Patent Publication Number: US-6216799-B1

Title: Subsea pumping system and method for deepwater drilling

Description:
This application claims the benefit of U.S. Provisional Application No. 60/060,042, filed Sep. 25, 1997, the entire disclosure of which is hereby incorporated by reference. 
    
    
     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&#39;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 and method is that it is not necessary to maintain strict synchronous operation of the supply and return lines. Another advantage is that working environment of the return pump and associated valves is materially improved, enhancing pump and valve life and performance. 
     A SUMMARY OF THE INVENTION 
     One aspect of the present invention is a method for offshore drilling which drives a bit mounted at a far end of a drill string, injects a drilling fluid into the drill string from surface drilling facilities, passes the drilling fluid through the far end of the drill string and flushes the borehole at the bit and entraining cuttings into the drilling fluid. The drilling fluid is treated through a subsea primary processing stage to removing the cuttings from the drilling fluid and the treated drilling fluid is returned to the surface with a subsea return pump system and passes to surface drilling facilities for injection. 
    
    
     A BRIEF DESCRIPTION OF THE DRAWINGS 
     The brief description above, as well as further objects and advantages of the present invention, will be more fully appreciated by reference to the following detailed description of the preferred embodiments which should be read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of one embodiment of a subsea pumping system for deepwater drilling; 
     FIG. 2 is a side elevational view of a one embodiment of a subsea pumping system for deepwater drilling; 
     FIG. 3 is a side elevational view of the dedicated riser section in the embodiment of FIG. 2; 
     FIG. 4 is a top elevational view of the dedicated riser section of FIG. 3; 
     FIG. 5 is a longitudinally taken cross sectional view of the drill string shut-off valve of FIG. 2 in a closed position; 
     FIG. 6 is a longitudinally taken cross sectional view of the drill string shut-off valve of FIG. 2 in an open position; and 
     FIGS. 7A-7C are longitudinally taken cross sections of another embodiment of a drill string shut-off. 
    
    
     BRIEF DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     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&#39;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 blowout 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 line through the water column, it may be convenient to have a drilling riser to run the blowout preventor and separation equipment discussed below. See FIG.  2 . 
     Returning to FIG. 1, another advantage of this embodiment is that gas resulting from a well control event is removed at 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. 
     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. In this embodiment, the bottom hole assembly includes DSSOV  20  as well as 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&#39;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  74 . 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  through subsea primary processing  22 , subsea return pump  26  and a second riser  50  serving as the drilling mud return line. In this embodiment, subsea primary processing  22  is illustrated with a two component first stage  22 A carried on the lowermost section of drilling riser  36  and a subsequent stage  22 B 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 intake line and over weir  74  to spill out over cuttings removal equipment such screens or gumbo slide  78 . Cuttings  76  too coarse to pass between bars or through a mesh screen proceed down 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 and is received in mud tank  80  and exits near the tank base. 
     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 (and perhaps the dump valve). 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. 
     FIGS. 3 and 4 illustrate in detail an alternate embodiment in which components of first and second stage processing  22 A and  22 B as well as gas separator  52  are mounted on a dedicated riser section  36 A. The dedicated riser needs to be sized to be run through the moonpool of the surface drilling facilities, preferably having a horizontal cross section no greater that the BOP stack outline  104 , illustrated in FIG. 4 in dotted outline  100 . 
     Components, here a pair of gumbo boxes  68  and a pair of horizontal gas/mud separators  58 , are mounted on frame  102  secured to dedicated riser joint  36 A. Cuttings discharge ditches  84 , jet pumps  86 , and cuttings discharge lines  88  are also mounted to this riser section. This allows connections between these initial components and the annulus within marine drilling riser  36  and BOP stack  38  to be fully modularly assembled on the surface before the drilling riser is made up to the subsea well. 
     Returning to FIG. 2, the illustrated embodiment also provides subsequent stage processing  22 B, here a further solids removal system  54 A, in the form of a second gumbo box arrangement  68 A in gas-filled ambient pressure dry chamber  72 A. The hydrostatic head of mud  32  within tank  80  drives the mud and over weir  74 A to spill out mud and entrained cuttings over more closely spaced bars or a finer mesh screen gumbo slide  78 A. 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  84 A and jet pump  86 B, as before, with the processed mud passing to mud tank  80 A. 
     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  82  at the very 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 mud tank  80 A. A liquid level control  90  in the mud tank or subsequent subsea mud reservoir activates return pump. 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 a 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. A return pump is provided to propel the mud up the return riser to the surface. A suitable pump may be deployed into the subsea environment or, as in this embodiment, the return pump can be 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 from subsea choke lines  33  associated with BOP stack  38 . The intake leads to 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 . 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 riser section  36  or, as illustrated in FIGS. 3 and 4, a dedicated riser section  36 A. 
     FIG. 5 details a DSSOV  20  deployed at the base of drill string  34  as part of bottom hole assembly  42  in FIG.  2 . The DSSOV is an automatic valve which uses ported piston pressures/spring balance to throw 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., 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. See FIG.  2 . 
     FIGS. 5 and 6 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 . In this illustrated embodiment it is ported to the bore below the valve. 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  and a plurality of o-rings  142  between valve  112  and body  120  isolate flow from drilling mud flow channels  126  except through ports  118 . 
     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, 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 . See FIG.  5 . 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 differences in density between seawater  48  and mud  32  over the distance between surface  14  and ocean floor  18 . See FIG.  1 . This holds the excess positive pressure in drill pipe  34 , keeping it from dissipating by driving drilling mud down the drill pipe and up annulus  46 , while isolating the excess pressure from borehole  16 . See FIG.  2 . 
     After a 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  overcome the bias of spring  110 , opening valve  112  and resuming circulation. See FIG.  6 . 
     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 kill speed and the circulating pressures are recorded. Kill speed is a reduced pump rate employed to cycle out well fluids while carefully monitoring pressures to prevent additional influx from the formation. The opening pressure, kill speed and circulating pressure are each recorded periodically or when a significant mud weight adjustment has been made. 
     With such current 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 ,  72 A, and  92  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 . 
     FIGS. 7A-7C illustrate another DSSOV embodiment, DSSOV  20 A, in full open, intermediate, and closed positions, respectively. The DSSCOV cylinder has three regions,  128 A,  128 B and  128 C. An additional profile in piston  116  provides paired large and small pressure faces as first pressure faces,  114 A and  114 B paired with corresponding second pressure faces  130 A and  130 B. Pressure faces  130 A and  114 A engage region  128 A of the cylinder during normal mud circulation. Pressure faces  130 A and  114 A have a greater area than pressure faces  130 B and  114 B. This means that a lower pressure differential will keep valve  112  open. However, when the balance shifts such that the DSSOV starts to close, pressure faces  130 A and  114 B disengage from a sealing relationship with the cylinder walls in region  128 A as the piston moves and these faces align with large diameter region  128 B. The smaller area pressure faces  130 B and  114 B are then aligned in a sealing relationship with a reduced region  128 C of the cylinder. 
     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. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.