The use of catenary steel compliant riser (SCR) systems has increased as hydrocarbon production has progressively moved further offshore and into deeper waters. The issue of fatigue damage caused by cyclic interaction of the riser with the seabed has gained prominence with the widespread use and lengthening of the spans. The problem involves a number of complex non-linear processes including trench formation, non-linear soil stiffness, soil suction, and breakaway of the riser from the seafloor. The analytical framework utilized in this research considers the riser-seafloor interaction problem in terms of a pipe resting on a bed of springs, the stiffness characteristics of which are described by non-linear load-deflection (p-y) curves. The P-y model allows for first penetration and uplift, as well as re-penetration and small range motions within the bounding loop defined by extreme loading. The backbone curve is constructed from knowledge of the soil strength, the rate of strength increase with depth, trench width and two additional parameters, while three parameters are necessary for the cyclic response.
