This invention relates generally to marine drilling apparatus and methods for deep water operations, Specifically the invention relates to a method and apparatus for preventing the displacement of drilling mud by formation gas in the drilling riser thereby preventing collapse of the riser due to the external pressure of the seawater.

The oil industry has had great success in pushing back the water depth frontiers for exploratory drilling offshore. Many technical challenges have been successfully met in the ^' course of this effort. Two areas that are particularly noteworthy in this respect are the extension of the capability of marine risers and the development of new well control tech- nigues. However, the problem of hydrostatic collapse of a marine riser in deep water has remained unsolved in this emerging technology.

On occasion a deepwater marine riser has col¬ lapsed at its lower segments. When the mud -column in the riser annulus about a drill pipe has been displaced by formation gas, the resulting low pressure in the riser annulus has rendered the pressure differential (ΔP) across the pipe wall so great that the collapse strength of the pipe has been exceeded. To combat such an occurrence, some deepwater risers have been equipped with riser fill valves which are intended to open the riser annulus to seawater influx before critical ΔP has been reached. Typically, such valves are fitted with controls that sense ΔP changes and automatically

open the valve. A manual override is usually provided. Because of the unpredictability of the formation pres¬ sures such inflexible means of controls have been uncer¬ tain and risky. At best, such apparatus provides only 5 one opportunity to restore equilibrium with critical dependence on the density of seawater. If the kinetic energy of the flowing fluid is sufficiently high, entering seawater may be ejected from the top of the riser.

]_ The invention provides a well control apparatus and method. for controlling the well when formation gas displaces drilling mud in a deepwater riser.

More specifically, the advantages and features of the invention are accomplished by providing a marine

]_5 riser well control system and method for its operation for deep water drilling operations. The system includes a telescopic joint connected to a floating vessel drilling rig below which is connected a riser control device having a housing with a vertical bore through it and at

20 least one outlet passage provided in its wall. The bore of the housing is in fluid communication with the tele¬ scopic joint to which it is connected. A packing element is disposed within the housing as well as an annular piston beneath the packing element which is adapted for

25 moving from a lower position to an upper position within the housing.

The wall of the piston in the lower position covers the outlet passage in the housing wall and pre¬ vents fluid communication from the bore of the housing

30 to the outlet passage. In the upper position, the wall of the annular piston at least partially uncovers the outlet passage and allows fluid communication from the bore of the housing to the outlet passage as the piston urges the packing element to close about an object

35 extending through the bore of the housing.

A blowout preventer stack having at least one ram blowout preventer is connected to the wellhead of

the borehole on the seabed. A riser string is disposed between the blowout preventer stack and the riser con¬ trol device. An auxiliary choke line is. connected bet¬ ween the outlet passage of the housing of the riser control device and a choke manifold of the drilling rig. A kill line is connected between the drilling rig mud pumps of the drilling rig and an outlet in the blowout preventer stack above the ram blowout preventer.

The method of using the well control system described above when formation gas rises above the blowout preventer, is to first close the ram blowout preventer, about the drill pipe which extends through it. The piston of the riser control device is then operably moved to the upper position to close the packing element about the drill pipe extending through its bore while opening the outlet passage in its housing wall to the annulus of the riser control device about the drill pipe. Drilling mud is then applied from the drilling rig mud pumps via the kill line to the annulus of the blowout preventer stack for circulating and forma¬ tion gas out of the riser via the riser control device outlet passage, said auxiliary choke line, and said choke manifold.

If the blowout preventers are open, or if they are not yet provided on the seabottom, circulation is directed down the drill pipe, up the riser annulus and through the choke manifold via the outlet passage in the riser control device after the annular packing element closes about the drill pipe extending through its bore. By maintaining a mud column in the riser annulus, the hazard of collapsing the pipe by external hydrostatic pressure near the lower end of the deepwater marine riser is avoided.

The control device used in the marine riser. well control system preferably includes an improvement whereby a connector is attached to the housing about

the outlet passage in the wall of the diverter housing. The connector has a passage defining a ninety-degree turn which has a lead target disposed therein for resisting erosion of pressurized well fluid out of the housing passage.

Further advantages and features of the invention will become more apparent by reference to the attached drawings wherein a preferred embodiment of the invention is shown of which: Figure 1 schematically illustrates a deepwater drilling system having a blowout preventer stack dis¬ posed on the sea floor and a drilling rig vessel con¬ nected thereto by means of a riser system;

Figure 2 illustrates schematically and partially in cross-section in more detail the marine riser well control apparatus provided according to the invention; and

Figure 3 illustrates an improved riser control device adapted for riser pressure control and used in the marine riser well control apparatus and method of Figure 2.

Figure 1 illustrates the environment in which this invention is used. A floating drilling vessel 2 is provided for drilling a borehole B through the seabed 6 beneath water surface 8. A blowout preventer stack 4 is disposed on the seafloor above a wellhead 44. A riser 10 and choke and kill lines 12, 14 are provided for well control between the floating vessel 2 and the blow¬ out preventer stack 4. A flow riser control device 20 is provided beneath the rig floor and the telescopic joint 21.

Figure 2 illustrates in more detail the marine riser well control apparatus according to the invention. The rotary table 52 is illustrated in drilling floor 53. Beams 60 beneath the drilling floor are attached to a

support 62 for supporting a flex joint 63 connected to the inner barrel 22 of the telescopic joint 21. A riser control device 20 according to the invention is connected to the outer barrel 24 of the telescopic joint 21. Packing 26 between inner barrel 22 and the outer barrel 24 of the telescopic joint 21 allows the inner barrel 22 to move up and down within the outer barrel 24 due to the heave of the floating drilling vessel 2 while drilling mud is normally passing through it to return to the mud pumps.

