The Extreme Conditions Modeling Workshop was organized and run by the National 

Renewable Energy Laboratory and Sandia National Laboratories with funding from the 

Wind and Water Power Technologies Program within the U.S. Department of Energy’s 

Office of Energy Efficiency and Renewable Energy. 

The National Renewable Energy Laboratory is a national laboratory of the U.S. 

Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by 

the Alliance for Sustainable Energy, LLC. 

Sandia National Laboratories is a multi-program laboratory managed and operated by 

Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for 

the U.S. Department of Energy’s National Nuclear Security Administration under 

contract DE-AC04-94AL85000.

Technical Report

NREL/ TP-5000-62305 - SNL/ SAND2014-16384R

July 2014

Contract No. DE-AC36-08GO28308

Extreme Conditions Modeling 

Workshop Report

R.G. Coe and V.S. Neary 

Sandia National Laboratories

M.J. Lawson, Y. Yu and J. Weber

National Renewable Energy Laboratory
 

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

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Sandia National Laboratory

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Technical Report

NREL/ TP-5000-62305, SNL/ SAND2014-16384R

July 2014

Extreme Conditions Modeling 

Workshop Report

R.G. Coe and V.S. Neary 

Sandia National Laboratories

M.J. Lawson, Y. Yu and J. Weber

National Renewable Energy Laboratory
NOTICE

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1

Executive Summary

Sandia National Laboratories (SNL) and the National Renewable Energy Laboratory (NREL) hosted 

the Wave Energy Converter (WEC) Extreme Conditions Modeling (ECM) Workshop in Albuquerque, 

New Mexico on May 13–14, 2014. The objective of the workshop was to review the current state of 

knowledge on how to numerically and experimentally model WECs in extreme conditions (e.g. large 

ocean storms) and to suggest how national laboratory resources could be used to improve ECM 

methods for the benefit of the wave energy industry. More than 30 U.S. and European WEC experts 

from industry, academia, and national research institutes attended the workshop, which consisted 

of presentations from WEC developers, invited keynote presentations from subject matter experts,

breakout sessions, and a final plenary session. 

The key findings and recommendations from the workshop were: 

• The wave industry understands that ECM is a critical step in the device design process.

• Numerical and experimental ECM methods developed by the offshore oil & gas and 

shipping industries, while useful, do not meet the needs of the WEC industry. 

Accordingly, research is needed to develop WEC-specific ECM methods. 

• Open-source experimental data sets are needed to validate WEC device design and 

analysis methods. The national labs could assist the industry by developing these data 

sets. 

• The WEC industry would benefit from a set of guidelines and best practices that 

describe how to numerically model WECs in extreme conditions.

• It is difficult to determine what meteorological and oceanographic conditions result in 

extreme loads on WEC devices. For example, it is not always the largest wave that 

causes the largest load, making it difficult to determine what wave conditions should be 

considered when performing survival analysis. The industry would benefit from 

research that helped develop methods of identifying when extreme events will occur.

• Certification bodies are starting to move towards risk-based certification of WEC 

devices and WEC developers should adopt a risk-based design approach. Many 

developers are already beginning to use this approach. 

• Uncertainty in how to design WEC devices to survive extreme conditions is slowing the 

pace of technology development by increasing the investment risk.
2

Table of Contents

Executive Summary ............................................................................................................... 1

Table of Contents................................................................................................................... 2

Nomenclature ......................................................................................................................... 4

1 Introduction .................................................................................................................... 5

1.1 Objectives...................................................................................................................................................................... 5

1.2 Definition of Extreme Conditions and Extreme Conditions Modeling ..................................................... 5

1.3 Report Format .............................................................................................................................................................. 6

2 Workshop Description .................................................................................................... 6

2.1 Developer Presentations........................................................................................................................................... 6

2.2 Invited Keynote Presentations................................................................................................................................ 7

2.3 Breakout Sessions....................................................................................................................................................... 7

2.4 Final Plenary Session................................................................................................................................................. 8

2.5 Post-Workshop Questionnaire ................................................................................................................................ 8

3 Workshop Findings and Recommendations................................................................... 8

3.1 Key Findings................................................................................................................................................................ 8

3.2 Recommendations to the National Laboratories.............................................................................................10

4 Conclusion..................................................................................................................... 11

Acknowledgements............................................................................................................... 12

References ............................................................................................................................ 13

Appendix A Workshop Agenda ....................................................................................... 14

Appendix B Workshop Attendees.................................................................................... 16

Appendix C Detailed Breakout Session Findings and Notes............................................ 17

C.1 Numerical Modeling................................................................................................................................................17

C.1.1 Participants....................................................................................................................................................................17

C.1.2 Key Findings and Recommendations...................................................................................................................17

C.1.3 Notes..................................................................................................................................................................................18

C.2 Met-ocean/Extreme Sea State Characterization..............................................................................................21

C.2.1 Participants....................................................................................................................................................................21

C.2.2 Key Findings and Recommendations...................................................................................................................21

C.2.3 Notes..................................................................................................................................................................................22

C.3 Physical Modeling ....................................................................................................................................................26

C.3.1 Participants....................................................................................................................................................................26

C.3.2 Key Findings and Recommendations...................................................................................................................26

C.3.3 Notes..................................................................................................................................................................................27

C.4 Design Standards and Extreme Loading Conditions.....................................................................................30
3

C.4.1 Participants....................................................................................................................................................................30

C.4.2 Key Findings and Recommendations...................................................................................................................30

C.4.3 Notes..................................................................................................................................................................................31

Appendix D Post-Workshop Questionnaire Responses................................................... 35
4

Nomenclature 

CFD Computational fluid dynamics

DLC Design load condition

DOE U.S. Department of Energy

DOFs Degrees-of-freedom

ECM Extreme conditions modeling

ESS Extreme sea state

FMEA Failure mode and effects analysis

IEC International Electrotechnical Commission

ITTC International Towing Tank Conference

JIP Joint industry project

Met-ocean Meteorological and oceanographic

NREL National Renewable Energy Laboratory

OWC Oscillating water column

O&M Operations & maintenance

PCC Power conversion chain

PTO Power take-off

QOI Quantity of interest

SNL Sandia National Laboratories

WEC Wave energy converter
5

1 Introduction 

Wave energy converters (WECs), which convert the kinetic energy in ocean waves into electricity,

are designed to maximize fluid-structure interactions between the device and the ocean wave 

environment in order to optimize energy production. As a result, the hydrodynamic loads WECs 

experience during both normal operation and extreme conditions must be carefully considered 

during the device design process. Under most operational conditions, the relevant fluid-structure 

interactions can be modeled using common numerical and experimental methods [1]–[6]. 

Conversely, under extreme conditions, WEC devices experience large amplitude motions, wave 

overtopping, wave slamming, and other physical phenomena that are difficult to accurately 

simulate. These extreme conditions often determine the maximum design loads, and accordingly, 

the prediction of extreme loads is a critical step in the device design process. 

The WEC industry has adopted extreme conditions design, modeling, and analysis techniques 

developed for offshore oil & gas and naval architecture applications. While leveraging these existing 

design and modeling methodologies has greatly benefited the WEC industry in its nascent stages of 

development, extreme conditions modeling (ECM) methods must be further developed in order to 

advance technologies towards commercial viability [7]. 

To assess the ECM needs of the wave energy industry, Sandia National Laboratories (SNL) and the 

National Renewable Energy Laboratory (NREL) held an ECM workshop in Albuquerque, New 

Mexico on May 13–14, 2014. ECM involves several different disciplines, including numerical, 

experimental, and meteorological and oceanographic (met-ocean) modeling. It also takes into

consideration design standards and identification of extreme loading conditions. As described in 

this report, the workshop was explicitly structured to address each of these topics.

1.1 Objectives

The overall goal of the ECM workshop was to identify research pathways through which national 

laboratory resources and expertise can be used to improve ECM methods for the benefit of the 

wave energy industry.

The specific objectives of the workshop were to: 

• Review current practices and the state-of-the-art in experimental and numerical modeling 

methods for predicting device loads, motions, and performance in extreme conditions

• Identify challenges and gaps in the current modeling methods

• Identify research pathways that have the potential to advance ECM methods. 

1.2 Definition of Extreme Conditions and Extreme Conditions Modeling

For the purposes of this document, extreme conditions are defined as met-ocean conditions that 

cause large loads (i.e. loads near or at the design limits) on a WEC device or any of its components. 

Accordingly, extreme conditions include large ocean storms where large device motions, wave 

slamming, and wave overtopping occur. Following this definition, extreme conditions may also

include operational met-ocean conditions that cause device resonance. 
6

Extreme conditions modeling methods are, therefore, considered to be any numerical or 

experimental methods that can be used during the design process to predict loads during extreme 

sea states. Because extreme conditions often result in hydrodynamic loads due to nonlinear 

phenomena (e.g., wave slamming and overtopping), most extreme modeling methods must be able 

to accurately model these nonlinear phenomena.

1.3 Report Format

The remainder of this document describes the workshop proceedings, findings, and 

recommendations. Section 2 provides a description of the workshop. Section 3 then presents the 

key findings and recommendations from the workshop. The workshop’s agenda, list of attendees, 

session notes, and the responses from the post-workshop questionnaire are presented in 

Appendices A – D, respectively. In addition, all workshop information, including presentation slides, 

can be accessed via the workshop website 

(http://en.openei.org/wiki/Wave_Energy_Converter_Extreme_Conditions_Modeling_Workshop). 

