OA11342A - Subsea pumping system for deepwater drilling. - Google Patents

Abstract

A method is disclosed for offshore drilling in which a bit (44) is driven at a far end of a drill string (34), drilling fluid (32) is injected into the drill string from surface drilling facilities, and drilling fluid flushes the borehole (16) at the bit and entrains drill cuttings (76). The drilling fluid is drawn off near the mudline and is treated through a subsea processing system (22) to remove the cuttings from the drilling fluid. The treated drilling fluid is then returned to the surface with a subsea return pump system (26) and passed to surface drilling facilities for injection and recirculation.

Description

1 011342 ν' :- - ί

SUBSEA PUMPING SYSTEM FOR

DEEPWATER DRILLING

The présent invention relates to a method and Systemfor offshore drilling. More particularly, the présentinvention is a method and System for handling thecirculation of drilling fluid in offshore drilling 5 operations.

Drilling fluids, also known as muds, are used to coolthe drill bit, flush the cuttings away from the bit'sformation interface and then out of the System, and tostabilize the borehole with a "filter cake" until newly 10 drilled sections are cased. The drilling fluid also performs a crucial well control function and is monitoredand adjusted to maintain a pressure with a hydrostatichead in uncased sections of the borehole that preventsthe uncontrolled flow of pressured well fluids into the 15 borehole from the formation.

In conventional offshore drilling fluid is circulated down the drill string and up through an annulus betweenthe drill string and the borehole below the mudline. Ariser surrounds the drill string starting from the 20 wellhead at the océan floor to drilling facilities at the surface and the return circuit for drilling fluidcontinues from the mudline to the surface through theriser/drill string annulus.

In such conventional System, the relative weight of 25 the drilling fluid over that of seawater and the length of the riser in deepwater applications combine to exertan excess hydrostatic pressure in the riser/drill stringannulus and the borehole/drill string annulus. Ü.S. patent 4,813,495 discloses a System to bring the 30 drilling fluid and entrained cuttings out of the annulus 2 011 342 at the base of the riser and te 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 5 and particularly so in light of the nature of the fluid with entrained cuttings that is handled in valves andpumps on the return segment of the circuit.

Thus, there remains a need for a practical method andSystem for reducing the excess hydrostatic pressure 10 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 thereis provided a method of drilling an offshore borehole inan earth formation, the method comprising 15 - drilling the borehole using a drill string extending into the borehole; pumping a drilling fluid from a surface drillingfacility through the drill string, the drilling fluidflowing from the drill string into the borehole whereby 20 cuttings resulting from the drilling operation are entrained into the drilling fluid; treating the drilling fluid by inducing the drillingfluid to flow into a subsea processing System so as toremove the cuttings from the drilling fluid; and 25 - returning the treated drilling fluid to surface by means of a return pump System.

The system for drilling an offshore borehole in anearth formation according to the invention comprises a drill string extending into the borehole; 30 - a pump for pumping a drilling fluid from a surface drilling facility through the drill string and from thedrill string into the borehole whereby cuttings resultingfrom the drilling operation are entrained into thedrilling fluid; 3 011342 ίο 15 20 25 30 a subsea Processing System for treating the drillingfluid by inducing the drilling fluid to flow into thesubsea processing System so as to remove the cuttingsfrom the drilling fluid; and a return pump System for returning the treateddrilling fluid to surface.

By treating the drilling fluid in the subseaProcessing System so as to remove the. drill cuttingstherefrom it is achieved that the drilling fluid issubstantially free’of cuttings before entering the returnpump System. The durability and reliability of the returnpump System is thereby greatly enhanced. '

Suitably the treated drilling fluid flows from thesubsea processing System into a réservoir from which thedrilling fluid flows to the return pump System via asuction line thereof. Thus, there is no need for accuratesynchronous operation of the pump of the surface drillingfacility and the subsea processing System, because theréservoir acts as a buffer and allows for variations ofthe drilling fluid level therein.

