Pump and a valve assembly therefor

A pump comprises a housing (141) having a cavity (142) with an inlet valve (143), an apparatus (144) for raising fluid pressure in the cavity (142) and a discharge valve (145), the discharge valve (145) comprises a valve head (160) movable between an open and closed position and a valve seat (167), wherein the pump further comprises a booster piston (182) arranged in a booster cylinder (183), the booster piston (182) has a first face (191) exposed to a low pressure zone (189) and a second face (194) exposed to fluid pressure in the cavity (142), and at least one of the booster piston (182) and the booster cylinder (183) is movable relative to the other to add an opening force to the discharge valve. The pump is preferably a mud pump for circulating drilling mud through a drill string in a wellbore or a cement pump which may be of the type for pumping cement into a wellbore. Also disclosed is a method for facilitating opening a valve comprising a valve head (160) and a valve seat (167), the valve head (160) is movable a predetermined distance from a closed position on the valve seat (167) to a fully open position distant on the valve seat, and the method comprises the step of adding a force to the valve head (160) along a small portion of the predetermined distance.

BACKGROUND

The present invention relates to a pump, a valve assembly for the pump and a method for facilitating opening a valve in a pump. The present invention particularly, but not exclusively relates to a mud pump, cement pump and a method for facilitating opening a discharge valve in a mud pump or cement pump.

In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic water or oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the returned drilling mud. The returned drilling mud flows from a wellhead located at a top of the well through a flow line. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved by flowing the returned drilling mud on to a screen of a vibratory separator to screen the returned drilling mud of large solids. The screened drilling mud flows into a series of partitioned sections in an active mud tank. Further processing equipment such as centrifuges and hydrocyclones may be used to further clean the mud of solids. Each piece of further processing equipment is located on top of the respective portioned section of the active mud tank, drawing pre-processed mud from the previous portioned section and outputting processed drilling mud into the partitioned section therebelow. The solids are: disposed of; cleaned and used as aggregate or the like; or certain solids, such as Lost Circulation Material is returned to clean drilling mud. It is not uncommon to have 30 to 100 m3of drilling fluid in circulation in a borehole.

The clean drilling mud is pumped into an additions unit of the active mud tank. Additives, such as weighting agents, viscosity control agents and lost circulation material are added to the clean drilling mud in the additions unit. The prepared drilling mud is now pumped into further testing unit of the active mud system and tested before being pumped into the suction tank of the active mud tank.

A mud pump is then used to pump the prepared and tested drilling mud from the suction tank into a top of the drill string to circulate drilling mud through the drill string to the drill bit and back to the surface through the annulus. The mud pump comprises a pumping section and a power section. The power section may comprise an electric motor, hydraulic motor or the like driving a reciprocating piston in the pumping section. The prepared and tested drilling mud flows from the suction tank through a supply hose or pipe, through an inlet valve in the pumping section of the mud pump into a cavity and is pushed out of the cavity through a discharge valve by action of the reciprocating piston into a further pipe or hose which leads to a goose neck on top of a top drive or swivel and into the top of a string of drill pipe extending into the borehole.

The borehole can be several kilometres long and several kilometres deep, so the mud pump is typically able to discharge drilling mud at between 200 bar and 1000 bar at between 300 and 4800 litres per minute.

The mud pump may comprise dual acting reciprocating pistons or single action. The mud pump may comprise a duplex reciprocating piston, triplex piston, or any other number of pistons. It is advantageous to maintain a constant flow of drilling mud through the drill string at a constant pressure. Surges in the flow of drilling mud and surges in pressure may cause problems downhole. Such problems may include: the constant return of drilled solids; build up of solids in the annulus; maintaining pressure in the well to inhibit collapse of the borehole; and maintaining a constant weight of mud in the well.

The flow of drilling mud through the drill string traditionally stops and starts when a section of drill pipe is added or removed from the string of drill pipe during tripping and drilling. However, continuous circulation systems while tripping and continuous circulation systems while drilling are becoming more popular. With these systems, flow of drilling mud downhole is more continuous and pressure can be maintained at a constant level more easily.

