Analytical model for flexible pipe axial compressive stiffness in wet and dry annulus conditions

Flexible pipes may be subjected to axial compression during installation and operation, yet existing theoretical models predominantly focus on the prediction of critical loads associated with lateral buckling and birdcaging, with limited attention given to the axial compressive stiffness itself. This paper presents an analytical model for evaluating the axial compressive stiffness of unbonded flexible pipes under both wet and dry annulus conditions. The analytical predictions are validated against experimental data and numerical simulations, showing good agreement. A linear compressive response is observed under wet annulus conditions, whereas the presence of external hydrostatic pressure gives rise to a bilinear behavior in the dry annulus condition. Parametric studies are conducted to investigate the effects of key design parameters, including the tensile stiffness and lay angle of the anti-birdcage tape, as well as the lay angle, width, and thickness of the tensile armour wires. The proposed model offers a computationally efficient and physically insightful tool for the stiffness design and response assessment of flexible pipes under axial compressive loading.