Master thesis : Deepwater Mooring Analysis for a 15 MW Seme-submersible FOWT located at the Morro Bay Wind Energy Area, California

Abstract

Mooring cables are crucial components of Floating Offshore Wind Turbine (FOWT) platforms. This study aims to analyze the mechanical behavior of a mooring system that is designed for a state-of-the-art floating offshore wind turbine structure that will operate in a deep-sea location (> 1000 m). To achieve this, the loading demands have been characterized by the environmental conditions of a specific region on the coast of central California (Morro Bay wind energy area). Afterward, these loading demands have been used to examine the long (fatigue) and short-term responses of the mooring system through high-fidelity coupled fluid-structure computational simulations followed by vortex-induced vibrations and multi-objective optimization studies. The proposed study begins with a brief presentation of the environmental, structural, and theoretical aspects of the mooring analysis. It is followed by an appraisal of various commercial and open-source simulation tools. Benchmark problem design and simulations have been carried out with a few software for different parts of the study. Firstly, a basic design demonstration has been made to cover the key concepts. A 5 MW spar type FOWT (Hywind) has been used as a reference. Uncoupled time domain analyses have been conducted in AQWA (ANSYS). Afterward, a preliminary mooring system has been designed for the state-of-the-art 15 MW semi-submersible (UMaine VolturnUS-S reference platform) floating offshore wind turbine. Then, coupled dynamic time domain simulations have been carried out in OrcaFlex (Orcina). Guidelines of the American Bureau of Shipping (ABS) and American Petroleum Institute (API) have been followed for the design and analysis of the mooring system. Pre-processing, processing, post-processing, and automation of these simulations have been explained. Results for the initial mooring design showed that the lines were over-conservative for the short-term damages, however, fatigue simulations showed that the upwind mooring line cannot reach the design life. Moreover, a 2D computational fluid dynamics (CFD) study has been conducted in Lily Pad software (Weymouth) to present the vortex-shedding characteristics for the designed mooring system in the ocean current. This part of the study aimed to highlight the vortex-induced phenomenon, which contributes to fatigue damage in the mooring systems. Finally, a multi-objective optimization study has been made with OrcaFlex and modeFRONTIER (ESTECO SpA) coupling via Python scripts. The purpose of this part of the study was to increase the fatigue life of the upwind mooring line. Initial solution sets presented high platform offsets; however, several improvements have been proposed for complete optimization.