Innovative Fatigue Design Improvement for Mooring Systems - Floating Offshore Wind Turbine Application

Abstract

Abstract Design of mooring systems for floating offshore wind turbine (FOWT) applications could greatly benefit from mooring system technology and knowledge that has evolved from the offshore oil and gas industry. Extensive studies in the offshore oil and gas industry have identified fatigue as one of the main causes of mooring failures. Based on this finding, there is certainly no reason to ignore this fatigue concern in FOWT industry. On the contrary, mooring systems in current FOWT industry are likely to be more vulnerable to fatigue due to following key reasons: Alongside the dynamic loading sources experienced by a typical oil and gas floating platform, a FOWT platform is also excited by additional major fatigue loadings attributed by the wind turbine operation. The desire for cutting down capital costs is forcing developers to further reduce the number of mooring lines and the size of mooring components. The light displacement FOWT platform moored in relatively shallow water with low to moderate pre-tension is exposed to slack in combination with snap loading of the mooring lines. This paper demonstrates how an innovative mooring component solution, i.e., polymer spring, can mitigate the fatigue issues mentioned above and improve the fatigue life of the mooring system for FOWT application. First, an overview of an ideal fatigue analysis procedure for FOWT is provided and why such a procedure is still not feasible or not economical based on the current computational capacity is explained. Then, a suitable fatigue analysis procedure is outlined utilizing the current state-of-the-art of analysis tools and hardware. This fatigue analysis procedure is based on first principal approach of the time-domain fully coupled dynamic analysis for a moored FOWT. It is tailored to enable the selection of the optimum parameters for the polymer spring with respect to reducing fatigue damages of the mooring lines. Using this fatigue analysis tool, the benefits of polymer spring mooring component in reducing the fatigue damages for FOWT mooring system are demonstrated. The following case studies are performed covering different floaters, turbine sizes, and mooring systems: Hywind Demo-2.3MW Spar with the as-installed mooring system. VolturnUS-15MW Semi-submersible with taut polyester mooring system. For each case study listed above, extensive fatigue analyses are performed for the base case mooring configuration, i.e., without the polymer spring, and the alternative configuration using the polymer spring components determined through the analysis procedure. The results in fatigue damages and fatigue life between each two configurations are then compared. Cost benefit assessment of utilizing polymer spring as mooring components is performed by comparing it with other feasible base mooring configuration without polymer spring. This cost assessment includes capex, i.e., components cost of the mooring line, as well as cost related to transport and installation. At the end of the paper, conclusions from the fatigue assessment results are drawn and some key benefits of polymer spring for FOWT mooring systems including the cost impact are highlighted.