Numerical study on the X80 UOE pipe forming process
Introduction
The UOE process is the most effective approach for manufacturing the longitudinal submerged arc welded pipes with large diameter, thick wall and high strength. UOE pipes have been widely used in oil and gas pipelines. The whole UOE forming process consists of a series of forming stages, including crimping, U-forming, O-forming, joining, welding, and expanding, as illustrated in Fig. 1. In the crimping stage shown in Fig. 1(a), the two longitudinal edges of plate are trimmed to obtain appropriate welding bevels, and then bent by using two involute-shaped tools, i.e., C-punch and C-die. The crimping length is determined by the horizontal position of C-die (Cd). The position of C-punch is determined by the tool offset between C-die and C-punch (Hd). Then the crimped plate is transferred to the U-press (Fig. 1(b)), where the plate is bent through moving the U-punch down. After the U-punch stops at the location of V-anvil (Vd), the side rollers move inward along the horizontal direction. The plate is deformed into a U-shaped profile in U-forming stage by adjusting the initial spacing (Rd) and movement of side rollers (RS). In the O-forming stage (Fig. 1(c)), the U-shaped plate is pressed into a pipe with an open-seam by using two semi-circular tools. Subsequently, the two ends of the open-seam are joined and welded to obtain a welded pipe (Fig. 1(d)). Open-seam is a key factor for product quality control, as well as an intermediate processing indicator in practical UOE manufacturing. In order to implement joining and welding, open-seam should be in a reasonable range. If an open-seam is too small, it is difficult to be cleaned and will result in inferior welding quality; an excessively large open-seam, on the other hand, will bring about much more difficulty to joining, sometimes even leads to crack of welding rollers. Finally, the welded pipe is expanded by a mechanical expander to acquire the desired pipe size (Fig. 1(e)).
Since UOE pipes manufacturing involves multiple forming procedures, the deformation of plate in a previous step can exert influence on subsequent steps. The traditional “trial and error” approach for process design could hardly quantitatively unveil the linkage of these operations, and leads to high expense of developing time and cost. Therefore, science-based analysis and design are essential for manufacturing high quality pipes.
Numerical simulation provides a scientific tool to study plate deformation and solve technological problems in pipe manufacturing operations. Kyriakides et al. (1991) evaluated the effect of UOE processing on collapse performance of pipes through experimental and numerical methods. Their study concluded that expansion process could significantly reduce collapse pressure. Huang and Leu (1995) developed an elastic-plastic incremental finite element code based on plane strain theory, and simulated the U- and O-bending processes. The influence of processing factors in U- and O-bending on plate geometries was investigated. Wen et al. (2001) simulated the welding process and investigated the heat transfer characteristics of heat affect zones. Palumbo and Tricarico (2005) analyzed the C–U–O forming and expansion steps by using both 2D and 3D finite element models. They found that the final circular shape not only depended on O-forming process parameters, but also was related to crimping operation. In addition, the 3D simulation highlighted that the pipe end profile during expansion stage was affected by the movement of expansion tools. Liu et al. (2007) discussed the deformation of a steel plate and its curvature variation in O-forming stage. The springback after unloading forming tools was not taken into account in their study. Herynk et al. (2007) investigated the influences of different process parameters on pipe quality through simulation. Their study showed that increase of expansion ratio could improve pipe ovality, but after the expansion ratio reaches a threshold, it had little influence on ovality. Raffo et al. (2007) developed a 2D FEM model to simulate the UOE forming process and analyzed the effect of strain hardening on pipe shape. Their study indicated that smaller hardening modulus could result in more uneven distribution of curvature along the circumferential direction. Han and Sun (2010) established a FEM model to simulate the UOE forming process by using MSC. Marc. Their study showed that the ovality of pipe was greatly improved as the expansion ratio increases from 0% to 0.5%. However, there was no significant difference in ovality when the expansion ratio exceeded 0.5%. Roza et al. (2008) analyzed the production process of a UOE pipe via a finite element model to evaluate the pipe mill's capability. Fan et al. (2012) discussed the effect of crimping parameters on the crimping operation by 2D finite element simulation. As the base radius of C-punch increased, the curvature of the crimped section increased monotonically and the length of non-deformed region decreased dramatically.
