english

High quality through high end technology on LSAW
large diameter pipes
A. Liessem
H.-G. Hillenbrand
T. Kersting
L. Oesterlein
N. Schoenartz
PRCI/EPRG/APIA, Technical Conference
May 15 - 20, 2005 Orlando, USA
TP64
EUROPIPE. The world trusts us.
Europipe GmbH, Mülheim, Germany
Europipe GmbH, Mülheim, Germany
Europipe GmbH, Mülheim, Germany
Europipe GmbH, Mülheim, Germany
Salzgitter Mannesmann Forschung, Duisburg, Germany
HIGH QUALITY THROUGH HIGH END TECHNOLOGY ON LSAW LARGE
DIAMETER PIPES
H.-G. Hillenbrand, T. Kersting, L. Oesterlein
EUROPIPE GmbH, Muelheim, Germany
A. Liessem
EUROPIPE GmbH, Ratingen, Germany
N. Schoenartz
Salzgitter Mannesmann Forschung, Duisburg-Huckingen, Germany
ABSTRACT
Large diameter longitudinally welded linepipe have to fulfil increasing technical requirements in order
to guarantee best performance during construction and longterm service. Therefore the linepipe
manufacturing steps from steel making to pipe production are continuously improved in order to
increase the quality level and the productivity by reducing non-quality costs. Besides forming and
expansion the welding process and its control is one of the most crucial operation during linepipe
manufacture. A welding process which parameters are steadily controlled and which weld seam
quality is inspected by highly developped non-destructive test methods is therefore a pre-requisite that
welding defects are primarily avoided or if not avoidable at least reliably detected. A closed quality
control loop between the NDT inspection and the welding process makes sure that corrective or
preventive actions are taken. In the last years EUROPIPE invested in the latest developments in
welding technology. Digital power sources open up new options of process control for multi-wire
welding. This investment now perfectly supplements the modernisation of the non destructive
inspection of the longitudinal weld seam by ultrasonic and radiographic test methods. In the first part of
this paper the main technical features of the new welding equipment will be presented and how this
new technology helps to control the welding process. The second part will focus on details of the non
destructive testing with particular regard to the internal quality control of the weld seam.
WELDING TECHNOLOGY OF LSAW PIPE
One of the most essential and challenging steps in the production of linepipe is the welding of the
longitudinal seam. A wide range of different quality requirements have to be achieved reliably and
under economic conditions. The production of a weld seam suitable for the purpose of high pressure
linepipe is more than just joining two plates edges. The following table 1 summarises the most
important quality aspect of a submerged arc welded longitudinal seam:
SAW quality aspects
Mechanical Properties
Corrosion Resistance
Weld shape
Imperfections/Defects
Strength
Matching ratio
Toughness
Hardness
HIC/SSCC resistance
Misalignment
Radial offset
Weld bead height
Weld contact angle
Weld bead width
Interpenetration
Weld linearity
Undercuts
Slag inclusions
Porosities
Lack of fusion/interpenetration
Cracks
Key factors:
(Welding parameters)
Consumables, Base metal
Key factors:
Welding parameters
(Consumables)
Key factors:
Welding parameters
(Consumables)
Table 1: Quality aspects for SAW seams
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Whereas the mechanical properties/corrosion resistance is predominantly defined by the selection of
appropriate welding consumables the weld shape and the occurrence of imperfections or defects is
strongly influenced by the welding parameters.
In the EUROPIPE Mill in Muelheim an der Ruhr up to 15 km of weld seam can be manufactured per
day. Stable welding processes are mandatory to guarantee lowest repair and rejection rates. To
further improve the process EUROPIPE invested in the latest technology of digital power sources.
Seven inside and the equal number of outside welding machines are equipped with the Lincoln Power
Waves (PW) (Figure 1).
Figure 1: View of the modular PW 500 for inside welding (left) and the PW1000 for outside (right)
The new power sources offer a wide range of additional possibilities compared to the former situation.
