Fabrication and installation of duplex stainless sealine

Sensitive handling and welding conditions necessary for N'kossa project

N. Saute
S.A.S./Inox Projects
M. Sauvage
Elf Aquitaine Production

Location of the duplex stainless steel pipeline installation.

Eighteen-inch outside diameter, 22.05 grade duplex stainless lines were fabricated and successfully installed in the N'Kossa Field off the Congo Republic recently. Both the laying operation and the two connections made by hyperbaric welding at a depth of 170 meters are unprecedented achievements.

The wellheads are supported by two fixed platforms, while the production equipment is mounted on a 220-meter long by 46-meter wide prestressed concrete barge. Two storage tankers - one for the oil and one for the liquefied gases (propane and butane) - transfer the products to export tankers. The residual water and gas are reinjected into the reservoir.

The 18-in. outside diameter pipeline, which has a 17.45 mm wall thickness, transports the two-phase oil and gas flow between the two platforms, and has a length of 3,240 meters.

The choice of 22.05 duplex stainless steel was dictated by the severity of the operating conditions involving a corrosive effluent (1% mol. carbon dioxide), high pressure (230 bars under closed wellhead conditions), and a high temperature (130 degrees C), at which commercially available corrosion inhibitors are ineffective.

The choice of pipe supplier was based on the results of prequalification tests. Manufacture of the grade 65-LC-22.05 pipes was specified to meet the API 5LC standard, with a number of additional requirements corresponding to control of the ferrite content, corrosion testing, low temperature impact testing, and a minimum nitrogen content in both the base metal and the weld filler metal. Furthermore, tighter tolerances were demanded on the inside diameter, wall thickness, and out-of-roundness, in order to ensure a maximum success rate during welding.

Pipe lengths of 12 meters were produced by butt-welding two 6-meter-long sections of longitudinally welded pipe. The longitudinal welds were made using the plasma arc (PAW) process with filler metal for the first pass, followed by filling and capping passes on both sides using the submerged arc (SAW) process. The filler metal employed was 22.05 grade duplex stainless steel, with argon as the shielding gas. After welding, the longitudinal welds were given a full (100%) inspection by X-ray radiography.

The 6-meter lengths were then given an annealing treatment, followed by pickling, passivation, rinsing in fresh water, and calibration. They were then assembled by girth welding, in the horizontal position, with the tubes rotating, using the gas tungsten arc (GTAW) process with filler metal for the first passes.

Filling and capping passes were then performed by the SAW process, before making the internal pass using the GTAW process, with the same 22.0.5 filler metal as for the first pass. The girth welds were fully inspected by X-ray radiography. Each 12-meter-long pipe was then subjected to a hydrotest, followed by drying.

Special precautions

The particular specifications imposed for welding included thorough cleaning of the bevel faces and neighboring zones with chlorine-free solvents before assembly, and the use of a duplex stainless steel filler metal with a nickel content greater than 7.7%.

The shielding gas used for making the girth welds (not annealed) had to contain 3-4% nitrogen to compensate for losses from the alloy, while the addition of hydrogen was not permitted. Monitoring of the oxygen content of the backing gas was required, with a maximum allowable level of 500 ppm during the first two passes of the girth welds. Preheating was not required, and the interpass temperature was limited to a maximum of 150 degrees C.

The fabrication and welding of pipes had to be carried out in a clean zone, and all tooling had to be made from stainless steel. All cutting operations had to be performed only with a plasma torch. During both manufacture and transport, the pipes must never come in direct contact with plain carbon-steel, in order to avoid all risk of iron contamination.

A total of 642 longitudinal welds and 321 girth welds were inspected by visual examination and X-ray radiography. Repairs to unacceptable defects were classified as either major or minor repairs, depending on their dimensions and their position with respect to the weld axis, since the specification included a clause limiting their number and length.

Repairs were made by manual welding using a coated electrode. The proportion of welds needing repairs represented 0.8% of the longitudinal welds (5 welds or tubes were rejected). and 0.7% for the girth welds (2 welds were eliminated and remade).

