NREL is a national laboratory of the U.S. Department of Energy, 

Office of Energy Efficiency & Renewable Energy, operated by the 

Alliance for Sustainable Energy, LLC. 

Contract No. DE-AC36-08GO28308

Analysis of Offshore Wind 

Energy Leasing Areas for the 

Rhode Island/Massachusetts 

Wind Energy Area

W. Musial, D. Elliott, J. Fields, Z. Parker, 

and G. Scott

Produced under direction of the Bureau of Ocean Energy 

Management (BOEM) by the National Renewable Energy Laboratory 

(NREL) under Interagency Agreement M13PG00002 and Task No 

WFS3.1000.

Technical Report

NREL/TP-5000-58091

April 2013
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Analysis of Offshore Wind 

Energy Leasing Areas for the 

Rhode Island/Massachusetts 

Wind Energy Area 

W. Musial, D. Elliott, J. Fields, Z. Parker, 

and G. Scott

Prepared under Task No. WFS3.1000

Technical Report

NREL/TP-5000-58091

April 2013
NOTICE

This manuscript has been authored by employees of the Alliance for Sustainable Energy, LLC (“Alliance”) under 

Contract No. DE-AC36-08GO28308 with the U.S. Department of Energy (“DOE”).

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors expressed 

herein do not necessarily state or reflect those of the United States government or any agency thereof.

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste
iii

Acknowledgments 

The National Renewable Energy Laboratory (NREL) would like to acknowledge the support and 

cooperation of the Bureau of Ocean Energy Management (BOEM). Additionally, as part of this 

work, NREL received cooperative assistance from the University of Rhode Island (URI) and 

others in the Rhode Island state-level working groups.

The following NREL reviewers and contributors are acknowledged: 

Sheri Anstedt, Ian Baring-Gould, Caroline Draxl, Fort Felker, Robert Hawsey, Pat Moriarty, 

Brian Smith, Suzanne Tegen 
iv

Executive Summary

The National Renewable Energy Laboratory (NREL), under an interagency agreement with the 

Bureau of Ocean Energy Management (BOEM), is providing technical assistance to BOEM on 

the identification and delineation of offshore leasing areas for offshore wind energy development 

within the Atlantic Coast Wind Energy Areas (WEAs) established by BOEM in 2012. This 

report focuses on NREL’s evaluation of BOEM’s Rhode Island/Massachusetts (RIMA) WEA

leasing areas. The objective of the NREL evaluation was to assess the proposed delineation of 

the two leasing areas and determine if the division is reasonable and technically sound. 

Additionally, the evaluation aimed to identify any possible technical consequences resulting 

from the BOEM delineation. 

As part of the evaluation, NREL researchers: 

1. Performed a limited review of relevant literature and RIMA call nominations 

2. Executed a quantitative analysis and comparison of the two proposed leasing areas 

3. Conducted interviews with University of Rhode Island (URI) staff involved with the URI 

Special Area Management Plan (SAMP)

4. Prepared this report summarizing the key findings 

NREL also reviewed information from the following sources: 

• Rhode Island Coastal Resource Management Council OCEAN SAMP (RI SAMP 2010) 

and interviews with URI staff 

• BOEM Wind Energy Area delineations, as described in the proposed sale notice, and 

economic considerations used by BOEM to inform the proposed sale notice 

• BOEM Call for Information and Nominations for Commercial Leasing for Wind Power 

on the Outer Continental Shelf (OCS) off of Rhode Island and Massachusetts (referred to 

throughout this report as the “Call”) and responses to the Call 

The current RIMA Wind Energy Area delineation is logical in terms of providing two distinct 

developable leasing areas, based on the criteria that NREL used to evaluate the Rhode Island

Wind Energy Area (see Table ES1). There are two proposed leasing areas: a north leasing area, 

with 97,498 acres, and a south leasing area, with 67,252 acres. The two areas differ significantly, 

with varying bathymetry, distance to shore, geotechnical, and wind access attributes. Together, 

these differing attributes lead to the north leasing area being a more competitive and costeffective area for near-term commercial development. As such, we would anticipate that the 

north leasing area may also be more highly valued. Although a full quantitative analysis of all 

siting criteria was beyond the scope of this report, NREL performed both quantitative and 

qualitative analyses of the critical technical siting criteria listed in Table ES1. 
v 

Table ES1. Evaluation Criteria used to Assess RIMA Wind Energy Areas

Quantitative Evaluation Criteria Qualitative Evaluation Criteria Considered

Resource area (acres and km2) Seabed soil conditions

Development capacity potential (megawatts) Distance from shore

Bathymetry (m) Technology challenges

Gross capacity factor (%) Development timing

Wind resource (meters per second) Development cost

Wind direction and prevailing conditions

Wake losses (%)

Table ES2 provides a comparison of the quantitative metrics for the RIMA Wind Energy Area

leasing areas as proposed by BOEM. The north leasing area was larger, shallower, and closer to 

shore than the south leasing area. The annual average wind resource slightly favors the south 

leasing area, as indicated by the annual average wind speeds and gross capacity factors (less 

estimated wake losses) shown in the table. Both areas could provide over 1,000 megawatts 

(MW) of developable wind area (1,955 MW in the north leasing area and 1,440 MW in the 

south), based on the NREL 5-MW reference turbine and a nominal 8 rotor diameters (8D x 8D) 

array spacing (Jonkman et al. 2009, Musial and Ram 2010). Using these criteria, wake losses

calculated using the AWS Truepower OpenWind Enterprise tool showed that the north leasing 

area was more burdened by wake losses than the south leasing area, but external and developerimposed internal buffers could mitigate these losses.

