The effects of second-order hydrodynamics on a semisubmersible floating offshore wind turbine

I Bayati,J Jonkman,A Platt,A Robertson

doi:10.1088/1742-6596/524/1/012094

Abstract

The objective of this paper is to assess the second-order hydrodynamic effects on a semisubmersible floating offshore wind turbine. Second-order hydrodynamics induce loads and motions at the sum- and difference-frequencies of the incident waves. These effects have often been ignored in offshore wind analysis, under the assumption that they are significantly smaller than first-order effects. The sum- and difference-frequency loads can, however, excite eigenfrequencies of a floating system, leading to large oscillations that strain the mooring system or vibrations that cause fatigue damage to the structure. Observations of supposed second-order responses in wave-tank tests performed by the DeepCwind consortium at the Maritime Research Institute Netherlands (MARIN) offshore basin suggest that these effects might be more important than originally expected. These observations inspired interest in investigating how second-order excitation affects floating offshore wind turbines and whether second-order hydrodynamics should be included in offshore wind simulation tools like FAST. In this work, the effects of second-order hydrodynamics on a floating semisubmersible offshore wind turbine are investigated. Because FAST is currently unable to account for second-order effects, a method to assess these effects was applied in which linearized properties of the floating wind system derived from FAST (including the 6x6 mass and stiffness matrices) are used by WAMIT to solve the first- and second-order hydrodynamics problems in the frequency domain. The method was applied to the Offshore Code Comparison Collaboration Continuation OC4-DeepCwind semisubmersible platform, supporting the National Renewable Energy Laboratory's 5-MW baseline wind turbine. In this paper, the loads and response of the system caused by the second-order hydrodynamics are analysed and compared to the first-order hydrodynamic loads and induced motions in the frequency domain. Further, the second-order loads and induced response data are compared to the loads and motions induced by aerodynamic loading as solved by FAST.

Highlights

Nowadays offshore wind energy is moving to deeper waters, where floating platforms seem to be the most suitable solution
The model analyzed in this work is the Offshore Code Collaboration Continuation-DeepCwind semisubmersible and the numerical tools considered are the aero-hydro-servo-elastic computer-aided-engineering tool FAST for the turbine’s time-domain modeling and the panel code WAMIT for the frequency-domain hydrodynamics
FAST is currently capable of including only first-order hydrodynamics as computed by WAMIT, and so a novel approach was taken to analyze the effects of second-order hydrodynamics without complete coupling between the two tools

Summary

Bayati

Presented at the Science of Making Torque from Wind Technical University of Denmark, Copenhagen June 17–20, 2014. NREL is a national laboratory of the U.S Department of Energy Office of Energy Efficiency & Renewable Energy Operated by the Alliance for Sustainable Energy, LLC This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The effects of second-order hydrodynamics on a semisubmersible floating offshore wind turbine

Introduction

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