An Overview of Offshore Wind Energy

An Overview of Offshore Wind Energy

by

C. N. Elkinton and J. G. McGowan

Renewable Energy Research Laboratory

Department of Mechanical and Industrial Engineering

University of Massachusetts

160 Governors Dr., Amherst, MA 01003 USA

1INTRODUCTION

This chapter will review the history, current technology, and state-of-the-art of offshore wind energy. Similar to land based wind systems, the topic of offshore wind energy is quite broad and encompasses technology, environmental issues, economics, etc.

The basic component of an offshore wind system is the offshore wind turbine which is defined as “a wind turbine with a support structure which is subject to hydrodynamic loading” [1]. The components of an offshore wind turbine are shown in Figure 1. This type of wind turbine consists of the following components [1]:

?Rotor-Nacelle Assembly – part of a wind turbine carried by the support structure o Rotor – part of a wind turbine consisting of the blades and hub*

o Nacelle Assembly – part of a wind turbine consisting of all the components above the tower that are not part of the rotor. This includes, principally, the drive train

(shafts, couplings, gearbox, generator(s), and brakes) and the nacelle enclosure.*

?Support Structure – part of an offshore wind turbine consisting of the tower, sub-structure and foundation

o Tower – part of an offshore wind turbine support structure which connects the sub-structure to the rotor-nacelle assembly

o Sub-Structure – part of an offshore wind turbine support structure which extends upwards from the seabed and connects the foundation to the tower

o Foundation – part of an offshore wind turbine support structure which transfers the loads acting on the structure into the seabed. Different foundation concepts

are shown together with the other main parts of an offshore wind turbine in Figure

1.

* This definition was contributed by the authors and is not found in the reference cited.

Figure 1. Components of an offshore wind turbine [1]

Offshore wind turbines have several advantages over onshore turbines [2]. These include:

?Greater area available for siting large projects

?Often, close proximity to cities and other load centers

?Generally higher wind speeds compared with onshore locations

?Lower turbulence intensities, which extends the fatigue life of the turbine components ?Lower wind shear, which allows shorter towers to be used, thus saving money

?Potentially less obvious visual impact, which may help with public acceptance issues There are, of course, some disadvantages as well, most notably:

?Higher project costs due to

o Necessity of specialized installation and service vessels and equipment

o More expensive support structures

?More difficult working conditions

?More difficult and expensive installation procedures

?Decreased availability due to limited accessibility for maintenance

?Necessity of special corrosion prevention measures

2HISTORY

The concepts of offshore wind turbines and offshore wind farms (consisting of multiple turbines) have existed for decades. In the US, Williams Heronemus, a naval architect and professor of Ocean Engineering at the University of Massachusetts, was the first to write seriously on the topic [3]. His ideas were often ahead of their time in the 1970s—for example, he envisioned floating, offshore wind turbines that produced hydrogen instead of electricity. As today’s society starts to contemplate a hydrogen-based transportation sector, this idea is certainly relevant.

In Europe, the first studies investigating offshore wind energy were published in the late 1970s and early 1980s [4]. These studies were focused on the offshore wind resource and on the feasibility of constructing turbines offshore. The concepts investigated in these early studies involved large (by today’s standards) farms, capable of generating hundreds of megawatts of electricity, tens of kilometers from shore. Like the early US studies, these concepts were ahead of their time. While no turbines were built and no electricity was generated during these early studies, they laid the groundwork for the first round of offshore projects.

The first offshore wind turbine was installed near Nogersund, Sweden, in 1990 (operation was terminated in 1998 after a fire) [5]. During the 1990, a handful of offshore projects were constructed, including the Vindeby wind farm in Denmark, which, with 11 turbines in 3-5 m of water 1.5 km from shore, was the first demonstration of a utility-scale offshore wind farm. The Middelgrunden wind farm, built in 2001 just outside Copenhagen harbor in Denmark, was the second utility-scale farm. In 2002 and 2003, the first large, utility-scale offshore farms were commissioned. The Horns Rev and Nysted wind farms, both in Denmark, were the first farms built with capacities exceeding 100 MW, and they remain the only such farms to date. Further details from the existing offshore wind farms is given in

Table 1. The history of offshore installation by country is given in Figure 2.

