Numerical modelling of plate anchors under sustained load: The enhancement of capacity from consolidation

Abstract

Embedded plate anchors can be used to moor floating offshore facilities. In taut mooring systems, a sustained tension carried by the anchors affects their long-term capacity due to consolidation effects in the soil surrounding the anchor. The resulting gain in capacity provides a potential basis for more efficient anchoring system design. To quantify this effect, and to understand the underlying mechanism, a systematic numerical study has been performed, to capture the effect of sustained loading and consolidation on soil failure mechanisms around an embedded plate anchor. The modified Cam clay model is used, to capture the effect of consolidation on soil strength, and also to faithfully model the process by which a water-filled gap forms under the plate, due to a loss of contact between the plate and the soil, with seepage into the gap zone. A critical observation is the load transfer process when such a gap forms beneath the plate after the applied tension causes the effective stress to fall to zero. The formation of this gap is shown to depend, in a systematic way, on the sustained tension, or preload, the soil strength ratio, the relative sharing of the anchor load between compression above the plate and tension below, as well as the embedded depth of the anchor. Simple relationships that capture these mechanisms are provided, linking the net change in soil strength above and below the anchor to the change in undrained capacity. As an example, it is shown that in soft normally-consolidated clays, a sustained preload of 60 % of the initial capacity, can create a 25 % gain in capacity, at which point the factor of safety is raised from 1/0.6 = 1.67 to 2. This gain is only available for anchors embedded by more than twice their diameter. At shallower embedment, the loss of strength in the unloaded soil beneath the anchor outweighs the gain in strength in the smaller zone of soil above the anchor, and the consolidation process causes a net weakening.