Assessing the impact of riser-soil interaction model on the fatigue life of large diameter SCRs

This paper investigates the effect of riser-soil interaction model selection on the assessment of steel catenary riser (SCR) fatigue life for realistic environmental conditions at a deep-water Australian North west shelf (NWS) site. Using a fatigue wave scatter diagram consisting of 100 metocean conditions at the site (combining irregular seas, swell and current), a dynamic timedomain finite element analysis is coupled with the rain-flow cycle counting algorithm in order to determine the fatigue life of SCRs due to first-order motions of the host floater. Rigid, linear elastic and nonlinear riser-soil interaction models are used in order to assess the impact of model selection on the fatigue life of example deep-water SCRs of varying diameter. It is shown that the use of a nonlinear riser-soil interaction model for a representative deep-water NWS site can give an almost two-fold increase in SCR fatigue life over a stiff linear seabed assumption, albeit at a significant computational expense. It is further shown that a recently developed method for calculating equivalent linear soil stiffness may be used in place of the computationally expensive nonlinear approach in order to estimate SCR fatigue life to a reasonable level of accuracy. A methodology for applying the equivalent linear stiffness method to irregular sea-states is proposed and general insight into the selection of the most appropriate soil-riser interaction model for the fatigue analysis of large diameter deep-water SCRs is provided.