Wave Load Mitigation by Perforation of Monopiles

Jacob Andersen,Torben Baun,Jesper Neubauer,Rune Abrahamsen,Morten Andersen,Thomas Andersen

doi:10.3390/jmse8050352

Jacob Andersen, Torben Baun + Show 4 more

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https://doi.org/10.3390/jmse8050352

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Abstract

The design of large diameter monopiles (8–10 m) at intermediate to deep waters is largely driven by the fatigue limit state and mainly due to wave loads. The scope of the present paper is to assess the mitigation of wave loads on a monopile by perforation of the shell. The perforation design consists of elliptical holes in the vicinity of the splash zone. Wave loads are estimated for both regular and irregular waves through physical model tests in a wave flume. The test matrix includes waves with Keulegan–Carpenter ( K C ) numbers in the range 0.25 to 10 and covers both fatigue and ultimate limit states. Load reductions in the order of 6%–20% are found for K C numbers above 1.5. Significantly higher load reductions are found for K C numbers less than 1.5 and thus the potential to reduce fatigue wave loads has been demonstrated.

Highlights

The capacity of offshore wind energy in Europe is increasing relatively more than the onshore [1]
Higher load reductions are found for KC numbers less than 1.5 and the potential to reduce fatigue wave loads has been demonstrated
The aim of the present paper is to examine the effect on mitigation of wave loads by perforation of monopiles

Summary

Introduction

The capacity of offshore wind energy in Europe is increasing relatively more than the onshore [1]. The relative increase in the offshore capacity over recent years may come as a result of the maturing of the offshore wind industry, where continuous optimization and development of know-how cause expenditures to drop. Levelized Cost of Energy (LCoE) for offshore wind energy has dropped more than 50% since 2013 [2]. In the offshore wind energy sector, monopiles are by far the most common foundation type. By 2018, more than 80% of the grid-connected offshore wind turbines in Europe were installed with monopiles [3]. The extensive use of monopiles has been key in driving down offshore wind LCoE. To utilize offshore wind power in the proximity of areas with challenging bathymetry or to locate wind turbines further offshore due to e.g., less visual impact or larger wind resources, wind turbines often need to be installed at locations with intermediate waters, 30 to 50 m, or even deep waters more than 50 m [3,5]

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