While some references are cited within this report, a more complete review of useful publications is 

presented in [8]. 

2 Workshop Description

The ECM workshop was attended by more than 30 wave energy experts from industry, academia, 

and national research institutes. A full list of participants and their affiliations is presented in

Appendix B. 

The workshop was divided into four sessions: (1) presentations from WEC developers, (2) invited 

keynote presentations, (3) breakout sessions, and (4) a final plenary session. The remainder of this 

section describes each session in detail. The full workshop agenda is presented in Appendix A. 

Other workshop documents can be downloaded from the workshop website 

(http://en.openei.org/wiki/Wave_Energy_Converter_Extreme_Conditions_Modeling_Workshop). 

2.1 Developer Presentations 

The workshop began with presentations from WEC technology developers Pelamis Wave Power, 

Resolute Marine Energy, Columbia Power Technologies, and WaveBob (see Table 1). These 

presentations focused on the specific ECM experience and design methods used by each developer. 

The developer presentations were followed by a presentation by Ryan Coe from SNL summarizing 

the NREL and SNL perspective on WEC survival analysis.

Table 1. Developer Presentation Details. 

Developer Speaker

Pelamis Wave Power Chris Retzler

Resolute Marine Energy Darragh Clabby

Columbia Power Technologies Pukha Lenee-Bluhm

WaveBob Jochem Weber
7

2.2 Invited Keynote Presentations 

Following the developer presentations, invited keynote presentations on numerical modeling, 

experimental modeling, met-ocean analysis, and offshore oil & gas experience were given by Armin 

Troesch (University of Michigan), Joop Helder (MARIN), Jim Thomson (University of Washington), 

and John Halkyard1 (Halkyard Associates), respectively. These presentations provided the 

attendees with background on the state-of-the-art WEC ECM methods and set the stage for the 

breakout sessions. 

2.3 Breakout Sessions 

After the developer and keynote presentations, the workshop participants formed four breakout 

groups based on their specific area of expertise. 

Numerical modeling: This group focused on all aspects of numerical modeling relevant to ECM. 

Numerical modeling tools, ranging from high-fidelity models, such as Navier-Stokes computational 

dynamics, to reduced-order methods, such as potential flow simulations, were discussed.

Physical model testing: This group focused on all aspects of wave tank testing of WEC devices in 

extreme conditions. Bench-top and dry testing methods were also discussed.

Design standards and extreme loading conditions: This group covered design standards as they 

apply to the design and analysis of WEC devices in extreme conditions. This group also considered 

the problem of how to define the conditions that cause extreme loads, addressing the wellrecognized issue that the largest loads on a device may not be caused by the largest wave. 

Met-ocean modeling: This group discussed current practices and new developments in estimating 

extreme sea states (ESS) through met-ocean measurements and modeling.

Each breakout group was tasked with answering the following questions with respect to their 

specific topic:

• What is the current practice and state-of-the-art? 

• What are the gaps and obstacles; how should they be prioritized?

• What are the desired outcomes of a concerted research and development (R&D) effort? 

• What specific activities are needed to move forward and in what timeframe? 

Each attendee selected a breakout session in which to participate. Attendees were encouraged to 

split their time between multiple breakout sessions if they felt they could contribute in more than 

one area. Each breakout session was led by an NREL or SNL staff member (see Table 22) who was 

responsible for taking notes during the session and for presenting the group’s findings to the entire 

workshop in the final plenary session (see Section 2.4). Key findings, recommendations and notes

from each breakout session are presented in Appendix C. 

 

1 Dominique Roddier gave John Halkyard’s presentation as Mr. Halkyard was unable to attend the workshop.
8

Table 2. Breakout Session Group Moderators.

Breakout Session Group Moderator

Numerical Modeling Ryan Coe (SNL)

Physical Model Testing Michael Lawson (NREL)

Design Standards Jochem Weber (NREL)

Met-ocean Vince Neary (SNL)

2.4 Final Plenary Session

Following the breakout sessions, the entire workshop reconvened to review the findings of the 

breakout sessions and for a final group discussion. During the plenary session, each breakout 

session leader gave a summary of the discussion and conclusions from their respective breakout 

session. The key findings and recommendations for ECM research, based on the discussion during 

the final plenary session, are provided in Section 3. 

2.5 Post-Workshop Questionnaire

At the conclusion of the workshop, the attendees were strongly encouraged to complete the online 

survey available at http://goo.gl/H0oRtv and shown in Appendix D, which also includes 

anonymized survey responses from the participants. Significant feedback from the survey is 

included in Section 3, Key Findings and Recommendations to National Laboratories. 

3 Workshop Findings and Recommendations 

The workshop presentations, notes from the breakout sessions and plenary session (Appendix C), 

and the responses to the post-workshop questions (Appendix D), were thoroughly reviewed by SNL 

and NREL to identify key findings and recommendations that can help the national labs direct 

resources in a manner that best addresses the ECM needs of the wave energy industry. This section 

presents these findings and recommendations in detail. Also note that key findings and 

recommendations from each breakout session are presented in Appendix C. 

3.1 Key Findings

1. The wave industry understands the importance of ECM during the design process: The 

industry is aware that ECM must be performed to estimate design loads, but research is 

needed to identify what conditions result in extreme loads and how to accurately model 

WECs in extreme conditions using numerical and experimental methods. 

2. Existing ECM methods do not meet industry needs: The WEC industry is currently using

ECM methods adopted from oil & gas, shipping, and naval architecture, but additional 

research is needed because the physics of wave energy devices are different from most 

traditional offshore devices (ships and oil & gas platforms). Specifically:

a. A dichotomy exists in the design drivers for WECs and traditional offshore devices.

WECs are generally designed to maximize fluid-structure interactions in order to 

maximize power performance, while traditional offshore devices are generally 

designed to minimize fluid-structure interactions for survivability. 
9

b. Ships and offshore platforms have a much larger scale than WECs, resulting in different 

Reynolds numbers and differences in the importance of viscous effects. In addition, the 

characteristic dimensions of WEC devices are typically smaller than a wavelength, 

whereas the characteristic dimension of ships and offshore platform are comparable to 

or larger than a typical wavelength.

c. WEC types and environments can vary greatly from moored surface deployments to 

seabed-mounted deployments, varying water depths, near shore and in shoaling 

conditions, thus requiring highly flexible modeling tools.

d. WECs tend to undergo large amplitude motions, and nonlinear hydrodynamic effects 

are therefore generally more important for WEC devices than other offshore 

structures.

3. Open-source experimental data sets are needed: Although there are several proprietary 

data sets from extreme conditions testing, there are no open-source (i.e. publically available) 

data sets. 

4. There are no guidelines for numerical modeling of WECs in extreme conditions:

Although there are a number of modeling methods capable of simulating the performance of 

WECs in extreme conditions (e.g. computational fluid dynamic [CFD] codes), there are no 

clear guidelines for how to perform ECM analyses. Most WEC companies are small, with 

limited in-house numerical modeling expertise and resources, and as such, the industry 

would benefit from guidelines on how to use existing numerical tools to model extreme 

events.

5. Extreme sea state characterization for WEC modeling is a challenge: Buoy observations 

are point measurements (they are therefore limited in spatial coverage) and may not 

represent extreme sea states at WEC deployment locations. Moreover, in order to get 

statistically significant data with which to estimate 25-, 50-, or 100-year storm conditions, 

long measurement histories (e.g. 20 years) are needed. Unfortunately these measurement 

histories do not typically exist at wave energy testing, demonstration, and potential 

commercial sites. 

6. Predicting the conditions that cause extreme loads is difficult: It is not always the 

largest wave that causes extreme loads and more often it is a specific wave train. For 

example, one wave may position a WEC into a vulnerable position and a following wave, that 

would not normally damage the device, may impact the device in a way that causes an 

extreme loading event. In other words, the largest wave is not always the most harmful, and 

the operational design space must be searched to identify the sea states and wave trains that 

will most likely cause a failure of the WEC system. This contrasts with the extreme 

conditions analysis for the offshore oil & gas industry, which are generally defined by the 

largest expected wave (often the 1,000- or 10,000-year wave). 

7. The uncertainty of how to design for extreme conditions is slowing the pace of 

technology development by increasing the investment risk: A device failure during a 

field deployment can cause serious harm to the reputation of wave energy companies and 

decrease the chance of private and public investment, even if the experience gained from 

device or system failures during testing is valuable. Accordingly, developing improved ECM 
10

methods that decrease the uncertainty associated with ECM will significantly benefit the 

wave energy industry. 

3.2 Recommendations to the National Laboratories

1. Create a U.S. “MARINET-like” program: The U.S. industry would benefit greatly from a 

testing program similar to the EU MARINET program (http://www.fp7-MARINET.eu/). This 

type of program would provide free access to wave tanks and expertise on how to perform 

experiments and numerical modeling in operational and extreme conditions. The vast 

majority of workshop attendees felt this type of program would be very valuable. The need 

for such a program may, to some extent, be met by the DOE WEC-Prize competition. 

2. Generate open-source WEC data sets: There are few, if any, high-quality publically 

available data sets that can be used for numerical model validation. The industry would 

benefit greatly from open-source data for WECs in both extreme and operational conditions. 

