Investigation of suction anchor pullout capacity under undrained conditions

Abstract

Floating units are dependent on reliable mooring systems to ensure safety during marine operations. Suction anchors have proved to be a technologically viable and cost-effective concept. They are capable of precision installation, re-use, and provide large resistive capacity. This thesis investigates load capacity and failure modes of suction anchors subjected to vertical, horizontal (lateral), and incline loading. Suction anchor design considerations, installation procedure, and associated challenges are discussed before reviewing analytical methods for calculating holding / pullout capacity. Analytical results are compared with solutions obtained from finite element analyses conducted with PLAXIS 2D. A Mohr- Coulomb failure envelope with undrained total stress parameters was used. The thesis is limited to loading conditions in undrained soil with a linear strength development. The soil characteristics correspond to clay in the Troll field, North Sea. Finite element analyses indicate that vertical loading of suction anchors in undrained soil will result in a reverse end bearing failure. They also indicate that the horizontal holding capacity is primarily a function of caisson vertical cross-sectional area and the soil strength profile. It was found that the mooring line attachment point greatly impacts the capacity of suction anchors in all load cases investigated.