Bottom Feeder: salvaging decks from the seafloor
An innovative invention is making light work of raising sunken steel structures in the US Gulf of Mexico, writes Anne Feltus
WHEN THREE hurricanes swept through the Gulf of Mexico (GOM) in 2004 and 2005, sending the topsides of more than 110 offshore platforms to the ocean floor, oil and gas companies had to cope with significant losses in equipment and production. In addition, the US Minerals Management Service (MMS) rules require them to plug permanently any wells on leases that are no longer productive and, in most cases, to clear away the sunken topsides and other obstructions.
Retrieving these steel structures, which weigh 1,000 tonnes or more, from their landing place, several hundred feet underwater, has created some unprecedented and costly technical problems.
Last year, a Louisiana-based specialised heavy-lifting company began offering a solution in the form of its Bottom Feeder vessel. The innovative apparatus, claims Versabar, can recover sunken decks for about half the cost of other retrieval options, at less risk and in considerably less time. It can operate in waters up to 400 feet deep and lift as much as 4,000 tonnes.
The deck-lifting system won one of this year's Offshore Technology Conference awards. It is Versabar founder and owner Jon Khachaturian's second: the first was last year for a system elevates the decks of fixed platforms high enough above the sea surface to minimise their risk of being struck by destructive waves. Within a year and a half of hurricanes Katrina and Rita, the University of Illinois engineering graduate had created the concept for the Bottom Feeder and had invested $30m to have it constructed just as the 2007 storm season was beginning.
The Bottom Feeder's appearance is as unique as its purpose. Like a pair of giant, bright-yellow bridges on pontoons, two 1,250 tonne, 113 foot-tall steel truss frames straddle two side-by-side barges, each 255 feet long and 72 feet wide. On each barge, one leg of the truss is attached with a universal joint and the other with a fixed hinge. This allows the barges to move up and down independently of each other, enabling the vessel to operate in rough seas.
Four pulleys hanging from the top of each of the trusses are equipped with 200 tonne, diesel-powered, hydraulic winches and 2¼ inch wire cables and can lift about 1,000 tonnes apiece. The blocks can be operated individually or in groups, giving the operator more flexibility in manoeuvring the deck during retrieval operations.
After the Bottom Feeder is moved by tugboats into position above the submerged platform and anchored in place, the cables are lowered. A remotely operated vehicle is used to attach them to custom-designed lifting hooks that have been welded by divers onto predetermined points around the deck. The structure is lifted slowly, intact, to the surface between the two barges, a process that can be completed in less than an hour. A cargo barge is moved under the trusses and the deck is lowered onto it to be towed to shore.
Booms on either side of the barges help contain any hydrocarbons that flow from the deck as it is being lifted out of the water. A containment and recovery system on the cargo barge helps prevent liquid pollutants from spilling into the sea during transport.
Until now, few options existed for fishing sunken platforms from the ocean floor. Only a few construction vessels had sufficient capacity and they are not only expensive, but have plenty of other work installing new platforms for the booming deep-water industry.
Furthermore, without knowing the weight, centre of gravity and structural condition of sunken topsides, retrieving them with a single-hook lift could be hazardous; the platform could become unstable and swing to the side. The Bottom Feeder's deck weight is distributed among multiple lift points, helping the vessel maintain stability.
More typically, sunken structures are salvaged by sending divers to cut them into pieces, which are retrieved individually. This solution is usually expensive and time-consuming and poses risks for the divers, who must remain at great depths for long periods of time.
The Bottom Feeder's first field operation took place on 12 June 2007, when it recovered a 1,000 tonne deck from 142 feet of water. By the end of that month, the vessel had conducted three more retrieval operations, raising topsides weighing as much as 1,250 tonnes from waters as deep as 273 feet – all about 50-75 miles from shore and all for the same unidentified operator.
On 29 August, the second anniversary of Hurricane Katrina's rampage through the GOM, the Bottom Feeder hoisted a 1,400 tonne deck owned by Chevron from 240 feet of water about 110 miles south of Lafayette, Louisiana. Several other projects planned for two separate operators last September were delayed by bad weather – not, says Versabar, because the Bottom Feeder could not handle the work, but because turbulent seas could have damaged the decks as they were being lifted. The operations were rescheduled for May.
Because only a fraction of the storm-damaged platforms have been pulled from the seafloor to date, there is still plenty of work for the Bottom Feeder: the MMS expects efforts to clean up the mess from the 2004 and 2005 hurricane seasons to continue for several years.