A method for predicting six degrees-of-freedom ultimate limit state of subsea mudmats

Rectangular mudmat foundations are extensively deployed in deep waters to support subsea infrastructure leading to renewed interest in optimizing the design of offshore shallow foundations. Offshore mdustry guidelines (e.g. API RP2GEO and ISO 19901-4) are based on classical bearing capacity theory of a plane strain strip foundation resting on the surface of a uniform Tresca material. More realistic conditions are accounted for through a range of superposed empirical modification factors and the effective width principle. In practice, subsea foundations experience complex loading in six degrees-of-freedom (vertical load, biaxial horizontal load, biaxial moment and torsion), due to expansion and contraction of connected pipelines and jumpers; they may be able to mobilize transient tensile capacity; and they are typically three-dimensional in plan, shallowly embedded and founded on soft, nonnally consolidated, soils with linearly increasing strength with depth. Accurate determination of the ultimate limit state of subsea mudmats is best achieved by considering the relevant foundation, soil and loading boundary conditions explicitly. In this paper, a simplified approach for predicting the ultimate limit state of mudmat foundations under six degrees-of-freedom, based on failure envelopes, obtained from extensive finite element analyses, is compared with the traditional bearing capacity methods as recommended in industry guidelines.