A Preliminary Assessment of the Behaviour of Drag Anchors in Layered Soils

This paper presents the results of a series of model drag anchor tests performed in the centrifuge on a saturated sample comprised of normally consolidated kaolin clay overlying dense silica sand. The initial focus of the paper is on the description of the test equipment and arrangement, outlining the model anchors used in the tests and the manufacture of a model instrumented anchor chain. A method of determining the orientation and embedment depth of the model anchor within the soil during each test, utilising a tracking probe attached to the anchor and “in-flight” video cameras , is also described. The results highlight the importance of the shank angle of the anchor in relation to the holding capacity and stability of the anchor/chain system.

Introduction

With the recent emergence of FPSO (Floating Production, Storage and Offloading) facilities as a preferred method of offshore hydrocarbon extraction, and the gradual shift of hydrocarbon discoveries towards deeper waters, greater attention has been focussed on appropriate anchoring and mooring systems . Drag embedment anchors provide a simple and economical anchoring solution, and can possess holding power to weight ratios exceeding 20.

Until recently, drag anchor design was largely empirical and based primarily on design charts developed from field test data. Furthermore, these charts describe the performance of drag anchors in homogeneous soils classified broadly as “sand” or ” clay”. They do not cater for drag anchor behaviour in layered soil profiles, like those encountered on the North Sea where silica sands underlie normally consolidated clays.

This paper outlines a procedure for testing model scale drag anchor and chain systems. The paper also presents the results of a number of model anchor tests using anchors with different fluke -shank angles. These tests were performed in the centrifuge in soil samples comprised of normally consolidated kaolin clay overlying dense silica sand.