Assuming that the boundaries of the WEA are fixed, NREL concluded that the BOEM leasing 

area delineation of north and south was a logical division that provided two leasing areas with 

natural buffers, but differing attributes. 

Table ES2. Rhode Island/Massachusetts BOEM Wind Energy Area Characteristic Comparison 

(Source: NREL)

Parameter North Leasing Area South Leasing Area

Total area (km2) 394.6 272.2

Total area (acres) 97,498 67,252

Potential installed capacity (MW) 1,955 1,440

Average annual wind speed (m/s) 9.1 9.2

Average depth (m) 35.7 45.9

Gross capacity factor including wake losses (%) 46.6 47.4

Wake losses (%) 11.3 10.2

Potential annual energy production (AEP) gigawatt-hours (GWh) 7,982 5,985

Assumptions: 

NREL 5-MW turbine with 8D x 8D array spacing

Below are the key findings of the NREL analysis and important considerations for policy makers 

and stakeholders involved in the RIMA offshore wind energy development process. 
vi 

• The north leasing area is more economically and technically developable because it has 

shallower bathymetry, is closer to shore, and has a larger area than the south leasing area. 

• Assuming that the boundaries of the wind energy area are fixed, the BOEM leasing area 

delineation of north and south was a logical division, but would not provide equal leasing 

areas in terms of potential development costs and resource access. 

• It may be possible to split the north leasing area into two smaller leasing areas that could 

provide a nameplate development capacity of up to 500 MW. 

• Both leasing areas are capable of supporting at least 1,000 megawatts (MW) of potential 

installed capacity. The potential installed capacity of the north leasing area is 1,955 MW 

and the south leasing area is 1,440 MW, based on dense 8D x 8D spacing with no internal 

buffers. 

• The north leasing area can implement current fixed-bottom wind turbine technology in 

the near term. The south leasing area will be challenged by limits to current experience 

with fixed-bottom turbine technology with 88% of the resource area at 40-m+ depths and 

where commercially viable technical solutions have not been fully demonstrated (Musial 

and Ram 2010). 

• The south leasing area may have a later project start of several years than the north

leasing area because of increased costs and technical challenges associated with the 

deeper water in the south leasing area. 

• The three islanded aliquots of the north leasing area are more difficult to develop because 

of their distance from (~10 km) the main lease blocks. The additional revenue from the 

remote turbines may not outweigh the additional cost of the electrical collection system 

and other infrastructure required to bridge the distance. 

• The ten islanded aliquots in the south leasing area are all in shallower depths that are 

more comparable to the north leasing area. As a result, these aliquots could be reallocated 

to the north leasing area to create a more depth-homogeneous project area. This 

reallocation would add up to 90 MW of capacity to the north leasing area. 

• Bathymetry variance within each leasing area may result in natural buffer zones within 

the WEAs, potentially reducing wake losses and project conflicts. The built-in 20-D 

external buffer between the north and south leasing areas will also help mitigate waking 

between the leasing areas (Barthelmie 2010).

• The wind resource is slightly better in the south leasing area, with estimated annual 

average wind speeds that are approximately 0.1 m/s higher, and with better exposure to 

predominant southwestern winds. In addition, this leasing area will have smaller potential 

wake losses. However, this benefit will fully offset higher development costs from deeper 

water and greater distance to shore. 

• Prevailing winds have a strong unidirectional component from the southwest, making it 

more straightforward to orient the array, a characteristic which could help optimize 

layout design for both leasing areas. 
vii 

Table of Contents

Acknowledgments ..................................................................................................................................... iii

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................. vii

1 Project Background ............................................................................................................................. 1

1.1 Rhode Island/Massachusetts WEA and Leasing Areas ................................................................... 1

1.2 Summary of Proposed Task Work................................................................................................... 2

2 Literature Review.................................................................................................................................. 3

2.1 Rhode Island Coastal Resource Management Council SAMP ........................................................ 3

2.2 BOEM WEA Delineations............................................................................................................... 4

2.3 Evaluation of RIMA Call Notice Responses ................................................................................... 4

3 NREL Methodology .............................................................................................................................. 7

3.1 Evaluation and Use of SAMP and Nominations Information.......................................................... 7

3.2 Characterization of Wind Resource and Wake Effects.................................................................... 7

3.3 OpenWind Enterprise Tool.............................................................................................................. 7

4 Results and Conclusions..................................................................................................................... 9

4.1 Summary of Results......................................................................................................................... 9

4.2 Leasing Area Development Capacity ............................................................................................ 10

4.3 Bathymetry..................................................................................................................................... 11

4.4 Wind Resource............................................................................................................................... 15

4.5 Gross Capacity Factor and Wake Effects ...................................................................................... 16

4.6 Qualitative Evaluation Criteria ...................................................................................................... 17

4.7 Key Findings.................................................................................................................................. 17

References................................................................................................................................................. 19

List of Figures

Figure 1. Proposed lease areas for RIMA WEA (north and south leasing areas)................................. 2

Figure 2. Rhode Island SAMP study area................................................................................................. 4

Figure 3. Turbine 8D x 8D layout and depth map for the north and south leasing areas.................. 11

Figure 4. Current offshore wind projects in Europe (installed, under construction, contracted and 

approved) as a function of water depth and distance to shore..................................................... 13

Figure 5. Offshore wind project development cost breakdown........................................................... 14

Figure 6. BOS development cost by depth............................................................................................. 14

