ABSTRACT

Mooring systems for offshore floating structures, such as the semi-submersible mobile offshore drilling units (MODUs) commonly used for hydrocarbon exploration, typically comprise eight to twelve mooring lines positioned about the structure. Caissons or plate anchors may be used to secure the mooring lines; this paper focuses on situations where drag embedment anchors (DEAs) are used, but some of the general concepts presented will apply to any anchor. When a DEA is loaded as intended, the following conditions are assumed to hold: (1) the anchor line load acts in the plane containing the shank of the anchor, a condition referred to as “in-plane” loading, and (2) the anchor chain (or anchor line if a wire line is used) lies in a vertical plane. During extreme storm events, MODUs will sometimes experience a partial failure of the mooring system. The consequent movement of the MODU following a partial failure will generally invalidate both assumptions stated above. Firstly, the anchor line force will no longer lie in the plane of the shank, and the anchor will be subjected to “out-ofplane” loading. Yang et al. (2010) analyze the effect of such out-of-plane loading on anchor load capacity. Secondly, the anchor chain will no longer lie in a vertical plane; the effect of this condition on the ultimate pullout capacity of a DEA is the primary focus of this paper.