When three strong earthquakes shook Tennant Creek in the Northern Territory on 22 January 1988, local welder Laurie Atkins had no idea that he would soon be called in to work on one of the most interesting and challenging pipeline repair jobs in Australian history.
The region had previously been considered seismically stable, but the quakes– escalating to 6.8 on the Richter scale – were strong enough to cause unexpected damage to the Amadeus Basin to Darwin natural gas pipeline.
“Someone called to let us know that [the damage] had happened, and we were asked how quickly we could mobilise,” Laurie says. “We started to act straight away.”
Laurie worked as part of a team of site managers, engineers and other personnel to weld and repair a section of the pipeline located above a reverse slip fault, which transgressed the line at right angles.
Dealing with the damage
The approximately 1,600 km long pipeline traverses the length of the Northern Territory from the Amadeus Basin to Darwin, principally of 12 and 14 inch diameter pipe.
The January 1988 shocks displaced a section of the mainline’s grade 5L X60 steel pipe, telescoping the pipeline so that it flowed plastically in a concertina-type formation. One of the main reasons that the pipeline did not rupture was because the Australian manufactured steel was of high cleanliness, resulting in a product with a high degree of ductility. This meant it was able to withstand a compressive strain of more than 20 per cent without cracking.
Operator NT Gas spearheaded the process of checking the pipeline system for faults, and when the compression failure was discovered near Tennant Creek, Laurie and his team were rushed in to ensure that it could return to supplying gas to Darwin and several other power stations in the region.
“Everyone had the same thought: they couldn’t understand how it had happened,” says Laurie. “It was all new to everybody and very new to me. It was the first time it had happened in Australia, I think, so no one had a lot of experience with it or knew what to expect.”
Laurie and his welding partner Tony Blew arrived on the site and met with NT Gas to establish a work plan for getting the pipeline back online as quickly and easily as possible.
At the site the pipeline was being progressively exposed toward the fault line in order to relieve the stress in the pipe. As this was relieved, the pipe snaked all over the place. The repair involved cutting out approximately 100 m of the stressed pipe and replacing it with pre-hydro tested thicker wall emergency pipe.
Laurie undertook preparatory work, and settled on a method for cutting the 14 inch diameter pipe at the location of the concertina-type failure.
“Because it was in a concertina shape, once it was exposed by backhoes, and still had some stress in it, nobody actually knew which way the pipe was going to swing or move when we cut it,” Laurie says.
“We had to drill a hole in it and light it up and then cut it live with oxy-acetylne cutting equipment, and as the flames were coming out, I got in the bucket of a backhoe to be lowered into the hole to cut the pipe.
“We asked everybody which way they thought the pipe might move, because you can never ask too many people. Cutting the bottom of the pipe was the hardest, because we had no idea which way it was going to go. If it had swung up it might have hit me.
If it went left and I was standing there it would squash me.”
Laurie and Tony cut the pipe on a taper, which allowed it to swing away from the taper side and away from themselves. Thankfully, the pipe swung as predicted; the stress was released, and the welders could continue working on the repair.
“You wouldn’t be allowed to do that these days, but we had no choice,” laughs Laurie.
Aftershock: repair undertaken in tough conditions
It was a difficult job in a tiny trench, completed under “one hell of a time strain” according to Laurie.
“We had a caravan onsite, and we caught a little bit of sleep whenever we could, when there was something else happening that we couldn’t work on,” he says.
Weather extremes of searing heat up to 45 degrees Celsius during the day, flies by the millions and over 2 inches of rain on the second night caused much discomfort. Laurie says “We couldn’t wait to get the weld out of the way so that we could pull our bloody shirts off for a minute, it was that hot and sticky. We were out in the desert sand in a ditch that was half-full of water, and we couldn’t pump it out but we still had to work.
“Aftershocks came through frequently, but we were really busy. I remember feeling a few of them but I was doing lots for the repair so didn’t even have time to take notice.
“After one of the aftershocks happened the excavator driver got out of the cab and started walking. No-one worried about it too much because it was one hell of a walk (50 km) to Tennant Creek. After a while he didn’t come back so we sent a vehicle to look for him. He might still be walking to Tennant Creek cause we never found him!”
Laurie recalls that an amazing support network helped the job to run reasonably smoothly.
“People put in a lot of help, from the workers’ wives who retrieved water, to the helicopter pilot who made sandwiches for us all – it was a team effort.
“I remember one of the blow-down stack valves got chewed out in 45 seconds. This happened during the blow down of the pipe section which was necessary to be able to perform the repair, and was caused by erosion of the steel by sand granules left in the pipeline from construction. I fabricated a new blow-down stack, and a guy flew to Darwin and got a valve, and at midnight that night I was putting it back in.
“Without other people on the sidelines helping, the repair wouldn’t have happened.”
Laurie credits Site Engineer Steve Dykes and Site Superintendent John Oxtoby for helping the repair job to reach a quick conclusion. From the time it was blown down to the time the gas was back on, it took just under 72 hours. He says Steve and John thoroughly consulted the welders before tackling repair tasks that had never been seen before in Australia.
“I’ve worked on quite a few tough ones, but that one was just so new,” Laurie says.
“I learnt a hell of a lot from it – the challenges were different. Nothing went wrong, nobody was hurt, and it went back online well without the lights going out in Darwin.
“I also heard later that an operator in Darwin had put in a huge effort constantly changing the filters at the city gate station. As the gas velocity increased in the pipe, the gas picked up all the dust from construction and blocked the filters in a matter of hours. Ronnie would just have finished replacing the cartridges in one filter and the other one was choked. NT Gas flew in a charter jet full of filter cartridges just to keep the gas flowing to the Channel Island power station.
“Celebrations were started immediately after the final weld but were subdued until the suspense of waiting on the x-ray result was lifted. The final x-ray passed and gas was slowly brought back online via the now repaired mainline block valve by pass. Brian’s Pub in Tennant Creek was in full swing for a night. A few cleansing ales were had with the local constabulary after closing time too! NT Gas flew in seafood from Darwin for the crew that night.”
A total of 93 m of thin wall pipe was removed and replaced with heavy wall, and the new section of the Amadeus Basin to Darwin pipeline was 970 mm shorter than the original. While other sites that showed surface indications of possible residual stress on the pipe were excavated for safety inspections and later backfilled, the site of the welding repairs was left exposed to allow the pipe to continue to flex and absorb any further movement along the fault line. The new section was wrapped with tape and painted to reduce ultra violet degradation of the coating.
The pipeline resumed normal operations on 30 January 1988, and Laurie resumed normal life as Managing Director for A&B Welding.
“It was very hard work, but we all look back on the repair and think we did a good job of it,” says Laurie.