Mud pumps are used on land rigs and offshore rigs. Mud pumps are generally located on or beside a land rig or on an offshore drilling platform. However, the mud pump may be located on the sea bed or between the seabed and a surface of the sea.

In deep wells or wells in formations which may easily collapse, it is common to line the borehole with casing. The casing is hung from a wellhead. Similarly, liner is used to case a borehole, except that the liner is hung from the bottom of an existing casing. The casing or liner is cemented in place by pumping cement down through the casing and up through an annulus between the casing and the borehole. To and bottom plugs are used to facilitate the cementing operation. A cement pump may be used to flow cement into the well and then flow of drilling mud or water behind a bottom plug may be used to push the bottom plug down and force the cement up through the annulus. The mud pump may be used to pump the drilling mud or water behind the bottom plug.

A cement pump may also be used in the general construction industry in pumping cement for: below ground structures, such as foundations; above ground structures such as car parks, housing, commercial buildings and sky scrappers; and in the marine industry in construction above and below water structures, such as quays and oil platforms.

SUMMARY AND STATEMENTS OF INVENTION

The inventors have observed that a build up in pressure in the cavity is required to crack the outlet valve open. This build-up of pressure then normalises to a stable pressure which the pump induces for the stroke of the reciprocating piston in the piston pumping section of the mud pump. The build-up of pressure and then sudden return in pressure to the normalised level creates a shock, which may induce a vibration in the mud pump. The vibration may cause an unwanted noise; may loosen fastenings in the mud pump; and may induce fatigue failure in parts of the mud pump or surrounding components. Furthermore, the inventors have observed that a more consistent pressure in the outlet flow of drilling mud is beneficial for controlling the combined flow once the outlet flows from several pumping sections are combined in a manifold to produce a consistent pressure in the outlet flow of drilling mud from the manifold.

The inventors have observed that a pump requiring having a smoother flow and more consistent pressure in the outlet would also be useful in a cement pump used in the general construction industry.

In accordance with the present invention, there is provided a pump comprising a housing having a cavity with an inlet valve, an apparatus for raising pressure in a fluid in the cavity and a discharge valve, the discharge valve comprising a valve head movable between an open and closed position and a valve seat characterised in that the pump further comprises a booster piston arranged in a booster cylinder, the booster piston having a first face exposed to a low pressure zone and a second face exposed to fluid pressure in the cavity, at least one of the booster piston and booster cylinder movable relative to the other to add an opening force to the outlet valve. Typically, the pressure in the cavity is in the order of 500 bar and the pressure in the low pressure zone is three bar.

Preferably, the pump further comprising a transfer member arranged between the valve head and the booster piston. Preferably, the member is a rod, which may be solid or hollow and of any suitable cross-section, such as circular, square or hexagonal.

Advantageously, the transfer member has a first end and a second end, the first end fixed to the valve head and the second end free. Alternatively, the transfer member has a first and a second end, the first end fixed to the booster and the second end free to selectively abut the valve head. Alternatively, the member has a first and a second end, the first end fixed to the cylinder and the second end free to selectively abut the valve head.

Preferably, the transfer member passes through the cylinder into the low pressure zone and the second end arranged in the path of movement of the booster in the cylinder.

Advantageously, the booster is free floating in the cylinder. Preferably, the booster is free floating between end stops to limit the range of movement of the booster, advantageously between a lip on the end of a cylinder and advantageously, with a hollowed out booster abutting a cylinder head. Advantageously, the range of movement is limited to between 2 mm and 30 mm and preferably, 5 mm to 10 mm.

