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Energy from Offshore Wind 

Preprint 

W. Musial and S. Butterfield 

National Renewable Energy Laboratory 

B. Ram 

Energetics, Inc.

To be presented at Offshore Technology Conference 

Houston, Texas 

May 1–4, 2006 

Conference Paper 

NREL/CP-500-39450 

February 2006 
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18355 (OER) 

Energy from Offshore Wind 

Walt Musial and Sandy Butterfield, National Renewable Energy Laboratory, Golden, CO 

Bonnie Ram, Energetics, Inc. Washington D.C. 

Copyright 2006, Offshore Technology Conference 

This paper was prepared for presentation at the 2006 Offshore Technology Conference held in 

Houston, Texas, U.S.A., 1–4 May 2006. 

 

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Abstract 

This paper provides an overview of the nascent offshore wind 

energy industry including a status of the commercial offshore 

industry and the technologies that will be needed for full 

market development. It provides a perspective on the status of 

the critical environmental and regulatory issues for offshore 

wind and how they are affecting the formation of the U.S. 

industry. The rationale provided describes why offshore wind 

has the potential to become a major component of the national 

electric energy supply. Future projections show this potential 

could result in over $100 billion of revenue to the offshore 

industry over the next 30 years in the construction and 

operation of offshore wind turbines and the infrastructure 

needed to support them. The paper covers technical issues and 

design challenges needed to achieve economic 

competitiveness for near term deployments in shallow water 

below 30-m depth. It also examines the requirements for 

future technologies needed to deploy systems in deeper water 

beyond the current depth limits. Although most studies to date 

indicate very low impacts to the environment, regulatory and 

environmental barriers have hindered the first offshore wind 

projects in the United States. A summary of these issues is 

given. 

 Introduction 

Over the past two decades, on-shore wind energy technology 

has seen a ten-fold reduction in cost and is now competitive 

with fossil and nuclear fuels for electric power generation in 

many areas of the United States. Wind energy installations in 

the United States have grown from about 1,800 MW in 1990 

to an estimated 9,200 MW at the end of 2005, and are 

expected to grow to 14,000 MW by the end of 2007 [1]. 

While onshore wind energy technology appears to be maturing 

rapidly by some measure, the need for further technology 

development still remains, as development booms have 

historically coincided with the existence of the 1.9-cent/kWh 

production energy tax credit for renewable energy sources. In 

addition, as wind energy penetrates a larger percentage of the 

grid, industry growth, dispatchability, and infrastructure, 

barriers will become critical long-term research issues. 

Initial onshore wind development in the United States focused 

on the windiest sites (Class 6 that average 7.4 m/s at 10 m 

above surface annually), but these sites are generally in the 

more remote areas of the west, and on a few ridgelines in the 

east. The DOE Wind Program has led an initiative to drive the 

cost of wind energy down further through sustained 

technology innovations that have been identified, but have not 

yet been fully implemented under a Low Wind Speed 

Technology Program [2,3]. As lower costs are achieved, more 

sites are becoming economically viable in areas closer to 

energy constrained load centers, giving a higher value to the 

delivered electricity [4]. The full extent of the vast land-based 

resource is limited by transmission line access and capacity on 

the grid, which is making transport of electricity from the 

windiest areas more difficult [5]. Efforts to lower the onshore 

cost of energy (COE) and integrate wind energy into the 

electric utility grid are major technology areas that must 

continue for the United States to take advantage of its onshore 

domestic wind energy supply, but the full domestic wind 

electric potential cannot be realized in the United States 

without a broader perspective that includes the wind resources 

over the ocean. 

Offshore wind generated electricity in the United States has 

the potential to become a major contributor to the domestic 

energy supply, on par with onshore wind, because it can 

compete in highly populated coastal energy markets where 
onshore wind energy is generally not available. Preliminary 

studies performed by the National Renewable Energy 

Laboratory (NREL) estimate the offshore resource to be 

greater than 1000 GW for the United States [6]. The wind 

blows faster and more uniformly at sea than on land. A faster, 

steadier wind means less wear on the turbine components and 

more electricity generated per turbine. The winds increase 

rapidly with distance from the coast, so excellent wind sites 

exist within reasonable distances from major urban load 

centers reducing the onshore concern of long distance power 

transmission. Figure 1 shows that in addition to the proximity 

to the load, the offshore resource tends to be geographically 

located nearest the states that already pay the highest electric 

utility rates in the United States.1

0

2

4

6

8

10

12

14

16

18

Pacific Noncontiguous

New England

Middle Atlantic

Pacific Contiguous

West South Central

U.S. Total 

South Atlantic

Mountain

East North Central

West North Central

East South Central

Source: EIA Census Division

Electricity Prices - All Sectors (cents/kWh)

Coastal States 

with Significant 

Offshore Wind

Inland States and States 

With No Significant 

Offshore Wind

Figure 1 – U.S. Electricity Rates by Region – 2005 (Source: EIA 

Census Division) 

Offshore Wind Energy Economics 

The estimated cost of offshore wind energy varies widely 

depending on the project, but some studies indicate that 

offshore projects cost significantly more than land-based 

turbine systems [7]. Much of the premium that is now being 

paid for offshore systems can be attributed to higher costs for 

foundations, installation, operation and maintenance. 

