Research On Calculation Method Of The Local Structure Behavior Of Unbonded Flexible Pipes

An unbonded flexible pipe is much complicated in structure, for which its local structure behavior is not only affected by the structural parameters of itself, but also affected by the load conditions. The objective of this research is to propose an approximate calculation method of the local structure behavior of unbonded flexible pipes.In this paper, multi-layer approach is adopted to model each layer of the pipe along with the interaction of them and then, summing up the contributions of all the layers in the cross-sectional properties. The conventional elastic thin-walled cylinder model is introduced simulate the inner fluid barrier layer, outer fluid barrier layer and anti-wear layers. In this case, the effects of the change of the thickness of these layers are also taken into account. The elastic helical steel strip model is adopted to simulate the carcass layer, pressure armor layer and tensile armor layers considering the contributions of EA, EI and GJ of the helical steel strip. When integrating the different layers into a composite beam model, these factors are also included that the radial deformations of all the layers are different, that layers separation occurs, that penetration doesn't happen, and that slip occurs between layers.Due to different deformation mechanisms of a flexible pipe under axisymmetric load (axial load and radial load) and bending load, the process of the specific modeling and analysis are divided into two phases. In the first phase, the deformations of the layers and pressure distribution between the layers under axisymmetric load are calculated, which are used as the initial pressure condition for the bending deformation. In the second phase, the hysteretic responses of the model applied cyclic bending loads are calculated.Based on the above-mentioned mechanical model, this paper presents a complete approximation calculation method of the axils stiffness, radial stiffness and bending stiffness of the cross-section of flexible pipe under axial load, radial load and bending load. The analytical formulation is coded in MATLAB (7.1) that is adopted to calculate a 63.2mm i.d. flexible pipe, and the program results are in great agreement with the experimental results.In the last part of the paper, the parameters that depend on the local structure behavior are studied with the above-mentioned program, and the influence of them on the local structure behavior are presented.