The proposed US “MARINET-like” program from recommendation 1 could supply these 

datasets if the data is required to be made publically available.

3. Initiate a WEC code-comparison project: The WEC modeling community would benefit 

from a WEC code-comparison project that is similar to the Offshore Code Comparison 

Collaborative projects (OC3, OC4, and OC5) that have verified a wide spectrum of wind 

turbine models/assumptions against baseline tank tests. As demonstrated by OC3, OC4, and 

OC5, code comparison projects are very useful not only for` validating codes, but also for 

identifying areas where existing modeling methods need to be improved.

4. Develop efficient methods to search the design space for expected met-ocean 

conditions that are likely to cause extreme loading: Methods are needed to efficiently 

search the operational and survival design spaces for scenarios that pose a threat to WEC 

survival. One example of this type of method is the Design Loads Generator developed at the 

University of Michigan [10].

5. Build on existing offshore marine structures experience, guidelines, and standards 

for extreme sea state characterization: For ESS characterization, current practices for 

offshore marine structures [11]–[15] can be directly adapted. These include: (1) Use of 

hindcast simulations or buoy observations of sufficient duration (20 years preferred) and 

appropriate locations; (2) Extreme value theory and models used for extrapolation to 

events more extreme than those observed in a shorter period of record; (3) Contours in the 

environmental space (usually significant wave height and peak period) from which sea 

states that elicit extreme response can be derived; (4) Identification of one or more ESS 

(e.g., Hs100, T100) to describe single extreme wave or wave group (with assumed wave 

spectrum) as input for numerical or physical model simulation.

6. Numerical and experimental modeling guidelines and design standards are needed:

Several numerical and experimental modeling methods are capable of predicting loads 

during extreme events. In addition, there are several design guidelines for WEC devices [10]–

[14]. However, there are no well-documented procedures for how to use the existing 

modeling methods or standards to design WEC devices. The wave energy industry would 

benefit greatly from the development of an ECM recommended practices document as well 

as example analyses. In addition, existing offshore standards should be reviewed to 
11

determine what standards can be used by the WEC industry and in what areas new 

standards are needed. This need is being partially met by IEC-TC114.

7. Encourage the WEC industry to adopt risk-based design methodologies from shipping 

and offshore oil & gas industries: The WEC industry and certification agencies are 

beginning to adopt risk-based design and device certification strategies pioneered by other 

offshore industries. The challenge of implementing a risk-based approach within the WEC 

industry lies in both the execution of the process and ensuring new frontiers related to WEC 

devices are considered and well understood. With a risk-based method, the likelihood and 

consequence of device and/or system component failures is evaluated with respect to the 

social, environmental, and economic impact of the failure. The risk-based design approach

should employ estimates from the best available ECM methods and predictions of 

operational loads.

4 Conclusion

The ECM Workshop provided a forum for experts from industry, academia and research institutions 

to discuss the current state of ECM knowledge and to suggest future research paths to benefit the

wave energy industry. Several shortcomings in existing WEC ECM methods were identified during 

the workshop and the workshop attendees identified and recommended research paths to help 

address the shortcomings. The recommendations developed during the ECM workshop span the 

sub-disciplines that comprise ECM (numerical and physical modeling, extreme sea state 

characterization and design standards). Successful execution of these recommendations will help

advance wave energy technologies towards commercial viability.
12

Acknowledgements 

The National Renewable Energy Laboratory and Sandia National Laboratories would like to thank 

all the workshop attendees, who generously took two days out of their busy schedules to share 

their knowledge and expertise in the area of WEC extreme events modeling. 

This study was supported by the Department of Energy (DOE), Office of Energy Efficiency and 

Renewable Energy (EERE), Wind and Water Power Technologies Office (WWPTO). 

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia 

Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of 

Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. 

The National Renewable Energy Laboratory 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. under contract No. DE-AC36-08GO28308. 
13

References 

[1] Kelley Ruehl, Carlos Michelen, Samuel Kanner, Michael Lawson, and Y. Yu, “Preliminary 

Verification and Validation of WEC-Sim, an Open-Source Wave Energy Converter Design 

Tool,” in Proceedings of OMAE 2014, San Francisco, CA, 2014.

[2] J. Lucas, M. Livingstone, M. Vuorinen, and J. Cruz, “Development of a wave energy converter 

(WEC) design tool – application to the WaveRoller WEC including validation of numerical 

estimates,” in ICOE 2012, Dublin, Ireland, 2012.

[3] A. Babarit, J. Hals, M. J. Muliawan, A. Kurniawan, T. Moan, and J. Krokstad, “Numerical 

Benchmarking Study of a Selection of Wave Energy Converters,” Renew. Energy, vol. 41, pp. 

44–63, May 2012.

[4] “Orcina: OrcaFlex.” [Online]. Available: http://www.orcina.com/SoftwareProducts/OrcaFlex/. 

[Accessed: 10-May-2013].

[5] G. Payne, Guidance for the experimental tank testing of wave energy converters. SuperGen 

Marine, 2008.

[6] M. B. Topper, Guidance for Numerical Modelling in Wave and Tidal Energy. SuperGen Marine, 

2010.

[7] NREL, “NREL MHK Workship Report,” 2013.

[8] R. G. Coe and V. S. Neary, “Review of Methods for Modeling Wave Energy Converter Survival in 

Extreme Sea States,” in Proceedings of the 2nd Marine Energy Technology Symposium, Seattle, 

WA, USA, 2014.

[9] “Interim Guidance on Hurricane Conditions in the Gulf of Mexico,” American Petroleum 

Institute, API BULLETIN 2INT-MET, 2007.

[10] L. K. Alford, D.-H. Kim, and A. W. Troesch, “Estimation of extreme slamming pressures using 

the non-uniform Fourier phase distributions of a design loads generator,” Ocean Eng., vol. 38, 

no. 5–6, pp. 748–762, Apr. 2011.

[11] DNV, Guidelines on design and operation of wave energy converters. Det Norske Veritas, The 

Carbon Trust, 2005.

[12] DNV, “Environmental Conditions and Environmental Loads,” Det Norske Veritas, DNV-RPC205, 2007.

[13] DNV, “Certification of Tidal and Wave Energy Converters,” Det Norske Veritas, DNV-OSS-312, 

Oct. 2008.

[14] DNV, “Position Mooring,” Det Norske Veritas, DNV-OS-E301, Oct. 2008.

[15] PCCI, “Wave and Current Energy Generating Devices Criteria and Standards,” PCCI, INC., Jun. 

2009.
14

Appendix A Workshop Agenda

Day 1 - Tuesday, May 13th

Time Topic (location) Speaker/s

7:15 – 8:15 

AM Breakfast (Majorca Foyer)

8:30 - 8:45 

AM Welcome and Opening Remarks (Majorca)

Daniel Laird, SNL

Ben Maurer, DOE

8:45 - 9 AM WEC Developer Experience: Pelamis Wave Power (Majorca) Chris Retzler

9 - 9:15 AM WEC Developer Experience: Columbia Power Technologies (Majorca) Pukha LeneeBluhm

9:15 - 9:30 

AM WEC Developer Experience: Resolute Marine (Majorca) Darragh Clabby

9:30 - 9:45 

AM WEC Developer Experience: NREL/WaveBob (Majorca) Jochem Weber

9:45 - 10:15 

AM National Lab Perspective & Discussion (Majorca) Ryan Coe

10:15 - 10:30 

AM Coffee break (Majorca Foyer)

10:30 - 11 

AM Keynote on Offshore Oil & gas Experience (Majorca) John Halkyard, Halkyard Assoc.

11 - 11:30 

AM Keynote on Tank Testing (Majorca) Joop Helder, MARIN

11:30 AM -

12 PM Keynote on Numerical Modeling (Majorca) Armin Troesch, U. of Michigan

12 - 1 PM Lunch (Casablanca)

1 - 1:30 PM Keynote on Met-Ocean (Majorca) Jim Thomson, U. 

of Washington

1:30 - 1:45 

PM Break-Out Group Logistics and Objectives (Majorca)

1:45 - 3 PM

Breakout Groups

Physical Model 

Testing

Numerical 

Simulation

The Role of Design 

Standards & 

Identification of Extreme 

Loading Conditions

Extreme Sea 

State 

Characterization

3 - 3:15 PM Coffee break (Majorca Foyer) 

3:15 - 5 PM

Breakout Groups

Physical Model 

Testing

Numerical 

Simulation

The Role of Design 

Standards & 

Identification of Extreme 

Loading Conditions

Extreme Sea 

State 

Characterization

5 - 5:15 PM Day 1 Debrief and Group Discussion (Majorca) NREL & SNL

7 – 9 PM Group Dinner
15

Day 2 - Wednesday, May 14th

Time Topic Speaker

7:15 – 8:15 

AM Breakfast (Majorca Foyer)

8:45 - 9 AM Review Day 2 Agenda and Objectives, Review Day 1 Findings and Readjust 

Groups as Needed (Majorca) NREL & SNL

9 - 10:30 AM

Breakout Groups

Physical Model 

Testing

Numerical 

Simulation

The Role of Design 

Standards & 

Identification of Extreme

Loading Conditions

Extreme Sea 

State 

Characterization

10:30 - 10:45 

AM Coffee break (Majorca Foyer)

