Axial Stiffness and Expansion Behaviour of a Flexible Flowline for Various Extents of Axial Restraint When Subject to Pressure and Thermal Loads

The conditions to which flexible pipes are being exposed are becoming increasingly severe. In particular, flexible flowlines that are fully or partially restrained by back fill or rock cover can become exposed to large axial compression forces under pressure and temperature loading. A high True axial compression force may lead to a radial outward displacement of the tensile armour which could eventually exceed the allowable limits of API spec 17J if the axial force exceeds a critical level. Furthermore, when the net Effective axial compressive pipe force exceeds a critical level, the pipe may undergo global buckling either upwards (upheaval buckling) or laterally, which needs to be addressed in detail at design stage, typically by using global finite element (FE) simulations in which equivalent mechanical properties are applied to pipe beam elements to capture the overall behaviour of the flexible pipe. The overall global mechanical properties of a flexible pipe, such as axial stiffness and thermal and pressure expansion coefficients, which are important parameters in capturing the global buckling behaviour, are dependent on the local behaviour of the various flexible pipe layers. In this paper an advanced local FE model developed using the industry standard numerical FE tool, Abaqus, together with the assessment methodology is presented in order to study local layer interactions and their influence on the equivalent axial mechanical properties.