Deepwater and gas-fired power projects have revitalized the pipelay sector, but this trend is adding to pressures on welding specialists. The pressures include the following:
- Speed up weld productivity for long distance pipelines
- Apply automated welding to small diameter lines
- Adapt techniques for J-lay installations
- Develop new systems compatible with alloys and higher strength steels.
The J-lay installation on Girassol necessitated fitting of a clamp for the Saturnax welding bugs.
Over the past year, Serimer Dasa, based north of Paris, has had to address all these issues for clients in the Gulf of Mexico, Middle East, South East Asia and the North Sea. Owner Stolt Offshore is its main customer, but since 1998 Serimer Dasa has been free to work for rival offshore pipelay contractors. They currently account for over half its revenue.
For offshore lines, the company offers the Saturnax and Saturne 8-torch systems. Saturnax, launched in 1988, is a GMAW bug-and-band system that features a dual-torch welding head. This is designed to increase substantially the normal rate of metal deposition in a single run. Additionally, the annealing effect of the second torch increases the weld's toughness while diminishing its hardness.
Saturnax can be used on pipelines with diameters of 6-in. and up. Welding is performed entirely from the outside of the pipe on a narrow J bevel, using a internal line-up clamp fitted with a backing ring that supports the weld pool during the root pass. All the system's elements are lightweight and all its bugs are identical. Welding stations are interchangeable; each is capable of completing one weld from root to cap, using the same shielding gas throughout.
Saturne 8-Torch is an automated, mechanically-guided mainframe system for pipelines of 24-in. diameter and upwards. The system consists of four arms, each fitted with a dual-welding head (hence the designation 8-torch). These bugs travel around the pipe simultaneously in a horseshoe-like format.
South Pars record
The 13% chrome line installation for Wintershall in the Dutch sector.
One of the company's higher profile assignments last year was on South Pars in the Persian Gulf, where operator TotalFinaElf was installing two 105 km, 32-in, diameter, 20.6 mm wall thickness (WT) lines between the South Pars platforms and a gas treatment plant at Assaluyeh. Hyundai Heavy Industries was the contractor, using its laybarge HD2500.
In January last year, Serimer Dasa inspected the vessel in Bombay prior to mobilization to size up the equipment required. By May, automated welding units had been installed on the barge which had since moved to Sharjah. Over the following month, welders were qualified for the work onboard the vessel.
After a slow start for the shore-end line pull, offshore laying accelerated at the end of July and was completed in November. TotalFinaElf had stipulated zero defects in the weld roots, due to the gas' corrosive hydrogen sulfide content. The peak achieved in one day was 251 welds - given that welding was undertaken simultaneously on a 4-in. piggyback line, this is thought to constitute a world record.
More importantly, the exercise was completed without a single cut-out. The operator had also demanded a hardness of below 248 Hv10 for all welds in these X65 steel pipelines, which was accomplished.
Early this year, Serimer Dasa achieved two more firsts in separate projects. For Bluewater's Ladybug installation in the Gulf of Mexico, it managed 563 welds of 6-in. diameter, 12 mm WT in one day via three Saturnax stations on Stolt's Seaway Falcon. (The previous record was 405 welds on a 6-in., 14.3 mm WT line).
The experience was Serimer Dasa's second working on the vessel, but this time weld productivity was 30% better. According to Sales Manager Nabil Aouad, each vessel's team has its own working procedures, and familiarization takes time. Extraneous delays can arise, for instance, due to barge move-up, late running of an anchor or winch operation, or a thruster failure.
The other milestone was set on Girassol off Angola in March, when Serimer Dasa undertook its first J-lay assignment, in 1,350 meters water depth. The 10 km of the two 12-in. lines laid so far featured a 0% weld repair rate (zero defects). This project also marked the debut for the pipelay vessel Polaris' new J-lay ramp. To operate in J-lay mode, some adjustments had to be made to the Saturnax system:
- The clamp had to have additional safety devices such as brakes fitted to prevent it from plunging off the moving pipe.
- In the event of a cut-out, the weld would have to be beveled into place. But this ran the risk of metal shards entering the pipe. The beveling machine had to be equipped with a shard collector.
- During vertical pipelay, welding must be made continuous through non-stop rotation of the bug around the weld bevel. On Girassol this was effected, leading to welding cycles of 10 minutes for a 12-in., 22.2 mm WT line.
- With one welding bug following another, there is a tendency for umbilicals and cables to get twisted. This was addressed through having just one bug turning three full, continuous runs before unwinding the umbilicals and embarking on subsequent runs.
- With standard lines using S-lay, a certain level of defect in the weld is acceptable. But for most deepwater lines (particularly steel catenary risers installed via J-lay), even a small flaw can prove detrimental, due to fatigue.
Serimer's R&D program introduced several procedures to improve the quality of the weld (fulfilling the zero-defect requirement) and to conserve the MIG-MAG process, which leads to higher productivity.
13% chrome solution
Last year, Serimer Dasa was drafted in at late stage to develop 6-in. internal clamps and ancillary equipment for a stainless steel application to weld 6-in. and 12-in. flowlines for Wintershall in the Dutch sector (L6-P4). It transpired that the originally nominated system lacked the capability to handle pipes with a 13% chrome content. According to Aouad, the normal weld rate for this alloy is 15 cm/min, but Serimer Dasa achieved 60cm/min, using a pulsed GMAW technique.
The market for 13% chromium is expanding, Aouad claims, as stainless steel flowlines are 3-4 times more expensive.
"We are now working on welding techniques for X80 and X100 steels. X80 lines are appearing now on land, and the offshore sector will follow. Manual welding with these materials is extremely difficult, so the operation will have to be automated. We are already qualified for X80. This material could be chosen for the proposed offshore gas line from Sakhalin Island to Japan."
