Fatigue Design of the Atlantis Export SCRS

Abstract

The Atlantis semi-submersible platform is located in 7040 feet water depth in southern Green Canyon, in the Gulf of Mexico. It supports the Mardi Gras 24-inch oil and 16-inch gas export steel catenary risers (SCRs), the deepest and largest diameter SCRs in the world. Fatigue performance was one of the critical and challenging aspects of the design due to the severe wave climate coupled with the large vessel motions and strong bottom currents below the Sigsbee Escarpment. Preliminary design showed that the fatigue life at the girth welds in the touch down point (TDP) region did not satisfy the design criteria. This paper presents the fatigue design measures that were adopted to improve the fatigue performance of the Atlantis export SCRs. These include the first-time use of manually relocating the host platform to reduce fatigue damage accumulation in the TDP region, and removing the weld caps over the most critical part of the TDP region and grind flush the caps and roots of the welds in the flexible joint assembly to achieve practical defect acceptance criteria. It is demonstrated that the narrow fatigue damage peak in the TDP region can be decreased significantly by spreading it over a wider region, and all the design requirements are satisfied.