A Design Method For an Anchor Pile in a Mooring System

Abstract

Abstract A procedure is described for the design of a pile in soft clay to provide an anchor for a moored vessel. A method is presented for computing the angle of incidence of the anchor chain with the pile, and theories are presented for computing the behavior of the pile. The procedure can be employed to compute the dimensions of the pile and optimum positioning of the chain bracket along the pile. Example computations are presented. Introduction In the mooring of floating vessels at locations where the bottom consists of soft soil, ship anchors frequently do not give sufficient holding power. A possible solution to the problem is shown in Fig. 1. The scheme involves the welding of a bracket to a tubular anchor pile and the attaching of the end of the anchor chain to the bracket. A pile is then driven with a follower to the desired penetration below the ground surface. The follower is subsequently retrieved and, when tension is applied to the chain, the chain assumes a position such as that shown in the figure. In the design of such a system the following information is normally at hand: water depth, shear strength and other properties of the soil, availability of construction equipment, tension on the anchor chain, and dimensions of the chain. The problems which are to be solved are to find the optimal length and diameter of the anchor pile, the position of the bracket along the pile, and the depth below the mudline to which the pile should be driven. In order to obtain solutions to the above problems, one must employ methods for analyzing the behavior of a pile under lateral and axial loading and for predicting the position of the anchor chain in the soil. The following paragraphs give brief presentations of those methods and give example computations. Behavior of a Pile Under Lateral Loading A model of the problem to be solved for behavior under lateral loading is shown in Fig. 2. The soil in Fig. 2 is represented by a set of mechanisms. These mechanisms are used to depict a set of curves which are varying in character along the length of the pile, with respect to x, and are such that the resistance p is nonlinear with pile deflection y. The concept of p-y curves can be explained by Fig. 3. In Fig. 3a a section is shown through a pile which has been driven into soil, with the soil stratum to be examined indicated at the depth below the ground surface x1. The earth pressure distribution around the pile after driving and prior to lateral loading is shown in Fig. 3b.