Influence of Bore Pressure on the Creep Behaviour of Polymer Barrier Layer Inside an Unbonded Flexible Pipe

This paper investigates the influence of bore pressure, combined with the nonlinear behaviour of the polymer material, on the creep behaviour of the polymer barrier layer inside an unbonded flexible pipe. Creep behaviour in the barrier layer may result in its reduction in thickness and is therefore an important design consideration in ensuring the structural integrity of this layer. It is meaningful to study the variation in creep behaviour in an unbonded flexible pipe under different bore pressures and temperatures, especially in high pressure pipelines for deep or ultra-deep sea applications. Creep behaviour in polymer material is complex, as it is governed by a number of variables such as the stress/strain state, temperature, and pressure for example. It is generally time-dependent and often associated with larger strains or states of deformation. Owing to the complexity of polymer material creep, an implicit time hardening creep model, based on the Maxwell viscoelastic model, has been selected to represent the creep behaviour in polymer materials and implemented into the Gap Span model, which is an in-house ANSYS based finite element model. The coefficients of this creep model were initially calibrated according to standard creep tests performed on polymer materials. The study presented in this paper focuses on the influence of bore pressure and high temperature on the creep behaviour of the polymer barrier layer. Comparisons between the simulation results of the calibrated Gap Span creep model and the corresponding small-scale creep tests demonstrate that these model predictions are overly conservative for the polymer material of the barrier layer inside an unbonded flexible pipe. Comparisons between the experimental test results and the finite element modelling results show good correlation.