Feeding off the bottom
  from: Offshore Engineer
  by: Steven McGinn
  Wednesday, April 09, 2008

An innovative custom-lifting system has been busy lately recovering hurricane-destroyed platform decks in the Gulf of Mexico. Steven McGinn looks at the rationale behind Versabar’s Bottom Feeder.

Versabar’s Bottom Feeder custom-lifting system got its first test in June. A Gulf of Mexico operator approached Versabar to perform a custom-lift rescue operation of four hurricane-toppled platforms located in nearly 300ft of water.

Manufactured by Corpus Christi, Texas-based Gulf Marine Fabrication, the Bottom Feeder is a dual-barge lifting structure with a catamaran system. Versabar president Jon Khachaturian says the company built the Bottom Feeder specifically to lift platforms from the sea floor. Bottom Feeder project manager Ian Todd oversaw the engineering, testing and construction of the vessel and execution of the four deck recoveries.

‘It went extremely well,’ Todd says. ‘What people don’t see is the preparation and planning to ensure a successful lift.’

Versabar initially surveyed all four platforms, which were 50-75 miles from the mouth of the Mississippi, to identify the best places to install custom lift-hooks. Saturation divers then placed hooks on each of the decks to a pattern identified during the survey. Oceaneering provided ROV support during the underwater operations.

On 12 June, Versabar lifted a 1000 ton deck from a depth of 242ft. Once out of the water, a barge moved under the trusses and the Bottom Feeder placed the deck on the barge.

‘The deck was then hauled off to a location provided by the operator,’ says Khachaturian. Spill rails installed on the barge prevented environmental hazards during haul-off.

Each subsequent lift followed a similar procedure.

‘Generally, we had two hooks per block on eight points per deck,’ Khachaturian says. ‘Once connected, the deck came off the bottom, and it took about an hour and a half to raise once everything was set.’

He says the operator emphasized a diver-less approach during the lift to eliminate the risk of human exposure to pipes, metals and other hazards. ‘The most challenging part during the lift was the diver-less approach, going out there and hooking on to this two and half million pound deck with nothing but a single ROV,’ Khachaturian emphasizes. ‘That worked better than I could hope.

‘No one was allowed to go on the barge once the deck was placed on it.’

Versabar raised the second platform from 262ft, hoisting the 1250 ton deck three days after the first raise. A week later, the Bottom Feeder raised the third deck, weighing 1100 tons, from a depth of 273ft. Finally, the lifting system raised the 1050 ton fourth deck on 30 June from a depth of 247ft, 18 days after the first lift’s completion.

‘Our operator was able to use pollution control booms on either side of the catamaran to capture any hydrocarbons that may have been spilled on the deck,’ Todd says. ‘When the decks came up through the water column, there was always the potential of a spill, the vessel letting go, or some pipe breaking.’

The Bottom Feeder’s dual-barge catamaran system contributed to the overall safety factor, and the booms contained any contaminants spilled during the lifts.

‘The idea behind the Bottom Feeder is to provide a stable platform with a lot of leverage for lifting,’ Khachaturian says. ‘Multiple hooks and lift blocks maintain stability as the deck is lifted, spreading the weight load over the entire system.

‘Unique to the system is its pin connections to the barge and multiple block lifting method,’ he says. ‘The system’s key feature is that it articulates in the X and Y directions so that the sea cannot break the structure, giving the flexibility to work in 10-12ft waves.’

According to the company, the 250ft long Bottom Feeder can operate in water depths up to 400ft, lifting up to 4000 tons. From deck of the barge to the top of the truss, the Bottom Feeder measures 113ft tall, and water line to truss bottom at 90ft.

Stabilizing the Bottom Feeder involves using a five-pin system for each truss with a four pin attachment to one barge and a single pin attachment to the other. The Bottom Feeder works by dropping cables underwater to the hooks placed on a submerged platform.

Because the center of gravity on the sunken decks is unknown, a standard single hoist would have been dangerous, allowing the deck to swing to the side and become unstable. Instead of using a single lift derrick barge, the Bottom Feeder uses multiple blocks installed on hooks spread around the deck.

‘What we found is that by distributing multiple lift points, we were able to lift the decks without them falling apart,’ Todd says. ‘You never know how the lift will happen because the decks fell almost 300ft to the floor following the storms, but we were able to successfully recover damaged decks without them failing on us.’

Envisioned years ago, Khachaturian says, the recent hurricanes provided the impetus for the Bottom Feeder to grow from a concept to a fact.

‘Existing construction equipment did not fit these types of salvage operations,’ Khachaturian says. ‘It’s a rather simple concept but the detail is an engineering challenge.’

Within one year, the company designed, built and the Bottom Feeder and conducted all four lifts. ‘The big achievement in this was successfully executing from a blank sheet of paper and actually sailing away with a 4000t system in a year,’ Todd remarks.

With high demand for salvage operations throughout the Gulf, Todd says the Bottom Feeder’s schedule is tightening up. He says the company is working with two or three operators, who indicate they wish to use the Bottom Feeder for other lifts.

‘Obviously with the successful recovery of four decks, that has raised a lot of interest within the industry,’ Todd says. ‘Since we raised them, our phones have not stopped ringing. I would say most availability is looking at next year, though.’

Plans are for use only in the Gulf of Mexico. ‘We’re going to continue on a deck-by-deck basis to lift pieces off the Gulf floor,’ Khachaturian says. ‘It’s strictly Gulf of Mexico right now. There’s plenty of work with hurricane recovery.’ OE

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