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Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Reference Model 5 (RM5): 

Oscillating Surge Wave Energy 

Converter

Y.-H. Yu, D.S. Jenne, and R. Thresher

National Renewable Energy Laboratory

A. Copping, S. Geerlofs, and L.A. Hanna

Pacific Northwest National Laboratory

Technical Report

NREL/TP-5000-62861

January 2015
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

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Reference Model 5 (RM5): 

Oscillating Surge Wave Energy 

Converter

Y.-H. Yu, D.S. Jenne, and R. Thresher

National Renewable Energy Laboratory

A. Copping, S. Geerlofs, and L.A. Hanna

Pacific Northwest National Laboratory

Prepared under Task No. WA14.1A01 

Technical Report

NREL/TP-5000-62861

January 2015
NOTICE

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Acknowledgments

This research was supported by the U.S. Department of Energy under contract number DEAC36-08GO28308 at the National Renewable Energy Laboratory and contract number DEAC05-76RL01830 at the Pacific Northwest National Laboratory. Funding for this work was 

provided by the DOE Office of Energy Efficiency and Renewable Energy.
iv

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Executive Summary

This report is an addendum to SAND2013-9040: Methodology for Design and Economic 

Analysis of Marine Energy Conversion (MEC) Technologies. This report describes an oscillating 

surge wave energy converter (OSWEC) reference model design and complements Reference 

Models 1–4 in the above report. 

A conceptual design for a taut, moored OSWEC was developed. The design had an annual 

electrical power of 108 kilowatts (kW), rated power of 360 kW, and intended deployment at

water depths between 50 m and 100 m. The study includes structural analysis, power output 

estimation, a hydraulic power conversion chain system, and mooring designs. The results were 

used to estimate device capital cost and annual operation and maintenance costs. The device 

performance and costs were used for the economic analysis, following the methodology 

presented in SAND2013-9040 that included costs for designing, manufacturing, deploying, and 

operating commercial-scale MEC arrays up to 100 devices. The levelized cost of energy 

estimated for the Reference Model 5 OSWEC, presented in this report, was for a single device 

and arrays of 10, 50, and 100 units, and it enabled the economic analysis to account for cost 

reductions associated with economies of scale. The baseline commercial levelized cost of energy

estimate for the Reference Model 5 device in an array comprised of 10 units is $1.44/kilowatthour (kWh), and the value drops to approximately $0.69/kWh for an array of 100 units. 
v

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Table of Contents

1 RM5 Description ................................................................................................................................... 1

1.1 Device Design and Analysis.......................................................................................................... 2

1.2 Array (Plant) Design and Analysis................................................................................................ 2

2 Module Inputs........................................................................................................................................ 4

2.1 Site Information............................................................................................................................. 4

2.1.1 Bathymetry and Bed Sediments........................................................................................ 4

2.1.2 Operational Wave Characteristics..................................................................................... 5

2.1.3 Extreme Sea States............................................................................................................ 6

2.1.4 Adjacent Port Facilities and Grid Options........................................................................ 7

2.2 Device/Array Information ............................................................................................................. 8

3 Device Design, Performance, and Analysis..................................................................................... 10

3.1 Design and Analysis Module ...................................................................................................... 10

3.1.1 Materials Specifications and Structural Analysis ........................................................... 10

3.1.2 Performance Analysis and AEP Estimation.................................................................... 14

3.1.3 Power Conversion Chain (PCC) ..................................................................................... 17

3.1.4 Foundation and Mooring Design .................................................................................... 20

3.2 Manufacturing and Deployment Strategy Module...................................................................... 21

3.2.1 Manufacturing Strategy and Costs.................................................................................. 21

3.2.2 Deployment Strategy and Costs...................................................................................... 23

3.3 Operations and Maintenance....................................................................................................... 25

3.3.1 Service Vessel Specifications......................................................................................... 25

3.3.2 Failure Rates ................................................................................................................... 26

3.3.3 Annual O&M Costs ........................................................................................................ 27

3.4 Environmental Analysis and Compliance ................................................................................... 28

3.5 LCOE Calculation....................................................................................................................... 30

3.6 Uncertainty in Design and Economic Analysis........................................................................... 33

4 Conclusion .......................................................................................................................................... 38

References................................................................................................................................................. 39
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List of Figures

Figure 1. RM5 oscillating surge wave energy converter design................................................................... 1

Figure 2. Side view of the RM5 OSWEC design ......................................................................................... 2

Figure 3. Planned array layout...................................................................................................................... 3

Figure 4. Device interconnection cable, riser cable, and junction box ......................................................... 3

Figure 5. Local site bathymetry plan and reference site grid interconnection options ................................. 4

Figure 6. 100-year contour for NDBC buoy #46212 .................................................................................... 7

Figure 7. NOAA nautical chart of Humboldt Bay, California...................................................................... 8

Figure 8. RM5 surge flap design................................................................................................................. 10

Figure 9. RM5 frame design ....................................................................................................................... 11

Figure 10. An example of pressure distribution (scaled by water density, gravity, and wave amplitude) 

from WAMIT (T=7 sec) when the flap is at its mean position.............................................. 12

Figure 11. Representation of mesh used for the FEA analysis of the RM5 flap and frame........................ 12

Figure 12. FOS plot for the RM5 flap......................................................................................................... 13

Figure 13. FOS plot for the RM5 frame...................................................................................................... 14

Figure 14. Model setup in WEC-Sim.......................................................................................................... 15

Figure 15. RM5 power conversion chain layout......................................................................................... 17

Figure 16. Hydraulic circuit for the RM5 design (Neary et al. 2014)......................................................... 17

Figure 17. PCC assembly and enclosure (three walls removed)................................................................. 18

Figure 18. Flow profile of the RM5 power conversion chain..................................................................... 19

Figure 19. Mooring design for the RM5 OSWEC...................................................................................... 20

Figure 20. RM5 structural cost breakdown ($/kW) per deployment scale ................................................. 21

Figure 21. Cost breakdown ($/kW) for the PCC components per deployment scale ................................. 22

Figure 22. RM5 PCC hydraulic component breakdown............................................................................. 23

Figure 23. Installation cost breakdown ($/kW) per deployment scale........................................................ 25

Figure 24. An example of a medium-sized workboat................................................................................. 26

Figure 25. The RM5’s annual OpEx costs ([$/kW] per array size) ............................................................ 28

Figure 26. High-level LCOE (cents/kWh) breakdown per deployment scale for the RM5........................ 30

Figure 27. RM5 CapEx contributions to LCOE (cents/kWh) per deployment scale.................................. 32

Figure 28. RM5 OpEx contributions to LCOE (cents/kWh) per deployment scale ................................... 33

Figure 29. RM5 LCOE for 1, 10, 50, and 100 units ................................................................................... 38
vii

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List of Tables

Table 1. Percentage Occurrence of Sea States at Humboldt Bay, California (NDBC #46212).................... 5

Table 2. Percentage of Total Energy of Sea States at Humboldt Bay, California ........................................ 6

Table 3. Wave Environments Along the 20% Inflated 100-Year Contour................................................... 7

Table 4. RM5 Design Specifications ............................................................................................................ 9

Table 5. List of Design Mass Properties..................................................................................................... 13

Table 6. Electrical Power Matrix for the RM5 OSWEC Design ................................................................ 16

Table 7. Rated Power and Power Output for FPA Device Predicted From WEC-Sim .............................. 16

Table 8. PCC Component Size Breakdown................................................................................................ 20

Table 9. RM5 Mooring System Component Cost Breakdown ................................................................... 23

Table 10. RM5 Manufacturing and Deployment Strategy Module Cost Assumptions.............................. 24

Table 11. Cost and Failure Rate Assumptions for WEC Components (Ten-Unit Cost)............................. 27

Table 12. RM5 Environmental Cost Summary........................................................................................... 29

Table 13. RM5 LCOE Breakdown by Cost Category (10-Unit Array)...................................................... 31

Table 14. Breakdown of RM5 CapEx Contributions to LCOE (10-Unit Array)........................................ 31

Table 15. Breakdown of RM5 OpEx Contributions to LCOE (10-Unit Array) ......................................... 32

Table 16. Uncertainty Matrix for RM5....................................................................................................... 34

Table 17. Assessment of Cost Uncertainty (CapEx)................................................................................... 36

Table 18. Assessment of Cost Uncertainty (OpEx).................................................................................... 37
1

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1 Reference Model 5 Description

Reference Model 5 (RM5) is a type of floating, oscillating surge wave energy converter 

(OSWEC) that utilizes the surge motion of waves to generate electrical power. Several OSWEC 

designs have been proposed by the wave energy technology industry, including Oyster, EBFrond, WaveRoller, Resolute Marine, and Langlee. Because of the potential issues surrounding

permitting and regulation at near-shore shallow water regions (Copping et al. 2014), combined 

with the superior wave resources at deep-water sites (Babarit et al. 2012), this study (conducted 

by researchers from the National Renewable Energy Laboratory [NREL] and the Pacific 

Northwest National Laboratory [PNNL]) focused on deep-water (50 meter [m]–100 m) surge 

designs, in which the devices were moored to the seabed. Figure 1 shows the orientation and 

dimensions of the RM5 OSWEC design. The flap was designed to rotate against the supporting 

frame to convert wave energy into electrical power from the relative rotational motion induced 

by incoming waves.

