Optimal skirt spacing for subsea mudmats under loading in six degrees of freedom

• Internal skirts were effectively modelled by kinematic coupling constraint.
• In-plane V–M–H loading was the governing case for determining the optimal skirt spacing.
• More internal skirts were required for smooth mudmats particularly at low embedment ratios.
• Plane strain analyses can be performed to predict the optimal skirt spacing for rectangular mudmats.
• Simple to use charts were provided to determine the optimal skirt spacing for mudmats under V–H2–M2–T loading.

Two- and three-dimensional finite element analyses are performed to identify the optimal internal skirt spacing for the maximum undrained capacity of subsea skirted mudmats. Fully three-dimensional loading (vertical, biaxial horizontal, biaxial moment and torsion) is considered for subsea mudmats with skirt embedment ranging from 5% to 20% of the foundation breadth in soil with a range of linearly increasing strength with depth. The results have identified the governing case for determining the optimal skirt spacing for mudmats subjected to fully three-dimensional loading. It is also shown that optimal skirt spacing for rectangular or square mudmats can be determined in plane strain conditions using the equivalent foundation embedment ratio. The number of internal skirts required to ensure soil plug rigidity under fully three-dimensional loading is presented as a function of skirt embedment ratio, soil heterogeneity index and vertical load mobilisation. Results also indicate that effects of skirt roughness become negligible as foundation embedment ratio increases in terms of determining the optimal skirt spacing.