Numerical modelling of rock anchor uplift capacity for offshore applications

Mooring and anchoring represent a significant part of the cost of wave energy converter (WEC) systems. The most common offshore embedded anchor solutions are inapplicable to rocky seabeds, which are likely in zones of strong currents/waves of interest for WECs. A new type of anchor was recently proposed for hard seabeds. It is composed a self-drilling head, which leads the anchor shaft into the rocky seabed. The anchor is then mechanically locked into the rock by applying a pre-tension. This work investigates the rock failure mechanism around the anchor, while subjected to uplift (axial) loading, and for which few models exist. Limit analysis was undertaken to calculate the failure load of the anchor in different configurations (3 rock types, varying depths and anchor geometries). The results indicate that the anchor capacity increases with depth until a certain limit is reached, corresponding to the creation of a local failure mechanism around the anchor, while a wedge failure type takes place at shallower depth. The underreamed contact area must be carefully controlled to maximise the uplift capacity related to the local failure mechanism.