Proceedings of the 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway
OMAE2017-61540 NUMERICAL SIMULATION OF JCO PIPE FORMING PROCESS AND ITS EFFECT ON THE EXTERNAL PRESSURE CAPACITY OF THE PIPE Giannoula Chatzopoulou Department of Mechanical Engineering University of Thessaly, Volos, Greece email:
[email protected]
Konstantinos Antoniou Department of Mechanical Engineering University of Thessaly, Volos, Greece
Spyros A. Karamanos Department of Mechanical Engineering University of Thessaly, Volos, Greece School of Engineering The University of Edinburgh, Scotland, UK ABSTRACT Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to external pressure, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. The resistance of offshore pipes against this instability mode strongly depends on imperfections and residual stresses introduced by the line pipe manufacturing process. In the present paper, the JCO pipe manufacturing process, a commonly adopted process for producing large-diameter pipes of significant thickness, is examined. The study examines the effect of JCO line pipe manufacturing process on the structural response and resistance of offshore pipes during the installation process using nonlinear finite element simulation tools. At first, the cold bending induced by the JCO process is simulated rigorously, and subsequently, the application of external pressure is modeled until structural instability is detected. For the simulation of the JCO manufacturing process and the structured response of the pipe a two dimensional generalized plane strain model is used. Furthermore, a numerical analysis is also conducted on the effects of line pipe expansion on the structural capacity of the JCO pipe. 1
INTRODUCTION
The “JCO” or “JCO-E” manufacturing process is a commonly adopted method for manufacturing thick-walled steel line pipes for deep water pipeline applications. It consists of four sequential mechanical steps, shown schematically in Fig. 1,
which are the crimping of the plate edges, the J phase where the plate is formed into a J-shape, the C phase, where the deformed plate is pressed into a quasi-round shape, O phase, where the deformed plate is pressed into a round shape, and both ends of the plate are welded, and finally, the plates undergoes the expansion phase (E: expansion), where the “circularity” of the pipe is improved through a mechanical expander. In contrast with the UOE pipe, which has been studied extensively [1] - [7] there exist very few publications on the JCO-E manufacturing process. Chandel et al. [8] examined the JCO-E manufacturing process and concluded that the dimensions of the tools/dies at every station of line pipe forming play a critical role in forming a line pipe. Furthermore, Gao et al. [9] examined numerically the effect of punch pressing on final geometry. Krishnan and Baker [10] examined the JCO-E and the JCO-C (C: compression) manufacturing process and their effect on structural performance. The effect of compressing a JCO pipe at the final step (JCO-C) on material properties of the formed pipe and the collapse pressure investigated by Reichel et al. [11]. In the present study, the JCO cold-forming process and the mechanical behavior of a JCO line pipe candidate for deep offshore pipeline applications under combined loading conditions are simulated using a rigorous finite element model. The pipe is made of X70 steel material, with nominal diameter equal to 609.6 mm (24 in) and plate thickness equal to 32.33 mm (1.273 in). The material and some geometric characteristics of the pipe, as well as the forming parameters are similar to those reported in [5]. Using the present simulation, initial imperfections, residual stresses and material anisotropy of the line pipe at the end of the JCO-E manufacturing process are
1
rigorously predicted. The initial imperfections refer to both outof-roundness and variation of thickness around the pipe circumference. Following the simulation of the cold-forming process, the analysis proceeds in simulating the mechanical behavior of the line pipe under external pressure to determine the ultimate pressure sustained by the pipe.