Table 1. Currently operating offshore wind farms [6]

Wind Farm Year built Total capacity

(MW) Number of turbines

Vindeby (DK) 1991 4.95 11 Lely (NL) 1994 2 4 Tun? Knob (DK) 1995 5 10 Irene Vorrink (NL) 1996 16.8 28 Bockstigen-Valor (SE) 1998 2.5 5 Blyth (UK) 2000 4 2 Utgrunden (SE) 2000 10 7 Middelgrunden (DK) 2001 40 20 Yttre Stengrund (SE) 2001 10 5 Horns Rev (DK) 2002 160 80 Sams? (DK) 2003 23 10 R?nland (DK) 2003 17.2 8 Frederikshavn (DK) 2003 10.6 4 Nysted (DK) 2003 165.6 72 Arklow Bank (IE) 2003 25.2 7 North Hoyle (UK) 2003 60 30 Scroby Sands (UK) 2004 60 30 Ems-Emden (DE) 2004 4.5 1 Kentish Flats (UK) 2005 90 30 Breitling (DE) 2006 2.3 1 Barrow (UK) 2006 90 30 TOTAL 2006 803.65 395

199199199199199199199199199199200200200200200200200Year

C a p a c i t y I n s t a l l e d [M W ]

C u m u l a t i v e C a p a c i t y [M W ]

Figure 2. Cumulative and annual offshore installed capacity [6]

3

CURRENT STATUS

3.1 Current Wind Farms

At the time of this writing, there are 21 offshore wind farms in operation with a combined installed capacity of 804 MW from 395 turbines [6]. These farms are located in 6 European countries with the distribution of installed capacity given in Figure 3.

18.8 MW 2%

3%

Figure 3. World-wide distribution of offshore wind farms to date [6]

The state-of-the-art of offshore wind turbines can be seen in several of the recently installed farms, e.g. Horns Rev, Nysted, Arklow Bank, North Hoyle, Kentish Flats, Scroby Sands, and Barrow. These farms all utilize turbines with rated capacities of or exceeding 2 MW, all are installed in water no more than 20 m deep, and all are less than 15 km from shore. Rotor diameters have been increasing with turbine capacity and are now around 90-100 m. The hub height is typically approximately 25 m greater than the rotor radius, a height presumably chosen to facilitate construction at a reasonable cost yet still allow adequate clearance for boat traffic.

The turbines in most of these farms are installed on monopiles, with the exception of Nysted, which uses gravity caissons made of concrete and filled with ballast. The arrangement of the turbines within the farm is typically a grid or skewed grid pattern. A summary of the state-of-the-art of offshore wind technology is provided in Table 2.

Table 2. State-of-the-Art of Offshore Wind Technology

Turbine rated power 2-3.6 MW

Farm rated power 60-165 MW

Rotor diameter 90-100 m

Hub height 70 m

Foundation Monopile or gravity caisson

Water depth < 20 m

Distance from shore < 15 km

Project cost $1450-2200/kW

Capacity factor 38-42%

Inter-turbine electrical system Medium voltage AC, 33 kV

Transmission system High voltage AC, 132-150 kV

The largest turbine designed for offshore siting in production is the GE Energy 3.6 MW, 104 m diameter turbine [7]. Siemens also has a 3.6 MW turbine, with a rotor diameter of 107 m, but it is still undergoing prototype testing.. The rotor-nacelle assemblies of these machines weigh approximately 200 metric tonnes. The components (especially the blades) are so large that, due to transportation limitations, they are primarily intended for offshore installation. As the industry continues to grow and technology continues to improve, larger turbines will be developed. Several manufacturers have 5-6 MW prototype turbines installed and operating [8].