Figure 1. RM5 oscillating surge wave energy converter design

The RM5 design is rated at 360 kilowatts (kW), uses a flap of 25 m in width and 19 m in height 

(16 m in draft), and the distance from the top of the water surface piercing flap to the mean water 

surface (freeboard) is 1.5 m. The flap is connected to a shaft with a 3-m diameter that rotates

against the supporting frame. The supporting frame is assumed to have an outer diameter of 2 m, 

and the total length of the device structure is 45 m. In addition, we found it effective to attach 

bottom plates to the frame, because the increased mass caused by the associated added mass 
2

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moment of inertia stabilized the frame, resulting in an improvement of the device’s power 

performance. The RM5 OSWEC was designed for deep-water deployment, at depths between 50 

m and 100 m, and was tension-moored to the seabed (Figure 2).

Figure 2. Side view of the RM5 OSWEC design

1.1 Device Design and Analysis

The first step in the device design and analysis process was to develop a conceptual design for a 

wave energy converter (WEC) device appropriate for the modeled reference resource site. Once 

the concept design was completed, detailed device design, performance analysis, and structure 

analysis were performed to estimate the levelized cost of energy (LCOE). The annual energy 

production (AEP) of the RM5 design was determined based on the characteristics of the wave 

resource statistic data from the reference site and the power matrix of the device that gave the 

estimated power prediction of the device for a range of sea states. Extreme loads were estimated 

using a finite-element analysis (FEA) model based on the pressure distribution under large 

operational conditions and the 100-year extreme sea state at the reference site.

1.2 Array (Plant) Design and Analysis

As described in the General Methodology section in the previous reference model report (Neary 

et al. 2014), detailed array design and analysis was not included, thus increasing the uncertainty 

in the array AEP estimates. In the RM5 OSWEC analysis, we assumed:

• A maximum of 100 units could be deployed at the reference site to take advantage of 

reduced costs through economies of scale, thus lowering the LCOE estimates. 

• The required spacing between the devices to accommodate moorings and avoid ocean 

vehicle traffic collisions was 600 m (more than 20 times of the flap width), as shown in 

Figure 3.

The bathymetry and the installation space availability at the deployment site were considered 

when planning the array layout. The chosen spacing also minimized the fluid dynamic 
3

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interaction between devices, ensuring that the loss of energy from the array interaction would be 

negligible (Babarit et al. 2012).

The total capacity of a 100-unit array is approximately 36 megawatts (MW). Similar to the 

Reference Model 3 point absorber project, as described in the previous reference model report

(Neary et al. 2014), the array layout in the RM5 OSWEC project assumed to have the following 

characteristics:

• A three-phase AC transmission cable with a voltage level of 30 kilovolts (kV) was 

selected for the main cable (cable to shore)

• All transmission cables included fiber optic lines to allow communication from each 

device to shore

• Groups of 10 devices (Figure 3) were connected with interconnection cables that ran

between individual units (Figure 4), and the electricity was transmitted to a junction box

via a riser cable; a trunk cable connected the junction boxes

• Cable landing was accomplished by directionally drilling a conduit that connected the 

cable to the first row of devices; this approach minimized installation and maintenance 

costs.

Figure 3. Planned array layout

(Note that the WEC devices are not drawn to scale)

Figure 4. Device interconnection cable, riser cable, and junction box
4

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2 Module Inputs

2.1 Site Information

The reference wave energy resource for RM5 was measurement data from a National Data Buoy 

Center (NDBC) buoy near Eureka, in Humboldt County, California. This reference site has a 

wave climate representative of the West Coast of the United States, with a spectrally computed 

annual averaged wave density of about 30 kilowatts per meter (kW/m) (Dallman and Neary 

2014). Of note, Pacific Gas and Electric Company selected the Humboldt site as one of the 

proposed locations in their WaveConnect Project after studying the possible wave energy 

potential, grid interconnection, and other infrastructure along the coast of California (Dooher et 

al. 2011).

2.1.1 Bathymetry and Bed Sediments

The reference site location for the RM5 OSWEC is the same as that of the RM3 point absorber. 

Figure 5 shows the bathymetry plan, as well as the reference site grid interconnection options.

Figure 5. Local site bathymetry plan and reference site grid interconnection options 

Source: Neary et al. 2014
5

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As mentioned in a previous reference model report (Neary et al. 2014):

• The deployment site has a gently sloping seabed, free of irregularities that could disturb 

the local wave field. Thus, it is likely that the wave field is homogeneous over the 

deployment area of interest.

• Sediment classification enabled a detailed seabed characterization at the reference site, 

which is a sedimentary shelf throughout the deep-water deployment zone. This 

characterization also helped assess the impacts that the RM5 device and array would have 

on the marine environment and ecosystem. 

• Most of the seabed in the near-shore proposed cable route and deployment area in the 

region of the Humboldt site consists of soft sediments (sand and clay) that are well-suited 

for subsea cable burial and anchoring. 

2.1.2 Operational Wave Characteristics

The behavior of irregular waves at a reference site may be provided in terms of a joint 

probability distribution (JPD) of possible binned sea states, with each characterized by

significant wave height Hs and peak period Tp (or energy period Te). The wave resource statistic 

data was measured from the NDBC buoy (#46212) near Humboldt Bay, California, from 2004 to 

2012 (Dallman and Neary 2014). Because the Humboldt site climate has a relative stable wave 

direction for operational waves, the analysis presented in the study assumed unidirectional

waves, and incident wave direction perpendicular to the RM5 OSWEC’s flap surface. Only the 

significant wave height and peak period were used to characterize the wave resource in this 

study. The percentage occurrence of each binned sea state at the reference site is shown in Table 

1. Inside the red zone are the sea states in which the OSWEC is assumed to be operational and 

generating power.

Table 1. Percentage Occurrence of Sea States at Humboldt Bay, California (NDBC #46212)
6

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According to linear wave theory, the wave energy flux for irregular waves in deep water is

Js = ρg2

64π

Hs

2Te

where Js is the wave energy flux per unit of wave-crest length for irregular waves, ρ is the water 

density, and g is the acceleration of gravity. Using the equation above and JPD calculations, the 

percentage of total incident energy at the reference site was determined. The incident energy for 

each binned sea state is shown in Table 2. Note that occurrences or contributions to annual 

power of less than 0.01% are not shown in the two tables.

Table 2. Percentage of Total Energy of Sea States at Humboldt Bay, California

2.1.3 Extreme Sea States

Based on NDBC data gathered off the West Coast of the United States, a typical 100-year 

significant wave height during storms is generally between 8 m and 13 m (Mackay et al. 2010). 

Data from the NDBC #46212 buoy (Dallman and Neary 2014) also contained specific extreme 

wave conditions during storms near the Humboldt site (station #46212). Figure 6 shows the 

scatterplot of measured conditions at NDBC #46212 from 2004 to 2012. Also shown are the 100-

year contour and a 20% inflated contour, accounting for the approximations in the extreme load 

simulations. Table 3 shows the selected wave environments on the 20% inflated contour (open 

circles in Figure 6), which were considered for design load analysis and device structure design 

(described in Section 3.1.1).
7

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Figure 6. 100-year contour for NDBC buoy #46212

Source: Dallman and Neary 2014

Table 3. Wave Environments Along the 20% Inflated 100-Year Contour

Significant Wave 

Height Hs (m)

Energy 

Period Te (s)

1 5.0 7.1

2 7.0 10.3

3 9.0 13.8

4 7.0 16.2

5 5.0 19.5

2.1.4 Adjacent Port Facilities and Grid Options

Figure 7 shows a nautical chart of the Humboldt Bay area from the National Oceanic and 

Atmospheric Administration (NOAA). As described in the previous reference model report 

(Neary et al. 2014), the advantages of port facilities and grid options include the following:

• The Humboldt Bay port is the only deep-water port on California’s north coast. 

