Hybrid subsea foundations (HSF) are combined foundation systems of mudmats and piles. The primary motivation of combining these two foundation types is to provide greater resistance to large horizontal loads in addition to vertical loads, for which use of mudmats alone will require it to be of impractically large size. The contribution from the piles in the lateral capacity helps to limit the size of the mudmat, which is critical in subsea environment. In a brownfield situation, this is sometimes a hard limit with only limited space available to place a new mudmat in the existing field layout. Also, in some cases, the HSF may prove to be a more economical option for resisting large horizontal loads compared to, for example, to suction piles. While the authors are aware of some scattered project-specific design and use of subsea mudmat-pile hybrid foundations by individual contractors and operators, there is no industry-wide publicly known best practice currently available. These designs of HSF appear to be generally based on simplified analytical approach that require superimposition of conventional shallow and deep foundation capacity calculation methods, hence violates the static and kinematic compatibility requirements fundamental for a sound and robust prediction procedure.
This paper attempts to provide some insight into the behavior of mudmat-pile foundations as a hybrid integrated system numerically using finite element modeling and analysis (FEA). The interactions between the mudmat and the piles in an HSF are complex and hence a FEA-based approach is considered most suitable. The FEA model in this study included the mudmat, the corner piles, the pile-mudmat connections and the seabed soil. Sensitivity of the HSF capacity to the size of the piles (length and diameter), the connection type of the piles to the mudmat, and the number of piles are selectively investigated and the results presented. Based on these results some pertinent observations relevant to design of HSFs are also given.
While the study is of limited scope, it offers important insights into the effects of the primary design variables on HSF’s capacities. Therefore, the authors hope the information herein will be of benefit to practicing subsea engineers who might have to face choices to consider mudmat-pile hybrid foundations as a real option for their projects.