Centrifuge for separating solids from solids laden drilling fluid

A flow apparatus for a separation apparatus for separating solids from a solids laden drilling fluid includes a feed conduit for feeding solids laden drilling fluid to the separation apparatus, a liquid discharge conduit for conveying a liquid phase discharge from the separation apparatus, and a solids discharge port for allowing discharge of solids phase from the separation apparatus. Also included is a parameter measuring apparatus connected to the feed conduit and the liquid discharge conduit for measuring at least one parameter of at least one of the solids laden drilling fluid and the liquid phase discharge.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of GB Patent Application No. 1309652.4, filed on May 30, 2013, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The invention relates to a centrifuge and a method for operating a centrifuge for separating solids from solids laden drilling mud.

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 commercial solids i.e. any purposely added solids, such as 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. It is not uncommon to have 30 to 100 m3of drilling fluid in circulation in a borehole. The drilling mud contains inter alia expensive synthetic 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 drilling mud. The first part of the process is to separate large solids and lost circulation material from the solids laden drilling mud. This is at least partly achieved with one or more a vibratory separators, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. The shale shakers may be cascaded in series of stages, such as three stages: a scalping deck having a large mesh screen suitable for removing colloidal material such as clumps of clay; a primary deck having fine mesh screen for removing large particles (but smaller than the colloidal material) which may include lost circulation material; and a secondary deck having a fine screen to remove small particles, mainly drill cuttings. The decks may be arranged in a single basket or in separate baskets and vibrated with a vibratory mechanism.

Further processing equipment such as a centrifuge may be used to further clean the drilling mud of smaller solids. The centrifuge may be used to remove large and medium size solids, although it is particularly suitable for removing small, heavy particles such as “barites” and thickening agents commonly referred to as “bentonites”. These particles are generally too small for a screen in a shale shaker to remove. The resultant drilling mud is returned to the active mud system of the drilling rig.

A mud engineer will analyze the resultant drilling mud and inter alia: dilute the drilling mud if it is too viscous; add more bentonites if the drilling mud is not viscous enough; and add more barites if the drilling mud is not dense enough for recirculation.

It should also be note that a centrifuge may be used without or ahead of the shale shakers or directly after only one or two stage of screening. Furthermore, the centrifuge may be used to clean drilling mud or other fluids on a rig which are not being continuously circulated in the well.

Centrifuges are typically used in any one of three modes of operation:

1. low gravity solids (LGS) removal, in water based mud (WBM) while meeting environmental discharge criteria, and in oil based mud (OBM/NAF) while meeting environmental discharge criteria

2. barite separation, which sometimes requires two centrifuges; and

3. dewatering, simply discharging as many solids as possible.

Typically, Low Speed Decanting is used for Barite removal. Separating factor 500-700, 4-7 micro-meter particle size. Barite is a dense mineral comprising barium sulfate [BaSO4]. Commonly used as a weighting agent for all types of drilling fluids, barites are mined in many areas worldwide and shipped as ore to grinding plants in strategic locations, where API specifies grinding to a particle size of 3 to 74 microns. Pure barium sulfate has a specific gravity of 4.50 g/cm, but drilling-grade barite is expected to have a specific gravity of at least 4.20 g/cm3to meet API specifications. Contaminants in barite, such as cement, siderite, pyrrhotite, gypsum and anhydrite, can cause problems in certain mud systems and should be evaluated in any quality assurance program for drilling-mud additives.

Typically, Medium Speed Decanting is used with a separating factor 800 for 5-7 micrometer separation.

SUMMARY

High Speed Decanter Separating factor 1200-2100 rpm for 2-5 micrometer separation.

The present invention may be used in any of the three modes of operation or for any other form of separation stage.

The inventors have noted that use of the centrifuge is not optimized. The centrifuge is adjusted manually to achieve desired results, which produces inconsistent results. The inventors noted that the centrifuge may be operated to produce drilling mud which does not need to be adjusted or only minimally for re-circulation. The inventors also observed that the price of clean drilling mud, bentonite and barites and the cost of processing used drilling mud vary, making it economically desirable to use the centrifuge in different ways according to these costs. Thus optimum performance of the centrifuge may vary according to these costs.

