Fatigue Analysis Of Mooring Systems And Compartment Optimization Of Pontoon For Floating Offshore Wind Turbine Platform

The development of platform for the large-capacity floating offshore wind turbine(FOWT)is the key to advance the exploitation of offshore wind power from shallow water to deep water.However,research on such platform for the FOWT is in the early stage in China and many challenging problems remain unsolved.Particularly,the reliable methodology of design and construction of the FOWT platform is still lacking,which is one of critical challenges restricting the development of offshore wind power in China.Meanwhile,due to the high construction cost of the FOWT,there is a urgent need to develop a safe,economic,and durable platform for the large-capacity floating offshore wind turbine(FOWT).This thesis presents two crucial analyses in the FOWT platform design: the fatigue analysis of the mooring systems and the compartmentation optimization of the pontoon.In the fatigue analysis,critical factors impacting the fatigue lives of the mooring systems of the submersible FOWT were identified and investigated;in the compartmentation optimization,the optimal design of pontoon compartmentation was proposed.The main procedures and findings of the paper are summarized as follows:(1)Existing research on the FOWT platform was reviewed and the limitations and remaining challenges were analyzed and summarized,which lay the groundwork for this thesis;(2)The effects of aerodynamic load,turbulence intensity,angle between wind and waves,and number of short-term sea states on the fatigue lives of the mooring systems of the submersible FOWT were studied.The coupled analytical model of a submersible FOWT was established using the FAST and the Orcaflex.Parameters were set to corresponding values to represent different cases in time domain,and the time histories of the tension in the mooring systems were obtained.Based on Miner's linear damage rule,rain flow counting algorithm,and the T-N curves of the mooring lines,the fatigue lives of the mooring systems in different cases were calculated and compared;(3)An optimization procedure of pontoon compartmentation for the FOWT platform based on Matlab was proposed.First,the result of the compartment damage stability of the FOWT platform calculated based on Matlab code was consistent with the result obtained with the industry software SESAM,indicating that the developed code is able to correctly reflect correct the structural behavior of the platform with damaged compartments.Then,the compartmentation design was optimized to achieve the minimum number of compartments while satisfying the requirements of the compartment damage stability.It was found that this optimization procedure for the pontoon compartmentation can generate satisfactory results fulfilling the requirements of the compartment damage stability in SESAM,and can be used to simplify the preliminary design of the FOWT platform.