Physical and Numerical Modelling of Installation and Pull-Out of Dynamically Penetrating Anchors in Clay and Silt

Abstract

Dynamically penetrating anchors (DPAs) are a recent type of mooring technology that have been shown to provide a cost-effective alternative to other forms of anchoring system in deep waters. This paper reports the results from a series of model tests undertaken to provide insight into the behavior of DPAs during dynamic installation and monotonic pull-out in normally consolidated clay and calcareous silt. The tests were carried out in a drum centrifuge at 200 g, varying the drop height and hence the terminal velocity. The pull-out angle at the mudline was also varied to encompass various mooring systems. This paper also reports the results from finite element (FE) analyses simulating dynamic installation of anchors. The 3D large deformation FE (LDFE) analyses were carried out using the Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit. A parametric study was undertaken, exploring a range of anchor geometry in terms of diameter, tip angle and number and length of fins; impact velocity and soil strength. For dynamic installation, two interesting aspects of the soil flow mechanisms were identified: (a) downward soil movement, concentrating around the advancing anchor, being reduced gradually with reducing penetration velocity and more rapidly with increasing number of fins and anchor projected area; (b) mobilisation of end bearing mechanism at the base of the anchor as well as fins, with the latter reduced significantly for shorter fins. The depth of anchor penetration increased as the drop height (and terminal velocity) increases and the soil strength decreases. For static pull-out, the mooring angle at the mudline influenced the pull-out mechanism and the capacity significantly. The anchor rotation and hence the anchor displacement required for attaining maximum capacity reduced as the angle increases.