Preferably, the pump further comprises an inlet for allowing fluid to flow to the inlet valve, wherein the booster cylinder further comprises an opening therein fluidly connected to the inlet to allow fluid in the inlet to flow into and from the low pressure zone in the booster cylinder. It is preferred to use fluid from the inlet to fill the low pressure cavity in the booster piston assembly, so that if there is any leak of fluid from the booster cylinder into the cavity, there will be no contamination. Furthermore, the pressure of the fluid in the inlet is at a suitable pressure, typically three bar. Alternatively, or additionally, the booster cylinder further comprises an opening therein connected to a low pressure source. The low pressure source may be a fluid reservoir open to atmospheric pressure or a bladder open to atmospheric pressure or an accumulator holding the fluid under a low pressure, such as 3 bar. The fluid is preferably water or may be a hydraulic fluid, such as oil.

Preferably, the pump is a mud pump for circulating drilling mud through a drill string in a wellbore and advantageously, the low pressure zone is filled with drilling mud.

Advantageously, the pump is a cement pump for pumping cement or the like. Preferably, for pumping cement into a wellbore in the construction of an oil or gas well, such as for cementing casing in a wellbore.

Preferably, the apparatus for raising pressure in a fluid in the cavity comprises a reciprocating piston. Advantageously, the apparatus for raising pressure in a fluid in the cavity comprises a motor. Preferably, the motor is an electric motor, which may be an AC motor or a DC motor. Advantageously, the motor is a hydraulic motor. Preferably, the motor is a diesel engine. Preferably, the motor drives a crank shaft to reciprocate a piston rod, with a piston head attached thereto.

Advantageously, the apparatus for raising pressure in a fluid in the cavity comprises a hydraulic pump. Preferably, the hydraulic pump comprises a piston and cylinder for reciprocating the reciprocating piston.

Preferably, the valve head of the outlet valve further comprises a spider. Preferably, the spider is arranged on the cavity side of the valve head. Advantageously, the spider comprises a plurality of legs, which may be two, three, four, five or more. Advantageously, the valve seat comprises a portion which extends into the cavity, the spider arranged in the portion of the valve seat. Advantageously, the portion is cylindrical, preferably of circular cross-section, but may be oval, square, pentagonal, hexagonal or have more sides. Preferably, the spider improves flow of fluid through the outlet valve, preferably normalizing the flow and distributing the flow through the valve. Advantageously, the spider guides the outlet valve, holding the valve head and transfer member in axial alignment.

Advantageously, the outlet valve further comprises a spring for biasing the valve head against the valve seat. Preferably, the spring is one of a: coiled spring; Belleville washer; leaf spring; or any other suitable resilient member. Advantageously, a valve stem extending from the valve head. Preferably, the valve stem is arranged in a valve guide.

Preferably, the inlet valve further comprises a valve head and a valve seat and advantageously, a spring for biasing the valve head against the valve seat. Preferably, the spring is one of a: coiled spring; Belleville washer; leaf spring; or any other suitable resilient member. Advantageously, a valve stem extending from the valve head. Preferably, the valve stem is arranged in a valve guide.

The present invention also provides a valve assembly for use in the pump of the invention, the valve assembly comprising a valve head movable between an open and closed position and a valve seat characterised in that the valve assembly further comprises a booster piston arranged in a booster cylinder, at least one of said booster piston and said booster cylinder movable relative to the other to add an opening force to the valve head.

The present invention also provides a method for facilitating opening a valve comprising a valve head and a valve seat, the valve head movable a predetermined distance from a closed position on the valve seat to a fully open position distant to the valve seat, the method comprising the step of adding a force to the valve head along a small portion of the predetermined distance. Preferably, for opening a valve in a pump and most preferably for opening a discharge valve in a pump for pumping drilling mud through a drill string in a wellbore and a cement pump.

Preferably, the small portion is between 1% and 95% of the predetermined distance. Advantageously, the small portion is between 5% and 33% of the predetermined distance.

DETAILED DESCRIPTION

Referring toFIG. 1there is shown a drilling rig generally identified by reference numeral1. The drilling rig1has a derrick2arranged on a drill floor3supported on legs4. The legs4are seated on a substructure5on ground6.

