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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

Technical Report

NREL/TP-5000-77264

July 2020

Survivability Enhancement of a Multi-Mode 

Point Absorber

Cooperative Research and Development Final 

Report

CRADA Number: CRD-16-631

NREL Technical Contacts: Yi-Hsiang Yu and Jennifer van Rij
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

Technical Report

NREL/TP-5000-77264

July 2020

Survivability Enhancement of a Multi-Mode 

Point Absorber

Cooperative Research and Development Final 

Report

CRADA Number: CRD-16-631

NREL Technical Contacts: Yi-Hsiang Yu and Jennifer van Rij

Suggested Citation

Yu, Yi-Hsiang and Jennifer van Rij. 2020. Survivability Enhancement of a Multi-Mode Point 

Absorber Final Report, CRADA Number CRD-16-631. Golden, CO: National Renewable Energy 

Laboratory. NREL/TP- 5000-77264. https://www.nrel.gov/docs/fy20osti/77264.pdf.
NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Water Power 

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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Cooperative Research and Development Final Report

Report Date: July 7, 2020

In accordance with requirements set forth in the terms of the CRADA agreement, this document 

is the final CRADA report, including a list of subject inventions, to be forwarded to the DOE 

Office of Scientific and Technical Information as part of the commitment to the public to 

demonstrate results of federally funded research.

Parties to the Agreement: Oscilla Power, Inc. (OPI)

CRADA Number: CRD-16-631

CRADA Title: Survivability Enhancement of a Multi-Mode Point Absorber

Joint Work Statement Funding Table showing DOE commitment:

Estimated Costs NREL Shared Resources 

a/k/a Government In-Kind

Year 1 $75,000.00

TOTALS $75,000.00

Abstract of CRADA Work:

The overall goal of the project is to design and validate a survival mode for the Triton wave energy 

converter (WEC), as shown in Figure 1, which allows for a reduction in system peak structural 

stresses by 50% compared to those experienced when the heave-plate is deployed in the operational 

configuration, while simultaneously allowing for a reduction of capital cost by 15% due to the 

elimination of overdesign to account for uncertainty. In addition, the project will seek to carefully 

understand performance of the design without survival mode engaged under extreme wave 

conditions so as to better understand how system loads vary and hence determine conditions where 

survival mode.
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Figure 1. TRITONTM WEC. Inset shows reaction body cross-sectional profile (elliptical ogive).

Summary of Research Results:

All project tasks, work plans, modeling results and findings are documented within the original 

CRADA tasks following below.

Phase I - Task 1: Mid-fidelity Model Hydrodynamic Coefficients

The hydrodynamic forces acting on an oscillating multi-degree-of-freedom wave energy converter 

reaction body are characterized using scaled experiments and computational fluid dynamics (CFD)

simulations. Curves indicating how the hydrodynamic coefficients vary with Keulegan-Carpenter 

number (KC) and Reynolds number (Re) at four different scales, ranging from 1:75 to 1:36, are 

developed for multiple degrees of freedom. Understanding scale dependence is identified as 

important, as although representative KC numbers can be generated, the dissimilitude between the 

laboratory and full-scale Reynolds numbers requires careful consideration when inferring fullscale coefficients from the experiments. To address and examine this, CFD is used to replicate a 

number of these model-scale tests and provide simulations at full-scale. These results provide an 

important validation of some scaling trends but also reveal interesting discrepancies at low KC. 

OPI with the National Renewable Energy Laboratory (NREL) have extracted directional 

hydrodynamic parameters from experimental and CFD measurements, resulting in relations that 

define the pitch and heave added mass and drag as a function of KC and Reynolds number. More 

details are described in [1].
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Figure 2. Experimental measurements for 1:75, 1:60, 1:50 & 1:36 scale geometries in pitch. CFD 

data points at 1:50 scale

Phase I - Tasks 2 & 3: Establish Triton Design Load Cases

WEC systems must survive in a wide variety of conditions while minimizing structural costs, so 

as to deliver power at cost-competitive rates. Although engineering design and analysis tools used 

for other ocean systems, such as offshore structures and ships, can be applied, the unique nature 

and limited historical experience of WEC design necessitates assessment of the effectiveness of 

these methods for this specific application. A study to predict extreme loading in a two-body WEC 

using a combination of mid-fidelity and high-fidelity numerical modeling tools is presented. Here, 

the mid-fidelity approach is a time-domain model based on linearized potential flow 

hydrodynamics and the high-fidelity modeling tool is an unsteady Reynolds-averaged Navier–

Stokes model. In both models, the dynamics of the WEC power take-off and mooring system have 

been included. For the high-fidelity model, two design wave approaches (an equivalent regular 

wave and a focused wave) are used to estimate the worst-case wave forcing within a realistic 

irregular sea state. These simplified design wave approaches aim to capture the extreme response 

of the WEC within a feasible amount of computational effort. When compared to the mid-fidelity 

model results in a long-duration irregular sea, the short-duration design waves simulated in CFD 

produce upper percentile load responses, hinting at the suitability of these two approaches.