In the previous studies on the finite element simulation of UOE pipes forming process, the steels of grade X70 and below are concerned. Nowadays, development of oil and gas transportation over long distances requires the usage of high-grade steels whose mechanical properties can bear substantial enhancement of the internal pressure (Tanguy et al., 2008). Considering the increasing application of high strength grades X80–X100 and induced larger springback and forming load, study on the UOE forming process of high strength steels is of great interest for expanding the knowledge in this field. Moreover, although open-seam practically is a key factor in UOE pipe manufacturing, so far there have been few studies focusing on it.
As for the finite element model, 3D models, theoretically, can simulate plastic forming processes more realistically than 2D models since they can take into account more details when setting boundary conditions. However, because of the huge size of UOE pipes, 3D simulation is extremely time-consuming and cannot satisfy the fast-response requirement in practical process design. Moreover, 3D simulation with implicit algorithm is not as robust as 2D simulation. It is often disrupted or stopped because convergence is hard to be continuously achieved when calculating contact problems.
In this study, a two-dimensional finite element model based on an implicit algorithm is established to simulate the whole forming process of an X80 UOE pipe, in which a kinematic hardening model is incorporated to describe the Bauschinger effect of the steel plates. The elastic–plastic deformation behaviors of an X80 steel plate in all the UOE forming stages are numerically studied step by step. The deformed configurations as well as the forming force in each stage are obtained. Specifically, the bending and reverse bending behaviors of plate in O-forming stage are investigated and the formation mechanism of open-seam is revealed. Good agreement between the simulation results and practical measurement demonstrates the validity of the model established. Furthermore, a parametric study of UOE forming is conducted to assess the effects of forming parameters, friction coefficient and material properties on open-seam and pipe ovality.
Section snippets
Finite element model
The 2D finite element model of the whole UOE forming process is established under the hypothesis of plane strain condition by ABAQUS software, as shown in Fig. 2. The nominal size of pipe is 1016 mm (outer diameter)*22.8 mm (thickness)*12,000 mm (length). The plate is X80 steel and the thickness is 23 mm. The forming parameters mentioned in Section 1 are listed in Table 1. Only a half of plate is modeled due to the symmetry of geometry, boundary and load conditions. The plate is discretized by
Crimping
During the crimping operation shown in Fig. 5, the plate is clamped by two blocks, and then crimped by moving C-punch upwards while keeping C-die stationary. As the involute-shaped C-punch moves upwards, the curvature of the bent section increases gradually. Along the thickness direction, the top surface is under compression while the bottom surface is subjected to tension. The equivalent plastic strain (PEEQ) distributes symmetrically about the neutral layer in the thickness direction. The
Parametric study of UOE forming
In this section, the UOE forming processes under different processing and material parameters are calculated with the validated finite element model, and the influence of these parameters on two forming quality indicators, open-seam and ovality, are numerically investigated.
Conclusions
In this study, a finite element model for the whole UOE pipe forming process is established. A kinematic hardening model is incorporated to describe the stress–strain response of X80 steel under loading–reverse loading deformation conditions. The deformation behaviors of an X80 plate in all the UOE forming stages are numerically analyzed. A parametric study of UOE forming is performed to assess the influence of forming parameters, friction condition, and material properties on open-seam and
Acknowledgments
Authors would like to acknowledge the support from Baosteel. In addition, authors thank Dr. Jianzeng Han, Engineers Xiaoxiu Wang, Xinwen Li and Yun Guo for their help in the experimental work.
Authors also would like to acknowledge the support of Major Projects of the Ministry of Education of China (no. 311017).
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2017, International Journal of Pressure Vessels and PipingCitation Excerpt :Nevertheless, to ease the reader's understanding, instead of a semi-elliptical crack in a pipe, we have considered a semi-elliptic surface flaw at the center of the flat plate (as depicted in Fig. 2) and evaluated the SIF by using three different methods. The aforementioned approximation considered by the authors is supported by the fact that most of the large diameter pipelines used in the O&G sector are manufactured from flat plates using the UOE forming process [43], therefore the SIF solutions for plates may be utilized to approximate the solution for pipes by introducing an appropriate bulging factor [29]. In other words flat plate solution is a good approximation for large diameter pipelines (i.e. thin-walled cylinders) as the hoop stress is constant in the aforementioned asset.