Research Of Mechanical Behavior Of Unbonded Flexible Risers

As human's growing demand of energy, deep-water exploration of oil and gas is an inevitable trend in the field of marine engineering. Unbonded flexible risers have been used widely in the field of exploration of offshore oil & gas. It can avoid the fatigue damage caused by VIV, and the high stress level as its excellent axial tensile and lateral bending performance.As for the numerical simulation of unbonded flexible risers, solid element can be usually used for high precision results and authenticity of mechanical behavior. But this method is very time-consuming. Based on the former achievement in research, this thesis proposed a simplified model composed of beam and shell element to conduct the analysis of unbonded flexible risers with consideration of key design parameters(section tensile stiffness of helical tape and its helix angle). Then a practicable design scheme of the unbonded flexible riser is established. The strength assessment under 100-year return period storm is conducted through OrcaFlex to check its validity.In local analysis this paper investigates the influence of section tensile stiffness of helical tape, the helix angle and inner pressure etc. on the mechanical property of riser under axial tension, bend moment and torsion. The conclusions can be drawn as follows:1) The tensile property, bending property and torsion property can all be influenced by section tensile stiffness of helical tape of helical armor layer. The axial tensile capacity are for the greatest, followed by the bending and torsion property. The bigger stiffness, the better performance. But the influence of stiffness will be reduced with its increment2) The helix angle of helical armor has a positive correlativity with resistance of axial tension and axial torsion, and negative correlativity with resistance of lateral bending. Optimal design should also be considered with axial rigidity and bend stiffness.3) Inner pressure has a not-so-remarkable contribution to the resistance of axial tension, lateral bending and axial torsion.A global analysis has also been conducted considering the upper floater and environmental load with OrcaFlex. It is observed that the hysteresis effect of flexible riser is the most evident around the touch-down zone, which fits the riser well with the severe environment, where the riser suffers from the most severe bending moment, and the hysteresis effect can also lower the stress level. And additional reinforcement should also be applied to this section Through the finite element analysis of flexible risers, this thesis assesses the effect of various parameters on the mechanical behavior, and can be the reference of design and research of unbonded flexible risers.