The invention will be described below in more detailand by way of example, with reference to the accompanyingdrawings in which: FIG. 1 is a schematic illustration of one embodimentof the subsea pumping System according to the invention; FIG. 2 is a side elevational view of anotherembodiment of the subsea pumping System according to the FIG. 1 is of the subsea FIG. 2 is embodiment of invention; FIG. 3 is of the drill position; and FIG. 4 is FIG. 3 is a longitudinally taken cross sectional viewof the drill string shut-off valve of FIG. 2 in a closed FIG. 4 is a longitudinally taken cross sectional viewof the drill string shut-off valve of FIG. 2 in an openposition. 4 011342 FIG. 1 illustrâtes schematically ône embodiment of a drilling fluid circulation System 10 in accordance, with the présent invention. Drilling fluid is injected into the drill string at the drilling rig facilities 12 above 5 océan surface 14. The drilling fluid is transported down a drill string (see FIG. 2), through the océan and downborehole 16 below mudline 18. Near the lower end of thedrill string the drilling fluid passes through a drillstring shut-off valve ("DSSOV") 20 and is expelled from 10 the drill string through the drill bit (refer again to FIG. 2). The drilling fluid scours the bottom ofborehole 16, entraining cuttings, and returns to mudline 18 in annulus 19. Here, near the océan floor, thedrilling mud is carried to a subsea primary processing 15 facility 22 where waste products, see line 24, are separated from the drilling fluid. These waste productsinclude at least the coarse cuttings entrained in thedrilling fluid. With these waste products 24 separated atfacilities 22, the processed drilling fluid proceeds to 20 subsea return pump 26 where it is pumped to drilling facilities above surface 14. A secondary processingfacility 28 may be employed to separate additional gas atlower pressure and to remove fines from the drillingfluid. The reconditioned drilling fluid is supplied to 25 surface pump System 30 and is ready for recirculation

Into the drill string at drilling rig 12. This Systemremoves the mud's hydrostatic head between the surfaceand the seafloor from the formation and enhances pumplife and reliability for subsea return pump System 26. 30 The.embodiment of FIG. 1 can be employed in both drilling operations with or without a drilling riser. Ineither case, the hydrostatic pressure of the mud returnthrough the water column is isolated from the hydrostatichead below the blow-out preventor, near the seafloor. 35 Indeed, with sufficient isolation the return path for the 5 011342 mud could proceed up the drilling risér/drill string annulus. However, it may prove convenient to hâve a separate riser for mud return whether or not a drilling riser is otherwise employed. Further, even if not used as 5 the mud return conduit through the water column, it may be convenient to hâve a drilling riser to run the blowoutpreventor and séparation equipment. FIG. 2 illustrâtes the subsea components of oneembodiment of drilling fluid circulation System 10, here 10 with a drilling riser that is not used for returning the mud through the water column. The drilling fluid ormud 32 is injected into drill string 34 which runs withinmarine drilling riser 36, through a subsea blow-outpreventor ("BOP stack") 38 near the mudline 18, through 15 casing 40, down the uncased borehole 16 to a bottom hole assembly 42 at the lower end of the drill string. Thebottom hole assembly includes DSSOV 20 and drill bit 44.

The flow of drilling mud 32 through drill string 34and out drill bit 44 serves to cool the drill bit, flush 20 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 newlydrilled sections. Drilling mud 32 also performs a crucialwell control function in maintaining a pressure with a 25 hydrostatic head in uncased sections of the borehole 16 that prevents the uncontrolled flow of pressured wellfluids into the borehole from the formation.

However, in this embodiment, the drilling mud is notreturned to the surface through the marine riser/drill 30 string annulus 46, but rather is withdrawn from the annulus near mudline 18, e.g., immediately above BOPstack 38 through mud return line 19. In thisillustration, with a drilling riser, the remainder ofannulus 46, to the océan surface, is filled with 35 seawater 48 which is much less dense than the drilling 6 011342 mud. Deepwater drilling applications may exert a thousandmeters or more of hydrostatic head at the base of marinedrilling riser 36. However, when this hydrostatic head isfrom seawater rather than drilling mud in annulus 32, theinside of the marine drilling riser remains substantiallyat ambient pressure in relation to the conditions outsidethe riser at that depth. The same is true for mud leavingthe well bore in riserless embodiments. This allows thedrilling mud spécification to focus more clearly on wellcontrol substantially from the mudline down.

Drilling mud 32 is returned to the surface indrilling fluid circulation System 10 including subseaprimary processing System 22, subsea return pump 26 and asecond riser 50 serving as the drilling mud return line.Subsea primary processing System 22 is illustrated with atwo component first stage 22A carried on the lowermostsection of drilling riser 36 and a subséquent stage 22Bon the océan floor.

In normal operation, solids removal System 54 firstdraws the return of drilling mud 32. Here solids removalSystem 54 is a gumbo box arrangement 68 which opérâtes ina gas filled ambient pressure dry chamber 72. Thehydrostatic head of mud 32 within the annulus 46 drivesthe mud through the mud return line and over weir 74 tospill out over cuttings removal equipment such as ascreen or gumbo slide 78. Cuttings 76 too coarse to passthrough the screen or through the gumbo slide, fall offits far edge beyond mud tank 80, and exit directly intothe océan through the open bottom of dry chamber 72. Themud, less the cuttings separated, passes through thegumbo slide into a mud tank 80 and flows from mud tank 80via a conduit 66 to a lower mud tank 80A.

