Offshore Technology Research Center


OTRC Project Summary

Project Title:

Upper Bound Analysis for Drag Anchors in Soft Clay

Prinicipal Investigators:

Charles Aubeny and Don Murff


Minerals Management Service and Industry Consortium

Completion Date:

December, 2005

Final Report ID#

A167(Click to view final report abstract)

Note: This study is part of a broader project "Suction Caissons & Vertically Loaded Anchors: Design Analysis Methods" (MMS Project 362).

The objective of the research proposed herein is to develop a simplified model for predicting the capacity of plate anchors embedded in cohesive soils under general conditions of loading. The analytical studies are based on upper bound plastic limit analysis methods. The upper bound method will be applied to determine the anchor trajectory and the anchor capacity at any point in the trajectory consistent with the compatible anchor line behavior. The embedded drag anchor components are idealized as simple symmetric plates and bars connected to each other at fixed angles. The failure mechanism involves a rotation of the rigid anchor about a center of rotation to be determined in the analysis.

The anchor line force for a given embedment depth and anchor orientation is determined using an upper bound limit analysis approach. The analysis considers the anchor to experience a virtual rotation about some center of rotation. The anchor line force is determined by equating the rate of work performed by the anchor line and known anchor weight to the internal rate of energy dissipation associated with the anchor moving through the soil. The total energy dissipation rate is determined by integrating the unit dissipation over the various anchor surfaces. The upper bound analysis procedure produces a curve relating anchor line force to anchor line inclination angle at the pad-eye. The intersection of this curve with the anchor line equation yields a unique solution for anchor line force. The location of center of rotation is optimized by systematically varying it to find the minimum anchor line load. The anchor is then advanced a small increment by rotating about the optimum center of rotation. If the center of rotation is an infinite distance from the anchor, then the anchor undergoes pure translation. If the center of rotation is near the anchor, then the anchor motion is primarily rotation.

This optimization process effectively identifies the specific failure mechanism that is as close to equilibrium as possible for the general mechanism in question. Because the failure mechanism selected includes all possible failure mechanisms, the optimized solution is the exact solution for the particular yield surface functions assumed. The proposed new method provides a practical means of estimating drag anchor load capacity and trajectory. In this study, results from the new proposed method will be compared with empirical methods, other equilibrium methods and field load tests.

Related Publications: Aubeny, C.P, Kim, B.M, and Murff, J.D. (2005) “Proposed upper bound analysis for drag embedment anchors, International Symposium on Frontiers in Offshore Geotechnics, Perth, Australia, pp. 179-184.


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