Mechanical Behavior And Optimum Design Of Ocean Flexible Pipes

Flexible pipes are extensively used for a variety of applications in the petroleum industry, such as the pipelines for the static and the marine risers for the dynamic.They link the satellite wells to central manifolds and bring oil and natural gas to the surface.In the deep-sea applications,because the bending stiffness when compared to the axial and the torsional stiffness is low,flexible pipes can suffer large displacements and deformations which make them to be the focuses.But,usually the flexible pipe is designed to meet the special requirements of users and many important parts as patent-protected products are belong to a few foreign enterprises.To promote the flexible pipe to be home-made,in this paper,the ANSYS Parametric Design Language(APDL) is used to model and analysis the influence of the dimension of main layers to the distribution of all stress.ANSYS Parametric Design Language(APDL) is the advanced analysis technology of ANSYS.All processes with parameters,such as modelling,applying load,solution and postprocessor,would be realized by APDL.In the progress of parametric finite element analysis,the user can directly modify the corresponding parameter of model to obtain all kinds of items with different dimension,loading and constrain.In this paper,program and atomization of modelling are realized based on the APDL.The finite element models of typical five-layer bonded flexible pipe,four-layer unbonded flexible pipe and five-layer unbonded flexible pipe are modelled and their dimension of main layers and working conditions are discussed.The validity of the two unbounded flexible pipe finite element model is verified by the uniaxial tension test.The conclusions based on the analysis of the 3-D finite element from this paper are shown as follows:1) For the typical five-layer flexible pipe bearing the sole internal or external pressure and the elastic strain territory,the radial stress of each layer is continued.When internal pressure, the radial stress of the most external layer is much smaller than other ones and the hoop stress of the helical armour layer is much bigger than others.But the distributions of stress for each layer are nearly the same with different helical angle.2) For the typical five-layer flexible pipe bearing the sole internal or external pressure and the elastic strain territory,with the increase of the thickness of tendon,the radial stress decrease slightly,but the hoop stress remarkable.So the design should consider the actual ocean condition and working demanding to find a suitable thickness of helical armour to ensure the highest flexibility.3) For the typical five-layer flexible pipe bearing the bending moment and the elastic strain territory,with the increase of helical angle,the average axial stress of the most two layers is decreasing and the inner three layers increasing.4) For the unbonded flexible pipe,when the loading is tension and the strain is elastic,the total stiffness of No.â… is nearly equal to the stiffness of most inner layer,and the total stiffness of No.â…¡is much bigger than No.â…¡.For the No.â…¡,the total stiffness is concerned with the helical angle of steel wires,and the minimum stiffness occurs when the angle is about 50 degree. Furthermore,the total stiffness can be enhanced by increasing the thickness of steel strip,the number of steel wires or the equal E_y of steel strip.