Influence of low-frequency vessel motions on the fatigue response of steel catenary risers at the touchdown point

Abstract

Fatigue design of a steel catenary riser (SCR) at the touchdown point (TDP) is a challenging problem. Many previous studies on this topic considered only the effect of wave-frequency (WF) vessel motions; the low-frequency (LF) motions are often neglected due to excessive computational costs. The LF vessel motion shifts the TDP, thus spreading the damage along the riser, but it can also increase the fatigue damage due to the bimodal stress response. Moreover, it may affect the trench depth and profile, and influence the WF dynamics. This paper investigates the above phenomenon to provide a better understanding. To avoid the high cost associated with simulating the combined WF and LF responses, this paper proposes a simple and efficient strategy for incorporating the LF motions. The approach is found to be satisfactory in accuracy, based on case studies that consider linear/non-linear soil models, as well as flat seabed and trench profiles.