Humboldt’s facilities are well-suited for installation and operational activities that would 

be required if a WEC plant was deployed.
8

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• Multiple piers within the bay would greatly facilitate the launching of any WEC 

installation project and provide some of the necessary infrastructure for operational 

activities.

• A 60-kV substation is located approximately 5 miles north of the Humboldt Bay inlet, 

which can easily serve as the interconnection point to the local electrical grid.

Figure 7. NOAA nautical chart of Humboldt Bay, California

Source: Neary et al. 2014

2.2 Device/Array Information

As summarized in Table 4, we determined the conceptual design specifications based on siting 

constraints, device structural calculations, and other considerations. More details are described in 

Section 2.
9

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Table 4. RM5 Design Specifications

Category &

Description Specification Justification Details Environment

Water depth

50 m‒100 m 

(modeled at 70

m)

Site resource 

characteristics

Sufficient depth for deep-water WEC 

design

Operational sea 

states

Te=5 sec~18 sec; 

Hs=0.75 m~6 m

Site resource 

characteristics

Based on the wave statistic data at 

the reference site

Wave energy 

density (flux) 30 kW/m Site resource characteristics Spectrally computed

Directionality Unidirectional Engineering 

judgment

Assuming unidirectional seas in 

power performance calculations

Device

WEC type Terminator WEC architecture Floating oscillating surge WEC

Structure Fiberglass and 

steel FEA

Designed for wave loads under 

extreme sea states (locked flap at 45 

degrees) and large operational 

waves (5 m)

Mooring

Taut mooring 

(four legs and

two lines per leg)

Power 

performance 

improvement

Designed for mooring loads under 

extreme 20% inflated 100-year 

return period sea states

Power 

conversion 

chain

Hydraulic Engineering 

judgment

Two dual-acting hydraulic cylinders 

with energy storage

Performance

Annual 

averaged 

electrical power

108 kW

Numerical 

modeling and 

literature

Used WEC-Sim to calculate the 

mechanical power matrix and 

assumed 82.5% power conversion 

chain efficiency (Neary et al. 2014)

Rated power 360 kW Literature 30% capacity factor (Previsic et al. 

2012; RenewableUK 2010)

Annual energy 

production

882 megawatthoursEngineering 

judgment

98% transmission efficiency; 95% 

device availability (Neary et al. 2014)

Deployment

Array 

configuration

Staggered with a 

separation larger 

than 20 times of 

device width

Literature and 

engineering 

judgment

Avoid device interaction (Babarit et 

al. 2012)
10

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3 Device Design, Performance, and Analysis

WEC devices must be designed to withstand extreme sea states during severe storms. To this 

end, the RM5 team used iterative processes including structure analysis and design performance 

simulations. We first used power-to-displacement-volume ratio calculations to select three 

potential OSWEC design dimensions for further analysis. We then simulated the three designs 

using WEC-Sim to estimate the design power output at the reference site, and compared the 

power-to-characteristic mass ratios (Yu et al. 2014). Based on those studies, design simplicity,

and engineering judgment, a taut mooring OSWEC design was selected for further LCOE 

estimation. This section presents the structural design and stress analysis for the RM5 OSWEC 

and the design power performance estimation, as well as discusses potential improvements to 

enhance power output and reduce LCOE.

3.1 Design and Analysis Module

3.1.1 Materials Specifications and Structural Analysis

This section presents a preliminary structural analysis for determining the mass properties of the

designed OSWEC components. As shown in Figure 1, the RM5 design contained a flap and a 

supporting frame that was made of steel tubes. The flap was assumed to be made of fiberglass 

tubes with steel ribs and connected to a steel rotational shaft.

Figure 8. RM5 surge flap design

The RM5 surge flap design utilizes steel and fiberglass to reduce costs, both capital and 

maintenance, while maintaining a desirable safety factor, as discussed further below. The flap 

structure is comprised of three steel upright sections with tapered profiles. The upright structures 

are attached to an additional steel shaft with a diameter of 3 m. The shaft is attached to the pivot 

point on the RM5 frame. The steel structure is designed to resist the bending moment that occurs
11

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during operation. Because the horizontal tubes do not experience the same magnitude of stress as 

the supporting frame, they do not require steel components. Instead, we installed fiberglass tubes 

horizontally in the upright sections to create the surface area required to capture wave energy. 

Using fiberglass reduces both mass and corrosion-associated maintenance. Figure 8 is a 

representation of the assembled surge flap.

The RM5 frame consists primarily of 2-m-diameter rolled steel tubing. The frame is designed to 

allow for a pivot location for a single flap, mounting locations for mooring, and to minimize 

movement of the frame relative to the seabed. The footprint is approximately 45 m by 28 m.

Figure 9 is a representation of the RM5 frame.

Figure 9. RM5 frame design

When a WEC is deployed, all structural components are subject to not only the stress from

hydrostatic pressure, but also stress from the force of waves passing over the device. To analyze 

these forces further, we evaluated two scenarios. The first focused on extreme sea states, and the 

second focused on large operational wave conditions. For extreme sea states, we assumed the 

RM5 design was not operating, that the flap was locked at 45 degrees, and that the structure was 

subject to 20% inflated 100-year extreme waves. For large operational wave conditions, we 

evaluated the pressure distribution for a flap at its mean position and subject to the operational 

sea states that gave the largest force. The structural calculations were performed using the 

pressure distribution data from WAMIT, and an example of the pressure distribution when the 

flap is at its mean position is plotted in Figure 10.
12

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Figure 10. An example of pressure distribution (scaled by water density, gravity, and wave 

amplitude) from WAMIT (T=7 sec) when the flap is at its mean position

The stress analysis for the flap and frame was performed using static FEA in SolidWorks. To 

reduce the number of elements, while still allowing for convergence, a curvature based mesh was 

used. This allows a nonuniform element size to be used in complex regions, such as where two 

components join. Even with a curvature-based mesh, the relatively small material thickness—in 

relation to the WEC size—requires a large number of mesh elements. The frame’s mesh has 

approximately 520,000 elements and the flap mesh has approximately 675,000 elements.

Figure 11. Representation of mesh used for the FEA analysis of the RM5 flap and frame

The FEA takes account for the nonlinear wave pressure distribution seen on both sides of the flap 

and frame, as well as a linear approximation of the hydrostatic pressure. The flap model was 

fixed using fixed-hinge connections at both the flap pivot locations, and the flap-hydraulic power 

conversion chain (PCC). The frame model was also fixed using fixed-hinge connections, but at 

the mooring pivot locations at each corner of the frame. A minimum Factor of Safety (FOS) of 

1.5 was assumed for all structural components. Figure 12 shows the FOS plot for the RM5 flap 

and Figure 13 shows the FOS plot for the frame, both of which scale FOS from 1 to 5. The final 

mass and Moment Of Inertia (MOI) for the design are given in Table 5.
13

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Table 5. List of Design Mass Properties

Fiberglass 

Mass 

(Tonnes)

Steel

Mass 

(Tonnes)

Center of 

Gravity (m)

MOI at Center of Gravity

(Tonnes-m2

)

Flap 72 428 [0, 0, -11.4]

17,328 0 0

0 56,248 2.93

0 2.93 40,156

Frame None 300 [0, 0, -14.1]

67,105 0 0

0 45,404 51.4

0 51.4 103,249

Figure 12. FOS plot for the RM5 flap
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Figure 13. FOS plot for the RM5 frame

3.1.2 Performance Analysis and AEP Estimation 

Numerical Model

A time-domain numerical model, WEC-Sim, was applied to simulate the hydrodynamics of the 

RM5 OSWEC to estimate its power performance under operational wave conditions. The 

numerical model solves the Cummins’ equation (Cummins 1962):

(m + m∞)ẍ= − � K(t − τ)ẋ

t

−∞

(τ)d − Fhs + Fe + Fv + Fee

where m is the mass matrix and m∞ is the added mass matrix at infinite frequency. The term 

− ∫ K(t − τ)ẋ t

−∞ (τ)d is the convolution integral that represents the resistive damping force on the 

body from wave radiation. K is the impulse response function. Fhs, Fe, Fv, and Fext are the 

hydrostatic restoring force, the wave excitation force, the viscous drag force, and the external 

forces, respectively.

