For deep water pipeline end manifold (PLEM) subject to complex loading conditions, a combined 3D finite element (FE) model has been developed by the authors to determine the foundation bearing capacity by studying the interaction between individual parts, including the PLEM structure, flowline, mudmat and soil. The advanced numerical techniques have proven that the resulting factor of safety against bearing failure is higher when compared to the classical approach given in API-RP-2A/2GEO. However, the minimum required safety factor of 2.0 is not reached. Therefore, risk assessment using reliability analysis becomes mandatory to assess the probability of failure of the mudmat. As an input to the risk assessment, this paper presents an investigation of the system’s modes of failure and failure consequences on the environment and project cost.
Using the developed 3D FE model, plausible excessive loads beyond design considerations are applied until a failure in soil foundation occurs. Then, loads are increased until the capacity of structural and mechanical components in the system is reached putting the containment of carried hydrocarbons at risk. A direct relationship between the hydrocarbon temperature and the capacity of structural/mechanical components is developed to evaluate the system flexibility and resistance to induced excessive deformations.
The paper demonstrates the advantages of numerical techniques in evaluating the ultimate capacity of a PLEM structure and foundation soil system. The methodology can be used in the design of a new system or to assess the stability of an existing system under future loadings.