 21 56333460.ang@shu.edu.cn (D. Huang).

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available space and fewer complaints about noise and visualintrusion. Besides, wind over the water is generally stronger, moreconsistent and much smoother than wind over land. The coastalregions are usually the most economically developed with highelectricity demand, thus the exploitation of offshore wind energycannot only help ease pressure on power supply in those areas butalso help reduce greenhouse gas emissions. Therefore, offshorewind power becomes one of today’s fastest growing energy tech-nologies and is going to be the future focus of development in manycountries around the world [2e4]. However, compared to onshorewind farms, offshore wind turbines are more expensive and diffi-cult to install and maintain due to the variable and rough seaconditions. At present, the major barrier to the deployment ofoffshorewind energy on amassive scale is the high costs of offshorewind facilities. Nevertheless, significant cost reduction in theoffshore wind sector could be achieved by using future advancedtechnology to optimize every stage of development, manufacture,installation and operation [5].

2. Current status of offshore wind energy development

2.1. European countries

Since the first offshore wind turbine was installed in 1990, inSweden, offshore wind energy development in Europe has expe-rienced three stages: initial research stage in 1980e1990; anexperimental testing stage in 1991e2000; and commercializationstage since 2001. After more than 30 years of development, Europe

Fig. 1. Installed offshore wind power capacity in Europe at the end of 2010 (from [7]).

Table 2The latest top 8 largest operational offshore wind farms in the world (from [9]).

Wind farm Capacity(MW)

Country Number ofturbines

Commissioned

Thanet 300 UK 100 2010Horns Rev II 209 Denmark 91 2009Rødsand II 207 Denmark 90 2010Lynn and Inner Dowsing 194 UK 54 2008Walney 1 184 UK 51 2011Robin Rigg (Solway Firth) 180 UK 60 2010Gunfleet Sands 172 UK 48 2010Nysted (Rødsand I) 166 Denmark 72 2003

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is well placed to lead the world in the offshore wind power tech-nology in nowadays and has become a front-runner in thecommercialization of these technologies. As of 30 June 2011, 1247offshore wind turbines have been fully grid connected in Europewith a total capacity of 3, 294 MW in 49 wind farms spread over 9European countries, in which the United Kingdom (UK), Denmark,Netherlands and Belgium are the top countries in offshore installedcapacity [6]. The installed offshore wind power capacity in those 9European countries at the end of 2010 is presented in Fig. 1 [7]. Thework at offshore wind farms in Europe for the half year period 1January to 30 June 2011 is summarized in Table 1 [6].

Today, European offshore wind farms show a trend towardlarger wind farms in deeper water at greater distances from theshore [8] and 22 out of 25 largest operational offshore wind farmsin the world are all located in Europe, in which the Thanet offshorewind farm operating in the UK is presently the largest offshorewind farm in the world at 300 MW [9], as shown in Table 2. Inaddition, they also have 9 out of 10 world’s largest offshore windfarms under construction in European waters now. Thus, if thestrong expansion of offshore wind farms continues, it is expectedthat between 40 GW and 55 GW of offshore wind farms will beintegrated into the grid and provide between 145 and 198 TWh ofelectricity to Europe [8] by 2020 which can meet 10% of Europe’selectricity demand. This figure could be boosted to 17% in 2030,according to the European Wind Energy Association (EWEA). SomeEuropean countries such as Germany and the UK with ambitiousoffshore wind plans have already become the world’s leadingoffshore wind market [10].

2.2. North American countries

Although wind power capacity in the United States (U.S.)currently accounts for more than 20% of the world’s installed windpower [11], its wind power utilization is largely been confined todevelopments on land and the United States is lagging far behindEurope in offshore wind energy production. As of 2010, there are

Table 1Summary of work at offshore wind farms between 1 January 2011 and 30 June2011(from [6]).

Belgium UK Germany Norway Total

Number of wind farms 1 7 2 1 11Number of foundations installed 4 108 16 1 129Number of turbines installed 0 101 6 1 108Number of turbines connected 0 68 32 1 101MW fully connected to the grid 0 244.8 103.3 0.015 348.1Total MW of projects (once

completed)148 2238 448.3 10 2844.3

still no offshore wind farms in U.S. waters despite the abundantresource off its coast. A wind energy resource map was drawn toassess potential wind farms sites in America and identified somelocations with high potential for offshore wind power such as theEast Coast of the United States and the Great Lakes region [12,13].However, as much of those potential sites are located in deeperwaters, the relatively high cost of offshore wind energy, the lack ofexperience and proprietary technology, and the complexity andresulting length of the permitting process are thus the main causesthat hinder the offshore wind farm development in America now[14]. At present, important offshore wind projects under consid-eration in America [15] include the Cape Wind project off the coastof Cape Cod, the Bluewater Wind project off the coast of Delaware,the LIPA Offshore wind park in Long Island and a project developedbyWind Energy Systems Technologies LLC in the Gulf of Mexico offthe coast of Texas, in which Cape Wind project has been approvedby U.S. government in April 2010 as the first offshore wind farm inAmerica and is due to start later in 2011. These planned offshorewind farms are important steps towards tapping this inexhaustibleand natural resource in America. In a major step forward, a nationaloffshore wind strategy [16] was published jointly by the U.S.Department of Energy and the U.S. Department of Interior onFebruary 7, 2011, in an attempt to stimulate offshore wind powerdevelopment. A target has been set to deploy 10 GW of offshorewind capacity by 2020 and increase this capacity to 54 GW by 2030.

As the largest country by total area in North America, Canada isalso about ready to dive into the offshore wind energy business.Four offshorewind farms are now in themost advanced stage of theplanning and regulatory process. Three of them will be located inthe Great Lakes, including Trillium Power Wind (414 MW), Wind-stream Energy (300 MW) and Toronto Hydro. The NaiKun project,which could have up to 110 turbines installed off the east coast ofHaida Gwaii, was thought to be the first offshore wind project to bedeveloped in Canada’s water but is hindered by a series of diffi-culties. This project is dropped for consideration by Provincialutility BC Hydro recently and also receives opposition from localresidents. Generating electricity from offshore wind turbines maynot be realized soon in Canada as all these proposed offshore windprojects generate environmental controversy and Ontario’s Liberalgovernment imposed a moratorium on all offshore wind farms inFebruary 2011 in order to conduct more research on the impacts ofoffshore wind farms on human health and environment [17].

2.3. China

The development of offshorewind power has a huge potential inChina as its offshore wind power resources at more than 750 GW ismuch higher than the 253 GW potential for land-based resources.According to a study conducted by China Wind Energy Association,Fujian, Guangzhou, Jiangsu, Hainan, Shandong and Zhejiang are theprovinces that have the greatest offshore wind energy generational

Fig. 2. Offshore wind resources in the major coastal provinces and cities of China (from[19]).

Table 3Some of the proposed offshore wind farms in China (from [19,20]).

Wind farm location Proposed installedcapacity (MW)

Guangdong Nan’ao 200Shanghai 600Zhejiang Daishan 200Zhejiang Linhai 200Jiangsu Rudong 150Jiangsu Dongtai 200Jiangsu Binhai 300Jiangsu Sheyang 300Jiangsu Dafeng 200

X. Sun et al. / Energy 41 (2012) 298e312300

potential (43%) in shallow waters below 30 m depth and there isalso 34% of the wind generational potential in depths beyond 50mfrom deep sea areas in China [18]. Fig. 2 shows the estimatedoffshore wind resources in some major coastal provinces and citiesin China.

In addition to focusing on the development of wind power onland, China now accelerates the pace of development andconstruction of offshore wind power and has made remarkableprogress. Located in the East China Sea, near Shanghai, 102 MWShanghai Donghai Bridge Wind Farm is the first offshore windpower demonstration project in China and Asia. The wind farmconsists of 34 3 MW wind turbines which are independentresearched and developed by China’s Sinovel Wind and expected tofuel more than 200,000 city households. Installation of all theturbines was completed in February 2010 and all the turbinesbegan transmitting power to the national grid in July of the sameyear. So far, the accumulated amount of electricity generated byDonghai Bridge wind farm has reached 200 million kilowatt-hours.Fig. 3 shows a photograph of Donghai Bridge offshore wind farm.Therefore, 2010 was a turning point for Chinese offshore windindustry, transiting from research and pilot project to commer-cialization operation. In the meantime, the exploitation of offshorewind power has also entered a phase of rapid development inChina. In 2011, another two offshore wind farms have beenapproved to be built in Shanghai, which are the second phase ofDonghai Bridge project and Lingang offshore wind farm close to the

Fig. 3. Donghai bridge offshore wind farm.

Donghai Bridge. These two offshore wind farms are expected to becompleted by 2015 and the city also expects that three to fiveinternationally competitive wind turbine equipment manufac-turers could be formed by then. In addition, the National EnergyAdministration (NEA) announced to kick off construction of 1 GWoffshore concession projects in the eastern Jiangsu Province,including four wind farms, in which two 300 MW offshore windfarms will be installed near shore and two 200 MW offshore windfarms will be built on tidal flats. The public tender for these fourwind farms has been completed on October 8, 2010.

Table 3 lists the proposed offshorewind energy projects going tobe built in China’s sea waters. According to the Chinese RenewableEnergy Industries Association (CREIA), China plans to expand itsoffshore wind power installed capacity to 5 GW by 2015 and 30 GWby 2020 in order to help the country meet its target to obtain 15% ofits energy mix from non-fossil energies by 2020.

