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.

In conventional offshore drilling fluid is circulated down the drill string and up through an annulus between the drill string and the borehole 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 fluid continues from the mudline to the surface through the riser/drill string annulus.

In such 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 and the borehole/drill string annulus.

U. S. patent 4,813,495 discloses a system 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.

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 method and system for reducing the excess hydrostatic pressure exerted by the mud column return in the riser/drill string annulus or in the borehole/drill string annulus.

In accordance with one aspect of the invention there is provided a method of drilling an offshore borehole in an earth formation, the method comprising -drilling the borehole using a drill string extending into the borehole; -pumping a drilling fluid from a surface drilling facility through the drill string, the drilling fluid flowing from the drill string into the borehole whereby cuttings resulting from the drilling operation are entrained into the drilling fluid; -treating the drilling fluid by inducing the drilling fluid to flow into a subsea processing system so as to remove the cuttings from the drilling fluid; and -returning the treated drilling fluid to surface by means of a return pump system.

The system for drilling an offshore borehole in an earth formation according to the invention comprises -a drill string extending into the borehole ; -a pump for pumping a drilling fluid from a surface drilling facility through the drill string and from the drill string into the borehole whereby cuttings resulting from the drilling operation are entrained into the drilling fluid;

-a subsea processing system for treating the drilling fluid by inducing the drilling fluid to flow into the subsea processing system so as to remove the cuttings from the drilling fluid; and -a return pump system for returning the treated drilling fluid to surface.

By treating the drilling fluid in the subsea processing system so as to remove the drill cuttings therefrom it is achieved that the drilling fluid is substantially free of cuttings before entering the return pump system. The durability and reliability of the return pump system is thereby greatly enhanced.

Suitably the treated drilling fluid flows from the subsea processing system into a reservoir from which the drilling fluid flows to the return pump system via a suction line thereof. Thus, there is no need for accurate synchronous operation of the pump of the surface drilling facility and the subsea processing system, because the reservoir acts as a buffer and allows for variations of the drilling fluid level therein.

The invention will be described below in more detail and by way of example, with reference to the accompanying drawings in which: FIG. 1 is a schematic illustration of one embodiment of the subsea pumping system according to the invention; FIG. 2 is a side elevational view of another embodiment of the subsea pumping system according to the invention; FIG. 3 is a longitudinally taken cross sectional view of the drill string shut-off valve of FIG. 2 in a closed position; and FIG. 4 is a longitudinally taken cross sectional view of the drill string shut-off valve of FIG. 2 in an open position.

FIG. 1 illustrates schematically one embodiment of a drilling fluid circulation system 10 in accordance with the present invention. Drilling fluid is injected into the drill string at the drilling rig facilities 12 above ocean surface 14. The drilling fluid is transported down a drill string (see FIG. 2), through the ocean and down borehole 16 below mudline 18. Near the lower end of the drill string the drilling fluid passes through a drill string shut-off valve ("DSSOV") 20 and is expelled from the drill string through the drill bit (refer again to FIG. 2). The drilling fluid scours the bottom of borehole 16, entraining cuttings, and returns to mud line 18 in annulus 19. Here, near the ocean floor, the drilling mud is carried to a subsea primary processing facility 22 where waste products, see line 24, are separated from the drilling fluid. These waste products include at least the coarse cuttings entrained in the drilling fluid. With these waste products 24 separated at facilities 22, the processed drilling fluid proceeds to subsea return pump 26 where it is pumped to drilling facilities above surface 14. A secondary processing facility 28 may be employed to separate additional gas at lower pressure and to remove fines from the drilling fluid. The reconditioned drilling fluid is supplied to surface pump system 30 and is ready for recirculation into the drill string at drilling rig 12. This system removes the mud's hydrostatic head between the surface and the seafloor from the formation and enhances pump life and reliability for subsea return pump system 26.

The embodiment of FIG. 1 can be employed in both drilling operations with or without a drilling riser. In either case, the hydrostatic pressure of the mud return through the water column is isolated from the hydrostatic head below the blow-out preventor, near the seafloor.

