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Slip initiation and progression in helical armouring layers of unbonded flexible pipes and its effect on pipe bending behaviour

Comparatively straightforward analytical formulations are given for determining the bending moment-curvature relationship for a helical layer in unbonded flexible pipes. The approach takes into account the non-linearity of the layer caused by sliding of individual helical elements between the surrounding layers. Bending stiffness of a helical layer is found to be a function of bending curvature, interlayer friction coefficients and interlayer contact pressures. The analysis presented here is based on the Coulomb friction model and is derived using the principle of virtual work. Theoretical results for a typical unbonded flexible pipe using the non-linear formulation for helical layers are compared with experimental data.