The development of renewable energy through wind turbine systems at sea induces new engineering challenges. Mooring systems for Floating Offshore Wind Turbines (FOWT) are one of the most critical issues for the design process of such units. Over the past decades, the offshore industry has created and developed innovative and cost-effective solutions for the mooring of Oil and Gas (O&G) units such as FPSOs, semi-submersible platforms or TLPs. However the specificities of FOWT need to be carefully considered when trying to adapt the actual state-of-the-art to these new types of units.
This paper aims to highlight some of the main differences between mooring systems of FOWTs and O&G units. Comparison studies have been performed between several standards and rules in order to emphasize key points in the design process. Specific constraints related to FOWT implantation and in-service operations have also been analyzed. Relevant key points, such as redundancy of mooring lines and extreme analysis assumptions, have been selected and specifically studied, in order to give recommendations to the designer.
The redundancy of the mooring components is a recognized requirement in the O&G offshore industry. However, for small units, such as FOWT, with a limited number of mooring lines, this requirement may become too restrictive. This paper proposes alternative solutions.
Some of the assumptions usually made to perform fatigue and extreme mooring analysis for large unit such as FPSOs may not be valid for smaller units as FOWTs. This paper studies the compliance with different usual assumptions, such as the line dynamic influence. It also proposes an alternative method for extreme tension calculations.
The main objective of this paper is to propose guidelines to the industry for the design of mooring systems of FOWTs according to the results of these analyses, through the Bureau Veritas NI572 "Classification and Certification of Floating Offshore Wind Turbines". Even though the knowledge coming from the O&G industry is an important background, this paper tends to show that many points of the design may require specific attention.