Suction Anchors

Abstract The development of self-burying suction anchors is described and data on pull-out forces presented for both inverted-cup and solid hemispherical types embedded in fine sand at depths up to three times anchor diameter. Observations made during sea trials of both anchor types are discussed. It is shown that the application of anchor suction increases the force necessary for pull-out of deeply embedded anchors and that suction anchors can have a low power requirement. Introduction Marine ground-anchors may be classified according to their capacity for withstanding uplift force and horizontal drag force. In recent years, many forms of uplift-resisting anchors have been proposed, each anchor type having its own proposed, each anchor type having its own characteristics and range of applications. One type, the suction anchor, has been investigated at a number of centres. Most of these studies were based on anchors in the shape of an inverted cup (Fig. 1), with a skirt and internal filter or porous stone between the soil and an internal cavity from which water is drawn by means of a suction line connected to a pump. The reduction of pressure within the anchor and the adjacent soil produces a downward-acting force which presses the anchor onto the soil bed and creates presses the anchor onto the soil bed and creates compressive increments in the effective stresses in the soil below the anchor. A vertical force is required to lift an anchor hanging freely in water. The extra force F required for pull-out of an embedded cup anchor comprises the resultant of the normal and shear stresses applied to the outer surfaces of the anchor by the soil and a force to cause failure of the soil below the anchor by further modification of the effective stresses. Schofield proposed an alternative to the cup anchor in the form of an anchor plate or an arrangement of interconnecting beams lying on the sea bottom and covered by an impermeable sheet. By creating a difference in hydrostatic pressure between the upper and lower sides of the sheet the anchor assembly was pressed onto the soil, thereby making the anchor capable of resisting uplift forces. All of these anchors are surface attaching devices with little or no overburden to contribute to the pull-out force. Also, the cup anchors have a very limited capacity for self burial, usually requiring a downward push into the soil. DEVELOPMENT OF SELF-BURYING ANCHORS At R.G.I.T., work on suction anchors was carried out by students Rosbak and Torbjornsen. By the end of 1974 Rosbak had tested a cup-type model (Fig. 1) in a submerged bed of fine sand and obtained results similar to Wang et al. In general, this model would not embed itself by suction alone. Sea trials with a simple anchor made from a small oil drum were encouraging though they confirmed the belief that suction alone would not give consistant embedment of a free anchor. If the anchor was part of a sea-bed structure then the weight of that structure might be sufficient to push the anchor into the soil. The weight of the anchor was unlikely to give sufficient initial penetration for suction to develop to complete the embedment. In order to give the anchor a self-burying action various arrangements of water jets were fitted to the original model. Effective sand fluidization was produced by a ring of downward-acting jets around the edge of the anchor skirt. A new model in acrylic with a brass jet ring was designed by Torbjornsen (Fig. 2). A secondary function of this ring was to give the model a low centre of mass and stability during burial. In the fine sand in the test tank the new model buried rapidly to filter level. Up to this time the suction anchor had been regarded as a surface attachment device, and it was a somewhat surprised Torbjornsen who on one occasion watched the model bury to filter level in the normal way and then disappear downwards into the sand bed. The forces required for pull-out increased significantly with burial depth. P. 139