A swivel7and raised and lowered on a travelling block8on wireline9, which passes over a crown block10located at a top of the derrick2and down to a drawworks11on the rig floor3. A section of drill pipe12depends from the swivel7, passes through a rotary table13and is connected at a lower end to a drill string14. The drill string14passes through a wellhead15into a bore hole16in formation17. A bottom hole assembly18is arranged on a lower end of the drill string14, which has a drill bit19on the lower end thereof. An annulus21is defined between the borehole16and the drill string14.

A flow line20is fluidly connected at one end to the annulus21at the wellhead15and the other end to an active mud system22. Returned drilling mud M flows from the annulus21, through wellhead15, into flow line20and to the active mud system22. The active mud system22comprises an active mud tank23and a series pieces of mud processing equipment, such as: a shale shaker24, a degassers25, a mud conditioner26, a centrifuge27. Further mud processing equipment may be used, such as gumbo separators and hydro cyclones. The flow line20flows returned drilling mud M directly on to screens of the shale shaker24. Screened drilling mud S flows from the shale shaker24into partitioned section27. Each piece of further mud processing equipment is located above a partitioned section27,28and29of the active mud tank23, being fed from the previous partitioned section using a small pump and flowing processed drilling mud into the partitioned section27,28,29therebelow.

The processed drilling mud then passes into additions section30, into which additives are added, such as bentonite, barite and lost circulation material. The processed and tested drilling mud P is tested and when acceptable, is pumped or allowed to flow into suction tank31. A mud pump32is located downstream of the suction tank31. The processed and tested drilling mud P flows from the suction tank31through a supply hose33to the mud pump32. The processed and tested drilling mud P flows into the mud pump32may flow directly from the suction tank31down to the mud pump under pressure provided by the head of drilling mud in the suction tank or may be pumped with a small pump to maintain a relatively constant supply pressure which pressure is typically is between 1 to 5 bar and preferably 3 bar. The processed and tested drilling mud P is then pumped by the mud pump32through a hose34to the swivel7where the hose is fluidly connected to a top of the drill pipe12and flows through the drill string14therefrom.

The mud pump32comprises: a pumping end35comprising an inlet manifold38and an outlet manifold39connected to a number of pumping sections, such as the pumping section shown inFIG. 4; and a power end36comprising at least one motor37driving piston rods, such as piston rod158of the pumping section140shown inFIG. 4.

In a top drive rig (not shown) the rotary table is replaced or supplemented by a top drive. In a top drive rig, the hose34is connected to a goose neck and rotary connection which is fluidly connected with a quill in the top drive which is in turn selectively connected to the drill pipe12and drill string14.

FIG. 2shows a prior art pumping section of a pumping end of a mud pump, generally identified by reference numeral40. The pumping section40has a valve block41with an inlet valve43, a reciprocating piston44and an outlet valve45defining a cavity42. A supply pipe46allows drilling mud P to flow from an inlet manifold38′ to an inlet connection47in the valve block41. The inlet manifold38′ receives drilling mud P from a suction tank, like the suction tank31shown inFIG. 1. A flow channel48in valve block41leads to an inlet face49of a generally circular valve head50of inlet valve43. The valve head50has a valve stem51fixed on a rear face52of inlet valve43. The valve stem51is slideably arranged in a stem guide53. The valve head50has a frusto-conical outer edge54. A spring55biases the valve head away from a ledge56, urging frusto-conical outer edge54against frusto-conical valve seat56of the valve block41to form a substantially fluid tight seal therebetween.

The piston44is slideably arranged in a cylinder sleeve57. A piston rod58is connected at one end to the piston44and at the other to a power end (not shown) of the mud pump. The power end of the mud pump may comprise any suitable means for reciprocating the piston44, such as a hydraulically driven ram or a motor rotating a crank shaft. Any suitable motor may be used, such as a diesel engine, an electric motor or a hydraulic motor.

