Table 1.

Baseline Parameters (Fixed/Floating)

Project Size 

(megawatts [MW])

600

Turbine Rating 

(MW)

6 

Rotor Diameter 

(meters [m])

155

Hub Height 

(m)

90

Distance to Shore 

(kilometers [km])

40

Distance to Installation Port 

(km)

60

Water Depth 

(m)

25/250

Array Spacing 

(rotor diameters)

9x9

Contacts: tyler.stehly@nrel.gov; michael.maness@nrel.gov

Introduction

Conclusions

Figure 3. Fixed BOS costs distributed over more megawatts results in a decrease in cost 

per megawatt-hour to a point. Eventually a steady increase in cost is caused by 

overestimated cable lengths connecting turbines to the offshore substation because

arrays are modeled in a rectangular grid layout and only one offshore substation is

assumed for all plant sizes. The saw tooth shape is a result of how turbines are located

within strings. Sharp cost increases happen when a new string of turbines is created that

contains only one turbine and as more turbines fill this string costs decrease until a new

string is needed. The larger, less frequent sharp changes in cost result from the model

optimizing, for lowest cost, array and export cable size selection (including installation

cost). Spikes in cost are a result from the model selecting array and export cables that are

capable of safely transferring generated power at the lowest cost, which, depending on

input parameters, can yield a rise in BOS cost, such as the spike near 500 megawatts. 

Figure 4. Offshore wind fixed substructure BOS costs decrease as turbine rating

increases, which is largely driven by the reduction in installation time required because

fewer turbines are needed to achieve the same plant capacity. The jagged shape of the

graph is also a result of how turbines are located within strings and how the model

optimizes for lowest cost cable size selection (including installation cost). 

Figure 5. Offshore wind floating substructure BOS costs also decrease with increasing

turbine size primarily because of reduced installation time. The cost decrease is more

gradual than fixed substructures because floating substructure and foundation costs are

more dependent on turbine size. The increase in substructure costs slightly offsets the

cost savings from the use of fewer turbines and a shorter installation time. 

Figure 6. Export cable costs are driven primarily by the minimum amount of export

cables required to safely transfer the power generated by the wind plant to the landbased substation. For example, three 1,000 square millimeter (mm2) cables are

necessary to transfer the same power as two 1,200 mm2 cables. The 1,200 mm2 cable

is more expensive per meter of cable length than the 1,000 mm2 cable, but the total

export cable length is much greater when more cables are required. Thus, the total

installed capital cost is greater for the smaller 1,000 mm2 cable size in this case. This

tradeoff between cable size and the number of cables required is represented in the

figure by the large peak followed by a reduction in total capital cost as the cable size 

switches from 1,000 mm2 to 1,200 mm2. 

Figure 7. As the water depth increases so does the offshore wind BOS cost. This

effect is mainly driven by the substructure and foundation cost, which increases

because of the need for larger, more robust support structures in deeper waters. The

overall BOS cost rises faster than for floating substructures because of increased

depth for fixed substructures; the economic viability of fixed substructures is more

dependent on water depth than floating substructures. 

Figure 8. Offshore wind BOS costs increase with deeper water. The floating bottom

relationship is relatively linear and primarily driven by electrical infrastructure and

assembly and installation costs rather than foundation costs. The escalation in

foundation component costs like mooring lines, which must expand in length with

deeper water, is relatively small compared to the rise in cost from electrical

components and assembly and installation. 

Offshore Wind Balance-of-System Cost Modeling

Michael Maness, Tyler Stehly, Ben Maples, Christopher Moné 

Development

Engineering and

Management 

Port & Staging

Substructure and

Foundation

Electrical Infrastructure

Assembly and Installation

Plant Commissioning

Figure 3. Sensitivity to plant size Figure 4. Sensitivity to turbine size Figure 5. Sensitivity to turbine size

Figure 6. Sensitivity to export cable size Figure 7. Sensitivity to water depth Figure 8. Sensitivity to water depth

Offshore wind balance-of-system (BOS) costs contribute up to 70% of

installed capital costs. Thus, it is imperative to understand the impact

of these costs on project economics as well as potential cost trends

for new offshore wind technology developments. As a result, the

National Renewable Energy Laboratory (NREL) developed and

recently updated a BOS techno-economic model using project cost 

estimates created from wind energy industry sources. 

Updates to the model help analyze both fixed and floating

substructure types. Other updates include a subsea cable cost 

optimizer and improved scaling relationships. Figure 1 and Figure 2

show the BOS cost breakdown by category for fixed and floating

substructures, respectively. Electrical infrastructure, substructure and

foundation, and assembly and installation categories dominate BOS

costs for both substructures. 

NREL performed analyses using common baseline parameters 

(Table 1) to see what effect(s) changing one variable at a time would

have on BOS costs. The results of this analysis are meant to

demonstrate the capabilities of the offshore BOS model rather than to

compare fixed versus floating substructures. The results of these

analyses should be taken as representative only, because of the high

level of variability of project parameters and site-specific elements. 

Figure 1. BOS cost breakdown

(fixed substructure)

Figure 2. BOS cost breakdown

(floating substructure)

Wind Plant Sensitivities Fixed Substructure Sensitivities Floating Substructure Sensitivities

The information contained in this poster is subject to a government license.

OFFSHORE WINDPOWER 2015

Baltimore, Maryland

September 29—30, 2015 

NREL/PO-6A20-64789