Remote maintenance within gumbo-box arrangement 68may be facilitated with a wash spray System to wash thegumbo slide with seawater and a closed circuit télévision 7 011342 monitor or other electronic data system in the dry chamber.

Cuttings 76 can be prevented from accumulation at thewell by placing a cuttings discharge ditch 84 beneath dry5 chamber 72 to receive cuttings exiting the dry chamber. A jet pump 86 injects seawater past a venturi with asufficient pressure drop to cause seawater and anyentrained cuttings to be drawn into cuttings dischargeline 88 from cuttings discharge ditch 84. The cuttings10 discharge line thên transports the cuttings to a location

J sufficiently removed such that piles of accumulatedcuttings will not interfère with well operations.

Another advantage of this embodiment is that gasresulting from a well control event is removed by means15 of a gas separator 52 and is expelled near seafloor 18.

Pump operation in such well events is critical. In a wellcontrol event in which large volumes of gas enter thewell, the overall System must handle gas volumes whilecreating an acceptable back pressure on the wellbore 1620 by pumping down heavier weight mud at sufficient volume, rate and pressure. Dropping below this pressure in a wellcontrol event will resuit in additional gas influx, whileraising pressure to excess may fracture the borehole. Theability to cycle through muds at weights suited to the25 immédiate need is the primary control on this critical pressure. However, multiphase flow is a challenge toconventional pumps otherwise suited to subsea return pumpSystem 26. Thus, only substantially gas free mud ispumped to the surface through subsea return pump30 System 26, facilitating pump operation during critical well control events. Additional gas may be removed at thesurface atmospheric pressure with an additional gasséparation System, not shown.

The gas separator 52 includes a vertically oriented35 tank or vessel 58 having an exit at the top which leads 8 011342 to a gas vent 60 through an inverted u-tube arrangement62 and a mud takeout 64 near its base which is connectedinto return line 66 downstream from solids removalSystem 54. 5 The subséquent stage processing System 22B is a further solids removal System, in the form of a secondgumbo box arrangement 68A in gas-filled ambient pressuredry chamber 72A. The hydrostatic head of mud 32 withintank 80 drives the mud over weir 74A to spill out mud and 10 entrained cuttings over more closely spaced bars or a

J finer mesh screen gumbo slide 78A. Mud separated inmud/gas separator 52 may join that from tank 80 in thissecond stage processing, A finer grade of cuttings isremoved and carried away with cuttings discharge ditch 15 84A and jet pump 86A, as before, with the processed mud passing to mud tank 80A.

It may also be désirable to provide the position ofnormal tank exit and a tank volume that allows settlingof additional cuttings able to pass through the gumbo 20 slide. A surface activated dump valve at the bottom of the mud tank may be used to periodically remove thesettled cuttings.

The suction line 94 of subsea return pump 26 isattached to the base of lower mud tank 80A. A'liquid 25 level control 90 in the lower mud tank 80A activâtes the return pump .26. The removal of the cuttings from the mudgreatly enhances pump operation in this high pressurepumping operation to return the cuttings from theseafloor to the facilities above the océan surface 30 through return riser 50. The return riser may be conveniently secured at its base to a foundation such asan anchor pile 98 and supported at its upper end bysurface facilities (not shown), perhaps aided by buoyancymodules (not shown) arranged at intervals along its 35 length. In this embodiment, the return pump 26 is housed 9 011342 in an ambient pressure dry chamber 92 which improves theworking environment and simplifies pump design andsélection.

In well control events, BOP stack 38 is closed and5 the gas separator 52 intakes fluid from subsea choke