As wind turbines are adapted for offshore, the process of 

achieving favorable economics depends less on reducing wind 

turbine costs and more on a full system life cycle cost 

approach. Figure 2 illustrates a typical breakdown of total 

system costs for an offshore wind farm in shallow water, from 

the wind turbine to the onshore utility connection, including 

the costs of operation and maintenance and decommissioning. 

Although the exact proportions of each cost category in this 

chart will vary with the specifics of each project, the purpose 

is to show that the cost of offshore wind energy is increasingly 

dominated by balance-of-station (BOS) and operating 

expenses (OPEX). The electrical and grid infrastructure, 

foundations and support structures, offshore construction, and 

operations and maintenance now represent the major fraction 

 

1

 Some localized Great Lakes regions are included in the 

inland states regions but were difficult to separate. 

of the total project cost. To be successful offshore, wind 

energy technologies must mature using the combined 

experiences and expertise of the cost-conscious wind industry 

and the sea-savvy offshore oil and gas and marine industries. 

Support Structure

24% Turbine

33%

O&M

23%

Management

2%

Grid Connection

15%

Decommissioning

3%

Figure 2 – Typical Cost Breakdown for an Offshore Wind Plant in 

Shallow Water [7] 

Oil and Gas Synergy 

Implicit in the large fraction of non-turbine system costs is the 

conclusion that the offshore oil and gas industry will have a 

dominant role in the implementation of offshore wind energy. 

The portion of the offshore turbine system below the waterline 

will largely be determined by experience and standards that 

were developed over the past four decades by the oil and gas 

industry. The design and installation of the electrical grid 

system, from interturbine cabling to laying the cable to shore, 

will be performed by the existing submarine cable industry. 

All offshore wind projects will employ the existing industry to 

perform site assessments and geotechnical engineering. 

Turbines will be installed and maintained using existing 

offshore vessels and equipment. Personnel access and service 

for turbines will use experienced offshore labor. In the United 

States where on-shore development has been largely 

unregulated, offshore turbines will have to undergo extensive 

structural evaluation and certification with an approval process 

under development at the Minerals Management Service 

(MMS) under the Energy Policy Act of 2005 [8]. This 

synergy between offshore wind and offshore oil is already 

underway in Europe where dozens of oil and gas and marine 

companies are already engaged in wind energy development. 

In the future, the partnerships between wind energy and 

offshore oil and gas will integrate further to maximize their 

mutual benefits resulting in sustainable commercial enterprises 

and the advancement of wind energy. 

The economic potential resulting from this union requires 

some speculation, but for the purpose of illustrating the 

potential, a moderately aggressive development scenario based 

on preliminary analysis performed internally by the U.S. 

Department of Energy2

, indicates a concerted research and 

development effort to develop offshore wind energy would 

result in 50 GW of installed offshore wind energy capacity in 

 

2

 A white paper with the details of this study is in the 

publication process at this time. 
the United States in the next 20 years. This represents 

approximately 5% of the nation’s current electric generating 

capacity. At current pricing, this represents approximately 

$100 billion of capital investment with at least half of this 

revenue going to offshore design and construction contracts. 

Further expansion of the offshore wind industry is expected to 

double this capacity to 100 GW over the subsequent 10-year 

period. This revenue will flow directly to companies that have 

experience with offshore construction and that will benefit 

from the growth of offshore wind. 

 

Offshore Wind: Current Situation 2006 

Offshore wind energy began in shallow waters of the North 

Sea where the abundance of sites and higher wind resources 

are more favorable by comparison with Europe’s land-based 

alternatives. The first installation was in Sweden with a single 

300-kW turbine in 1990 and the industry has grown slowly 

over the past 15 years. There are now 18 operating projects 

with an installed capacity of 804 MW. Figure 3 shows a 

breakdown of where the installed capacity is located as a 

percentage of the total capacity and by country. The majority 

of the capacity is now located in Denmark and the United 

Kingdom, using mostly Danish turbine technology. 

Over 11 GW of new offshore wind projects are planned before 

the year 2010 [9]. Most development will take place in 

Germany and the UK, but at least 600 MW of offshore wind is 

in the permitting process in the United States. All 

installations have been in water depths less than 18 m and 

distances from shore range from 1 km out to 14 km. The 

largest installations are operating off the coast of Demark with 

two 160-MW power plants; Horns Rev in the North Sea, and 

Nysted in the Baltic. 

Sweden

3%

Netherlands

2%

Ireland

3%

Germany

1%

Denmark

53%

United 

Kingdom

38%

Figure 3 – Offshore Wind Projects Installed through 2005 

Based on a Total of 804 MW 

The largest offshore turbine installed to date is shown in 

Figure 4. This turbine sits in the Iris Sea at the 25.2-MW 

Arklow Banks wind farm. Each of the seven turbines has a 

rotor diameter of 104 m with a hub height of over 70 m. A 

single unit has an output capacity of 3.6 MW and can make as 

much as 15,000 MWh per year. The total turbine weight is 

about 290 tons. Current trends indicate that offshore turbines 

will grow much larger than this turbine in the future, as there 

are several 5-MW class turbines in the prototype stage. 

Figure 4 – Typical Offshore Wind Turbine (courtesy of GE) 

Offshore Wind Technology Development 

The Path to Deeper Water. The offshore wind industry is 

likely to develop along the path illustrated in Figure 5. 