Figure 7. RIMA WEA showing the annual average wind speed in 0.1 meters per second (m/s) 

increments and wind roses for each leasing area .......................................................................... 15

Figure 8. RIMA WEA annual average wind rose showing strong prevailing winds from the 

southwest ............................................................................................................................................ 16

List of Tables

Table ES1. Evaluation Criteria used to Assess RIMA Wind Energy Areas ........................................... v

Table ES2. Rhode Island/Massachusetts BOEM Wind Energy Area Characteristic Comparison...... v

Table 1. Summary of Nomination Statistics from the BOEM Rhode Island/Massachusetts Wind 

Energy Area Call................................................................................................................................... 5

Table 2. Evaluation Criteria used to Assess RIMA Wind Energy Areas................................................ 9

Table 3. Rhode Island/Massachusetts BOEM Wind Energy Area Characteristic Comparison......... 10

Table 4. Potential Installed Wind Capacity by Leasing Area and Depth ............................................. 12

Table 5. Capacity Factor (less wake losses) and Wake Losses by Leasing Area.............................. 17
1 

1 Project Background

Since 2009, the U.S. Department of the Interior’s Bureau of Ocean Energy Management 

(BOEM) has been working to create and collaborate with intergovernmental task forces to 

identify the most appropriate areas for commercial wind energy leasing on the Outer Continental 

Shelf (OCS) off the Atlantic Coast. To date, BOEM has identified six Wind Energy Areas 

(WEAs) on the OCS that are appropriate for commercial offshore wind energy development, 

with the goal of minimizing conflicts with existing uses and the environment. BOEM is currently

considering issuing leases for five WEAs through a competitive process: 1) Virginia, 2) Rhode 

Island/Massachusetts, 3) New Jersey, 4) Maryland, and 5) Massachusetts. Except for Virginia, 

BOEM intends to offer more than one lease within each WEA. WEAs that have multiple leasing 

areas (all but Virginia) within their boundaries require analysis; using engineering tools and 

available WEA site characteristics to ensure that the leasing areas are appropriately divided. 

1.1 Rhode Island/Massachusetts WEA and Leasing Areas 

The National Renewable Energy Laboratory (NREL), under an interagency agreement with

BOEM, is providing technical assistance to identify and delineate offshore leasing areas for wind 

energy development within the Atlantic Coast WEAs. The work being performed by NREL for 

each WEA depends on the specific area requirements, available information provided by BOEM, 

and a predetermined scope of work. This report focuses on NREL’s evaluation of BOEM’s 

Rhode Island/Massachusetts leasing area delineation. Background information and the scope of 

work for the Rhode Island/Massachusetts WEA are provided below. 

 

BOEM has been working with the Rhode Island and Massachusetts Renewable Energy Task 

Force since 2009 to identify the most appropriate areas for offshore leasing in Rhode Island and 

Massachusetts. In July 2010, the two states signed a memorandum of understanding establishing 

the area of mutual interest on the OCS for joint coordination and collaboration of offshore wind 

energy development. Working with the joint task force, BOEM refined the area of mutual 

interest and published a Call for Information and Nominations for Commercial Leasing for Wind 

Power on the OCS off of Rhode Island and Massachusetts (referred to in this document as the 

“Call”) in the Federal Register in August 2011. In response to the Call, BOEM received eight 

nominations of interest wishing to obtain a commercial lease for offshore wind energy 

development. The Bureau then refined the area further and announced the identification of the 

Rhode Island/Massachusetts Wind Energy Area (RIMA WEA) in February 2012. BOEM intends 

to hold a lease sale for the RIMA WEA in 2013, with plans to issue leases corresponding to the 

entire WEA. The Bureau initially proposed to delineate the RIMA WEA into two leasing 

areasnorth and southfor the lease sale. The north leasing area comprises 97,498 acres and 

the south leasing area comprises 67,252 acres, as shown in Figure 1. 
2 

Figure 1. Proposed lease areas for RIMA WEA (north and south leasing areas) 

(Source: BOEM)

1.2 Summary of Proposed Task Work 

BOEM has proposed that the RIMA WEA be delineated into a north leasing area and a south 

leasing area, and provided NREL with these leasing area delineation coordinates for review. 

NREL assessed BOEM’s recommendation and the proposed sale notice, as well as the economic 

information that informed the proposed sale notice. NREL evaluated the proposed delineation on 

the basis of several technical criteria, including (but not limited to) total energy production 

capacity, wake losses, and water depth. This technical report summarizes the findings of the 

NREL analysis and identifies any potential technical consequences resulting from BOEM’s 

leasing area delineation. 

BOEM also provided NREL with the eight nominations submitted in response to BOEM’s 

Federal Register Notices for use in NREL’s evaluation of the RIMA WEA leasing area

delineation. These nominations, in many instances, included detailed project information such as 

siting constraints, project layout specifications, turbine type and size, foundation type, project 

capacity, development schedule, interconnect points, and various cost indices. NREL determined

which (if any) of the provided information should be considered in the development of the 

leasing area identification and delineation methodology. As part of that process, NREL

researchers considered factors such as meteorological information and potential wake effects 

between leasing areas.
3 

2 Literature Review

During this project, NREL researchers reviewed the following technical documents and 

communications: 

• Rhode Island Coastal Resource Management Council OCEAN SAMP (RI SAMP 2010) 

and interviews with University of Rhode Island (URI) staff. 

• BOEM Wind Energy Area delineations, as described in the proposed sale notice, and 

economic considerations used by BOEM to inform the proposed sale notice. 

