Pipe Anchor
Authors: Hogervorst J.R. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
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Abstract
Field experiments with pipe anchor models in sand and soft clay bottom revealed that in the former the anchor can readily be buried by fluidisation, but that in the latter artificial burial (cement injection) is required. In both cases the ratio of anchor weight to holding power is high (1:60 and 1:25). Comparison of test results with those obtained by means of a mathematical model provides reasonable design data for anchors of provides reasonable design data for anchors of different holding powers.

Introduction
In general floating structures such as mooring terminals, crude oil transfer systems, semi-submersibles, guyed Conductors, wave rider buoys, etc. have to remain anchored offshore for many years. The anchoring is required to be effective, low in cost and capable of standing forces as may occur in marine operations under a 100 years storm condition.
Under these extreme conditions the use of conventional ship's anchor is sometimes ruled out. An example is a situation where an anchor cannot dig itself in sufficiently deep or when the anchor is exposed to vertical forces. Piles that can be loaded laterally as well as vertically tend to be too costly for deep-water moorings. This means that there is an urgent need for an effective alternative anchoring method. This induced us to investigate the capabilities of a so-called embedment anchor.
An embedment anchor combines light weight with high holding power provided it is buried sufficiently deep. Of the techniques applied for anchor burial in erodable soils the jetting technique is the most attractive. This technique enables the anchor to be embedded in sand by natural resedimention, while it can be artificially buried in clay by cement injection via jetting nozzles. This feature led to the design of the so-called pipe anchor, a square-shaped anchor made from steel pipe (Fig. 1).
To test the performance of the pipe anchor, experiments were conducted with scaled anchors in sand and soft clay bottoms.
In the following sections the main results of the experiments are summarised and compared with theoretical estimates.

EXPERIMENTAL PROCEDURE
The experiments started with jetting in of the anchor to the required depth. In cases where the pit was jetted into a sand bottom, settling of the pit was jetted into a sand bottom, settling of the grains occurred after jetting was stopped, so natural burial of the anchor took place (Fig. 1b). In clay bottoms, however, the soil particles were washed out, which necessitated the anchor to be embedded artificially, e.g. by means of cement injection (Fig. 1d).