Abstract

The principles and development of suction anchors from 1958 to the present day are reviewed. Data from vertical pull-out tests on laboratory-scale suction anchors, buried at depths up to eight times anchor diameter in a submerged sand, are presented and discussed. The data include force, displacement, pressure and water flow measurements at various anchor embedment depths. Non-dimensional parameters are introduced to permit comparison of the model tests with the results of field trials with larger scale anchors. The data are used to predict the vertical break-out force for a 2 m diameter anchor. It is estimated that break-out forces of the order of 10 MN (1000 tones) are attainable with an anchor of this size at a 12 m burial depth.

Introduction

The demand for improved mooring continues on a world scale due to the growth of ocean operations and construction which, over the last two decades in particular, has resulted in the increased application of structures anchored in shallow or deep water. A substantial mooring is often required in deep water, by reason of extreme storm conditions, where the use f conventional marine anchors may be ruled out by the need for precise station keeping and the reaction of uplift forces. The quest is for anchors that have a high resistance to pull-out forces, are reliable, of minimum cost, size and weight, precise in their application and simple to handle and maintain. This quest has led to the introduction of many new anchor types.

The marine "suction" or "hydrostatic" anchor is' an example of innovation in anchor design. A variety of suction anchor types have been developed but all types make use of the same fundamental concept. This concept is the generation of force by modification of the pressure gradients that exist in stationary fluids and give rise to vertical buoyancy forces. A marine suction anchor is a device which can change the magnitude and direction of the hydrostatic pressure gradient within a region. The region encloses the device and an impermeable surface, such as the side of a ship or a slab of rock or concrete, or a permeable surface such as the sea bed. By changing the hydrostatic pressure gradients, the suction anchor produces forces that act inwards on the items within the region, pressing them together. These forces can be of very large magnitude. Suction anchors in the form of inverted cups have been the subject of the greatest investigation. The largest anchor of this type is the Shell/S.B.M. Suction Pile4. In this case the weight of the anchor is sufficient to make the rim of the cup (known as the skirt) penetrate the top soil layers so that, when the water pressure within the cup is reduced by pumping, the hydrostatic pressure difference drives the suction pile into the sea bottom. The pump is removed after anchor installation and the pressure difference no longer maintained. Tests on a 6DO mm diameter inverted cup anchor fitted with skirt water jets to assist penetration were reported by Hinson and Sahota23,24.

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