Novel Fixed Tension Mooring Theory and Application for Floating Offshore Wind Turbines in Shallow Water

Abstract

Abstract Fundamentally, existing mooring systems are based on the conventional theory of "Length-Fixed Tension-Varying" (LFTV), i.e., the physical length of the mooring line between the mooring fairlead on the floater and the anchor on the seabed is fixed and the mooring line tension varies as the floater moves. For the existing LFTV mooring system, the line tension is highly dynamic. It can increase exponentially as the offset of the floater increases and the mooring line is tightened to its full physical length under extreme wind and wave loading. This causes significant challenges in mooring system design particularly for floating offshore wind turbines (FOWT) in shallow water of 50~100m depth. Another challenge is that the existing catenary mooring system typically has a large footprint on the seabed which may have negative impacts on the marine environment. In this paper, a new fixed tension mooring theory of "Length-Varying Tension-Fixed" (LVTF) is proposed to overcome the shortcomings of the existing mooring systems. Based on the new mooring theory, novel LVTF mooring configurations are developed for shallow water FOWT applications. Discussions are given on the mooring system restoring force for the novel LVTF mooring concept in comparison with existing catenary mooring. A simplified two-line system is used to illustrate the new mooring theory. Conceptual design of a three-line single shared anchor (SSA) LVTF mooring system is performed and numerical analyses are conducted for a semi-submersible platform supporting the IEA 15 MW wind turbine in shallow water. The results show that the novel ultra-small footprint SSA LVTF mooring concept is highly feasible for station-keeping of the FOWTs. It provides a highly cost-effective and environmental-friendly solution with improved safety and efficiency to the mooring challenges for shallow-water FOWTs. The next step is to conduct proof-of-concept model tests and prototype pilot tests to gain further insights and validate the novel LVTF mooring system conceptual design.