Indeed, with sufficient isolation the return path for the

mud could proceed up the drilling riser/drill string annulus. However, it may prove convenient to have a separate riser for mud return whether or not a drilling riser is otherwise employed. Further, even if not used as the mud return conduit through the water column, it may be convenient to have a drilling riser to run the blowout preventor and separation equipment.

FIG. 2 illustrates the subsea components of one embodiment of drilling fluid circulation system 10, here with a drilling riser that is not used for returning the mud through the water column. The drilling fluid or mud 32 is injected into drill string 34 which runs within marine drilling riser 36, through a subsea blow-out preventor ("BOP stack") 38 near the mudline 18, through casing 40, down the uncased borehole 16 to a bottom hole assembly 42 at the lower end of the drill string. The bottom hole assembly includes DSSOV 20 and drill bit 44.

The flow of drilling mud 32 through drill string 34 and out drill bit 44 serves to cool the drill bit, flush the cuttings away from the bit's formation interface and to stabilizes the uncased borehole with a"filter cake" until additional casing strings 40 are set in newly drilled sections. Drilling mud 32 also performs a crucial well control function in maintaining a pressure with a hydrostatic head in uncased sections of the borehole 16 that prevents the uncontrolled flow of pressured well fluids into the borehole from the formation.

However, in this embodiment, the drilling mud is not returned to the surface through the marine riser/drill string annulus 46, but rather is withdrawn from the annulus near mudline 18, e. g., immediately above BOP stack 38 through mud return line 19. In this illustration, with a drilling riser, the remainder of annulus 46, to the ocean surface, is filled with seawater 48 which is much less dense than the drilling

mud. Deepwater drilling applications may exert a thousand meters or more of hydrostatic head at the base of marine drilling riser 36. However, when this hydrostatic head is from seawater rather than drilling mud in annulus 32, the inside of the marine drilling riser remains substantially at ambient pressure in relation to the conditions outside the riser at that depth. The same is true for mud leaving the well bore in riserless embodiments. This allows the drilling mud specification to focus more clearly on well control substantially from the mudline down.

Drilling mud 32 is returned to the surface in drilling fluid circulation system 10 including subsea primary processing system 22, subsea return pump 26 and a second riser 50 serving as the drilling mud return line.

Subsea primary processing system 22 is illustrated with a two component first stage 22A carried on the lowermost section of drilling riser 36 and a subsequent stage 22B on the ocean floor.

In normal operation, solids removal system 54 first draws the return of drilling mud 32. Here solids removal system 54 is a gumbo box arrangement 68 which operates in a gas filled ambient pressure dry chamber 72. The hydrostatic head of mud 32 within the annulus 46 drives the mud through the mud return line and over weir 74 to spill out over cuttings removal equipment such as a screen or gumbo slide 78. Cuttings 76 too coarse to pass through the screen or through the gumbo slide, fall off its far edge beyond mud tank 80, and exit directly into the ocean through the open bottom of dry chamber 72. The mud, less the cuttings separated, passes through the gumbo slide into a mud tank 80 and flows from mud tank 80 via a conduit 66 to a lower mud tank 80A.

Remote maintenance within gumbo-box arrangement 68 may be facilitated with a wash spray system to wash the gumbo slide with seawater and a closed circuit television

monitor or other electronic data system in the dry chamber.

Cuttings 76 can be prevented from accumulation at the well by placing a cuttings discharge ditch 84 beneath dry chamber 72 to receive cuttings exiting the dry chamber. A jet pump 86 injects seawater past a venturi with a sufficient pressure drop to cause seawater and any entrained cuttings to be drawn into cuttings discharge line 88 from cuttings discharge ditch 84. The cuttings discharge line then transports the cuttings to a location sufficiently removed such that piles of accumulated cuttings will not interfere with well operations.

Another advantage of this embodiment is that gas resulting from a well control event is removed by means of a gas separator 52 and is expelled near seafloor 18.