An outlet valve45comprises a valve head60of generally circular shape in plan view, the valve head60having an outlet face61exposed to the cavity42. The valve head60has rear face63to which a valve stem62is fixed. The valve stem62is slideably arranged in a stem guide64. The valve head60has a frusto-conical outer edge65. A spring66arranged in an outlet passage68biases the valve head60away from a top portion of the valve block41, urging frusto-conical outer edge65against frusto-conical valve seat67of the valve block41. A discharge passage68is provided for conveying flow of drilling mud into a discharge manifold39′.

Any number of pumping sections40are coupled together with the inlet manifold and outlet manifolds (not shown), so that large quantities of drilling mud can be pumped. Any suitable number of pumping sections may be coupled, typically: two, such as with duplex mud pumps; three, such as with triplex mud pumps; or six, as with hex mud pumps; but may be four, five or more. The position of the pistons44along each cylinder sleeve57may be staggered to obtain a smooth flow of drilling mud through the outlet manifold and into the drill string.

In use, the pumping cycle starts with outlet valve45closed and drilling mud P flowing through supply pipe46under low pressure (such as 3 bar) being drawn into the cavity42by action of the piston44retracting, drawing drilling mud P through inlet valve43. Thus very low pressure in the cavity42and the head of drilling mud provided by the level of mud in the suction tank provides sufficient force to open inlet valve43. Once the reciprocating piston44reaches a fully retracted position, the drilling mud P in the cavity42is no longer under a very low pressure. The spring force provided by spring55overcomes the force on the inlet face49of the valve head50provided by pressure in the drilling mud in inlet passage48moving the valve head to abut valve seat56to close the inlet valve43.

The reciprocating piston44is pushed forwards through the cylinder sleeve57by the power end moving the piston rod58, reducing the volume of cavity42and pressurising the drilling mud therein, indicated by the steep slope71of trace line70in the graph inFIG. 3. When the force on outlet face61overcomes the force on the rear face63, the outlet valve45should open. The force on the outlet face61is provided by the pressure of the drilling fluid over the area of the outlet face61. The force on the rear face63is provided by pressure in drilling mud in the discharge passage68over the area of the rear face63, from spring force supplied by spring66and a minimal effect of the weight of the valve head60may be taken into consideration. However, the inventors have observed that a large pressure differential across the outlet valve45is needed to crack the outlet valve45open, as shown in portion72of trace line70in the graph ofFIG. 3. A pressure P-Crack is noted before the outlet valve45opens, typically 675 bar. Once the outlet valve45is open, the pressure in the drilling mud normalises, indicated by the horizontal portion73of the trace line70at a pressure P1in the graph. P1is the drilling mud pressure required in the outlet flow, which is typically 500 bar, (although may be anywhere between 100 and 2000 bar). Thus P-Crack is typically 20 to 30% higher than the pressure P1. When the force on the rear face63of valve head60overcomes the force on the outlet face61and frusto-conical edge65provided by the pressure in the drilling mud in the cavity42, the outlet valve45will close. This will typically occur when the piston44reaches the end of its stroke and starts to retract, as the inlet valve43begins to open, repeating the pumping cycle.

The inventors noted that the pressure spike P-Crack is seen in both the cavity42and the outlet flow, which can induce unwanted vibrations in the mud pump and associated components. The inventors have noted it is beneficial to obtain a constant pressure P1in the cavity42and the outlet passage68.

Referring toFIG. 4, there is shown a pumping section of the present invention, with similar parts referred to with reference numerals in the one hundred series.

The pumping section140has a valve block141with an inlet valve143, a reciprocating piston144and an outlet valve145defining a cavity142. A supply pipe146allows drilling mud P to flow from the manifold38to an inlet connection147in the valve block141. A flow channel148leads to an inlet face149of a generally circular valve head150of inlet valve43. The valve head150has a valve stem151fixed on a rear face152. The valve stem151is slideably arranged in a stem guide153. The valve head150has a frusto-conical outer edge154. A spring155biases the valve head150away from a ledge156, urging frusto-conical outer edge154against frusto-conical valve seat156of the valve block141.

