Fatigue reliability analysis of a steel catenary riser at the touchdown point incorporating soil model uncertainties

Abstract

Fatigue design of a steel catenary riser (SCR) at the touchdown point is a challenging problem. The touchdown point attracts the worst bending stresses and is subject to the greatest uncertainties, such as those arising from the riser-seabed contact. Design codes typically recommend generic safety factors on the design life, but the heightened uncertainty renders the use of such safety factors questionable. This paper addresses these uncertainties through a systematic reliability analysis. A sensitivity study is first conducted to select the most critical random variables. The soil model uncertainties are characterized by three variables representing stiffness, suction and trench. The efficient first-order reliability method (FORM) is used in conjunction with the response surface method to estimate the failure probability without considering soil uncertainties. Subsequently, an inverse-FORM (IFORM) analysis is performed to determine the alteration of the safety factor for the same level of reliability, when the soil variables are incorporated. A simple method is described to estimate the probability density of the fatigue life. This work reveals, among other things, that the soil uncertainties significantly influence the SCR fatigue reliability, and that FORM and IFORM are fairly accurate when compared against Monte Carlo simulation.