Trajectory prediction for drag embedment anchors under out of plane loading
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Partial failure of mooring systems for floating structures will subject drag anchors to loads having an appreciable component outside of the intended plane of loading. Partial failures of mooring systems during hurricanes in recent years have generated an interest in understanding drag anchor performance under these conditions. This paper presents a formulation for prediction of three dimensional trajectories of an anchor system subjected to an out-of-plane load component. The analysis shows that, if the self-weight of the anchor chain is neglected, the anchor chain configuration will lie always within a plane, although for general conditions of out-of-plane loading, the anchor chain will lie in an oblique plane defined by the direction of the anchor chain at the pad-eye and the mudline. Therefore, existing formulations for predicting anchor chain configuration and anchor trajectory during drag embedment for in-plane loading can readily be adapted to out-of-plane loading provided that appropriate adjustments are made to account for rotation of the coordinate frame. Illustrative examples of three-dimensional trajectories for anchor systems subjected to out-of-plane loading are presented in the paper. For the conditions simulated in the example analyses, the anchor experienced a modest amount of continued embedment following partial failure of the mooring system; however, the ultimate embedment and capacity of the anchor are much less than what would have developed if the anchor had continued in its original trajectory within the plane of intended loading. 2011 Taylor & Francis Group, London.