3.2Proposed Wind Farms

Several thousand megawatts of offshore wind capacity have been proposed in Europe and in the US. According to the speculations of the Danish Energy Authority [9], the largest concentration of proposed projects through 2009 are in the UK and Germany, with Denmark, Spain, Ireland, the US, France, The Netherlands, Canada, Sweden, and Belgium contributing as well.

Among these proposed offshore farms are extensions of the existing Horns Rev, Nysted, and Arklow Bank farms. The Horns Rev II and Nysted II projects have proposed capacities of 200 MW each and new additions to the Arklow Bank farm would bring its total capacity to 520 MW. Also proposed are several German farms in an area that has been specially designated for offshore development, and several farms in the US.

4TECHNOLOGY

In this section we will review technological trends in wind turbine design for offshore applications, the offshore wind resource and climate, offshore support structures, electrical and interconnection issues.

4.1 Technology Trends

Offshore turbines may look like onshore turbines installed in the water, but there are some significant differences between these two types of machines. These differences include

Onshore

Offshore

The support structure is subject to horizontal loading from wind forces only

The support structure is subject to horizontal wind, wave, and current loads

The size of components (e.g. blades) is limited by the site access roads.

The size of components is limited by the lifting capacity of the specialized offshore crane vessels.

Rotational speeds are reduced to reduce the

aerodynamic noise from the turbine at the expense of efficiency.

Aerodynamic noise is not a concern, so rotational speeds can be increased for optimal efficiency.

As shown in Figure 4, the current trends in offshore wind farm design have been the use of larger wind turbines and the movement of offshore wind farms to locations further offshore.

Year

R o t o r D i a m e t e r [m ]

(a) Year

D i s t a n c e f r o m S h o r e [k m ]

(b)

Figure 4. Trends in rotor diameter (a) and the distance from shore (b) from offshore wind farms over time [6]

A wind turbine designed for an offshore installation must be more robust than an on-land design. That is, it must be designed for offshore environmental conditions (e.g., saline and damp climate) and the effects of wind and wave interactions on the entire structure. The initial installations of offshore wind turbines in Europe featured the use of land based wind turbines that were modified for use in the offshore environment (defined as the marinisation approach). Thus, the wind

turbines used in the first offshore installations featured such modifications as larger generators, a higher instrumentation specification, and component redundancy [2]. As more machines have been installed in European offshore sites, turbine manufacturers now offer machines specifically designed for the offshore market. For example, a description of the General Electric 3.6 MW wind turbine that was expressly designed for the offshore market is given by Fric, et al [7]. As pointed out by these authors, in addition to special designs for offshore wind turbines

(marinization), site-specific analyses are being used to assess offshore turbine accessibility and wind/sea conditions.

Since the design requirements for offshore wind turbines differ from those on land, other design modifications may include [2]:

?Larger machines, up to 5 or 10 MW;

?Faster rotational speeds than on land, where noise restrictions generally mean that the turbine operates slightly below optimum speed;

?Larger generator for a specific rotor size, this enables additional available energy to be obtained;

?High voltage generation, possibly in DC instead of AC.

In addition, the wind turbine/ support structure must be designed to withstand the offshore environment.

The future trends in offshore wind technology have been recently addressed in a paper by Jamieson [10]. As shown in Figure 5, the technical characteristics (for both onshore and offshore wind turbines) have evolved from the original wind turbine designs with stall regulated, fixed speed, and geared transmission. Today, the current utility-scale wind turbines feature pitch regulated, variable speed, and direct drive or geared transmissions.

?

Figure 5. Technical Characteristics of Utility-Scale Wind Turbines Evolution with Time [11]

Jamieson also notes that for offshore wind turbines the newest risk is accessibility and that to overcome this technical problem real (re-engineered) offshore wind turbines are required.