In accordance with the present invention, there is provided a flow apparatus for a separation apparatus for separating solids from a solids laden drilling fluid, the flow system comprising a feed conduit for feeding solids laden drilling fluid to the separation apparatus, a liquid discharge conduit for conveying discharged liquid phase from said separation apparatus and a solids discharge port for allowing discharge of solids phase from said separation apparatus, characterized in that said flow system comprises a parameter measuring apparatus connected to said feed conduit and said liquid discharge conduit.

Preferably, the flow apparatus comprises at least one valve to selectively allow solids laden drilling fluid or discharged liquid phase through the parameter measuring apparatus and advantageously, a plurality of valves, which may be controlled from a control system, such as a computer and be actuated using an electric, hydraulic or pneumatic means, such as an electric stepper motor.

Advantageously, the at least one valve is a two-way valve and preferably a plurality of two-way valves, such as six. Alternatively, or additionally, the at least one valve comprises a three-way valve, preferably a plurality such as three three-way valves.

Preferably, the flow apparatus comprises a feed tank. The feed tank receives solids laden drilling mud directly from the well or preferably from other solids control apparatus such as a shale shaker, degasser, hydrocyclone, mud cleaner or a further centrifuge. Preferably, the feed tank forms part of a tank system, wherein separate tanks are arranged between each of or some of the other solids control equipment.

Preferably, the parameter measuring apparatus comprises a coriolis meter for measuring mass flow or other apparatus for measuring mass flow. Advantageously, the parameter measuring apparatus may also or alternatively comprise: a temperature sensor for measuring temperature; spaced apart pressure sensors for use in measuring a pressure differential and calculating density therefrom or other density measuring apparatus; a velocity sensor; and viscosity sensor.

Preferably, a control system is provided for receiving data from said parameter measuring apparatus and advantageously controlling its operation.

Advantageously, the separation apparatus is a centrifuge. The centrifuge preferably having a bowl for retaining a pond of solids laden drilling mud and a conveyor, an inlet for solids laden drilling mud to be introduced to the bowl, a solids discharge outlet and a drilling mud discharge outlet, a bowl drive for driving the bowl and a conveyor drive for driving the conveyor.

The present invention also provides a method of flowing fluid to a separation apparatus, the method comprising the steps of flowing a solids laden drilling fluid through a feed conduit into a separation apparatus, conveying discharged liquid phase from said separation apparatus through a liquid discharge conduit and discharging a solids phase from said separation apparatus through a solids discharge port, characterized in that a parameter measuring apparatus is connected to said feed conduit and said liquid discharge conduit, such that the parameter measuring apparatus selectively takes a parameter reading of the solids laden drilling fluid and the liquid discharge phase.

Preferably, parameter reading of the solids laden drilling fluid and the liquid discharge phase are logged. Advantageously, the parameter reading of the solids laden drilling fluid is logged once every two to sixty minutes, preferably every five to thirty minutes and advantageously every ten to twenty minutes.

Preferably, the parameter reading of the liquid discharge phase is logged once every ten to sixty minutes, preferably every five to thirty minutes and advantageously every ten to twenty minutes.

Advantageously the method further comprises the step of activating at least one valve to select flow of solids laden drilling fluid or the liquid discharge phase through the parameter measuring apparatus.

Thus, the invention describes techniques for using automated two way control valves or three way directional control valves and piping arrangements to facilitate the use of a single mass flow meter to measure the mass flow of the process slurry sent to a centrifuge and the mass flow of the liquid discharge from the centrifuge. The two measurements cannot be taken simultaneously. The valves can be connected to actuators which can be operated by either hydraulic, pneumatic or electric means. The actuators are connected to a control system that positions the valves to direct either the process slurry or the liquid discharge from the centrifuge through the mass flow meter. The valves can have an output signal which will tell the control system the current position of the valve.

DETAILED DESCRIPTION

FIG. 1shows a centrifuge generally identified by the reference numeral10. The centrifuge10has a bowl12, supported for rotation about its longitudinal axis. The bowl12is in the form of a hollow solid walled cylinder of circular cross-section preferably having a first bowl portion12′ having an internal diameter which reduces in a tapering fashion towards a distal end12aand a second bowl portion12″ having a substantially constant internal diameter from the first bowl portion12′ to a proximal end12d. The bowl12has an opening at each of the distal and proximal ends12aand12d, the distal end12ahaving a drive flange14fitted into the opening which is connected to a drive shaft21for rotating the bowl12. The drive flange14has a longitudinal passage which receives a feed tube16for introducing a feed slurry, such as solids laden drilling mud, into the interior of the bowl12.