• BOEM Call for Information and Nominations for Commercial Leasing for Wind Power 

on the OCS off of Rhode Island and Massachusetts (the “Call”) and responses to the Call.

2.1 Rhode Island Coastal Resource Management Council SAMP

The Rhode Island Special Area Management Plan (SAMP) document contains large amounts of 

technical detail on the proposed offshore wind areas including wind resource, bathymetry, 

geotechnical, marine transport and shipping, recreation and tourism, fisheries, and environmental 

considerations. In addition, the document highlights several key issues for offshore wind energy 

development, but most of those issues could not be evaluated for this analysis. For example, the 

SAMP report discusses the presence of glacial moraine in the soils, with higher levels found in 

the south leasing area. This presence could limit the type of turbine foundation that can be used 

in these areas. However, the report did not provide enough data on where these soils are present 

and how much area is affected, so it was not possible to make a strong case for modifying the 

leasing area on this criterion alone. 

The original SAMP document contains information for the area bounded by the coordinates 

shown in Figure 2. This SAMP area is significantly larger than the current proposed RIMA WEA

shown in Figure 1. 
4 

Figure 2. Rhode Island SAMP study area 

(North: 41.497420°; South: 40.912180°; West: -71.907426°; East: -70.848987°)

(Source: RI SAMP 2010)

2.2 BOEM WEA Delineations

The current WEA under consideration, shown in Figure 1, evolved significantly over time as 

input was received by BOEM from various stakeholder groups and as a result of the SAMP 

process. NREL used the final north and south leasing areas shown in Figure 1 for RIMA WEA to 

perform the analysis in this report. It should be noted that the leasing areas are not equal in terms 

of area, depth, and distance to shore, and most likely will be valued differently by offshore wind 

developers. However, assuming that the boundaries of the WEA are fixed, the BOEM proposed 

delineation offers two distinct leasing areas, each with advantages and disadvantages that are 

separated by external buffer zones. 

NREL was not asked to evaluate alternative delineation methods as part of this report, but a 

qualitative NREL assessment suggests that an additional delineation, from north to south, 

through the north leasing area could create two approximately equal leasing areas in terms of 

area, potential power production, and bathymetry. This alternative would subdivide the shallower

north leasing area into two areas with approximately equal near-term development potential, but 

with a developable capacity only up to 500 megawatts (MW) per leasing area. 

2.3 Evaluation of RIMA Call Notice Responses

NREL was provided with confidential access to the eight industry responses to the Call. These 

nominations served to provide insight into the commercial sector considerations for offshore 

development and wind energy leasing area delineation. 
5 

The Call responses addressed the following topics as part of their RIMA development 

recommendations: 

• Wind resource 

• Bathymetry and geotechnical attributes

• Marine transport and shipping 

• Recreation and tourism 

• Fisheries and commercial interests

• Environmental factors. 

Key findings from NREL’s review of the nominations are shown in Table 1. Because of

confidentiality requirements, the project data from the industry responses were reduced to 

statistical averages, and maximum and minimum values and area compared to the nominal 

values determined from the NREL analysis. 

Table 1. Summary of Nomination Statistics from the BOEM Rhode Island/Massachusetts Wind 

Energy Area Call (Source: NREL)

Average Maximum Minimum NREL Values

Project nameplate capacity (MW) 1,038 2,000 501 3,395

Turbine nameplate capacity (MW) 4.76 (Model is 5 MW) 7 3 5

Avg wind resource (meters per 

second) at 90 meters (m)

9.1 9.6 8.4 9.2

Proposed project area (km2) 482 750 262 667

Array spacing 

(rotor diameters)

12D x 12D 15D x 15D 9D x 9D 8D x 8D

Array power density (MW/km2) 2.2 1.4 3.9 4.9

Number of turbines 209 334 140 679

Notes: 

1. NREL used the RIMA WEA (8/8/12) for its analysis, which differs from the WEA considered by 

developers during the call for nominations. 

2. Array power density computation assumes 126-m NREL reference turbine dimensions and 

nameplate power capacity. 

Note that NREL’s assessment used conservatively high turbine array density (5 MW/km2 and 8D 

x 8D turbine spacing) that is consistent with the gross resource estimations carried out in 2010 

(Schwartz et al. 2010), but may not reflect current best practices. This differs from the nominal 

proposed offshore wind projects for the RIMA WEA. However, NREL wanted to remain 

consistent with past analysis and used a high density array to induce enhanced wake effects for 

the purposes of this investigation. Array densities proposed by developers ranged from 1.4 to 3.9 

MW/ km2

. This turbine spacing tends to reflect an industry trend toward more cautious siting 

practices to avoid large wake losses. This trend can be contrasted against early wind projects like 

the 80 turbine Horns Rev project in Denmark, which used higher array power densities (6.4 MW/ 

km2 and 7D x 7D turbine spacing). Although Horns Rev is not the highest density offshore array, 

its performance is well-documented with array losses over 10% (Hansen et al. 2012). 
6 

During the nomination evaluation, some prospective developers expressed concerns about the 

economic and technical viability of development in deeper water [40 or more meters (m)], that is 

prevalent in the south leasing area. These findings are generally consistent with typical offshore 

project proposals, industry experience, and NREL’s prior experience, and did not significantly 

influence the conclusions of this report or the NREL evaluation of the proposed BOEM 

delineation of the RIMA WEA. 
7 

3 NREL Methodology

NREL’s technical assessment of the leasing area delineation included the use of input data from 

a variety of sources to model and compare key parameters such as maximum development 

capacity, wind speed, wind direction, and wake losses. To model these parameters, gridded 

layouts representative of maximum turbine deployment were created and evaluated with the 

OpenWind Enterprise tool developed by AWS Truepower. 

