An Analytical Model for the Unbonded Flexible Pipe Stress Analysis With Consideration of Nonlinear Material Properties for Metal Layers

This paper presents an improved analytical model for the unbonded flexible pipe stress analysis with consideration of nonlinear material properties for metal layers. Analytical methods have often been used to analyse the stress and strain of flexible pipe systems because of their low cost and efficiency compared with detailed finite element modeling. Most of these kinds of models only consider the deformation of pipes within the elastic region. Such linear models can not be used directly to assess pipe failure modes such as the pipe burst strength, where the nonlinearity of the metallic material plays an important role in governing the pipe deformation and pipe structural capacity. The improved analytical model presented in this paper has fully considered the nonlinearity of metal layers such as the pressure armour and tensile armour layers because of their importance in resisting internal pressure and tension loads. Non-associative elasto-plastic stress strain curves obtained from experiments are used to simulate the metal layers. Von Mises stress is adopted in the model as the yield criterion of the metal layers. Radial return method (Simo and Taylor 1985 [1], Simo and Hughes 1998 [2]) is used to solve the plastic stress and strain of metal layers beyond the yield point. Due to its high nonlinearity from both system equations and material properties, Newton-Raphson method is adopted in the model as the solving method. The proposed study here considers tension, torque and pressure loads only for a straight pipe. The model predictions have been compared against measurements from Wellstream burst tests and failure tension tests performed over the full scale pipe samples. The prediction and experiment results agree.