A hollow flanged shaft19is disposed in the opening in the proximal end12dof the bowl12and preferably fixed thereto with a plurality of bolts8(shown inFIG. 4). The hollow flanged shaft19receives a drive shaft20of an external planetary gear box32for rotating a screw conveyor18in the same direction as the bowl12at a selected speed, which may be at a different speed from the bowl12.

The screw conveyor18is arranged within the bowl12in a coaxial relationship thereto and is supported for rotation within the bowl12between a distal hollow stub axel14aof the drive flange14and a distal hollow stub extending from flanged shaft20. The screw conveyor18has a hollow solid walled cylindrical body17of circular cross-section, a first part17aof which has a tapering external diameter and a second part17bwhich has a constant diameter. The screw conveyor18has a flight13of substantially constant pitch, although as an alternative may be a variable pitch (not shown). As an alternative, a further flight may be provided for a double-start flight (not shown). Annular depth of the flight13is preferably substantially constant along second part17bof the screw cylindrical body17, advantageously reducing in a tapering fashion in a linear taper or alternatively, non-linear taper (not shown), towards and along first part17a. Openings (not shown) are formed in the cylindrical body17of the screw conveyor18in the region identified by reference numeral15.

The solids laden drilling fluid flows from an inlet18aof the feed tube16so that the centrifugal forces generated by the rotating bowl12move the slurry radially outwardly through the openings in region15in the solid walled cylindrical body17and into an annular space13′ between the solid walled cylindrical body17and the bowl12. The liquid portion of the slurry forms a pond11and is displaced to the proximal end12dof the bowl12. Entrained solid particles11ain the slurry settle towards the inner surface12′″ of the bowl12due to the G forces generated, and are scraped and displaced by the screw conveyor18back towards the distal end12aof the bowl12for discharge through a solids discharge outlet(s) which may be a plurality of solids discharge ports12cformed through the wall of the bowl12near its end12a. The solids discharge ports12care arranged closer to the centerline of the bowl than the pond depth, thus only the solids are displaced through the discharge ports12cwith very little or no drilling mud. A cowling10a(shown inFIG. 2) is provided about the bowl12to collect and direct the solids to a discharge pipe120into a discharge system, such as a skip, trough or solids conveying apparatus.

A liquid discharge outlet is provided in the bowl12, which liquid discharge outlet may comprise liquid discharge ports19′ provided in flange19″ of hollow flanged shaft19. The flange19″ is bolted to the bowl12with bolts8(shown inFIG. 4). The liquid discharge ports19′, preferably five ports, but may be any suitable number such as one to twenty, are spaced in a concentric circle about the flange19″, each spaced from the inner surface12′″ of the bowl12at an equal distance. Thus the liquid discharge ports19′ act as weirs, controlling the depth of the pond11.

The liquid discharge ports19′ are shown in more detail inFIG. 4and an alternative embodiment inFIG. 5. The liquid discharge ports19′ each comprise a circular hole20′ in the flange19″, although the hole20′ may be of any shape, such as a polygon, oval and may take the form of a slot. The hole20′ is approximately five to ten centimeters in diameter. A disc gate20ais pinned to the flange19″ with a pin20b. The pin20bis rigidly fixed to the flange and may be welded thereto. The pin20bmay be placed at or close to a point on the same radius as the center of the hole20′, the radius taken from the center of the flanged shaft19. The disc gate20ais movable about the pin20b. A toothed cog27engages with a splined opening25in the disc gate20aabout the pin20b. A drive shaft23of a control motor29is rotationally fixed to the toothed cog27. The disc gate20ahas a circular opening20ctherein, although the opening20cmay be of any shape such as a polygon or oval and may take the form of a slot. Upon activation of the control motor29, the drive shaft23rotates rotating the disc gate20aabout the fixed pin20b. The disc gate20ais movable about the fixed pin20bto vary the effective weir height. The control motor29moves disc gate20ain small increments or linearly in response to commands from a control system PM.