3. Technical, economic and environmental aspects ofoffshore wind power

3.1. Economics of offshore wind

Although current offshore wind technologies build on onshorewind technology, it still remains relatively immature. Located inremote areas and operating under the harshest weather conditions,robust offshore technology is needed to ensure the safety, reliabilityand survivability of offshore wind power plants. At present,offshore wind farm is more expensive than onshore wind andlarger investments are required for offshore wind farms [21]. Asummary of investment costs, which greatly depend on the size ofwind farm and total power output, for some offshore wind farmsnow operating in Europe is given in Table 4. The relatively high costof electricity from the offshore wind farm due to large investmentsmakes offshore wind power less competitive with other sources in

Table 4Summary of investment costs for some offshore wind farms in Europe (Source: RisøDTU).

Offshore wind farm Number ofturbines

Turbinesize (MW)

Capacity(MW)

Investmentcosts (V million)

Middelgrunden (Demark) 20 2 40 47Horns Rev I (Demark) 80 2 160 272Samsø (Demark) 10 2.3 23 30North Hoyle (UK) 30 2 60 121Nysted (Demark) 72 2.3 165 248Scroby Sands (UK) 30 2 60 121Kentich Flats (UK) 30 3 90 159Barrows (UK) 30 3 90 e

Burbo Bank (UK) 24 3.6 90 181Lillgrunden (Sweden) 48 2.3 110 197Robin Rigg (UK) 60 3 180 492

Fig. 4. Composition of total system costs for an offshore wind farm in shallow water(from [22]).

Table 6The variation in investment costs of offshore wind power at different water depth(V/kW, from [23]).

Water depth (m)

10e20 20e30 30e40 40e50

Turbine 772 772 772 772Foundation 352 466 625 900Installation 465 465 605 605Grid connection 133 133 133 133Others 79 85 92 105Total cost 1800 1920 2227 2514

X. Sun et al. / Energy 41 (2012) 298e312 301

the energy market. For instance, it is estimated that the cost rangefor the electricity generated by existing offshore wind farmsmay bebetween 6 and 12 Vct/kWh compared with 3e8 Vct/kWh for theonshore sites [22]. Fig. 4 illustrates the composition of total systemcosts for an offshore wind farm in shallow water. Different fromonshore wind farm, much higher foundations, installation, elec-trical connections and operation and maintenance (O&M) coststake up a large proportion of the total cost of an offshore wind farm.Those costs will also be considerably increased as the futureoffshore wind farm will be far away of the coast and operate indeeper water. Increase in offshore investment costs as functionof water depth and distance to the coast are estimated in Tables 5and 6 respectively. It can be seen that the installation and gridconnection costs increase with distances rapidly due to operate inmuch hasher weather and require energy transmitting for the longdistances. On the other hand, foundation as well as its installationcosts can be strongly influenced by water depth. For instance, thefoundation cost for offshore wind turbines with capacity between 1and 1.5 MW can increase from V317,000 at 7 m depth to V352,000at 16 m depth, which has increased by 11% [23]. In addition, theO&M costs of an offshore wind farm whose main componentsinclude insurance, regular maintenance, repair, spare parts andadministration [24] are also substantially higher than onshoreprojects due to the cost of accessing offshore wind farm andmaintaining turbines in difficult weather. According to the latestWind Energy Update Operations and Maintenance report, O&Mcosts of offshore wind farms could reach as high as V100,000 toV300,000 per year, per turbine and this figure might be increasedby an additional 20% over the short-term once the innovative 5MWvariable-speed and direct-drive turbines are put into operation.

Although offshore wind today is expensive, a substantial costreduction can be expected over the long-term through economiesof scale, learning effects and R&D efforts. The experience curveconcept has been widely applied to predict the future trend ofoffshore wind energy costs, which expresses cost reduction asa function of increased cumulative installed capacity and has been

Table 5The variation in investments costs of offshore wind power at different distancesfrom the coast (V/kW, from [23]).

Distance to coast (km)

0e10 10e20 20e30 30e40 40e50 50e100 100e200 >200

Turbine 772 772 772 772 772 772 772 772Foundation 352 352 352 352 352 352 352 352Installation 465 476 476 500 511 607 816 964Grid

connection133 159 159 211 236 314 507 702

Others 79 81 81 84 85 87 88 89Total cost 1800 1839 1878 1918 1956 2131 2534 2878

introduced and discussed in detail in Ref. [25]. Economies of scalecan be achieved by the use of either larger capacity turbines oroffshore wind farms. Thus, when total installed offshore windpower doubles, it is estimated by using the experience curves thatthe costs per produced kWh can decrease by between 9 and 17%[26]. Further cost reduction in offshore wind also depends on R&Defforts including the development of both new techniques andmaterials. Key areas where research is urgently needed are inno-vative and efficient wind turbine design, offshore electricitytransmission, innovative offshore foundation concepts and instal-lation, and new O&M strategies. In addition, policy support is alsobelieved to be important for offshore wind cost reduction as thosepolicies will help achieve economies of scale and promote R&Ddevelopment [26e28]. According to the European Commission’sstrategic energy review of 2007, it is assumed that the capital costsof offshore wind will follow the same cost reduction trend foronshore wind in the future and might drop to 1274 V/kW in 2020and 1161 V/kW in 2050, as shown in Table 7.

3.2. Offshore wind turbine

3.2.1. The development of larger capacity wind turbinesWind turbine is the electricity generating component of an

offshore wind power plant and is installed on top of a supportstructure. Offshore wind turbines look similar to onshore althoughseveral modifications have to be designed specifically for theoffshore environment, such as corrosion protection, internalclimate control, high-grade exterior paint and built in servicecranes. The current offshore wind turbines in operation typicallyhave three-bladed horizontal axis, yaw-controlled, active blade-pitch-to-feather controlled, upwind rotors whose diameter canrange from 65 to 130m and capacity is nominally between 1.5 MWand 5MW [29]. The size of land-based wind turbine is often limitedby land space constraints on transportation of its components andsubstructure, for instance, the width and weight of roadways andbridges during delivery. In contrast, offshore turbine design canhave very large rotors in order to take full advantage of higheroffshore wind speeds and extract more total energy for a givenproject site area. On the other hand, the costs of non-turbineproject elements including the substructure, installation, O&M,and the grid and electrical infrastructure can considerably decreasewith the increase of offshore turbine size [29], as shown in Fig. 5.Hence, for these reasons, it has become an inevitable trend foroffshore wind farms to use larger capacity wind turbines.

Table 7Cost reduction prospects for onshore and offshore wind power (from [26]).

Wind power V/kW in 2020 V/kW in 2030 V/kW in 2040 V/kW in 2050

Onshore 826 788 770 762Offshore 1274 1206 1175 1161

Fig. 5. Possible reduction of non-turbine project elements with the increase of turbinesize in the offshore wind energy project (from [29]).

Fig. 6. 10 MW Aerogenerator X developed by UK firm Arup (from [34]).

Fig. 7. 10 MW vertical axis offshore wind turbine developed by Vertax (from [35]).

X. Sun et al. / Energy 41 (2012) 298e312302

Rated output from most of offshore wind turbines installed arebetween 2 MW and 3 MW now but major wind turbine manufac-tures have already started engaging in the research and develop-ment of even larger offshore wind turbines. 5 MW wind turbineshave been developed by BARD Engineering, Multibrid and REpowerwith rotor diameter of 116 m to 122 m. Wind blade manufacturerLM Wind Power and energy producer Alstom announced onFebruary 8 2011 that they will start to develop the longest windturbine blade ever produced, designed to fit Alstom’s new 6 MWwind turbine for European offshore wind market. Although thelength of the new blade has not been specified, it is reported thatthe blade made out of polyester and glass fiber can be relativelymuch lighter and also generate an additional annual energyproduction of 4e5% compared to standard blades. Sinovel, China’sbiggest wind turbine manufacture has successfully produceda 6 MW prototype with 128 m blades in diameter in June 2011 andother Chinese wind turbine companies including Huayi, Goldwindand Guodian United Power all have set out to develop 6 MWturbines [30].

Giant 10 MW offshore wind turbines are currently beingdesigned and developed by the UK, Norwegian and Americancompanies and also a Spanish group led by Gamesa announced inNovember 2011 it now working on a 15 MW offshore turbine for2020. According to the latest report from the European-fundedUpWind project [31], a 20 MW wind turbine is feasible and couldbe a solution for the expanding global offshore wind capacity,providing several times more electricity at lower costs than today’swind turbines. It is believed that the global energy market could betransformed by those large-capacity turbines within the next fewyears because of their economies of scale [32].

3.2.2. Vertical axis offshore wind turbinesAlthough horizontal axis wind turbine is the presently domi-

nant turbine type in the wind power industry, researchers find thatvertical axis offshore turbine may perform even better than theirhorizontal axis counterparts and be more cost-effective in manylocations like in deep water. In addition, the characteristics such assimple structure, rotation regardless of wind direction, relativelylow center of gravity and low maintenance costs make the verticalaxis wind turbines highly suitable for offshore installation. On theother hand, it is claimed that the size and power of vertical axiswind turbine might be able to get much bigger compared to

horizontal axis technology and thereby generate electricity at evenlower cost [33].

