Research On The Ultimate Pull-out Capacity Of The Vertically Loaded Anchor (VLA)

With the exploration for oil and gas reaching deeper waters, the use of floating production, storage and offloading (FPSO) facilities has increased, identifying a need for high-capacity anchoring systems. Conventional catenary systems have become too costly, and this led to the introduction of taut-wire mooring systems, where anchors are subjected to significant vertical loads. While large drag embedment anchors can have very high holding capacities and offer an economical and simple means of anchoring for a catenary system, their suitability for taut wire systems remains in question. As a result, drag-in plate anchors, or vertically loaded anchors (VLAs), were introduced by anchor manufacturers. VLAs are installed like conventional fluke anchors by pulling up to a target installation load. The loading direction is then changed such that it becomes normal to the fluke, leading to a significant increase in anchor resistance. In this normal loading mode, the anchor acts as an embedded plate.The purpose of this article is to present calculations of ultimate pullout capacity for VLA foundations, accounting for embedment, embedment angle, rate of increase of strength with depth, and width of the foundation. The soil is assumed to be elestic-perfect plastic, with yield determined by the Tresca condition with an undrained strength su. The method of upper bound and limit equilibrium calculations based on assumed mechanisms is used for the ultimate pullout capacity calculation. Finite element analysis of the fluke-soil interaction behaviour of VLA drag anchors in undrained soft clay has allowed calculation of plastic yield loci for characterisation of anchor failure states. The VLA fluke-soil interaction are introduced into the MSC.MARC software .The soil is assumed to be elestic-perfect plastic, with yield determined by the Von Mises condition.The plastic limit formulation is evaluated through comparisons with finite element simulations and the methed using conventional bearing capacity theory.