Unes 33 associated with BOP stack 38. In such a wellcontrol event, gas separator 52 permits removal of gasfrom mud 32 so that subsea pump System 26 may operatewith only a single phase component, i.e., liquid mud. The 10 gas separator 52 may be conveniently mounted to the lowermost section of riser 36. FIG. 3 details DSSOV 20 deployed at the base of thedrill string 34 as part of the bottom hole assembly 42 inFIG. 2. The DSSOV is an automatic valve which uses ported 15 piston pressures/spring balance to close a valve 112 for containing the hydrostatic head of drilling fluid 32within the drill string when the bottom hole assembly isin place and the normal circulation of the drilling fluidis interrupted, e.g. in order to make up another section 20 of drill pipe into the drill string. In such instances the DSSOV closes to prevent the drilling fluid fromrunning down and out of the drill string and up theannulus 46, displacing the much lighter seawater untilequilibrium is reached. 25 FIGS. 3 and 4 illustrate DSSOV 20 in the closed and open positions respectively. The DSSOV has a mainbody 120 and may be conveniently provided with connectorssuch as a threaded box 122 and pin 124 on either end tomake up into the drill string in the région of the bottom 30 hole assembly. The body 120 présents a cylinder 128 which receives a piston 116 having a first pressure face 114and a second pressure face 130. First pressure face 114is presented on the face of the piston and is ported tothe upstream side of DSSOV 20 through channel 132 passing 10 011342 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 5 20. Further, the first and second pressure faces of piston 116 are isolated by o-rings 136 slidingly sealingbetween the piston and the cylinder.

Body 120 also has a main flow path 140 interrupted byvalve 112, but interconnected by drilling mud flow10 channels 126. A plurality of o-rings 142 between valve 112 and body 120 isolate flow from drilling mudflow channels 126 except through ports 118 of valve 112.

The DSSOV is used to maintain a positive surfacedrill pipe pressure at ail times. When the surface mud15 pump System 30 (see FIG. 1) is shut off, e.g., to add a section of drill pipe 34 as drilling progresses, atensile valve shut-off spring 110 shuttles valve 112 to aclosed position in which valve ports 118 are taken out ofalignment with drilling mud flow channels 126 in20 body 120. The spring 110, the surface area of first pressure face 114, and the surface area of the secondpressure face 130 of piston 116 are balanced in design toclose valve 112 to maintain the pressure margin createdby the différence in density between seawater 48 and mud25 32 over the distance between water surface 14 and océan floor 18. Thus the excess positive pressure in drill pipe34 is kept from dissipating by driving drilling mud downthe drill pipe and up annulus 46, while isolating theexcess pressure from borehole 16? 30 After the new drill pipe section has been made up or drilling is otherwise ready to résumé, surface pumpSystem 30 (FIG. 1) is used to build pressure on valve 112until the pressure on face 114 of piston 116 overcomesthe bias of spring 110, opening valve 112 and resuming35 circulation. See FIG. 4. 11 01 1 342 DSSOV 20 also facilitâtes 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 5 observed as an interruption of pressure building despite continued pump operation. This signais that flow has beenestablished and the pressure is recorded as the pressureto open the DSSOV. Surface pump system 30 is then broughtup to a reduced pump rate employed to cycle out well 10 fluids while carefully monitoring pressures to prevent

J additional influx from the formation. The openingpressure, the reduced pump rate and the circulatingpressure are each recorded periodically or when asignificant mud weight adjustment has been made. 15 With such information, the bottom hole pressure can be determined should a well control event occur. Shuttingof surface pump system 30 after a flow is detected willclose off DSSOV 20. The excess pressure causing theevent, that is the underbalanced pressure of the 20 formation, will add to the pressure needed to open valve 112. Pump pressure is then reapplied and increasedslowly, monitoring for a leak-off signaling theresumption of flow. The pressure différence between thepre-recorded opening pressure and the pressure after flow 25 is the underbalanced pressure that must be compensated for with adjustments in the density of mud 32. The killmud weight is then calculated and drilling andadjustments are made accordingly in the mud formulation.

In the illustrated embodiment, some of the components 30 of the subsea primary processing system 22 are provided on the marine drilling riser 36 and others are setdirectly on océan floor 18. As to components which areset on the océan floor, it may be useful to deploy aminimal template or at least interlocking guideposts and 35 receiving funnels to key components placed as subsea 12 011342 packages into secure, prearranged relative positions.

This facilitâtes making connections between components placed as separate subsea packages with remotely operated vehicles ("ROV"). Such connections include electric 5 Unes, gas supply Unes, mud transport Unes, and cuttings transport Unes. A System of gas supply Unes(not shown) supply each of the dry chambers 72, 72A, and92 with gas tô compensate for the volumétrie compressionof gas in the open bottomed dry chambers when air trapped 10 at atmospheric pressure at the surface is submerged to great depths. Other combinations of subsea primaryProcessing components and their placement are p'ossible.Further, some components may be deployed on the returnriser 50 analogous to the deployment on marine drilling 15 riser 36.