 

Onshore Shallow water 

0 m to 30 m 

Transitional Water

30 m to 60 m Deep Water 

60 m to 900 m

Figure 5 – Technology Progression for Offshore Wind Turbines 
Preliminary mesoscale weather model assessments of the 

offshore wind indicate a sharp increase in wind speed with 

distance to shore. Siting options also improve with distance 

from shore, as there are more viable high wind sites with less 

visual impacts and competing uses for the seabed. These 

matters primarily, will gradually attract developers to deeper 

waters. 

 

This progression to deeper water will make its way from 

experience gained from more sheltered projects in shallow 

water, similar to the petroleum industry’s march into deep 

water during the twentieth century. As a result, much of the 

technology to do this has already been developed by the 

existing oil and gas industry, and a concerted effort to transfer 

that technology is already underway in the wind industry 

today. However, new technology is still needed to make wind 

energy economically competitive over a broad range of deeper 

water sites. 

Offshore Substructures. The most critical aspect in the 

development and expansion of offshore wind energy lies with 

the substructures. As water depth increases, it is likely that the 

cost of offshore foundations will increase due to the added 

complexity and resources needed below the waterline. One of 

the goals of a new USDOE research and development program 

is to develop new substructure technologies and make them 

commercially available as the current designs reach their depth 

limits, and thereby minimize the water depth cost penalty. 

Figure 6 gives a conceptual view of how these technologies 

may evolve. 

0

1

0 20 40 60 80 100 120 140 160

Water Depth (meters)

Substructure Cost

Monopiles 

Gravity Foundations

Tripods, Jackets, Trusses

Floating Structures

Shallow Water 

Technology

Transitional 

Technology

Deep Water 

Technology

Figure 6 – Cost of Offshore Wind Turbine Substructures with 

Water Depth [10] 

Tripods, jackets, and truss-type towers will replace 

monopiles and gravity bases, initially using conventional oil 

and gas offshore practices, but later implementing new 

strategies that can take advantage of the lower environmental 

and safety risks, and higher production volume associated with 

offshore wind turbines. At some depth, fixed bottom 

foundations will be replaced by floating systems that have a 

high potential for site independence, mass production, and 

wide-ranging wind turbine innovation. 

Shallow Water Foundations. All offshore wind power plants 

thus far are in shallow waters between 5 m and 18 m. These 

projects use marinized versions of proven land-based turbine 

designs, with upgraded electrical systems and corrosion 

systems, placed on free standing concrete gravity bases or 

steel monopile foundations. The estimated 98 GW of shallow 

water wind energy potential in the United States (5 to 50 nm 

offshore) will provide the first step for the U.S. wind industry 

to develop the infrastructure, technical capabilities, and 

experience to advance into deeper waters [6]. 

 

Monopile Gravity Base Suction 

Bucket 

Figure 7 − Shallow Water Foundation Technology – Current 

Options 

Figure 7 shows the range of shallow water foundations that 

are being deployed today. Monopiles are used in shallow 

depths because of their simplicity and minimal design 

developments required to transition from onshore to offshore, 

as well as their minimal footprint on the seabed. Monopiles 

are used in most offshore installations including the 160-MW 

wind farm at Horns Rev off the west coast of Denmark [11]. 

Monopiles are depth-limited due to their inherent flexibility. 

This limit occurs when the natural frequency of the 

turbine/support structure system is lowered into a range where 

coalescence with excitation sources such as waves and rotor 

frequencies becomes unavoidable. To maintain adequate 

monopile stiffness in deeper waters, a volumetric (cubic) 

increase in mass and therefore cost is required. This means 

the monopile length, diameter, and thickness are all growing 

to accommodate greater depths. At the same time, installation 

equipment such as pile hammers and jack-up vessels become 

more specialized and expensive, and eventually the required 

hammer capacities and jack-up depth limits cannot be reached. 

These limits are thought to be somewhere between 20 and 

30m [12]. 

An alternative to the monopile is the gravity base 

foundation, also shown in Figure 7. These foundations have 

been successfully deployed at the 160-MW Nysted project in 

southeastern Zeeland in Denmark and at Samsoe in 

northeastern Jutland in Denmark to name a couple of 

examples. These foundations can overcome the flexibility 

issues of monopiles but will grow in cost very rapidly with 
water depth as well, although the use of concrete may provide 

some advantage in extending favorable economics [13]. 

Gravity base foundations require significant preparation of the 

seabed to assure a level substrate within 20 mm, however, 

installation effort is reduced once this preparation is complete 

[14]. Extensive site-specific soil analysis is required for each 

gravity base to assure homogeneous soil properties and 

compaction to minimize uneven settling. 

Suction bucket foundations have not yet been used as an 

alternative to shallow water foundations but significant 

development research has been carried out and this new 

technology shows promise for some shallow water sites, 

especially in avoiding the limitation of large pile drivers 

presented by monopile type foundations [15]. 

Transitional Technology. The shallow water support 

structures will be replaced by fixed bottom systems that use a 

wider base with multiple anchor points like those frequently 

used in the oil and gas industry. Transitional substructure 

technology can be deployed up to depths 60 m or greater, as 

shown in Figure 8. 

Figure 8 − Transitional Substructure Technology 

Foundation types are identified numerically in the figure (from 

left to right): 1) tripod tower [16], 2) guyed monopole, 3) fullheight jacket (truss), 4) submerged jacket with transition to 

tube tower [17], 5) enhanced suction bucket or gravity base. 

