Stability and Performance Evaluation of Hybrid Foundation System for Subsea Applications

Abstract A hybrid foundation system defined as the combination of shallow and deep foundations has been developed as an alternative to large diameter suction piles, to tackle ever increasing flowline walking loads in deep water subsea field developments. In the current environment of cost effective measures and continued expansion of offshore developments, a hybrid subsea foundation, typically consisting of a mudmat and corner pin-piles, can offer significant fabrication and installation cost savings, compared to traditional large diameter suction piles, as flowline restraint anchors. Since a hybrid foundation combines shallow foundation (i.e. mudmat) with deep foundation (i.e. pin piles); the current industry standards / guidelines do not provide any specific recommendations as to the design requirements of this complex foundation system. This study is primarily aimed to elucidate the performance and stability evaluation of the hybrid foundation based on the full 3 dimensional (3D) finite element analysis (FEA) results. Since the emergence of this relatively new hybrid foundation concept, a limited number of technical papers have been published based on analytical, experimental as well as numerical approaches with no clear consensus in design principles. This comprehensive numerical study aims to bridge the gaps in the previous studies by developing 3D failure envelops (yield surface) to provide better understanding of the effects of multi-directional loadings on the capacity and displacements of a complex hybrid foundation system. The study also investigates the pattern of the soil resistance mobilization under various loading conditions to understand the soil-structure failure mechanism as well as displacements of the hybrid subsea foundation.