4.2Wind Resource and Climate

The ocean surface is, generally, smoother than the surface of the land. This leads to a lower boundary layer which, in turn, results in lower turbulence levels and wind shear. Low wind shear means that the wind speed does not change much with height—over the face of the rotor, say—compared to sites with high shear. At a site with high wind shear, the large turbines must vary the blade pitch constantly in order to maintain uniform thrust over the rotor face. This constant pitching causes the pitch mechanisms to require more frequent maintenance. In area with low shear, the need for constant pitching is reduced, which ultimately reduces the operation and maintenance costs.

Another benefit of low wind shear is the availability of sufficient wind speeds at low elevations. This provides good energy capture (capacity factors generally greater than on land) without requiring tall towers. In fact, the difference between the hub height and the rotor radius is usually smaller offshore than onshore.

The low turbulence levels generally measured offshore is both advantageous and detrimental to offshore wind farm development. On the one hand, low turbulence intensities result in low fatigue damage and longer component life. On the other, the low turbulence levels extend the wake effects of turbines requiring greater separation between turbines within a farm. The turbines at the Horns Rev farm are separated by 7 rotor diameters (7 D) in the prevailing

direction and 7 D in the crosswind direction, while those at Nysted have separations of 10.5 D in the prevailing direction and 5.8 D in the crosswind direction. For comparison, onshore, a turbine separation on the order of 8 – 10 D in the prevailing wind direction and 5 D in the crosswind direction will typically result in array losses of less than 10% [12]. The difference in wake effects between onshore and offshore sites is easily seen in the following example.

Several turbine wake models have been developed (see [13] or [14] for descriptions of several of the most popular wake models). The mathematical model first developed by [15] and refined by [16] is based on conservation of momentum and only takes into account wind speed deficits.

20121!

"#$%

&+=!!"#$$%

&'=R x k b U U Deficit

where U is the downstream wind speed, U 0 is the free-stream wind speed, b is the axial induction factor, k is the wake entrainment factor, x is the distance downstream, and R is the rotor diameter. An empirical relationship for k was developed by Frandsen which relates k to the hub height, z H and the surface roughness length, z 0 [17].

!!"

#$$%&=

0ln 5

.0z z k H

Using b = 0.1, z H = 80 m, and D = 90 m, Figure 6 shows the wind speed deficit for four different values of z 0 corresponding to calm sea, rough sea, pasture, and crops (the

roughness values for each of these conditions are given in [18]). The figure shows that, in this case, it takes almost twice the distance for the wind speed deficit to drop below 10% for the “calm sea” case as for the “crops” case. This is entirely due to the difference in turbulence intensity.

4.3Support Structures

Significant information on the design and construction of offshore support structures has been learned through collaboration with the oil and gas industry. Wind turbine support structures differ from typical offshore oil and gas structures, however, in terms of the loads that the structures are designed to withstand. For wind turbines, the horizontal loads (wind and wave loads) and overturning moment are greater than the vertical loads (weight of the structure), whereas it is the opposite for oil and gas platforms [19].

The two types of offshore support sub-structures that have been used in wind farms to date are the monopile and gravity caisson, or gravity base. The monopile has proven to be the sub-structure of choice because is has been the most economical solution at the sandy sites where most farms are currently located. Reference [11] summarizes the sub-structure concepts currently employed and those under serious consideration in the near future. Illustrations of the more popular sub-structures are shown in Figure 7 [20].

Table 3. Support sub-structure concepts [11]

Sub-Structure Type Application Advantage Disadvantage

Monopile Most conditions, preferably

shallow water and not deep

soft material. Up to 4 m

diameter. Diameters of 5-

6 m are the next step Single, light, versatile. Of

lengths up to 35 m.

Expensive installation due

to size. May require pre-

drilling a socket. Difficult to

remove.

Multiple piles (e.g. tripod) Most conditions, preferably

not deep soft material.

Suits water depth above

30 m

Very rigid and versatile. Very expensive

construction and

installation. Difficult to

remove.