Transitional depth technology is an important step in the 

progression toward floating systems and access to the full 

offshore wind resource. Preliminary resource assessments for 

the United States have shown that the transitional depth 

resource (30-m to 60-m depth, and 5-nm to 50-nm distance 

from shore) for Class 5 wind and above exceeds 250 GW [18]. 

The first offshore wind turbines in transitional water depths 

are currently being deployed at a demonstration project 

consisting of two 5-MW wind turbines at 42-m depths in the 

North Sea [17]. 

Floating Technology. At some water depth, a floating 

substructure may be the best option. A floating structure must

provide enough buoyancy to support the weight of the turbine 

and to restrain pitch, roll, and heave motions within acceptable 

limits. A primary difference between the load characteristics 

of a floating wind turbine and a floating oilrig is that, for a 

wind turbine, large wind-driven overturning moments 

dominate the design while an oilrig’s design is payload and 

wave driven. System-wide interactions such as coupled 

turbine/platform dynamics could potentially impose additional 

inertial loading requiring more dynamically tolerant turbines. 

Any added complexities must be offset by higher offshore 

winds, and greater public acceptance due to lower visual and 

environmental impacts. 

No full-scale floating systems have been deployed yet but 

some private groups in Norway claim to be working on fullscale prototypes [19,20]. 

1 2 3 4 5 6

Figure 9 – Floating Deepwater Platform Concepts 

Figure 9 shows a wide range of platform architectures that 

are being considered for floating offshore platforms. Platform 

types are labeled numerically in the figure (from left to right): 

1) semi-submersible Dutch Tri-floater [21], 2) barge, 3) sparbuoy with two tiers of guy-wires [22], 4) three-arm mono-hull 

tension leg platform (TLP), 5) concrete TLP with gravity 

anchor [23], and 6) deep water spar [19,20]. 

1 2 3 4 5

Some preliminary studies have been done already to assess 

floating systems but none of the public studies to date have 

attempted to optimize the platform cost and geometry [21,24].

The wind turbine platform and mooring system should 

provide the most potential for system cost reduction because 

the application is new and the most significant cost saving 

design tradeoffs have not yet been explored. However, a solid 

basis from which to determine the optimum design has not yet 

been established. 

Many of the same issues that govern oil and gas platforms 

will also be present in the design of wind platforms, but the 

importance of each variable will be weighted differently. 

There are a vast number of possible offshore wind turbine 

platform configuration permutations when one considers the 

variety of available anchors, moorings, buoyancy tanks, and 

ballast options in the offshore industry. Unfortunately, 

designers will find that most of the resulting topologies will 
have some undesirable aspects that could drive the system cost 

out of range for wind applications. The optimum platform 

probably does not exist due to real-world constraints, but there 

are many features that such a platform would embody that 

most designers could agree on. To narrow the range of 

options, a study is now underway at the National Renewable 

Energy Laboratory (NREL) to compare each platform design 

to features that an optimized platform should have. From this 

comparison, we can begin to determine the key issues that 

limit each platform type and that will direct future study in this 

area. Some of the variables to be assessed are identified by 

Butterfield et al [25] and are given below: 

3⁄4 Requirements for design tools and methods- controls 

complexity 

3⁄4 Buoyancy Tank Cost/Complexity/Material options 

3⁄4 Mooring Line System Cost/Complexity/Options 

3⁄4 Anchors Cost/Complexity/Options 

3⁄4 Load Out Cost/Complexity/Options/Requirements 

3⁄4 On-site installation requirements 

3⁄4 Decommissioning and maintainability 

3⁄4 Corrosion Resistance requirements – coatings, 

cathodic protection, etc. 

3⁄4 Depth Independence/ Specify depth range 

3⁄4 Sensitivity to Bottom Conditions / Specify limitations 

3⁄4 Required footprint (as a function of depth) 

3⁄4 System Weight Sensitivity/ CG sensitivity 

3⁄4 Induced Tower Top Motions - Wave SensitivityAllowable heal angle 

3⁄4 First Order Costs for Candidate Configurations 

Butterfield also provided a framework for assessing 

various platform concepts on the basis of how a platform type 

achieves static stability. This approach argues that static 

stability and a range of associated operational and technical 

factors largely determine the first order economics of a 

floating platform. An optimization study to determine the 

lowest cost platform architechture will follow from these 

analyses. 

 

Technologically, it is recognized that the commercial 

undertaking of floating wind turbines will be a bold step, but is 

necessary to unlock an additional 500-GW of offshore wind 

energy potential in the United States and become one of the 

major contributors to the world’s electric grid [18]. However, 

it will require substantial experience in shallower water, with 

parallel and substantive research and development initiatives 

to realize this technology over the next 15 years. 

Other Offshore Technology Challenges 

Aside from the matter of substructures and foundations, a key 

area for development lies in the adaptation of the technology 

to the offshore environment. Current offshore wind 

installations in Europe are, ostensibly, extensions of landbased turbine technology and operating strategies that are 

functionally sound, but are not optimized for operation at sea. 

Offshore turbines must ultimately be re-engineered to 

accommodate multiple offshore environmental and logistical 

factors that address the effect of the offshore operating 

environment on the wind turbine, and the difficulty in working 

in it. Below, some of these areas are summarized. 

