WELDING A CRA-LINED PIPELINE - MAGNATECH LLC USA

WELDING A CRA-LINED PIPELINE

Many gas and oil fields worldwide are both deep and sour, containing high percentages of

dangerous and corrosive hydrogen sulfide (H2S) and carbon dioxide (CO2). This use of steel

pipes with injected inhibitors is not feasible due to severe corrosion and cracking problems

brought about by high temperatures and "sour" conditions.

Solid pipe manufactured of nickel-based corrosion-resistant alloys (CRA) has been used in the

past, but the high cost has led to the development of steel pipe with a CRA lining.

CRA-lined pipe is a double-wall bimetallic pipe, which can be fabricated by several methods.

One method is to sleeve a CRA tube inside a carbon steel pipe, and hydraulically expand it.

The liner is shorter than the steel pipe, and a length of 80-100mm on both ends are clad with an

overlay of a CRA alloy using the Gas Tungsten Arc Welding (GTAW) process

Two "Bugs" simultaneously weld joint

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THE HARWEEL PROJECT

The Sultanate of Oman has many small oil reservoirs with unique geological features. One

such case is the Harweel Cluster in the far southwest corner of the country which is estimated to

contain 2 billion barrels of reserves. Oil is found in multiple reservoirs trapped in nonporous salt

deposits more than 4 km below the surface. (The Cluster is composed of 8 different fields.) Oil

and natural gas are mixed with hydrogen sulfide (H2S) and carbon dioxide (CO2) at very high

pressures. Producing the oil using conventional techniques would lead to a rapid loss of

pressure and reduce the amount of oil that could be extracted to 10% of the total. The decision

was made to turn the liability of large amounts of toxic and corrosive H2S and CO2 into an asset,

by separating them from the sour natural gas at a processing facility, and reinjecting them back

into the reservoir. This technique maintains pressure, and it is projected that 40% of the

existing oil can be produced using this enhanced recovery technique. The project expects to

deliver 125,000 bb/d with gas injection.

Harweel Reservoir Cluster (Oman) Map Courtesy of PDO Fact File 2008

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Petroleum Development of Oman (PDO) is developing the Harweel fields using a shared central

processing facility. The pipelines leading from individual wells to the processing facility are

transporting untreated sour gas. The main contractor for the Harweel Cluster Phase II facilities,

gas and oil pipeline project was Petrofac, with Tekfen Construction and Installation chosen to

undertake detailed engineering and construction of both carbon steel and CRA lined flow lines,

trunk lines and the main pipeline system for gas and oil. This involved welding approximately

20 km of 14" (8 and 9mm wall thickness carbon pipe with a 2mm internal CRA sleeve) and 8 km

of 16" (7.9mm with 2mm liner). The base material was API 5L Gr. X65, with a CRA sleeve of

Alloy 825 (a nickel-iron-chromium alloy with additions of molybdenum, copper, and titanium).

On both ends of a pipe, the shorter sleeve transitions to a weld overlay of Alloy 625.

A CRA project starts at the pipe mill. Pipe ends must be "calibrated" after manufacture, or

expanded past yield to achieve tight (high/low) diameter tolerances. This is dictated by the

need to achieve pipe end fit-up tolerances of no more than 1.0mm when ready for welding.

WELDING PROCESS

The use of faster, traditional weld processes (Shielded Metal Arc Welding, Gas Metal Arc

Welding) are generally prohibited. Gas Tungsten Arc Welding is the preferred process: slower,

but producing consistent welds to the highest quality standards. The GTAW process is always

required for the initial root pass, and at least several subsequent passes. The root pass must

smoothly join the CRA cladding without dilution by the base carbon pipe material. Subsequent

passes must not repenetrate the root pass, or contaminate the root pass with carbon steel.

PDO required the entire weld to be made with the GTAW process.

PREPARATION FOR FIELD WELDING

Pipe ends are machined (faced) on the jobsite, rather than attempting to protect mill bevels from

damage during shipment. A "J" bevel is used for GTAW welding. Preparing the pipe ends is a

two-step operation. A special ID mounting machining tool is used to counterbore the clad ends.