OrcaFlex simulations were performed by OPI, and we used the time history estimate the wave and 

response spectra, from which the Frequency Response Functions (FRFs) for the tendon and 

mooring line responses were determined.
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Figure 3: Example of a focused wave and expected response for tendon tension

Phase II - Tasks 1~3: Set-up & Validate CFD Models of DLCs, Run CFD Models & Process

Results

Sandia developed the CFD models in STAR-CCM+ of the Triton WEC with the support of NREL 

team. Figure 4 shows the boundary condition settings and the rendering of a selected case. To 

better investigate behavior in the tail region, we thus consider the complementary cumulative 

distribution functions (CCDF). The tendon tension responses are shown in Figure 5, and more 

details of the study are presented in [2].

Figure 4. Dimensions of computational domain for CFD simulation of Triton WEC Inlets (left) and 

rendering of StarCCM+ simulation (right)
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Figure 5. Complementary cumulative distribution functions (CCDFs) along with the results from 

CFD simulations shown by vertical lines. Regular-wave results are shown by dashed lines (- - -); 

focused-wave results are shown by dash-dot lines (- ⋅ -)

Finally, a comparison was presented between the design loads predicted by the mid- fidelity model, 

the CFD model and the physical model tests, and based on this comparison. Figure 6 shows a 

comparison the physical model and CFD data for two regular wave case. In general, the results 

suggest reasonable agreement between the physical model and CFD.

Figure 6. Comparison between experiments and CFD for two regular wave cases

Figure 6 summarizes the maximum bow tendon load produced by all experimental and numerical 

approaches. Percent differences for each method are compared to the equivalent long-duration 

irregular physical model test, which is taken here to be the benchmark. The varying levels of 

accuracy between the design loads produced by CFD and the long-duration irregular physical 

model tests could be attributed to two explanations: inaccuracies in the CFD calculation, or 

inability of the simplified design waves to reliably predict the maximum loads in the corresponding 

irregular sea state. The results from this study also indicate that the CFD model compares fairly 

well against the physical model tests in several cases, suggesting that the simplified wave 
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

approaches may have some shortcomings, where long-duration irregular wave tests are required. 

A complete comparison of numerical and experimental approaches was presented in [3].

Phase III - Task 1: Prepare Final Report

As agree by all three participants (i.e., NREL Sandia and OPI), to better report and disseminate 

the outcome of the project, all the technical results/accomplishments and findings that will benefit 

the WEC community are documented in the three conference and journal papers [1-3], listed in 

the References.

Reference 

[1] Mundon, T. R., Rosenberg, B. J., & van Rij, J. (2017). Reaction Body Hydrodynamics for a 

Multi-DoF Point-Absorbing WEC. 12th European Wave and Tidal Energy Conference 

(EWTEC), Cork, 1–10. Available for purchase at 

https://proceedings.ewtec.org/product/ewtec2017-cork-ireland/

[2] Coe, R. G., Rosenberg, B. J., Quon, E. W., Chartrand, C. C., Yu, Y.-H., van Rij, J., & 

Mundon, T. R. (2019). CFD design-load analysis of a two-body wave energy converter. 

Journal of Ocean Engineering and Marine Energy, 5(2), 99–117.

[3] Rosenberg, B. J., Mundon, T. R., Coe, R. G., Quon, E. W., Chartrand, C. C., Yu, Y., & Rij, 

J. Van. (2019). Development of WEC design loads: A comparison of numerical and 

experimental approaches. 13th European Wave and Tidal Energy Conference.

Subject Inventions Listing:

None

ROI #:

None

Responsible Technical Contact at Alliance/NREL:

Yi-Hsiang Yu, yi-hsiang.yu@nrel.gov

Jennifer van Rij, Jennifer.VanRij@nrel.gov

(for Al LiVecchi and Arielle Cardinal on the agreement)

Name and Email Address of POC at Company:

Brian J. Rosenberg, rosenberg@oscillapower.com

Tim Mundon, mundon@oscillapower.com

DOE Program Office:

U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, Water Power 

Technologies Office