Pipe protection

A 6-mm thick coating of polypropylene was chosen to protect the pipe external surfaces from the seawater, taking into account its resistance at the effluent temperature of 130 degrees C. After shot blasting the surfaces, the pipes were heated and a 60-80 micrometer thick primer coating was applied, immediately followed by a 250-300 micrometer thick layer of polypropylene adhesive.

The final layer of polypropylene was applied by hot extrusion and the coating was subsequently cooled with water. The pipe ends were not coated because of the welding operations to be performed on the pipe-laying barge.

In order to avoid damage due to mechanical shocks and prevent the entry of dirt, the pipe ends were temporarily protected by polyethylene caps with small ventilation holes.

Pipeline assembly

The pipeline was laid from Saipem's Castoro 8 barge. The operations were supervised by Elf Congo and CEP (Controlle et Prevention). The pipe sections were joined by automatic pulsed arc orbital TIG welding with filler metal for the first two passes. This choice was made because of the good reproducibility of this process and the need to precisely control the welding parameters.

These first two passes were made at station 1 on a narrow gap bevel, with two welding machines moving on a rail encircling the tube, in downhill welding position. The filling and capping passes were performed at stations 3 to 6 by two manual TIG welders, using filler metal, in downhill welding position. Because of the specified metallurgical requirements, a 25.07 super-duplex grade was used as filler metal for the first two passes and a 22.05 grade for the filling and capping passes. Argon was employed for the shielding gas.

Welding conditions

In order to attain the desired quality and productivity objectives, it was necessary to suitably adapt the pipe-laying equipment. After inspecting the barge, certain modifications to equipment liable to be in contact with the pipes were found to be necessary (tension block shoes, transfer line rolls, etc.). A final acceptance inspection of the barge was performed before the start of production welding.

The equipment used for performing the girth butt welds had to be specially adapted, particularly with regard to the following requirements:

A chamfering machine enabling perfect machining of the pipe ends
The internal alignment clamp was modified to allow the introduction of the shielding gas necessary to avoid oxidation of the weld during the first two passes
An oxygen analyzer was provided for monitoring the oxygen level, which had to remain less than 500 ppm
An automatic welding machine, with preprogrammed welding parameters, only the height and lateral position of the torch being adjustable by the welding operators
X-ray radiographic inspection of the first two weld passes.

Production welding

The pipes were carefully stored on the deck of the barge and handled with slings equipped with aluminum hooks. They were brought one by one to the transfer line as needed. After thoroughly cleaning the ends to be joined, each new section was rigidly assembled by the internal alignment clamp. After verification of the perfect adjustment of the joint, the argon flow was started and welding was begun as soon as the oxygen level fell below the 500 ppm maximum limit.

Welding was performed continually, 24 hours a day, in two 12-hour shifts for the automatic welding, with two operators per shift, and in three 8-hour shifts for the manual welding, with two operators per station per shift. This organization was adopted to take into account the difficult working conditions on the site (heat and humidity).

The average time necessary for the first two welding passes was about 20 minutes. The total cycle time, during which the barge remained immobile, was about 35 minutes, determined by the following series of operations: abutment of the new pipe section, alignment, stabilization of the argon flow, performance of the first two weld passes, advance of the pipe to station two for radiographic inspection, radiographic exposure, and development and interpretation of the film.

The filling and capping passes at stations 3 to 6, the final radiographic examination at station 7, the fixing of sacrificial anodes at station 8 and plastic coating of the weld zones at station 9 can be considered to take place in parallel with the above operations.

These times enabled a maximum rate of 2.3 welds per hour (55 welds per day) to be attained. This cruising speed was reached only after four days. For the whole operation, the overall average rate was about 1.25 welds per hour (30 welds per day).

Altogether, 3,240 meters of pipeline were laid in 8.5 days.

Non-destructive inspection

A total of 267 welds were inspected at station 7 by visual examination and X-ray radiography. The defects observed can be divided into two categories, corresponding to point defects and repetitive defects.

