Abstract

The authors have presented a summary of the various methods that are available to determine the holding capacity and trajectory of drag embedment anchors. One of the drawbacks of the methods that are available today is that these are generally only suited for a single type of soil. The authors are currently working on a methodology that will predict the performance of anchors in varying soil conditions. The initial findings and the methodology are presented.

Introduction

Drag embedment anchors are a common method of fixing floating structures to the seabed. Over the years numerous methods have been developed to determine the holding capacity and trajectory of the anchor in the seabed. The most common of these, are the design graphs developed by the various anchor manufacturers. More recent developments are the development of methods which can predict the anchor holding capacity and trajectory based on geotechnical methods.

The prediction of the anchor holding capacity and trajectory can be split into a number of distinct parts, being:

  1. The static holding capacity of an anchor penetrated to a certain depth into the seabed, both in cohesive and cohesionless soils.

  2. The profile that the mooring line makes in the seabed (the inverse catenary).

  3. The kinematic behavior of the anchor and mooring line in the seabed.

To accurately predict the anchor performance, these 3 parts should be combined into a single methodology.

This paper will discuss the various methods that are available to determine the performance of the drag embedment anchor. The design graphs, static holding capacity, mooring line profile and anchor kinematics will be presented in details showing the various methods that have been developed. The final part of the paper will discuss a method that the authors are working on, which in their opinion gives an accurate method of predicting the anchor performance. While the authors have tried to give a complete view of the material that is available, they do not claim that the material presented in this paper is complete.

Design graphs

The most common method of predicting the holding capacity of drag embedment anchors is the use of design graphs. Typically these graphs are shown on double logarithmic scale, with the weight of the anchor on the horizontal axis and the holding capacity of the anchor on the vertical axis. Holding capacity curves are generally presented for sands and soft clays.

The basis of these design graphs for a specific anchor type is the tests performed with a number of small scale anchors (typically up to a weight of 3 metric tons) which have been tested up to the ultimate holding capacity. The ultimate holding capacity being defined as the maximum horizontal steady pull which can be resisted by the anchor at continuous drag. Based on the tests with the small scale anchors, the performance of the larger size anchors has been extrapolated. These design graphs are supplied by the various anchor manufacturers for their specific anchors.

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