crimping C-shape J-shape
to the circular shape configuration of the pipe, restraining outof-plane displacements, as well as the subsequent application of pressure in order to examine the structural behavior of the JCOE pipe during deep-water installation. A user-defined material subroutine (UMAT) is used for the description of the material behavior under severe plastic loading conditions, as presented in section 2.2. The pipe is discretized using four-noded, reduced-integration generalized plane-strain continuum finite elements, denoted in ABAQUS as CPEG4R, whereas the forming dies for the four steps are modeled as analytical rigid surfaces. The radius of curvature of the rigid parts for the crimping and JCO steps are calculated according to expressions (1) below. t tot Dm E
Rm
expansion Fig. 1: Schematic representation of: Crimping, J-shape, Cshape, O-shape and Expansion phases. To model steel material behavior, a cyclic plasticity material model is used, based on the von Mises plasticity formulation and the nonlinear kinematic hardening rule. The model has been introduced elsewhere [7] and is capable at modelling both the yield plateau at initial yielding and the Bauschinger effect upon reverse plastic loading. The material model is inserted within the finite element model using a material-user subroutine. A parametric analysis is also conducted on the effects of the amount of expansion at the final stage of the JCO-E process, on the ultimate capacity of the pipe. The numerical tools developed in the present study can be employed for optimizing the JCO-E manufacturing process in terms of pressure capacity. 2
(1) t Rin Rm 2 t Rou Rm 2 The geometry of the mandrels for the expansion is similar to those employed by [5] for a UOE pipe of similar dimensions and material grade. It is important to underline that the developed finite element model is capable of simulating both the manufacturing process and the subsequent application of external pressure, considering an appropriate sequence of loading steps. Furthermore, a sensitivity analysis of collapse pressure to the number of elements through thickness is performed. The mesh convergence study concluded that 8 elements are used through pipe wall thickness and the size of elements along the plate is equal to 4 mm. 1.0
normalized collapse pressure (PCO/Py)
O-shape
t 2 tot
NUMERICAL MODELING
0.8
0.6
0.4 JCO-E pipe X-70 steel D=24in t=1.273in
0.2
0.0
2.1
6
Finite element modeling description
8
10
12
number of elements through wall thickness
A generalized two-dimensional model is developed in the general-purpose finite element program ABAQUS. The model describes the cross-sectional deformation of the pipe under generalized plane strain conditions. This allows for the simulation of both the manufacturing process from the flat plate
Fig. 2: Sensitivity analysis on the collapse pressure with respect to the number of JCO steps.
2
14
Expansion
JCO
Crimping 2.2
y
Description Plate thickness (mm) Plate width (mm) Steel yield stress (MPa)
Value 32.33 1803 498
D Dm
Pipe diameter (mm) Dm=D-t
Rin
Internal crimping radius (mm)
609.6 577.2 7 265.4
Rout
External crimping radius (mm)
259.5 3
Rin
Punch radius (mm)
259.5 3
Mandrel radius (mm)
260
Expansion value (mm)
7.75
Number of mandrel segments
8
t W
600 400
nominal stress (MPa)
Plate/Pipe
Table 1: Geometric parameters of the JCO-E manufacturing process for the 24-inch-diameter X-70 line pipe.
200 0
Measured
-200
Fit
-400 X-70 steel -600 -1
0
1
2
3
4
5
6
7
8
engineering strain (%)
Fig. 3: Test and material modeling for uniaxial X-70 stress – strain curve [5].
3
NUMERICAL RESULTS FOR JCO-E MANUFACTURING PROCESS
Constitutive modeling
The accurate simulation of material behavior under reverse (cyclic) loading conditions is of major importance for modeling the JCO-E process and for the reliable prediction of line pipe structural capacity. In the course of JCO-E manufacturing process, the pipe material is deformed well into the plastic range, and is subjected to reverse loading, characterized by the appearance of the Bauschinger effect. In the present study, a modified Amstrong-Frederick model, described elsewhere [7], is used. The material model is calibrated with a uniaxial stress-strain curve from uniaxial tensile testing of a steel coupon as shown in Fig. 3 [5] [7]. The yield stress of steel material σy is equal to 498 MPa (72 ksi), corresponding to X-70 steel grade. The capability of the present material model to reproduce the experimental uniaxial stress-strain curve of the X-70 steel is shown in Fig. 3; both the plateau region after initial yielding and the Bauschinger effect are described satisfactorily. It is noted that the accurate description of the stress-strain curve in terms of the local rigidity of the steel material has significant effect on the resistance of the steel line pipe against external pressure collapse.
During each step of the manufacturing process the plate is deformed in different area in order to reach the final shape. In the first step the edges are crimped and all the other plate parts remain almost undeformed. After crimping, the J-step follows in which one side of the plate is bent in order to obtain the Jshape. Afterwards, the plate bents from the other side until the shape of C is formed. Finally, punch pressed the plate at the middle so it forms as O. Last, expansion is conducted in order to improve the circularity of the pipe. Fig. 4 depicts the contour plots of the Von Mises stresses from the different steps of the simulation illustrating the areas which deform in every step of the process. In the present study, a parametric analysis is conducted in order to examine the effect of number of the JCO steps on the structural performance. Three cases are considered, 9 steps 15 steps, and 19 steps respectively.
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O-shape crimping
J-shape
expansion
C-shape Fig. 4: Numerical results of forming process in different steps.