10:45 AM - 

12 PM

Breakout Groups

Physical Model 

Testing

Numerical 

Simulation 

The Role of Design 

Standards & 

Identification of Extreme 

Loading Conditions

Extreme Sea 

State 

Characterization

12 - 1 PM Lunch (Casablanca)

1 - 1:30 PM Physical Model Testing Group Presentation and Discussion (Majorca) Michael Lawson

1:30 - 2 PM Numerical Simulation Group Presentation and Discussion (Majorca) Ryan Coe

2 - 2:30 PM Design Standards Group Presentation and Discussion (Majorca) Yi-Hsiang Yu

2:30 - 3 PM Identification of Extreme Loading Conditions Group Presentation and 

Discussion (Majorca) Jochem Weber

3 - 3:30 PM Extreme Sea State Characterization Presentation and Discussion (Majorca) Vince Neary

3:30 - 5 PM Final Discussion and Next Steps (Majorca) NREL & SNL
16

Appendix B Workshop Attendees

Name Organization

Paul Liu NOAA Great Lakes Environmental Research Laboratory

Lance Manuel University of Texas at Austin

Eric Paterson Virginia Tech

Arnie Fontaine Penn State - Applied Research Lab

Rob Kuntz Penn State - Applied Research Lab

Pukha Lenee-Bluhm Columbia Power Technologies

Darragh Clabby Resolute Marine

Chris Retzler Pelamis

John Halkyard Halkyard Associates

Dominique Roddier Principle Power

Marco Masciola American Bureau of Shipping

Jarett Goldsmith DNV GL

Gaizka Zarraonandia DNV GL

Thomas Fu Office of Naval Research

Armin Troesch University of Michigan

Jim Thomson University of Washington

Aurélien Barbarit Ecole Central de Nantes

Joop Helder MARIN

David Newborn Naval Surface Warfare Center - Carderock

Miguel Quintero Naval Surface Warfare Center - Carderock

Ken Weems Leidos

Ronan Costello National University of Ireland, Maynooth

Solomon Yim Oregon State University

Mirko Previsic Re Vision

Bryson Robertson University of Victoria

Vince Neary Sandia National Laboratories

Ryan Coe Sandia National Laboratories

Daniel Laird Sandia National Laboratories

Diana Bull Sandia National Laboratories

Yi-Hsiang Yu National Renewable Energy Laboratory

Michael Lawson National Renewable Energy Laboratory

Jochem Weber National Renewable Energy Laboratory
17

Appendix C Detailed Breakout Session Findings and Notes 

This appendix presents the notes and presentation materials from the breakout sessions. The notes 

were lightly edited after the workshop for clarity and grammar, but care was taken not to modify

the technical content. While some references are cited within this report, a more complete review 

of useful publications is presented in [8]. 

C.1 Numerical Modeling

C.1.1 Participants

The numerical modeling breakout group was composed of the following participants:

Participant Organization

Aurélien Barbarit Ecole Central de Nantes

Ryan Coe (moderator) Sandia National Labs

Rob Kuntz Penn State ARL

Daniel Laird Sandia National Labs

Pukha Lenee-Bluhm Columbia Power Technologies

David Newborn Naval Surface Warfare Center, Carderock

Eric Paterson Virginia Tech

Mirko Previsic Re Vision

Kelley Ruehl (note-taker) Sandia National Labs

Armin Troesch University of Michigan

Solomon Yim Oregon State University

Ken Weems Leidos

C.1.2 Key Findings and Recommendations

C.1.2.1 Key Findings

• Lack of experimental data for validation: Relatively little data is publically available from 

tests of WECs in extreme conditions with which numerical models can be validated. Publically 

available datasets from well-documented experimental test campaigns would allow for better 

validation and thus greatly improve confidence in numerical models.

• Quantitative relative performance of numerical models: While there are a range of 

numerical models available for analysis of a WEC in extreme conditions (see Section Error! 

Reference source not found. of the Appendix of this report), their relative performance is not 

well characterized. To understand how these models should best be used in various scenarios, a 

more quantitative understanding of their relative performance (including efficiency and 

accuracy) is needed.

• Efficient search of survival space: Testing a survival condition with a model (numerical or 

experimental) is challenging partially because the most dangerous condition for a specific 

device cannot be known in advance. Thus part of the problem becomes searching a very large 

space of possible scenarios for those that pose a threat. The scenarios must be searched are 

comprised of irregular sea states, winds and currents, each with a distinct angle of incidence. 

The number of scenarios that must be evaluated prohibits the exclusive use of high-fidelity 
18

numerical and physical models. These more expensive and time-consuming models must be 

accompanied by more efficient lower-fidelity models, which can help reduce the search space to 

a more manageable size.

C.1.2.2 Recommendations 

A. Numerical model comparison: A formal “code comparison” project could be used to provide a 

quantitative comparison of the various numerical models available for analysis of WECs in 

extreme conditions. This comparison would focus on a set of representative WEC designs (e.g., 

DOE Reference Models). Researchers from various institutions would then be asked to use their 

respective numerical codes to perform a predefined set of analyses. The results of these 

analyses would then be compared using a common set of metrics. If possible, physical 

experiments should also be conducted to provide a validation point.

B. Development/demonstration of efficient survival space search method(s) – This effort 

would focus on development and/or demonstration of methods to more efficiently analyze the 

survival of a WEC for a given deployment location. Two paths were suggested by workshop 

participants:

o Lower-fidelity models can be employed to develop response surfaces on which point 

evaluations can employ higher-fidelity models.

o High-fidelity models can be employed to find scenarios that may cause failures of the 

device; searches can then be expanded in that vicinity with lower-fidelity models, using 

extreme sea state characterization methods to determine event frequency.

C. Numerical model improvement – Potential improvements to numerical models include:

o Viscous damping parameters can be predicted via CFD for use in lower-fidelity models.

o Wave propagation in CFD simulations can be improved, through domain decomposition 

or improved boundary condition formulations.

C.1.3 Notes

C.1.3.1 Quantities of Interest

The choice of a numerical model and decisions made in its implementation must take into account 

the nature of the specific WEC being considered. These decisions can be informed by identifying 

quantities of interest (QOIs). A general set QOIs for WEC survival include

• Dynamic response – The dynamic motion of the various bodies of the WEC as determined 

from rigid-body inertial properties and global loads. The position of the WEC relative to the 

free surface is needed to determine the effect of various physical phenomena (e.g., 

slamming and breeching). This is particularly important for oscillating water columns (due 

to the importance of sloshing and flooding in the moon pool).

• PCC & joint loading – Forces and moments applied at joints and within the power 

conversion chain are needed for structural analyses.

• Mooring connection points – Forces induced mooring connection points on device and sea 

floor are needed for analysis of a device’s mooring system.
19

• Spatial pressure distribution – Local loads on the hull of the device are needed to inform 

structural analyses. When necessary, these may include peak impact pressure due to 

slamming.

C.1.3.2 Key Physical Phenomena

Accurate prediction of these QOIs is dependent on an adequate representation of the physical 

phenomena that dominate those QOIs. A list of physical phenomena key to the dynamics of WEC in 

survival conditions are

o Environment – The environment at a WEC deployment can be seen as the input to a 

numerical model. 

o Waves – Waves are thought to represent the dominant forcing factor for most WEC 

survival scenarios. A numerical model must be capable of producing regular and 

irregular waves with arbitrary directionality.

o Wind – Wind loading on WECs is generally considered to be a marginal component 

for WEC survival. However, for WECs with large solid areas above the free surface 

(e.g., OWCs) wind may play a larger role.

o Current – Current, which may occur in a direction different than the primary wave 

spectrum, must be considered.

• Hydrodynamics – Hydrodynamic phenomena are considered to be the dominant 

component influencing the above listed QOIs.

o Wave modeling – Beyond specifying accurate wave conditions as the input to a 

model, some models (e.g., free surface Navier-Stokes) require additional 

consideration as to accurate propagation of waves.

o Ideal flow – Phenomena captured by ideal flow theory must be included.

 Hydrostatics 

 Froude-Krylov

 2nd-order drift force

 Radiation 

These phenomena can be characterized using frequency-domain boundary element 

method (BEM) codes (e.g., WAMIT). However, parameters obtained via BEM codes 

are only truly applicable for small amplitude motions. Models that rely on 

frequency-domain codes for hydrodynamic parameters can employ additional 

approximations, in which hydrodynamic parameters are approximated as a function 

of submerged geometry.

o Viscous effects 

o Slamming – Slamming during water reentry and due to steep or breaking waves 

should be considered, especially when spatial pressure distributions on a WEC’s hull 

are of interest. 

o Greenwater – Overtopping can influence local pressures and the dynamic response 

of a WEC

• Mooring – A WEC’s mooring system can have a large influence on the dynamic response. 

Likewise, a mooring system must be designed to withstand loads created by the motions of 
20

the WEC. Additionally, localized loads are mooring connection points that may be of 

interest. 

• Power conversion chain (PCC) – Numerical models used in survival analysis must be 

capable of including arbitrary forcing from a PCC. Many WECs are currently designed to 

employ survival strategies in the PCC is either “locked” or allowed to “freewheel” to reduce 

loading. However, research is currently underway to investigate the effectiveness of the lifeextending controls (LECs) on WECs, in which the PCC force can be modulated to reduce 

system loading.