Main features are:
Feature
Old power sources
Power Waves (PW)
Net compensation ± 10%
7
E
Power source presets
7
E
Adjustable ramp function
7
E
AC and DC Mode
7
E
Frequency control
7
E
CC-Mode
E
E
CV Mode
7
E
Phase angle control
7
E
Wave Designer
7
E
Table 2: Comparison between old and new power sources
The advantages are based on the new digital design. The Power Wave uses the inverter technique.
As these inverters are primary and secondary switched they allow a wide range of parameter settings.
It is possible to switch between CC- (constant current) or CV-(constant voltage) mode, Sine- or square
wave mode, constant or dropping characteristics by simple software changes (Figure 2)
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U(t), I(t)
U(t), I(t)
t Offset
Frequency
0 - 100 Hz
t
Figure 2: Examples for the parameter options of the PW
In addition the modular design allows a flexible layout of the welding station. Higher capacities can be
achieved by simply connecting an additional modul. The smaller size of the moduls facilitates the
maintenance.
The use of presets allows a fast setting of the power source well adapted to the wall thickness to weld.
A preset contains all necessary information for a single welding head such as wire diameter, stop/start-parameters and so on. This allows narrow tolerances for the welding parameters leading to a
stable process. Welding speed can be improved to reach high productivity and low heat input without
the risk of increasing defect rates. Figure 3 shows an example of the optimisation of the heat input
while increasing the deposition rates at a given wall thickness. A reduction of 11% heat input with a
parallel increase of deposition rate of 4% was realised by using the power waves options.
250
115%
Deposition rate
Heat Input
110%
200
Welding speed
105%
100%
95%
90%
ID 4 Wire
ID 3 Wire
150
100
50
85%
80%
0
V0
V1
V2
V3
V4
V5
V6
V7
V8
V9 V10 V11 V12
0
Welding variant
10
20
30
40
50
WT [mm]
Figure 3: Optimisation of deposition rate, heat input (left) and welding speed (right)
The inside welding machines limited the performance of the mill especially at higher wall thickness.
The installation of the new power sources went hand in hand with the introduction of 4-wire welding
inside and 5-wire welding outside. The resulting increase of welding speed (Figure 3,right) of 25- 30%
allowed a reduction from eight to seven welding lines. Nevertheless the welding capacity could be
improved by 15%.
But increase of the mill performance was only one part of the project. The further improvement of
quality by reducing the weld defect rate was the second one. The higher reliability and the more stable
welding have a direct impact on those process deviations that lead to start-/stop repairs (Figure 4).
These are the so called "burn throughs" and the "weld stops". Both having as consequence time
consuming and therefore expensive rework on the pipe.
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100%
100%
90%
90%
80%
80%
70%
70%
60%
60%
50%
50%
40%
40%
30%
30%
20%
20%
Burn Through
Weld stops
10%
10%
0%
0%
2002
2003
2004
2002
2005
2003
2004
2005
Figure 4: Development of Weld Stops (left) and Burn Through (right) since 2002
To keep the defect rate at a low level and to improve quality further on a tight process control is
necessary. The information has to be available as fast as possible and as easy as possible. Therefore
EUROPIPE installed an efficient control circuit immediately on and behind the welding machines. The
best way to higher quality is to avoid any defect. The new control environment of the welding
machines allows a close follow up of all components influencing the welding itself (Figure 5). Beside
the visualisation of the essential welding parameters such as current, voltage and speed the operator
has a feedback about heat input, flux system, transportation system, seam tracking and so on.
Figure 5: Examples of control screens for welding
An analyzer software connected to the welding equipment provides the possibility to evaluate the
welding process (Figure 6). This leads to systematic improvement and deeper understanding of the
measures to avoid defects.
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Figure 6: Analyzer Tool: Improvement of weld stop on the run-off tab ends (left), welding parameters
leading to a slag inclusion (right)
The next level of this process control is the fast feed back between NDT and welding to avoid
systematic deviations. EUROPIPE relies on a combination of visual inspection, automated UT and Xray as described in the following.
