Research On Cross Sectional Mechanical Characteristics And Typical Failures Analyses Of Marine Unbonded Flexible Pipe

The development of offshore oil and gas is facing more challenges with the exploitation moving towards deep sea and ultra-deep sea.At the same time,it needs more professional and high-tech equipment to meet the challenges.As a typical high-tech offshore oil and gas exploitation equipment,more and more unbonded flexible pipes are used in the development of offshore oil and gas resources with its special structures and superior mechanical properties.Both academia and industry pay close attention to it since its appearance.The design,production and application of deep-water flexible pipes in China at present basically rely on imports,which is expensive and has a long supply cycle.That seriously restricts the development process of Chinese deepwater oil and gas resources.Therefore,it's important to carry out the mechanical characteristics analysis and typical failure research of flexible pipe to to realize the localization of flexible pipe production,break the technical blockade,and improve the R&D ability of marine equipment for China.At the same time,it can also provide the useful reference for the mechanical analysis and typical failure assessment of flexible pipes in terms of academic research.Based on a systematic summary of current research of flexible pipes,theoretical and numerical analysis were taken as the main research methods to study the cross-section mechanical properties,collapse failure and fatigue failure of flexible pipes.The main work is as follows.1)Theoretical analysis of cross sectional mechanical characteristics of flexible pipes.In terms of axisymmetric load,all layers of flexible pipe divide into polymer layers,interlocked metal armor layers and tensile armor layers according to the material and structural type of each layer.Based on the principle of energy,the balance equations of each layer under the axial tension,torque and internal and external pressure were derived.Then the overall balance equation of flexible pipe were assembled.As for the bending load,the mechanical characteristics of helical wires in tensile amor under no-slip and slip stage were analyzed.And the expressions of bending moment and bending stiffness of flexible pipe under different bending stages were given.The research can provide a theoretical basis for the establishment of simplified numerical model of flexible pipe and the analysis of cross sectional mechanical properties under different loads conditions.2)Development of simplified numerical model of flexible pipe and research on cross sectional mechanical properties under different loads.The 3D refined numerical models of carcass layer and pressure armor layer were established and the corresponding mechanical properties were analyzed under different loads.According to the principle of stiffness equivalence,a calculation method was proposed and verified,which equates the four layers in a flexible pipe(carcass layer,inner sheath layer,pressure armor layer and anti-friction layer)into a cylinder with orthotropic material properties.Considering the custom-designed fact of flexible pipe in industrial application and the research convenience,the parametric modeling codes of flexible pipe based on Python were developed to improve the modeling efficiency of flexible pipe.And the corresponding graphical user interface(GUI)were developed.Based on the model,study on cross sectional mechanical properties of flexible pipe under axisymmetric loads,bending loads and combined loads were carried out.Then the model was verified by comparing the the pipe equivalent stiffness and tensile armor layer helical wires strain value with experiment.The work can provide the reference for the mechanical properties analysis and optimization design of flexible pipes.3)Equivalent calculation of critical collapse pressure of flexible pipes and parametric analysis of collapse influence factors.An equivalent layer method was proposed in this work at first.The method is easy to calculate the equivalent thickness of carcass and get the critical collapse pressure.The real cross-section shape of carcass was considered in the method.In addition,the contact between the steel strips in the carcass layer and the contact between carcass and adjacent inner sheath layer were also considered.Then,the two-dimensional(2D)numerical collapse model including carcass and inner sheath layer was established based on the corresponding equivalent parameters.Next,results based on 2D model were compared with three-dimensional(3D)full solid model to verify the proposed equivalent layer method.Moreover,critical collapse pressures from different equivalent layer methods were discussed.Finally,based on the verified 2D model,the collapse parametric analysis of flexible pipes was carried out by arc length method.The collapse characteristics and post-buckling behavior of the flexible pipes under different initial imperfection,initial ovality,material elastoplasticity and constraints from external layers were analyzed.4)Global static and dynamic response analysis and fatigue life evaluation of flexible riser.The Global coupling analysis model including floating platform,mooring system and flexible riser was established by Sesam/sima.Then the line type and the distribution characteristics of tension and curvature along the pipe length of lazy wave flexible riser were analyzed.In addition,the influence of buoyancy factor,length and starting point position of buoyancy segment on pipeline types,tension and curvature distribution was also investigated.Acccording to the actual ocean parameters,considering the wave,current and upper platform motion,the nonlinear time-domain dynamic response of riser was calculated,and the load response time-history curves at the hot spots of the pipe were obtained.Combined with the proposed simplified numerical model of the flexible pipe,the stress response time-history curves of the tensile armor layer of flexible pipes were obtained.And then based on the S-N curve and Miner linear cumulative damage theory,fatigue life of different tensile armor layer in flexible riser was calculated.The research gives a flexible pipes fatigue life evaluation method and the correspongding results can support for the line type design and fatigue life evaluation of flexible risers.