As shown in Figure 14, the model for the RM5 OSWEC was set up in WEC-Sim by connecting 

the flap to the frame with a defined linear damping coefficient that simulates the PCC system, 

and connecting the frame to the seabed using a 3 degree-of-freedom (DOF) floating joint

connection, which was used to model a floating WEC device and allowed the body to move 

freely in heave, surge, and pitch.
15

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Figure 14. Model setup in WEC-Sim

The simulation was performed for a range of irregular sea states, each with characteristic Hs and 

Tp and each represented using a Brechtschneider spectrum. The hydrodynamic response of the 

device was simulated in the time domain for a duration of 125 Tp with a ramp time of 25 Tp and 

a maximum time step size of 0.01 Tp. Note that the mooring lines were modeled using a linear 

quasi-static mooring stiffness. For simplicity, the weight of the mooring line was not considered 

in the simulations. The pretension was equal to the total net buoyancy force from the flap and 

shaft and the frame. For the simulations, an optimal mooring stiffness was selected based on the 

device power performance for a typical sea state (Te = 8.7 sec; Hs = 1.75 m) from the reference 

site. An optimal velocity-dependent PCC damping force for each sea state was determined by 

adjusting the PCC damping coefficient and selecting the value with the best device power 

performance.

Power Matrix and Estimated AEP Calculation

The annual energy production (AEP) for a WEC can be determined from the wave statistics at a 

reference site, the device performance power matrix, and the loss between absorbed 

hydrodynamic power and electrical power output. As discussed earlier, the wave statistics at the 

reference site are often represented by the JPD of the waves, characterized by Hs and Te (or Tp). 

The JPD at Humboldt Bay, California, (Table 1) was used in the study.

The power generation performance of WECs in irregular seas is often represented using a power 

matrix, which we obtained by modeling the RM5 OSWEC at each given binned sea state and 

then calculating the averaged power performance. To estimate the electrical power matrix, we 

multiplied the mechanical power matrix with a PCC conversion efficiency and limited the 

maximum electrical power output at the rated power. The electrical power that can be generated 

by the OSWEC design under each given sea state can be obtained from

Pe = Pm × η1
16

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where Pm and Pe are the estimated mechanical and electrical power at each binned sea state, and 

η1 is the PCC efficiency that accounts for the losses between absorbed hydrodynamic power and

electrical power output.

We assumed that a hydraulic PCC system with a conversion efficiency of 82.5% was used for 

the OSWEC designs, which followed the assumption used in RM 3 (Neary et al. 2014). The 

rated power was estimated based on a capacity factor of 30% (Previsic et al. 2012; 

RenewableUK 2010). The resulting electrical power matrix for the RM5 design is shown in 

Table 6.

Table 6. Electrical Power Matrix for the RM5 OSWEC Design

The annual averaged electrical power Pae was then obtained by summing the product of the 

electrical power matrix and the JPD for the reference site. Then, the estimated AEP (in 

megawatt-hours [MWh]) was calculated by

AA = Pa × 8766 (hours) × β

where β=η2xη3 is the parameter accounting for the losses caused by transmission efficiency

η3=0.98 (98%) and device availability ηs=0.95 (95%) (Neary et al. 2014).

Table 7. Rated Power and Power Output for the Floating Point Absorber Device Predicted from 

WEC-Sim

Device Performance Per Unit

Rated power 360 kW

Annual averaged electrical power 108 kW

AEP 882 MWh
17

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3.1.3 Power Conversion Chain 

To characterize the effects of the PCC on LCOE, we established a conceptual design. We chose a

hydraulic design because of the oscillating motion inherent to any oscillating surge device. 

Figure 15 shows the layout of the two hydraulic rams and the PCC enclosure installed on the 

RM5.

Figure 15. RM5 power conversion chain layout

Figure 16. Hydraulic circuit for the RM5 design (Neary et al. 2014)
18

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The general layout of hydraulic components is similar to that of RM3 (Neary et al. 2014), but 

with an additional hydraulic ram to reduce torsion across the surge plate. Figure 16 shows a 

schematic of RM5’s hydraulic circuit.

The hydraulic circuit consists of two double-acting hydraulic rams that are connected to the 

surge plate at one end and the frame on the other end. As the surge plate oscillates back and 

forth, the hydraulic rams extend and compress. The oscillating extension and compression of the 

cylinder forces the hydraulic fluid through the system, therefore creating pressurized flow. The 

energy in the hydraulic fluid is then converted into mechanical energy via a hydraulic motor. The 

pressure and flow through the hydraulic motor create torque and angular velocity that can be 

converted into electricity via an electric generator. To reduce pressure fluctuations at the 

hydraulic motor, a set of hydraulic accumulators are used to smooth the peaks associated with 

the oscillating motion of the rams.

The PCC was designed as a fully enclosed unit that can be lifted out of place for complete device 

replacement or overhaul; however, the hydraulic rams are separate and would be disconnected 

prior to PCC removal. Figure 17 shows the PCC with the walls removed to display the layout of 

components within the assembly.

Figure 17. PCC assembly and enclosure (three walls removed)

The PCC components were sized based on the device’s rated generator output of 360 kW (482 

horsepower. A 373-kW four-pole (1,800 rpm) generator was used for sizing the hydraulic 

components. To eliminate the need for a gear reduction system, the hydraulic motor was sized to 

match the generator input. The hydraulic motor is a positive fixed-displacement axial piston 

type, with a displacement of 500 cc/revolution (0.132 gal/rev). Therefore, operating the hydraulic 

motor at 1,800 rpm requires an input flow of approximately 240 gallons per minute (gpm). Using 

the equation below, through the relationship between pressure, flow, and hydraulic power 

(Lapeyrouse 2002), we can approximate the required system pressure that will create the needed

torque. To maintain consistency with RM3, the hydraulic system efficiency of the RM5 PCC was 
19

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assumed to be 82% (95% hydraulic ram efficiency and 86.5% hydraulic motor efficiency) 

(Neary et al. 2014)

H = P ∗ Q ∗ η1

1714

where HP is hydraulic power (horsepower), P is hydraulic pressure (PSI), and Q is the flow rate 

(gpm).

After solving for pressure, we found that the required system pressure is 4,123 PSI, which is 

within the operating limits of the selected hydraulic motor. It should be noted that even though

the system pressure is within the operational limits of the hydraulic motor, high-pressure systems 

have more inherent design challenges. These difficulties could be mitigated by using a larger 

displacement hydraulic motor, or by increasing the generator speed. Given the current selection 

of off-the-shelf hydraulic components, these options were not explored. The hydraulic ram 

dimensions were designed so that the stroke length is less than 2 m during normal operation 

(±20°), while still allowing the device to lay down past 45° during an extreme event. The 

cylinders chosen have a piston diameter of 7.5 in (19 cm) and a shaft diameter of 4.7 in (12 cm). 

This configuration was designed based on a flow profile of 9-second (s) wave periods and a ±20° 

flap angle during normal operation.

Figure 18. Flow profile of the RM5 power conversion chain

As shown in Figure 18, the flow distribution is not perfectly mirrored, because the hydraulic 

rams are not centered on the device. This creates a slight variation in piston velocity relative to 

flap angle; however, it can be seen that during normal operation, the system is exceeding the

average flow and is below the average flow in quarter-period increments. To smooth the flow so 

that the hydraulic motor sees a constant value equal to the average rate, a set of hydraulic 

accumulators should be utilized. Although there are only 2.25 s of accumulator discharge and 

2.25 s of charge under normal operation, our PCC was designed for 20 s of discharge with an 

allowable pressure drop of 66% (2,721 PSI discharge pressure). This is similar to the RM3 

design. Following the recommended original equipment manufacturer accumulator precharge 

pressure (Hydac 2013), it was determined that 260 gallons of accumulator volume is needed. 
20

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Table 8 lists the critical PCC components and their respective size and mass values for a 360-kW 

rating.

