BP decommissions damaged platform

An anchor-handling vessel fitted with a 500-ton winch pulled the platforms into the water on May 6.

When Hurricane Lili blew through the Gulf of Mexico, passing over EI 322, it reached Category 4 status with 140 mph winds. The platform had been producing at marginal levels, and BP planned to use it to test new shallow water drilling technology. The platform could not withstand the hurricane's force and collapsed, leaning to one side with damage to both decks, jacket, and bridge. The platform consisted of two, adjacent, four-pile structures linked by a bridge. One served as a drilling platform, the other as a production structure. The bridge connecting the two held flowlines and accommodation units.

The EI 322 platforms stood in 235 ft of water and were designed to withstand 70-ft waves and 125-mph winds. A wind-wave-current hindcast of Lili revealed only 56-ft waves during the storm.

BP's initial inspection using an ROV revealed one crushed cruciform joint in an X-bay and one compressed vertical diagonal joint on the drilling jacket. BP discovered no impact damage on the jacket, and focused instead on a possible structural deficiency. A computer analysis showed that the shim pile connection could have failed during Lili. A visual inspection revealed a weld at a height considered less than required. The analysis also suggested that one of the tension piles could have pulled out during the hurricane.

Decommissioning plan

BP assembled its experienced team of decommissioning personnel and set out to develop a decommissioning plan, considering the high risk involved in performing the work during hurricane season and the environmental factors at stake. The initial plan called for assessment of the platform to determine its state, partial clearing of the platform to accommodate a crane, strengthening the deck and jacket, abandoning the remaining wells and a 10-in. pipeline, clearing and cleaning the platform for reefing, and toppling both platforms.

"It was fortuitous that the site had two platforms," Tom Straub, BP decommissioning manager, GoM shelf, said. "The drilling platform was damaged, but the production platform was structurally undamaged. That gave us a base of operations from which the crane could work."

A visual inspection aboard the platform revealed that the leaning wellheads were accessible from the drill deck and were undamaged. Crews cleared part of the production deck and loaded a 60-ton crane on board. It cleared the rest of the deck, including the inter-connecting bridge, and provided well access.

BP grouted the jacket leg/pile annulus and installed a partial steel shell to strengthen the buckled deck leg above the failed pile-to-jacket leg connection. Additional strengthening was carried out to the main deck beams and deck-beam to deck-leg connections. The team then strengthened the foundation using skirt pile templates and piles, one of which had to be re-designed after BP found a pile that was completely severed below the mud line. The repairs would save the platform from further damage if another hurricane blew in.

The platform failed at two locations. The first was severance of one pile at about 25 ft below mudline and the other was failure of the opposite jacket leg to the pile shim plate connection welds.

"We believe (the pile) may have been severed some time before Hurricane Lili, but we will probably never know," Patrick O'Connor, BP senior advisor for structural and offshore engineering, said. "The hurricane hit the platform from a bad direction with respect to the defective pile. There are two possible failure scenarios. The pile failure occurred first, resulting in failure of the opposite jacket leg-to-pile connection, or the jacket leg-to-pile connection failed first, leading to failure of the opposite pile. Either way, the ultimate deflected shape of the platform would have been the same; i.e., moving the top of the deck 84 ft in the horizontal direction and pulling the failed pile approximately 20 ft out of the sea bed."

End of an era

BP chose a rig-less plugging and abandoning plan to end the life of 11 of the original 13 wells, plus one subsea well. The drive pipe and casing strings were left in place because of the uncertain loading conditions on the platform.

BP carried out all the operations simultaneously, which called for considerable communication and teamwork. "The contractors put together a super tight team that regardless of color hardhat, had one objective," Straub said. "At one time, we had simultaneous operations on platform wells, facilities work on a platform, underwater saturation diving associated with placing the skirt piles, a derrick barge, a 116 class drilling rig right up against the platform to provide a base of operations, and all the activity on the platform."

BP also received permission from the Minerals Management Service and the Louisiana Department of Wildlife and Fisheries to reef-in-site both EI 322 platforms. "We are not opposed to using pre-established reef sites," Straub said. "What makes this unique is the extensive damage of the platform. It would have been problematic to disassemble and move it to different location. It would have placed additional risk to divers and construction workers."