3.1 Evaluation and Use of SAMP and Nominations Information

NREL evaluated both the SAMP and industry nominations, as described in Section 2. These

evaluations were used to guide the analysis, along with NREL’s own expertise, available data, 

and numerical models. Note the following important caveats about these documents: 

• The SAMP analysis is useful in informing the current WEA; however, the SAMP 

analysis area is much larger than the current WEA established by BOEM in February 

2012. Therefore, the conclusions drawn from SAMP are broader and may be less 

applicable to the WEA in its current configuration.

• The current WEA established in February 2012 is also smaller than the area referenced in 

the Call, released in August 2011. Therefore, broader conclusions drawn from the 

nominations may also have diminished applicability to the WEA in its current 

configuration. 

NREL researchers utilized the available information to guide our analysis and ensure that the 

analysis included all potential scenarios from the developers. Specifically, researchers focused 

on the wind resource, array spacing, and nominated areas to inform the analysis. NREL is unable 

to present additional specifics from the Call documents because of their proprietary nature; 

however, many of the documents included perceptions on economic drivers, which generally 

agreed with the conclusions provided in this document. 

3.2 Characterization of Wind Resource and Wake Effects

To evaluate the wind resource across the RIMA WEA, NREL researchers used time-seriesmodeled wind data from AWS Truepower. Researchers also determined the wind speed gradient 

across the WEA and the directional distribution. Wake losses were evaluated using the Deep 

Array Fast Eddy-Viscosity Wake Model (DAWM Fast Eddy-Viscosity) in OpenWind Enterprise 

(see Section 3.3.2) and the gross capacity factor was determined for each leasing area by 

accounting for only the losses that were caused by wake effects. 

3.3 OpenWind Enterprise Tool

The OpenWind Enterprise tool is a wind energy facility design tool created by AWS Truepower 

(validated in 2010) and licensed to NREL (AWS Truepower 2010). The tool can be used for 

layout design, flow modeling, wake modeling, and energy assessment; and it was selected for its 

interoperability with geographic information system (GIS) data as well as its ability to model 

deep array wake effects. OpenWind Enterprise is intended for commercial applications. The 

following paragraphs describe the primary OpenWind components. 
8 

3.3.1 WindMap Flow Model

The WindMap flow model within OpenWind is based on the NOABL code (Phillips 1979) and 

solves the conservation of mass equation to generate a three-dimensional wind flow map. The 

model accounts for moderate changes in terrain and surface roughness when used in conjunction 

with measured time series meteorological data. 

3.3.2 Wake Model

Wind turbine wake modeling is an emerging science and carries a relatively high uncertainty. 

Uncertainties include measurement uncertainty, the effect of wake meandering, and even 

fundamentals such as the correct choice of free-stream wind speed profile (Barthelmie et al. 

2005). As a result, it is difficult to make an accurate comparison of the different wake models 

that are currently available. Wake models and scientific approaches are evolving rapidly. More 

computationally intensive research methods that are used to calculate wakes are currently applied 

in research laboratories that are not yet practical for commercial use (e.g., large-eddy 

simulations) (Churchfield et al. 2012). Even though these methods may produce more accurate 

results, they are still under development and are computationally too expensive to be used for 

wind energy evaluations like the one conducted for this report. As of the writing of this report, 

the OpenWind DAWM model is one of the most widely used and accepted tools in 

industry. NREL’s prior experience (mostly land-based) indicates that the OpenWind DAWM 

model performs better than other models currently available. 

The DAWM Fast-Eddy Viscosity within OpenWind (AWS Truepower 2010) is a combination of 

the open-source standard Eddy-Viscosity (EV) model and a roughness effect associated with 

each turbine. 

3.3.3 Layout Design

The gridded turbine layer function within OpenWind was used to create maximum capacity 

layouts to fill the WEA zones. An assumption of 8D x 8D (rotor diameter) turbine spacing was 

used by NREL as the reference spacing for the analysis, based on an assumed 5-MW/km2

resource capacity, when calculating the global gross offshore wind resource potential of the OCS

(Musial and Ram 2010). As noted in Section 2, potential RIMA developers prefer larger array 

spacing. Square or triangular tiling is used with manually adjusted bearing, obliquity, and offset 

to obtain the maximum number of turbines. 

3.3.4 Energy Assessment

The energy capture function in OpenWind sums the energy produced by the turbines using 72 

direction sectors and 71 wind speed steps. It calculates and stores the energy yield, capacity 

factor, and wake losses associated with each turbine. Losses other than those caused by the wake 

effect can be accounted for by directly entering assumptions or calculating from other layers.
9 

4 Results and Conclusions

4.1 Summary of Results

The comparison of the north and south leasing areas of the Rhode Island/Massachusetts (RIMA)

Wind Energy Area (WEA) shows important differences between the two leasing areas in terms 

of potential development challenges and cost. Generally, researchers from the National 

Renewable Energy Laboratory (NREL) found that the north leasing area, as delineated by the 

Bureau of Ocean Energy Management, had more favorable characteristics with respect to 

maximum potential project size and ease of development, considering the criteria listed in Table 

2. 

Overall, the south leasing area had more favorable access to the southwesterly winds and would 

have a better wind resource and lower wake losses. However, the better wind and unobstructed 

exposure in the south leasing area is unlikely to offset the increased costs and technical 

challenges of its deeper water.