Table 8. PCC Component Size Breakdown

PCC Component Sizing

Component Name Component Size Unit Mass [kg]

Generator 373 kW 908

Hydraulic motor 500 cc 155

Accumulators 260 gal 9,502

Hydraulic rams 7.5”x4.7”x78” Dp x Ds x Stroke 2,500

Filters - - 240

Frequency converter 373 kW 1,201

Step-up transformer 400 kW 1,590

Other (valves, couplings, and so on) - - 5,723

PCC enclosure - - 10,500

3.1.4 Foundation and Mooring Design

We used a taut mooring design to reduce the motion of the frame and improve the RM5 OSWEC 

design’s power performance. The design consists of four taut moorings as shown in Figure 19.

The mooring stiffness was determined by selecting the value with the best device power 

performance based on the dominant sea state at the reference site. We chose nylon as the main 

material for the mooring rope because it is lightweight and relatively inexpensive. For simplicity, 

the present mooring line was perfectly vertical so we did not consider a horizontal reaction 

component to mitigate the surge motion. An advanced mooring design can be developed to 

further improve the design power output.

Figure 19. Mooring design for the RM5 OSWEC
21

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Following RM4’s anchor design (Neary et al. 2014), we chose a suction embedment plate anchor 

(SEPLA) to anchor the mooring lines in this study. SEPLA anchors have the advantage of very 

high holding capacity-to-weight ratios, which gives a low cost-to-performance ratio. In addition, 

they can be efficiently installed using a submerged pile driver.

We analyzed the baseline mooring design of RM5 OSWEC using WEC-Sim, with the flap fully 

submerged and locked at 45 degrees, under extreme 20% inflated 100-year return period sea 

states. We assumed two mooring lines and anchors per leg, and the maximum load on the 

mooring was about 17,000 kilonewtons (kN). The maximum mooring load was then used to 

determine the mooring rope design and size of the SEPLA. The simulations were performed to 

determine a baseline mooring design for the LCOE estimate. In reality, these anchor choices are 

largely driven by the sedimentation type at the reference site and the directional spreading of 

incoming waves. These choices need to be refined in a more detailed mooring analysis.

3.2 Manufacturing and Deployment Strategy Module

3.2.1 Manufacturing Strategy and Costs

The manufacturing costs of system components for RM5 at different array scales (1, 10, 50, and 

100 units) are summarized in the figures and tables below. Figure 20 shows the cost breakdown 

of the device structure subcomponents, which includes the main surge frame and flap (fiberglass 

horizontal tube sections, two outside upright support structures, center support structure, and 

torque tube). Approximately two-thirds of the cost comes from the surge flap. Although there is 

some reduction because of volume discounts at 100-unit deployment, with a structural mass of 

approximately 800 tonnes, a large portion of the cost is a result of the price of raw steel.

Figure 20. RM5 structural cost breakdown ($/kW) per deployment scale

$0

$2,000

$4,000

$6,000

$8,000

$10,000

$12,000

1 10 50 100

Structure Component Cost [$/kW]

Array Scale [# of Units]

Torque Tube

Upright Support Structures -

Center

Upright Support Structures -

Side (2)

Fiberglass Tubing

Surge Frame
22

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Figure 21 shows the cost breakdown for the PCC components of the RM5. Because it uses a 

hydraulic PCC design, we expect that the majority of the PCC costs come from the hydraulic 

components. The cost breakdown of the PCC’s hydraulic components (Figure 22) shows that

approximately 80% of the cost comes from four critical components: the hydraulic reservoir, 

hydraulic rams, plumbing, and accumulators. Because the high operating pressure of the RM5 

drives the cost of each of these components, a cost reduction may be possible with a higher flow, 

lower pressure PCC design. It may also be possible to significantly lower the cost by reducing 

the accumulators’ discharge time. For example, reducing the discharge time in half, from 20 to 

10 s, would also halve the cost of the accumulators.

Figure 21. Cost breakdown ($/kW) for the PCC components per deployment scale

Mooring component costs, including mooring lines, chains, anchors, subsurface buoys, and 

hardware connections were estimated at about $1,264,000 for one device deployed as a single 

unit, or $728,000 per device at a deployment of 100 units. The cost of the mooring system in 

RM5 is dominated by the anchor’s taut mooring design, which is critical for energy capture. 

Approximately 47% of the mooring and foundation costs are the result of anchor prices for 

single unit deployment. These costs decrease quickly because of the reduction in engineering and 

development costs as deployment moves to 10 or more units. At 10 units, anchors account for 

32% of the mooring costs, and at 100 units they account for only 26%. Table 9 shows the 

estimated cost of the mooring system components at different deployment scales.

$0

$200

$400

$600

$800

$1,000

$1,200

$1,400

$1,600

$1,800

1 10 50 100

PCC Component Cost Breakdown [$/kW]

Array Scale [# of Units]

Other

PTO Mount

Assembly, Testing, & QA

Bearings

Control System

Step up Transformer

Frequency Converter

Couplings

Hydraulic Components

Generator
23

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Figure 22. RM5 PCC hydraulic component breakdown 

Table 9. RM5 Mooring System Component Cost Breakdown

 

1-Unit 

Deployment 

[$/kW] 

10-Unit 

Deployment 

[$/kW] 

50-Unit 

Deployment 

[$/kW] 

100-Unit 

Deployment 

[$/kW] 

Mooring lines/chain $974 $877 $877 $877 

Polyester line $899 $809 $809 $809 

Wire rope to subsea buoys $75 $67 $67 $67 

Anchors $2,084 $1,030 $805 $737 

Buoyancy $167 $150 $150 $150 

Connecting hardware $284 $256 $256 $256 

Total $4,482 $3,189 $2,963 $2,896 

3.2.2 Deployment Strategy and Costs 

The deployment strategy for the RM5 is similar to the one prescribed for the RM3 (Neary et al. 

2014) and accounts for the installation of the mooring system, subsea cable infrastructure, and 

the devices themselves (including commissioning). Because the masses of the RM5 and RM3 are 

comparable, the same “DP-2 class vessels” that were specified for the RM3 installation (Neary et 

al. 2014) are assumed for the RM5. We assumed that the vessel would be mobilized from the 

Gulf of Mexico region and used for the mooring installation. A separate cable installation vessel 

28%

21%

16%

15%

6%

6%

4% 1%

1%

1% 1% 0% 0%

PCC Hydraulic Component Breakdown

Reservoir

Hydraulic Cylinder

Plumbing (Including Flex Lines)

High Pressure Accumulators

Fluid

Hydraulic Motor

Check Valves

Solenoid Valves

Low Pressure Filter

High Pressure Filter

Valve Subplate

Pressure Transducers

Relief Valves
24

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would be used for installing the cable. The device1 would be connected to its mooring system 

and commissioned using the same workboat/custom service vessel that will be used for operation 

and maintenance (O&M) activities. Because of the taught mooring line system that is specified 

for the RM5, there is significantly less mooring line length than what was prescribed for the 

RM3. Therefore, the vessel used for the RM3 was assumed for cable installation.

Table 10 lists the total installation costs using the assumed day rates for these three types of 

vessels and the assumed durations for the key steps in the process.

Table 10. RM5 Manufacturing and Deployment Strategy Module Cost Assumptions

1-Unit 10-Unit

Operation Detail No. 