During the numerical simulation of the manufacturing process, the value of the expansion displacement constitutes a key parameter. In the present work, it is considered equal to zero at the point where the first mandrel reaches contact with the inner surface of the pipe. This is referred to as “JCO” case. After that stage, the mandrels continue to expand and circular plate is accommodating itself to this expansion causing expansion and bending deformation on the pipe wall. At the stage where all mandrel segments are in contact with the pipe walls, the pipe has reached a quasi-rounded shape. Upon increased outward displacement of the mandrels, the circularity of the pipe is further improved. This outward movement of the mandrels induces net hoop strain, denoted as , and defined by the following equation:
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CE CO (2) CO where CE and CO are the mid-surface lengths of the pipe circumference after the Expansion phase and after the O-ing phase (JCO case) respectively. A similar definition for the hoop expansion has also been suggested in [5]. Note that the values of CE is measured after the mandrels are removed, therefore it may be considered as a “permanent” expansion hoop strain, which accounts for the small “elastic rebound” at the end of the expansion. It should also be noted that this expansion hoop strain is quite different than the local hoop strain of the pipe material after the JCO process; the value of should be considered as a “macroscopic” parameter to quantify the size of expansion. Fig. 5 depicts the relation between the value of the expansion displacement u and the corresponding permanent expansion hoop strain . In the present analysis, the value of u is considered to range from u =0 mm (corresponding to the JCO case) to u =13 mm for the case of 9 JCO steps and u =9mm for the other two cases, which corresponds to a value of permanent expansion hoop strain equal to 2.39%, 1.38% and 1.48 respectively . The results in Fig. 5 show that the permanent expansion hoop strain is a non-linear function of the expansion displacement u .
3.1
Line pipe ovalization and out-of-roundness
An important parameter for assessing the mechanical behavior of offshore pipes subjected to external pressure is the ovalization of pipe cross-section after the manufacturing process, also referred to as “cross-sectional ovality”. Ovalization is a geometric imperfection of the pipe and may have a significant effect on the ultimate capacity under high external pressure, causing premature collapse. To quantify pipe cross-sectional ovality, the ovality parameter 0 is defined as:
0
| D1 D2 | D1 D2
(3)
where D1 and D2 are the horizontal and vertical outer diameters of the pipe cross-section respectively, measured at the end of the JCO-E process. The effect of expansion hoop strain on the ovalization parameter 0 is shown in Fig. 6 for the three different cases. As the expansion increases, the value of the ovality parameter drops rapidly; it obtains quite small values (less than 0.2%). Further increase of the applied expansion hoop strain causes additional decrease of ovalization, so that the roundness of the pipe is improved.
1.4
2.0
ovalization (%)
expansion hoop strain (%)
1.6
9 JCO steps 15 JCO steps 19 JCO steps
1.2
9 JCO steps 15 JCO steps 19 JCO steps
1.8
1.4 1.2 1.0 0.8
1.0 0.8 JCO-E pipe X-70 steel D=24in t=1.273in
0.6 0.4
0.6
JCO-E pipe X-70 steel D=24in t=1.273in
0.4 0.2
0.2 0.0 0.0
0.0 0
2
4
6
8
10
12
uE[mm]
0.3
0.6
0.9
1.2
expansion hoop strain (%)
Fig. 6: Ovality parameter in terms of permanent expansion hoop strain.
Fig. 5: Variation of the induced (permanent) hoop expansion strain E in terms of the expansion displacement value uE of the formed JCO-E pipe.
5
1.5
32.40 32.25
tnom 32.33mm (1.273in)
thickness(mm)
32.10 31.95 31.80 31.65 31.50
JCO-E pipe X-70 steel D=24in t=1.273in
31.35 31.20
taverage
31.05 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
expansion hoop strain (%)
Fig. 7: Effect of the expansion hoop strain E on the average thickness tave of the JCO pipe. 3.2
Pipe Thickness Due to the manufacturing process, the thickness of a JCO-E pipe after forming is somewhat different than the thickness of the initial steel plate. The average thickness of the pipe tave around the cross-section is computed at the end of the JCO-E process (including unloading) and the results are shown in Fig. 7 with respect to the expansion hoop strain . The plot presents the results from the case 1, in which 9 JCO steps are performed, and it indicates that the mean thickness of the formed pipe decreases with increasing values of hoop expansion in a quasi-linear manner. The average thickness for a pipe with zero expansion 0% , corresponding to the JCO case, is equal to 31.99 mm, very close to the original plate thickness, whereas for 1.4% the average thickness reduces to 31.69 mm, corresponding to a 2 % reduction with respect to the initial plate thickness. 3.3
from the longitudinal and the hoop direction of the pipe, in order to examine material anisotropy in the two principal directions of the pipe. These strip specimens are subjected to uniaxial testing, so that the corresponding stress-strain curves are obtained [1]. A numerical simulation of this experimental procedure is attempted in the present study. More specifically, a circumferential location of 90º from the weld region is selected in the numerical model. Two integration points are selected at this location, one at the external surface of the pipe and the other at the internal surface in order to examine (a) the compression response in the hoop direction at the “intrados” and “extrados” of the pipe and (b) the material anisotropy in the hoop and the axial direction. Throughout the simulation of the forming process, all material state parameters (stresses, strains) are recorded at those integration points. Subsequently, a “unit cube” finite element model is considered and the material parameters from each selected integration point are introduced as initial state variables. First a step with zero external loading is performed, simulating the extraction of the strip specimen from the pipe. At this step, residual stresses are reduced to zero but the plastic deformations due to the forming process are maintained. Finally, a second step is performed where the “unit cube” is loaded under uniaxial compression in the hoop direction of the pipe or under uniaxial tension in the direction parallel to the pipe axis. Fig. 8 depicts compression stress-strain response in hoop direction at the intrados and the extrados of the pipe. The results show that there is a change compared with the initial material as shown in Fig. 2. Fig. 9 illustrates the axial tensile stress-strain response and the hoop compressive stress-strain response. The results show that the manufacturing process induces anisotropy in the line pipe. Moreover, the results show that the compressive yield stress in the hoop direction is lower than the corresponding tensile yield stress in the longitudinal direction of the pipe, which indicates anisotropy of the yield surface due to work hardening from the JCO-E cold bending process.
