Drilling mud-line cellars for OCS drilling
A blowout preventer, a tall stack of valves and other devices designed to rapidly shut down a well in the event of an oil blowout, is an essential piece of safety equipment that has to be installed at the surface end of a well whenever a drilling operation is in progress. And when drilling on the outer continental shelf, the blowout preventer would sit on the sea floor.
But in the Arctic offshore, such as on the outer continental shelf of the Beaufort and Chukchi seas, the prevalence of sea ice, much of it in constant motion, gives rise to the possibility of an ice keel hitting a blowout preventer, causing major damage to the device and raising the risk of an oil spill.
To avoid this eventuality, all blowout preventers on the Arctic OCS have to be installed in mud-line cellars, cylindrical holes in the sea floor, typically 40 feet deep, Cody Teff, Shell engineering team lead in Alaska, told the U.S. Minerals Management Service Arctic Technologies Workshop in Anchorage on Oct. 13.
The first step in designing a mud-line cellar is the acquisition of multibeam sonar images of the seabed, a technique that uses acoustic signals to generate detailed profiles of the sea-floor surface. The sonar images enable gouges to be identified and measured, thus setting parameters for the required mud-line cellar depth, ensuring that the top of the blowout preventer will sit well below the deepest scour.
A typical blowout preventer is 20 feet tall and about 16 feet in diameter, weighing about 500,000 pounds, Teff said.
After the gouge depth measurement is complete, a 20-foot diameter, hydraulically powered rotary bit, with teeth in the form of inward-angled plow blades, carves out a cellar, an operation that typically takes anywhere from two to 10 days to complete. The plow blades direct debris from the operation towards the center of the bit, from where compressed air pushes the debris up through the tubular riser that holds the bit in place on the sea floor, Teff said.
—Alan Bailey
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