Gas permeation in flexible multilayer pipes: Multiphysics modelling and experimental validation

Flexible pipes used in oil and gas and emerging energy applications are subject to gas permeation through polymeric layers, leading to the accumulation of species such as CO 2 , CH 4 , H 2 O, and H 2 S in the annulus and potentially driving corrosion-related failures. Accurate prediction of annulus conditions is therefore critical for design and life assessment. This study presents a computationally efficient multiphysics modelling framework that integrates heat transfer, mass diffusion, fugacity-based thermodynamics, and pH-dependent chemical effects to predict gas transport and annulus chemistry in flexible multilayer pipes under variable pressure and temperature conditions. The framework is implemented using the Finite Difference Method (FDM) within a unified design tool that complies with API 17J (5th Edition) and enables integration with industrial design workflows. Validation is conducted through comparison with finite element analysis (FEA) models and against a subset of full-scale experimental tests representative of high-CO 2 environments, including pre-salt and carbon capture applications. The results show good agreement with experimental observations, supporting the modelling of non-ideal thermodynamics, phase behaviour, and chemical reactions governing annulus conditions.