Nova research project, which was launched in 2009, has beenundertaken by a UK-based consortium of Wind Power Limited,OTM Consulting, Cranfield University, the University of Strathclyde,Sheffield University, James Ingram & Associates, CEFAS and QinetiQ,in order to examine the economic, environmental and technicalaspects of a 5 MW or 10 MW offshore vertical axis turbine calledAerogenerator X that mimics a spinning sycamore leaf as displayedin Fig. 6. This type of turbine is half the height of an equivalenthorizontal axis turbine and its center of gravity is concentrated atthe base of the structure [34]. It is expected that the AerogeneratorX can generate 20 MW or even more and might be turned intoa full-scale prototype by 2013. Another UK-based company Vertax[35] is also developed a 10 MWoffshore vertical axial wind turbinethat is presented in Fig. 7 and is now in the second phase of itsengineering development. In addition, French oil and gas engi-neering company Technip and wind power startup Nenuphar havelaunched the Vertiwind project recently for the purpose of testinga pre-industrial prototype of a floating vertical axis offshore windturbine which is characterized by no massive tower and nacelle,yaw or pitch system, gearbox and complex blade geometry [36]. Itis claimed that this concept design can float on the sea surface ona platformmoored to the seabed as shown in Fig. 8 and be deployedin water deeper than 50 m. Its 2 MW offshore prototype turbine isexpected to be tested at sea in 2013. Marine and wind energy

Fig. 8. Vertiwind turbines (from [36]).

Fig. 10. Structure of SeaTwirl (from [40]).

X. Sun et al. / Energy 41 (2012) 298e312 303

research group at Shanghai University, China has recently devel-oped a novel vertical axis wind turbine which has several flat platessurrounded around the turbine rotor. In addition to having all theadvantages of vertical axis wind turbines, this type of turbine alsocan achieve the higher wind energy utilization efficiency andstructural stability than conventional vertical axis wind turbines.Thus, this design could have great potential for offshore application.Fig. 9 shows a 5 kW prototype of this turbine which has been tested

Fig. 9. A novel vertical axis wind turbine for offshore application.

on land in Zhejiang province [37e39]. SeaTwirl [40] is a floatingvertical wind turbine that is currently being developed by Swedishcompany Ehrnberg Solutions AB and has a large flywheel appear-ance, as shown in Fig. 10. This unique design can allow the systemto kinetically store the wind energy harvested without therequirement of any energy conversion and also keep producingenergy even if the wind stops blowing. In addition, as SeaTwirl usessea water as low-friction roller bearing, the system can thus losethe excess weight caused by gearbox, transmission line or roller-bearings which are not needed for the system. A one-fiftieth-scale prototype SeaTwirl has been tested off the coast of Swedenin August 2011 and it is reported that the system has proven toperform well.

Some new concepts have also appeared in vertical axis offshorewind turbine design with an attempt to simplified the system andreduce the cost of offshore wind energy generation. Fig. 11demonstrates a new concept called Deepwind, which has beenbrought out and is being studied by Risø DTU, Technical University

Fig. 11. Artistic view of Deepwind concept (from [42]).

X. Sun et al. / Energy 41 (2012) 298e312304

of Denmark. This concept consists of a Darrieus rotor and a longvertical rotating tube extended into the water and connected to theseabed by a mooring system, which gives the system a very simplestructure and appearance. Its generator is installed in the bottom ofthe long tube and can be used to regulate the rotor during opera-tion. Thewhole structure can be tilted by changing the ballast in thetube and thus is not sensitive to inclined air flow [41,42]. At present,this invention is at the proof of concept stage. Another conceptualstudy on a floating axis offshore wind turbine (FAWT) has beenconducted and is introduced in Ref. [43], in which the proposedtechnique allows large inclination of the wind turbine floating axisand the inclination angle is passively adjustable according to windspeed, as shown in Fig. 12. As a large tilt angle can be allowed, thetotal weight of the system is reduced. Some major componentssuch as generator can be mounted above the water surface and areconvenience for maintenance. Although there are only preliminaryestimation and comparison conducted, the technique’s inventorsbelieved that this new technique can lead to develop a break-through to reduce the present high energy cost of offshore windpower generation [43].

3.3. Support structure

Offshore wind turbine support structures consist of towers andfoundations. The offshore wind turbine support structures needspecial design considerations different from the onshore windturbine support structures due to strong offshore winds and badsea conditions.

3.3.1. TowerThe turbine tower carries the nacelle and the turbine rotor, and

is fixed to the foundation on the sea floor. Due to the effect of strongwind and wave, offshore wind turbine towers must be sufficientlystrong to withstand cyclonic wind gusts, rigid so as not to damagethe wind turbine generator system or itself through vibration andbending, resistance to fatigue stress and resonant vibration [44]. A

Fig. 12. Concept of a floating axis wind turbine (a) with straight blades; (b) withcurved blades (from [43]).

tubular steel tower is currently the most common form of offshorewind turbine tower and its diameter increases downwards towardthe base of the turbine thereby forming a conical shape in order toincrease the strength of the tower. Alternatively, lattice tower canalso be used. Lattice tower are lighter than tubular steel tower asthe materials for truss towers cost much less than those for tubulartowers. In addition, truss towers also have the advantages of lessforce from wave, smaller foundation and flexible design [45,46].The Space Frame tower, a new concept for wind turbine towers, isbeing developed byWasatchWind LLC. Instead of a solid steel tube,the Space Frame tower consists of five custom-shaped legs andinterlaced steel struts. With this design, this technology can be usedat wind farm site that requires hub heights of 100m or more andmeanwhile its weight and cost can be less than traditional steeltube towers [47]. Characteristics of several offshore wind turbinetowers have been introduced in Ref [48].

At a given site, the selection of suitable tower type depends onsuch factors as space and cost. In order to reduce the cost ofoffshore wind turbine towers, the current research efforts focusedon other construction materials instead of steel. The trend towardsincreasing generating capacity of offshore wind turbines makesconcrete a competitive material. The concrete tower is durable,reliable and needs less maintenance. Through the use of admixtureand special reinforcements, the strength and the resistance of theconcrete tower can be further improved. The design of concretetower can also be flexible as concrete can be tailor-made to meetspecific requirements [49]. According to studies conducted by twoDutch companies Mecal and Hurks Beton [50,51], a concrete/steelhybrid tower could be economically viable, focusing on whole-lifecosting for a wind farm. Their studies also suggested that in spiteof initial higher investment required for the hybrid tower, itsreturns will be greater as this type of tower can be built taller andthus allow the turbine to generate more power. In addition, anAmerican based company Pyramatrix Structure Inc. has developeda composite wind tower which is made of composite materialswoven into lattices of reinforcing pyramids. According to thecompany, this tower can be 76% lighter than aluminum, 93% lighterthan steel and 25 times stronger than steel [44].

3.3.2. FoundationThe foundation costs approximately between 15% and 40% of the

total cost of current offshorewind farm projects. Hence, it is of greatimportance to select or design the most cost-effective turbinefoundations according to offshore site conditions. Offshore windturbine foundation type and design are considerably affected by seafloor soil properties, water depth, wave heights and currents.Currently, offshore wind farms are built primarily in shallowwatersless than 30 m and close to shore. Therefore, relatively simple typesof foundations such as monopole and gravity based foundationhave beenwidely used. However, the increasing number of plannedprojects in far-offshore and deeper water areas has motivatedresearch and pilot installations for more complex and cost-effectiveoffshore wind turbine foundations. Suction caissons currentlybeing studied are a new form of offshore foundation and arethought to be an economically attractive alternative [52,53]. Theyresemble large upturned buckets, as shown in Fig. 13 and areinstalled into the seabed either by pushing or by using ‘suction’ inorder to pump water out of the caisson and create a pressuredifferential across the top of caisson. This type of foundation hasadvantages of being quicker to install and also easier to removeduring decommissioning. However until now this technology doesnot arise in a wide range of commercial applications.

Technologies for floating foundations for offshore wind turbinesare being developed now, which are designed especially forapplications in deep water areas, where the water depth is greater

Fig. 15. An artist’s impression of a combined floating type platform for offshore windturbines (from [55]).

Fig. 13. Suction caisson foundation for offshore wind turbine (a) monopod, (b) tripod/tetrapod (from [54]).

