Analysis Of Subsea Foundations Subjected To Significant Torsion

The interest in the adequacy of shallow foundations to withstand the forces in the subsea realm is growing. Subsea structures such as PLETs and PLEMs often experience significant torsion loads in addition to vertical, horizontal and moment loads which are typically supported by shallow foundations. In this paper, the interactions between torsional and sliding loads for shallow foundations on sand were evaluated using available analytical methods as well as by a comprehensive 3-dimensional finite element analysis (PEA). The calculated pure sliding and pure torsional resistances of un-skirted mudmat calculated by simple analytical method compared well with PEA results. There appears to be a lack of analytical methods to include torsional resistance generated by skirts. It was evident from PEA that even shallow mudmat skirts can contribute significant torsional resistances. The PEA results indicate that torsional load on foundation can significantly reduce pure sliding resistance, and the foundation design should consider sliding (H) - torsion (T) interaction, particularly when significant torsion load is present. Finally, a case history is presented involving existing shallow foundations that were deemed inadequate to support the new torsional and sliding forces. Additional clump weights required to improve overall foundation stability were determined considering H-T interactions. A comprehensive ABAQUS model of an existing PLEM foundation under combined uplift, compression, sliding forces, torsion and bending moments was analyzed to confirm the improvement in foundation stability.