Session: Mooring

Paper Number: 165135

165135 - On the Mooring Loads of a Novel, Light-Weight Semi-Submersible FOWT Platform 

Abstract: 

This research investigates the mooring loads on a novel, light-weight semi-submersible type floating offshore wind turbine (FOWT) platform. The design of the platform and motion RAOs were presented and published at the OMAE 2024 conference [1]. It was found that the platform utilises 15% less steel and achieves a 30% reduction in displacement compared to the NREL DeepC 5 MW semi-submersible platform [2].

The core of this research addresses the challenges associated with the selection of mooring lines for platforms characterised by light displacement.  This is specifically, due to their reduced displacement, optimizing the weight of catenary mooring lines becomes critical, as excessive weight can significantly influence the platform’s design draft. The stiffness required for the mooring system predominantly depends on the mass of these lines and the pretension. Furthermore, lighter platforms are more prone to experiencing slack in mooring lines, consequently increasing the risk of snap loadings due to abrupt increases in tension [3]. This study calculates the design loads for the mooring lines of this lightweight platform, adhering to the ultimate limit state (ULS) design guidelines set forth by the DNV design standards. The mooring analysis incorporates both first-order and second-order mean and low-frequency hydrodynamic loads to evaluate the system comprehensively under different sea states. Additionally, the effects of rotor thrust and current drag on the mooring lines are examined. Hydrodynamic coefficients and load transfer functions were generated using WAMIT (https://www.wamit.com/) and Ansys AQWA (https://www.ansys.com/products/structures/ansys-mechanical#tab1-2), facilitating the development of coupled models for aero-hydro-servo-elastic time-domain simulation using OpenFAST, OrcaFlex and Ansys AQWA.

The mooring tension results were benchmarked against industry-standard simulation tools, OrcaFlex and Ansys Aqwa, and demonstrated strong agreement. Preliminary analyses reveal that turbine thrust accounts for over 50% of peak cable tension, while combined wind and current loads result in a doubling of line tensions compared to scenarios that only involve wave impact. The directions of wind and current substantially impact the platform’s mean surge offset, which in turn significantly elevates cable tensions under dynamic loading conditions. The full paper will present a comparative analysis of mooring response under first and second-order hydrodynamic loading.

Keywords: FOWT, Mooring Analysis, ULS, Coupled simulation, Hydrodynamic Loading, Snap Loading

[1]    T. Islam and V. Venugopal, ‘On the Design, Hydrodynamic Modelling and Response Analysis of a New Concept FOWT Platform’, in Volume 7: Ocean Renewable Energy, Singapore, Singapore: American Society of Mechanical Engineers, Jun. 2024, p. V007T09A036. doi: 10.1115/OMAE2024-126638.

[2]    A. Robertson et al., ‘Definition of the Semisubmersible Floating System for Phase II of OC4’, NREL/TP-5000-60601, 1155123, Sep. 2014. doi: 10.2172/1155123.

[3]    W. Hsu, K. P. Thiagarajan, M. Hall, M. MacNicoll, and R. Akers, ‘Snap Loads on Mooring Lines of a Floating Offshore Wind Turbine Structure’, in Volume 9A: Ocean Renewable Energy, San Francisco, California, USA: American Society of Mechanical Engineers, Jun. 2014, p. V09AT09A036. doi: 10.1115/OMAE2014-23587.

 

Presenting Author: Md Touhidul Islam The University of Edinburgh

Presenting Author Biography: Md Touhidul Islam is a PhD Candidate at the Centre for Doctoral Training (CDT) in Wind and Marine Energy Systems and Structures (WAMSS) in the University of Edinburgh (with Uni Strathclyde and Uni Oxford), UK. Before starting his PhD, he was an Assistant Professor (teaching) at the department of Naval Architecture and Marine Engineering (NAME) in Military Institute of Science and Technology (MIST), Bangladesh. Prior to that he worked as a professional Naval Architect for over ten years at home and abroad.

Authors:

Md Touhidul Islam The University of Edinburgh
Vengatesan Venugopal The University of Edinburgh