X. Sun et al. / Energy 41 (2012) 298e312 305

than 50 m. Significant research and development efforts started atthe turn of this century [54] and so far, many different concepts forfloating foundations have been proposed which can be divided intothree main categories: spars, tension leg platforms (TLPs) andsemisubmersible/hybrid systems. At present, some floating foun-dation designs have moved from their concept stage to develop-ment of a fully functional prototype. Norwegian company StatoilHydro developed a prototype of a floating wind park using spartechnology, as shown in Fig. 14(a) and it can help deploy windturbines in water depths of up to 700 m. A 2.3 MW Hywind systemin water 220 m deep off the coast of Norway is the first full-scalefloating wind turbine in the world and has been tested for overtwo years delivering electricity to the grid since September 2009.Another Norwegian company Sway is currently immersed in an

Fig. 14. Prototypes of floating wind turbines (from [57]) (a) Hy

attempt to build the world’s largest floating 10 MW wind turbine,using a floating tower which is a pole filled with ballast beneath thewater creating lower center of gravity. The floating tower needs totake up an equilibrium tilt angle around 5e10� and turn aroundwith the wind, as shown in Fig. 14(b). Blue H Group TechnologiesLtd. (Blue H), incorporated in the UK, has developed a tension legplatform system and the prototype was installed and tested insouthern Italy in 2007, as shown in Fig. 14(c). Now, the company isdeveloping a 2 MW floating foundation which is planned to beinstalled in 2012. A combined type floating offshore wind powertechnology is being developed by Shandong Changxing WindPower Technology, a hi-tech company based in Shandong provinceof China. This technology provides a floating platform for offshorewind turbines which has a regular octagon shape, as shown inFig. 15. This whole structure is composed of nine pontoon typebases for offshore wind turbines which are all connected togetherthrough support columns and support components. This tech-nology uses the stability triangle to uniformly distribute all theloads over the structure and therefore can withstand the complexloads and extreme operating environments common to offshorewind turbines. It is reported that the demonstration program forthis combined floating platform will be conducted in the waters ofthe East Sea [55]. Similar floating offshore wind platform is alsobeing developed by Sweden’s Hexicon company [56]. Otherpromising floating foundation concepts being studied now alsoinclude WindFloat [57] designed by Principle Power and WindSeadeveloped by FORCE Technology [58].

Wind (spar) (b) Sway (tension leg/spar) (c) Blue H (TLPs).

Fig. 16. Wake clouds behind the Horns Rev offshore wind farm in Denmark (Photocourtesy of Vattenfall).

Fig. 17. Characteristics of various optimization algorithms (from [68]).

X. Sun et al. / Energy 41 (2012) 298e312306

The first floating offshore wind farm may be established inScotland, UK, which will consist of Hywind turbines developed byStatoil Hydro. Two potential sites have been selected now and anenvironmental impact assessment for Statoil will start to beginrecently.

3.4. Wake losses and offshore wind farm layout optimization

3.4.1. Wake lossesAs offshore wind farms increase in size, wake effects within

large offshore wind projects can influence considerably on theirenergy production and mechanical loads on individual turbine. Inaddition, offshore wind turbine wake can also propagate overlonger distances than over land owing to low ambient turbulenceconditions. Fig. 16 shows wake clouds observed behind the HornsRev offshore wind farmwest of Denmark. It is estimated that whenaveraged over different wind directions, losses of approximately12% for offshore farms can be expected [59] and the wind recoversto within 2% of its ambient speed over a downstream distance of5e14 km [60]. Hence, accurate simulation of interactions betweenthe individual turbines, the atmosphere and neighboring turbinesis essential to precisely predict the power output before wind farmconstruction, assess production losses due to wake effects andassist with optimizing offshore wind farm layout.

The wake models used for offshore wind farms now are modi-fied on the basis of onshore wind farm wake models in order toimprove their predictions of the wake development in offshoreconditions, especially the low ambient turbulence and the effect ofatmospheric stability [61]. However, the absence of measurementdata makes it very difficult to validate these numerical models, asinstalling meteorological masts can be expensive and technicallychallenging. Advanced measurement instruments such as sodarand doppler lidar start to be used in Europe [62] and then researchhas been conducted regarding the validation of wake models usedfor offshore wind farms. One good example is the Efficient Devel-opment of Offshore Wind farms (ENDOW) project headed by RisøNational Laboratory in Denmark and it attempts to link wake andboundary layer models to better predict wind speed and turbulenceprofiles within large offshore wind farms [63,64]. Evaluated wakemodels including not only complex CFD models, but also simpleanalytical models were selected from ten organizations in Europeand varied in complexity. Measurement data from two offshorewind farms was used for comparison purpose. The results obtained

from this study indicated that no one model currently has, intotality, an advantage over others. POW’WOW project funded bythe European Commission has developed a virtual laboratorywhich makes measurement data from offshore wind farms andresults from some wind farmmodels public. Other users can accessand use this information in order to compare with their own wakemodeling.

3.4.2. Offshore wind farm layoutThe offshore wind farm layout is a relatively complex problem

as it usually involves some tradeoffs that mainly depend onparameters like distance of wind farms from the coast and turbinespacing [65]. For instance, turbines in wind farms should be placedfarther apart from each other in order to minimize the wakeinterference and achieve the best possible energy output. On theother hand, the large spaced intervals will increase the cost of theinter-turbine array cables. Therefore, optimizing the offshore windfarm layout is the process of achieving a balance among the variousfactors at play in order to minimize the cost of energy (COE) whilemaximizing the energy production of the wind farm. Convention-ally, a turbine cost model and a wake model are combined ina conjunction with an optimization routine for the wind farmlayout design [66]. However, due to the characteristics of offshorewind farms, factors such as O&M and availability also need to beconsidered for offshore wind farm optimization design. Selectionsof appropriate objective function and optimization algorithm aretwo keys to optimizing offshore wind farm layout. The objectivefunctions used now mainly include maximization of energyproduction, maximization of profit and minimization of the cost ofenergy, among which minimization of the cost of energy seeksbalance between energy output and cost of the wind farm and thusis believed to result in the best solution from the perspective ofratepayers [67]. There are a number of optimization algorithmsbeing studied and some of the most popular ones include GreedyHeuristic Algorithms (GHA), Gradient Search Algorithm (GSA),Genetic Algorithms (GA), Simulated Annealing Algorithms (SAA)and Pattern Search Algorithms (PSA) [68,69]. According to the timethe algorithm needs to obtain the solution and the quality of thatsolution, the characteristics of these algorithms are shown in Fig.17.As general practice, turbines in current offshore wind farms aretypically arranged in rows perpendicular to the prevailing winddirection. The spacing between turbines aligned in a row is usuallyon the order of 5e10 rotor diameters, and spacing between rows isbetween 7 and 12 rotor diameters [70].

3.5. Protection measures for offshore wind turbines

3.5.1. Anti-corrosion protectionAnti-corrosion is a very important aspect in protecting offshore

wind turbines from damage caused by erosion. Some types ofcorrosion damage to offshore wind turbine are illustrated in Fig. 18[71]. Unlike other offshore buildings like oil platforms, repair andmaintenance the anti-corrosion measures for offshore windturbines could be very difficult and expensive. Therefore, the anti-

Table 8Anti-corrosion measures for steel structures in different areas of the supportstructure of offshore wind turbine (from [71]).

Areas Anti-corrosion method

Atmospheric zone Paint coatingThermal sprayed metallic coating

Splash zone Paint coatingThermal sprayed metallic coatingCorrosion allowance

Submerged zone Cathodic protectionPaint coating

Seabed zone Cathodic protection

X. Sun et al. / Energy 41 (2012) 298e312 307

corrosion protection measures for offshore wind turbines shouldhave the characteristics of excellent anti-corrosion effect, simpleconstruction method, long-term service life and without mainte-nance and management work [72].

Many offshore wind turbines operating now are equipped withan anti-corrosion package and these measures must preventcorrosion on both the interior and exterior of the turbine, whichmainly are its nacelle and support structure. The anti-corrosionprotection on the inside of the nacelle can be achieved bykeeping the air dry. For wind turbines used for Nysted wind farm inDemark, a local air conditioning system is applied to the inside ofthe nacelle in order to ensure a low relative humidity and water-tight tower. The anti-corrosion inside the wind turbine nacelles inMiddelgrunden wind farm is realized with the aid of the improvedpanting system and creating a dry condition inside the turbine. Thewhole machine is sealed and the gear and generator are cooled byheat exchangers recycling the air used in the air-cooling system,instead of conventional air-cooled components on earlier turbines.The interior relative humidity can be remained within the limit ofany steel corrosion risk limit with the aid of de-humidifying devicesinstalled in the turbine tower and nacelle. In addition, someimportant electric components such as generator and controlsystems are also equipped with standby heating systems in order toprevent condensation even during a rapid change in ambienttemperature [73]. Ref. [74] discussed different deign-basedapproaches for the corrosion protection of the interior of offshorewind turbine.

According to the different corrosion extent of the steel structurein different locations, the offshore wind turbine support structurethat requires protection can be divided into atmospheric area,submerged area, splash zone and seabed zone [72], in whichcorrosion of steel structure happened in the splash zone is mostserious due to the combined effects of sea water and salt-laden airof marine environment [75]. The current methods used for theexterior corrosion protection of offshore wind turbine are mainlycomprised of increasing corrosion allowance, cathodic protection,coating and spray. Different anti-corrosion measures have to betaken in different zones, as summarized in Table 8. Painted coatingsare the most common anti-corrosion treatments, which aredeveloped for offshore platforms and now adapted to the offshore

Fig. 18. Some types of corrosion damage

wind turbine. Much research on effective, long lasting and envi-ronmentally friendly coating materials and systems for offshorewind farm has been executed over the years. Surfaces with inte-grated functions, multifunctional coatings and nano-based layersystem are some examples of pioneering technological solutionsthat are currently being developed. Additionally, there are alsomany more coating solutions that have been investigated for useoffshore in order to improve efficiencies during the fabricationprocess, such as fewer coats and faster cure leading to improvedproductivity [76]. A detailed description of anti-corrosion coatingsis given in Ref. [77]. Cathodic protection is another frequently usedmethod for protecting offshore turbines at present. There are twobasic methods of applying cathodic protection which are known assacrificial anodes and impressed current cathodic protection (ICCP).It has been found that sacrificial anodes can pollute the marineenvironment by releasing thousands of tons of zinc or aluminuminto water bodies every year. Thus, being environmentally safe, andelectrically efficient, ICCP system is now the most elegant solutionfor offshore wind turbines. A performance comparison of thevarious cathodic protection systems has been conducted inRef. [78], based on extensive computer modeling.