Table 2. Evaluation Criteria used to Assess RIMA Wind Energy Areas

Quantitative Evaluation Criteria Qualitative Evaluation Criteria Considered

Resource area (acres and km2) Seabed soil conditions

Development capacity potential (megawatts) Distance from shore

Bathymetry (m) Technology challenges

Gross capacity factor (%) Development timing

Wind resource (m/s) Development cost

Wind direction and prevailing conditions

Wake losses (%)

Table 3 provides a comparison of the quantitative metrics for the RIMA Wind Energy Area 

leasing areas as proposed by BOEM. The north leasing area is larger, shallower, and closer to 

shore than the south leasing area. The annual average wind resource slightly favors the south 

leasing area, as indicated by the annual average wind speeds and gross capacity factors (less 

estimated wake losses) shown in the table. Both areas could provide over 1,000 megawatts 

(MW) of developable wind area (1,955 MW in the north leasing area and 1,440 MW in the 

south) based on the NREL 5-MW reference turbine and nominal 8 rotor diameters (8D x 8D) 

array spacing (Jonkman et al. 2009, Musial and Ram 2010). Using these criteria, the wake loss 

analysis that was calculated using the AWS Truepower OpenWind Enterprise tool showed that 

the north leasing area was more burdened by wake losses than the south leasing area, but natural 

and developer-imposed internal buffer zones could mitigate these losses.

Assuming that the boundaries of the WEA are fixed, NREL concluded that the BOEM leasing 

area delineation of north and south was a logical division that provided two leasing areas with 

external buffers, but differing attributes. 
10 

Table 3. Rhode Island/Massachusetts BOEM Wind Energy Area Characteristic Comparison

(Source: NREL)

Parameter North Leasing Area South Leasing Area

Total area (km2) 394.6 272.2

Total area (acres) 97,498 67,252

Potential installed capacity (MW) 1,955 1,440

Average annual wind speed (m/s) 9.1 9.2

Average depth (m) 35.7 45.9

Gross capacity factor including wake losses (%) 46.6 47.4

Wake losses (%) 11.3 10.2

Potential annual energy production (AEP) gigawatt-hours (GWh) 7,982 5,985

Assumptions: 

NREL 5-MW turbine with 8D x 8D array spacing

4.2 Leasing Area Development Capacity

The development capacity of the two leasing areas was evaluated by creating layouts that 

maximized the developable area of the leasing zones using the NREL 5-MW reference turbine 

(Jonkman et al. 2009) with 8D x 8D spacing. In creating these layouts, no additional buffer zones 

were added other than the external buffer (~20D) between the two leasing areas was applied. The 

potential nameplate capacity in megawatts, as a function of depth, was also analyzed and 

compared. For this analysis, it was assumed that all areas were developed, regardless of potential 

technology challenges in the deeper aliquot portions. As shown in Figure 3, the entire WEA was 

filled with 5-MW turbines in a gridded array to maximize the 8D x 8D turbine density for the 

OpenWind analysis. This allowed the researchers to make a determination of the upper bound for 

development potential and wake losses. 

We discovered that both areas could support large-scale offshore wind project development with 

over 1,000 MW of developable wind area. The north leasing area has a greater maximum 

capacity with 1,955 MW, representing about 58% of the total capacity of the WEA, than the 

south leasing area, which has 1,440 MW, representing about 42% of the WEA. 

Some additional observations by NREL that may affect development capacity are as follows:

• The three islanded aliquots of the north leasing area may be more difficult to develop due 

to their distance (~10 km) to the main lease blocks. The additional revenue from the 

turbines may not outweigh the additional cost of the electrical collection system and other 

infrastructure required to bridge the distance.

• The 10 islanded aliquots in the south leasing area are all in shallower depths and 

therefore more comparable to the north leasing area. These aliquots could be reallocated 

to the north leasing area to create a more depth-homogeneous project area. This 

reallocation would add up to 90 MW of capacity to the north leasing area.
11 

Figure 3. Turbine 8D x 8D layout and depth map for the north and south leasing areas 

(Source: NREL)

4.3 Bathymetry 

Water depth is a significant driver in the cost of offshore wind and was a primary criterion that 

was used by NREL to evaluate the BOEM delineation of the RIMA Wind Energy Area. Table 4

shows the maximum deployment capacity that can be supported by the two proposed leasing 

areas by depth. The analysis shows (quantitatively) that the south leasing area is significantly 

deeper.

North Zone

South Zone

External Buffer Zone ~20D

North Zone

Islanded Aliquots

South Zone 

Islanded Aliquots
12 

Table 4. Potential Installed Wind Capacity by Leasing Area and Depth (Source: NREL)

Depth 

Range

North Leasing Area South Leasing Area Total Wind Energy Area

Capacity 

[Megawatts 

(MW)] 

Turbines Capacity (MW) Turbines Capacity (MW) Turbines

25−30

meters (m)

45 9 0 0 45 9 

30−35 m 990 198 55 11 1,045 209

35−40 m 685 137 110 22 795 159

40−45 m 220 44 400 80 620 124

45−50 m 15 3 590 118 605 121

50−55 m 0 0 285 57 285 57

Total 1,955 391 1,440 288 3,395 679

The north leasing area corresponds to an average water depth of about 35 meters (m), with 

significant capacity in waters below this depth. However, 96% of the capacity of the south 

leasing area corresponds to a depth of greater than 35 m, with 88% above 40 m in depth. 

