Analysis On Mechanical Properties Of Fibre Glass Reinforced Flexible Pipes

Subsea pipelines are widely used in the exploitation for marine oil and gas resources.Fibre glass reinforced flexible pipe is a new kind of submarine flexible pipe with good performances.Fibre glass reinforced flexible pipes(FGRFP)are excellent corrosion resistance,endurable in high pressure,excellent flexibility,lower production costs and easy to transport and installation.The above characteristics make FGRFP have better industrial application potential and development value in emerging field of submarine pipelines research and development.During the transportation,installation and subsea service period,the basic load conditions of FGRFP include external pressure,bending,tensile load,internal pressure and torsion.The try-production of FGRFP is carried out through experience and there are no corresponding theoretical analysis on the mechanical properties of FGRFP.In summary,the paper systematically studies and validates the mechanical characteristics and failure mechanism of FGRFP under the basic load conditions.The main research contents of this paper are as follows:(1)Based on the nonlinear buckling theory and virtual work principle,the paper introduces the ovality as the initial imperfection and establishes mechanics equilibrium equations which are solved by Newton-Raphson under external pressure,considering the elastic-plastic of materials and cross-section deformation.The paper also builds finite element model of FGRFP and designs external pressure test.The results show that the analytic results,the FEM results and the experimental results are in good agreement with each other,verifying the feasibility of analytic solution and FEM.On the basis,the buckling mechanism of FGRFP are discussed through the initial imperfection,geometric characteristics and material properties,which provides a reference for the section design of FGRFP.(2)Based on the nonlinear buckling theory and virtual work principle,the paper simplifies the infinite FGRFP into a two-dimensional plane model,introduces the ovality as the initial imperfection and establishes mechanics equilibrium equations under bending at any loading time,considering the elastic-plastic of materials.The paper also builds finite element model of FGRFP and designs four-points bending test.The results show that the analytic results,the FEM results and the experimental results are in good agreement with each other,verifying the feasibility of analytic solution and FEM.On the basis,the buckling mechanism of FGRFP are discussed through the ovality and the diameter to thickness ratio.(3)A new method for transforming the fibre glass of reinforced layers into the helically armored cables based on the Knapp's theory about tension and torsion is proposed in this paper.The material nonlinearity,cross section deformation and cable deformation are taken into account and the equilibrium equations are obtained.According to the necessary assumptions,the finite element model is established and stress variety of fibre glass are analyzed emphatically.The load-displacement curve is obtained by tensile test.The analytic results,the FEM results and the test results are compared to analyze the relative error source.The winding angles,the diameter to thickness ratio and the thickness of the inner and outer layers of FGRFP are analyzed and the mechanical properties of FGRFP are obtained.(4)Based on the anisotropy of fibre glass and the uniform continuous characteristics of FGRFP,it is proposed that FGRFP is divided into inner layer,outer layer and fibre glass reinforced layers.A corresponding elastic mechanical model is established and solved by numerical method to get the variation of stress and the burst pressure under internal pressure.The three-dimensional numerical simulation method of FGRFP is discussed.The short-term burst pressure test verifies the analytic solution and FEM.The analysis of sensitivity such as winding angles,the diameter to thickness ratio and the number of reinforced layers of FGRFP are carried out.(5)Based on the anisotropy of fibre glass and the constitutional characteristics of FGRFP,it is proposed a elastic mechanical model which is solved by numerical method to get the ultimate torque under torsion.Aiming at the material composition of different layers of FGRFP,the finite element model is established.The torsion test verify the analytic solution and FEM.Analyze the effects of different winding angles,the diameter to thickness ratio and the number of reinforced layers on the mechanical properties of FGRFP.