The piston144is slideably arranged in a cylinder sleeve157. A piston rod158is connected at one end to the piston144and at the other to a power end (not shown) of the mud pump. The power end may be any suitable power end, such as any discussed herein.

An outlet valve145comprises a generally circular valve head160having outlet face161exposed to the cavity42. The valve head160has a valve stem162fixed on a rear face163exposed to the outlet flow. The valve stem162is slideably arranged in a stein guide164. The valve head160has a frusto-conical outer edge165. A spring166arranged in an outlet passage168biases the valve head160away from a top portion of the valve block141, urging frusto-conical outer edge165against frusto-conical valve seat167of the valve block141. The valve head160moves approximately 30 mm between a closed position and a fully open position. A transfer rod169is located substantially concentrically with the valve head160fixed and projecting from the outlet face161. The transfer rod169is preferably 30 mm in diameter, although may typically be 10 mm to 75 mm in diameter and extends preferably 75 mm from the outlet face161into the cavity142. The transfer rod169is rigid and able to withstand compressive forces so that a force can be applied thereto to push on the valve head160to facilitate opening.

The outlet valve145is provided with a booster piston assembly generally identified by reference numeral81for facilitating the application of an additional force to the valve head160to facilitate opening and then preferably, removing the additional force.

The booster piston assembly181comprises a booster piston182slideably arranged in a cylinder183. The cylinder183is fixed in the cavity142in a retainer184beneath and concentrically in line with outlet valve head145. The retainer184comprises three arms (only one shown185), spaced at 120 degrees from each other about the cylinder183fixed at one end to the cylinder183and at the other to cavity wall186of the valve block141. Three spaces between the three arms185allow free flow of drilling mud P through the cavity142.

The transfer rod169passes through a seal187in an opening in a cylinder head188of the cylinder183. The transfer rod169extends into the cylinder approximately 75 mm. A low pressure zone189is defined by the cylinder head188, cylinder wall190, an upper face191of the piston182. The low pressure zone189is in fluid communication with drilling mud inlet flow channel148via a fluid line192, so that drilling mud P can flow freely therebetween. The fluid line192is of a small diameter, such as 5 mm, but may be any suitable size such as 3 mm to 20 mm. The fluid line192may comprise a channel bored through the valve block141or any suitable pipe or hose.

The piston182has a seal193arranged between the upper face191and lower face194, which isolates the cavity142from the low pressure zone189and allows the piston to slide along the cylinder183smoothly. The piston182is retained in cylinder183by a lip195. The piston182has a range of movement of approximately 2 to 10 mm and may be less than the opening distance of the outlet valve145. Preferably, no more than 30 percent of the opening distance of the outlet valve145. Advantageously, the range of movement is between 2 to 5 mm.

An opening196in the cylinder is arranged out side of the range of movement of the piston182, which is fluidly connected to the fluid line192.

In use, the pumping cycle starts with outlet valve145closed and drilling mud P flowing through supply pipe146under low pressure (such as 3 bar) being drawn into the cavity142by action of the piston144retracting, drawing drilling mud P through inlet valve143. Thus very low pressure in the cavity142and the head of drilling mud provided by the level of mud in the suction tank31provides sufficient force to open inlet valve1143. Once the reciprocating piston144reaches a fully retracted position, the drilling mud P in the cavity142is no longer under a very low pressure. The spring force provided by spring155overcomes the force on the inlet face149of the valve head150provided by pressure in the drilling mud in inlet passage148moving the valve head150to abut valve seat156to close the inlet valve143.