• Rigid-body motion – Global loads must be coupled with rigid-body properties to predict 

the dynamic motion of a WEC. Models should be capable of simulating constrained motion, 

in which only a subset of the full six degrees-of-freedom (DOFs). Numerical models should 

also allow for relative motion of two coupled rigid bodies.

• Structural response – In some cases, it may be necessary to consider the structural 

response of a WEC. It is believed that, in most cases, structural analyses can be carried out 

without direct coupling with hydrodynamic models. Exceptions include highly flexible 

structures (e.g., Anaconda device) and slamming analyses.

C.1.3.3 Available Numerical Models

A range of numerical models are available for use in the WEC survival analysis process. In general, 

the cost to run a model (which must include licensing as well as computational and personnel 

resources) increases with model fidelity. Even with free access to today’s best computational 

resources and highly-skilled engineers, using the highest-fidelity models for all of the simulations in 

a WEC survival analysis is impractical, as the models would require many years to run.

• Frequency-domain (same as time-domain Cummins’) – These models assume a linear 

system in which reactions due to radiation and diffraction can be linearly decoupled and

superimposed. These assumptions are only valid for bodies in regular waves undergoing 

small amplitude motion.

• Semi-empirical time-domain (Cummins’ Eq. w/ nonlinear effects) – These models have 

their basis in the same theory used for frequency domain models, but employ Fourier 

transforms to operate in the time-domain. While time-domain models are based on the 

assumption of small amplitude motion, augmentations to account for large amplitude 

motions and nonlinearities are possible. 

o Nonlinearities – Nonlinearities that can be added to these models include

 Hydrodynamic & hydrostatic – These phenomena can be made a function 

of body position.

 PCC (control) – Arbitrary control of the PCC with force and motion 

saturation.

 Mooring – Mooring models can vary from simple catenary models to fully 

dynamic models.

o Free surface representation – Nonlinear wave formulations provide a better 

model of real ocean waves.

• Nonlinear potential flow – These models solve an initial boundary value problem by 

employing a potential flow panel method. Viscous effects must be included empirically.
21

• Free surface Navier-Stokes – These are generally referred to as computational fluid 

dynamics (CFD) models. Free surface Navier-Stokes models represent the highest fidelity of 

numerical model applicable to analysis of WECs in extreme conditions. Given the proper 

configuration, these are capable of capturing the full range of phenomena that affect a WEC 

in extreme conditions.

C.2 Met-ocean/Extreme Sea State Characterization

C.2.1 Participants

The participants in the Met-Ocean breakout group are listed below:

Participant Organization

Annie Dallman (note-taker) Sandia National Labs

Tom Fu Office of Naval Research

Lance Manuel University of Texas at Austin

Chris Retzler Pelamis

Bryson Robertson University of Victoria

Vincent Neary (moderator) Sandia National Labs

Jim Thomson University of Washington

C.2.2 Key Findings and Recommendations

C.2.2.1 Key Findings

• Current practices for characterizing extreme sea states exist: Current practices for offshore 

marine structures, [11]–[15], can be directly adapted for extreme sea state characterization.

• Return periods for WECs are different that for other structures: Although 100-yr return 

period events are common for marine structures, lower return periods can be used, if 

acceptable for survivability, when the design service life is less than 100 years [DNV 2005]. The 

acceptable level of risk for WECs can be quantified through a rigorous risk-based assessment 

that depends on the design service life, cost of device, and consequence class. 

• Improved wave measurement capabilities are needed: Additional measurements and 

measurement capabilities are needed to more accurately predict the true distribution of 

extreme sea states. Buoy observations are point measurements and are therefore limited in 

spatial coverage, and may not represent ESS at the WEC deployment location. They typically 

have shorter periods of record than desired for accurate estimates of ESS (less than 20 years). 

Altimeters and synthetic aperture radar (SAR) measurements offer better spatial coverage, but 

at a low spatial and temporal resolution, which low pass filters large waves and underestimates 

ESS. Validated long-duration simulations may be the most practical approach to ESS 

characterization, but they are also known to under-bias prediction of large waves and 

underestimate ESS. 

• Ongoing research will improve ESS characterization methods: Expected advances within 

the next 5 to 10 years should improve the accuracy of ESS characterization. These advances 

include the development of improved and more affordable monitoring instruments, improved 

measurement techniques (e.g., buoy arrays), adoption of higher order spectral methods (e.g. 
22

Janssen and Herbers), improved prediction of breaking waves, improved hindcast models and 

models that can simulate the effects of WEC device interaction in arrays.

C.2.2.2 Recommendations

• The WEC industry should leverage existing standards: Current practices for offshore marine 

structures [11]–[15] can be directly adapted by the WEC industry, but a consolidation of the 

existing standards in one location would make the standards easier to understand and use.

• Research is needed to do the following: 

• Improve wave measurements and include better quantification of uncertainty of buoy 

measurements

• Determine appropriate spectra for modeling extreme wave environments

• Improve time-domain analysis of buoy data to identify individual waves and wave 

groups

C.2.3 Notes

The group defined four objectives that aligned with the overarching workshop objectives:

• Define the current state of the art for extreme sea state characterization (“current practice”)

• Identify & prioritize challenges and gaps (limitations of current practices)

• Recommend research pathways to most effectively advance ECM methods, including immediate 

recommendations for improvement, R&D needs and specific R&D activities (with time frame)

C.2.3.1 Current practices

• Use of hindcast simulations or extreme value analysis to extrapolate buoy measurements

• Extreme value theory and models used for extrapolation to events more extreme than those 

observed in a shorter period of record

• Use of Inverse FORM technique to define ESS; contours in the environmental space (usually 

significant wave height and peak period) along which contours of an extreme response with 

a given return period should lie [12] . Generalized Extreme Value (GEV) distribution, to 

estimate one variable at a time (not recommended)

• Identify one or more ESS (e.g., Hs100, T100) to describe single extreme wave or wave group 

(with assumed wave spectrum) as input for numerical or physical model simulation

Due to time constraints, the discussion did not include recommended practices and guidelines for 

ESS characterization, including guidelines and applicable design standards for offshore structures 

cited in Reference [11]. These are listed below: 

• Guidelines for characterizing ESS are given in Ref. [11], and there are recommended practices 

and standards for offshore structures listed in Ref. [15] that can be directly adopted. 

• Reference [11] defines three WEC deployment environments, bed-deployed-shoreline (surf 

zone), bed-deployed-shore (shallow water), and moored floating-offshore (deep water). 

• Based on design criteria API RP 2A WSD – Fixed Offshore Structures, the recurrence interval 

of the ESS should be several times the planned service life of the WEC [15]
23

• Based on design criteria API RP 2SK – Station keeping Systems for Floating Structures, the 

recurrence interval design condition for permanent moorings should be determined by a 

risk analysis taking into account the consequence of failure [15]

• Based on design criteria API RP 2SK – Station keeping Systems for Floating Structures, 

environmental loads must consider a combination of wave, current and wind conditions 

causing the extreme load in the design environment [15]

• Based on DNV design guidelines for wave energy converter design (Section 13.9) [11], 

design of the device should be based on the most severe environmental loads the structure 

may experience

• Based on design criteria API RP 2SK – Station keeping Systems for Floating Structures, the 

most severe directional combination of wind, wave and current forces should be specified 

for the permanent installation [15]

• Based on DNV design guidelines for wave energy converter design (Section 13.9) [11], The 

proper combinations and joint occurrences of waves and current conditions, as given in 

DNV-OS-C101 (Design of Offshore Steel Structures, General), should be applied [15]

• Based on DNV design guidelines for wave energy converter design (Section 15.2) [11], for 

WECs with a dynamic response to wave loading and or novel mooring configuration, a 

complete time domain simulation combined with tank testing may be necessary

• Based on EMEC guidelines on Reliability, Maintainability and Survivability of Marine Energy 

Conversion Systems (5.2.2.1), near-shore corrections should be considered before metocean data used to represent a site. Wave loading through the water column during storms 

is of particular importance [15]

• Based on DNV design guidelines for wave energy converter design (Section 15.2) [11], for 

WECs with a dynamic response to wave loading and or novel mooring configuration, a 

complete time domain simulation combined with tank testing maybe be necessary

• Based on EMEC – Wave Energy Tank Testing guidelines, (3.1), planned testing should 

include extreme wave loading of structure [15]. 

C.2.3.2 Limitations 

• ESS characterization in shallow shoaling surf regions with breaking waves is especially 

challenging

• Buoy data are point measurements

• Extrapolated return periods of ESS are sensitive to the extreme theory or model used

• Wave buoys use different measurement technologies and have different accuracies and 

resolution

• Sea state measurements are typically over short periods of record less than 20 years

• Archived buoy data only have first five spectral moments (cannot reconstruct 2D phaseresolved wave group, e.g., crossing-seas)

• Long duration hindcast simulations under-bias extreme waves (See comparison of buoy and 

hindcast Hs plot, Fig. 2.3, M. Prevosto, Extreme and long term extrapolation, EquiMar 

Deliverable D2.6, January 2011)
24

• Estimating the loads on offshore structures in extreme irregular waves is sensitive to the wave 

kinematics model used (C.T. Stansberg, O.T. Gudmestad, S.K. Haver2008. Kinematics under 

extreme waves. Trans. of ASME, J. of Offshore Mech. And Arctic Engrg. 130, )

• Archived hindcast simulations are not sufficiently archived (e.g. NOAA only archiving bulk 

parameters in WW3). Cannot reconstruct spectral shape and random wave field

• Not accounting for ‘rogue waves’ H>2.2Hs (Rayleigh Distribution), large wave events, breaking, 

Cahill PhD Dissertation.

