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Unlocking energy from the deep ocean
A double-deck drilling rig, innovation in the use of technology on the seabed, and a first-of-a-kind link-up to an existing pipeline network made first oil production possible in ultra deep-water at the Perdido development in the Gulf of Mexico.
The water depth of nearly three kilometres (1.9 miles) and isolated location of the Perdido platform posed major challenges to first oil production. So did the dispersed nature of the reservoirs and low pressure in the porous rock that hinders the flow of oil and natural gas.
Cutting deep-water drilling time
Only a few floating rigs can drill down far enough to reach the reservoirs: the Noble Clyde Boudreaux is one of them. Retrofitted specifically for the Perdido project, the Noble Clyde Boudreaux pre-drilled Perdido’s 22 direct vertical access wells to just above the oil and gas layers. This limited the time the crew would spend drilling once the Perdido spar platform was installed, speeding up production. It takes six 13,500-horsepower generators to power the drills on the 29,000-tonne Noble Clyde Boudreaux that soars six stories above the water.
With two drilling decks instead of one, it operates faster than regular rigs. It can work on one well and simultaneously drill a new one. Precision drilling at these depths demands great skill — operators on the rig use joysticks to guide the drill-bits below, driving them up to three kilometres (1.9 miles) beneath the seabed to hit a target about the size of a garbage bin lid.
The location of the fields, 320 kilometres (200 miles) from the nearest supply port in Galveston, Texas means that every litre of fuel and piece of machinery must be shipped in on a 23-hour journey. Keeping the platform compact and reducing the amount of equipment needed in this isolated location is vital.
Until recently, in a typical deep-water development each well was connected to an individual flexible pipe called a riser, which carries oil and gas to the surface. But at the Perdido project, due to its incredible depth, each long riser weighs 500 tonnes and the cylindrical hull, or spar, would have needed to be four times bigger to support each of the 22 wells directly below. So Shell engineers pioneered a system to boost oil and gas production from below the sea, connected to a network of pipelines on the ocean floor that linked the wells to just five risers.
This innovative approach helped to reduce the weight and size of the platform and risers, making Perdido the first economic development at these water depths. Other projects have since adopted the same approach.
Deepest pipeline link-up
Adding to the challenges, the Perdido project’s wells sit in rugged seabed terrain, with an underwater Grand Canyon to the north, and protected sea life colonies to the south and east. That complicates the task of building a pipeline to carry Perdido’s oil and gas to shore. “We’ve really only got one path out,” says Mike Dupre, pipelines and flow lines engineer.
Instead of building a new 381-kilometre (237-mile) pipeline to the shore, engineers tapped into existing pipelines on the seabed less than half that distance away already transporting oil and gas for other fields to the north. Operators on the surface guided two remotely operated vehicles (ROVs) below the surface to install new pipelines to the nearest existing ones — a 172-kilometre (107-mile) gas pipeline and a 124-kilometre (77-mile) oil pipeline. The greatest challenge was creating the connection between the new oil pipeline and the existing one at nearly 1,400 metres (4,600 feet) below sea level.
Engineers had to accomplish the whole operation without letting any oil leak into the sea. They first halted the flow of oil in the existing pipe. Then they used water to flush the section of pipe where they planned to create a new junction. Using the underwater robots, operators sawed through and removed an 8.6-metre (28-foot) section of the pipe.
If the team had detected an oil leak, they were ready to seal the cuts with clamps and pump in more clean water. As a further precautionary measure, they installed a giant tent over the area to keep in any fluid potentially released and transfer it to a reservoir.
Finally, the robots installed a new connection point and welded it into place. It was the deepest such tie-in ever. “We achieved this record-breaking feat without spilling a drop of oil,” says Don Nelson, subsea pipeline installation manager during the construction of Perdido.