In practice, different standards and criterions for preventingcorrosion of offshore steel structure have been established andfollowed in various countries, among which NORSOK M-501 [79],ISO 20340 [80] and ISO 12944-6 [81] are the most frequently usedstandards in the protective coatings field for pre-qualification dueto their validity and authority.

to offshore wind turbine (from [71]).

X. Sun et al. / Energy 41 (2012) 298e312308

3.5.2. Anti-typhoonA powerful typhoon can pose a significant risk to offshore wind

farm development. Extreme wind speed, unusual turbulence andsudden change in wind direction are the main characteristics oftyphoon and also the main causes of damage to wind turbines vitalcomponents [82], destroying turbine blades, nacelles and eventowers. Therefore, the design of offshore wind turbines mustcomply with anti-typhoon requirements in areas with a highfrequency of typhoon. At present, the design of wind turbines in theworld must be in accordance with certain professional standards inorder to ensure wind turbines continue operating reliably at stan-dard operational environmental conditions. However, most ofthese applicable technical and professional standards are set basedon Europe’s climate and environment, as Europe has been the earlywind turbine technology adopter and also become the early leadersin this technology development. Wind turbines designed andcertified according to these standards may not be able to operatesafely in extreme environment conditions like typhoons whichoccur most frequently in a region of the western Pacific and eastAsia, especially China, Taiwan and Japan [83]. The offshore windpotential varies along China’s coast, with the greatest overallpotential in Guangdong, Fujian and Hainan, however, these prov-inces are also the areas most severely affected by Typhoons.Therefore the development of offshore wind farms in China has toovercome the challenging hurdle of designing equipment that canwithstand the destructive wind speeds [84] in order to fully exploitthe vast offshorewind energy resources in these areas. Studies haveshown that extremely high wind loads and time-lag in controlsystem are two critical problems [85,86] and presentlymany Asian-based wind turbine manufacturers are engaged in developing newanti-typhoon technologies to tackle these problems.

Mitsubishi Heavy Industries Ltd. in Japan has come up witha new concept called Smart-Yaw Concept for reducing the loads atthe time of strong winds. This technique utilizes properties similarto weather vanes which naturally face down wind. If there isa strong wind, the turbine nacelle can rotate to face down, therebyalleviating the load, as shown in Fig. 19. In addition, since the yaw isdriven by the wind load acting on the rotor, this technique can thusalso ensure effective wind turbine yaw control even duringpower interruption [87e89]. Mitsubishi 1000 kW wind turbine

Fig. 19. Operational principle of Smart-yaw concept (from [89]).

MWT-1000A equipped with this system has successfully withstoodthe gust exceeding the maximum instantaneous wind speed of70 m/s and survived from typhoons hits [88]. Various anti-typhoonwind turbines are also being developed by Chinese wind turbinemanufacturers such as Dongfang Electric (2 MW FD87ANo1 andFD87ANo2), Shanghai Electric and Guangdong Mingyang WindPower (2.5 and 3 MW SCD). However, the key technologiesinvolved and the performances of these turbines during typhoonsare not available to the public yet. Some Chinese experts havesuggested that when the typhoon comes, temporarily suspendingturbine operations [90] or applying standby power [91] can beadopted to ensure the optimal control of wind turbine systems.However, suspending turbine operation during typhoons will havea negative effect on the economics of wind farms and the use ofstandby power for turbine control still needs further study toensure its validity. Other ongoing researches also focus on thetyphoon resistant wind turbine tower [92e94] and blades.

3.6. Environmental impacts of offshore wind power

Although wind power is considered to be environmentallybenign compared to conventional energy technologies, windturbines are, after all, man-made structures and previous studieshave suggested that the existence of the wind farm may haveadverse effects on the local environment. However, offshore windfarms located far from shore have a lower noise and visual impact,compared to onshore wind farms.

3.6.1. Impacts on birdsWith the rapid expansion of offshore wind power, the influence

of offshore wind farms on birds has aroused great public concernand the main potentially detriment effects of offshore wind farmsinclude collision fatalities, barriers to birds movement and short-term or long-term habit losses due to human disturbance at thestages of construction or maintenance [95e97]. The modernoffshore wind turbines have larger rotor diameters and thus tallertowers, which increase the risk of birds colliding with the turbinesor being injured in the associated turbulence vortices. Accurateestimates of bird mortality rates at wind farms are important assome particular bird species susceptible to collisions may becomeendangered or have declining populations. However, informationabout bird mortality due to offshore wind farms is still limited atpresent because of the difficulty in detecting collisions and deadbodies of birds at sea. Radar, thermal imagery and acoustic detec-tion are some of techniques which are currently under develop-ment for the study of bird behavior in relation to offshore windfarms [98,99]. Some research studies conducted for existingoffshore wind farms by using these techniques have suggested thatbirds can avoid the wind farms by flying around the outside of thewind farm rather than between the turbines [100] and therebycollision rates per turbine may be relatively low [27,101] duringdays of good visibility. According to the study of birds at Nysted andHorns Rev offshore wind farms in Denmark, it was estimated that41 to 48 out of 235,000 passing birds could be killed by turbines ina single autumn at the Nysted offshore wind farm, which onlyaccount for 0.018e0.020% [102]. Nevertheless, the health of localand migrating bird populations could still be threatened by theadditional energetic expenditure required for their avoidanceresponse and increased travel distances, especially if more windfarms would be built in the foraging areas or along the migrationroutes of birds and together cause significant contributions tocumulative impacts, as suggested by some studies [103,104].

Offshore wind farms may also harm birds through disturbanceand habitat loss or damage. Disturbance to birds can occur duringwind farm construction and continue due to the post-construction

X. Sun et al. / Energy 41 (2012) 298e312 309

O&M activities required on a regular basis. These disturbances canlead directly to expulsion and thus loss of territory for certainspecies of birds. For instance, studies at Horns Rev offshore windfarm found that changes in distributions of divers, Common Scoterand Common Guillemot/Razorbills were observed and thesespecies of birds tend to avoid the wind farm site and even the zonesof 2 and 4 km around the wind farm. Conversely, some species suchas gull and tern showed a preference for thewind farm area [105] inthese studies. Displacement of birds from favorable habitat into lesssuitable habitat may adversely affect the sustainability of theirpopulations. Although more knowledge is needed on the overalleffects of offshore wind farms on bird populations, offshore windfarms should not be placed in the protection areas for birds.

3.6.2. Impacts on marine mammals and fishAs most marine mammals rely to some extent on sound and

hearing for essential activities including communication, naviga-tion, foraging and predator detection, excessive man-madeunderwater sound can negatively affect these marine mammalsover very large distances and cause temporary or permanenthearing impairment, behavioral disturbance and possible stress formarine mammals [106]. Many activities required in the process ofconstructing an offshore wind farm can generate intense noisepollution during a limited period of time, such as pile-driving ofmonopile foundations which are commonly used as foundationsfor shallow water turbines. Monopile installation needs hydraulichammers to hit the pile repeatedly and push it into the seabed withenormous force. Thus, noise emission from large piles installationcan reach 205 dB re 1 mPa at 100 m from the pile-driving and onlybecome indistinguishable from background noise at 80 km away,according to Ref. [107]. Such high-intensity sound could causehearing loss in marine mammals like seals [27] and impact variousmarine mammals over a large area. For bottlenose dolphins, theycan still detect the sounds from pile-driving at 1 kHz up to 40 kmfrom the site [108]. Even after the offshore wind farm is put intooperation, the underwater noise can still be continuously producedby operational wind turbines and associated maintenance activitiesand will last over the lifetime of the wind farm [108]. But theseoperational noises are considered to have a small impact on marinemammals due to the low-intensity and low-frequency of the noise[108]. Although more data are still needed to fully understand theimpact of offshore wind farm on different species of marinemammals, changes in behavior of seals and harbor porpoises wereobserved at the Nysted and Horns Rev offshore wind farms whenthey were constructed and the number of these two species ofmarine mammals were found to be reduced in the areas around thewind farms [27].

Fish can react to sound. Therefore, anthropogenic noisemay alsohave negative effects on fish, such as hearing damage, maskinginterference on acoustic communication and a change of habitat ora disturbance in routine and behavior [109]. Since fish species reactvery differently to sound [110], lack of adequate knowledge andlong-term monitoring data makes it difficult to draw firm conclu-sions as to the likelihood of health effects on fish from exposure tonoise emitted by the offshore wind farms at the present stage ofstudies [111]. In addition, numerous fish species can be verysensitive to magnetic field changes like elasmobranches (sharks,rays and skates) [112]. Natural magnetic fields are used by some fishlike a compass to navigate and detect their prey. Therefore, artificialelectromagnetic fields induced by the underwater cables which areused to collect power from wind turbines and transmit it to shoremay be able to cause abnormal migration and even changes in thefish’s physiological mechanism [113]. Although field studies on theimpact of underwater cables on fish are still limited now, someresults suggested that the cables could retard migration of some

fishes in the area of Nysted wind farm [27] or slow down the speedof migrating sliver eels between the island Oland and the Swedishmainland [113]. On the other hand, it has been found that offshorewind farmsmay also have the potential to produce positive impactson fish and fisheries. Like offshore oil and gas platforms, offshorewind turbine foundations can also act as artificial reefs and fishaggregating devices, increasing marine biomass and attractingdifferent kind of fish species [114,115]. According to a field studyconducted at Yttre Stengrund and Utgrunden offshore wind farmsin Sweden [115], fish aggregating around the offshore turbines havebeen recorded and the average density of fish around the turbinesite was found significantly increased compared to that on theneighboring seabed.

