Theoretical Analysis And Numerical Simulation On Collapse For Marine Unbonded Flexible Pipe

Unbonded flexible pipe, which is made of the combination of metal material layer and polymeric material layer in the form of not binding, is an important equipment in the exploitation of offshore oil and gas, it allows the pipe sliding between layer and layer. Compared to other forms of marine pipelines, flexible tube structure have better mechanical properties, which are more suitable for deep sea oil and gas development. Today, offshore oil development is gradually moving towards deep sea, where the marine environment is more complex, so the unbonded flexible pipe may withstand excessive hydrostatic pressure loads which can cause structural damage. Because of the spiral winding characteristic of armored steel wires, it may also sustain excessive tensile load, and the tensile armor layers will produce excessive radial pressure acting on the skeleton layer, resulting in rupture of the tube body. These are called unbonded flexible pipe crush failure.In practical engineering, due to the manufacturing defects, the role of tensioner and other factors, there is inevitably an initial pipe ovality in the skeleton layer, and it is difficult to achieve a perfect circle state, while the skeleton layer is mainly made of steel, the material nonlinear characteristic of steel may have an impact on its ability to resist collapse, and the adjacent sliding of carcass layer surfaces will have some impact on the strength of the flexible pipe structures. However, the existing theoretical approaches usually assume no relative sliding between adjacent surfaces, and ignoring the effect of the initial ovality and material nonlinear characteristic on the crushing failure, making the results when applied to the actual project has certain bias.In response to these shortcomings, this paper describes a detailed summary of the structure and characteristics of the skeleton layer, and the crushing failure mechanisms, crush design requirements in industry design specification and the theoretical crush model of unbonded flexible pipe. On this basis, this paper uses the finite element method building model of an 8-inch diameter skeleton layer in three contact ways, and the radial displacement values and critical crush values under perfect circle state, the existence of initial geometric imperfections ovality state and consider the material inelastic state three case are calculated, comparing the results with theoretical calculations, and researching unbonded flexible pipe crush behavior, drawing appropriate conclusions, which can provide a certain significance to unbonded flexible pipe pressure collapse analysis.