The greater overall depth of the south leasing area represents a significantly higher cost of 

development (with an estimated 20%−40% increased substructure cost) and would require 

technology that has not yet been demonstrated on a large scale in European wind facilities. 

Figure 4 shows the current projects installed, under construction, contracted, or approved in 

Europe as a function of water depth and distance from shore. 
13 

Figure 4. Current offshore wind projects in Europe (installed, under construction, contracted and 

approved) as a function of water depth and distance to shore

(Source: NREL)

Figure 4 shows that most of the technology for support structures has not been deployed beyond 

the 40-m depth. A large part of this is due to the fact that shallow waters are more abundant in 

the North Sea and European offshore wind developers have not yet had to venture into deeper 

waters. 

Figure 5 shows that approximately half the cost of offshore wind projects is related to balance of 

station (BOS). BOS is defined as the balance of equipment on a wind project that is not the 

turbine, namely foundations and support structures, installation costs, substations, and the 

electrical collection system. Therefore, some of the biggest cost drivers for offshore wind 

projects are factors affecting BOS. 

Deeper waters would increase the amount of steel required for a given support structure and 

could limit the feasibility of some support structure options. The depth of the waters in the 

RIMA WEA, particularly for the south leasing area, could also limit vessel options and increase 

the cost of the vessels needed by dictating more specialized capabilities. As such, NREL 

anticipates that the south leasing area may have a later project start of several years compared to

the north leasing area because of increased costs and technical challenges that are a consequence 

of deeper water. 
14 

Figure 5. Offshore wind project development cost breakdown 

(Source: NREL)

Figure 6 shows a trend of rising cost as water depth increases (Maples 2012). Although this 

figure only extends to a depth of 30 m, this preliminary analysis led researchers to conclude that 

the model would likely show a cost trend that continues to rise, and perhaps rise more steeply at 

greater depths. 

 

Figure 6. BOS development cost by depth 

(Source: Maples 2012)
15 

4.4 Wind Resource

The RIMA WEA wind resource is dominated by strong southwest winds, with an annual average 

wind speed shown in Figure 7 (approximately 9.1 m/s in the north leasing area compared to 9.2 

m/s in the south, both at 90 m hub height). The southeast has slightly higher winds than in the 

northwest. The wind speed gradient across the entire RIMA WEA is small and only about 0.2 

m/s. The prevailing winds, shown by the wind rose in the center of Figure 7, and in more detail 

in Figure 8, come from a well-defined prevailing southwest component. 

Figure 7. RIMA WEA showing the annual average wind speed in 0.1 meters per second (m/s) 

increments and wind roses for each leasing area 

(Source: NREL)
16 

4.5 Gross Capacity Factor and Wake Effects

Figure 8 shows a wind rose of the annual wind distribution for the RIMA WEA. The prevailing 

wind comes from the southwest direction. Therefore, there is potential for the north leasing area

to experience greater wake effects from a fully developed south leasing area. These interarray 

effects would be greatest among the north leasing area turbines that are placed adjacent to the 

external buffer dividing the two leasing areas, although the OpenWind model predicts that the 

overall wake losses will differ by just over 1% between the two leasing areas. Generally, the 

strong unidirectional component from the southwest prevailing winds may make array 

orientation more straightforward, which could help developers optimize layout design for both 

leasing areas.

Figure 8. RIMA WEA annual average wind rose showing strong prevailing winds from 

the southwest (Source: NREL)

Table 5 provides an assessment of average wake losses predicted by OpenWind for both leasing 

areas assuming that they are fully developed and using 8D x 8D spacing. All stated capacity 

factors include wake losses but do not account for other losses such as electrical transmission 

losses.
17 

Table 5. Capacity Factor (Less Wake Losses) and Wake Losses by Leasing Area 

(Source: NREL)

Parameter North Leasing Area South Leasing Area 

Capacity factor 46.6% 47.4% 

Wake loss 11.3% 10.2% 

Developer imposed increases in array spacing and internal buffer zones may reduce wake losses 

considerably. Also, variations in bathymetry within both leasing areas may inhibit uniform grid 

spacing in which could provide internal buffer zones within the WEAs, having the benefit of 

potentially reducing wake losses and project conflicts. The built-in 20-D external buffer between 

the north and south leasing areas will also help mitigate waking between the leasing areas 

(Barthelmie 2010).

4.6 Qualitative Evaluation Criteria

Several qualitative criteria were considered in this evaluation including seabed conditions, 

distance from shore, development cost and timing, and technology challenges. 

Concerns regarding seabed soil conditions have been raised in the Special Area Management 

Plan and by the University of Rhode Island. Glacial moraine substrates that may prevail in some 

areas could be unsuitable for certain foundation types, thereby further impacting development 

costs, especially in the south leasing area. If certain foundation types (e.g., pile-driven 

substructures) are eliminated because of unsuitable soil conditions, the impact on development 

cost could be significant, but not enough is known at this time to draw this conclusion. 

The south leasing area is significantly further from the shore than the north leasing area, which 

would also contribute to increased development costs. Distance from shore would increase costs 

by lengthening travel time for vessels moving to and from the wind facility as well as by 

increasing the overall length of the export cable. NREL does not have data to quantify the effect 

of distance from shore for this analysis, but the impact on cost is not expected to be as significant 

as water depth.

Section 4.3 concludes that water depth will be a major cost driver in the development cost and 

timing of the RIMA WEA. Some analysis is provided that supports this conclusion, but a full 

quantitative analysis is beyond the scope of this report. 

