Boring or penetrating the earth – Boring a submerged formation
Reexamination Certificate
1998-09-24
2001-07-24
Bagnell, David (Department: 3673)
Boring or penetrating the earth
Boring a submerged formation
C175S007000, C175S317000, C175S324000, C166S367000
Reexamination Certificate
active
06263981
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to drilling systems and operations. More particularly, the present invention is a method and system for handling the circulation of drilling mud in deepwater offshore drilling operations.
Drilling fluids, also known as muds, are used to cool the drill bit, flush the cuttings away from the bit's formation interface and then out of the system, and to stabilize the borehole with a “filter cake” until newly drilled sections are cased. The drilling fluid also performs a crucial well control function and is monitored and adjusted to maintain a pressure with a hydrostatic head in uncased sections of the borehole that prevents the uncontrolled flow of pressured well fluids into the borehole from the formation.
Conventional offshore drilling circulates drilling fluids down the drill string and returns the drilling fluids with entrained cuttings through an annulus between the drill string and the casing below the mudline. A riser surrounds the drill string starting from the wellhead at the ocean floor to drilling facilities at the surface and the return circuit for drilling mud continues from the mudline to the surface through the riser/drill string annulus.
In this conventional system, the relative weight of the drilling fluid over that of seawater and the length of the riser in deepwater applications combine to exert an excess hydrostatic pressure in the riser/drill string annulus.
Systems have been conceived to bring the drilling fluid and entrained cuttings out of the annulus at the base of the riser and to deploy a subsea pump to facilitate the return flow through a separate line. One such system is disclosed in U. S. Pat. No. 4,813,495 issued Mar. 21, 1989 to Leach. That system requires complex provisions to ensure the closely synchronous operation of the supply and return pumps critical to the approach disclosed. However, the durability and dependability of such a mud circulation system is suspect in the offshore environment and particularly so in light of the nature of the fluid with entrained cuttings that is handled in valves and pumps on the return segment of the circuit.
Thus, there remains a need for a practical means for reducing the excess hydrostatic pressure exerted by the mud column return in the riser/drill string annulus.
An advantage of the present system is that the excess pressure is isolated from formation during operations and that ambient pressure is maintained when pump operations cease.
A SUMMARY OF THE INVENTION
One aspect of the present invention is a method a method for controlling the mud circulation system for deepwater marine drilling operations in which a drill string is run from surface facilities, through a blowout preventor and into a borehole. Mud is injected into the drill string, up a borehole and through the blowout preventor adjacent the sea floor. There the mud is withdrawn from a mud exit return line near the blowout preventor and the hydrostatic head from the mud in the drill string is isolated from the relatively lesser ambient pressure at the sea floor seen at the mud exit return line with a pressure activated drill string shut-off valve when mud circulation is interrupted.
Another aspect of the present invention is a drill string shut-off valve system for controlling the mud circulation system for deepwater marine drilling operations. The drill string shut-off valve system uses a drill string run from the surface, through a blowout preventor and down a wellbore. Mud is in the drill string, the wellbore and the blowout preventor and a mud exit return line is provided above the blowout preventor, near the sea floor. The bottom hole assembly at the end of the drill string has a drill bit and a drill string shut-off valve suitable to selectively isolate the hydrostatic head from the mud in the drill string from the relatively lesser ambient pressure from seawater at the mud exit return line when circulation is interrupted.
Yet another aspect of some practices of the present invention is a method of well control to overcome formation pressure in a well control event.
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Bagnell David
Lee Jong-Suk
Shell Offshore Inc.
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