In deepwater drilling a conventional large diameter riserrequires drilling vessels with huge weight and spacecapacities, large mud volume to circulate through a riser, andmany casing points because of narrow gap between pore andfracture pressures. A large number of casing points alsorequire a larger wellhead and a larger marine riser. Theseproblems are inter-related and intensify as the water depthincreases. Although there are some successes to set a newdrilling record on water depth, it is impractical to extrapolatecurrent technology with a marine riser to 10,000 ftwater depth.Subsea mudlift drilling(SMD) is a term used to describe anunconventional technique using a relatively small diameterpipe as a mud return line from the sea floor instead of a largediameter marine riser. The scheme also balances internal andexternal pressures at the sea floor by reducing the internalpressure to make a dual pressure gradient possible. It haspotential advantages of cost and time savings and rig upgradesfor deepwater applications.Kick detection and well control will not be hurdles forfield applications of SMD. If the circulation rate is less thanthe maximum free fall rate, there will be a severe delay in kickdetection. In this case, we may need to increase the circulationrate or use a drill string valve that provides a positive surfacepump pressure. In case of transient U-tubing or fill up, weshould avoid other operations for easy kick detection and wellcontrol, unless we have an accurate prediction andmonitoring tool.
展开▼
