Three-dimensional modelling of drag anchor penetration using the material point method

Drag embedment anchors are a key threat to buried subsea linear infrastructure, such as power/data cables and pipelines. For cables, selecting a burial depth is a compromise between protecting the cable from anchor strike and the increased cost of deeper installation. This paper provides an efficient large deformation, elasto-plastic Material Point Method-based soil–structure interaction predictive tool for the estimation of anchor penetration based on Cone Penetration Test (CPT) site investigation data. The tool builds on earlier work by the authors supplemented by three key developments: modelling assemblies of rigid bodies (necessary for articulated anchors), a partitioned domain approach to enable accurate and efficient modelling of long anchor pulls, and an improved means of modelling rotational inertia. The numerical model is calibrated using CPT data and then used to predict the penetration behaviour of two different drag anchors across a range of relative density sands under drained conditions with validation against scaled geotechnical centrifuge physical tests. Numerical simulations both confirm assumptions in, and identify key issues with, the UK Carbon Trust’s Cable Burial Risk Assessment (CBRA) approach for estimating anchor penetration. In particular, the results confirm that anchor penetration scales linearly with fluke length but also that the penetration of drag anchors is highly dependent on both the relative density of the sand and the full geometry of the anchor. The numerical model presented in this paper enables site-specific anchor-penetration assessment along cable routes and can be used to evaluate the performance of different anchor designs and sizes in varied soil conditions.