Improved empirical formulation for seabed trench and its influence on fatigue performance of steel catenary risers under directional waves and currents.

Steel catenary riser (SCR) offers a cost-effective solution to deep water deployments. Hanging from a floater, an SCR is usually subjected to large tensions at hang-off location and large bending moments at touchdown zone (TDZ) which leads to fatigue damage. Field observations showed that a trench gradually formed in seabed with the depth might reach to five times of SCR's diameter. However, flat seabed was often assumed for the modeling of SCRs to avoid the time-consumingly dynamic simulation of trench development. Few studies have been conducted to evaluate the influence of trench on SCR fatigue performance, and led to contradictory conclusions, i.e., some studies suggested that fatigue damages of SCRs at TDZ were reduced, while, others proved that they were increased, if considering the presence of trench. This contradiction may be explained by factors including inappropriate trench model and different sea states assumed by different researchers. An iterative procedure initially developed by Wang and Low and further improved in the present work was used to estimate the position and the length of a trench and, then, an improved empirical formulation was generated to predefine the trench model for the finite element analysis of SCR. Additionally, dynamic simulations were conducted to study the influence of trench on fatigue performance of SCRs encountering directional waves, currents and floater offset. Results showed that presence of trench always reduced fatigue damages of SCR at TDZ compared with the flat seabed. [ABSTRACT FROM AUTHOR]