The disc gate20amay alternatively be solid, the outer perimeter of the disc gate20ais used for the weir and thus controlling pond depth.

Another alternative automatic weir is shown inFIG. 6. The disc gate20atakes the form of a radially slideable gate20a′ arranged in a track25′. The gate20a′ is slideable over the hole20′ with a linear actuator motor29′. Activation of the linear motor29′ thus controls the position of the gate20a′ over the hole20′ by extending and retracting an arm23′ fixed to the gate20a′. An end31of the gate20a′ acts as the weir and its position and thus weir height are controlled by the linear actuator motor29′. The linear actuator motor29′ is controlled by control system PM.

The centrifuge as shown inFIGS. 1 and 2is enclosed in a cowling10ain a conventional manner to collect and divert the flow of separated liquid into a fluid discharge pipe105and to collect and divert the solids into a solids discharge pipe120.

As shown inFIG. 2, a drive shaft21forms an extension of, or is connected to, the drive flange14and is supported by a bearing22. A variable speed AC main bowl drive motor24has an output shaft24awhich is connected to the drive shaft21by a drive belt26and therefore rotates the bowl12of the centrifuge at a predetermined operational speed. The flanged shaft19extends from the interior of the conveyor18to a planetary gear box32and is supported by a bearing33. A variable speed AC back conveyor drive motor34has an output shaft34awhich is connected to a sun wheel35by a drive belt36and the sun wheel is connected to the input of the gear box32. The conveyor drive motor34rotates the screw conveyor18of the centrifuge through the planetary gear box32which functions to establish a differential speed of the conveyor18with respect to the bowl12. A coupling38is provided on the shaft of the sun wheel35, and a limit switch38ais connected to the coupling which functions in a conventional manner to shut off the centrifuge when excessive torque is applied to the gearbox32.

For receiving and containing the feed slurry being processed, there is a tank40and a conduit42connected to an outlet opening formed in the lower portion of the tank to the feed tube16. An internal passage through the shaft21receives the conduit42and enables the feed slurry to pass through the conduit and the feed tube16and into the conveyor18.

The tank40may form part of a mud tank system (not shown) and comprises a series of tanks. A first tank is fed with underflow of screened solids laden drilling mud from a shale shaker. The first tank comprises a sand trap, such that sand settles therein on a pan. The sand is tapped off after sufficient build up. The screened solids laden drilling mud is then pumped from the first tank through a degasser to remove at least a portion of any gas which may be present in the screened solids laden drilling mud and flows into a second tank. The screened and degassed solids laden drilling mud is pumped from the second tank through a hydrocyclone to further remove sand particles. The screened, degassed and hydrocycloned solids laden drilling mud flows into a tank, such as tank40for further processing with the centrifuge10. The first tank may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The tank40may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank may comprise an impeller to agitate to inhibit solids from settling. The inventors observed that the impellers in the second tank and the tank40tend to mix incoming flow with the solids laden drilling mud already in the respective second tank and tank40.

The slurry is pumped from the tank40by a powered pump44which is connected to the conduit42and is preferably driven by a motor and most preferably having driven by a variable frequency drive unit46, which pumps the slurry through the conduit42and the feed tube16, and into the centrifuge10. Optionally, a control valve52disposed in the conduit50controls flow through the conduit. Two variable frequency (“VFD”) drives54and56are respectively connected to the motors24and34for driving the motors at variable frequencies and at variable voltages. The drive unit46may also be a variable frequency drive. Preferably, the VFDs46,54and56are connected to and controlled by the control system PM.

Optionally, the VFD54is also electrically connected to the input of a magnetic starter58, the output of which is connected to the drive unit46. The VFD54supplies a control signal to the starter58for starting and stopping the drive unit46, and therefore the feed pump44.

The control system PM may be a computer provided which contains computer programs stored on a computer readable media, such as a ROM, RAM, in the computer itself containing instructions for controlling the operation of the drilling mud system: the centrifuge10and preferably the feed pump44. To this end, the control system PM has several input terminals two of which are respectively connected to the VFDs54and56for receiving data from the VFDs, and two output terminals for respectively sending control signals to the VFDs. The control system PM thus responds to the input signals received and controls the VFDs54and56in a manner so that the drive units can continuously control the system and vary the frequency and the voltage applied to the respective AC motors24and34, to continuously vary the rotation and the torque applied to the drive shaft21and to the sun wheel35, respectively.

