Instead of bringing two redundant platforms to shore, a Gulf of Mexico operator chose to redeploy them on new discoveries in greater water depth by adapting the jacket legs with plinths to raise their height.
ATP Oil and Gas has since received official recognition for using the platform plinths - selected out of more than 100 entrants - with an Offshore Energy Achievement Award for innovation and technology.
Two platforms, a four-leg jacket and a tripod were due to be abandoned by ATP at its Vermilion 410 field complex in the Gulf of Mexico. The company was considering its options when the opportunity to redeploy them on two other discoveries at Garden Banks 142 and Ship Shoal 358 became available.
Instead of removing the Vermilion platforms and shipping them ashore for disposal, and then building two new ones for the new discoveries, a concept for lengthening the existing Vermilion platform jackets by providing them with pedestals or plinths was conceived which would allow them to stand in greater water depth on the new discoveries.

Aside from the 25% construction costs saved by not building two new taller platform jackets, reusing the existing platforms also allowed the production schedule for the new discoveries to be accelerated. ATP also took the opportunity to install platform drilling rigs on refurbished decks at its new developments, allowing the option of more wells without the extra cost of a semisubmersible or jackup drilling unit to drill production wells later.

Robert Shivers, projects vice president for ATP, was the engineer responsible for reusing the two Vermilion gas platforms by re-engineering them with plinths for greater water depth. "It is just a pedestal," he said. "One plinth was 60 ft (18.3 m) and the other 180 ft (54.9 m) tall," explained Shivers. This allowed both platforms at ATP's Vermilion 410 field, in a water depth of 365 ft (111 m), to be redeployed to Garden Banks 142 and Ship Shoal 358.

Building the plinths emerged as a solution both to an abandonment problem at Vermilion and a desire to keep costs down on the new developments.

Leland Tate, chief operating officer at ATP, said, "The solution came from the issue of having to abandon platforms at Vermilion, having reached the end of their economic life. We were just getting ready to abandon them. And then we had this reuse opportunity and Robert looked into the engineering of how to reuse the structures." ATP's desire to tap new discoveries at Ship Shoal and Garden Banks matched the timing for abandonment at Vermilion almost perfectly. What was needed was a compressed project schedule to ensure the new plinths would be ready in time for first gas at the new developments.
"Pinnacle Engineering in Houston has this core strength of structural steel and we knew we had to do something with Vermilion and we had another two fields that needed development," Tate said.
It was a perfect opportunity to marry the demands of the three assets together.

Building a new platform for each of the new developments was clearly a high cost option, while reusing a larger facility suited to the water depth of the new developments was equally difficult - unless the redeployed platforms can correctly match the depths at Ship Shoal 358 and Garden Banks 142, 419 ft and 542 ft (128 m and 165 m), respectively.

"It is so difficult to redeploy jackets," Tate said. "They have to have piles and sometimes these piles cannot be reused."

While redeployment in shallow water, in 50 ft (15.24 m) for example, is easily possible with a small platform which can be lifted on a hook by a small derrick barge, it is a different question in much greater depth.

And ATP had a schedule to do it all in 180 days, whereas designing, engineering and building a new jacket would normally take twice as long.

Shivers said the Vermilion jackets were picked up on the hook and wet towed to their new locations suspended from derrick barges where they were set down on newly built plinths. "The plinths are more like separate, stand-alone structures," he said.

One of the first moves was to send divers into the water to check the existing Vermilion jackets to make visual checks and take field measurements for comparison with the original design engineering drawings. "That was to ensure that the plinths would mate with the jackets," said Shivers. Major work got underway in March 2003. Vermilion platform decks were removed first and shipped back to shore for refurbishment.
Heerema Marine Contractors used its Balder deepwater construction vessel to install the new plinths, designed by Pinnacle and built by Twin Brothers Marine in Louisiana, which were secured with skirt piles.
The jackets were then picked up by Global Industries derrick barges, lifted 20 ft (6 m) from the seabed and wet-towed at a speed of 1 to 2 knots to their new locations, one to Ship Shoal 358 and another to Garden Banks 142.

Benign weather conditions were essential to the success of the operation. Planning the tow routes involved detailed surveys undertaken by Thales, including planning for adverse weather events and set down points. "We had to have the ability to set the jackets down on the seabed and get loose of them," said Shivers. "We were never more than 30 ft (9.1 m) away from a spot where we could set the jackets down." Effort was also put into evaluating the dynamic responses of the 400-ft (121-m) high steel jackets in water during towing. It took 4 days to move one of the jackets 100 miles (160 km) to its new location. The other move was a distance of 50 miles (80 km). Global Industries performed both moves, with its 800t derrick barge Cherokee.

Reduction

Overall ATP estimates it saved 25% of the cost of new facilities, but the biggest benefit was the reduction in the schedule to first gas production for the new developments.
"We had to get them into production and all of this took about 180 days," Tate said. "There is just no way you can construct two new platforms for 480-ft (146-m) and 520-ft (158-m) water depth in 180 days - that would take more than a year. And it solved our abandonment conundrum over Vermilion."