4.7 Key Findings

Below are the key findings of the NREL analysis and important considerations for policy makers 

and stakeholders involved in the RIMA offshore wind energy development process. 

• The north leasing area is more economically and technically developable because it has 

shallower bathymetry, is closer to shore, and has a larger area than the south leasing area. 
18 

• Assuming that the boundaries of the wind energy area are fixed, the BOEM leasing area 

delineation of north and south was a logical division, but would not provide equal leasing 

areas in terms of potential development costs and resource access.

• It may be possible to split the north leasing area into two smaller leasing areas that could 

provide a nameplate development capacity of up to 500 MW. 

• Both leasing areas are capable of supporting at least 1,000 megawatts (MW) of potential 

installed capacity. The potential installed capacity of the north leasing area is 1,955 MW 

and the south leasing area is 1,440 MW, based on dense 8D x 8D spacing with no internal 

buffers. 

• The north leasing area can implement current fixed-bottom wind turbine technology in 

the near term. The south leasing area will be challenged by limits to current experience 

with fixed-bottom turbine technology with 88% of the resource area at 40-m+ depths and 

where commercially viable technical solutions have not been fully demonstrated (Musial 

and Ram 2010). 

• The south leasing area may have a later project start of several years than the north

leasing area because of increased costs and technical challenges associated with the 

deeper water in the south leasing area. 

• The three islanded aliquots of the north leasing area are more difficult to develop because 

of their distance from (~10 km) the main lease blocks. The additional revenue from the 

remote turbines may not outweigh the additional cost of the electrical collection system 

and other infrastructure required to bridge the distance. 

• The ten islanded aliquots in the south leasing area are all in shallower depths that are 

more comparable to the north leasing area. As a result, these aliquots could be reallocated 

to the north leasing area to create a more depth-homogeneous project area. This 

reallocation would add approximately 90 MW of capacity to the north leasing area. 

• Bathymetry variance within each leasing area may result in external buffer zones within 

the WEAs, potentially reducing wake losses and project conflicts. The built-in 20-D 

external buffer between the north and south leasing areas will also help mitigate waking 

between the leasing areas (Barthelmie 2010).

• The wind resource is slightly better in the south leasing area, with estimated annual 

average wind speeds that are approximately 0.1 m/s higher, and with better exposure to 

predominant southwestern winds. In addition, this leasing area will have smaller potential 

wake losses. However, this benefit will fully offset higher development costs from deeper 

water and greater distance to shore. 

• Prevailing winds have a strong unidirectional component from the southwest, making it 

more straightforward to orient the array, a characteristic which could help optimize 

layout design for both leasing areas. 
19 

References 

AWS Truepower, LLC. (2010). OpenWind Theoretical Basis and Validation. 

http://www.awsopenwind.org/downloads/documentation/OpenWindTheoryAndValidation.pdf. 

Accessed April 6, 2013.

Barthelmie, R.J., Folkerts, L., Larsen, G.C., Rados, K., Pryor, S.C., Frandsen, S.T., Lange, B., 

Schepers, G. ( 2005). Comparison of wake model simulations with offshore wind turbine wake 

profiles measured by sodar. Wind Energy, 23,888-901. 

Barthelmie, R.J., Pryor, S.C., Frandsen, S.T., Hansen, K.S., Schepers, J.G., Rados, K., Schlez, 

W., Neubert, A., Jensen, L.E., and Neckelmann, S. (2010). Quantifying the impact of wind 

turbine wakes on power output at offshore wind farms. Journal of Atmospheric and Oceanic 

Technology, 27(8), 1302–1317. DOI: 10.1175/2010JTECHA1398.1. 

Churchfield, M. J.; Lee, S.; Michalakes, J.; Moriarty, P. (2012). “A numerical study of the 

effects of atmospheric and wake turbulence on wind turbine dynamics.” Journal of Turbulence, 

13, N14. http://dx.doi.org/10.1080/14685248.2012.668191.

Hansen, K.S., Barthelmie, R.J., Jensen, L.E., Sommer. A. (2012). The impact of turbulence 

intensity and atmospheric stability on power deficits due to wind turbine wakes at Horns Rev 

wind farm, Wind Energy, 15(1), 183–196. DOI: 10.1002/we.512. 

Jonkman, J.; Butterfield, C.P.; Musial, W.D.; Scott, G. (2009). “Definition of a 5-MW Reference 

Wind Turbine for Offshore System Development,” National Renewable Energy Laboratory. 

Maples, B. (2012). “U.S. Balance-of-Station Cost Drivers and Sensitivities.” Podium 

presentation at AWEA Offshore Windpower Workshop 2012, Virginia Beach, VA, October 10, 

2012. 

Musial, W.; Ram, B. (2010). “Large-Scale Offshore Wind Power in the United States: 

Assessment of Opportunities and Barriers.” NREL/TP-500-40745. Golden, CO: National 

Renewable Energy Laboratory, 240 pp. 

Phillips, G.T. (1979). “A Preliminary User’s Guide for the NOABL Objective Analysis Code.” 

DOE/ET/20280-T1. 

RI Coastal Resource Management Council OCEAN SAMP – Rhode Island Ocean Special Area 

Management Plan (RI SAMP). (2010). http://www.crmc.ri.gov/samp_ocean.html. Volume I&II, 

October 2010. 

Schwartz, M.; Heimiller, D.; Haymes, S.; Musial, W. (April 2010). Assessment of Offshore 

Wind Energy Resources for the United States. NREL/TP-500-45889. Golden, CO: NREL.