PSI - Issue 57

Baran Yeter et al. / Procedia Structural Integrity 57 (2024) 133–143 Baran Yeter & Feargal Brennan/ Structural Integrity Procedia 00 (2023) 000 – 000

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The representative long-term structural response is obtained by weighing the short-term response with the joint probability of occurrence of each considered sea state (load scenario). Subsequently, the nominal stresses are combined with the stress concentration factor to derive hotspot stresses, which are then counted as a set of constant amplitude hotspot stresses by the rainflow counting (RFC) method. The next step is the structural integrity assessment which simulates a cycle-by-cycle FCG considering the overload induced retardation for hotspot stress time history established by the RFC method. Finally, the Monte Carlo simulation generates a new time history for each trial using the established long-term distribution of stress ranges. 3. Time-domain Load and Structural Response Analyses 3.1. Reference Structure and Critical Structural Detail In the present study, the “ UMaine VolturnUS-S ” semisubmersible platform is taken as the reference floating support structure developed to support the 15MW wind turbine developed by IEA (Allen et al., 2020). Further, the welded-tubular connection at the tower base can be considered one of the most fatigue-critical structural details since the predominant wind-induced loading is distributed to the side columns through these tubular connections. Thus, the structural integrity assessment is carried out for this critical welded-tubular joint. Fig. 2 shows the 15MW “UMaine VolturnUS- S” FOWT and the offshore site planned to install the FOWTs. Table 1 shows the general system properties as well as the geometrical configuration of the analysed structural detail.

Fig. 2. Studied FOWT with the fatigue-critical structural detail (left) and Scottish Sectoral Marine Plan - NE8 offshore wind farm (right)

Table 1. General System Properties of 15MW with “UMaine VolturnUS - S” semisubmersible platform (Allen et al., 2020) Parameter Units Value Wind turbine type - IEA-15-240-RWT Turbine Rating MW 15 Hub Height m 150 Rated Power m/s 10.6 Freeboard (Tower base location) m 15 Chord – Thickness (at tower base) mm 80 Chord – Outer Diameter (at tower base) m 10 Brace – Thickness (at tower base) mm 32 Brace – Outer Diameter (at tower base) m 3