This paper studies on the typical design load cases that dominate the characteristics of structural stress distributions. The OC4-DeepCwind conceptual semi-submersible substructure with the 5.0MW floating offshore wind turbine (FOWT) is adopted as the target structure. Parametric finite element method (FEM) is employed for idealized numerical modelling. Different environmental load cases controlled by random variable parameters such as wave directions, phases, heights and periods are imported into static probabilistic design system (PDS) as samples. Core areas with localized stress concentration based on probability statistics and corresponding typical design load cases are summarized. This study presents a method of effectively simplifying the complicated dynamic strength analysis procedures and would serve as a reference of reasonable optimization of main dimensions of the semi-submersible FOWTs.


Due to the depletion reserves and negative environmental influences of fossil fuels, human beings have been forced to seeking for alternative energy sources. According to the report of Intergovernmental Panel on Climate Change (IPCC), nearly 80 percent of the world's energy supply could be provided by renewable energy resources in 2050, and wind energy would make up one of the largest contributions to the energy system by then (Sun, Huang and Wu, 2012). Nowadays wind energy industry has moved its interest offshore. Reference shows that offshore wind power will cover 14% of European electricity demand by 2030 (Athanasia, Anne-Bénédicte, and Jacopo, 2012). In the first half of 2017, developers have totally installed about 6.1GW of capacity, including 1.3GW in Europe. The activity in the offshore market is 2.6 times higher than for the first half of 2016 (WindEurope, 2017).

Most offshore wind farms so far are installed and operating in shallow waters (<30m), where bottom-fixed foundations with simplified structure concepts such as monopile and gravity concrete caisson are widely used (Failla and Arena, 2015). At water depths between 30m and 60m, multi-foot foundations such as tripod or jacket support are considered (Lozano-Mjinguez, Kolios and Brennan, 2011). For the benefits of relatively unrestricted space, lower social impacts and rich wind resources, wind farms are pushed into deeper waters. For cost-effective solutions, floating offshore wind turbines (FOWTs) become feasible options to extract energy (Meng, Lou and Shi, 2014).

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