In an alternative embodiment the first and secondstage processing Systems and the gas separator aremounted on a dedicated riser section. The dedicated risersection needs to be sized to be run through the moonpool 20 of the surface drilling facilities, preferably having a horizontal cross section no greater than the BOP stackoutline. The components of such system, e.g. a pair ofgumbo boxes and a pair of horizontal gas/mud separators,are mounted on a frame secured to the dedicated riser 25 section. Cuttings discharge ditches, jet pumps, and cuttings discharge Unes can also be mounted to thisriser section. This allows connections between thesecomponents and the annulus within the marine drillingriser and the BOP stack to be fully modularly assembled 30 on the surface before the drilling riser is made up to the subsea well.

Other modifications, changes, and substitutions arealso intended in the foregoing disclosure. Further, insome instances, some features of the présent invention 13 011342 will be employed without a correspondihg use of otherfeatures described in these illustrative erabodiments.

Claims (10)

14 011 34 2 NEW CLAIMS

1. A method of drilling an offshore borehole in an earthformation, the method comprising drilling the borehole using a drill string extendinginto the borehole; 5 - pumping a drilling' fluid from a surface drilling facility through the drill string, the drilling fluidflowing from the drill string into the borehole wherebycuttings resulting from the drilling operation <areentrained into the drilling fluid; 10 - treating the drilling fluid by inducing the drilling fluid to flow into a subsea processing System so as toremove the cuttings from the drilling fluid; and returning the treated drilling fluid to surface bymeans of a return pump System, wherein the drilling fluid 15 flows into the subsea processing System via an outlet opening provided in a drilling riser, characterised inthat the wellbore is provided with a casing and a blow-out preventer (BOP) arranged between the casing and saidoutlet opening of the drilling riser, and that the 20 drilling fluid flows to the drilling riser via said casing and said BOP.

2. The method of claim 1, wherein the treated drillingfluid flows from the subsea processing system into aréservoir from which the drilling fluid flows to the 25 return pump system via a suction îine thereof.

3. The method in accordance with claim 1 or 2, furthercomprising passing the returned drilling fluid to surfacedrilling facilities for reinjection.

4. The method in accordance with any one of daims 1-3 30 wherein the step of treating the drilling fluid comprises passing the drilling fluid into an ambient pressure gas 15 011342 chamber near the sea floor through a weir, separating the cuttings at gumbo rails and passing the drilling fluid to a collection basin, and transporting the cuttings away from the subsea Processing System for disposai.

5. The method in accordance with claim 4 wherein transporting the cuttings away from the subsea processingSystem for disposai comprises dropping the cuttings offthe end of the gumbo rails into the océan via an openbottom of the ambient pressure gas chamber, collecting 10 the cuttings in a discharge ditch below the open bottom of the ambient pressure gas chamber, and drawing thecuttings out of the discharge ditch with a je£ pump andpropelling the cuttings to a dump site away from thesubsea processing System through a cuttings discharge 15 line.

6. The method in accordance with any one of daims 1-5,wherein the step of treating the drilling fluid comprisesseparating any gas entering the drilling fluid from theearth formation during a well event upstream of the 20 return pump System.

7. The method in accordance with any one of daims 1-6,further comprising treating the drilling fluid afterreturn to the surface in a surface secondary processingSystem to remove gas and cutting fines therefrom.

8. The method in accordance with any one of daims 1-7, further comprising selectively isolating the hydrostatichead of drilling fluid in the drill string from therelatively lesser fluid pressure in the borehore by meansof a pressure activated drill string shut-off valve 30 arranged in the drill string, when drilling fluid circulation is interrupted.

9. A System for drilling an offshore borehole in anearth formation, the System comprising a drill string extending into the borehole; I 10 15 -16- 011342 a pump for pumping a drilling fluid from a surfacedrilling facility through the drill string and from thedrill string into the borehole whereby cuttings resultingfrom the drilling operation are entrained into thedrilling fluid; a subsea processing System for treating the drillingfluid by inducing the drilling fluid to flow into thesubsea processing System so as to remove the cuttingsfrom the drilling fluid; and a return pump system for returning the treated ! drilling fluid to surface, the System further comprisinga drilling riser provided with an outlet opening for flowof the drilling fluid to the subsea processing system,characterised in that the wellbore is provided with acasing and a blow-out preventer (BOP) arranged betweenthe casing and said outlet opening of the drilling riser.

10. The system of claim 9, further comprising a pressureactivated drill string shut-off valve arranged in thedrill string, which valve closes upon a pressuredifférence across the valve due to interruption ofdrilling fluid circulation through the drill string so asto prevent outflow of drilling fluid from the drillstring into the borehole. 20