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The mudslide area of the Gulf of Mexico - where the Mississippi River meets the Gulf - is home to enormous platforms. When this area was developed, the instability of the seabed necessitated construction of structures capable of withstanding this shifting floor environment.
Built in 1978, the South Pass 78A platform was one of these imposing structures. A monstrous four-pile, straight-leg platform, it was located just 12 miles (19 km) south of the mouth of the Mississippi River in approximately 230 ft (70 m) of water.
Unique project challenges
This structure had 91-in. diameter jacket legs with wall thickness of 2 in. grouted to 84-in. diameter piles with a wall thickness of 2.5 in. (pile diameters usually range from 30 in. to 48 in.).
Jacket legs extended 90 ft (27 m) past the mudline. In most installations, legs extend only 3 ft to 5 ft beyond mudline. The jacket weighed 4,800 tons (about 1,500 tons came from 12 wells drilled and grouted within two jacket legs). Harsh environment conditions included a shifting sea floor and strong current.
In the fall of 2002, Tetra Applied Technologies was given the challenge of decommissioning this imposing platform. With its core focus on well abandonment and decommissioning and its specialized skill set, which allows it to manage such projects from cradle to grave, the company has tackled a multitude of extensive abandonment and decommissioning operations - many involving multiple structures and considerably more wells than the South Pass project - but none involving a structure with these particular engineering challenges.
To date, very few platforms have been removed from the mudslide area, so the challenges presented by such structures are still unique. After an extensive project evaluation, the company presented a turnkey abandonment solution that addressed the complexities of the project and met the needs of the customer.
Planning and engineering
Because few such removals have been undertaken, the project team approached the South Pass removal with an open mind. After looking at the complexities, team members relied on experience to outline an initial plan and used creativity to refine and perfect it as the project progressed. A great deal of discovery and planning was undertaken to understand and prepare for this removal project. The team reviewed records and interviewed original installation team members. They also brainstormed among themselves and consulted with experienced salvage contractors to develop a set of options and, ultimately, determine the best course of action.
To plug the 12 wells, the company mobilized a plug and abandonment (P&A) crew with the necessary equipment, including one of the company's rigless P&A packages, crew quarters and generators. Working 24 hours a day for approximately 36 days, this phase of the project was completed on schedule, on budget and without incident.
Installation records indicated that, due to its location in the mudslide region, the platform's 12 wells had been installed within its two northernmost legs. According to these records, after the wells were drilled and completed, the annulus of each leg, with the conductors inside, was filled with grout extending from below the sea floor to the surface. To minimize risk during the jacket cutting and removal, it was determined that the best approach would involve removing the wells prior to severing the jacket. This would dramatically minimize the complexity of the jacket leg/pile cut in that it would no longer be necessary to sever the conductors or the grout filling each annulus. Removing the wells would also reduce the jacket weight from 4,800 tons to a manageable 3,100 tons, meaning the jacket could be left in one piece and local service providers could perform the lift.
The project team initiated a discovery process to see if removing the wells was feasible. They analyzed the installation records and made site visits to verify information. Additionally, a dive team was mobilized to assess the present day underwater situation. They made small openings through the jackets, grout and piles, and utilized probes to determine the state of the grout and location of the conductors. It was ultimately determined that the wells could be extruded from the platform legs. In October of 2003, one of the P&A crews with mechanical cutting equipment and casing jacks was deployed to the location. The grouted wells were removed without incident over the course of 42 days.
The deck and appurtenances consisted of structural steel weighing approximately 1,450 tons; a 30-man, three-story crew facility with a helideck weighing close to 300 tons; a 400-ton production module; and a 120-ton drill deck section. A derrick barge and two material barges were deployed for this phase. The deck and modules were successfully removed, loaded, secured, brought to shore and properly disposed of. Before the barge was demobilized, the jacket was prepared for the severing and lifting operations. Scaffolding and a tugger platform were installed to create a safe and functional work area. Pad eyes, installation aids, plus lifting and de-ballasting trunnions were installed.
Jacket removal presented the greatest challenge in this project. As the jacket legs extended well beyond the mudline, rather than having to sever only the piles, as is normally the case, this project involved cutting through the legs, a layer of grout and the piles (a matter of 6.5 in. of material). Given this diameter and thickness, conventional abrasive and explosive methods were not feasible. After considering several options, diamond wire cutting technology was selected.
Working with the Minerals Management Service (MMS), the project team had been given 5 ft (1.5 m) of relief, allowing Tetra to sever the legs at 10 ft (3 m) below the mudline, rather than the standard 15 ft (4.5 m). Because the diamond wire tool can only be used externally, Tetra decided to cut each jacket leg twice. The first cut at the mudline would facilitate the removal of the bulk of the 3,100-ton structure, and the second cut would allow the removal of the remaining required 10 ft (3 m) sections of jacket leg/pile from below the mudline.
In severing a structure of this magnitude, a great deal of planning and preparation was necessary. The project team worked closely with the cutting contractor, Cutting Underwater Technologies (CUT), and the dive contractor, Cal Dive International (CDI), to ensure that this phase was well orchestrated. In designing the cutting procedure for South Pass 78A, engineers from CUT determined two wires would safely and effectively sever each leg. In the weeks prior to mobilization, the dive team was familiarized with the project. The divers were trained on operating the diamond wire tool and on the procedure for changing the wire to be certain they could perform this task underwater without difficulty.
The severing process required between 12 and 18 hours with one wire change at the halfway point for each leg. During this operation, the cut was inspected and shims were installed to keep the cut from closing and pinching or damaging the wire. After each leg was severed, the tool was brought to the surface, inspected and fitted with a fresh wire. Once 31/2 legs were severed, the derrick barge was mobilized. The final half cut was made as the barge was setting up in the field. Within 18 hours of the derrick barge's arrival, the jacket was on the hook. This operation, by far the most difficult and expensive phase of the project, went like clockwork. Extensive planning, preparation and diver training, as well as the high degree of professionalism and teamwork demonstrated by CUT and CDI personnel, was credited
for this success.
Because of its proximity to a shipping fairway - within 500 ft (152.5 m) - and its water depth, the structure could not be reefed on site. Instead, working with the Louisiana Department of Wildlife and Fisheries and the US Coast Guard, Tetra permitted the jacket for reefing at West Delta Block 134, approximately 24 miles (38 km) from the installation site. The jacket was moved to the reef site, lowered to the sea floor and pulled over to settle on its side in its new home, where it will provide acres of living and feeding habitat for Gulf of Mexico marine life. From initial lift to resting on the Gulf floor, this operation took just over 30 hours.
Still remaining at this point was the removal of the final 10 ft (3 m) of legs/piles from below the mudline. This presented its own challenges, as the MMS had stipulated use of a cofferdam to evacuate the mud and create a work area. To allow enough room for tool operation and diver, 10-ft (3-m) high coffer dams 20 ft (6 m) in diameter with large internal and external stabbing guides and jetting nozzles were fabricated. Once the cofferdams were jetted down to depth and the mud was evacuated, the procedure used to make the initial mudline cuts was repeated. The four remaining leg/pile sections were severed, removed and sent to shore.
The importance of planning such a project cannot be overstressed. The successful removal of South Pass 78A proved that extensive discovery, engineering and preparation, combined with an open-minded approach, can pay off on a complex decommissioning operation.