Buckle Propagation Of Undersea, Thermal Insulation Composite Sandwich Pipelines

There are huge oil and gas resources buried undersea, mining processes are gradually from shallow to deep water. Compared to shallow water, deep sea has a more complex marine environment, lower temperature and bigger pressure. Temperature of Oil mined from the undersea is high, while that of outside the pipeline is low, leading to blocking caused by the solidification inside the pipe using single wall pipe. The greater pressure undersea put forward higher requirements on structure. To adapt to such a complex environment, the thermal insulating pipeline is manufactured with inner tube and outer tubes made of alloy steel materials to withstand load, in between a sandwich core made of polyurethane for insulation. Among many failure modes the most worthy one is the buckling and buckle propagation of the insulating pipe. Current research on buckle propagation of insulation pipes focus primarily on single-wall pipe, while few studies of insulated pipelines are published. In this thesis the insulation pipes are simplified as sandwich cylindrical shells. Buckle propagation problem are analyzed using nonlinear shell theory, theory of plasticity, stability theory. Experiment investigation and finite-element analysis are also carried out. The main content concludesThe nonlinear equations for thermal insulation composite sandwich pipelines are establish. A buckle propagation pipeline consists of three areas: collapsed area, transition area and undistorted area. The transition area will spread along the pipeline on both sides if the external pressure is greater that a critical value. First-order shear deformation theory and rotation angle are introduced to establish the nonlinear geometric equations. The differential equations of equilibrium and the compatibility equation are set up by analyzing the deformation mechanism.The formulas to calculate the buckle propagation pressure is derived. The displacement field of the transition area in an insulation pipeline obtained in previous study is introduced to analyze derive the strain field. Assuming linear elastic linear hardening material properties to consider strain hardening effect, the stress distributions in the transition area are obtained using deformation theory. Plastic stable theory is used to establish energy formula. According to energy conservation the analytical formulas for buckling propagation pressure of insulated pipelines are obtained. Parametric analyses are conducted to investigate the effect of radius-thickness ratio, face-sheet- core thickness ratio on strengthen factor, yielding factor and buckle propagation performances.Experimental analysis is performed. High-pressure vessels and other equipment are set up to carry out buckling propagation experiments. Two sets of pipes with diameter-to-thickness ratio of 17, 32 are used. 810 Material Test System(MTS) is used to perform the tensile test on the specimens cut from the pipe and to obtain the material properties. Non-linear Ramberg-Osgood model is applied to fit the stress-strain curve and to acquire the stress-strain relationship of the pipeline.Finite element simulation is conducted. Finite element model of an insulation sandwich pipe is built in ABAQUS software. Buckling propagation pressures of insulating pipelines as well as the propagation processes are obtained. Parametric analyses are conducted to study the effects of initial imperfections, diameter to thickness ratio, thickness of sandwich panels, young's modulus, shear modulus, yield stress, etc., on the buckling propagation pressure. Finally, results of the buckling propagation pressure values from FEA and from theoretical analysis are compared with each other and are found in good agreement with each other.