PROCESS CONTROL OF LONGITUDINAL SEAM WELDING
As it is shown in Figure 7 the visual testing immediately follows the outside welding. Both internal and
external pipe surfaces including weld seam are controlled. All kind of surface defects are detected and
entered in PRODIS, the integral production data information system. Beside real defects such as
undercuts the testing registers process deviations i.e. irregular weld shape or width and heights of the
weld seam. An automated 100%UT of the weld seam reports all internal imperfections. UT indications
are confirmed by filmless radiography and/or verified by manual UT as described more in detail in the
following chapter.
Weld Process Control
Forming
Visual
Inspection
Welding
NDT
UT/X-Ray
PRODIS
SPC
Figure 7: Weld process control by interaction of different test methods (internal testing before
expansion)
All test results are available in PRODIS. A list with all defect codes is on-hand at any time to the
welding operators. At a low defect level each defect becomes a single event. Systematic statistic
evaluation of a defect type becomes mandatory to trace it back to its origins. To facilitate that
evaluation and to deliver the required information directly to the process supervisors at the shop floor a
statistical process control (SPC) has been developed and integrated in PRODIS (Figure 8).
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Figure 8: PRODIS: Defect statistics for a single welding machine (left) and an example for SPC (right)
The consequent process control in combination with a fast access to the essential information at any
time and any place in the mill lead to a permanent reduction of rework and losses due to weld defects
(Figure 9).
100%
100%
95%
90%
90%
85%
80%
80%
70%
75%
70%
60%
Repair Rate
Pipe Losses
65%
60%
50%
2002
2003
2004
2005
2002
2003
2004
Figure 9: Development of repair rate (left) and pipe losses (right)
QUALITY CONTROL OF LONGITUDINAL WELD SEAM
In order to fulfil highest quality requirements a highly developed system of complementing nondestructive test methods has been established for the internal quality control, see Figure 10.
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PRODIS
SPC/UT-Alarm
Quality Control Flow
Smart
UT evaluation
System.
PRODIS
UT-Alarm
FLORAD
No
OK?
Repair
Yes
Yes
UT
Alarm?
No
AXION
Assistant
No
OK?
Yes
Archive
UT Manual
No
accepted?
Yes
Fig. 10: NDT Quality control flow of pipes before expansion
After welding all pipes are visually tested inside and outside in order to find as soon as possible
surface imperfections or defects on the pipe body and in the weld seam area.
Hereafter the complete weld seam is tested by an automatic UT-equipment. The system has been
installed in 2001 based on an expert system condensing 25 years of experience with automated UT
and data evaluation. The equipment is able to reach a high reliability even under difficult inspection
conditions typically with non expanded pipes [1]. This was realised by a smart computer based
evaluation system. The sensitivity for the UT signals is set to the highest level technically reasonable.
Significant UT indications will launch a so called UT-Alarm. The smart UT-evaluation system is linked
to the PRODIS system in order to enable a closed loop to the welding machines as rapid process
control.
Pipes with an UT indication will be x-rayed by a digital film-less radiographic system called FLORAD
(FLORAD = FilmLOse RADiographie (German) = Filmless Radiography (English)) which was installed
in 1998 [2]. Since that time more than 250 thousand images have been made and the film technique
has been successfully substituted by the image converter system. Digital systems have the great
advantage of image processing like contrast adaptation, filtering etc. Based on the high amount of
radiographs an automated x-ray image evaluation system (AXION) was developed to support the
evaluator in order to increase the reliability of detection. This system works on basis of an intelligent
expert system and is also linked to the smart UT evaluation system to reach best performance.
UT-indications which are confirmed by the FLORAD are repaired and rechecked. If X-ray results are
not consistent to the UT-Alarm pipes will be tested by manual UT which makes the final decision.
Finally only pipes fulfilling the stringent internal quality requirements will be expanded. As described
above the results of the non destructive testing supplemented by a statistical process control is fed
back to the welding machines for process control purposes.
To use the full benefits of film less radiography it is planned in the near future to invest in the latest
digital x-ray technique based on high quality flat panel detectors. Finally two x-ray chambers with two
flat panel detectors will substitute three x-ray chambers with four x-ray tubes.
