Performance of Suction Anchors in Fine-Grained Calcareous Soils

Abstract

Abstract Anchor caissons, or suction anchors as they are often referred to because of the method of installation, are an attractive alternative to conventional high-capacity drag anchors in soft seabed sediments. The paper is concerned with the use of such anchors to withstand quasi-horizontal loads, such as are applied through catenary mooring chains. A new threedimensional upper bound analysis is described, for calculating the limiting geotechnical capacity of the anchor for different depths and angles of chain attachment. Results are also presented from centrifuge modelling of an anchor of 5.4 m diameter, and embedment ratio of 2.3, installed in fine-grained calcareous soils with a high proportion of clay-size particles. The paper addresses issues such as the extent to which suction occurs on the trailing edge of the anchor, the rapid reduction in capacity (and change in motion of the anchor) that occurs if that suction is lost, the vertical and horizontal capacities, and the response under monotonic and cyclic loading with a typical chain configuration. Introduction Suction caissons are being used more widely as alternatives to drag anchors, offering various advantages (Colliat et al, 1997):the vertical and horizontal capacity of suction caissons can be defined more precisely than for drag anchors, in terms of conventional soil strength parameters;suction caissons are relatively straightforward to install, with no requirement to proof load to a high proportion of their design loads;the final location of suction caissons on the seabed is fixed, allowing well-focused site investigation, whereas for drag anchors uncertainty in the required drag-length leads to uncertainty in the final anchor locations. To date, the geometry of suction caissons has tended to be relatively low aspect ratio, with lengths less than 3 times the diameter. However, higher aspect ratio caissons can be installed by combined deadweight and suction, and may offer improved anchoring capacity for a given weight of caisson, particularly where the soil strength increases with depth. The main focus of this paper is on the capacity of suction caissons when subjected to quasi-horizontal loading from a mooring chain. This type of loading is rather different from the quasi-vertical loading of suction caissons for TLP anchorages (Dyvik et al, 1993; Anderson et al, 1993; Clukey et al, 1995), and has received rather less attention although small field trials have been reported by Keaveny et al(1994). A three-dimensional upper bound analysis is presented, based on the flow mechanism proposed by Murff and Hamilton (1993) for laterally loaded piles. This mechanism is for a single cylindrical caisson, although it can also be extended to pairs of caissons or square groups of four caissons. The attachment point (or pad-eye) for the anchoring chain is generally attached at a depth of 50 to 70 % of the total embedment of the suction caisson, in order to optimise the lateral resistance. This means that the anchoring chain will subtend a small angle to the horizontal (typically 10 to 20°), as shown in Figure 1, imparting a significant vertical component of force.