Establish a Design Basis for Offshore Turbines. The best 

way to establish the requirements for an offshore turbine is to 

demonstrate the best technologies, down-selected from earlier 

optimizations at sea. This initiative should be done on a 

turbine test bed of representative scale to measure the true 

MET Ocean environment, loads, and actual permitting and 

environmental impacts. Ideally, this initiative should be 

conducted in the public domain to maximize its benefit to a 

wide industry base, including potential new entries from the 

offshore oil and gas industry. The output from such an effort 

would yield critical design methods and codes, uniform 

standards for structural reliability, design specification 

guidelines, industry accepted safety margins, and valuable 

data to validate design models, codes, and assumptions. 

Offshore Design Codes and Methods. The development of 

accurate offshore computer codes to predict the dynamic 

forces and motions acting on turbines deployed at sea is 

essential before turbines can reliably be deployed. One of the 

immediate challenges common to all support structure designs 

is the ability to predict loads and resulting dynamic responses 

of the coupled wind turbine and support structure when 

subjected to combined stochastic wave and wind loading. 

Where the offshore oil industry primarily is concerned with 

wave loading when extrapolating to predict extreme events, 

offshore wind turbine designers must consider wind and wave 

load spectrums simultaneously [26, 27]. 

Additional offshore loads arise from impact of floating 

debris and ice and from marine growth buildup on the 

substructure. Offshore turbine structural analysis must also 

account for the dynamic coupling between the translational 

(surge, sway, and heave) and rotational (roll, pitch, and yaw) 

platform motions and turbine motions, as well as the dynamic 

characterization of mooring lines for compliant floating 

systems [28]. 

Minimize Work at Sea. Much of the success of land-based 

turbines can be attributed to the ability of designers to reduce 

the initial capital cost of turbine components while 

maintaining or improving overall reliability. The experience 

from on-shore wind development demonstrated good 

accessibility to turbines and therefore relative ease of 

maintenance. Therefore, added capital investment to reduce 

the long term O&M burden is not generally tolerated for onshore systems due to the lack of a demonstrated life-cycle 

payback model and the desire to keep initial project costs low. 

However, work at sea is significantly more costly and time 

consuming and the on-shore O&M model will not provide the 

best economical option. New offshore strategies must be 

developed that minimize work done at sea, requiring a 

paradigm shift that takes into account all aspects of the 

project. Materials must be selected for durability and

environmental tolerance. Designs, starting with the 

preliminary concepts, must rigorously place a higher premium 

on reliability, float-out deployments, and in-situ repair 
methods. Fabrication facilities must be strategically located 

for mass production, on-shore assembly, and rapid deployment 

with minimal large vessel dependence. Operators must be 

remotely equipped with intelligent turbine condition 

monitoring and self-diagnostics systems to manage O&M 

weather windows, minimize downtime, and reduce the 

equipment needed for up-tower repairs. Ultimately, a new 

balance between initial capital investment and long term 

operating costs will be established, which will have a broad 

impact on the COE for offshore wind technology. 

Low Cost Anchors and Moorings. Cost-saving opportunities 

arise for wind power plants in deeper water with both fixed 

bottom and floating turbine foundations, as well as for existing 

shallow water designs where value engineering cost reductions 

can be achieved. Fixed bottom systems that favor rigid 

lightweight substructures, automated mass production 

fabrication facilities, and integrated mooring/piling 

deployments systems are envisioned as a possible low cost 

option. For floating systems, platforms that do not depend on 

mooring line tension as their primary means for achieving 

stability may be able to develop and capitalize on a new 

generation of drag embedment type anchors or vertical load 

anchors (VLA) to lower overall platform cost. Deployable 

gravity anchors show promise for all platform types. When 

multiple turbines are installed, economies-of-scale will allow 

developers to streamline repetitive installation procedures and 

look for innovative tooling that could not be justified under 

customized single installations. 

Offshore Wind-Wave Measurements 

Remote Measurement Systems. Unlike onshore data 

where extensive fixed-station continuously recorded 

anemometer data is available, the available measurements 

made for offshore wind are thinly distributed and more 

uncertain. The typical onshore method of making wind 

measurements from a MET mast is not feasible for a large 

number of sites due to the increased offshore cost. 

Anemometer based offshore windspeeds measurement 

systems consist of a sparsely distributed system of buoys and 

fixed C-MAN stations at the National Data Buoy Center 

(NDBC) operated by the National Oceanogrphic and 

Atmospheric Adminstration (NOAA). Buoy data is usually 

taken at 5 m above sea and is insufficient for regional 

mesoscale model validation, or to characterize the wind 

regime at a particular site. Alternative methods are needed to 

measure windspeed at multiple locations, and to determine 

wind shear profiles up to elevations where wind turbines 

operate. This will require adaptation of equipment such as 

SODAR, LIDAR, and coastal RADAR based systems 

combined with more stable buoy systems or fixed bases. 

Some systems are under development but have not fully been 

proven in these applications. 

 

Hybrid Windspeed Database. Validation of offshore 

mesoscale models and site assessments to determine energy 

capture potential for a given site will require the use of a wide 

range of alternative databases [29]. Data from different 

agencies and entities can be compiled to characterize the

coastal and offshore wind regime. Ultimately, the industry 

may depend on more accurate mesoscale models to make 

siting decisions and to assess wind project risk. 