The "as welded" clad ends must be bored to produce a uniform surface finish at a specified

diameter. A laser-based templating system is used prior to boring to determine actual pipe ID,

OD, and wall thickness. This data is used for the counterbore operation. The tool must be

extremely rigid to achieve the precise bore with the extremely hard, difficult-to-machine Alloy

625.

Typical Clad Surface Before Machining

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The second operation is to machine the actual "J" bevel geometry on the pipe end. This is done

with a second tool which clamps in the pipe ID. It is important to note that the land extension of

the "J" bevel is entirely composed of the Alloy 625 material - any carbon steel pipe must be

completely machined away during the beveling process. Any ferritic material in close proximity

to the bevel end will melt during root pass welding, and unacceptably dilute the root bead

deposit, severely reducing corrosion resistance of the weld during service. The land extension

must therefore be of adequate dimension to ensure that the carbon steel side walls do not melt

during root pass welding.

The importance of a detailed, comprehensive pipe specification when ordering the pipe, and

proper QC oversight during manufacture, can be understood in light of the above requirements.

If the Alloy 625 buildup after deposition are below limit, it will be impossible to achieve a root

face (land thickness) of 1.4 ± 0.2mm solely composed of the CRA. If the pipe end calibration at

the mill is inadequate and results in unacceptable dimensional variation and out-of-roundness, it

will not be possible to achieve this 1.4mm CRA-only land extension during field beveling, and

acceptable high/low during fit-up.

After beveling, pipe ends must be cleaned using a lint-free cloth wetted with an approved

solvent.

The machined bevel must be checked to verify that no carbon steel (ferrite) particles adhere to

the clad material surface. A solution of copper ammonium chloride is used. If any trace carbon

steel particles are found on the clad surface, the cleaning and ferrite test procedure must be

repeated.

Once it is determined that the clad portions of the pipe end bevel are free from ferrite

contamination, the bevel is checked using a Liquid Penetrant Examination (PT) in accordance

with an approved procedure. This serves as a final check that there is no lack of fusion

Typical Joint Design for GTAW Welding

Joint Fit-up is Critical

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between the clad ID and the carbon pipe. Sulfur-free cleaners, penetrants, and developers

must be used in PT.

CRA lined pipe is often associated with magnetic fields high enough to interfere with welding.

All arc welding processes are subject to disruption in a magnetic field. This is caused by

deflection of the arc, which is forced to follow a curved path when passing through the magnetic

field (known as 'arc blow'), leading to a number of weld defects. Some welding processes are

more sensitive to arc blow than others. The effects of arc blow are reduced by welding at higher

currents which produces a stiffer arc. GTAW welding tends to be more sensitive because of the

lower arc voltages used.

The lined pipe ends must be checked using a Gaussmeter. If the residual magnetic field is

more than 10 Gauss, the pipe ends must be demagnetized. An automatic degaussing system

generally accomplishes this task.

FIT-UP AND GAS PURGING

The prepared pipe ends are fit up using an internal clamp. Bevel misalignment (High/Low) can

be no more than 1.0mm. While ID clamps are commonly used for pipeline applications, CRA

lined pipe dictates several special requirements. The clamping components which come in

contact with the pipe ID must be fabricated of austenitic stainless steel to avoid contamination of

the CRA.

Demagnetization is done using an automatic system.

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When a root pass is made on carbon steel pipe, no special provisions are necessary to shield

the ID root bead from oxidation. However, elimination of an oxidizing interior atmosphere is a

requirement for root bead welding of CRA clad pipe, including Alloy 625. An inert backing gas,

most commonly welding grade argon, protects the ID weld bead and adjacent CRA material

from oxidation during welding. Purging a pipe first displaces air from the pipe and prevents air

re-entry, and prevents oxidation of the root bead surface.

The time required to completely purge a pipe length (and argon cost) is not acceptable for field

pipeline welding. The internal lineup clamp is equipped with a gas purge delivery device built

into the clamping components, allowing delivery of argon directly to the immediate weld area.