4. Conclusions and perspectives

Despite their successes and continuing projected growth, manycountries are approaching the limits of their land-based windpower production. It is believed that the future of wind energy liesoffshore. In recent years, offshore wind technology has beenundergoing rapid development. This paper gives a brief introduc-tion to several key aspects of offshore wind power and their recentdevelopment. Currently, high cost is still the main barrier pre-venting the successful implement of offshore wind power. If itscosts cannot be considerably brought down in time, offshore windcould lose its attractiveness to the market. Although it is expectedthat cost reduction in offshore wind will follow a similar path toonshore wind, it is a long-term process and thus patience andperseverance are essential, which are reflected in sustainablefinancial and political support. As offshore wind is still in itsinfancy, further development depends on technology innovation.However, almost all of today’s offshore wind farms are an adapta-tion of onshore wind technology and a horizontal axis wind turbineon a high tower is still the mainstream concept for offshore windturbines. Since the meteorological conditions offshore are quitedifferent compared to the onshore conditions, optimal offshorewind technology needs to be developed in order to adapt to themarine environment and meanwhile provide high efficiency,robust and reliable performance. This is leading to the developmentof novel wind turbine concepts specifically designed for offshoreuse, such as floating wind turbines and different types of verticalaxis wind turbines. Offshore wind turbines are human-madestructures and would have an impact on wildlife and thesurrounding environment. However, environmental impacts ofoffshore wind farm are still controversial issues that have been indebate, due to a limited amount of field monitoring data andinconsistent results. Even though some results suggest the impactsof offshore wind farms on the environment are minor, the long-term and cumulative effects caused by offshore wind farms arestill not clear now. In addition, necessary knowledge improvementsneed to be acquired inmany other issues, especially those related tofar-offshorewind energy: wind resources assessment, optimizationof the electrical connection, support infrastructure, maintenanceand operation, etc.

Future development of offshore wind turbines may show thefollowing trends:

� Offshore wind turbines will get much bigger both in size andpower as the world community continues developing greaterproduction capacity for wind energy. With the increase in theweight and size of individual rotor blade, control of largeoffshore wind turbines becomes more and more difficult.Therefore, ‘Smart Turbine Blades’ technology will become oneof the most important technologies for the future in order tohelpmaximize the amount of electricity generatedwind power

X. Sun et al. / Energy 41 (2012) 298e312310

while ensuring longer life spans for wind turbines, and willeventually lead to the next generation of large, smartlycontrolled wind turbines. At the same time, the design speci-fication for an offshore wind turbine will also become localizedin order to meet different climatic, environmental andeconomic requirements.

� Future development of offshore wind power will move evenfurther away from the coast and in deeper waters (�60 m).However, in such locations, the large amount of turbulence willcause problem for horizontal turbines and it will also beextremely difficult to install very large horizontal axis windturbines at such sea depths. Thus, vertical axis wind turbinesare more likely to become a dominant design that is adopted inthose areas, as they can be more technically feasible andproduce cheaper electricity because of their ability to usesignificantly smaller flotation system and thus reduce the cost.

� Except the high cost, there are still many technological chal-lenges in the transmission and grid integration of electricityfrom the wind further away from the shore. Therefore, off-gridwind power applications will become important. For instance,using offshore wind power to produce hydrogen is a goodoption.

� In addition, the use of advanced materials like carbon fiber toproduce bigger, lighter andmore durable offshorewind turbineblades and direct-drive generator technology are also expectedto expand with future development of offshore wind.

These improvements will undoubtedly lead over time to largecost reductions, making offshore wind more competitive and itsutilization more important in the coming years.

Acknowledgments

This work was financially supported by National Natural ScienceFoundation of China Grant No.50836006, Shanghai Science andTechnology Committee with Grant No. 09JC1405800, Program forChangjiang Scholars and Innovative Research Team in Universitywith Grant No. IRT0844, the Special Fund for Selection and Trainingof Outstanding Young Teachers from Shanghai Universities andShanghai Pujiang programme.

References

[1] GWEC forecasts further growth for wind industry. Available from: http://www.renovablesmadeinspain.com/noticia/pagid/128/titulo/GWEC%20pronostica%20que%20la%20e%C3%B3lica%20seguir%C3%A1%20creciendo/len/en/.

[2] Esteban MD, Diez JJ, López JS, Negro V. Why offshore wind energy?Renewable Energy 2011;36(2):444e50.

[3] Zhang Xianping. Current status and trends of offshore wind energy. ElectricAge 2011;(3):46e8.

[4] Kaldellis JK, Zafirakis D. The wind energy revolution: a short review of a longhistory. Renewable Energy 2011;36(7):1887e901.

[5] Edwards I. Overcoming challenges for the offshore wind industry andlearning from the oil and gas industry. POWER cluster. Report No.012345eR001A; 2011. Available from: http://www.power-cluster.net/Portals/6/Overcoming%20OW%20Challenges%20summary.pdf.

[6] TheEuropeanWindEnergyAssociation. TheEuropeanoffshorewind industry-key trends and statistics: 1st half 2011. Report; 2011. Available from: http://www.ewea.org/fileadmin/ewea_documents/documents/publications/statistics/EWEA_OffshoreStatistics_2010.pdf.

[7] The European Wind Energy Association. The European offshore windindustry-key trends and statistics 2010. Report; 2011. Available from: http://www.ewea.org/fileadmin/ewea_documents/documents/00_POLICY_document/Offshore_Statistics/110120_Offshore_stats_Exec_Sum.pdf.

[8] McConnel N. Offshore wind power. Alstom power. Report. Available from:http://www.alstom.com/assetmanagement/DownloadAsset.aspx?ID¼fe843e9d-bb33-4ed6-8da8-0c8b57e9337a&version¼512cb99298434c93a4d84aba3ec23bae8.pdf; 2011.

[9] Wikipedia. List of Offshore Wind Farms. Available from: http://en.wikipedia.org/wiki/List_of_offshore_wind_farms.

[10] Jones AT, Westwood A. Recent progress in offshore renewable energytechnology development. Power engineering society general meeting, vol. 2.San Francisco, USA: IEEE; 2005. 2017e2022.

[11] AmericanWind Energy Association. Industry Statistics. Available from: http://www.awea.org/learnabout/industry_stats/index.cfm [accessed 08.04.2011].

[12] Tobias D. Long Island’s offshore wind farm. Report. Available from: http://web.mit.edu/1.011/www/finalppr/dtobias-Term_Paper_-_Tobias.pdf; 2011.

[13] Musial W, Butterfield S. Future for offshore wind energy in the United States.In: International conference and exposition on renewable ocean energy;28e29 June, 2004 [Florida, USA].

[14] U.S. Department of Energy. Creating an offshore wind industry in the UnitedStates: a strategic work plan for the United States. Predecisional Draft; 2010.

[15] Breton SP, Moe G. Status, plans and technologies for offshore wind turbinesin Europe and North America. Renewable Energy 2009;34(3):646e54.

[16] U.S. Department of Energy. A national offshore wind strategy: creating anoffshorewindenergy industry in theUnitedStates.Report; 2011.Available from:http://www1.eere.energy.gov/wind/pdfs/national_offshore_wind_strategy.pdf.

[17] The Canadian Press. Ontario imposes moratorium on offshore windfarms. Available from: http://www.globaltoronto.com/Ontarioþimposesþmoratoriumþoffshoreþwindþfarms/4267666/story.html [accessed 02.11.2011].

[18] Enslow R. China, Norway and offshore wind development: a win-win rela-tionship? Presented at offshore wind China conference and exhibition. Avail-able from: http://www.norway.cn/PageFiles/391359/Azure%20International%20-%20Rachel%20Enslow.pdf; 15e16 March, 2010 [Bergen, Norway].

[19] Zhao Shiming, Jiang Bo, Xu Huifen, Li Shouhong, Ding Jie. Exploration andapplication of ocean wind energy resources in coastal sea of China. OceanTechnology 2010;29(4):117e21.

[20] Chen Jinjin. Development of offshore wind power in China. Renewable andSustainable Energy Reviews 2011;15(9):5013e20.

[21] Green R, Vasilakos N. The economics of offshore wind. Energy Policy 2011;39(2):496e502.

[22] Junginger M, Faaij A, Turkenburg WC. Cost reduction prospects for offshorewind farms. Wind Engineering 2004;28(1):97e118.

[23] Bilgili M, Yasar A, Simsek E. Offshore wind power development in Europeand its comparison with onshore counterpart. Renewable and SustainableEnergy Reviews 2011;15(2):905e15.

