Fatigue Life Improvement of Steel Catenary Risers due to Self-Trenching at the Touchdown Point

Abstract

Abstract The fatigue problem associated with the Touchdown Point (TDP) of a SteelCatenary Riser (SCR) may be less serious than is commonly understood, particularly for "soft" seafloor sediments typical of the Gulf of Mexico. Forsuch soft sediments, a SCR will dig for itself a trench having a curvedvertical profile that minimizes the shear forces in the SCR at the TDP andconsequently reduces the fatigue damage in the vicinity of the TDP. In thispaper this trench is modeled as a rigid curved surface upon which the SCRlands. Quasi-static analysis predicts that the fatigue life of a SCR with the TDPtouching down on a rigid curved trench is much greater than that of an SCRlanding on a rigid flat seafloor. The maximum fatigue damage either occurswithin the trench past the TDP, where riser motions are small, or occurs up theriser away from the TDP, where shear forces are small. Dynamic analysesutilizing finite element computer models are less clear. One early FE analysisincorporating a rigid curved trench showed a fatigue life improvement factor ofabout 2.3, whereas other analyses were inconclusive. Therefore, it is too earlyto draw general conclusions about potential fatigue life improvements untilfurther research is done. Introduction Since 1994 the population of Steel Catenary Risers has grown from zero tomore than 60 worldwide, connecting marine pipelines and flowlines with TLPs, SPARs, Semisubmersible FPSs, Ship-Shaped FPSOs, and other fixed and floatingplatforms. Many more such risers are planned in sizes ranging from 4" to 24", and water depths ranging from about 1000 feet to more than 6000 feet. The primary design criteria for Steel Catenary Risers are the usual internaland external pressure requirements, and a requirement that the predictedminimum fatigue life of the riser must exceed the field life by a specifieddesign factor. (The usual factor of 10 is thought to realistically cover themany unknowns in material strength and oceanographic loadings.) Fatigue damageis caused by cyclic bending of the riser, induced by the combination ofplatform motions and direct wave and current loadings on the riser, includingvortex-induced vibrations. Fatigue analysis of a SCR involves counting thebending cycles that will be applied to the riser over the full range of theoceanographic loading conditions that will occur during its lifetime. Fatigue damage is concentrated in the girth welds at both the upper end ofthe SCR where the riser is connected to the platform, and at the TouchdownPoint where the SCR first touches down onto the seafloor. This paper focuses onthe conditions at the TDP.