The riser control device 20 to be described in detail below with respect to Figure 3 includes an L shaped connector 100 to an outlet in the riser control device housing wall. An auxiliary choke line 7 is "connected between the outlet connector 100 and a choke manifold 56 of the floating drilling vessel 2.

A riser string 10 is connected to the bottom of the' flow controller 20. The riser string extends' in some cases thousands of feet deep below the water surface to a blowout preventer stack 4 attached to a wellhead 44 of the well being drilled. The blowout preventer stack 4 provided for marine drilling typically includes four pipe rams and an annular blowout preventer as illustrated in Figure 2. Ram pipe blowout preventers 40, 38 and 36 are provided along with a shear ram blow¬ out preventer 32. An annular blowout preventer is illus¬ trated schematically as element 30. Hydraulic connectors 51 and 42 connect the connecting spools of the stack to the wellhead 44 and to the drilling riser 10 respectively, Choke line 12 may be connected as illustrated in blowout preventer stack 4 and extends to the surface choke manifold 56 (.although for simplicity not illus¬ trated) . Kill line 14 is illustrated as extending down¬ wardly along the telescopic joint and the riser 10 and is connected to the spool to the blowout preventer spool above the lower ram blowout preventer 40. The kill line

14 is connected to the mud pumps 54 for providing pres¬ surized drilling fluid to the blowout preventer stack above the ram blowout preventer 40. A length of drill pipe 58 is illustrated as extending down through the telescopic joint 21, the riser control device 20 and the riser 10 and the blowout preventer stack 4 and into the borehole below for drilling the borehole. As illustrated in Figure 2 the marine riser string 10 as well as the riser control device 20 and the telescopic joint 21 may be run through the rotary table for ease of installation.

Figure 3. illustrates the construction details of the improved riser control device 20 of the marine riser well control apparatus according to the invention. The riser control device 20 includes a cylindrical housing 82 with a lower body 84 and an upper head 80 connected to the outer body 82 by means of bolts 97 and 96. Disposed within the housing is an annular packing unit 88 and a piston 90 having a conical bowl shape 92 for urging the annular packing unit 88 radially inwardly upon the upward movement of piston 90. The lower wall 94 of piston 90 covers an outlet passage 86 in the lower body 84 when the piston is in the lower position. When the piston moves upwardly to force the packing element 88 inwardly about a drill pipe extending through the bore of the diverter 20, the lower end of the piston 94 moves upwardly and opens the outlet passage 86.

A connector 100 has a ninety-degree turn passage 102 which communicates with the outlet passage 86. A "lead "target" plate 98 is provided in line with the outlet 86 to withstand the pressurized fluid flow which may flow out of outlet 86. When piston 94 is moved up¬ wardly lead plate 98 withstands the highly erosive effect of the pressurized mud flow. A threaded connection 104 is provided to connect the auxiliary choke line 7

extending upwardly as illustrated in Figure 2 to the choke manifold 56.

The outer dimension D as illustrated in Figure 3 of the housing 82 of the riser control device 20 is pro- vided to be less than the outer dimension of the remove- . able inserts of the rotary table 52. The L shaped con¬ nector 100 is also designed to insure that the outer dimension of the entire riser control assembly 20 may be run through the rotary table for ease of installation. As illustrated in Figure 2, accumulator bottles 70 may be attached directly to the drilling riser string beneath the flow diverter 20 to provide rapid hydraulic operation of the piston 90 of the riser control device 20 during an emergency kick of formation gas into the riser string. Figure 2 serves to illustrate the operation of the marine riser well control apparatus according to the invention. In the event of excessive formation gas flow, the upward flow path of the riser annulus is closed by actuating the riser control device. Actuation °f the riser control device 20 causes the piston 90 to move upwardly thereby causing the packing element 88 to move radially inwardly to seal about a drill pipe 58 through its vertical flow path. As the piston 90 moves up, the outlet 86 is uncovered by the lower portion 94 of the piston 90. Rapid closing may be assured by the use of large multiple hydraulic control lines and a bank of dedicated accumulator bottles 70 which are remotely rechargeable and manifolded together and mounted on the riser string 10. Preferably, the riser control device 20 is designed for two thousand PSI working pres¬ sure compatible with the riser string pressure rating and anticipated formation pressures. Having safely closed the annulus and having opened the auxiliary choke line 7 via riser control device outlet 86, the bottom most ram blowout preventer 40 is closed and pressurized

drilling mud from pumps 54 is applied via kill line 14 to the annulus of the stack above the ram blowout pre¬ venter 40.

The kill mud is then pumped into the annulus between the interior of the riser string 10 and the exterior of the drill pipe 58. The drilling mud pro¬ vides return flow circulation through the choke mani¬ fold until a normal well pressure is restored. By rapidly providing drilling mud into the annulus of the riser string 10 sufficient working pressure is main¬ tained within its interior so that the pressure external to the riser string 10 from deep seawater will not col¬ lapse the drilling riser 10.. Following kill operations, the annular packer 88 of the flow diverter 20 can be - opened and the auxiliary choke line 7 shut off and routine drilling operations resumed.

Where the blowout preventer stack 4 is not pro¬ vided to the wellhead 44 but the drilling riser 10 is connected directly there-to, the kill line 14 of course is not provided but control over pressure in the deep- water riser string 10 is provided by pumping drilling mud through the interior of the drill pipe 58. The mud is pumped out the end of the drill pipe in the borehole and returns to the annulus of the drill string 10, where upon operation of the riser control device 20 as described above, the formation gas in the drill string 10 may circulate out via the auxiliary choke line 7 to the choke manifold 56. Again, following kill operations, the annular packer 88 of the riser control device 20 may be opened and the choke line shut off and routine drilling operations resumed.