Pull-out Parameters for Buried Suction Anchors

Abstract

ABSTRACT Data obtained from a model of a self-burying suction anchor tested in a granular soil bed in the laboratory and from larger anchors tested at sea are presented. Scaling parameters for comparing the performance of buried suction anchors are proposed and discussed and a preliminary investigation made of an anchor group with a 1000 kN (225000 lbf) vertical pull-out capacity. The procedures used and some of the problems encountered during sea trials with suction anchors are described. INTRODUCTION Suction anchors should be considered as a family of marine devices with capacity for resisting uplift force, each device having its own characteristics and field of application. Anchors in the form of inverted suction cups have been investigated extensively and have been shown to be capable of application in sands, silts and clays 1,5,11. The use of large cup-type anchors as footings on a gravity structure has been proposed by Wang et al 10 with the intention that additional anchoring capacity should be developed by soil pore pressure reduction during extreme storm conditions. Large suction anchors have been tested by the Shell Rijswijk laboratory5,6 and the maximum allowable lateral force for a 3.8 metre (12.5 ft) diameter anchor reported to be 160 tonnes. Cup-type anchors may be regarded as surface attaching devices, even though the anchor skirts penetrate the sea bed, since there is usually no soil above the anchor to contribute to pull-out resistance. A suction anchor with non-penetrating skirts of flexible material has been proposed by Schofield4 and investigated in the laboratory by Al-Awadi l Cup type anchors, which are of rigid construction, require some soil depth for penetration of the skirts but the non-penetrating anchors can be attached to slabs of rock or concrete. The development of self-burying suction anchors of hemispherical shape has been described by Wilson and Sahota12 and it has been observed that, for a given anchor size and suction pressure, the buried anchors can develop greater pull-out resistance than the other types due to the effect of the soil above the anchor. All suction anchors have certain features in common. A cavity within the anchor is connected to the water in the soil voids via a filter or porous part of the anchor body, so that a reduction in water pressure within the cavity, created by pumping, is transmitted to the soil pore water. The flow of water which develops between the surface of the submerged soil bed and the anchor filter creates a distribution of force in the soil, directed in the flow direction and at right angles to the equal-pressure or equipotential surfaces (Fig. 1). The flow lines and equipotential surfaces form a flow net which develops with time from initiation of pumping but which tends towards a steady state condition if no perturbations occur. The generally favourable increments in the soil effective stresses that are produced increase the pull-out resistance of the suction anchor.