The reciprocating piston144is pushed forwards through the cylinder sleeve157by the power end moving the piston rod158, reducing the volume of cavity142and pressurising the drilling mud therein, indicated by the steep slope171of trace line170in the graph inFIG. 5. When the force on outlet face161overcomes the force on the rear face163, the outlet valve145should open. The force on the outlet face61is provided by the pressure of the drilling fluid over the area of the outlet face161and an additional force is applied to the outlet face161by movement of the transfer rod169. The large pressure, such as 500 bar (but may be 100 to 1000 bar) in the drilling mud in cavity142acts on second face194of the booster piston182, which is greater than the force provided by the lower pressure zone (typically 3 bar) acting across the first face191of the piston182. The resultant additive force moves the piston182towards an end of the rod169, expelling low pressure fluid from the low pressure zone189through fluid line192. The piston182meets the transfer rod and pushes on the transfer rod169, applying this additional force to the outlet face161. The piston182moves forward approximately 4 to 10 mm, pushing the valve head160open approximately 2 to 8 mm, whereupon the transfer rod169leaves the piston182and opens to a fully open position after approximately 30 mm of travel. Thus no additional force is applied to the valve head160during the final part of the valve head's travel. The valve head160begins to unseat itself when the total force on the outlet face is greater than the force on the rear face163. The force on the rear face163is provided by pressure in drilling mud in discharge passage168over the area of the rear face163and from spring force supplied by spring166. The additional force applied by the booster piston assembly181provides the additional force required to crack the outlet valve open without a large spike in pressure in the drilling mud, as shown by the smooth transition172from pressure build up in cavity142to the discharge pressure P1shown by horizontal line173in the graph inFIG. 5. Thus the pressure in the cavity142is substantially the same as the pressure in the drilling mud in then discharge passage168during opening of the discharge valve145.

Once the outlet valve145is open, the pressure in the drilling mud normalises, indicated by the horizontal portion173of the trace line170at a pressure P1in the graph. P1is the drilling mud pressure required in the outlet flow, which is typically 500 bar, (although may be anywhere between 100 and 2000 bar). When the force on the rear face163of valve head160overcomes the force on the outlet face161and frusto-conical edge165provided by the pressure in the drilling mud in the cavity142, the outlet valve145will close. This will typically occur when the piston144reaches the end of its stroke and starts to retract, as the inlet valve143begins to open. The booster piston182will fall back on to lip195, sucking drilling mud in from the inlet148into the low pressure zone189. The pumping cycle then repeats.

A part of a booster piston assembly in accordance with the invention is shown inFIGS. 6, 7 and 8, which is generally similar to the pumping section ofFIG. 4, with similar parts referred to with reference numerals in the two hundred series.

A free floating booster piston282is slideably arranged in a booster cylinder283. The booster piston282has an upper face291exposed to a low pressure zone289and a lower face294exposed to drill mud pressure in cavity242. The booster cylinder283comprises a lip295at a lower end to retain the booster piston282in the booster cylinder283. The booster cylinder282comprises a main body283′ with three arms285,285′ and285″ extending therefrom to a valve body241to centralize the main body283′ in the cavity242. The arms285,285′ and285″ are spaced about the body at approximately 120 degrees and define flow paths242′242″ and242′″ through which drill mud can flow. Three raised lugs297,297′ and297″ are spaced about the main body283′ to rigidly fix the booster piston assembly to the valve block241.

Referring toFIG. 9, an outlet valve245comprises a generally circular valve head260having outlet face261exposed to the cavity242. The valve head260has a valve stem262fixed on a rear face263exposed to the outlet flow. The rear face263comprises a planar ring portion263′ and a concave portion263″. The valve stem262is slideably arranged in a stem guide (not shown). The valve head260has a frusto-conical outer edge265, with a frusto-conical seal member265′, which may be an elastomeric, machined metal or nylon material. A spring266arranged in an outlet passage268biases the valve head260away from a top portion of the valve block (not shown), urging frusto-conical seal member265′ against frusto-conical valve seat267of the valve insert241′ which forms part of or fitted in valve block241. The valve head260moves approximately 30 mm between a closed position and a fully open position. A transfer rod269is located substantially concentrically with the valve head160fixed and projecting from the outlet face261. The transfer rod269is preferably 15 mm in diameter, although may typically be 10 mm to 75 mm in diameter and extends preferably 75 mm from the outlet face261into the cavity242. The transfer rod269is rigid and able to withstand compressive forces so that a force can be applied thereto to push on the valve head260to facilitate opening. The transfer rod269passes through a seal nipple287on cylinder head288. The seal nipple comprises a cap287′ surrounding the transfer rod269. The seal nipple287also comprises a number of seal elements287″ and guide rings to provide a drilling fluid tight seal about the transfer rod269, between the cavity242and the low pressure zone289during a pressure differential of typically 500 bar and may be between 100 bar and 1000 bar.

