Fatigue Mitigation Design of Deepwater Steel Catenary Risers

Abstract

With the development of deepwater offshore industries, Steel Catenary Riser (SCR) concept offers great advantages over others and has been widely deployed worldwide. More than 100 SCRs have been installed in the deepwater fields including West of Africa, Gulf of Mexico, and Offshore Brazil. Fatigue is still the number one governing design criterion of the deepwater SCR regardless the floater types (SPAR, TLP, SEMI, and FPSO). Typical SCR fatigue damages include vortex induced vibration (VIV), host floater motion induced, floater vortex induced motion (VIM) fatigue, floater vertical motion induced VIV, installation, and other kinds of dynamic loading imposed to SCR. The success of the SCR design depends primarily upon the fatigue design and its mitigation methods. This paper summaries the viable fatigue mitigation design for deepwater SCR. Detailed discussion is given to different kind of fatigue damages of SCR. Focus of fatigue design of each type of deepwater floater is pointed out. Detailed discussion is given to the selection of design methods, SN curves, stress concentration factors, influence of sweet and sour service, and post-processing of fatigue sensitive welds. Different potential and practical fatigue design mitigations have been evaluated and discussion. The most viable options have been pointed out. Some examples based on actual projects experience are given to demonstrate the design principles and fatigue mitigation methods.