An Experimental Study of Drag Anchors and Implications for OTEC
Authors: Carchedi D.R. , Nacci V.A. , Pekin O. , Morgan R.J. 
Published: October 2008

Selection of an anchoring system for the Ocean Thermal Energy Conversion (OTEC) Project has become a major design problem at the limits of current technology. OTEC anchors must be able to withstand mooring loads 1arger than ever previously encountered. High loads coupled with deep water and a sloping bottom render the use of empirical relations or extrapolation from existing anchor data to design the mooring system impossible.

In-situ soil properties and soil mechanics concepts have rarely been applied to the selection or design of an offshore anchoring system. Empirical relationships (1) such as the rating of an anchors efficiency (ratio of anchor holding power to anchor weight) have long been the standard, yet inadequate, means available for anchor selection.

Laboratory Model drag anchors have been pulled through dry sand in an attempt to identify important parameters and to develop a working theory which will aid in the design of offshore anchor systems. Model anchors were tested with eight different shank lengths as well as with several shanks of different cross-sectional areas and shapes. Each configuration was also investigated at several different shank to fluke (S-F) angles. A 'standard' anchor configuration was developed and tested by varying sand density, S-F angles, anchor roughness, and bottom slopes. The effect of each factor on anchor holding power is presented.

INTRODUCTION

A need for a sound and rational approach for accurately predicting deep sea anchor capacities became apparent during the initial stages of the OTEC anchor selection. Extrapolation of anchor performance from existing anchors or the use of empirical equations to design such large anchors is at best questionable. Anchor efficiency also becomes unreliable for very large anchors (2), especially with the recent use of high strength steels and hollow sections.

The goals of this study were to investigate the parameters and forces involved in the setting and ultimate holding power of a drag anchor. Several hundred model tests have been performed in the laboratory in an attempt to at least qualitatively understand some of the many factors that affect an anchor?s performance. Lastly, the most important factor studied was the effect of bottom slope on drag anchors. As a result of the requirement of deep water and close proximity to land, the OTEC plant by necessity will be positioned above a sloping bottom, thus the effectiveness of anchors pulling on slope has become paramount.

THEORY

The mechanics of anchor resistance although extremely complex, is related to earth pressure theories, from figure 1. it can be seen that earth pressure against the flukes of the anchor is resolved in two directions, a force, N, normal to the flukes and a force, T, tangent to the flukes. The anchor is in equilibrium if the forces N and T are less than the ultimate value that the soil can generate. If the ultimate values are exceeded, the anchor will either penetrate deeper or will fail as the soil wedge resistance in front of the flukes is overcome.