The control system may be a Programmable Logic Controller having readable media containing instructions for controlling the operation. Alternatively, the control system may be a single board computer. Alternatively, the control system PM may comprise a dumb terminal DT having access, wired or wireless to an intranet I via a network connection NE or internet on which the instructions for controlling the operation are stored and/or executed. The sensors of the type set out herein may transmit their data through a wire back to bus connection of the dumb terminal DT from which the data is collected and passed through a network connection or wirelessly to the internet. Thus, the instructions for controlling the operation may be in a cloud PM′. An HMI apparatus (human-machine interface, e.g. the touch screen system)54dprovides a visual display of the system operation and a tactile means of control54dfor the control system. The HMI apparatus54dis shown inFIG. 3as part of or attached to a laptop computer CP, but may be arranged as part of dumb terminal DT on a skid10bof the centrifuge and/or in a control room of a drilling rig and/or a remote control room RS distant from said drilling rig. The instructions for controlling the operation may alternatively be in a computer readable media PM″ in the laptop CP. The centrifuge10could also be remotely monitored. This could be done by apps on a portable device such as a smart phone with different user profiles for technicians, customers, etc.

The control system PM has another input terminal connected to the drive unit46with a motor46afor receiving data from the drive unit46. Another output terminal of the control system PM is connected to the drive unit46for sending control signals to the drive unit46. The control system PM thus responds to the input signals received from at least one the VFDs54and56and can send corresponding signals to the drive unit46for varying the operation of the feed pump44. Another input terminal of the control system PM is connected to the limit switch38awhich provides a signal in response to excessive torque being applied to the gear box32.

Mounted on the outer surface of the bowl12is a vibration sensor62, such as an accelerometer which is connected to the control system PM, and responds to excessive vibrations of the centrifuge for generating an output signal that causes the control system to send signals to the VFDs54and56to turn off the motors24and34, respectively, and therefore shut down or slow down the centrifuge.

Near the bearings22and33are connected a pair of accelerometer sets64a,64b,64cand64deach set advantageously including two accelerometers for respectively measuring certain operational characteristics, particularly, but not exclusively, at high frequencies of the drive shafts and20and their associated bearings, gearbox32and equipment skid10bon which the centrifuge10sits. The accelerometer sets64a,64b,64cand64dare connected to the control system PM for passing their respective output signals to the control system PM for processing. The accelerometer sets64a,64b,64cand64dcan be of the type disclosed in U.S. Pat. No. 4,626,754, the disclosure of which is hereby incorporated by reference.

Each accelerometer set preferably includes two or more accelerometers having orthogonal axes that are placed on the frames of the bearings22and33for detecting vibrations caused by the rotating bowl12and screw conveyor18, as well as the drive shaft21and the sun wheel35. The accelerometer signals provided by the accelerometers of each set64a,64b,64cand64dare passed to the control system PM where a computer program contained therein analyzes the signals for the presence of specific predetermined frequency signatures corresponding to particular components and their status, which could include a potentially malfunctioning condition. The computer program is designed to provide instructions to produce an output in response to any of these frequency signatures being detected. The accelerometer signals are analyzed by the control system PM and upon using the evolutionary operation parameter change method as set out below, if the accelerometer signals pass a predetermined threshold, are regarded as constraints on the system and the control system may regard any change in the parameter as a performance not improved status. The back current to the drive units24and34, are proportional to the loading of the bowl12and the conveyor, respectively, the values of which is fed back to the control system PM.

The control system PM has conventional devices including, but not limited to, programmable media, computer(s), processor(s), memory, mass storage device(s), video display(s), input device(s), audible signal(s), and/or programmable logic controller(s), access to storage on the internet and cloud, such that any computer program used by the control system PM may be stored and/or run on in the cloud. Optionally, e.g. in field applications, a generator is provided which generates electrical power and passes it to a breaker box which distributes the power to the VFDs54,56, and46. Optionally, the VFD54(and any VFD of the system10and any VFD disclosed herein) can have a manual potentiometer apparatus for manually controlling a motor; a display or window in a display for displaying inter alia torque; a display or window in a display54cfor displaying rpm/speed display apparatus; and/or an HMI apparatus (human-machine interface, e.g. the touch screen system)54dwhich provides a visual display of the system operation and a tactile means of control.

