Mooring System Optimization for Floating Wind Turbines using Frequency Domain Analysis

TLDR

The study introduces a new tool to optimize catenary mooring systems for floating wind turbines on semi‑submersible platforms. The tool uses frequency‑domain analysis to compute the platform’s linear response, derives mean mooring stiffness from wind and current forces, and applies spectral loading matrices for stochastic wind, swell, and turbulent wind to produce a site‑specific mooring design that accounts for load directionality. The tool enables rapid evaluation of many scenarios, proving useful for preliminary design, and the comparison of two North Sea sites demonstrates that spectral wind loads must be considered in mooring system design.

Abstract

This paper describes a new tool for the optimization of catenary mooring systems for floating wind turbines with a semi-submersible support structure. Each source of environmental loading -wind, current and waves -is spread over a range of compass directions, based upon probability distributions determined by meteorological and oceanographic measurements. Frequency domain analysis is applied to determine the linear response of the platform. Mean forces due to wind and current are determined to find the mean position of the platform and thereby the linearized mooring stiffness matrix. Subsequently, the stochastic loads from wind sea, ocean swell and turbulent wind are applied in the form of spectral loading matrices. The resulting mooring system design is site-specific, accounting for the directionality of environmental loads. A large number of different scenarios can be evaluated quickly and approximately. Hence the tool is useful for preliminary design of floating wind turbines.As an example application, a symmetric semi-submersible design with three columns is analyzed. Results for two sites in the North Sea, with different water depths and distinct directionality of environmental loads, are compared and indicate that spectral wind loads should be considered when designing mooring systems for floating wind turbines.