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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: gihatzop@uth.gr
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
2
rigorously predicted. The initial imperfections refer to both out-
of-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
J-shape
O-shape
C-shape
expansion
Fig. 1: Schematic representation of: Crimping, J-shape, C-
shape, 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 NUMERICAL MODELING
2.1 Finite element modeling description
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
to the circular shape configuration of the pipe, restraining out-
of-plane displacements, as well as the subsequent application of
pressure in order to examine the structural behavior of the JCO-
E 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.
2
2
2
tot m
mtot
in m
ou m
t
DE
t
R
t
RR
t
RR
(1)
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.
6 8 10 12 14
0.0
0.2
0.4
0.6
0.8
1.0
normalized collapse pressure (PCO/Py)
number of elements through wall thickness
JCO-E pipe
X-70 steel
D=24in
t=1.273in
Fig. 2: Sensitivity analysis on the collapse pressure with respect
to the number of JCO steps.
3
Table 1: Geometric parameters of the JCO-E manufacturing
process for the 24-inch-diameter X-70 line pipe.
Description
Value
Plate/Pipe
t
Plate thickness (mm)
32.33
W
Plate width (mm)
1803
y
Steel yield stress (MPa)
498
D
Pipe diameter (mm)
609.6
Dm
Dm=D-t
577.2
7
Crimping
Rin
Internal crimping radius
(mm)
265.4
Rout
External crimping radius
(mm)
259.5
3
JCO
Rin
Punch radius (mm)
259.5
3
Expansion
Mandrel radius (mm)
260
Expansion value (mm)
7.75
Number of mandrel
segments
8
2.2 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.
-1 0 1 2 3 4 5 6 7 8
-600
-400
-200
0
200
400
600
Measured
nominal stress (MPa)
engineering strain (%)
X-70 steel
Fit
Fig. 3: Test and material modeling for uniaxial X-70 stress –
strain curve [5].
3 NUMERICAL RESULTS FOR JCO-E
MANUFACTURING PROCESS
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 J-
shape. 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.
4
crimping
J-shape
C-shape
O-shape
expansion
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:
5
EO
O
CC
C
(2)
where
E
C
and
O
C
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
E
C
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
.
0 2 4 6 8 10 12
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
9 JCO steps
15 JCO steps
19 JCO steps
expansion hoop strain (%)
uE[mm]
JCO-E pipe
X-70 steel
D=24in
t=1.273in
Fig. 5: Variation of the induced (permanent) hoop expansion
strain
E
in terms of the expansion displacement value
E
u
of
the formed JCO-E pipe.
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:
12
012
||DD
DD
(3)
where
1
D
and
2
D
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.
0.0 0.3 0.6 0.9 1.2 1.5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 9 JCO steps
15 JCO steps
19 JCO steps
ovalization (%)
expansion hoop strain (%)
JCO-E pipe
X-70 steel
D=24in
t=1.273in
Fig. 6: Ovality parameter in terms of permanent expansion
hoop strain.
6
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
31.05
31.20
31.35
31.50
31.65
31.80
31.95
32.10
32.25
32.40
average
t
thickness(mm)
expansion hoop strain (%)
32 33 1 273. ( . )
nom
t mm in
JCO-E pipe
X-70 steel
D=24in
t=1.273in
Fig. 7: Effect of the expansion hoop strain
E
on the average
thickness
ave
t
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
ave
t
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 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
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.
7
inner
outer
0.000 0.002 0.004 0.006 0.008 0.010
0
100
200
300
400
500
600 inner
outer
stress (MPa)
strain
JCO-E pipe
X-70 steel
D=24in
t=1.273in
plate
Fig. 8: Compression hoop stress-strain response of pipe at
intrados and extrados.
0,000 0,002 0,004 0,006 0,008 0,010
0
100
200
300
400
500
600
hoop compression
stress (MPa)
strain
JCO-E pipe
X-70 steel
D=24in
t=1.273in
axial tension
Fig. 9: Axial tensile and hoop compression stress-strain
response of JCO-E.
4 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
CO
P
, 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
with increasing value of
as shown in Fig. 6, resulting to an
increase of
CO
P
, 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
CO
P
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.
0.0 0.5 1.0 1.5
26
28
30
32
34
36
38
40
42
9 JCO steps
15 JCO steps
19 JCO steps
collapse pressure PCO (MPa)
expansion hoop strain (%)
JCO-E pipe
X-70 steel
D=24in
t=1.273in
Fig. 10: Response of JCO-E pipes under external pressure for
different values of expansion hoop strain for the three cases of
JCO steps.
8
(a) (b)
(c)
Fig. 11: Deformed (collapsed) shape of the JCO-E pipe under
external pressure.
5 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-inch-
diameter (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.
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