Failure Envelopes of Helical Anchors in Sand

Helical anchors have emerged as an attractive alternative supporting offshore floating structures due to their low noise installation, high pullout capacity, and potential reusability. This paper investigates large-diameter (Dh=1–2 m) helical anchors subjected to inclined pullout at the mooring line using a critical-state-compatible bounding surface sand plasticity model (Sanisand04) within a three-dimensional finite-element (3D-FE) framework. With careful numerical implementation of the sand model, validation studies were performed with comparisons with theoretical solutions from the literature. A range of sand densities of practical interests were explored while documenting the sand state and displacements surrounding the anchor under complex V−H (vertical-horizontal) loading conditions. The model successfully captures the transition from shallow to deep failure mechanisms and provides visual evidence of the sand mobilized zone under varying mooring load angles over the range of sand densities studied. The affected zone decreases with increasing embedment depth and increases with higher relative density. A stepwise calculation framework is proposed for routine assessment of the V−H failure envelope for a single-helix anchor. An existing anchor pullout model for shallow embedment is systematically extended to deep helical anchors, allowing quick estimation of the vertical pullout capacity.