In deepwater drilling a conventional large diameter riser requires drilling vessels with huge weight and space capacities, large mud volume to circulate through a riser, and many casing points because of narrow gap between pore and fracture pressures. A large number of casing points also require a larger wellhead and a larger marine riser. These problems are inter-related and intensify as the water depth increases. Although there are some successes to set a new drilling record on water depth, it is impractical to extrapolate current technology with a marine riser to 10,000 ft water depth. Subsea mudlift drilling(SMD) is a term used to describe an unconventional technique using a relatively small diameter pipe as a mud return line from the sea floor instead of a large diameter marine riser. The scheme also balances internal and external pressures at the sea floor by reducing the internal pressure to make a dual pressure gradient possible. It has potential advantages of cost and time savings and rig upgrades for deepwater applications. Kick detection and well control will not be hurdles for field applications of SMD. If the circulation rate is less than the maximum free fall rate, there will be a severe delay in kick detection. In this case, we may need to increase the circulation rate or use a drill string valve that provides a positive surface pump pressure. In case of transient U-tubing or fill up, we should avoid other operations for easy kick detection and well control, unless we have an accurate prediction and monitoring tool.
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