ABSTRACT

Plate anchors are commonly used in offshore industry to anchor floating facilities. They are installed vertically using a follower and subsequently rotated, through a process called keying, to exhibit their maximum area normal to the loading direction. This paper describes a series of centrifuge tests performed in order to investigate the influence of the keying on the overall performance of the anchor. Tests were conducted against a Perspex window in plane strain chambers containing normally consolidated clay. Particle Image Velocimetry (PIV) provided insights into the different failure mechanism generated during keying and information about the load inclination and load eccentricity minimising the loss of embedment and hence maximising the anchor performance. The loss of embedment after keying for different load inclinations and different load eccentricities was also quantified.

INTRODUCTION

The gradual exhaustion of hydrocarbon reserves in shallow waters has forced the offshore oil and gas industry into water depths now approaching 2000 m (Aubeny et al. 2001, Ehlers et al. 2004). This transition to deep water and ultra deep water has necessitated the replacement of fixed platforms by floating facilities that are anchored to the seabed by vertical tethers or relatively taut mooring lines, both of which require anchoring systems designed to withstand significant vertical components of load. One such anchoring system is the Suction Embedded Plate Anchor (SEPLA - Dove et al., 1998). It has been conceived to combine the advantages of suction caissons (known penetration depth and geographical location) and vertically loaded plate anchors (geotechnical efficiency and low cost) (Wilde et al. 2001). The SEPLA uses a suction caisson to embed a plate anchor that is slotted vertically into its base. The caisson is lowered to the seabed where it is allowed to penetrate under self weight.

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