Stress Analysis of a Cryogenic Corrugated Pipe

One method to develop offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG via tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a flexible cryogenic hose whose main body is a pipe-in-pipe hose made of two concentric corrugated 316L stainless steel pipes (C-pipe) with flanged terminations. Thermal insulation is achieved by maintaining vacuum between the inner and outer corrugated stainless steel pipes. In addition, the hose assembly contains two outer layers of helical armor wires to sustain the axial load. Given the complexity and novelty of the transfer system, a finite element study was performed on the inner C-pipe — the critical fluid containment layer. The effects of strain hardening of corrugations due to cold forming and temperature were modeled. Finite element (FE) analyses of the C-pipe under axial, bending, and internal pressure loading were carried out to evaluate global load-deformation and local stress responses. Comparisons of full-scale tests at room and cryogenic temperatures to simulation predictions including the novel material model showed good agreement. However, fatigue life predictions for the C-pipe that were based on local stresses and sheet metal fatigue S-N curves did not agree with the full-scale fatigue test results. The results indicated that the spatial variation in strain hardening due to corrugation forming and biaxial local stresses during pipe deformation could play important roles in the fatigue response of the C-pipe.