Line Pipe Material properties
The structural performance of the line pipe depends on its material properties. The manufacturing process introduces significant changes to the stress-strain response of the plate. Furthermore, the material anisotropic behavior of the hoop direction with respect to axial direction induced by the manufacturing process is another important factor affecting the mechanical behavior of offshore pipes. The JCO-E forming process introduces significant stresses and deformations, the material enters the strain hardening region and, consequently, its initial stress–strain response is modified. In practice, the change of the steel material at the end of the JCO-E forming process is evaluated by extracting two strip specimens from the extrados the intrados in the hoop direction of the pipe to examine the response in two areas. Furthermore two specimens are obtained
6
600
inner
plate 500
stress (MPa)
400
outer
300
outer
200
inner
100
0 0.000
JCO-E pipe X-70 steel D=24in t=1.273in
0.002
0.004
0.006
0.008
0.010
strain
Fig. 8: Compression hoop stress-strain response of pipe at intrados and extrados. 600
axial tension 500
hoop compression
stress (MPa)
400
300
200
JCO-E pipe X-70 steel D=24in t=1.273in
100
0 0,000
0,002
0,004
0,006
0,008
0,010
strain
Fig. 9: Axial tensile and hoop compression stress-strain response of JCO-E.
with increasing value of as shown in Fig. 6, resulting to an increase of PCO , as presented in Fig. 10. The same conclusion is observed for all the cases (9, 15 and 19 JCO steps). One should note that at relatively small values of ovalization, which means large values of the expansion hoop strain increases, significant residual stresses are induced by the cold-forming process, resulting to a decrease of the maximum collapse pressure, attributed to the reduction of the corresponding compressive strength of the material due to the Bauschinger effect. This counteracts the effects on the collapse pressure capacity. The numerical results (Fig. 10), show that a maximum value of PCO is reached respectively each case under consideration, at a value of equal to 1.45% ( 9 JCO steps), 0.46% (15 JCO steps) and 0.26% (19 JCO steps) and with further increase of the corresponding the collapse pressure reduces progressively. Therefore, there exist an optimum expansion at which the highest resistance against external pressure instability (buckling) is achieved, an observation consistent with the one reported in [10]. It is worth mentioning that the case with 15 JCO steps reaches higher values of collapse pressure than the case with 19 JCO steps. This may attributed to the fact that 15 JCO steps are adequate to deform the plate to pipe, whereas more steps lead to overlapping of the deformed areas, reducing the pressure capacity. Fig. 11 depicts the deformed shape of the JCO-E pipe under external pressure. Note that the post-buckling configuration of flattening mode is non-symmetric with respect to θ=90° plane due to the presence of the weld. Finally, the above numerical results are in agreement with the relevant conclusions reported in [11], supporting the argument that the JCO-C process (using a compressive final step), which counteracts the Bauschinger effect, may have a beneficial effect on the external pressure capacity of the JCO pipe.
