In deepwater drilling a conventional large-diameter riser requires drilling vessels with huge weight and space capacities, a large mud volume to circulate through a riser, and many casing points because of the narrow gap between pore and fracture pressures. The large number of casing points also requires a larger wellhead and a larger marine riser. Subsea mudlift drilling (SMD) is an unconventional technique that uses a relatively small-diameter pipe as a mud-return line from the seafloor instead of a large-diameter marine riser. The scheme also balances internal and external pressures at the seafloor by reducing the internal pressure to make a dual pressure gradient possible.