Concrete gravity base Virtually all soil conditions. Float-out installation. Expensive due to large

weight.

Steel gravity base Virtually all soil conditions.

Deeper water than

concrete. Lighter than concrete.

Easier transportation and

installation. Lower expense

since the same crane can

be used as for erection of

turbine.

Costly in areas with

significant erosion.

Requires a cathodic

protection system. Costly

compared with concrete in

shallow water.

Mono-suction caisson Sands, soft clays. Inexpensive installation.

Easy removal. Installation proven in limited range of materials.

Multiple suction caisson (e.g. tripod) Sands, soft clays. Deeper

water.

Inexpensive installation.

Easy removal.

Installation proven in limited

range of materials. More

expensive construction.

Floating Deep waters – 100 m. Inexpensive foundation

construction. Less

sensitive to water depth

than other types. Non-rigid,

so lower wave loads. High mooring and platform costs. Excludes fishing and navigation from most areas of farm.

(a) Monopile

(b) Gravity base

(c) Tripod

(d) Floating

Figure 7. Support sub-structure concepts [20]

4.4 Electrical Interconnection

The electrical interconnection system in an offshore wind farm has two components. Within the farm, the electrical collection system usually connects the turbines to a central node, often a step-up transformer. From this central transformer, the wind farm is connected to land by the electrical transmission system.

The current state-of-the-art electrical collection system is comprised of medium voltage

(10-100 kV) AC cables. Each cable typically connects several, but not all, of the turbines to the central transformer. Each collection cable has its own circuit breaker system so that if one cable fails, the rest of the farm can remain in operation.

Electrical transmission systems are typically high voltage systems and can be comprised of multiple sets of cables. Multiple cables allow for redundancy. A fault in the transmission system can be costly to repair. If it also requires stopping production from the wind farm, revenue is lost as well. Using multiple transmission cables is one way of reducing this type of revenue loss.

The collection and transmission cables are usually buried below the surface of the sea floor through a process of trenching and backfilling. This process is usually performed using an underwater plow, although saws may be required to cut through rock. Directional drilling is sometimes used when minimum surface disturbance is required, such as when the cable come ashore in areas with sensitive habitats.

Additional information on offshore cable laying and the cables themselves is available in [21; 22; 23; 24]

5ECONOMICS

Several studies and reviews of the economics of offshore wind energy have been published in the last decade. These studies discuss a much broader range of topics than can be covered here and the reader is encouraged to consult these papers for more information [25; 26; 27; 28].

5.1Comparison with Onshore Economics

Offshore project costs offshore are currently higher than they are for onshore installations, due principally to more expensive support structures, electrical interconnections, and installations. Junginger et al. summarized the cost differences between onshore and offshore [29]. They found that typical onshore installed costs ranged from 800-1100 €/kW (approximately 950 – 1300

$/kW, assuming 1.2 $/€), compared with offshore costs of 1200-1850 €/kW (1450-2200 $/kW). The higher energy yield offshore does not quite balance out these higher project costs. The net result is a higher cost of energy from offshore projects: 6-12 €ct/kWh (7-14.5 ￠/kWh) compared with 3-8 €ct/kWh (3.5-9.5 ￠/kWh) onshore.

The most noticeable difference between the economics of onshore and offshore wind is the distribution of the costs. Looking at the project costs, which do not include operation and maintenance (O&M) costs, there are four major cost categories for a wind farm: the turbines, the support structures, the electrical interconnection with the grid, and the installation. Figure 8 shows the typical contribution of each of these categories to the total project cost [20]. It should be noted that, in the onshore case, the installation cost has been included in the other four categories shown. Onshore, the turbines comprise the large majority of the project cost, while offshore, they represent less than half of the total cost. Going from onshore to offshore causes the contributions of the support structures and grid connections to almost double.

