A Time-Domain Methodology for Nominal Stress-Based Fatigue Assessment of Semi-Submersible Floating Wind Turbine Hulls at Different Offshore Sites

This paper deals with a time-domain methodology for nominal stress-based, screening-level fatigue assessment of semi-submersible FWT hulls, using a 10-MW semi-submersible FWT as a case study. A comprehensive procedure is outlined for both short- and long-term fatigue analysis, emphasizing the influence of wind and wave loads, as well as the probability distributions of environmental conditions. A fully coupled dynamic analysis of the FWT, employing a multibody floater, is conducted to compute internal global loads and time-domain nominal stresses on the hull structure. Short-term fatigue damage is evaluated across various wind-wave directions, environmental conditions, and random wind and wave samples, identifying critical loading scenarios. For long-term assessment, 10,182 one-hour time-domain simulations are conducted across three wind-wave directions for five offshore sites in the North Sea and one site in the China Sea. Fatigue damage at different locations of the hull structure is estimated for each offshore site, with results discussed in the context of screening-level nominal fatigue assessment and identification of fatigue-critical regions. The insights gained from this study form a basis for validating simplified and computationally efficient fatigue analysis procedures in an accompanying paper. Additionally, the findings support the design optimization of hull structures. Limitations of the present study are identified, pointing to future research directions aimed at mitigating fatigue risks.