The removal of subsea wellheads and the subsequent plugging of the well can often be compromised by materials that have settled not only around, but also inside the structure - from muds, sands and other sediments to shells and small pieces of debris. Normally diver-operated dredges have proven effective in removing these materials, particularly where the buildup is around the outside of the seabed structure. What has proven more difficult is to remove materials that have settled inside the wellhead. - The Bullet dredging tool removes sediment from the wellbore prior to plugging and abandonment.
Andrews Genflo believes it has a downhole dredgehead for their Barracuda jetpump system that can accomplish the task. The dredgehead complements the standard diver-operated handpump.
The internal head, called "The Bullet," consists of a 100 mm jetpump within an aluminium housing. It is designed to pass through the concentric wellhead casings without snagging. With a diameter of 7 in., it can dredge within the 9 5/8 in. central casing of a wellhead. The dredge provides a primary suction action capable of removing soft sediments at a rate of 30-50 tons/hour plus a jetting action (eight agitation jets) to dislodge difficult materials in the hole.
A second dredgehead (Barracuda) is a lightweight diver-operated, jet pump dredge for dredging outside the wellhead. It weighs 10 kg under- water and can remove most soft sands and sediments. It can operate at typical production rates of around 30 tons/hour, to as high as 50 tons/hour in free-flowing sands in 30 meters water depth.
Water is passed to the dredgehead via a submersible electric pump held just below the vessel. The Barracuda system is different to conventional diver-operated dredges. It is designed to reduce deck space and keep handling to a minimum. There are no topside holding tanks and primary lift pumps to provide motive water nor large frameworks to hold hydraulic or electrical water pumps. All are replaced by a compact control/deployment unit. Gone is the usual seabed pumpset frame requiring heavy lifting.
The Barracuda is powered, controlled, and handled from a topside unit weighing 2000 kg. The spooler, dredge controls, and a small hoist driven by compressed air, have been integrated into a single frame. The only other requirement for users may be a small crane to handle hoses, particularly when dredging at greater depths.