• Rarely coupling wave, wind and current loads

• No accounting for spatial variation (arrays) of rogue waves

• No shoaling wave environments.

C.2.3.3 Session overview

WEC survival can be compromised in low and high sea states. This project narrowed the scope to 

focus on modeling the WEC response to an extreme sea state (ESS) because this entails special 

considerations and challenges in analysis, including numerical and physical modeling; not only 

challenges analyzing WEC response, but challenges modeling the extreme sea state as the model 

input. 

The inverse-FORM method is applied to define an extreme sea state condition for a desired WEC 

deployment location. Device response in these conditions is then assessed with numerical and/or 

physical models. Numerical models generally consist of low to mid-fidelity codes (see “Semiempirical time-domain” models in Section C-4.3 of the Appendix of this report). Numerical 

modeling efforts are generally backed up with model-scale physical testing.

Extreme sea state characterization challenges: Buoy observations are point measurements, are 

limited in spatial coverage, and may not be representative of ESS at the WEC deployment 

location. They typically have shorter periods of record (less than twenty years) than desired for 

accurate estimates of ESS. Altimeters and synthetic aperture radar (SAR) measurements offer 

better spatial coverage, but at a low spatial and temporal resolution, which low pass filters large 

waves and underestimates ESS. Validated long-duration hindcast simulations may be the most 

practical approach to ESS characterization, but they are also known to under-bias prediction of 

large waves and underestimate ESS.

For ESS characterization, current practices for offshore marine structures [11]–[15] can be directly 

adapted. Nevertheless, the ESS breakout group outlined the limitations of current practice 

methods, recommendations for improving ESS characterization, expectations for advancing ESS 

measurement and modeling techniques and recommended research pathways.

During the breakout group discussion, the following key findings and recommendations were 

discussed:

• Quantify uncertainty with buoy measurements (e.g., JCOMM)

• Use full (well resolved) spectral information

• reconstruct complete wave field (nonlinear preferred, but linear ok); At least 100 crests
25

• Or partition to different unique wave trains (separate swells and wind waves); Way to 

improve inverse FORM method

• If buoy data can be obtained, use time-domain analysis of buoy data to identify individual waves 

and groups

• Use wave time series and zero crossing analysis to identify large waves (e.g., Tucker and 

Pitt – Cahill Thesis)

• Include wave steepness as additional parameter

C.2.3.4 Expected R&D Advances (time frame)

• 2D phase resolved descriptions (sequence and timing of waves in wave groups), measurement 

and modeling, 10-year time frame

• Need to reduce cost of instruments, 5-years

• New measurement techniques (Thomson): Doppler radar, scanning LiDAR, buoy arrays, 5-years

• Deployment & adoption of newer measurement devices, 5-years

• Adoption of higher order spectral methods (e.g., Janssen and Herbers), 5-years

• Predictive tools for breaking (measurement and modeling), 5-years

• Improved hindcast models (better wind inputs) to better resolve extreme waves, 5-years

• Array effects on ESS and environmental loading, 5-years

• Improvements in characterizing ESS in near-shore environments, 5-years

C.2.3.5 Research Paths

• Complete review of current practice in seakeeping, offshore oil & gas, offshore wind

• Demonstration project (DOE Reference Model WEC)

• Improved wave measurement techniques

• Rough/approximate description of 2D phase resolved wave field

• Use partitioned spectra to perform Inverse FORM method for site/device pair (DOE 

reference model, Humboldt Bay)

• Coupling of wave, current and wind loads

• Evaluate value of recommendations

• 5-10 year effort

• Working Group, international Technical Specification on WEC survivability analysis (e.g., IEC, 

IEA, ISSC, ITTC)

• Identify other international researchers working on extreme sea state analysis
26

C.3 Physical Modeling

C.3.1 Participants

The participants in the Physical Modeling breakout group are listed below:

Participant Organization

Michael Lawson (moderator) National Renewable Energy Laboratory

Diana Bull (co-moderator) Sandia National Laboratories

Chris Rensler Pelamis Wave Power

Dominique Roddier Principle Power

Joop Helder MARIN

Miguel Quintero Naval Surface Warfare Center - Carderock

Ronan Costello National University of Ireland, Maynooth

C.3.2 Key Findings and Recommendations

C.3.2.1 Key Findings

• Experimental methods being used are from offshore oil & gas and shipping industries:

Developers are using experimental methods developed by the shipping and offshore oil & gas

industries. 

• Extreme loads are not necessarily the result of the largest wave: Extreme loading events 

are typically a series of waves from a specific wave train as opposed to simply the largest wave.

Accordingly, focused waves in a wave tank test may not be the most appropriate way to test 

device performance in extreme conditions. 

• WECs will not be designed to the same level as ships or offshore platforms: The industry 

needs a way to determine what condition to design to (see the risk-based design in the 

Standards breakout session). In other words, depending on cost and consequence class, should 

extreme conditions be defined as a 20-year storm, 50-year storm, or 100-year storm. In 

addition, it is important to develop appropriate safety factors to apply during the extreme loads 

design process and these factors may be different than those used by the offshore oil & gas and 

shipping industries.

• Testing devices with deformable bodies is difficult: It is difficult to test WECs that rely on 

deformable body dynamics because of material scaling properties. It is unclear whether there 

are enough developers perusing this type of technology to justify investment in developing 

experimental modeling techniques.

C.3.2.2 Recommendations

• Develop improved methods of modeling power take off systems at small scale: Powertake-off systems are difficult to test accurately at small scale and R&D and testing methods 

improvement are needed and should be developed

• Consolidate existing standards and design guidelines: There are several design and testing 

guidelines currently under development (e.g. International Electrotechnical Commission, 

International Towing Tank Conference, ABS, DNV-GL). However it is not always clear what 
27

standards should be followed and the existing standards should be consolidated in one location 

(perhaps OpenEI) for clarity.

• Insure that all measurements are of appropriate time resolution and are time synced: 

During testing, all experimental measurements should be time synced.

• Calibrate wave tanks before testing: Calibration of wave tanks is critical and it should be 

confirmed the tank is accurately calibrated before testing begins.

• Perform numerical simulations at the same time as experimental tests: Numerical 

simulations should always be performed simultaneously with wave tank tests so that any 

problems with the experiments can be identified before testing is complete.

• Insure PTO systems are accurately modeled in scale experimental tests: PTO system 

scaling is very difficult. Scale PTO systems should be constructed very carefully to insure proper 

performance, or electric motors should be used in place of scale PTO systems to insure the 

forces acting on the device are correct.

C.3.3 Notes

C.3.3.1 Current state-of-the-art

• Extreme wave conditions are difficult to define. Tests are often performed with 100-year 

wave or wave environment that developers identify as particularly harmful to their device

• Water current and wind is not considered at this point. For some (but likely not all) devices, 

these effects may be important.

• Instrumentation of critical systems and/or failure points. Detailed measurements (e.g. 

pressure distributions) are not typically made due to associated cost and complexity.

• Experimental methods developed for oil & gas and shipping industry are heavily leveraged. 

Little methods development has been performed by the wave industry. 

• What are the important experimental design considerations?

o Important that the mass properties (mass and moments of inertia) are accurately 

represented in the experimental tests

o Testing different survival scenarios may require different scale models and/or tests

o Instrumentation can be problematic for small device scales, as the required 

instrumentation may effect inertial properties

• Need to think about what the purpose of experimental tests are before the tests are 

performed. Need to ask, what do we want to measure/validate/verify. Then a test plan

should be developed. This type of systematic approach is not always the standard in the 

WEC industry.

• Real-time processing of data is important.

o Real-time data analysis should be performed during the experiments

o Data processing ensures the results are useful

o Simulations should also be run real time to find numerical model and experimental 

test problems so they can be addressed while in the tank – this is not possible after 

testing has been completed.

• Video is important

• Some redundant measurements make sense given the expense of tank testing
28

C.3.3.2 Gaps

• Small scale is desirable for Froude scaling/tank limitations, but makes PTO modeling 

difficult and causes problems with Reynolds number not scaling properly.

• Model PTO systems are not typically repeatable  needs more attention and electronic PTO 

systems show promise.

• Effective ways to test different failure modes in tank tests – consider when designing tank 

test program test conditions where there are PTO failures / test when PTO is operational, 

but grid connection is lost, i.e. fault conditions / what happens if device does not go into 

“survival mode” during survival condition

• How to identify failure modes and conditions that cause them (i.e. limit states for the 

system) – well defined for oil & gas, but not yet for WECs

• Test fault conditions

• Developers have traditionally used focused waves to test extreme conditions performance 

of device. This is not best practice and a response base analysis (i.e. determination of what 

wave trains cause the largest loads) should be used

• Calibration of wave tanks is critical and not all tanks are well calibrated – especially the low 

budget tanks that are typically used by smaller developers (which is a larger percentage of 

the wave energy industry). Wave tank characterization includes the characterization of 

reflections, basin coverage, energy density, etc. 

o Running numerical models at the same time that experimental measurements are 

performed. Numerical simulations can help identify for existing tanks

o Testers should always run and check tank with no model present – seems to be 

standard practice – developers and testers need to account for calibration time 

when they are developing test plans

• There are other necessary testing conditions – e.g. tow out, dry lift, installation, O&M, 

mooring installation, arrays, accidents, boat strike. These conditions are not typically 

considered as seriously as extreme loads that occur after the installation is completed, even 

though they are just as important.

