Shear Strength Along Inside of Suction Anchor Skirt Wall in Clay
Authors: Andersen Knut H. , Jostad Hans Petter 
Published: October 2008

The paper discusses the mechanisms governing the shear strength along the inside of the skirt wall of suction anchors with and without stiffeners during and after installation by underpressure. Methods to calculate the shear strength along the skirt wall are proposed and used to calculate the shear strength after installation for a range of clays. The results are used to propose a simplified method to estimate the shear strength along the inside of the skirt wall after installation.

Introduction

Suction anchors are cylindrical steel units normally closed at the top and open at the bottom (e.g. Andersen and Jostad1). They typically have a diameter of the order of 4-5m, a depth to diameter ratio of 5, and a cylinder wall thickness of 25-35mm. The anchor may be equipped with stiffeners, like inside ring stiffeners and bulk head plates, and increased wall thickness over a height around the load attachment point.

Suction anchors are installed by penetrating the cylinder wall, also called "skirt", into the seabed. The first part of the penetration is achieved by the self-weight of the anchor. Further penetration requires application of an underpressure below the top lid inside the anchor to generate an additional driving force.

The shear strength along the skirt wall will during installation be reduced to the remolded shear strength. In cases with stiffeners, the shear strength above and between the stiffeners may be reduced even further. After installation, the shear strength along the skirt wall will in most cases increase again due to increased total normal stresses during skirt penetration, redistribution and dissipation of installation pore pressures, and thixotropy. A method to determine the shear strength along the outside of the skirt with time after installation was presented by Andersen and Jostad. They showed that the shear strength along the outside of the skirt may be reduced due to the installation effects, even after full set-up has occurred.

This paper focuses on the shear strength along the inside of the anchor wall during and after installation by underpressure. This shear strength will influence the penetration resistance during installation, and it will influence the holding capacity during operation for anchors without a top lid, with leakage in the upper part of the anchor, and in cases with long term loads. The paper first discusses the mechanism governing the shear strength during installation. Anchors with and without ring stiffeners are considered. The conditions at the end of penetration are defined and used as initial conditions to determine the shear strength with time after installation, which is discussed afterwards.

Behavior Inside Anchor during Installation.

The behavior of the soil plug inside the anchor can depend strongly on whether the anchor is equipped with inside stiffeners. In this section, the behavior of anchors without inside stiffeners is presented first, and then it is discussed what influence the stiffeners may have.

Anchor without stiffeners.

The clay plug inside an anchor penetrated into clay will have many similarities with a clay sample in a soil sampling tube. The photo of a split clay sample in Fig. 1 shows that the sample can have a thin zone with extreme shear strains at the periphery along the cylinder wall, a transition zone with large shear strains, and an inner zone that deforms quite uniformly.