Fatigue study of unbonded flexible risers for ocean thermal energy conversion based on the joint distribution of wind, wave, and current

• Used copula to model joint probability of wind, wave, and current, improving load assessments. • Introduced a grid-based method reducing computational loads while maintaining high confidence. • Proposed a calculation new method for designing fatigue life of OTEC risers. • Combining grid-based method with copula method can reduce the time required for fatigue analysis. This paper investigates flexible risers attached to the base of a semi-submersible platform. Using meteorological and oceanographic data, the wind-wave-current boundary distribution was fitted, and the c-vine copula method established the joint probability density function of wind, wave, and current. An improved grid-based method was used to account for these loads, selecting conditions that meet the confidence level by calculating occurrence probabilities. Time-domain analysis yielded the flexible riser stress time history, and the Miner linear damage accumulation method calculated damage at critical points. The overall lifespan pattern of the riser was analyzed with the joint probability distribution. Results indicate that large heave response amplitude and higher current speeds significantly reduce the riser's fatigue life. The improved grid-based method effectively reduces computational load, providing a feasible approach for designing the fatigue life of ocean thermal energy conversion risers under wind, wave, and current loads.