Research On Three-roll Continuous Straightening Process For LSAW Pipes

As the development of oil and natural gas in the world has gradually transferred to unconventional resources,the remote and poor development environment has put forward higher requirements for pipeline construction.Longitudinally Submerged Arc Welding(LSAW)pipes are made of high quality wide plates,supplemented by reasonable forming processes and welding methods.They are particularly suitable for long-distance oil and gas pipeline transportation and piping requirements in cold or deep seabed.Due to the influence of forming equipment and welding thermal stress,straightening is a necessary process in the production of LSAW pipes.At present,the manufacturers still use the traditional three-point mould straightening method with “positioning by taut wire,loading by experience”.Due to lack of theoretical guidance,the accuracy of straightening is low and efficiency is difficult to guarantee.Due to lack of theoretical guidance,the accuracy of straightening is low and efficiency is difficult to guarantee.Therefore,the market demand is in urgent need of the technical innovation to improve product quality and production efficiency,and then improve the quality of Chinese manufacturing.For the problem that straightening accuracy in traditional processes is highly dependent on operator experience,this paper firstly proposes the over-bend straightening theory of plane curved beam with symmetrical section based on springback equation of small curvature plane bending previously developed,and the essence of the cornering straightening process is revealed,that is,the loading process of the theoretical straightening moment or the realizing process of the theoretical straightening curvature base on the over-bend straightening theory.Given this,the straightening mechanism of the existing three-point multi-step mould bending and the multi-point one-off mould bending is revealed.Since the straightening process has not achieved the complete loading of theoretical straightening moments,a new three-roll continuous straightening process for LSAW pipes has been proposed.According to the annular section characteristics of the pipe,the calculation formula of the theoretical straightening moment distribution required for the straightening of the plane bending pipe based on the initial curvature distribution isgiven,which provides theoretical guidance for the calculation of the straightening load of the new process.Based on the three-roll continuous straightening process and its device characteristics,a deflection detection method based on a laser displacement sensor is proposed,accompanied by a global deflection calculation model related to local deflection.The experimental study on deflection detection of curved pipes by the sensor and CMM shows that the overall deflection error obtained by the new technique is less than 3.50%,which can meet the engineering application requirements,and the new deflection detection method is particularly suitable for on-line detection and off-line inspection of the straightness of a large plane-bending pipe with a length exceeding 10 m.In order to accurately calculate the straightness,a piecewise curve fitting algorithm with constraints is established,and the an ideal order of the fitting polynomial of the middle curve segment is determined to be 10-th.In order to improve the prediction accuracy of the theoretical straightening moment by limiting the cross section deformation,the cross-section distortion in the straightening process of the thin-walled curved pipe is studied.The prediction model for cross-section distortion during the elastic-plastic bending process of curved beams is established,and the four-point bending of the pipes with different initial curvature and relative thickness is performed numerically and physically,which verified the reliability and applicability of the prediction model.According to the prediction model,it is recommended that the straightening roller should follow the principle of equal-diameter pass design,that is,the hole diameter should be equal to or slightly larger than the outer diameter of the pipe to be straightened.For the case of LSAW pipe produced by the company,four curvature calculation models are established,and the principles for determining the roll shape and span are analyzed.Based on the numerical simulation technology,the bending and rolling models are established.Through the analysis of straightness and section ovality of the pipe after straightening,the principle reliability of equal-diameter pass design is verified,the empirical formula of span determination is given,and the rationality of the calculation models based on low-order simple-fitting curvature and local homogenization curvature isconfirmed.The three-roll continuous straightening finite element analysis of the welded pipe example shows that,the straightness of the pipe with reasonable process parameters can be controlled within 1.5‰,and the ovality of the section is less than 0.5%,both meeting the standard requirements.Finally,a three-roll continuous straightening experiment system for LSAW pipes,including a mechanical system,an electro-hydraulic control system,and a detection device,is established,which can simultaneously realize the deflection detection and continuous straightening.Through the numerical simulation and physical experiments of the continuous straightening process of curved pipes with multi specification,the low-order simple-fitting curvature calculation model is determined as the optimal initial curvature calculation model,and the straightness of the experimental pipes is corrected within1.50‰,which proved the feasibility and reliability of the new approach.Based on this,the overall deflection control strategy based on the initial curvature distribution is further established,and the straightening process analysis software is developed,which laid the foundation for realizing the intelligent control.