Flexible high temperature rubber "dams" create a small chamber on either side of the weld,

minimizing the volume of pipe (and thus time) that must be purged. Argon gas is supplied via

an inlet hose. An outlet hose allows air to exhaust and prevents pressure build up. An oxygenmeasuring

device located at the end of the outlet hose indicates when the oxygen percentage is

low enough to begin welding as the typical maximum level of oxygen permissible is under 100

ppm, an oxygen monitor must be suitably accurate to measure under 100 ppm (0.001%).

PREHEATING

Generally preheating is done manually with rosebud torches to 60°C, sufficient to eliminate any

residual moisture.

WELDING

Welding was done using a Magnatech orbital GTAW welding system supplied by the Pipeline

Group, Magnatech International, based in The Netherlands. Factors contributing to this

decision included the lack of skilled labor, the very high welding quality requirements, and the

project timetable. The Pipemaster 516 and T-Head is a "bug and band" system similar to

Internal Line-Up Clamp designed for inert gas purging of Root Pass bead

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GMAW and FCAW systems commonly used for carbon steels, but is designed for the

requirements of multi-pass GTAW. The bands are sized for a specific pipe diameter and clamp

360 degrees around and to one side of the weld joint. The "bug" (head) clamps on the band

and mounts a 300-amp water-cooled torch. The head incorporates mechanisms to move the

torch in similar fashion to a manual welder: torch rotation around the joint; an on-board wire

feed device; and electronic Arc Voltage Control (AVC) to maintain a stable programmed arc

length; and an electronic weave mechanism, with independent control of torch weave width,

speed, and end point dwell.

A typical pipe welding system consists of the weld head, and a small remote control, both

connected with an 8-15m cable to the controller/power source.

The controller, Magnatech's Pipemaster Model 516, is a digital programmable system. It is

designed to operate with a commercially available power source of suitable amperage output.

The entire weld program can be stored and called up from a library. Very importantly, the

welder can be prevented from indiscriminately changing parameters. Weld supervision or QC

can restrict parameter changes on an individual basis. For example, the operator can be

allowed to vary rotation speed and weave width by +/- 5% to compensate for field fit-up, but be

given no ability to change amperage. The Pipemaster Model 516 has a data output port,

allowing data logging of amperage, voltage, rotation speed, and wire speed. A 165-kVA

generator supplied each weld station.

PDO required the weld made for WPS/PQR qualification be data-logged. Magnatech supplied a

custom-built data-logger to capture and store parameter data.

A 1.0mm filler wire (AWS A5.14 ERNiCrMo-2) was used for all passes. Pulsed DC current was

used, and a 150°C maximum interpass temperature limit was observed.

Welding is performed with an orbital GTAW process welding system.

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FIELD INSPECTION

Welding was done in multiple stations, with two weld heads per station, each welding 180° of

the joint circumference.

Weld Sequence

As GTAW is a gas-shielded process, a shelter (tent or rigid) is required as a wind barrier.

Tekfen used metal 'huts' or shelters of their own design. Two welding systems were mounted in

each hut. The first station welded 3-4 passes (approximately 6mm weld deposit thickness)

before moving, and three stations completed the weld.

Welding Shelter. Note canvas "floor" to minimize dust

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Completed welds underwent an immediate Visual Examination (VE). Every weld was subjected

to both radiographic and ultrasonic Testing to API 1104 (amended), in addition to VE from the

inside using a boroscope. Future projects are specifying laser-profiling inspection of the ID

bead.

"Although Tekfen has significant experience in cross country pipeline projects in the Middle

East, this project being the first onshore CRA-clad pipeline construction in the world was a

challenge for all disciplines and parties in Harweel. With the potential of many more of these

projects in the near future, Tekfen felt that Harweel would be the important forerunner.

Magnatech, along with their knowledge and technical personnel, partnered with us in solving the

problems that arose during the project and contributed to the project success", states Cem

Ozbay, Engineering Manager. The project was successfully completed in 2010.

John Emmerson, Magnatech LLC, USA