Pipe-in-pipe (PIP) systems are proposed for platform risers subjected to extra high pressure high temperature (XHPHT) shut-in condition, to meet the flow assurance and stringent strength and thermal criteria, and to mitigate design issues associated with wet insulation application. To further satisfy the corrosive fluid environments, the inner pipe of the PIP system is metallurgically clad with a Corrosion Resistant Alloy (CRA). These complex design challenges require advanced numerical simulation to correctly capture the complex PIP behaviour and clad-pipe effects in order to avoid overly conservative design, and to provide a robust and optimised solution. The equivalence of CRA clad pipe was investigated and analytically deduced, especially on the thermal expansion behaviour under the XHPHT environments. An advanced numerical simulation based on Finite Element Analysis (FEA) was subsequently carried out. A systematic family of FE models was developed to meet the design complexity, namely: global PIP platform riser model to capture the global behaviour, local PIP centraliser model to address contact behaviour, local bulkhead design model for PIP bulkhead design and optimisation, local girth weld model to address mismatches (high-low misalignment, thickness and material strength). In addition, a modal analysis was conducted based on a PIP model to ensure that the analysis accounts for centralisers, pre-stress and deformation effects. The eigenvalue computing is then used for free span analysis. Due to lack of limit state design codes for pipe bends and the fact that the allowable stress criteria can be overly conservative, a bend collapse capacity deduced from FEA was applied in accordance with DNV local buckling criteria. The analysis procedures developed are outlined and a XHPHT PIP platform riser design is presented. This paper aims to provide a robust solution to aid design by the application of advanced numerical simulation.
Numerical Simulation of Material Strength Deterioration due to Pitting Corrosion