[24] Morthorst PE. Volume 2-‘Costs and Prices’ in wind energy-the facts.Denmark: Risø National Laboratory; 2004. pp. 100.

[25] Junginger HM, Faaij A, Turkenburg WC. Global experience curves for windfarms. Energy Policy 2005;33(2):133e50.

[26] Blanco MI. The economics of wind energy. Renewable and SustainableEnergy Reviews 2009;13(6e7):1372e82.

[27] Snyder B, Kaiser MJ. Ecological and economic cost-benefit analysis ofoffshore wind energy. Renewable Energy 2009;34(6):1567e78.

[28] Heptonstall P, Gross R, Greenacre P, Cockerill T. The cost of offshore wind:understanding the past and projecting the future. Energy Policy 2012;41:815e21.

[29] Musial W, Ram B. Large-scale offshore wind power in the United States-assessment of opportunities and barriers. National renewable energy labo-ratory report; 2010. Available from: http://www.nrel.gov/wind/pdfs/40745.pdf.

[30] Xinhua news. Wind turbine manufactures rush to build larger facilities.Available from: http://www.china.org.cn/business/2011-06/03/content_22713420.htm [accessed 06.03.2011].

[31] Fichaux N, Beurskens J, Jensen PH, Wilkes P. UpWind: design limits andsolutions for very large wind turbines-A 20 MW turbine is feasible. Europeanwind energy association. Report; 2011. Available from: http://www.ewea.org/fileadmin/ewea_documents/documents/upwind/21895_UpWind_Report_low_web.pdf.

[32] The Guardian. Engineers race to design world’s biggest offshore windturbine. Available from: http://www.guardian.co.uk/environment/2010/jul/26/offshore-turbine-britain [accessed 07.26.2010].

[33] Dvorak P. Is the vertical-axis turbine key to offshore success? Available from:http://www.windpowerengineering.com/featured/business-news-projects/is-the-vertical-axis-turbine-key-to-offshore-success/ [assessed 03.18.2010].

[34] Arup. 10 MW wind turbine unveiled. Available from: http://www.arup.com/News/2010_07_July/27_Jul_2010_Arup_and_Wind_Power_Limited_unveil_10MW_Wind_Turbine.aspx#!; 2010 [accessed 07.26.2010].

[35] Vertax wind Ltd. Available from: http://vertaxwind.com/images.php.[36] Wind powermonthly. Work starts on prototype floating vertical-axis turbine.

Available from: http://www.windpowermonthly.com/news/1050768/Work-starts-prototype-floating-vertical-axis-turbine/; 2011 [assessed 01.21.2011].

[37] Yin Caozi, Huang Diangui. Wind turbine with wind collection-shield boards.Journal of Engineering Thermophysics 2008;29(6):960e2 [in Chinese].

[38] Zhou Wanli, Yin Caozi, Huang Diangui. Experiment research on the charac-teristics of wind wheel with wind collection-shield pattern. Journal ofEngineering Thermophysics 2009;30(4):598e600 [in Chinese].

[39] Jijun Feng, Zhaoyi Deng, Lei Jiang, Diangui Huang. Optimization design ofa 5 kW lift type vertical axis wind turbine with wind collection-shieldpattern. Accepted to be published by Journal of Engineering Thermophy-sics [in Chinese].

[40] Ehrnberg Solutions AB Ltd. Available from: http://seatwirl.com.[41] Vita L, Paulsen US, Pedersen TF, Madsen HA, Rasmussen F. A novel floating

offshore wind turbine concept. European wind energy conference andexhibition (EWEC); 16e19 March, 2009. Marseille, France.

X. Sun et al. / Energy 41 (2012) 298e312 311

[42] Vita L, Zhale F, Paulsen US, Pedersen TF, Madsen HA, Rasmussen F. A novelconcept for floating offshore wind turbines: recent developments in theconcept and investigation on fluid interaction with the rotating foundation.In: Proceedings of the ASME 29th international conference on ocean,offshore and arctic engineering; 6e11 June, 2010 [Shanghai, China].

[43] Akimoto H, Tanaka K, Uzawa K. Floating axis wind turbines for offshorepower generation- a conceptual study. Environmental Research Letters2011;6(4):1e6.

[44] Magoha PW. Design, manufacture, transportation, and foundationconstruction of wind turbine towers. In: World renewable energy congressVIII; 29 Auguste3 September, 2004 [Denver, USA].

[45] Engels W, Obdam T, Savenije F. Current developments in wind-2009; 2009.ECN Report, ECN-E-09e96. Available from: ftp://energie.nl/pub/www/library/report/2009/e09096.pdf.

[46] Kurian VJ, Narayanan SP, Ganapathy C. Towers for offshore wind turbines. In:Proceeding of the 10th Asian international conference on fluid machinery;21e23 October, 2009. Kuala Lumpur, Malaysia.

[47] GE completes acquisition of new-generation wind turbine towertechnology. Available from: http://www.businesswire.com/news/home/20110211005482/en/GE-Completes-Acquisition-Next-Generation-Wind-Turbine-Tower [assessed 02.11.2011].

[48] Berril T. Wind energy conversion systems: resource book. Renewable EnergyCenter, Brisbane Institute of TAFE, ISBN 1 876880 112; 2001.

[49] Singh AN. Concrete construction for wind energy towers. The IndianConcrete Journal-Point of View; 2007:43e9.

[50] Brughuis FJ. Advanced tower solutions for large wind turbines and extremetower heights. In: Proceedings of the 2003 European wind energy confer-ence and exhibition; 16e19 June, 2003 [Madrid, Spain].

[51] Brughuis FJ. Improved return on investment due to larger wind turbines. In:Proceedings of the 2004 European wind energy conference and exhibition;22e25 November, 2004 [London, UK].

[52] Byrne BW, Houlsby GT. Investigating novel foundations for offshore wind-power generation. In: Proceedings of the 21st international conference onoffshore mechanics and arctic engineering OMAE’02; 23e28 June, 2002[Oslo, Norway].

[53] Byrne B, Houlsby G, Martin C, Fish P. Suction caisson foundations for offshorewind turbines. Wind Engineering 2002;26(3):145e55.

[54] Houlsby GT, Ibsen L-B, Byrne BW. Suction caissons for wind turbines. In:Proceedings of international symposium on frontiers in offshore geo-technics; 19e21 September, 2005 [Perth, Australia].

[55] Development and application of Changxi combined offshore floating platform. Available from: http://www.cnwp.org.cn/fenxi/show.php?itemid¼813; 2011 [in Chinese, accessed 08.05.2011].

[56] Chen L, Ponta FL, Lago LI. Perspective on innovative concepts in wind-powergeneration. Energy for Sustainable Development 2011;15(4):398e410.

[57] Roddier D, Cermelli C, Aubault A, Weinstein A. WindFloat: a floating foun-dation for offshore wind turbines. Journal of Renewable and SustainableEnergy 2010;2(3). 033104-1-34.

[58] Du Xiuli, Zheng Jianjun, Yan Weiming, Li Yue, Zhang Jianwei. Researchdevelopment and key technical on floating foundation for offshore windturbines. Advanced Materials Research 2012;446e449:1014e9.

[59] Sanderse B, Pijl SP, Koren B. Review of computational fluid dynamics forwind turbine wake aerodynamics. Wind Energy 2011;14(7):799e819.

[60] Christiansen MB, Hasager CB. Wake effects of large offshore wind farmsidentified from satellite SAR. Remote Sensing of Environment 2005;98(2e3):251e68.

[61] Lange B, Waldl HP, Guerrero AG, Heinemann D, Barthelmie RJ. Modelling ofoffshore wind turbine wakes with the wind farm program FLaP. WindEnergy 2003;6:87e104.

[62] Barthelmie RJ, Folkerts L, Ormel FT, Sanderhoff P, Eecen PJ, Stobbe O, et al.Offshore wind turbine wakes measured by Sodar. Journal of Atmosphericand Oceanic Technology 2003;20(4):466e77.

[63] Barthelmie RJ, Folkerts L, Rados K, Larsen GC, Pryor SC, Frandsen S, et al.Comparison of wake model simulations with offshore wind turbine wakeprofiles measured by Sodar. Journal of Atmospheric and Oceanic Technology2006;23(7):888e901.

[64] Barthelmie R, Larsen G, Bergström H, Magnusson M, Schlez W, Rados K, et al.ENDOW: efficient development of offshore windfarms. Wind Engineering2001;25(5):263e70.

[65] Lackner MA, Elkinton CN. An analytical framework for offshore wind farmlayout optimization. Wind Engineering 2007;31(1):17e31.

[66] Szafron C. Offshore windfarm layout optimization. 2010 9th internationalconference on environment and electrical engineering; 16e19 May 2010[Prague, Czech Republic].

[67] Elkinton CN, Manwell JF, McGowan JG. Algorithms for offshore wind farmlayout optimization. Wind Engineering 2008;32(1):67e83.

[68] Elkinton CN. Offshore wind farm layout optimization. PhD dissertation. 2007,University of Massachusetts, Amherst, USA.

[69] Baños R, Manzano-Agugliaro F, Montoya FG, Gil C, Alcayde A, Gómez JJ.Optimization methods applied to renewable and sustainable energy:a review. Renewable and Sustainable Energy Reviews 2011;15(4):1753e66.

