Static Analysis Of Flexible Risers

Unbonded flexible risers are being increasingly used in the oil. The main advantage of unbonded flexible risers with respect to other risers is they can endure large deformations under loads induced by the sea current, vortex-induced vibrations, the motion of the floating-vessel and those seen during installation. However, the response of unbonded flexible risers is extremely difficult to analyze due to their complex structure. In order To capture the many important aspects of the structural response, including the energy dissipation due to frictional slip between layers, the hysteretic response and the fatigue damage, efficient and accurate tools are required.This paper presents the total stiffness matrix of an unbonded flexible riser based on the plane-stress assumption. Antiwear layers and sheath layers can be modeled as an isotropic layer in plane-stress. The structure of the carcass layer causes the stiffness in the axial direction to be totally different compared to the circumferential one so that an orthotropic model is a reasonable choice. The total stiffness matrix of the whole model is founded by summing the stiffness matrix contributions of the isotropic layers, the carcass layer, and those of the helical armor layers.But analytical model ignores effects of friction which is the main reason of the unlinearized response of unbonded flexible riser.The finite element model of the riser is developed using ABAQUS for friction between layers. The elements used in the model are of eight-noded linear brick type with reduced integration and hourglass control for the sheath, antiwear, and the helical armor layers and four node doubly curved thin shell type with reduced integration and hourglass control for the carcass layer.3D contact interaction is introduced between all layers so that the layers are allowed to slide with respect to each other during all stages of loading. Contact is simulated using a penalty contact method based on Coulomb friction model.Comparison is made between the analytical and the numerical results for various load cases that include axial, torsional, and bending loads in this paper. Comparison is made between the result of the finite element model without friction between layers and the result of the finite element model with friction between layers. At last, the influence of friction between layers to bending moment-curvature curves is discussed.