Optimisation of Screw Anchor Lateral Capacity in Sand for Offshore Renewable Energy Applications-Paper

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The paper presents a design-optimisation method for screw anchors used to moor floating offshore renewable-energy devices in dense sand. Screw anchors can be installed with relatively little noise, but their lateral capacity and practical design are constrained by both soil behaviour and installation and structural requirements.<br /><br />The method combines simplified analytical and semi-analytical models to assess installation torque and vertical force, core strength and buckling, helix bending, and lateral capacity. It identifies the maximum embedment depth achievable for a given anchor geometry and available installation torque. Lateral resistance is evaluated using short-pile and long-pile failure mechanisms; capacity increases with depth until the transition between these mechanisms limits further benefit.<br /><br />For dense HST95 sand, the authors vary core diameter, wall thickness and maximum installation torque to identify designs that maximise lateral capacity while minimising anchor weight. In the example with a 5 MNm torque limit, an anchor reaches a maximum depth of 6.1 m and a lateral capacity of 1.53 MN. Increasing torque generally enables larger, stronger anchors and greater capacity, although the benefit diminishes at higher torque levels. The governing limits can shift from short-to-long-pile transition to insufficient installation torque or inadequate core strength.<br /><br />The study provides design envelopes and a chart linking optimal capacity and anchor weight to available installation torque. It demonstrates that the thickest or largest anchor is not necessarily the most material-efficient choice. The framework is intended as a preliminary design tool: it excludes safety factors, cyclic loading and interaction between vertical and lateral loads, and relies on simplified models that may require refinement for other soil conditions and installation procedures.

Keywords

screw anchors

offshore renewable energy

dense sand

design optimisation

installation torque

lateral capacity

embedment depth

helix bending

core strength

mooring systems