Modelling And Robust Design Of High Strength UOE Pipe Forming Process

Large diameter welded pipes used in oil and gas pipelines are primarily manufactured by the UOE process.With the increase of oil and gas exploitation in remote area under harsh conditions,UOE pipes with higher dimensional accuracy and mechanical properties are required to enhance the pressure capacity.Therefore,high strength grade pipeline steels(from X80 to X120)have been increasingly used in pipeline construction in recent years.However,due to severe springback,strong Bauschinger effect and high inhomogeneity and variation of material properties,application of high grade pipeline steels brings challenges to UOE forming process.In order to solve these challenges in quality control,e.g.O-forming gap,mechanical properties and ovality of UOE pipe,deeply research is carried out based on modelling,analysis and robust design of UOE forming: 1.Research on the constitutive model of high grade pipeline steelsThe steel plate experiences complicated deformation in UOE process,involving multiple loading-unloading-reverse loadings.In order to establish constitutive model of high grade pipeline steels,cyclic loading tests are performed on three kinds of pipeline steels,i.e.X70,X80 and X90,according to the deformation characteristics of plate in UOE process.The yielding and hardening properties of these three kinds of pipeline steels are analyzed.A modified Chaboche model that can describe Bauschinger effect,yield plateau and elastic modulus variation accurately is proposed.Succeeding studies on the modelling and robust design of UOE process are based on the proposed model.2.Research on modelling of UOE forming process and optimization of O-forming gapUOE forming process is modeled based on the proposed constitutive model and elastic-plastic theory.A semi-analytical method is proposed for accurate and efficient prediction on the geometric configuration and O-forming gap of steel plate after C-U-O-forming as well as mechanical properties of UOE pipe.In comparison with FEM,current method has evident advantage in the calculation efficiency but with the same accuracy level.Effects of process parameters on O-forming gap are numerically investigated.The radius of the lower part of U-mould and compressing ratio of O-forming are the two important factors to determine O-forming gap.The gap width is enlarged with the increasing of the above mentioned two factors.Parameter optimization for C-,U-and O-forming processes combining the semi-analytical model and genetic algorithm provides theoretical guidance for O-forming gap control.3.Research on mechanical properties evolution from plate to pipe in UOE processThe evolution of the yield strength and Ys/Uts ratio(ratio of yield strength to ultimate tensile strength)of high-strength pipeline steel in UOE forming and flattening process is investigated by the proposed semi-analytical model,and the influence of expansion ratio & t/D ratio on yield strength & Ys/Uts ratio is analyzed.The variation of strain history at different locations of UOE pipe causes uneven yield stress distribution through the cross section and this inhomogeneity is enlarged by the Bauschinger effect.In C-and U-forming,the yield strength of the steel plate decreases slightly.In O-forming stage,the tensile yield stress shows evident decreasing trend due to the circumferential compressing loading.Expansion increases and homogenizes the tensile yield stresses.Flattening operation can reduce yield strength of the specimens taken for measuring pipe strength.Ys/Uts ratio of UOE pipe is positively correlated with expansion ratio and t/D ratio.High expansion ratio increases the yield strength but Ys/Uts ratio is also enlarged at the same time.4.Research on robust design of UOE processConsidering the variation and inhomogeneity of material properties,the robust design method for UOE forming process is applied to resolve the problem of poor quality and low qualified rate.Material partition method is proposed to describe and quantify the random distribution of material properties.The robust optimization method for UOE forming process is established based on support vector regression and sequential response surface.The optimization method is verified by a subject of large diameter,high grade pipeline steel UOE pipe.The results show that the new method can reduce the ovality and increase the robustness of UOE forming process as well.Compared with deterministic optimal method,the robust optimization method increases qualified rate by 11.7%.