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NUMERICAL SIMULATION OF THE STRUCTURAL BEHAVIOR OF JCO-E PIPES
Following the simulation of the forming process, the JCO-E pipe under consideration is subjected external pressure. So that the ultimate capacity under external pressure is determined. Each of the forming parameters of the JCO-E manufacturing process has an important effect on the response of JCO-E pipes under external pressure. In the present section the effect of expansion process on the ultimate pressure and the effect of the number of JCO steps are presented through a short parametric analysis. In Fig. 10, the predicted collapse pressure of the pipe, denoted as PCO , for the three cases of JCO steps under consideration, is presented with respect to the applied expansion hoop strain . It is important to note that starting from the JCO case ( 0% ), the initial ovality drops sharply
collapse pressure PCO (MPa)
42 40 38 36 9 JCO steps 15 JCO steps 19 JCO steps
34 32
JCO-E pipe X-70 steel D=24in t=1.273in
30 28 26 0.0
0.5
1.0
1.5
expansion hoop strain (%)
Fig. 10: Response of JCO-E pipes under external pressure for different values of expansion hoop strain for the three cases of JCO steps.
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[2] Kyriakides, S., Corona, E., Fischer, F. J., 1991. "On the Effect of the UOE Manufacturing Process on the Collapse Pressure of Long Tubes", Offshore Technology Conference Vol. 4, OTC 6758, pp. 531–543. Also, published in Journal Engineering for Industry, Vol. 116, pp. 93–100.
(a)
(b)
[3] Gresnigt, A.M., Van Foeken, R.J., Chen, S., 2000. "Collapse of UOE Manufactured Steel Pipes", Proceedings of the Tenth International Offshore and Polar Engineering Conference, Vol. 2. pp. 170-181, Seattle, Washington. [4] DeGeer, D., Marewski, U., Hillenbrand, H.-G., Weber, B., Crawford, M., 2004. "Collapse Testing of Thermally Treated Line Pipe for Ultra-Deep Water Applications", 4th International Conference on Pipeline Technology, Ostend, Belgium.
(c) Fig. 11: Deformed (collapsed) shape of the JCO-E pipe under external pressure.
[5] Herynk, M.D., Kyriakides, S., Onoufriou, A., Yun, H.D., 2007. "Effects of the UOE/UOC Pipe Manufacturing Processes on Pipe Collapse Pressure", International Journal of Mechanical Sciences, Vol. 49, pp. 533–553.
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[6] Toscano, R. G., Raffo, J., Mantovano, L., Fritz, M., Silva, R. C., 2007. "On the Influence of the UOE Process on Collapse and Collapse Propagation Pressure of Steel Deep-Water Pipelines Under External Pressure", Offshore Technology Conference, Houston, Texas.
CONCLUSIONS
The present paper presents a rigorous simulation of the JCO-E manufacturing process and its effect on the mechanical behavior of offshore pipes, subjected to high external pressure, using advanced finite element simulation tools. A plasticity model, capable of describing the nonlinear elastic–plastic material behavior (introduced elsewhere) is used within the finite element model, using a material user subroutine. In the first part of the paper, the JCO-E cold-bending manufacturing process steps are simulated in detail, considering a 24-inchdiameter (609.6 mm) pipe, with nominal thickness equal to 32.33 mm (1.273 in). A parametric analysis is conducted focusing on the effects of JCO-E manufacturing process, and in particular those of the expansion stage on the overall pipe behavior against pressure. The numerical results show that the increase of the expansion hoop strain value leads to minimization of pipe out-of-roundness, but beyond a certain expansion value, the collapse pressure resistance of the pipe is reduced due to the Bauschinger effect. As a result, there exists an optimum expansion at which highest resistance against external pressure loading can be achieved.
REFERENCES [1] Kyriakides, S., Corona, E., 2007. Mechanics of offshore pipelines, Buckling and Collapse, Vol. 1, Elsevier.
[7] Chatzopoulou, G., Karamanos, S. A., Varelis, G. E., 2016. "Finite Element Analysis of UOE Manufacturing Process and its Effect on Mechanical Behavior of Offshore Pipes", International Journal Solids and Structures, Vol. 83, pp. 13–27. [8] Chandel, J. D., Singh, N. L., 2011."Formation of X-120 M Line Pipe through J-C-O-E Technique", Engineering, Vol. 3, , pp. 400-410. [9] Gao, Y., Li, Q., Xiao, L., 2009. "Numerical Simulation of JCO/JCOE Pipe Forming", World Congress on Computer Science and Information Engineering. [10] Krishnan, V.R., Baker, D.A., 2014. "Enhanced Collapse Resistance of Compressed Steel Pipes". Proceedings of the 33rd International Conference on Ocean, Offshore and Arctic Engineering, California, USA. [11] Reichel, T., Pavlyk, V., Beissel, J., Kyriakides, S., Jang, W.Y., 2011. “New Impander Technology for Improved Collapse Resistance of Large Diameter Pipe for Deepwater Applications”, Offshore Technology Conference, Houston, Texas.
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