 Wind Farm Turbulence and Array Effects. To achieve 

high penetrations of offshore wind, multiple wind power 

arrays will be installed over significant portions of the ocean. 

To supply 10% of the U.S. electric energy supply would 

influence an ocean area of approximately 20,000 km2

. 

Interference from one wind farm to another could introduce 

downstream turbulence and upset energy capture predictions. 

Flow models and measurements to determine the turbine-toturbine influence and the aggregate influence of multiple 

turbines are needed to accurately determine the economic and 

structural consequences of wind large-scale offshore wind 

deployment. 

Offshore Turbine Weight Reductions. Future offshore 

turbine designs may take advantage of several methods to 

reduce weight (and cost) that have been rejected for landbased systems because of acoustic emissions or aesthetics. 

For floating systems, the benefits of weight reduction are 

multiplied as a reduction in weight aloft also reduces the 

weight of the buoyancy and mooring systems. Offshore wind 

turbines will enable relaxed constraints on aerodynamic blade 

noise allowing higher tip-speeds, and resulting in lower 

nacelle weights. Higher rotational speeds allow smaller blade 

planform and lower blade weight for the same energy output. 

Higher speeds mean lower input torque and lower gear ratios, 

and hence, smaller shafts and gearboxes. Direct drive 

generators can be made smaller for higher rotational speeds 

and show promise for future turbines. The heaviest 

component above the water-line is by far the steel tower. 

Lower thrust loads and alternative lightweight materials and 

designs may significantly lower tower weight. Weight 

reductions may also be realized in the platform and foundation 

where, for example, lightweight concrete aggregates can 

reduce weight 40% below standard mixtures. 

Electric Grid and Systems Integration. The offshore grid 

will have unique characteristics that will warrant in-depth 

studies and modeling including fault and stability analysis. A 

systems approach to wind farm turbulence, array stability, and 

array aggregation will be the focus of extensive research 

needed for wind plant optimization. Offshore wind 

forecasting and the reliable prediction of power fluctuations 

will also be critical. New grid code and security standards will 

be established for offshore wind. Control and communication 

systems of large offshore wind farms will need greater 

attention due to the inaccessibility of remote offshore turbines, 

which may lead to new grid architechures.

Ultra-large Turbines. No consensus exists yet on how large 

offshore wind turbines will become although most wind 

engineers agree that there is no hard physical limit preventing 

10-MW turbines or greater. With turbine costs representing 

only one third of the life cycle cost of the wind project, and 

transportation and erection limits eased offshore, turbine 

growth will continue until overall system costs are minimized. 

The substructure and foundation costs appear to favor larger 

turbines, as mobilization of the installation and service
equipment is a major cost driver. Fewer turbines means lower 

geotechnical costs, fewer electrical terminations, more 

generating capacity per ocean area, less interturbine cable 

length and trenching, and fewer service trips to and from 

towers. 

As turbine size increases, new opportunities will become 

available and should be evaluated. For example, control 

systems and sensors that monitor and diagnose turbine status 

and health will not grow in cost as turbine size increases. But 

for the same cost fraction, larger turbines will enable a much 

higher level of controls and condition monitoring intelligence. 

In a similar way, larger turbines may also allow wind to move 

into other technologies such as light-weight super-conducting 

generators that become more cost effective with size. 

Larger turbines will have significant implications for 

future research needs, test facilities, and infrastructure 

requirements. For example, no facilities currently exist 

anywhere that can perform the necessary testing for a 10-MW 

wind turbine blade, although the first 100-m blade test facility 

is under construction in Denmark at this time [30]. 

Environmental and Regulatory Issues 

By comparison to other forms of electric power generation, 

offshore wind energy is considered to have relatively benign 

effects on the marine environment, according extensive 

analyses conducted by the European Community3

. However, 

regulatory and environmental uncertainties have hindered the 

approvals for the first offshore wind projects in the United 

States and for the early years of development may have a 

greater influence over the pace of industry growth than the 

technical issues presented above. 

Regulatory Framework for Wind Energy. Though the 

offshore wind industry has over a decade of experience in 

Europe, the United States did not have any project proposals 

until 2001 with the Cape Wind Associates project in 

Nantucket Sound [31]. Moreover, there were no firm national 

policies for offshore wind developments until the summer of 

2005 with the passage of the Energy Policy Act of 2005. This 

is a case where projects were proposed before policies were in 

place. 

On August 8, 2005, President Bush signed into law the 

Energy Policy Act (EPAct 2005, PL 109-58) granting the 

Minerals Management Service (MMS) within the Department 

of the Interior (DOI) new responsibilities over renewable 

energy and alternate uses of offshore public lands [8]. Prior to 

the passage of this legislation, the U.S. Army of Corp of 

Engineers (ACE) assumed the lead for coordinating the 

approval process for the first applications. For the last 40 

years, MMS has regulated the offshore oil and gas industry 

and other mineral extraction activities in federal waters, also 

known as the Outer Continental Shelf (OCS). This new 

regulatory authority granted to MMS will evaluate compatible 

 

3 For detailed analyses comparing the lifecycle costs of fossil, nuclear 

and wind power, see the EC reports at http://www.externe.info. 

uses of the OCS, establish fair economic compensation for 

projects in federal waters, evaluate potential impacts to marine 

resources, and involve other federal and state agencies in the 

review and approval of future wind power permits. 