Figure 8. Typical cost comparison between onshore and offshore wind [20]

The Opti-OWECS study, which was the definitive work on the subject of offshore wind when it was published in 1998, suggested that for an offshore wind system, the turbine, support structure, and electrical interconnection would contribute equally to the project cost. As turbines are installed further from shore, they speculated that the relative contribution of the turbines will decrease [30].

Another significant factor in offshore wind economics is the cost of O&M. O&M costs factor into the cost of energy, or levelized production cost (LPC), which is usually given in units of cost per kWh. Research in The Netherlands found that the O&M costs accounted for approximately 25-30% of the LPC, compared to 10-15% onshore [31].

5.2Market

Several recent studies have pointed to the large and mostly untapped offshore wind resource. The potential outside Europe, which is the only part of the world to have started to capitalize on its offshore wind resource, has been estimated at an annual 4,600 TWh, which was more than one quarter of the electric demand world-wide [27].

In the U.S. The National Renewable Energy Laboratory (NREL) released a study in 2004 which estimated the available offshore wind energy resource, within 50 nautical miles (92.6 km) of shore, at almost 1,000 GW, which is exceeds the total U.S. electric demand [32]. If all of this capacity were installed, assuming a reasonable production cost of 5 ￠/kWh and a capacity factor of 35%, revenue in the US market would exceed $150 billion.

In Europe, an estimated 150,000 square km of shallow ocean exists on which offshore wind farms might be built [33]. Depending on the technology and the specific site conditions, this area could equate to a capacity exceeding 500 GW which, if the same economic conditions are assumed, suggests revenue on the order of $85 billion or greater.

At the present time, most future forecasts for offshore wind development have been confined to the European market. A number of the most recent estimates are shown in Figure 9 [11; 34]. As shown, the latest European Wind Energy Association (EWEA) goals [35] are for 70,000 MW of

offshore wind energy capacity to be installed by 2020 (compared to 110,000 MW installed on land).

Figure 9. Projected Growth of Offshore Wind Energy in the European Market [34]

According to the Danish Energy Authority [9] it is expected that there will be a major expansion of offshore wind farms in the next 5 years with the UK and Germany becoming major markets. These predictions are shown in Figure 10 (the yearly MW totals are not cumulative and represent a single year’s addition). It should be noted that the predictions shown in this figure are for single years and are not cumulative. Their stated predictions also forecast that in 2009 offshore wind farms will account for about 7% of the total global capacity.

Figure 10. Expected Global Offshore Wind Power Expansion to 2009 [9]

6ENVIRONMENTAL ASPECTS

Two types of environmental impact have been observed at offshore wind farms: disturbances during construction and maintenance activities, and permanent alteration of the local habitat due to the introduction of the foundations and scour protection. These impacts affect all aspects of

marine life in the area either temporarily or permanently. The major types of impacts and the latest research and data on them are described below.

Environmental impacts are a topic of much discussion and many offshore wind energy conferences have sessions dedicated to this discussion. These conference papers (e.g., see the Proceeding of the 2006 EWEC conference [36]) will provide the reader with the latest information about the environmental aspects of offshore wind. The two largest offshore wind farms, Horns Rev and Nysted, have been the focuses of several environmental impact studies before, during, and after their construction [37]. Another excellent source of information is available on the Offshore Windenergy Europe website [38].

In the following sections we will briefly review important aspects of the following environmental impacts of offshore wind systems: birds, fish, marine mammals, benthos, and global aspects.

6.1Birds

Bird collision is usually the most important environmental impact for any wind power project. It can be expected that there will be collisions by migrating birds or birds feeding in the area. The turbines may also become a barrier within migration routes or between the normal feeding and nesting grounds. The presence of turbines may also cause some birds to leave their traditional feeding or nesting areas.

Data on bird behavior and interaction with turbines were taken on several of the older farms and a considerable effort has been made to take data on the newer farms. Studies using radar, airplanes, and telescopes have been conducted before, during, and after construction to monitor the impact of the wind farms on bird behavior. As shown in Figure 11, early data from the studies at Horns Rev and Nysted, while not conclusive, suggest that migrating birds tend to detect the presence of the wind farm at least 1 km before reaching it and either correct their trajectory so as to fly between the turbines or avoid the area all together [37].

