Breaking phase focused wave group loads on offshore wind turbine monopiles

A. Ghadirian,M. Dixen,H. Bredmose

doi:10.1088/1742-6596/753/9/092004

A. Ghadirian, M. Dixen + Show 1 more

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https://doi.org/10.1088/1742-6596/753/9/092004

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Abstract

The current method for calculating extreme wave loads on offshore wind turbine structures is based on engineering models for non-breaking regular waves. The present article has the aim of validating previously developed models at DTU, namely the OceanWave3D potential flow wave model and a coupled OceanWave3D-OpenFOAM solver, against measurements of focused wave group impacts on a monopile. The focused 2D and 3D wave groups are reproduced and the free surface elevation and the in-line forces are compared to the experimental results. In addition, the pressure distribution on the monopile is examined at the time of maximum force and discussed in terms of shape and magnitude. Relative pressure time series are also compared between the simulations and experiments and detailed pressure fields for a 2D and 3D impact are discussed in terms of impact type. In general a good match for free surface elevation, in-line force and wave-induced pressures is found.

Highlights

Cost-reduction for the substructures is an important part in reducing the cost of offshore wind energy
We provide further validation of the OceanWave3D and the coupled solver against new systematic measurements of focused wave group impacts on a monopile
The monopile was mounted on two force transducers — one at the top and one at the bottom — to measure the in-line force and the moment

Summary

Introduction

Cost-reduction for the substructures is an important part in reducing the cost of offshore wind energy. The current method for calculating such loads is to use the stream function theory [1] combined with a background sea state time series. This method is easy to implement and benefits from limited complexity in the parameters that should be chosen. The stream function theory is associated with assumptions such as 2D wave motion, symmetry in the crest of the waves, periodicity and a flat sea bed. This approach neglects the effect of breaking waves which is the focus of this study. These models are not still vastly used in the industry, partly because of only limited validation against design cases

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