Global Analysis And Finite Element Simulation Of End Fitting Effect For Unbonded Flexible Risers

With the development of oil field enters deeper water, unbonded flexible risers are widely utilized in the current oil and gas exploitation projects, because of the advantages of flexibility, easy installation, good dynamic performance, corrosion and high pressure resistance. The stick-slip effect a flexible riser has results in the distinctive non-linear bending response and hysteresis effect. As for global analysis, current practice consists of assuming the global behaviour of the flexible riser to be linear. This leads to an over-estimate of the response and the fatigue damage in the riser, and the design not being as efficient as what it could be. In addition, end fitting is an essential component of flexible risers. In dynamic applications, large bending effects take place near the end fitting owing to the joining of rigid and flexible system elements. Therefore, stress concentrations and higher fatigue will result.The objectives of this thesis are to look for a suitable method for unbonded flexible risers' global analysis in the deep-sea oil production and to perform an investigation of behaviour in helically wound tensile armour wires on a flexible riser under the influence of an end fitting.There are many different forms of damping, or energy dissipation, experienced by a flexible riser in a subsea environment. Typical sources of damping are hydrodynamic drag, seabed friction, friction between layers in a composite structure and internal material damping. This thesis focuses on the hysteresis damping caused by friction between layers, which can be modeled by two methods:Equivalent Viscous Damping Method and Orcina Model Method.According to the modeling methods of hysteresis damping and bending moment-curvature relationships, three different constitutive models are investigated:(1) Linear bending moment-curvature relationship and Equivalent Viscous Damping Method model, EVDM-L Model; (2) Non-linear bending moment-curvature relationship and Equivalent Viscous Damping Method model, EVDM-N Model; (3) Non-linear bending moment-curvature relationship and Orcina Model Method model, OM Model. Comparing the dynamic response of the three, OM model is the most suitable method in global analysis for unbonded flexible risers.A finite element model is given to analyses of slip and stress in tensile armour wires at the end fitting under axial tension and uniform bending. FE results in the absence of friction are compared with theoretical results and encouraging correlations are found. End restraint could cause a significant stress increase in the armor wire at the end fitting vicinity and the response to curvature is linear. When riser is bent, the transverse bending stress is related to the wire polar angle. The maximum bending stress is found to occur at the compressive extreme fiber position. The transverse stress is decreased with the friction considered but increased with the friction coefficient increased. In addition, interlayer friction has little effect on axial slip but could be sufficient to prevent the lateral displacement of tensile armour wires.