• Testing deformable devices (AWS, anaconda) is a challenge – relevant when deformations 

are large enough to radiate waves – also important when deformation is critical to PTO 

operation due to scaling issues and when the performance of deformable components need 

to be tested at scale.

• Syncing and data logging – need a “data unifier” for inexpensive tanks – important if you 

want to look at results real-time – can save post-processing time and make results more 

useful after testing is compete. 

o Possibly attach data logging requirements to DOE funding for tests? 

o Approval of test plans? 

o More automation in testing could help – sensors must be compatible with 

measurement systems

o follow ITTC procedures

• Difficult to measure pressure distributions for numerical validation – point measurements 

are standard, spatially resolved measurements are a more “research-level” topic and are 

typically very expensive
29

• Material scaling is typically not considered at model scale  loads are passes to 

FEA/structural software for more detailed analysis

• Small scale tanks (~100K) – there is a company that makes one of these – the cost of tank 

testing is mostly the experts required to run the tests/setup instrumentation

C.3.3.3 What are the next steps?

• Develop a U.S. (likely DOE) program similar to MARINET (http://www.fp7-MARINET.eu/). 

Critical components of this program would be:

o Access to wave tanks

o Characterization of wave tanks prior to testing

 Reflections

 Directional waves

 Basin coverage – i.e. where are the “dead spots” and what is the energy 

distribution 

o Technical/engineering support for test planning, model construction, mooring 

system, PTO system development, and post-processing – move responsibility for 

model construction away from developers/testers towards tanks – important that 

testing center is involved with the design and fabrication of the test model – models 

should be built with the test facility in mind – approval of test plan/model design

o Require publication in peer-reviewed conference/journals so that the tank testing is 

not just a “box checking” exercise

o Explore possibilities of requiring numerical modeling prior to wave tank testing

 relevant to do before going into the tank to check for design problems

 should be performed by a third party to verify design specifications

 can help develop experimental test plan

 leverage DOE/National Lab computing facilities

o Example test programs to represent some set of likely deployment locations – these 

guidelines would include extreme seas

o Requirements/test plan should reflect the level of development/cost of the device 

and test

o Give priority to developers who ‘need’ the test

• Testing guidelines – consolidate existing guidelines – e.g. ITTC

o Require that tests follow guidelines or provide rational/justification for not 

following guidelines

o Example test programs (super program and everyone tests a subset)

o Recommended test conditions to cause extreme loading – require to be simulated 

numerically before it is tested experimentally

o Require several different extreme conditions to be considered – e.g. short steep 

waves, long waves, etc

o Having a reviewed and approved test plan is perhaps better than requiring 

developers to exactly follow ITTC standards – by DOE determined group (e.g. DVNGL/National Labs/testing facilities)
30

• Development of generic PTO systems (e.g. linear rotary) that enable the testing of scale 

devices – this is challenging because there is very little convergence within the industry

o Important because friction is a big problem at model scale – very relevant to a U.S. 

MARINET program

o Important to do bench-top testing of PTO / dynamic calibrations to determine 

response – is it linear, non-linear, etc. – complete before test

o PLC development that can be used with different motors

• Develop methods to experimentally recreate wave trains that are appropriate to replicate 

realistic extreme sea states (met-ocean breakout group will look into this in more detail)

• Open-source data processing tools for making sense of raw data

o Some useful tools exist – DIWASP, WAFO

• Ability to easily model non-linear waves in wave tank tests

C.3.3.4 What tanks can be used for ECM testing?

• University of Maine (coming soon)

• OSU

• Carderock

• MARIN

• ECN

• And many others depending on acceptable scale

C.4 Design Standards and Extreme Loading Conditions

C.4.1 Participants

The participants in the Design Standards and Extreme Loading Conditions breakout group are 

listed below:

Participant Organization

Jochem Weber (moderator) National Renewable Energy Laboratory

Jarett Goldsmith DNV-GL

Yi-Hsiang Yu National Renewable Energy Laboratory

Diana Bull (note-taker) Sandia National Laboratories

Gaizka Zarraonandia DNV-GL

Arnie Fontaine Penn State ARL

Marco Masciola ABS

C.4.2 Key Findings and Recommendations

C.4.2.1 Key findings

• No standards exist: Although there are guidelines (DNV OSS 312, Equimar, and Carbon Trust), 

applicable standards, and experience from offshore wind, naval architecture, and oil & gas, 

there are no dedicated standards for WEC design, particularly for extreme conditions. Wave 

energy developers often use offshore oil & gas standards as a starting point.
31

• Failure modes effects analysis (FMEA): Not every WEC developer had conducted a detailed 

FMEA analysis and identified extreme/ultimate limit states for their design during the 

development process.

• Lack of data/experience. The WEC industry is at the nascent stage. Only a limited number of 

WEC devices have been deployed and tested in the water. The WEC community lacks data, 

including WEC system information, detailed environmental conditions, structural loads and 

associated conditions, failures and root causes, overall experience and safety factors used. 

• Uncertainty in determining extreme loading conditions: WEC developers often face the 

question of how to estimate the extreme loading conditions, which can be different for different 

WEC devices and different device components. It is challenging to identify the extreme 

conditions, which can be extreme sea states or a sequence of operational waves with relatively 

large amplitudes. In addition, numerical/experimental methods may still result in high 

uncertainty, including scaling issues, appropriate representation of all coupled systems, and the 

limitation of methods to represent accidental/survival/temporary conditions. 

C.4.2.2 Recommendations 

• Develop design standards: Currently, DNV and other certifying bodies are working to develop 

guidelines and the development of standards, and the IEC is working on the draft of a design 

requirements document. A risk-based approach has been suggested and the standards 

development will also rely on other industry experience where applicable (e.g. particular 

systems, subsystems for the WEC devices).

• Use a risk-based approach for design standards: One key difference between WECs and 

other offshore structures is that the risks (loss of investment, loss of life, environmental impact) 

associated with the failure of a WEC are much less than for most offshore structures (with the 

notable exception of offshore wind turbines) and ships. A risk-based design approach is 

suggested, which evaluates the design, impact, and failure probability of the device components

relative to the cost. The steps include defining modes of operation, technology assessment, and 

FMEA analysis and specification of design load cases. For the design standard development, the 

interim goal is to lead to a “Statement of Feasibility” and the final goal is to use for certification.

• Share WEC industry data and experience. To gather more data from all the WEC testing 

projects, it is essential to ensure all prototypes are instrumented to capture detailed 

environmental condition and load data and give the WEC developer access to resources in 

modeling and testing (e.g. access to expertise/computing resources/basins). The MARINET

project is a successful example, and Joint Industry Projects (JIPs) are also an option. Test 

centers and the leasing agency (Bureau of Ocean Energy Management) can play an essential 

role in pushing for data and experience sharing in the WEC community. 

• Improve met-ocean/extreme sea state, numerical and physical modeling: See Appendix 

C.2

C.4.3 Notes

C.4.3.1 Design Standards

• What are the existing standards and are they relevant to the WEC industry?
32

• No standards exist, although IEC TC114 is in the process of developing a set of wave and 

tidal device standards. There are guidelines: DNV OSS 312, Equimar, Carbon Trust, 

applicable standards and experience from offshore wind and oil & gas, (shipping) 

• Risk-based approach is applied 

• Defining modes of operation 

• Technology assessment and failure mode identification

• Specification of design load cases 

• Interim goal: To lead to “Statement of Feasibility”

• Final goal: Certification of compliance

• Are there gaps in the existing standards?

• Yes, lots. The whole approach is different to a traditional standards approach.

• Scope of design basis 

• Deployment met-ocean conditions 

• Modes of operation 

• Configuration of all systems

• System specifications 

• Structural 

• Mechanical

• Electrical 

• Can the developers be provided with advice when best to start the process?

• Roughly when the developer plans for sea going tests and will undertake relevant and 

same design tank testing campaign for this sea going campaign

• Specifically when they can provide all the information required for the establishment of 

the design basis 

• Certification is not compulsory

• To ensure best practices & reduce risk. 

• Is there need for additional standards development?

• Yes, but the diversity of WECs requires a flexible approach 

• Many load cases need to be considered: Thousands 

• Baseline safety factors need to be derived for specific cases and risks

• Gap to be filled: Collection of data to help determine safety factors. 

• How does the topic relate to other breakout groups?

• What are the critical design load conditions (DLCs), including identification of extreme 

loads?

• Key survival conditions may not be associated with extreme wave events 

• Well-defined met-ocean characterization needed

• Understanding of uncertainty from met-ocean characterization, physical modeling, and 

numerical modeling methods applied

• Physical understanding of key risks and consequences for each WEC device (FMEA)

• Once design basis and DLCs are known (i.e. from FMEA), appropriate 

numerical/physical modeling techniques need to be applied 

• The certification process can help developer progress through design, including 

reliability and survivability considerations. 
33

• How can standards be written for an industry that has not converged on a winning design 

archetype?