Figure 11. Flight paths of birds, observed using radar, though and around the Nysted offshore wind farm, autumn

2004 [37]

6.2Fish

Impacts on fish populations are important both from an environmental perspective and from the perspective of fishermen. The potential detriments to fish include noise and vibration during construction, operation, and maintenance; increased sedimentation during construction; alteration of the distribution of species due to changed habitat; and effects from electromagnetic fields generated by the electrical cables.

6.3Marine Mammals

The studies at Horns Rev and Nysted focused on wind farm impacts on porpoises and seals. Both could be affected by the construction and operation of the farm as well as the presence of the turbines in their habitat. During construction, especially when piles or sheet piles were driven, deterrents were used to make the animals to leave the construction area temporarily so as to avoid hearing damage. When the pile driving ceased and the deterrents were deactivated, the animals returned within a few hours. Data from the first few years of operation have been collected and the populations of porpoises and seals and their hunting and breeding activities do not appear to have been affected by the introduction of the wind farms.

6.4Benthos

Dredging for the electrical cables and construction of the support structures is expected to cause the most disturbance to benthic plants and animals. These activities cause an increase in the amount of sediment suspended in the water, also known as turbidity, which can clog small organisms’ feeding mechanisms and shade sea floor vegetation. Studies at Horns Rev and Nysted do not show permanent damage to the benthic communities, although again, the data are insufficient to draw general conclusions at this time. At Horns Rev, the most common species

have returned but there is conflicting evidence about whether their numbers have grown or have simply returned to pre-farm levels. At Nysted, while the area disturbed by the cable laying had an estimated width of 10 m, the species studied are expected to recover fully [39].

6.5Global

As is the case with onshore wind, the most obvious global benefit is the ability to produce electricity without producing toxic pollutants, e.g. CO2, SO2, and NOx. The ultimate question of whether this benefit balances out the detrimental impacts such as bird collisions, has yet to be fully answered. The debates over this broad and important topic will doubtless continue into the foreseeable future.

7MAJOR OBSTACLES

Significant improvements in offshore turbine technology have lead to reduced cost and increased production, but several major hurdles still exist before offshore wind energy could be considered to be a mainstream source of electrical energy. Synergies between the offshore wind industry and the oil and gas industry are being developed and it is anticipated that many advances will be made through this process.

7.1Offshore support structures

The support structures are one of the most expensive components of an offshore wind farm, potentially comprising 25-35% of the total project cost. Reducing their costs would equate to lower project costs and lower costs of energy. Various means of such cost reduction are under investigation including:

?choice of concrete or steel, depending on market prices for each material (see, for example, [40]),

?different sub-structure geometries including tripods, quadripods, and guyed or lattice towers [11; 41]

?different types of foundations e.g. suction caissons

?reducing the size of components so that they can be installed by standard vessels, not the custom vessels that are currently required.

7.2Installation and Maintenance

Limited site accessibility makes installation and maintenance difficult, time-consuming, and expensive. Crane vessels capable of operating in waters with significant wave heights greater than the current 1.5-2 m maximum would help to increase a farm’s availability. One of the largest time-saving measures employed to date has been the reduction of installation work at sea. By assembling most of the components at the dock, then transporting them to the wind farm, the time required at sea, and the corresponding expense, is reduced.

For routine maintenance, when a large vessel is not required, small boats are usually used to access the turbines. The boats used at Horns Rev, for example, can provide access to the

turbines when the significant wave height is less than 1.1 m [42]. At Horns Rev, this equates to approximately 60% of the year. To gain access to the turbines during periods of larger seas, helicopters have been used. Helicopters require wind speeds less than 15-20 m/s, which means that the Horns Rev turbines are accessible by helicopter 90% of the year. For this farm, the addition of helicopter platforms on the backs of the nacelles has resulted in a significant increase in farm availability and, therefore, energy production. In this case, the increased production outweighed the increased cost of the helicopter.