Straub said BP had considered other alternatives, but explosives above the mudline would have caused damage to the environment, and the platforms could not have been moved to an artificial reef site in one piece. The platforms would have had to be cut and moved in several pieces, but because they were structurally compromised, there was no way to predict how they would react.

"The regulatory agencies, together with the Louisiana Department of Wildlife and Fisheries, appreciated that there was potential exposure that was unnecessary," Straub said. "That allowed us to reef on site."

BP chose to mechanically cut the jacket legs using environmentally benign diamond wire cutting technology. The engineers chose cutting locations to achieve a minimum 85-ft clearance from sea surface to the top of the toppled structure, thus maximizing the environmental fish habitat benefit.

Crews cut sixteen 51-in. steel columns and eight legs, working 24 hr/d for five days. An anchor-handling vessel fitted with a 500-ton winch then pulled the platforms into the water. They were toppled as planned on May 6.

Because the EI 322 decommissioning project was considered to carry a high-exposure to accident because of the damaged condition of the structures, BP assembled a project management team to ensure safe operation. In fact, to deal with such situations, BP maintains a network called D-Net (Decommissioning Network) that includes operators and contractors involved in decommissioning.

"People tap us all the time to learn safe practices," Straub said. "Certain contractors realize that this is a core line of business and are tendering services in accordance to that with their own expertise. I think we were just a little ahead of the game."

MMS to assess aging rigs

More than 4,000 fixed platforms are operating on the US Gulf of Mexico shelf, and almost 1,200 are more than 30 years old. Doubling the original design life, 400 of these are 40-years old or older. Aging platforms, possibly ones that are corroding, fatigued, or survived hurricanes, are producing some of the older fields.

The US Minerals Management Service (MMS) is requiring that all platforms more than five years old must undergo an assessment process to ensure they are within an acceptable risk level. This requirement was put in place last month and must be completed by November 2006.

Rig-to-Reefs program benefits everyone

Obsolete drilling platform jackets used to be towed to shore and sold for scrap metal. Not only does the scrap value hardly pay for shore-based dismantling and disposal, but also the existing artificial reef habitat created by the platform is destroyed, eliminating the associated biological community that thrived there. The Minerals Management Service recognized the benefits of converting obsolete, nonproductive offshore oil and gas structures to designated artificial reefs and began the Rigs-to-Reefs program in 1985.

The natural characteristics of the Gulf of Mexico include flat and sandy terrain with very little coral, rock, or natural reefs. A typical 200-ft jacket – the underwater support structure of an offshore petroleum platform – provides several acres of living and feeding habitat for thousands of underwater species. Invertebrates and plants attach to petroleum platforms and, according to the MMS, within six months to a year after installation, a newly placed rig is completely covered by marine life. This, in turn, attracts mobile invertebrates and fish species, forming a highly complex food chain.

Of all the possible reef-building materials, offshore platforms are considered most desirable because of their size, shape, and design, Ben Mostkoff, Florida's Dade County artificial reef program coordinator, told the MMS. The open configuration of a petroleum structure allows for water circulation and easy mobility for fish, he said. Plus, offshore platforms originally were constructed for the marine environment, and for that reason are secure and reliable.

In May, the Louisiana Department of Wildlife and Fisheries and BP announced the creation of a new artificial reef in the Gulf of Mexico, approximately 80 mi south of Morgan City, Louisiana. The department and BP developed this new site with the decommissioning and toppling of the Eugene Island block 322 platform. The structure is the newest addition to the Louisiana Artificial Reef Program that creates reefing sites from offshore platforms taken out of service. The structure lies in 230 ft of water where it will provide a complete marine ecosystem and serve as an attractive spot for marine scientists and for recreational diving and fishing.

During the decommissioning of EI 322, BP set up an underwater camera to track the cutting progress. "You can tell from underwater video that there is quite an aquatic habitat there," Tom Straub, BP Gulf of Mexico shelf decommissioning manager, said. "Subcontractors that move platforms say that if you do move a platform, the fish will follow it. They seem to line up waiting for the best spot."