• Risk-based approach (e.g. DNV OSS 312) can be used.

• Rely on other industry experience where applicable (e.g. particular systems, 

subsystems)

• Is the work being performed by IEC TC 114 on target?

• The 62600-2 Design Requirements document is still being drafted (unknown if ‘on 

target’)

• IEC TC 114 is discussing this risk-based approach as the appropriate way to go forward.

• How should the standard development community work to best help the WEC industry and vice 

versa?

• Lack of data is a problem. WEC industry needs to share their experience.

• WEC industry can help by providing data.

• WEC system information, detailed environmental conditions, structural loads and 

associated conditions, failures and root causes, overall experience and safety factors 

used

C.4.3.2 Identification of Extreme Loading Conditions

• How is the WEC industry currently identifying conditions where extreme device loading 

occurs?

• Each developer has their own processes

• Often using offshore oil & gas standards (as starting point, or as sole source of 

environmental condition recommendations)

• Identification of conditions through experimental (often) and numerical modeling 

campaigns

• Careful application of techniques (see other groups), and awareness of 

limitations is critical

• What gaps in existing methods are holding the industry back?

• Not everyone is identifying extreme/ultimate limit states through FMEA (risk 

assessments) – Living document.

• Numerical/experimental methods may still result in high uncertainty

• Scaling issues

• Appropriate representation of all coupled systems

• Representation of sea states in physical modeling

• Limitation of test basins/measurement instrumentation

• Limitation of numerical methods to represent accidental/survival/temporary 

conditions

• Validation of numerical models

• Not everyone has well-defined test plans or follow them

• What research could help address these gaps?

• Training in risk-based design approaches

• E.g. example of detailed FMECAs (applied to reference model or publically 

known design)
34

• Ensure all prototypes are instrumented to capture detailed environmental condition 

and load data

• Including full-scale deployments

• Role of test centers and leasing agency (BOEM)

• Giving access to resources in modeling and testing (e.g. access to expertise/computing 

resources/basins) – MARINET model

• But need to consider sharing of data

• Joint Industry Projects (JIPs) and sharing of data

• Under what conditions do WECs experience extreme loads?

• Depends on each individual WEC

• Potential for some similarities/trends (particularly among similar WEC types)

• E.g. connection points, end stops, resonance, system memory or time 

accumulated effects, breaking wave impacts, high accelerations, steep high 

frequency waves, tow out, installation, directionality issues, superposition of 

different environmental conditions, slap/slam, green water. 

• Potential to learn from other industries (e.g. moorings loads) 

• What type of numerical and experimental methods can be used to model devices under the 

different type of extreme loading conditions?

• Experimental: See other Experimental group

• Numerical: See other Numerical group
35

Appendix D Post-Workshop Questionnaire Responses

Following the workshop, attendees were encouraged to fill out the online questionnaire presented 

in the Figure D-1. Table D-1 presents the attendees responses to the questions. Note that the 

responses are anonymized. 

Figure D-1. Post-workshop questionnaire that attendees were asked to complete.

 
36

Table D-1. Response to the post-workshop questionnaire.

 Please summarize what you feel where the major conclusions of 

the workshop.

Response 1 There is still a lot too be done, and technology developers still need a 

lot of help!

Response 2

- Extreme WEC conditions not confined to extreme wave conditions

- Low fidelity numerical models (e.g. linear frequency and semi nonlinear time domain models) often incapable of representing extreme 

conditions

- High fidelity numerical models (CFD) are often as costly as tank 

testing. Furthermore, it is likely that they will need to be verified against 

model measured in tank.

- Numerical modelling offers more complete simulations as it is not 

subject to instrumentation limitations (pressure distributions, PTO 

simulation).

Numerical and physical models should be developed in a 

complementary fashion.

Identification of extreme wave conditions from MetOcean data by 

extrapolation of probability distribution (Weibull) may underestimate 

extreme conditions.

Response 3

* standardization of design for survival of WECs is difficult as there is 

such a wide variety of devices

* modeling tools, appropriate for extreme condtions / response, that 

balance accuracy and speed/cost, are not necessarily available. a 

hybrid approach where mid and high fidelity tools are used may be 

necessary

* the data/tools necessary for accurate, site specific extreme conditions 

estimations are not necessarily available

Response 4

- There is considerable amount of work to be done in this field.

- Experience and knowledge can be imported from other ocean 

engineering fields yet must be tailored for the wave energy industry.

- Additional measurements and measurement capabilities to determine 

the true distribution of extreme wave events are required.

- There are numerical tools at the disposal of researchers, yet a detailed 

description and independent analysis of each is vital to efficient use to 

time and resources.
37

Response 5

- Extreme behaviour/conditions are technology dependent. Need to be 

identified by developer first.

- End stops are critical regarding survivability of WECs. Should be 

avoided whenever possible. End stops may be showstoppers actually.

- US MARINET like project sounds like a great idea, however issues 

identified with the EU MARINET needs to be adressed.

- CFD is not a pratical option for WEC developers. CFD needs experts 

for correct use. Developers do not have the required resources.

What breakout session did you attend and what were the major 

conclusions from your session?

Response 1

Physical Model Testing:

Need to create a system in the US similar to the mariner project in 

europe to help small companies develop their ideas technically better 

before going to the prototype stage. Additionally to lab time, giving 

those companies support time from expert engineering companies on 

modeling would deb a good thing.

Response 2

Physical Model Testing:

-A Marinet style framework in the US to assist developers in the 

successful implementation of tank test campaigns.

- While standards and recommended practises exist for tank testing 

WECs, they have not been adopted. This should be encouraged.

 

PTO simulation is very difficult to implement in physical model tests. A 

generic PTO simulator would be ideal. However, given the wide range 

of WEC configurations, A truly generic solution may not be feasible.

Response 3

Numerical Modeling: 

* modeling tools, appropriate for extreme condtions / response, that 

balance accuracy and speed/cost, are not necessarily available. a 

hybrid approach where mid and high fidelity tools are used may be 

necessary

* CFD tools may be used judiciously to support a robust modeling 

program

Response 4 Sea State Characterization: Reported in the break-out group notes.

Response 5

Numerical Modeling, Physical Model Testing:

- CFD is not a pratical option for WEC developers. CFD needs experts 

for correct use. Developers do not have the required resources (people 

and computational resources)

- US MARINET like project sounds like a great idea, however issues 

identified with the EU MARINET needs to be adressed.

Response 6 Sea State Characterization
38

What research and/or resources are most needed by the WEC 

industry in the area of extreme conditions design and modeling?

Response 1 Funding, and dissemination of know how.

Response 2

1. Developer access to tank facilities

2. Assistance in the development of physical models (instrumentation, 

PTO simulation)

3. Reduction in cost of high fidelity numerical modelling tools ... or 

improvement in fidelity of low cost numerical modelling tools!

Response 3

* evaluation and bench marking of available numerical modeling 

packages for a variety of WEC types in extreme conditions / response

* methodology for identifying conditions (and instances) resulting in 

potential failure

Response 4

- Better ability to determine the true distribution of extreme wave events 

for each geographic locations.

- A quantitative understanding of the risks WEC developers are willing 

to undertake in order to minimize the costs of designing for 

100/500/1000 year events.

Response 5 - Access to experimental facilities and numerical resources.

- Access to expertise to make good use of these resources.

Do you have any suggestions on how to improve future workshops 

of this type?

Response 1 Well let see if the workshop has any impact on DOE, if it does, we ought 

to have more.

Response 2

This workshop (like many others in this field) was dominated by 

representatives from research institutions, with only three 

representatives from WEC developers (Pelamis, Columbia Power, and 

RME). 

In addressing the conclusions drawn by this workshop (particularly the 

development of a Marinet style project) success will require a more 

balanced approach between those on the supply side (research 

institutions, tank facilities) and those on the demand side (developers).

Having said this, it seems that the three developers in attendance were 

given precedence as they were each invited to give presentations at the 

start of the workshop. Perhaps the poor representation of developers 

was due to unwillingness on their part to engage?

Response 3 a little more time, and a little more focus (ie reining in of wandering 

conversations)
39

Response 4

- Excellent conference in general.

- An additional workshop to disseminate the findings from each group. 

There was considerable need to understand the events taking place in 

other groups in order to determine the best path forward to your group. 

All "break-out" groups are connected and information flow is important.

Response 5

It was all very good. Maybe it would have been better to engage more 

WEC developers, or to survey what they believe are their issues prior to 

the workshop.

How valuable did you find the workshop?

Response 1 Very valuable

Response 2 Valuable

Response 3 Valuable

Response 4 Very valuable

Response 5 Very valuable

Response 6 Valuable

Other comments on workshop value and/or success.

Response 1

Good to bring people from the various part of the industry, and 

discipline together, only something good can come out of that..

I feel that some of the recommendations to DOE that cam out of the 

conclusions are very good and hope they can be implemented

Response 2

The format of this workshop made for very productive discussion as 

there was a good mix of presentation and discussion. Workshops 

dominated by presentations often risk isolating the majority of the 

participants, while those dominated by discussion may descend into 

long unstructured rambling! This workshop's mix of these two elements 

made for a very efficient format.

Response 3 it was a huge topic, perhaps one additional day would have been 

beneficial