Days

Vessel Day 

Rate Cost No. Days

Vessel Day 

Rate Cost

Mooring Installation (DP-2 Vessel)

Transit (5,000 miles) 68.7 $58,754 $4,039,062 68.7 $58,754 $4,039,062

Mob/demob of vessels 4.0 $422,000 4.0 $422,000

Dockside support $7,350 $735,000

At dock landing 0.4 $70,485 $26,079 3.7 $70,485 $258,445

Transit to site and back 0.4 $76,610 $27,580 3.6 $76,610 $275,796

On-site working 0.4 $73,810 $27,310 3.7 $73,810 $270,637

Total 74 $279,659 $4,549,381 84 $279,659 $6,000,940

Cable Shore Landing

Horizontal drilling $667,000 $767,200

Cable Installation (Using Cable Install Vessel)

Mob/demob cable install vessel 11.0 $66,350 $729,850 11.0 $66,350 $729,850

Load cable 0.7 $75,625 $53,694 3.4 $75,625 $257,125

Transit to site 2.0 $101,275 $202,550 2.0 $101,275 $202,550

Install cable and surface lay 0.6 $101,075 $55,591 5.5 $101,075 $555,913

Cable burial and excavation 3.1 $101,075 $313,333 3.1 $101,075 $313,333

Contingency 1.7 $87,855 $152,516 2.5 $87,855 $221,395

Total 19 $533,255 $1,507,534 28 $533,255 $2,280,165

Device Installation (Same Workboat Used for 

O&M)

Mob/demob $181,750 $181,750

Installation 1.0 $66,775 $66,775 10.0 $66,775 $667,750

Contingency 0.1 $66,775 $6,678 1.0 $66,775 $66,775

Total 1 $133,550 $255,203 11 $133,550 $916,275

 

1 This analysis assumed devices could be assembled in a suitable fabrication facility in Oregon and barged down to 

the installation site about 300 miles south.
25

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Figure 23 shows the total installation cost normalized by installed power at different deployment 

scales. The single unit deployment cost is dominated by the cost of installing the mooring system 

and the cable shore landing. The dollars per kilowatt ($/kW) cost of installation is significantly 

higher for the deployment of a single unit (about $16,000/kW) than for an array (about 

$2,500/kW for a 10-unit deployment and just over $1,000/kW for a 100-unit deployment). 

Figure 23. Installation cost breakdown ($/kW) per deployment scale 

3.3 Operation and Maintenance 

3.3.1 Service Vessel Specifications 

Like other wave reference models, we assumed that a dedicated service vessel would become 

feasible with larger unit deployment. The workboat specified for the RM3 would likely be 

sufficient for typical RM5 service intervals. The boat shown in Figure 24 represents a similar 

workboat that would be used for service and installation. The figure shows the boat pulling an 

offshore wind platform that is similar in size to a WEC device. The requirements for the vessel 

are: 1) sufficient deck space to handle mooring lines and cable repair; 2) dynamic positioning 

(DP-1) to allow for more effective operation; and 3) crane lifting capacity of 5 tonnes at a 20-

foot radius. Total cost estimates for a new RM3 service vessel range from $4M to $5.5M. 

Although the mass of the RM3 and RM5 is similar, we assumed the conservative estimate of 

$5.5M to account for the larger crane capacity that would be required for the RM5’s PCC 

removal. A crew of about 10 would be required to operate the vessel and carry out repair and 

maintenance activities. Operations would take place only during daylight hours (12 hours per 
26

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day) and the vessel would return to port at night. The cost of marine operations is based on the 

number of interventions and the cost of the vessels used.

Figure 24. An example of a medium-sized workboat

(Photo from Siemens AG, NREL 27837)

Based on the failure rate assumptions (see Section 3.3.2) and operational frequency, we found 

that the device would require a total of two interventions per year. There are two major types of 

interventions: those requiring device recovery and those requiring only PCC recovery. The 

vessel day rates are the same for both types of interventions with the exception of the cost of fuel 

and consumables; the rate is higher for PCC removal. It is assumed that PCC retrieval requires 

either three vessels or a single vessel with three times the capacity of the vessel used in the RM3

project. The operational cost would be expected to drop if the WEC plant used a custom-built 

service vessel that was purchased as part of the project, rather than employing a vessel of 

opportunity.

3.3.2 Failure Rates

Table 11 provides first-order approximations of failure rates based on rates estimated for the 

RM3. The mean failure rate (L50) was assumed to be the mean time before the subsystem 

required complete replacement. Only components that would not require complete device 

retrieval were considered for replacement. The cost of replacement parts was assumed to equal 

the value of the part/subsystem of the original device. Annual replacement part costs were 

calculated from the part cost and the estimated number of failures per year (Table 11).
27

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Table 11. Cost and Failure Rate Assumptions for WEC Components (Ten-Unit Cost)

Ten-Unit Device Failure Rates

$/Unit

# 

Units L50 $/Year

# 

Failures/Year

Hydraulic System

Hydraulic cylinder $25,532 2 8 $6,382.88 0.13

Hydraulic motor $13,222 1 5 $2,644.34 0.20

High-pressure accumulators $2,726 13 12 $2,953.45 0.08

Relief valves $162 4 5 $129.85 0.20

Check valves $1,049 8 

Solenoid valves $3,051 1 

Valve subplate $2,241 1 

Pressure transducers $719 1 8 $89.83 0.13

High-pressure filter $866 3 4 $649.69 0.25

Low-pressure filter $2,644 1 4 $661.08 0.25

Reservoir $66,050 1 

Electrical System

Generator $29,115 1 10 $2,911.53 0.10

Frequency converter $34,917 1 7.5 $4,655.63 0.13

Step-up transformer $14,844 1 15 $989.61 0.07

Mechanical Components

Motor-to-generator coupling $278 1 20 $13.92 0.05

Hinge/flap bearings $6,549 2 20 $654.89 0.05

Riser cable $88,000 1 10 $8,800.00 0.10

Mooring $833,021 1 50 $16,660.42 0.02

Total $ 1,124,986 $ 48,197.10 1.75 

3.3.3 Annual O&M Costs

Based on the estimated number of interventions and replacement part values, the annual O&M 

cost was computed at different scales of deployment. Figure 25 shows the breakdown of the 

estimated annual cost per WEC device. Increasing the unit scale of the plant would have a 

dramatic impact on reducing operational costs because the costs for the service vessel and the 

crew would increase at a slower rate as the deployment scale goes up. Insurance estimates for the 

RM3 were adopted for the RM5, which is a percentage of the summation of infrastructure, 

mooring/foundation, device structure, PCC, subsystem integration, and installation costs. It is 

assumed that deployment insurance is 2% for single- and 10-unit projects, 1% for 50-unit

projects, and 0.5% for 100-unit projects. Note that the postinstallation monitoring is a part of the 

environmental monitoring and regulatory compliance cost under the environmental compliance 

module (see Section 3.4) and is included in the total operational expenditure (OpEx) costs. Initial 
28

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environmental compliance and monitoring activities prior to start up would fall under capital 

expenditures (CapEx), as explained in the following section.

Figure 25. The RM5’s annual OpEx costs ([$/kW] per array size)

3.4 Environmental Analysis and Compliance

Although the specific sites and technologies will have a major influence on the costs for any 

project, there are many commonalities driven by regulatory requirements and information needs. 

For the RM1, RM2, and RM3, PNNL researchers derived cost ranges using the best available 

information from existing and planned marine and hydrokinetic projects by consulting with 

developers and the consultants supporting them. In this report, we have also relied on the best 

professional judgment of researchers and natural resource management agency staff. For the 

RM5 (surge WEC), the basis for the costs of environmental studies and processes were 

developed through extrapolation from the previous three models. Even though the surge WEC 

model differs considerably from the RM3 (point absorber WEC) in size, mooring, and operation, 

the two devices have similar potential for animal interaction. The impact of anchors and mooring

lines on marine habitats in the RM5 is analogous to that of the lines and anchors proposed for the 

RM3. Because of the similar ocean space occupied by the RM5, the National Environmental 

Policy Act (NEPA) processes and study costs can be extrapolated using information from other 

near-shore marine and hydrokinetic projects, and in consultation with experts in the area

(Polyage et al. 2011).

In the RM5 project, the design strategy was altered because of environmental permitting and 

regulation concerns. The original OSWEC design proposed in the reference model project was a 

fixed-bottom system, in which the flap rotated around a frame rigidly connected to the seabed. 

$0

$500

$1,000

$1,500

$2,000

$2,500

$3,000

$3,500

1 10 50 100

$/kW

Array Scale [# of Units]

Consumables

Replacement Parts

Shoreside Operations

Marine Operations

Insurance

Post-Installation Monitoring
29

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From the power generation performance point of view, the fixed-bottom OSWEC designs have 

better power generation performance (Yu et al. 2014); however, the design was changed to a 

floating system before significant engineering or cost analysis was conducted, based on concerns 

raised by the environmental assessment (Copping et al. 2014). The potential risk to the marine 

environment from the devices could have a significant effect on near-shore marine animals, 

habitats, and ecosystem processes, particularly for a commercial-scale deployment in shallow 

water (10- to 20-m water depth) with a sandy bottom, where shoreline sediment drift and shoreform creation also depends on the incoming waves. As commercial feasibility was central to this 

project, the OSWEC design was changed to a floating system.