Point defects represented only a very small percentage, and were practically unavoidable, due to the large number of possible causes. The repetitive defects, whether permissible or otherwise, had to be rapidly analyzed in order to determine their origin and immediately take the necessary steps to repair them and if possible to prevent their future occurrence. In all, 14 repairs were made, corresponding to 5.24% of the total number of welds

Most of the repairs (10) corresponded to lengths between 10 mm and 70 mm, with four others between 100 mm and 200 mm, with depths between 9 and 12 mm. Because of the depth, the first two passes were not of concern. The repairs were carried out by manual TIG welding at station 7. Typically, each repair stopped the barge for about one hour.

An aluminum anode was fixed to the pipeline every six sections, at station 8, corresponding to a total of 46 anodes. The anodes were welded to the pipeline with the aid of two 22.05 duplex stainless steel tabs. These operations took about 26 minutes to perform.

The protective plastic coating had to be made in the weld zones after welding, This was performed at station 9 by first of all applying a primer, then winding four layers of strip, followed by a final strip for mechanical protection. The time necessary for these operations was about 10 minutes. The integrity of the weld coatings was inspected using a holidays detector.

Hyperbaric welding

The two connections of the pipeline were made at a depth of 170 meters from a dynamically positioned floating support vessel, the Seaway Harrier. The welds were performed in downhill welding position using a single-head automatic orbital TIG machine, with filler metal. Because of the heliox (helium plus oxygen gas atmosphere in the welding chamber, a gas-tight hood had to be placed over the weld zone to enable an argon cover to be used during welding. A super-duplex filler metal was used for all the welding passes.

The welding procedure had previously been established with the aid of simulation installations, reproducing the real seafloor working conditions, with the same equipment (current generator, umbilical connections, welding head, etc.).

Since iron contamination due to contact with carbon steel had to be carefully avoided, such heavy items such as the welding chamber and lifting gear were equipped with protection pads made of nylon or similar material, preventing direct contact between the steel and the pipeline.

In the welding chamber, all machining, measuring, alignment, and welding tools were specially adapted for use with duplex stainless steel, being either protected at their points of contact or made from stainless steel.

A ventilation hole was drilled in the 12 o'clock to 2 o'clock segment of the weld root to allow the pipe to breathe during the first four passes, to avoid the possibility of collapse due to potential differences in pressure. This hole was subsequently plugged. The oxygen content of the backing gas was monitored to make sure it did not exceed the maximum permissible level of 500 ppm at atmospheric pressure, corresponding to 27 ppm at a depth of 170 meters.

The two welds were inspected by visual examination and by gamma source radiography. The radiographic inspection took an average of two hours per weld, due to the necessity for the diver to leave the chamber during the exposures (6 films per weld). The acceptance criteria were the same as those applied to the pipeline welds. The defects revealed were considered to be acceptable and not to require repairs.

The protective coating was rebuilt in the weld zone with the aid of two layers of polypropylene strip, without either prior application of a primer or a final mechanical protection layer.

The pipeline was first of all cleaned with the aid of a scraper pig equipped with stainless steel brushes, following the injection of a fresh water plug. The line was then calibrated with a pig equipped with a 12-mm thick aluminum gage plate, with a diameter equal to 90% of the nominal inside pipe diameter.

After inspection and acceptance of the aluminum gage plate, the line was filled with fresh water and a test manifold was mounted on the flange at the top of the riser chosen for the pressurization. A full flange with a venting valve was mounted on the top of the other riser.

The pressure was then applied gradually. After thermal and mechanical stabilization for about six hours, the test pressure was maintained for 24 hours. The test was then considered conclusive and the pipeline was then depressurized, emptied, and dried.

Conclusions

The laying and underwater joining of a duplex stainless steel pipeline at a depth of 170 meters represented two world first achievements at the time.

Special specifications were drawn up for the project, with additional requirements compared to those of standard procedures, including corrosion tests, measurements of ferrite content, low temperature impact bending tests, and the imposition of a minimum nitrogen content in both the base metal and the filler material.

Although the development of appropriate procedures and specific tooling were essential factors, probably more important was the control of the welding environment, due to its immediate influence on weld quality. High performance automatic welding systems played a major role in the success of the operation.

The personnel involved in the manufacture and welding of the 22.05 duplex stainless steel pipes must be highly trained, since any deviation from the prescribed procedures can lead to defects and the need for repairs, with deleterious consequences on pipelaying operations.