Simulating Extreme Directional Wave Conditions

Samuel Draycott,David Ingram,Thomas Davey

doi:10.3390/en10111731

Abstract

Wave tank tests often involve simulating extreme wave conditions as they enable the maximum expected loads to be inferred: a vital parameter for structural design. The definition, and simulation of, extreme conditions are often fairly simplistic, which can result in conditions and associated loads that are not representative of those that would be observed at the deployment location. Here we present a method of defining, simulating at scale, and validating realistic site-specific extreme wave conditions for survival testing of wave energy converters. Bivariate inverse-first order reliability method (I-FORM) environmental contours define extreme pairs of significant wave height and energy period ( H m 0 – T E ), while observed extreme conditions are used to define realistic frequency and directional distributions. These sea states are scaled, simulated and validated at the FloWave Ocean Energy Research Facility to demonstrate that the site-specific extreme wave conditions can be re-created with accuracy. The presented approach enables greater realism to be incorporated into tank testing with survival sea states. The techniques outlined and explored here can provide further and more realistic insight into the response of offshore structures and devices, and can help make important design decisions prior to full-scale deployment.

Highlights

Offshore structures and devices, including wave energy converters (WECs), are tested in wave tanks to assess behaviour, performance and loads at scale prior to deployment at sea
It is paramount to identify the expected extreme conditions, which often initially focusses on the identification of extreme significant wave heights
Due to slight under-generation, the incident wave spectrum is corrected using frequency dependent amplitude based correction factors as defined in Equation (14). This is repeated until mean spectral errors, e (Equation (15)), are below 5%, which happens for all six sea states after a single iteration

Summary

Introduction

Offshore structures and devices, including wave energy converters (WECs), are tested in wave tanks to assess behaviour, performance and loads at scale prior to deployment at sea. It is paramount to identify the expected extreme conditions, which often initially focusses on the identification of extreme significant wave heights. This is achieved using extreme value distributions to extrapolate the observed data to the desired return period and is suggested by Goda [1] to be the first step in coastal structure design. To define and simulate sea states, appropriate wave period values are required These can be obtained using joint probability methods such as the inverse-first order reliability method (I-FORM) technique (implemented in Haver and Nyhus [5], Winterstein et al [6], Berg [7]) recommended by the EquiMar

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Discussion

Conclusion