Investigation On Analysis And Design Of Radial Pressure Resistance Of Unbonded Flexible Pipe Structure

With the gradual depletion of terrestrial energy and the rapid growth of the energydemand,the extraction of offshore oil and gas resources has gradually attracted much attention from industries.Unbonded flexible pipe is an important transportation equipment for offshore hydrocarbon resources exploitation,which is composed of metal layers and non-metal layers easily sliding between each other.There will be a variety of failure modes such as crushing and bursting caused by radial load during storage,transportation,installation and incumbent operation,which will cause huge economic losses.Therefore,it is necessary to design and analyze the radial pressure resistance structures of unbonded flexible pipes.Firstly,the structures and failure modes of the unbonded flexible pipe and the research status of the radial pressure resistance structures done by scholars both at home and abroad are summarized systematically.The radial load can be divided into two forms: external pressure and internal pressure.In terms of external pressure behavior,based on the bending stiffness section equivalent method and the theoretical model of the cross-sectional compression on the thin-walled cylinder,a theoretical prediction method for the critical cross-sectional compression on the carcass is presented.Moreover,a three-dimensional finite element model is established,and the results show that theoretical results are more dangerous than the numerical results because of neglecting the change of inertia moment during the loading process,which verifies the feasibility of the theoretical methodology.It is also found that the carcass evolves into the deformation mechanism of four plastic hinge under the compression.Then,the entire finite element model of the pipe section is further presented,and the contribution of the different structural layer to the of compression resistance capability is discussed.The results show that the contribution ratio of the carcass is only 40%.The pressure armor and the tensile armor play important roles in the radial compression resistance of the pipe.The traditional design method of the compression resistance capability of the unbonded flexible pipe during installation will be conservative,if only considered the carcass.In the aspect of internal pressure,the theoretical prediction methods of internal pressure failure of unbonded flexible pipes are investigated,and the applicability of the theoretical methods are discussed through the three-dimensional simulation.It is found that all the resultsobtained from the theoretical method are dangerous.The theoretical method which combines the "filling factor" section equivalent with the elastic mechanics theory is the most conservative.Subsequently,the influence of machining gap on the pressure resistance capability of the structure is further discussed.A theoretical method for calculating the critical inner pressure resistance capability of the pressure armor considering gaps is derived.The theoretical method is verified by the finite element model with different gaps.The theoretical and numerical results match well.It is found that the machining gap between layers will significantly reduce the pressure resistance capability of the structure,which provides a useful reference for the design of pressure resistance structure of unbonded flexible pipes.