Experimental and Theoretical Studies of Deep Penetrating Anchors

Abstract

Abstract Dynamic anchors have been shown to provide a cost-effective alternative to other forms of anchoring system in deep water. These anchors are torpedo shaped and are designed so that after release form a designated height above the seafloor, will penetrate to a target depth in the seabed by the kinetic energy obtained through "free-fall" and through the self-weight of the depths are 2 to 3 times the anchor length and expected anchoring capacities lie in the range 3 to 5 times the dry anchor weight. Although suggested in concept in 1999, and uncertainties in respect of the penetration depth and resulting capacity for a given seabed strength profile, and how these parameters are affected by the geometry of the anchor and its flukes. This paper presents results from a series of centrifuge model tests undertaken in order to evaluate different methods of predicting the anchor performance. Prediction models incorporating viscous enhancement of the soil resistance, and drag coefficients based on measured free-fall terminal velocities, are shown to give reasonable agreement with the centrifuge model results. Introduction The geotechnical focus of the offshore oil and gas industry has recently shifted to the development of cost-effective anchoring systems that meet both the geotechnical and economic extraction in deep water. One such system, referred to as a Deep Penetrating Anchor (DPA), has been proposed by Lieng et al. (1999). The DPA is a rocket shaped anchor with a dry weight of 500-1000kN and a height of 10-15m, which after release from a designated height above the seabed (typically 20-40 m), penetrates to a target depth in the seabed by the kinetic energy obtained through free-fall and self-weight of the anchor. The uplift forces due to environmental loading are mainly resisted by the friction developed along the soil-structure interface (i.e. between the soil and the shaft and "flukes" of the anchor). A more detailed description of various aspects of the DPA concept is given in Lieng et al. (1999) and Lieng et al. (2000). Reduced scale field trials have recently been performed in Trondheim, Fjord (Norwegian Sea) on instrumented and non-instrumented DPAs of length 4.4 m, shank diameter 0.4 m and with 4 flukes, 0.475 m wide and 2.0m long(see Figure 1). A schematic and photograph of the DPA are shown on Figures 1a and 1b, and the installation procedure is summarised on Figure 1c. Medeiros (2001, 2002) describes tests performed in the Campos Basin on a conceptually similar Torpedo Anchor (see Figure 2). These specific types of this anchor, referred to in this paper as Types I, II and III, have been proposed as a means of anchoring flexible risers, mobile offshore drilling units and floating production systems respectively. Type I anchor is essentially a 30 inch (762 mm) diameter hollow cylindrical pile of length 12 m, filled with high density scrap material to maximise self weight and with a conical tip to aid penetration.