AUTOMATIC WELDING SYSTEM GETS FIRST USE IN NORTH SEA

Allseas Marine Contractors SA's new automatic welding system "Phoenix" has been first used aboard the company's Lorelay pipelay vessel for Amoco Netherlands as part of a gas development project in the Dutch North Sea. The system was used to weld approximately 40 km of 26 in. x 16 mm (0.630 in.) W.T. carbon steel gas-export pipeline between the P15D platform and the tie-in to the landfall section at the Hook of Holland, near Europort in The Netherlands. Repair rates were lower than for

Nov. 1, 1993

4 min read

The system was used to weld approximately 40 km of 26 in. x 16 mm (0.630 in.) W.T. carbon steel gas-export pipeline between the P15D platform and the tie-in to the landfall section at the Hook of Holland, near Europort in The Netherlands.

Repair rates were lower than for any existing system, says Allseas.

PROGRAMMABLE SYSTEM

Phoenix uses gas-metal arc (GMA) or flux-cored arc processes for welding offshore pipelines of 6 in. OD and greater.

The system consists of the following components (Fig. 1):

The welding converter unit (WCU). The 450-amp constant voltage welding converter supplies more power than needed and provides good arc stability at low voltage ranges.
The wire feed unit (WFU). The WFU is powered by a digitally controlled motor which monitors and adjusts the actual wire feed speed continuously, as defined in the preprogrammed welding parameters.
The unit is mounted on the welding trolley, a working platform on rails, located in each welding station which travels with the pipe to compensate for pipe movement.
The bug-control cabinet (BCC). The BCC standalone computer supervises and controls the welding process and contains the preprogrammed parameters. The welding parameters are predefined and controlled for each pass as a function of the location of the bug on the pipe circumference.
These parameters are initially programmed, then qualified, and finally stored and locked in the computer.
This offers the advantage of their repeated use on any welding station in the firing fine, every possible pass, and each joint.
The bug. The compact bug is clamped onto the band by a quick-release system; it manipulates the torch.
The torch is water-cooled to reduce spatter and to improve the contact tip fife in production, therefore reducing downtime. All bugs are identical and can operate without reprogramming on both the starboard and the port side while positioned on the same band.
The positioning band. The bug travels on a hinged shell band consisting of two halves, with a quick-release coupling.
The band has been designed so that elastic adjustment to the out-of-roundness of the pipe is possible, within limits.
The bug remote control (BRC). The bug's movements, the preprogrammed welding parameters, and any controlled adaption to these parameters, within preset limits, are operated with the use of the hand-held BRC.
This allows the welder to seek optimum position with respect to the puddle and pipe position without coming into contact with the bug or the pipe.

The system is capable of producing high-quality welds in carbon or stainless steels, san,s Allseas.

Because the system follows preprogrammable welding parameters, as stated, and therefore allows repeated use of these parameters for each joint, adherence to welding parameters is ensured, and weld quality guaranteed, says Allseas.

All passes are applied externally, with the root pass using backing shoes positioned on the internal lineup clamp.

Either solid wire or flux-cored wire in both the uphill and downhill welding directions can be used.

If required, different shielding gasses can be used. Most commonly, Argon-CO, mixtures are used.

OPERATION

The welding operator uses the BRC (Fig. 2) to select the pass number, position the torch, and start and stop the process. Minor adjustment to some parameters is permitted in within the limits defined by the welding procedure to allow for small variations in the geometry of the aligned pipe-ends.

In order to reduce downtime and ensure optimum reliability, Allseas subjected the electronic components of the system to extensive testing which simulated field conditions.

Allseas says its personnel, who have all received inhouse training, operate, maintain, and support the system.

OFFSHORE OPERATIONS

Allseas says that presentations of the system were made to Amoco's specialist welding personnel and project management representatives and, after a qualification program, Amoco consented to first application of the system on the 26-in. pipeline.

Before mobilization to the P15 field and to simulate offshore operations, the firing line aboard the Lorelay was the site of a trial run that used concrete-coated line-pipe of the same material grade and dimensions.

The trial was purposes included in the operations schedule to reduce risk of start-up problems with the welding system, says Allseas. Start-up of the 26-in. pipeline would occur in conditions that could cause problems for dynamic positioning strong currents in limited water depth.

Acceptance criteria for pipeline welds made with short circuit GMA welding stipulated a maximum length of 16 mm on radiographically and 10 mm on ultrasonically detected lack of-side-wall fusion defects. This criterion imposed a strict control on welding parameters.

Within 2 dan,s after start-up, the estimated production rates were achieved and gradually expanded to top production rates greater than expected.