Polymer Mooring Component for Offshore Renewable Energy

Abstract

The paper presents a cost benefit analysis of using polymer mooring components in the mooring system design of the Maine Aqua Ventus I (MAV1) floating offshore wind turbine (FOWT) project. Polymer components in mooring lines can offer new mooring system responses, which can be tailored to the needs of challenging mooring systems. For FOWT deployments these can deliver mooring system responsiveness at the thrust loads of the turbine, despite the high background mooring loads. MAV1 will deploy two 6MW floating offshore wind turbines off the coast of Maine and this paper compares the existing mooring system design against a polymer mooring component solution, undertaking dynamic analysis of the mooring systems across multiple sea states (including ultimate limit states, fatigue limit states and accidental limit states). The Aqua Ventus platform is modelled in Orcaflex, with multiple mooring system designs containing different polymer component responses modelled and contrasted. Results are analyzed and load analysis data used to undertake a cost benefit analysis. Cost reductions are shown across the mooring system (anchors, lines, connectors), as well as the platform and tower structures. Fatigue analysis is undertaken using a Rainflow analysis of the sea states to be experienced by the platform over its life, for both the existing and the polymer mooring configurations. Polymer mooring components which can be used throughout the renewable energy and offshore industries to manage mooring loads, and are capable of mooring any sized platform, in any challenging conditions. While the specific components modelled in this work are targeted at FOWT or tidal platforms with novel stress-strain response curves designed to suit the high background thrust load conditions, other component responses are available to deliver design load and fatigue reductions on existing catenary or TLP moored platforms. Components can be easily retrofitted into existing mooring lines or deployed in new lines. Using polymer mooring components can dramatically reduce the peak loads experienced by the platform. Previous work has looked at an OC4 FOWT model in hypothetic conditions, whereas the current paper presents new work related to a real US FOWT deployment. The paper demonstrates that >50% reductions in design loads are possible. Cyclic loads are also substantially reduced, resulting in >60% reduction in wave induced fatigue in extreme sea states. This results in operational and maintenance cost savings. The patented mooring components have been developed initially for aquaculture and wave energy applications and have now been scaled to the MN loads required by the FOWT and offshore industries. Components are certified to relevant standards and delivered to projects globally.