Drag Embedment Anchor Performance Prediction in Soft Soils

ABSTRACT A rational approach to calculating the drag embedment performance of marine anchors in cohesive soils is presented. The method considers the geotechnical and gravity/buoyancy forces acting on all components of the anchor system including various areas on the anchor and the attached mooring line which may be either chain or wire. The solution proceeds in a piece-wise linear manner until an equilibrium position is found for the anchor. This equilibrium position is described in terms of the anchor burial depth, the anchor drag distance, and an anchor angle of rotation in the soil. The equilibrium position may be such that the anchor holds the applied line load and no further movement occurs, or it may be such that the anchor can dig no deeper and horizontal movement is predicted with a given maximum resistance. The solution technique requires a computer program. Such program has been developed and the results from the program have been compared with large scale tests performed in the Gulf of Mexico in 1990. HISTORICAL ANCHOR DESIGN This paper is concerned primarily with relatively large marine drag embedment type anchors which areused in the offshore industry. However, it is worth considering the evolution of anchors in general. The first patent that was filed for describing an anchor was that by Hawkins in 1821. This patent described an anchor consisting of two flukes which were articulated about a pin perpendicular to the shank of the anchor. This concept has remained in use today and falls now into two main categories of stockless anchors and stock anchors. Stockless anchors are widely used today on warships and on merchant ships. They are not typically used inthe offshore industry. Articulated stock anchors are more widely used incommercial applications today than stockless anchors. The Danforth anchor is a typical example developed in the late 1930s. These anchors are used on barges and semi-submersible drilling rigs. Names in common use include Danforth, LWT, and Offdrill. The requirements of the offshore industry to provide ever larger capacities of anchors led to a scaling up ofexisting models. Stockless and stock anchors weighing up to 30 tons in air have been built. Special anchor handling vessels have been developed specifically for the placement and retrieval of these anchors. In the last decade, the increasing use ofpermanent mooring systems for floating production has resulted in the design of anchors specialized to certain types of soil conditions. In the soft cohesive soils typical of many deep water areas in the world, increasing use of anchors fabricated from steel plates has occurred. These fabricated anchors replace cast steel anchors. They are more efficient in terms of their holding capacity compared to their weight. However, they are very specific to individual applications and cannot be regarded as general purpose anchors.