Differential equations for dynamic response of deep-ocean steel in the touchdown area

Steel catenary risers (SCR) are amongst the most attractive elements in development of offshore oil and gas fields. The fatigue life assessment of SCRs in the touchdown zone (TDZ) is a challenging design issue due to contribution of several non-linear and interactive mechanisms in cyclic soil stiffness degradation, the formation of a trench under the SCR, and consequently the variation of the SCR catenary profile. In this study, different mathematical models and their impact on riser dynamic performance were reviewed. A boundary layer method (BLM) was developed to predict the effect of touchdown point relocation on ultimate fatigue damage within different conditions via developing user defined elements (UEL) and DISP subroutine implemented into ABAQUS for finite element analysis. Mathematical and numerical approaches showed that the touchdown point (TDP) oscillation amplitude could be potentially considered as an index to approximate the cyclic cross-sectional stress range and consequently the ultimate fatigue damage in SCR