The new authority does not supersede or modify existing 

authority of any other federal agency. It does not change any 

of the exclusions of the moratoria areas for oil and gas drilling. 

In addition, the regulatory regime will not apply to areas 

designated as National Marine Sanctuaries, National Parks, 

National Wildlife Refuges, Sanctuaries, National Parks, 

National Wildlife Refuges, or any National Monument. The 

project siting process will have to take into account these 

exclusion zones and a range of legal authorities affecting the 

OCS [32]. 

Given their experience in the oil and gas program and sand 

and gravel mining, MMS has a wealth of experience in siting 

and managing activities on the OCS. They do not, however, 

have a depth of understanding about wind resources or the 

wind energy industry. DOE and MMS will be signing a 

memorandum of understanding to facilitate cooperation 

between the two government entities for exchanging technical 

information relating to offshore wind energy R&D activities, 

engineering principles of wind turbines and their components, 

and certification procedures for the turbines and the entire 

structure. 

In addition to the MMS and ACE, there are two other key 

federal agencies involved with ocean boundary jurisdictions, 

including the independent Federal Energy Regulatory 

Commission (FERC) and the National Oceanic and 

Atmospheric Administration (NOAA). FERC jurisdiction 

stems from their authority regarding electric transmission 

rights for approval of power supply contracts and connecting 

to the landfall cable. Most recently, FERC has assumed 

additional authority over ocean technology projects with a 

license requirement for wave power. NOAA, within the 

Department of Commerce, has jurisdictional authority to 

protect and manage marine sanctuaries. Any projects in or 

around a marine sanctuary or any protected area will be 

subject to NOAA review and approval. 

There is a multitude of other federal and state agencies 

involved with ocean uses and management that have a role in 

the approval process for offshore wind projects. Generally, 

these roles and responsibilities are well defined, but there are 

numerous areas where these responsibilities overlap and even 

conflict. This could create a web of approvals and 

consultations that would delay projects and not necessarily 

contribute to better siting or management of offshore wind 

energy projects. Now MMS has the authority to strike a better 

balance between the development of the offshore wind 

resources and competing commercial and natural resource 

interests. 

The new MMS authority to develop a new regulatory 

paradigm for offshore wind facilities, based upon Section 388 

of the EPAct 2005, includes the following responsibilities: 
3⁄4 Act as the lead agency for permitting offshore renewable 

energy projects, including wind.4

3⁄4 Ensure consultation with states and other stakeholders. 

3⁄4 Grant easements, leases or rights-of-ways for uses of the 

OCS on a competitive basis. 

3⁄4 Pursue appropriate enforcement actions in the event that 

violations occur. 

3⁄4 Require financial surety to ensure that facilities 

constructed are properly removed at the end of their 

economic life (decommissioning). 

3⁄4 Regulate, monitor, and determine fair return to the nation 

with a reasonable payment for sharing revenue among 

coastal states within 15 miles of a project. 

MMS is under a congressional mandate to develop new 

regulations by May 2006. The Advanced Notice of Proposed 

Regulation was just issued in December 2005 and public 

comments are being sought. 

Potential Environmental and Socio-Economic Issues. The 

full range of potential environmental impacts from offshore 

wind is unknown today in the United States, since no projects 

have yet been installed. The only project evaluation thus far is 

the 3800-page Cape Wind draft environmental impact 

statement (DEIS) prepared by Cape Wind Associates, under 

the leadership of the ACE New England District. The 

document, released in November 2004, did not identify any 

significant impacts, but a range of specific mitigation 

measures and monitoring studies are proposed. The ACE held 

several public hearings, coordinated with 17 public agencies, 

and received over 5000 public comments. The extensive 

public involvement requirements along with the transfer of 

jurisdiction to MMS have slowed the permitting process 

significantly. Recently, MMS required that the Cape Wind 

DEIS be expanded to include construction and operational 

procedures, personnel safety, and decommissioning that fit a 

broader “cradle-to-grave” approach -- reflecting the new MMS 

program authority. 

The only peer-reviewed information on potential 

environmental impacts from offshore wind is based upon 

lessons learned from land-based projects and European beforeand-after-control-impact (BACI) studies for installed projects. 

Though there is over 15 years experience with offshore wind 

facilities in Europe, most of the projects were quite small (less 

than 10 turbines) and there were not scientifically credible 

siting criteria, study methodologies, and mitigation strategies 

established. Given the higher growth rate in Europe and 

significant deployment plans for the next 10 years, there is 

now a proliferation of studies and standards. 

The most credible and broad-based environmental studies 

in Europe for commercial facilities are based upon the Horns 

Rev and Nysted projects in Denmark. These 2 sites have 80 

and 72 turbines, respectively. Both sites have government-

 

 

4

 The new authorization also includes jurisdictional authority over 

alternative energy, such as wave, solar, and current power as well as 

marine related uses of the existing infrastructure and a coastal 

assistance program that are not addressed in this paper. 

sponsored BACI studies with oversight from an international 

scientific panel reviewing the methods, design plans, and 

findings from three-year post-construction evaluations. The 

Danish studies did identify several significant temporal 

impacts during the construction phase. The pile driving and 

increased transportation requirements, for example, created 

noise and disturbance to the marine environment. 

