Reliability assessment of a subsea pipe-in-pipe system for major failure modes

The structural reliability of a pipe-in-pipe system operating in deep-waters under high internal pressure and high temperature is assessed. First, the major failure modes are identified and the respective limit state functions are formulated. In particular, the collapse of the outer and inner pipes, the local buckling of the inner and outer pipes under combined loading, the burst of the inner pipe and the buckle propagation are addressed. Additionally, a serviceability limit state function is formulated for the lateral buckling response due to excessive axial force on the outer pipe. Appropriate resistance and load models for the pipe-in-pipe configuration are included in the limit state functions. Uncertainties in the structure, operating and environmental conditions are incorporated in the limit state functions through probabilistic modelling. First-Order and Second-Order, reliability methods and Monte Carlo Simulation techniques are used to assess the structural reliability and the effect of limit state variables on the probability of the PiP failure modes. System reliability bounds for combined failure modes are estimated through a second-order series bounds approach. The importance of basic variables of the limit states is evaluated through sensitivity analysis. Several parametric studies are conducted to show the influence of the limit state variables on the reliability of each failure mode and on the bounds of the system reliability.