Costs for each of the RM5 studies and processes have been developed for pilot and commercial 

projects, as described. Although the size of a pilot project differs from one technology and 

location to another, we have assumed that the RM5 pilot project consists of one device, totaling 

less than 5 MW of generation capacity, and could be deployed for up to 5 years. The scaling 

rules used in the RM1‒4 projects were applied to the RM5 to generate a range of costs for both 

small and large commercial-scale projects (10 and 100 devices, respectively).

Each stage of study development (e.g., scoping and siting, preinstallation assessment, and

postinstallation monitoring) requires documentation and adherence to processes designed to meet 

regulatory requirements. These processes include conducting public meetings, filing necessary 

permitting paperwork, and performing periodic checks with government agencies. Our estimates 

account for the associated costs of these processes. It is assumed that many of the siting and 

permitting processes that drive costs are included under NEPA. Other regulatory drivers include

the Endangered Species Act of 1973, Clean Water Act of 1977, Marine Mammal Protection Act 

of 1972 (as amended), Magnuson-Stevens Fishery Conservation and Management Act, and the 

Migratory Bird Treaty Act of 1918, as well as state and local regulatory requirements.

Table 12. RM5 Environmental Cost Summary

Information 

Need

Pilot Small-Scale 

Commercial

Large-Scale Commercial

Low High Low High Low High

Siting and

scoping

$240,000 $430,000 $67,000 $105,000 $77,000 $105,000

Preinstallation 

studies

$846,000 $1,583,000 $770,000 $1,555,000 $595,000 $1,615,000

Postinstallation $320,000 $610,000 $780,000 $2,460,000 $780,000 $1,860,000

NEPA and

process

$725,000 $1,125,000 $70,000 $150,000 $70,000 $150,000

Total $2,131,000 $3,748,000 $1,907,000 $3,760,000 $1,742,000 $3,820,000

The overall costs for environmental studies and associated processes required for the RM5 are 

summarized in Table 12. Detailed spreadsheets, references, standardized protocols, and an indepth explanation are available for all parts of the environmental costing process for the RM5 

(Copping et al. 2014). It should be noted that the costs listed here are not intended to make 
30

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recommendations about which studies should be carried out or how much they should cost, 

rather, they reflect cost data representative of projects carried out to date and professional 

judgment regarding how the costs associated with the RM5 may differ. Real-world costs may be 

significantly lower or higher depending on site characteristics, regulatory concerns, and 

stakeholder dynamics. Costs are also expected to be reduced over time. The numbers here 

represent a conservative estimate, and are not intended to inform study plan negotiations between 

developers and regulatory agencies. 

Costs shown in Table 12 summarize total costs expected at the pilot phase and each commercial 

phase. Small- and large-scale commercial costs have been calculated under the assumption that 

information collected during permitting at the pilot phase would be used for permitting in the 

commercial phase as well, thereby achieving cost savings; these costs were calculated as 

incrementally adding to those of the pilot scale. 

3.5 LCOE Calculation 

As shown in Figure 26, the baseline commercial LCOE estimate (i.e., an array comprised of 10 

units) for the RM5 device is $1.44/kWh. For an array of 100 units, this value drops to 

approximately $0.69/kWh. These values are based on the device’s AEP, CapEx, OpEx, and 

prescribed fixed charge rate (FCR). Based on these values, it is critical that the device cost must 

decrease and/or the device performance must increase for this technology to become 

economically viable. 

Figure 26. High-level LCOE (cents/kWh) breakdown per deployment scale for the RM5 
31

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Table 13 provides a breakdown for a commercial deployment of 10 units. The cost of 

manufacturing and deployment contributes 48% of the total LCOE for RM5, and O&M 

contributes another 36%. As expected, these two categories are the dominant cost drivers for the 

WEC design. To make the RM5 design cost competitive with other renewable energy 

technologies, it is essential to conduct research and development that is geared toward reducing 

the cost of manufacturing, deployment, and O&M.

Table 13. RM5 Levelized Cost of Energy Breakdown by Cost Category (Ten-Unit Array)

cents/kWh % of Total LCOE

Development 10.0 7.0%

Manufacturing and deployment 69.4 48.2%

Subsystem integration and profit margin 3.9 2.7%

Contingency 8.3 5.8%

O&M 52.4 36.4%

Total 144.0 100.0%

The total CapEx for single-unit deployment is estimated to be approximately $52,000/kW. This 

value drops to $21,000/kW for a 10-unit deployment, and $13,800/kW for a 100-unit array. 

Figure 27 shows the CapEx contributions to the final LCOE for the RM5. It is true that there are 

some cost savings associated with volume production in components such as the PCC, and 

installation and permitting in mooring systems; however, large cost reductions to the device 

structure will still need to be made even as installation and permitting costs diminish. The 

detailed breakdown of CapEx cost categories in terms of LCOE, as well as the percentage 

breakdown for a 10-unit array, is provided in Table 14.

Table 14. Breakdown of the RM5 CapEx Contributions to LCOE (Ten-Unit Array)

cents/kWh % of Total CapEx

Design 2.4 2.6%

Site assessment 0.4 0.4%

Permitting and environmental compliance 7.3 7.9%

Infrastructure 9.3 10.2%

Mooring/foundation 10.2 11.1%

Device structural components 32.9 35.9%

PCC 5.8 6.3%

Installation 11.1 12.2%

Subsystem integration and profit margin 3.9 4.2%

Contingency 8.3 9.1%

Total 91.6 100.0%
32

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Figure 27. RM5 CapEx contributions to LCOE (cents/kWh) per deployment scale

Annual operating cost was estimated at $3,169/kW for a single unit device. The amount falls to 

$1,283/kW for a 10-unit array, and $202/kW for a 100-unit array. Figure 28 shows how OpEx 

costs contribute to the RM5 LCOE, and a breakdown of the RM5 OpEx contributions to LCOE 

is provided for a 10-unit array in Table 15.

Table 15. Breakdown of RM5 OpEx Contributions to LCOE (10-Unit Array)

cents/kWh % of Total OpEx

Marine operations 8.60 16.4%

Shoreside operations 4.54 8.7%

Replacement parts 0.55 1.0%

Consumables 1.53 2.9%

Insurance 13.55 25.9%

Postinstallation environmental monitoring 23.65 45.1%

Total 52.42 100.0%

0.0

50.0

100.0

150.0

200.0

250.0

1 10 50 100

cents/kWh

Array Scale (# of Units)

Project Design, Engineering,

and Management

Site Assessment

Permitting and Environmental

Compliance

Infrastructure

Mooring/Foundation

Device Structural Components

PCC

Installation

Subsystem Integration & Profit

Margin

Contingency
33

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Figure 28. RM5 OpEx contributions to LCOE (cents/kWh) per deployment scale

3.6 Uncertainty in Design and Economic Analysis

The preliminary RM5 design, like all other previously presented reference model designs, has 

inherent uncertainties surrounding the device’s performance, design, and economics. A 

qualitative uncertainty assessment was performed for the device design and performance, shown

in Table 16. The table assigns levels of uncertainty to various aspects of the model, from low to 

very high, depending on whether the aspect was assessed using test/field data, modeled data, or 

engineering judgment. Aspects that were not addressed were assigned a “very high” level of 

uncertainty. Some of the categories, such as device performance, were broken into subcategories. 

The subcategories are represented using blue text. Device performance is broken into WEC 

performance, reliability, and array wake effects. Each of these categories has a different level of 

uncertainty because of the different levels of data availability. All categories with gray text 

represent the data requirements that would be mandatory for that category, implying that in the 

case of device performance, a validated model (e.g. tank testing and ocean testing) is required 

before the uncertainty can be considered low. The judged design uncertainty level for each of the 

categories and subcategories of the RM5 design are identified in Table 16 through crosshatching.

