The fatigue life of risers loaded by wave kinematics, vessel movements, and vortex-induced vibrations, is one of the critical issues when designing floating production systems comprised of a large floating structure attached to the seabed by vertical tethers or mooring lines. It is especially difficult to estimate fatigue stresses due to the interaction between the seabed/soil and the riser because of the high non-linearity of soil response. Adding to the complexity the touchdown zone where the riser contacts the soil often proves to be the critical location for the fatigue life, since the maximum bending stresses usually occur in this part of the riser. Previous studies have also shown fatigue damage to be sensitive to soil stiffness. Although linear elastic soil models provide very useful insights about pipe-soil interaction, they cannot fully describe the complex interaction problem including: trench formation, non-linear soil properties, soil suction, detachment of the pipe from the soil, pipe re-contact with the soil and degradation of soil stiffness, which are evident from full-scale experimental testing and field surveys.

The background for this study is to explore the effect of going beyond the common linear soil model, calculate the riser-soil interaction effects in more detail and identify which interaction effects are the most important for riser response.

Based on consolidating knowledge from conference publications, joint development projects and bearing capacity theory a pipe-soil interaction model accounting for the seemingly most important interaction mechanisms is developed and implemented in ABAQUS. The importance of the many interaction effects is explored through sensitivity studies. Based on the importance rating recommendations are given on what level of model sophistication to choose for different applications e.g. fatigue analysis.

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