Pump operation in such well events is critical. In a well control event in which large volumes of gas enter the well, the overall system must handle gas volumes while creating an acceptable back pressure on the wellbore 16 by pumping down heavier weight mud at sufficient volume, rate and pressure. Dropping below this pressure in a well control event will result in additional gas influx, while raising pressure to excess may fracture the borehole. The ability to cycle through muds at weights suited to the immediate need is the primary control on this critical pressure. However, multiphase flow is a challenge to conventional pumps otherwise suited to subsea return pump system 26. Thus, only substantially gas free mud is pumped to the surface through subsea return pump system 26, facilitating pump operation during critical well control events. Additional gas may be removed at the surface atmospheric pressure with an additional gas separation system, not shown.

The gas separator 52 includes a vertically oriented tank or vessel 58 having an exit at the top which leads

to a gas vent 60 through an inverted u-tube arrangement 62 and a mud takeout 64 near its base which is connected into return line 66 downstream from solids removal system 54.

The subsequent stage processing system 22B is a further solids removal system, in the form of a second gumbo box arrangement 68A in gas-filled ambient pressure dry chamber 72A. The hydrostatic head of mud 32 within tank 80 drives the mud over weir 74A to spill out mud and entrained cuttings over more closely spaced bars or a finer mesh screen gumbo slide 78A. Mud separated in mud/gas separator 52 may join that from tank 80 in this second stage processing. A finer grade of cuttings is removed and carried away with cuttings discharge ditch 84A and jet pump 86A, as before, with the processed mud passing to mud tank 80A.

It may also be desirable to provide the position of normal tank exit and a tank volume that allows settling of additional cuttings able to pass through the gumbo slide. A surface activated dump valve at the bottom of the mud tank may be used to periodically remove the settled cuttings.

The suction line 94 of subsea return pump 26 is attached to the base of lower mud tank 80A. A liquid level control 90 in the lower mud tank 80A activates the return pump 26. The removal of the cuttings from the mud greatly enhances pump operation in this high pressure pumping operation to return the cuttings from the seafloor to the facilities above the ocean surface through return riser 50. The return riser may be conveniently secured at its base to a foundation such as an anchor pile 98 and supported at its upper end by surface facilities (not shown), perhaps aided by buoyancy modules (not shown) arranged at intervals along its length. In this embodiment, the return pump 26 is housed

in an ambient pressure dry chamber 92 which improves the working environment and simplifies pump design and selection.

In well control events, BOP stack 38 is closed and the gas separator 52 intakes fluid from subsea choke lines 33 associated with BOP stack 38. In such a well control event, gas separator 52 permits removal of gas from mud 32 so that subsea pump system 26 may operate with only a single phase component, i. e., liquid mud. The gas separator 52 may be conveniently mounted to the lowermost section of riser 36.

FIG. 3 details DSSOV 20 deployed at the base of the drill string 34 as part of the bottom hole assembly 42 in FIG. 2. The DSSOV is an automatic valve which uses ported piston pressures/spring balance to close a valve 112 for containing the hydrostatic head of drilling fluid 32 within the drill string when the bottom hole assembly is in place and the normal circulation of the drilling fluid is interrupted, e. g. in order to make up another section of drill pipe into the drill string. In such instances the DSSOV closes to prevent the drilling fluid from running down and out of the drill string and up the annulus 46, displacing the much lighter seawater until equilibrium is reached.

FIGS. 3 and 4 illustrate DSSOV 20 in the closed and open positions respectively. The DSSOV has a main body 120 and may be conveniently provided with connectors such as a threaded box 122 and pin 124 on either end to make up into the drill string in the region of the bottom hole assembly. The body 120 presents a cylinder 128 which receives a piston 116 having a first pressure face 114 and a second pressure face 130. First pressure face 114 is presented on the face of the piston and is ported to the upstream side of DSSOV 20 through channel 132 passing

through the piston. Channel 132 may be conveniently fitted with a trash cap 134.

Second pressure face 130 is on the back side of piston 116 and is ported to the downstream side of DSSOV 20. Further, the first and second pressure faces of piston 116 are isolated by o-rings 136 slidingly sealing between the piston and the cylinder.