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DEVELOPMENT OF FILMLESS RADIOGRAPY
As described above the high level of UT testing is complemented by sophisticated X-ray testing
systems and software. Since 1998, EUROPIPE has been operating the FLORAD system for the X-ray
testing of unexpanded pipes (Figure 11). This system has been supplemented in 2003 with the AXION
software assistant which supports the evaluator in the assessment of the X-ray Figures (AXION =
Automatic X-ray Image evaluatION) [3].
Figure 11: View of X-ray chamber with FLORAD system
The FLORAD system provides a lot of advantages compared to the classical X-ray film (Table 2):
X-ray film
FLORAD
High film costs
No film costs
High costs for film storage
Low costs for electronic storage
High disposal costs for chemistry
No chemistry
Processing time 10 minutes
No processing required
Very high image quality
Comparable image quality
Available only at one place
Available throughout the mill and beyond
Only manual evaluation possible
Computer aided evaluation
Table 2: Advantages of filmless radiography (FLORAD) compared to conventional X-ray film
Of course, the fundamental question is if the image quality of the FLORAD system is sufficient to
detect all relevant defects which can be detected with X-ray film. This question was examined
independently by RWTÜV (a German testing organisation), by Shell Global Solutions and by DNV,
Norway. All reports concluded that (in the wording of DNV) “the EUROPIPE Deutschland GmbH
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Filmless Radiographic System (FLORAD) has been found to give at least the same detectability of
weld defects as conventional film radiography, and is suitable to replace film radiography.”
However, X-ray images are important documents which must be preserved for at least 12 years. X-ray
film can easily fulfil this requirement when stored properly. Furthermore, X-ray film testing is a proven
and reliable technique for decades. The same level of security and availability is mandatory for any
filmless system. With the digital technique it is possible for third party inspectors to have the images
rapidly at any place without danger of scratches on the film which would make the film invalid as
document. All digital x-rays images will be stored in the same way as the X-ray films for at least 12
years. The retrieval and the handling will be facilitated by use of a digital image data bank.
For the FLORAD system, these features are reached by a redundant cluster of OpenVMS computers
with shadowed disks attached to a shared SCSI bus. Long term storage of the images is performed on
ISO 9660 formatted CD-ROM disks. In this configuration, the cluster gives the required availability
while through the shadowed disks the images are preserved also in case of a hardware failure. In the
seven years of operation more than 250,000 images have been recorded without any unplanned
downtime due to a computer hardware failure.
In parallel to the operation of the system the AXION software assistant was developed. This software
examines each image recorded with FLORAD and checks it for possible defects. In the current field
test phase, AXION is invoked after the evaluator has made his assessment. While the evaluator draws
a red rectangle around the defects he has found, suspicious areas found by AXION are marked with a
green rectangle and the defect is classified using a database (Figure 12). This procedure gives the
possibility to verify the operation of AXION against the evaluator with a large number of images taken
in the regular mill production. The final goal is that AXION sorts out all images which certainly have no
defect, leaving only the remaining images for the evaluator for the final decision.
Figure 12: Example of a FLORAD X-ray image assessed by human evaluator (red) and by AXION
(green)
The detector used with the FLORAD system is an image converter the heart of which is a rather bulky
image intensifier tube. Because of its size it was not possible to use the detector inside the pipe.
Instead, it had to be mounted on a carriage above the pipe and the X-ray tube had to be mounted
inside the long spar within the pipe (see Figure 11). This set-up is mechanically complex and reduces
the choice of X-ray tubes.
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In the meantime a new class of X-ray detectors has emerged, the so called flat panel detectors or
digital detector arrays. An example is shown in Figure 13 and some technical data for the detector
chosen by EUROPIPE are given in the following table 3:
Detector size
267 mm x 318 mm (10.5 in. x 12.5 in.)
Detector thickness
51 mm (2 in.)
Pixel area
195 mm x 244 mm (7.68 in. x 9.6 in.)
