Load capacity of caisson anchors exposed to seabed trenching

Abstract

Floating structures are often secured in position with a taut mooring system and suction caissons. Large seabed trenches have been observed adjacent to some suction caisson anchors with taut-line mooring systems. The trenches may jeopardise the geotechnical capacity of the caissons and in turn the stationkeeping of the floating structures. Finite-element method is employed to examine the geotechnical capacity of suction caissons in a trenching seabed. The results show that the reduction in the geotechnical capacity becomes more significant with increasing trench width due to the loss of soil support and a change in failure mechanism as the caisson rotates into the trench. For a given trench width, the reduction in capacity becomes more significant as the load inclination angle to the horizontal decreases. However, the shape of the normalised failure envelopes for combined vertical and horizontal load is insensitive to trench width. A strategy to design for inevitable trenching by moving the padeye shallower to reduce the depth of trench formation is not straightforward. The gain from a shallower trench may often be outweighed by the reduction in capacity from rotation of the caisson at failure for loading angles typical of taut moorings.