Saturday, December 15, 2018

Jaws of steel

November 23, 2012

When a major operator with extensive holdings on the US Gulf of Mexico shelf wanted to scale back the use of divers while ramping up decommissioning and platform removal activities, it turned to contractor Versabar, the company behind the Bottom Feeder heavy lift system.Russell McCulley gets the lowdown on Versabar's latest big lifter, the Claw.

The deadly hurricanes that swept through the Gulf of Mexico in the middle of the last decade left dozens of downed and damaged platforms on the Outer Continental Shelf, creating headaches for operators and opportunities for companies that specialize in decommissioning and salvage. Versabar rolled out the Bottom Feeder, a lift system that can remove sunken platforms in one piece, in 2006; four years later, the company introduced the VB10,000, a significantly larger system that can perform heavier lifts for above-water decommissioning or seafloor salvage. One of the VB 10,000's more notable innovations was the use of large steel ‘baskets' used to lift entire sunken topsides, many of which were too damaged to remain intact if lifted without the supporting baskets.

The twin lifting devices that make up the Claw are suspended from twin gantries on Versabar's VB 10,000.First, however, a structure must be lifted enough to be placed atop the basket, which requires the use of divers to attach hooks. To reduce reliance on divers, Versabar in late 2010 began working on what would become the Claw. The device was built to operate exclusively from the VB10,000, says Versabar sales and marketing manager Tom Cheatum.

Versabar engineers produced a series of AutoCAD drawings in early 2011 that would be used to create documents for the fabrication yards. In March, steel was cut for the Claw at Gulf Marine Fabricators in Aransas Pass, Texas; meanwhile, Versabar's fabrication yard in Belle Chasse, Louisiana began work on four new ‘cradles' to be used in conjunction with the new equipment. Each of the cradles, completed last May, measure 104ft long by 75ft wide by 22ft high. The Claw itself—two identical assemblies, one for each gantry of the VB 10,000 – went to work in early August, lifting five damaged topsides weighing a cumulative 3000 tons in four days offshore Louisiana in 250-350ft water depths, Cheatum says.

After fabrication, the dual Claw elements were positioned aboard a barge for a floatover installation on the VB 10,000.The Claw is raised and lowered from two main hoist blocks; a second set of blocks controls the opening of each independently controlled half of the device. When opened, the Claw can be positioned to insert its tines into a sunken structure and closed as tension in the blocks is released. An angle indicator on the VB10,000 girder allows for optional ROV monitoring during operations.

A diagram illustrates how the Claw uses tines to lift a sunken, damaged topside from the seafloor. The structure is then placed on a large cradle so it can be lifted intact tot the surface. The system is rated to lifts of as much as 7500 tons. For the initial five-platform lift, the Claw was configured with one claw attached to one gantry to perform underwater lifts of the topsides and a custom rigging affixed to the second gantry was used to lift the cradles. The two claws can be used in tandem, however, for heavier lifts. As configured, the Claw is rated to work in 450ft water depths, but can be reconfigured for greater depths, Cheatum says.

While the Claw will likely stay in the Gulf of Mexico, ‘the technology translates, and is scalable', he says. ‘There is always interest in heavy lift in other markets, whether it's installation or removal.' OE

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