Pixel pitch
127 µm
Number of pixels
1920 x 1536
Dynamic range
12 Bit (4096 grey levels)
Table 3: Technical data of flat panel detector
Figure 13: Flat panel detector (left) with command processor and power supply (right)
A detector of this size can even be used inside a 20” pipe. In this configuration, the X-ray tube is
mounted outside the pipe, which results in a simplified mechanical set-up and gives the chance to use
larger, more powerful X-ray tubes. The higher power of these tubes extends the wall thickness range
that can be tested. Compared to the image converter the spatial resolution of the selected flat panel is
remarkably higher, resulting in a once more improved image quality. In Figure 14, the double wire pair
9 (wire diameter = wire distance = 130 µm) can be clearly separated, proving the excellent spatial
resolution. The large active area of the flat panel reduces the number of images that have to be taken
for a given weld length. A comprehensive comparison between the image converter used in the
current FLORAD set-up and the flat panel based system is given in the following table 4:
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Image converter
Flat panel
Assembly of opto-electronic and mechanical
components
Compact solid-state device, no mechanical parts
Customised X-ray tube required
Standard high-power X-ray tube
Special designed bar
Simplified set-up
100 mm weld length in one image
200 mm weld length in one image
Max. wall thickness 42 mm
Max. wall thickness 50 mm
Expensive
Very expensive
Table 4: Comparison of image converter and flat panel based system
Figure 14: Part of flat panel X-ray image demonstrating the high spatial resolution
Considering the long experience and positive results obtained with the FLORAD system, EUROPIPE
plans to replace the X-ray film completely in the Muelheim works using flat panel technology. A first
installation with a flat panel system is currently tested under operational conditions in the Dunkerque
works of EUROPIPE. The detailed concept for the integration of the flat panel technology in the
Muelheim mill is simultaneously developed by EUROPIPE together with the Salzgitter Mannesmann
Forschung (SZMF).
CONCLUSION
The ever increasing requirements on the weld seam properties can reliably fulfilled only by use of a
welding process which parameters are steadily controlled and which weld seam quality is inspected by
intensive non-destructive test methods. This constitutes a pre-requisite in order that welding defects
are primarily avoided or if not avoidable at least reliably detected. Therefore EUROPIPE replaced the
existing welding equipment by the latest generation of digital power sources. The new digital design
offers a comprehensive variety of parameter control and pre-settings. As a result of the welding
parameter optimisation both quality and productivity could be improved significantly.
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In order to reach best performance of the welding process with high economic efficiency it is
necessary to install a closed process control loop between welding and inspection where results of
NDT are fed back to the different welding machines. This is possible by use of modern computer
controlled machines in a network of data exchange and automated evaluation systems.
In order to fulfill highest quality requirements a highly developed system of contemplating nondestructive test methods has been established using visual inspection and the latest technology of
ultrasonic testing and filmess radiography. The applied quality control system achieves high
performance with regard to detection of defects and imperfections.
The existing filmless radiography using image converter has been proven to be a further development
for the conventional X-ray film technology. Based on the experience gathered on pipes tested
according to the stringent internal quality requirements before expansion, the system will be transfert
to the inspection of the expanded pipes in near future. As meanwhile a new class of X-ray detectors
has emerged, the actually used image converter system will be replaced by the new flat panel
detectors providing various technical advantages.
REFERENCES
[1]
[2]
[3]
Kersting, T., Oesterlein, L., Kauth, G., Finger , G.: Neue Ultraschallanlage zur Prüfung von
UP-längsnahtgeschweissten Grossrohren bei EUROPIPE in Muelheim a.d. Ruhr, DGZfPJahrestagung 2002, Weimar
Kersting, T., Oesterlein, L., Schoenartz, N.: Application of Filmless Radiography during the
th
Production of Large Diameter Pipes, 15 WCNDT, Rome 2000
Liessem, A., Grimpe, F., Oesterlein, L.: State-of-the-art Quality Control during the Production
th
of SAW Linepipe, 4 International Pipeline Conference, Calgary, 2002, IPC2002-27141
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