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

1 10 50 100

cents/kWh

Array Scale (#of Units)

Marine Operations

Shoreside Operations

Replacement Parts

Consumables

Insurance

Post-Installation Environmental

Monitoring
34

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Table 16. Uncertainty Matrix for RM5

Uncertainty Device 

Performance

Structural 

Design PCC Design Resource Assessment

Environmental 

Compliance Economic

Low Validated 

model 

Validated 

model

Validated 

model or 

original 

equipment

manufacturer

parts

Actual data Actual data Actual data

Medium 

Model 

simulation, no 

scaled test or 

field dataWEC 

performance

Model 

simulation, 

no scaled 

test or field 

data

Nonvalidated 

model 

simulation, 

no test dataExperience 

lacking 

submersed 

PCCs

Validated 

model

Validated 

model

Model 

simulation

High

Engineering 

judgmentreliability

Engineering 

judgment

Engineering 

judgment

Engineering 

judgment

Engineering 

judgment

Data from 

similar 

renewable 

energy 

technology, 

manufacturing 

and 

distribution, 

O&M

Very High

Issue not 

addressedarray wake 

effects 

Issue not 

addresseddynamic 

loads and 

fatigue

Issue not 

addressed 

Issue not 

addressed 

Issue not 

addressed 

Issue not 

addressed 

Here we discuss the economic uncertainties of each cost breakdown category, for both CapEx 

and OpEx, in more detail. As with previous reference model devices, there is a lack of validated 

and public industry data for oscillating surge devices. To capture the CapEx and OpEx 

uncertainties, a qualitative assessment of the RM5 uncertainties was performed. A tabulated 

assessment of CapEx is provided in Table 17 and an OpEx assessment is provided in Table 18. A 

detailed description of the RM5’s CapEx and OpEx uncertainties is described below:

• Development. PNNL has performed an initial study on environmental compliance;

however, postinstallation monitoring has considerable uncertainty. Aside from 

postinstallation monitoring, the costs associated with preinstallation monitoring depend

on site location and final array size. Until a pilot device is installed, there will be 

uncertainty regarding the selected designs, leading to a medium- to high-level of 

uncertainty.
35

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• Infrastructure. Cost estimates for cables and connections were obtained based on 

previous reference model work. The taut mooring design used in the RM5 will enable 

less device movement, and therefore cable movement, but the lack of operational data 

results in a medium level of uncertainty. There is a high uncertainty regarding the vessel 

estimates because of the large mass and lack of purpose-built installation vessels. 

Conservative cost estimates have been utilized and it is likely that these costs can be 

optimized in future designs.

• Foundation/Mooring. There is a high level of uncertainty in the foundation and mooring 

estimates because of their dependency on seabed conditions. The benefits of taut mooring 

lines, and the associated anchor cost, will likely require optimization to maximize their

performance and minimize LCOE.

• Device Structural Components. A high level of uncertainty on the device structure is 

caused by the lack of dynamic and fatigue considerations that will likely affect the final 

design. There is less uncertainty in the methods used to estimate the costs of the design. 

Fabricated steel costs, similar to those used on the RM3, were used to estimate costs for 

the RM5 structure.

• PCC. There is a medium level of uncertainty with the PCC because of the assumptions 

that were made regarding the hydraulic rams. Although initial performance and structural 

analyses were performed on the cylinders, they are not off-the-shelf components, and 

parametric estimations were used for cost.

• Installation. Installation estimates were based on time and material (vessels included) 

estimates. There is uncertainty regarding the specific vessels used during installation,

therefore installation has been assigned a medium- to high-level of uncertainty. 

• Subsystem Integration and Profit Margin. There is a high level of uncertainty because 

of a 10% factor added to the device cost. 

• Contingency. There is a high level of uncertainty because of a 10% factor added to the 

device cost.

• Decommissioning. Decommissioning costs are assumed to have equivalent costs 

associated with installation, therefore they are ranked as having a medium level of 

uncertainty.

• Marine Operations. There is a high level of uncertainty when using a simplified O&M 

model.

• Shoreside Operations. There is a high level of uncertainty because of a lack of longterm performance data on fully submerged hydraulic PCCs. This lack leads to an 

uncertainty in failure rates and annual maintenance labor rates.

• Replacement Parts. Replacement part cost is based on original part cost. The high level 

of uncertainty is the result of an uncertainty in failure rates of submersible PCCs.

• Consumables. There is a high level of uncertainty around submersible hydraulic PCC 

systems.
36

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• Insurance. Insurance is based on knowledge gained from oil and gas and offshore wind 

projects and therefore has a low level of uncertainty.

• Postinstallation Environmental Monitoring. The analysis, as performed by PNNL, led 

to a low- to medium-level of uncertainty. The uncertainty will be higher if a different 

location is selected.

Table 17. Assessment of Cost Uncertainty (CapEx)

Cost Breakdown 

Structure

Subcategory 

(If Applicable) Result Maturity/Fidelity Uncertainty

Development

Siting and scoping

Preinstallation studies

Postinstallation studies

NEPA and process

site assessment

Based on data from similar studies 

and/or engineering judgment

and/or data from PNNL study

Medium to high

Design and engineering

Technology Readiness Level 2 

design and analysis High

Infrastructure

Cables and connectors Conceptual layout, generic 

hardware ID, and estimates Medium

Dockside and vessel Generic for dockside and generic 

vessel ID High

Foundation/ 

mooring N/A Anchor and mooring have high fluctuations based on the seabed High

Device structural 

components All

CAD designs, conceptual designs, 

steel cost estimates; high 

uncertainty is the result of

dynamic and fatigue loads that are 

not addressed

High

PCC All components

Primarily off-the-shelf hydraulic 

components, with custom 

hydraulic cylinders; medium 

uncertainty as a result of hydraulic 

cylinder limitations

Medium

Installation N/A

Time and material estimates for a 

specific resource location

(includes labor)

Medium to high

Subsystem 

integration and

profit margin

N/A Assumed to be 10% of machine 

cost High

Contingency N/A Assumed to be 10% of installed 

capital cost High

Decommissioning N/A Assumed to be the same as 

installation cost Medium
37

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Table 18. Assessment of Cost Uncertainty (OpEx)

Cost Breakdown 

Structure

Subcategory 

(If Applicable) Result Maturity/Fidelity Uncertainty

Marine 

operations N/A

Large uncertainties with 

respect to maintenance and a 

simplified O&M model

High

Shoreside 

operations N/A

Large uncertainties in failure 

rates for submersible, 

hydraulic PCC assemblies 

High

Replacement 

parts N/A Limited failure rate data, based on original part cost High

Consumables N/A Annual High

Insurance N/A Based on offshore oil and gas 

projects Low

Postinstallation 

environmental 

monitoring

N/A Based on PNNL study Low to medium
38

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4 Conclusion 

This report describes the design and economic analysis of a floating oscillating surge WEC using 

the methodology developed by Neary et al. (2014). To maintain consistency, the methodology 

for estimating performance and costs is similar to the methodology used for the RM3 project, 

including the same reference resource and cost categories; however, the RM5 is intended to 

represent an oscillating surge device. Caution should be taken when comparing this device with 

other reference model designs—the inherent variation in performance, loading, and permitting 

will skew final LCOE values.

The primary purpose of this study was to identify key cost drivers to focus on future research and 

development (R&D) efforts. Figure 29 shows that, at early development stages, a very large 

percentage of LCOE is the result of development costs (permitting, site assessment, and 

design/engineering); however, this is an area that still has large data gaps. The high uncertainties 

associated with the early stages of marine energy have driven the development costs up. As 

knowledge is gained and the industry matures, there will be opportunities to reduce development 

costs. It was also observed that, as the project moves to commercial scale, the device structure, 

PCC, and mooring/foundation become the dominant contributors. These categories not only 

suggest areas that can impact cost, but their ability to drive device performance. Therefore, R&D 

in these areas is likely to have the greatest impact on LCOE when considering WECs of this 

type. It should also be pointed out that the RM5, along with the previous reference model WECs, 

is primarily constructed of steel. Because of the inherently high loads that are seen in the ocean 

environment, steel is a relatively inexpensive solution to design devices, based on current 

technology, that can withstand the extreme loading under various sea states; however, the LCOE 

from WECs is not yet cost competitive with other forms of renewable energy. Nevertheless, 

there may be opportunities to reduce LCOE by developing innovative material that is more cost 

effective (in terms of the material-strength-to-cost ratio) or device designs that require less 

structural loading per unit of energy produced. 

Figure 29. RM5 LCOE for 1, 10, 50, and 100 units 
39

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