Soil-Riser-Flowline Interaction

The response of the riser touch down point (TDP) varies due to vessel    motion (surge, sway and heave), current drag and vortex induced     vibrations (VIV), subjecting the soil-riser interface to oscillatory motions      and loads. The TDP point will be moving constantly relative to the       magnitude of the motions of the riser system. The TDP movement is        restrained by the rigidity of the riser and the soil resistance developed         between the seabed soils and the riser. The soil-riser interaction          could develop in such a way that large bending occurs at the TDP           and the riser becomes significantly overstressed. Fatigue damage            can also occur due to the cyclic loads. Hence, a soil-riser and soil-             riser-flowline interaction analysis should be carried out to               investigate the effect on the riser integrity at the critical touch                down point.

                         GeoLine is capable of providing engineering advice and
                          carry out advanced finite element analysis (FEA),
                            modelling the complex soil-riser and soil-riser-
                              flowline interaction.

                                 
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In very soft soils the riser and flowline will likely become self buried, making the soil interaction with the riser and flowline complex. The constantly moving TDP will create a zone of heavy frictional fluid (soil) surrounding the riser TDP. The size of the zone depends on the magnitude of the cyclic amplitudes and soil characteristics.

Significant movement of the riser TDP will result in the riser/flowline being dragged sideways through and across the seabed, affecting the integrity of the riser and flowline. The riser is more likely to become overstressed when the TDP is buried, due to the lateral passive soil force developed, when the riser is pulled through the soil. Fatigue damage is affected by the seabed stiffness, hence likely to occur when the seabed soils are stiff.