Continuum mechanics and implicit material point method to underpin the modelling of drag anchors for cable risk assessment

Abstract

The last years have seen an extraordinary expansion of the wind offshore industry towards new markets. With this development, wind farms are being pushed further offshore, leading to new challenges in maintaining their infrastructures. Data have revealed that one of the assets most susceptible to risks are power cables transporting the electricity generated offshore to the onshore transmission system. Whenever there is no alternative to shielding the cables, these are buried in the seabed to ward off the threat of drag anchors. Understanding the embedment process of drag anchors deployed by ships for mooring purposes is critical to determining the appropriate burial depth of cables. In turn, this protection strategy must include the influence exerted by the seabed conditions.

Historically, studies have focused on lab and field tests, whose results have been recently called into question. The lack of an appropriate scientific tool to investigate the anchor cable interaction has recently fostered this process examination via numerical methods. Among these, the Material Point Method (MPM) is well-placed to master large deformation mechanics without mesh distortion and retains all of the advantages of a Lagrangian method. As a matter of fact, the MPM is the main object of investigation of this thesis, whose ultimate goal is to:
1. include inertia forces in the context of finite strain elasto-plasticity for solid materials;
2. expand the above point to handle the presence of water in the porous seabed; and
3. model the friction between the anchor and the surrounding soil.

This work has dedicated particular attention to the compliance of the MPM discretised algorithms with the underlying continuum formulation. In this sense, this work’s primary contributions comprise:
• a conservation law consistent MPM algorithm for solid mechanics;
• a constitutive relationship for porous materials respectful of the solid mass conservation; and
• a rigorous assessment of the frictional contact formulations available in the MPM literature.