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

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

This paper was prepared for the SPE-European Spring Meeting 1976 of the Society of Petroleum Engineers of AIME, held in Amsterdam, The Netherlands, April 8–9, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

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).