A novel helix contact model for predicting hysteretic behavior of unbonded flexible pipes

This paper presents novel equivalent layered contact numerical and analytical models to predict the bending hysteretic behavior of unbonded flexible pipes. A double helix contact beam model, instead of surface-to-surface contact algorithm, is proposed to simulate normal contact and tangential friction behavior between tensile armor tendons and supporting layers. The spatial beam distance is evaluated based on Hertz contact theory. The proposed model is validated through comparison with bending moment-curvature relations obtained from experimental data, a full layered finite element model, and existing analytical models considering or not shear interactions. The shear stiffness is determined by accounting steel armor layers as boundary conditions. The initial contact pressure is introduced into analytical models by accounting residual stress in cylindrical layers and residual strain in helical layers. The bending stresses and relative slip displacements of tensile armor tendons are cross validated between the proposed model and available data. The results show that the proposed model could give consistent bending hysteretic consequences, while shorten simulation time due to significantly reduction of total elements. Sensitivity studies including definition of helix contact beams, equivalent material properties of inner core layer and tangential stick stiffness between layers are performed to validate feasibility of the proposed model.