Consequently, they documented short-term impacts to marine 

mammals as they dispersed away from the area when noise 

levels increased. In order to mitigate these temporal impacts, 

pingers were used before construction began to scare away 

any mammals in the area to reduce the impacts of the 

construction noise. Satellite tracking devices and porpoise 

detectors were attached to the seals and porpoises to verify 

their movements. Since the mammals returned to the area 

during the operational phase, these impacts were considered 

“insignificant.” The actual impact to the mammals for feeding 

and molting is considered unknown since it is very difficult to 

ascertain the physical impacts on mammals in the wild and the 

subjects would have to be tracked for several seasons for a 

more definitive survey5

. 

There are now thousands of pages of scientific material 

relating to the ecological effects of offshore wind sites in 

Europe and the United States. A discussion of the range of 

environmental effects and findings along with issues related to 

the competing uses of the ocean is beyond the scope of this 

paper. To give the reader a sense of community priorities, 

public opinion may shed some light. A recent survey of 

residents of Cape Cod, MA near the proposed Cape Wind 

project conducted by the University of Delaware identified the 

following as the most important concerns [33]: Impacts on 

marine life, aesthetics, fishing impacts, boating and yachting 

safety. Unfortunately, some of these public concerns have 

been heightened by poorly researched media anecdotes rather 

than documented factual information. 

The installation of wind turbines also provides some 

beneficial effects to the local community and ecosystem. The 

turbine foundations placed onto or buried into the seabed 

create artificial reefs or breeding grounds that have a 

beneficial effect on local fish populations and benthic 

communities. Danish studies indicate that socio-economic 

impacts may be positive. Over 80% of the respondents in a 

recent Danish study have a “positive attitude towards the 

establishment of new offshore wind farms.” There were, 

however, some concerns about the visual externalities of 

turbines when they can be seen from the shore (generally, less 

than 10 km). In the case of the Horns Rev wind site, over 1700 

man-years of local jobs were created during the construction 

period and 2000 man-years created over the 20-year life of the 

projects. Approximately, one fourth of these jobs were locally 

based. The multiplier effects are associated with the 

construction activities and the manufacturing of materials as 

well as indirect effects from demands of inputs from goods 

and services. 

5

 For details on these studies see the Horns Rev environmental 

reports at http://www.hornsrev.dk/Engelsk/default_ie.htm . 
Realistically, there is no form of electric generation that 

can claim to be completely benign with respect to the 

environment. To provide a fair assessment of the alternatives, 

the environmental impact of a generating facility should be 

compared to the impact of an equivalent power plant using a 

competing fuel source with the same capacity. When this 

comparison is conducted, the potential impacts of offshore 

wind to the environment appear to very benign [34]. 

Summary 

An overview of the present status offshore wind energy 

showed the industry in its infancy but with the potential to 

become a major contributor in the U.S. electric energy market. 

Since over half of the cost of an offshore wind energy power 

plant is outside the wind turbine itself, the offshore industry 

would be the primary beneficiary from this new energy source. 

If offshore energy predictions are achieved, offshore wind 

could result in over $100 billion of revenue to the domestic 

offshore industry over the next 30 years. The offshore would 

receive this revenue in the form of construction, site 

assessments, subsea electrical, inspections, service, and 

operation contracts. The technical issues and design 

challenges needed to achieve economic competitiveness for 

near term deployments in shallow water below 30-m depth 

were described, as well as the requirements for future 

technologies needed to deploy systems in deeper water beyond 

the current depth limits. New regulatory authority was granted 

to the Minerals Management Service in 2005 and this new 

regulatory system was discussed. Offshore wind shows very 

low impacts to the environment but regulatory and 

environmental barriers have hindered the first offshore wind 

projects in the United States. 

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Final Report”, Concerted Action on Offshore Wind Energy in 

Europe, December 2001, NNE5-1999-562, Report Reference: 

Duwind 2001.006. 

8. See the EPAct 2005 full text of section 388 of the Act at 

http://energy.senate.gov/public/_files/ConferenceReport0.pdf

9. Website Offshore Windenergy Europe, 

http://www.offshorewindenergy.org/ Last accessed November 

9, 2005. 

10. Figure adapted from presentation by Dolan, D., MMI 

Engineering at Deepwater Wind Energy Workshop, Washington 

D.C. October 26-27, 2004. Website: 

http://www.energetics.com/deepwater.html 

11. Horns Rev Website: http://www.hornsrev.dk/ accessed January 

23, 2006. 

12. Presentation by Mohamed A. GE Wind Energy at Deepwater 

Wind Energy Workshop, Washington D.C. October 26-27, 

2004. Website: http://www.energetics.com/deepwater.html 

13. V∅lund, P. “Concrete is the Future for Offshore Foundations” 

Proceedings Copenhagen Offshore Wind 2005, 26-28 October 

2005, Copenhagen, DK. 

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Collaboration”, page 28, July/August 2003 

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Delaware, December 2005. 
F1147-E(12/2004) 

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Energy from Offshore Wind: Preprint 

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14. ABSTRACT (Maximum 200 Words)

This paper provides an overview of the nascent offshore wind energy industry including a status of the commercial 

offshore industry and the technologies that will be needed for full market development. It provides a perspective on 

the status of the critical environmental and regulatory issues for offshore wind and how they are affecting the 

formation of the U.S. industry. 

15. SUBJECT TERMS

offshore wind energy; ocean wind technologies; floating platforms 

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