Influence of Water Depth Variation on the Mooring Line Design for FOWT in Shallow Waters

Abstract

This thesis presents a study about the modeling and optimization of mooring systems for floating offshore wind turbines (FOWT) for different water depths in Taiwan western offshore areas A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCwind platform and the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine Extended analysis on the materials types and configurations of the mooring line are presented in this thesis in order to select the feasible and reliable option for the given water sites The mooring system proposed consists of a catenary mooring with studless chain and a drag embedment anchor (DEA) Three nominal sizes of the mooring chain links are taken into account: diameter of 95 mm 115 mm and 135 mm With this configuration a total of five mooring designs for different water depths (i e 50 m 60 m 70 m 80 m 100 m) are analyzed according to the rules and regulations of the two certification institutions Det Norske Veritas and Germanischer Lloyd (DNV GL) and American Petroleum Institute (API) Considering ultimate limit state (ULS) fatigue limit state (FLS) and maximum operating sea state (MOSS) based on a storm with a 50-year return period and current with a 10-year return period 25-year design life as well as 1-year return period respectively long-term predictions of breaking strength fatigue and stability are performed The software OrcaFlex version 10 3d is used to simulate and design the mooring systems For the practical reasons all designs consider the same metocean conditions seabed profile soil type floating platform wind turbine anchor size and mooring material As a result of this study the shallow mooring design of 50 m water case presents heaviest chains among the other water cases driving their mooring costs On the other hand 100 m waters design has much longer mooring lines making this parameter the cost driving ones This fact leads to a minimum range is from 60 m to 80 m water depth

Date of Award	2020
Original language	English
Supervisor	Ray-Yeng Yang (Supervisor)