A spider298is rigidly fixed or integral with the outlet valve head260and depends therefrom into cavity242. The spider298is slideable with the outlet valve head260in a cylinder241″ formed in a lower part of valve insert241′. The spider comprises four legs298′,298″ and298′″ spaced at 90 degrees about the transfer rod269. The spider298helps hold the outlet valve245and the transfer rod269centralized during opening and closing. Furthermore, the spider facilitates obtaining a better of flow of drilling mud through the outlet valve245.

The low pressure zone289is defined by the cylinder head288, cylinder wall290, upper face291of the piston282. The low pressure zone289is in fluid communication with drilling mud inlet flow channel (not shown, but generally similar to flow channel148) via a fluid line292, so that drilling mud P can flow freely therebetween at a pressure of approximately three bar.

The piston282has a seal293arranged between the upper face291and lower face294, which isolates the cavity242from the low pressure zone289and allows the piston to slide along the cylinder283smoothly. The piston282has a range of movement of approximately 2 to 10 mm and may be less than the opening distance of the outlet valve245. Preferably, no more than 30 percent of the opening distance of the outlet valve245.

In use, the free-floating booster piston282initially sits on lip295of the cylinder283, as shown inFIG. 10. As pressure in drilling mud in cavity242builds up, the pressure differential across the booster piston282moves the booster piston282upwards to contact a foot of the transfer rod269, as shown inFIG. 10. As the pressure in the drilling fluid in cavity242reaches the pressure in the drilling fluid in outlet passage268, an additional force provided by the booster piston282on transfer rod269facilitates cracking the outlet valve head260from the valve seat267, as shown inFIG. 11. The booster piston282may only push the transfer rod2692 mm in order to accomplish this. The pressure differential on the outlet valve head260and the flow of drilling mud therethrough pushes the outlet valve head260open further, whereupon the foot of the transfer rod269parts from the upper face291of the booster piston282. The outlet valve head260fully opens, which is usually in the order of 30 mm, as shown inFIG. 12.

The opening296allowing fluid communication between the low pressure zone289and the flow channel (such as148) through fluid line292, is covered by the piston282, although the seals293are located in a lower portion of the booster piston282, so that the seals293are prevented from being damaged by the opening296.

During the part of the mud pump's cycle in which drilling fluid is sucked into the cavity242, the pressure in the cavity242reduces to below the pressure of the fluid pressure in the low pressure zone289. The free-floating booster piston282seats itself on the lip295. The outlet valve head260returns to a closed position with seal265′ seated on valve seat267.

Referring toFIG. 13, there is shown a part of a pumping section of the present invention, which is generally similar to the pumping section ofFIG. 4, with similar parts referred to with reference numerals in the one hundred series.

There is shown an outlet valve345and booster piston assembly380. The booster piston assembly380is generally similar to the booster piston assembly180, save for the booster piston382, which has a transfer rod369fixed thereto which has a free upper foot399which selectively engages lower face361of outlet valve head360. A fluid line392is provided with water therein in fluid communication with low pressure zone389. The water is supplied from a constant pressure source399a, which may be an accumulator primed to a pressure of preferably 3 bars.

Other alterations and modifications are envisaged, such as using a stationary piston and a moving cylinder for pushing on the valve head.