Offshore oil production is big business, so whenever oil companies need to make repairs or schedule upgrades to their platforms, they look for the fastest and most efficient ways possible to minimize shutdowns and maintain production. A specialty contractor recently traveled offshore to an oil platform in the Gulf of Mexico to cut a series of 30-inch-diameter piles 15 feet below the sea bed and show why wire sawing was an ideal cutting technique for this application.
The Mactech Offshore office of CSDA member Mactech, based in Red Wing, Minnesota, was contracted by a major independent oil and natural gas exploration and production company to cut and remove six concrete-filled steel piles from one of its offshore platforms. The platform was to be removed from the Gulf of Mexico as part of a strategically planned decommissioning project after the well ran dry. The steel casing of the piles was 1.5 inches thick and helped secure the platform jacket to the sea bed. Due to federal regulations, the piles had to be cut 15 feet below the sea bed—or mud line as it is known in the industry. In addition, the weight and handling limitations available at the platform meant the jacket structure also had to be cut just above a ‘mud mat’ on the mud line before dredging and cutting below.
Aside from internal cutters or the use of explosives, other methods had been considered and ruled out including shears and inline diamond wire sawing. Shears were simply too big and heavy for the job and also left a large footprint, while inline wire sawing was deemed too costly. In the end, the team from Mactech was able to offer a custom wire sawing solution that met the customer’s needs. The company has recently received patent-approval status for an articulating wire saw system its engineers designed for subsea and topside cutting. The saw utilizes an articulating cutting arm for minimal clearance during cuts where other setups may not be able to fit or operate. Needing only 2 feet of clearance around the object being cut and having the cut set at the bottom of the saw, there was no need for extra dredging. Adjustable guide arms clamped around the piles and the diamond wire ran through a series of pulleys on the unit. This design reduced setup, installation and removal times, as the guiding arms were closed loosely around the piles to guide the saw down to cutting level before clamping on. This also increased safety on the job as the contractor was able to minimize the need for divers in the water. Power was provided by a diesel hydraulic power unit driving two hydraulic motors.
A plan was developed by Mactech for the cutting of the six 30-inch-diameter pile sections above the mud mat structure to free the jacket and platform. These sections would take less time to cut than those below the mud line because they were hollow. Three articulating wire saws were used at the same time to cut piles on one side of the platform, then all three were moved to the other side to cut the three remaining piles. Each saw was installed by a diving team and each cut took an average of four hours and 15 minutes, including setup and takedown, with a 24-foot length of diamond wire supplied by Husqvarna. Once cut, the entire jacket section was rigged and removed from the water by crane and placed on a materials barge. This portion of the work was done in about 48 operational hours.
After the majority of the jacket structure was lifted away, Tusk Subsea of Lafayette, Louisiana was brought in to excavate and dredge the areas around the remaining piles. Each area was dredged to a depth of 15 feet below the mud line and 2 feet around the piles. As each area was cleared, Mactech began setting up one of its saws to be lowered into each dredged area ready for cutting. As these pile sections were filled with concrete, it took the wire sawing team an average of one hour and 40 minutes to complete each cut and a total of five days to finish all tasks.
While the cutting contractor’s articulating wire saws did minimize the use of divers, there was still a need to have divers in the water and so safety was critical. Once in the water, each diver had to find and set up a saw with limited visibility. Only one diver was allowed in the water at any time. Above the surface, the team from Mactech kept a close eye on sawing progress and the position of the wire through the water. There was a concern that other parts of the platform structure positioned close to the cutting loop would pinch the wires, so operators made regular adjustments to avoid this issue.
Utilizing these custom wire saws, Mactech was able to make 12 underwater cuts through 30-inch-diameter pile sections in nine days, helping to remove hundreds of tons of steel and concrete. The solution offered by the cutting contractor brought the work in ahead of time and within budget.
“Our customer was able to realize substantial time and cost savings from this job. The results of the cooperative effort exceeded everyone’s expectations,” said Marcks. “The saws we used to cut the steel and concrete piles were proven technology, but we had never employed them simultaneously or had to set them up so quickly to meet such a tight time frame before. We were extremely happy with the results,” he concluded.