Body 120 also has a main flow path 140 interrupted by valve 112, but interconnected by drilling mud flow channels 126. A plurality of o-rings 142 between valve 112 and body 120 isolate flow from drilling mud flow channels 126 except through ports 118 of valve 112.

The DSSOV is used to maintain a positive surface drill pipe pressure at all times. When the surface mud pump system 30 (see FIG. 1) is shut off, e. g., to add a section of drill pipe 34 as drilling progresses, a tensile valve shut-off spring 110 shuttles valve 112 to a closed position in which valve ports 118 are taken out of alignment with drilling mud flow channels 126 in body 120. The spring 110, the surface area of first pressure face 114, and the surface area of the second pressure face 130 of piston 116 are balanced in design to close valve 112 to maintain the pressure margin created by the difference in density between seawater 48 and mud 32 over the distance between water surface 14 and ocean floor 18. Thus the excess positive pressure in drill pipe 34 is kept from dissipating by driving drilling mud down the drill pipe and up annulus 46, while isolating the excess pressure from borehole 16.

After the new drill pipe section has been made up or drilling is otherwise ready to resume, surface pump system 30 (FIG. 1) is used to build pressure on valve 112 until the pressure on face 114 of piston 116 overcomes the bias of spring 110, opening valve 112 and resuming circulation. See FIG. 4.

DSSOV 20 also facilitates a method of determining the necessary mud weight in a well control event. With the DSSOV closed, pump pressure is slowly increased while monitoring carefully for signs of leak-off which is observed as an interruption of pressure building despite continued pump operation. This signals that flow has been established and the pressure is recorded as the pressure to open the DSSOV. Surface pump system 30 is then brought up to a reduced pump rate employed to cycle out well fluids while carefully monitoring pressures to prevent additional influx from the formation. The opening pressure, the reduced pump rate and the circulating pressure are each recorded periodically or when a significant mud weight adjustment has been made.

With such information, the bottom hole pressure can be determined should a well control event occur. Shutting of surface pump system 30 after a flow is detected will close off DSSOV 20. The excess pressure causing the event, that is the underbalanced pressure of the formation, will add to the pressure needed to open valve 112. Pump pressure is then reapplied and increased slowly, monitoring for a leak-off signaling the resumption of flow. The pressure difference between the pre-recorded opening pressure and the pressure after flow is the underbalanced pressure that must be compensated for with adjustments in the density of mud 32. The kill mud weight is then calculated and drilling and adjustments are made accordingly in the mud formulation.

In the illustrated embodiment, some of the components of the subsea primary processing system 22 are provided on the marine drilling riser 36 and others are set directly on ocean floor 18. As to components which are set on the ocean floor, it may be useful to deploy a minimal template or at least interlocking guideposts and receiving funnels to key components placed as subsea

packages into secure, prearranged relative positions.

This facilitates making connections between components placed as separate subsea packages with remotely operated vehicles ("ROV"). Such connections include electric lines, gas supply lines, mud transport lines, and cuttings transport lines. A system of gas supply lines (not shown) supply each of the dry chambers 72,72A, and 92 with gas to compensate for the volumetric compression of gas in the open bottomed dry chambers when air trapped at atmospheric pressure at the surface is submerged to great depths. Other combinations of subsea primary processing components and their placement are possible.

Further, some components may be deployed on the return riser 50 analogous to the deployment on marine drilling riser 36.

In an alternative embodiment the first and second stage processing systems and the gas separator are mounted on a dedicated riser section. The dedicated riser section needs to be sized to be run through the moonpool of the surface drilling facilities, preferably having a horizontal cross section no greater than the BOP stack outline. The components of such system, e. g. a pair of gumbo boxes and a pair of horizontal gas/mud separators, are mounted on a frame secured to the dedicated riser section. Cuttings discharge ditches, jet pumps, and cuttings discharge lines can also be mounted to this riser section. This allows connections between these components and the annulus within the marine drilling riser and the BOP stack to be fully modularly assembled on the surface before the drilling riser is made up to the subsea well.

Other modifications, changes, and substitutions are also intended in the foregoing disclosure. Further, in some instances, some features of the present invention will be employed without a corresponding use of other features described in these illustrative embodiments.