Study On Cross-Sectional Mechanical Properties And Typical Failure Characteristics Of Unbonded Flexible Risers

Unbonded flexible risers consist of metallic and polymeric layers. Different layers have different geometric forms, material properties and functions, which ensure structural safety of the risers during installation and in use. Adjacent layers of unbonded flexible risers are not bounded; what is more, there are even gaps between them, so separation and relative slip can occur between adjacent layers when risers under external loads. Due to special structural forms and manufacturing process, unbonded flexible risers have large axial tensile stiffness and small bending stiffness, so they can withstand large platform and ship motion, and have been largely used in deep-sea oil and gas resources development. Meanwhile, structural complexity also brings challenges in characteristics analysis, such as hysteresis characteristic of helical strips under bending moment, collapse of carcass layer and pressure armor layer, radial and lateral buckling of tensile armor wires. Currently, cross-sectional mechanical properties and typical failure characteristics of unbonded flexible risers can not be fully grasped by international marine engineering, therefore, it is necessary to conduct in-depth research on such risers.Unbonded flexible risers are expensive offshore structures, so study on this kind of riser can not be completely depend on experimental method. In this paper, based on the latest international research trends of unbonded flexible risers, mechanical properties and typical failure characteristics of unbonded flexible risers are deeply studied by theoretical and numerical methods, and positive results are obtained. The main works of this dissertation are as follows:(1) Cross-sectional mechanical properties of unbonded flexible risers under axisymmetric loads and bending moment are predicted by theoretical and numerical methods. In the theoretical model, all layers are divided into cylindrical shells and helical strips. The influence of local bending and torsion of helical strips is also taken into consideration. Equilibrium equations of forces and displacements of layers under axisymmetric loads are deduced by energy method. Finally, combined with equilibrium equations of axial force and torsion, and relationship between adjacent layers, deformation of each layer and contact pressures between adjacent layers under axisymmetric loads are predicted. In the detailed three-dimensional numerical model, cross-sectional profiles and lay angles of carcass layer and pressure armor layer are included. Meanwhile, two numerical models of anti-friction tapes are developed, the former is the helix model which takes actual geometry into consideration, while the latter is the cylindrical model with sectional shape and lay angle ignored. To avoid convergence problem and excessive calculating time due to contact and geometric nonlinearities of the detailed numerical model, ABAQUS/Explicit quasi-static and mass scaling are adopted to conduct the analysis. An eight layer unbonded flexible riser is chosen for the case study, and cross-sectional mechanical properties of the riser under axisymmetric loads and bending moment are predicted by theoretical and numerical models, respectively. The effect of local bending and torsion of helical strips, models of anti-friction tapes, direction of load and boundary conditions are analyzed. By comparison with experimental results and results got from different organizations and scholars, the theoretical and numerical methods in this article are verified.(2) Collapse of carcass layer under uniform external pressure and crushing load are predicted by numerical method. The actual cross-sectional shape of carcass layer is considered in the numerical model. Carcass layer is simulated as helix model and cylinder model, separately, the former includes the lay angle of carcass layer, and the latter with lay angle ignored. Three contact conditions are chosen to simulate contact between adjacent surfaces of carcass layer: bonded coupling, coulomb friction with friction coefficient and frictionless. Effect of lay angle, contact conditions between adjacent surfaces, material nonlinearity and initial ovality on failure characteristics are analyzed. By comparing the numerical and theoretical results, scope of theoretical approach is analyzed.(3) Radial buckling of tensile armor wires under axial compressive load is predicted by numerical method. The inner sheath is simplified as connection elements with the same stiffness. Meanwhile, all tensile armor wires are simulated in the numerical model. Taking a 2.5 " unbonded flexible riser under axial compressive displacement as example, by comparing axial compressive stiffnesses got from the simplified numerical model, detailed numerical model and analytical model, the validity of the simplified numerical model is verified. Then a 6" unbonded flexible riser is chosen for the radial buckling analysis, and the impact of the number of tensile armor wires, friction coefficient and thickness of external polymeric layer on radial buckling of tensile armor wires are analyzed.