Loading History Effects for Deep-Water S-Lay of Pipelines

Abstract

For economic reasons S-Lay is often preferred to J-Lay. However in very deep water S-Lay requires a high curvature of the stinger to achieve the required close-to-vertical departure angle (or a large, low curvature stinger). Choosing the high curvature stinger can lead to plastic deformations of the pipe. The high top tension increases the plastic deformations in two ways: firstly it adds an overall tensile component to the strains, thereby increasing the strains at the 12 o’clock position. Secondly, it increases the strain concentrations, which arise due to discontinuous support of the pipe on the stinger. Typically, the pipe is guided over a series of roller beds. The high top tension tends to straighten the spans between the rollerbeds. To accommodate this (so that the pipe can still follow the stinger), higher curvatures occur at the roller beds. Analytical and numerical solutions are provided to quantify this effect. The analytical solution is fully developed for an arbitrary pipe material models, provided that: (i) the moment-curvature relation for the pipe under tension is known, and (ii) no cyclic plastic ratchetting occurs due to repeated bending of the pipe over the roller beds and straightening in the spans between roller beds. Agreement between the analytical and numerical (finite element) results is excellent. Proper loading history must be used in the numerical simulation, otherwise the level of strain concentration can be overpredicted.