Repeated loading and unloading of the seabed

Abstract

In many contexts, offshore systems repeatedly load and unload the seabed in the same place. A catenary riser, for example, is repeatedly lifted from the seabed and lowered back, and progressively indents the bed, to the extent that it may cut a trench 2m deep. This is vitally important to the mechanical behaviour of the riser, because the region near the touchdown point is a fatigue hotspot where the stress range is particularly high. Moreover, a deep trench constrains the riser from moving sideways, which can lead to large lateral curvatures if the floater moves off station. Similar problems occur during laybarge and reelship pipelaying, and when jackup spudcans are lifted and lowered. Our research applies a combination of centrifuge modelling, numerical modelling byABAQUS and plasticity theory. It shows that this kind of interaction with the seabed cannot usefully be modelled by soil springs and that the interaction is much more complex. Repeated loading and unloading do not lead to shakedown to a condition at which the seabed response is elastic, and hysteresis continues to absorb energy from the system, though that may have a usefully favourable effect on the system dynamics. Another option is to pave the seabed to eliminate the formation of deep trenches. © 2011 Taylor & Francis Group, London.