Estimating Sliding and Penetrating Resistances of a Tolerably Mobile Subsea Mudmat: Large Deformation Modeling and Theoretical Framework

A tolerably mobile mudmat suffers from long-term maintained self-weight and periodic sliding events with intervening consolidation during its whole life, resulting in changes in seabed strength and mudmat capacity. In this paper, the performance of a mobile mudmat founded on fine-grained soil is investigated through results of a coupled large deformation finite-element (LDFE) analysis, with focus on the consolidation effect induced from the installation. A series of LDFE simulations of several typical field scenarios is undertaken, which provide the basis for developing an extended effective stress framework. The main development is the extended reconsolidation solution allowing coupled dissipation of excess pore pressure that remains from partial installation consolidation and is progressively generated from cyclic shearing. Dissipations of the excess pore pressures caused by vertical self-weight load and horizontal sliding are calculated respectively. Existing centrifuge tests on kaolin clay are reproduced to validate the performances of both LDFE and framework simulations. Variations in stress state and strength profile are discussed to interpret the changing sliding and penetrating resistances. A shorter installation period followed by frequent slide events is found to contribute to a rapid hardening of resistance. The framework approach is further examined through retrospective simulations of centrifuge tests performed on an offshore carbonate silt, which demonstrates its robustness in practice.