A Reliability-Based Assessment Framework For Drag Anchors

Drag anchors are often employed in offshore floating facility moorings. The standard drag anchor design approach is based on a deterministic Load and Resistance Factor Design (LRFD) framework that considers characteristic design ‘low’ and ‘high’ estimates of soil strength and other geotechnical parameters, combined with code-specified partial factors. A disadvantage of this approach is that the resulting anchor designs may not achieve a consistent level of reliability. This paper describes a study that addresses this limitation by developing and demonstrating a generalised drag anchor probability of failure analysis framework for inclusion in a Reliability Based Assessment (RBA) of a mooring. A feature of the study is the inclusion of consolidation and cyclic loading effects in the anchor analysis. The study highlights the benefits an RBA approach can offer to the drag anchor design process, including reduced anchor size and preloading requirements, and increased confidence in the anchor design and estimate of anchor performance. For temporarily moored facilities this approach offers the potential to exploit expanded weather windows for operations. For permanently moored floating offshore wind developments this approach may allow adoption of reduced levels of target reliability, thereby reducing costs for systems with a large number of anchors.