A general model to predict torsion and curvature increments of tensile armors in unbonded flexible pipes

Abstract

An analytical model is proposed to investigate the local torsion and bending behavior of tensile armors in unbonded flexible pipes. The general expressions of torsion and curvature increments, including the effects of axial strain and twisting rotation of tendon cross-section, are firstly derived based on the generalized Frenet–Serret equations. Then the expressions corresponding to the strained helix assumption are obtained, which are further simplified by ignoring the twisting rotation. Afterwards, explicit expressions are given for axisymmetric case and tendon stick and slip states when bending is included. The developed model is finally verified with a finite element simulation. The results show that the model prediction of normal curvature increment is in good agreement with the numerical solution while the changes in torsion and transverse curvature cannot be obtained accurately through simple analytical formulas. Several earlier models are also discussed briefly and are demonstrated to be special cases of the present general model.