[70] AWS Truewind Offshore Wind Technology. Offshore wind technologyoverview. Albany, New York: Prepared for the Long Island-New York CityOffshore Wind Collaborative by AWS Truewind, LLC; 2009.

[71] van der Mijle Meijer H. Corrosion in offshore wind energy: a major issue.Den Helder, Netherlands: Presented at The Offshore Wind Power Confer-ence. Available from: http://www.we-at-sea.org/docs/Conference/Session%204/3%20Harald%20vd%20Mijle%20Meijer.pdf; 12e13 February, 2009.

[72] Ge Yan, Zhu Xi-chang, Yan Li. Anti-corrosion protection strategies forsupport structures and foundations of wind turbines of offshore wind farms.In: International conference on sustainable power generation and supply(SUPERGEN); 6e7 April, 2009 [Nanjing, China].

[73] Middelgrunden, Denmark. Available from: http://www.power-technology.com/projects/middelgrunden/.

[74] Siegfriedsen S. US patent 6,439,832 B1, 2002.[75] Momber A. Corrosion and corrosion protection of support structures for

offshore wind energy devices (OWEA). Materials and Corrosion 2011;62(5):391e404.

[76] Thick J. Offshore corrosion protection of wind farms. European RenewableEnergy Review; 2006:44e50.

[77] Sørensen PA, Kiil S, Dam-Johansen K, Weinell CE. Anticorrosive coatings:a review. Journal of Coatings Technology and Research 2009;6(2):135e76.

[78] Bortels L, Van den Bossche B, Parlongue J, de Leeuw J, Wessels B. Designvalidation of ICCP systems for offshore wind farms. In: Corrosion 2010conference and expo; 14e18 March, 2010 [San Antonio, Texas, USA].

[79] NORSOK M-501. Surface preparation and protective coating.[80] ISO 20340. Paints and varnishes- Performance requirements for protective

paint systems for offshore and related structure.[81] ISO 12944. Paints and varnishes-Corrosion protection of steel structures by

protective paint systems.[82] Wu Jincheng, Zhang Rongmiao, Zhuang Xiuzhi. Anti-typhoon design for

offshore wind turbines. Engineering Sciences 2010;12(11):25e31.[83] Ueda Y, Suguro Y. High performance MW class wind turbine for Asian region.

In: Proceedings of 3rd world wind energy conference and renewable energyexhibition-the 2nd wind power Asia; 31 Octobere4 November, 2004 [Bei-jing, China].

[84] World Wildlife Fund. China: an emerging offshore wind development hot-spot. Report; 2010. Available from: http://www.wwf.no/?30761.

[85] An urgent need for three main technologies in China’s offshore windpower development. Available from: http://newenergy.in-en.com/html/newenergy-1327132791627991.html [assessed 04.20.2010].

[86] Zhang Lida, Lachun Ren, Jinduo Mao, XuWang, Qingxiang Meng. The analysison influence of wind turbine generating units in trocious weather conditions.In: Proceedings of ISES solar world congress 2007: solar energy and humansettlement; 18e21 September, 2007 [Beijing, China].

[87] Kuroiwa T, Karikomi K, Hayashi Y, Shibata M, Ueda Y. New products andtechnologies of Mitsubishi wind turbines. Technical Review 2004;41(3):1e5.

[88] Ueda Y, Shibata M. Development of next generation 2MW class large windturbines. Technical Review 2004;41(5):1e4.

[89] MITSUBISHI. Design Technology in Wind Turbines: “Smart Yaw”-the newconcept in load reduction. Available from: http://www.mhi.co.jp/en/power/news/sec1/2004_mar_05.html [accessed 03.2004].

[90] Wang Jingquan, Chen Zhengqing. Analysis of risks and measures on the bladedamage of offshorewind turbine during strong typhoons-enlightenment fromRed Bay wind farm. Engineering Science 2010;12(11):32e4 [in Chinese].

[91] Wang Shuai. Analysis of impact on safety operation of wind generating setby natural environment. Journal of Safety Science and Technology 2009;5(6):214e8.

[92] Yan Hongwen, Cai Quan, Tian Hongqi. Pilot study on anti-typhoon of towerdesign. Machinery Design and Manufacture 2011;2:34e6 [in Chinese].

[93] Tang Weiliang, Qi Yuan, Zhonghe Han. Withstanding typhoon design of windturbine tower. Acta Energiae Solaris SINCIA 2008;29(4):422e7.

[94] Garciano L, Koike T. A proposed typhoon resistant design of a wind turbinetower in the Philippines. Journal of Construction Engineering and Manage-ment 2007;63(2):181e189l.

[95] Exo KM, Huppop O, Garthe S. Birds and offshore wind farms: a hot topic inmarine ecology. Wader Study Group Bulletin 2003;100:50e3.

[96] Fox AD, Desholm M, Kahlert J, Christensen TK, Petersen IK. Informationneeds to support environmental impact assessment of the effects of Euro-pean marine offshore wind farms on birds. The International Journal of AvianScience 2006;148:129e44.

[97] Huppop O, Dierschke J, Exo KM, Fredrich E, Hill R. Bird migration studies andpotential collision risk with offshore wind turbines. The International Journalof Avian Science 2006;148:90e109.

[98] Drewitt AL, Langston RHW. Assessing the impacts of wind farms on birds.The International Journal of Avian Science 2006;148:29e42.

[99] Desholm M, Fox AD, Beasley P, Kahlert J. Remote techniques for counting andestimating the number of bird-wind turbine collision at sea: a review. Inwind, fire and water: renewable energy and birds. The International Journalof Avian Science 2006;148(suppl.1):76e89.

[100] Desholm M, Kahlert J. Avian collision risk at an offshore wind farm. BiologyLetters 2005;1(3):296e8.

[101] Desholm M. Thermal animal detection systems (TADS). Development ofa method for estimating collision frequency of migrating birds at offshorewind turbines. NERI technical report No. 440. Rønde, Denmark: NationalEnvironmental Research Institute; 2003. Available from: http://www2.dmu.dk/1_viden/2_publikationer/_fagrapporter/rapporter/fr440.pdf.

[102] Petersen IK, Christensen TK, Kahlert J, Desholm M, Fox AD. Final results ofbird studies at the offshore wind farms at Nysted and Horns Rev, Denmark.

X. Sun et al. / Energy 41 (2012) 298e312312

Denmark: Report to Dong Energy and Vattenfall A/S, National EnvironmentalResearch Institute; 2006. Available from: http://www.folkecenter.net/mediafiles/folkecenter/pdf/Final_results_of_bird_studies_at_the_offshore_wind_farms_at_Nysted_and_Horns_Rev_Denmark.pdf.

[103] Masden EA, Haydon DT, Fox AD, Furness RW. Barriers to movement:modeling energetic costs of avoiding marine wind farms amongst breedingseabirds. Marine Pollution Bulletin 2010;60(7):1085e91.

[104] Masden EA, Haydon DT, Fox AD, Furness RW, Bullman R, Desholm M.Barriers to movement: impacts of wind farms on migrating birds. ICESJournal of Marine Science 2009;66(4):746e53.

[105] Petersen IK, Clausager I, Christensen TJ. Bird numbers and distribution in theHorns Rev. offshore wind farm area. Denmark: Report to Elsam EngineeringA/S, National Environmental Research Institute; 2004. Available from: http://www.hornsrev.dk/Miljoeforhold/miljoerapporter/horns%20rev%20bird%20numbers.pdf.

[106] Richardson WJ. Marine mammals and man-made noise: current issue.Proceedings of the Institute of Acoustics 1997;19(9):39e50.

[107] Bailey HR, Senior BL, Simmons D, Rusin J, Picken G, Thompson PM. Assessingunderwater noise levels during pile-driving at an offshore windfarm and itspotential effects on marine mammals. Marine Pollution Bulletin 2010;60(6):888e97.

[108] Tougaard J, Madsen PT, Wahlberg M. Underwater noise from constructionand operation of offshore wind farms. Bioacoustics 2008;17(1e3):143e6.

[109] Slabbekoorn H, Bouton N, van Opzeeland I, Coers A, ten Cate C, Popper A.A noisy spring: the impact of globally rising underwater sound levels on fish.Trends in Ecology and Evolution 2010;25(7):419e27.

[110] WahlbergM,Westerberg H. Hearing in fish and their reactions to sounds fromoffshore wind farms. Marine Ecology Progress Series 2005;288:295e309.

[111] Thomsen F, Lüdemann K, Kafemann R, Piper W. Effect of offshore wind farmnoise on marine mammals and fish. Biola, Hamburg, Germany on behalf ofCOWRIE Ltd.; 2006.

[112] Petersen JK, Malm T. Offshore windmill farms: threats to or possibilities forthe marine environment. A Journal of the Human Environment 2006;35(2):75e80.

[113] Öhman MC, Sigray P, Westerberg H. Offshore windmills and the effects ofelectromagnetic fields on fish. A Journal of the Human Environment 2007;36(8):630e3.

[114] Inger R, Attrill MJ, Bearhop S, Broderick AC, Grecian WJ, Hodgson D, et al.Marine renewable energy: potential benefits to biodiversity? An urgent callfor research. Journal of Applied Ecology 2009;46(6):1145e53.

[115] Wilhelmsson D, Malm T, Ohman M. The influence of offshore windpower ondemersal fish. ICES Journal of Marine Science 2006;63(5):775e84.