7.3Offshore Environment

Environmental conditions impact the cost of offshore wind turbines. The most important design considerations resulting from these conditions include:

?sealed and/or slightly pressurized nacelles to repel the salty, humid air

?scour protection around the bases of support structures to minimize damage caused by ocean currents

?corrosion protection to minimize the corrosive effects near the sea-air interface

?properly applied marine coatings to prolong the life of components in this corrosive environment

7.4Design Standards

Internationally adopted design standards for offshore wind turbines do not yet exist, but there are standards in place that have been widely adopted [43]. The design standards that do exist are the following:

?International Electrotechnical Commission (IEC) 61400-3: Wind turbines – Part 3, Design requirements for offshore wind turbines, draft [44]. This standard in a work in

progress and is speculated to be widely adopted in Europe when it is published. Whether or not it will be adopted in the U.S. is still unclear.

?IEC 61400-1: Wind turbines – Part 1, Design requirements, 2005 [45]. This standard deals with onshore turbines and is the foundation of the 61400-3 standard.

?DNV offshore standard DNV-OS-J101: Design of offshore wind turbine structures, 2004

[46].

?Germanischer Lloyd (GL): Guidelines for the Certification of Offshore Wind Turbines, 2005 [47].

7.5Regulatory Hurdles

The work and expenditure required during the regulation and permitting process for an offshore wind projects depends largely on where the project is proposed. A detailed description for each country where offshore wind is being considered is beyond the scope of this work, so two case studies, one from the U.S. and one from Europe, will be presented. The difference in the duration of the two permitting processes is remarkable.

In the U.S., the question of jurisdiction over the regulatory process for offshore wind is in a state of flux. The first and longest-standing proposed offshore wind farm is the Cape Wind project off

the coast of Massachusetts. This first proposed project has brought with it a multitude of questions about the regulatory process including the question of who has jurisdiction over offshore property in federal waters. When the Cape Wind project was proposed in 2001, the U.S. Army Corps of Engineers was the lead regulatory agency for all offshore installations. In 2005, the Minerals Management Service of the Department of the Interior assumed the responsibility for offshore renewable energy projects in federal waters. They also regulate offshore oil and gas projects. Before the Cape Wind project can proceed, it must get approval from 17 state and federal agencies [41]. They currently project that the permitting process will be complete by the end of 2007 [48].

The Middelgrunden wind farm was the largest offshore farm when it was built in 2001. It is located 3 km off the coast of Copenhagen, Denmark. The permitting process began in June, 1997 and the appropriate approval was given in November, 1999. By the end of January, 2001, the wind farm was operational.

8SUMMARY

This chapter has shown that offshore wind energy has established a good platform for future growth. In general, its future success will depend on a concerted world wind research and development effort. To be successful, the offshore and onshore research should be integrated as closely as possible. The European Community [11] established a set of research objectives which are still applicable:

?Combined wind and wave loading studies

?Design studies of systems rated above 5 MW for offshore, possibly including multi-rotor systems [10]

?Development of alternative, and deep water, support structures

?Higher tip speed designs, as noise issues are less significant offshore

?Legal research into offshore ownership in coastal waters. Exclusive economic zones, etc.

?Minimization of O&M related downtimes. The distance offshore and the water depth at the site have significant impacts on O&M

?Monitoring of environmental impacts of near and far offshore projects

?Offshore meteorology: Short and long-term forecasting; Hardware for measurements ?Research on potential conflicts of interest: defense, fisheries, shipping, oil and gas exploration and pipelines, etc.

?Special designs of systems and components for erection, access, and maintenance of offshore wind turbines

As evidenced by recent work in this area, it appears that most, if not all, of these research and development areas for offshore wind development are in progress.

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