In use, the storage tank40receives the slurry, (which, in one particular aspect, is a mixture of drilling fluid and drilled cuttings). The control system60sends an appropriate signal, via the VFD54, to the starter58which functions to start the VFD46and activate the pump44. The slurry is pumped through the conduit42and into the interior of the bowl12under the control of the control system60. The bowl drive motor24is activated and controlled by the VFD54to rotate the drive shaft21, and therefore the bowl12, at a predetermined speed. The conveyor drive motor34is also activated and driven by the VFD56to rotate the sun wheel35, and therefore the screw conveyor18, through the planetary gear box32, in the same direction as the bowl12and at a different speed. As a result of the rotation of the bowl12, the centrifugal force thus produced forces the slurry radially outwardly so that it passes through the inlet18ain the conveyor and into the annular space between the conveyor and the bowl12. The drilling fluid portion of the slurry is displaced to the end12bof the bowl12for discharge from the weirs19ain the flanged shaft19. The entrained solid particles (drilled cuttings) in the slurry settle towards the inner surface of the bowl12due to the G forces generated, and are scraped and displaced by the screw conveyor18back towards the end12aof the bowl for discharge through the discharge ports12c.

The control system PM receives signals from the VFD46or flow meter113corresponding to the pumping rate of the feed pump44, and signals from the VFDs54and56corresponding to torque and speed of the motors24and34, respectively. The control system PM contains instructions which enable it to process the above data and control the VFDs. The control system PM controls the VFDs54and56to vary the frequency and voltage applied to the motors24and34, as needed to control and/or continuously vary the rotational speed of, and the torque applied to, the drive shaft21and the sun wheel35, to maintain predetermined optimum operating conditions. The control system PM also monitors the torque applied to the sun wheel35from data received from the VFD56and maintains the torque at a desired level. In the event one of the inputs to the control system PM changes, the system contains instructions to enable it to change one or more of its output signals to the VFDs54and56and/or the VFD46, to change their operation accordingly. The accelerometer sets64a,64b,64cand64drespond to changes in rotational speed of the drive shaft21and the sun wheel35, and therefore the bowl12and the conveyor18, in terms of frequency, as well as changes in the drive current to the motors24and34in terms of amplitude which corresponds to load. In the event the centrifuge becomes jammed for whatever reason the control system PM will receive corresponding input signals from the VFDs54and/or56and will send a signal to the starter58to turn off the feed pump44and thus cease the flow of the feed slurry to the centrifuge.

The control system PM of the invention attempts to optimize performance of the centrifuge. The optimization requires an “optimal” operation. The definition of “optimal” operation is programmed into the control system and is preferably either of the following algorithms:

1. Maximize the economic benefit of the centrifuge by maximizing the equation: E=D−B−F−b, where E=the net economic benefit ($) from operating the centrifuge D=the cost of the base fluid that would otherwise have to be used to dilute the used drilling mud if the centrifuge wasn't operating to remove the fine drilled solids. The % drilled solids in the mud should be kept below a certain threshold or problems such as slow drilling and stuck pipe can occur.

B=cost of the barite that is lost via the centrifuge solids discharge

F=cost of the base fluid that is lost via the centrifuge solids discharge

b=cost of the bentonite gel (thickening agent) that is lost via the centrifuge solids discharge

2. Minimize the operational cost of the mud system based on the sum of the following costs

a. mud dilution (new mud that has to be added to the working volume to reduce the percentage of solids in the mud)

b. chemical additives that must be replaced because the centrifuge discarded some of them (barite, chlorides, well bore stability materials, lost circulation material (LCM), etc.)

3. ignore the loss of chemical additives and just minimize the mud dilution costs by maximizing the LGS removal rate while still meeting environmental discharge requirements

4. achieve either of the two above objectives while minimizing maintenance costs

The parameters in a centrifuge in accordance with the present invention comprise:1. bowl speed (directly affects acceleration or g-force)2. conveyor speed (differential−relative difference in speed between the bowl and conveyor)3. slurry feed rate4. pond depth
Bowl speed is varied by the control system PM by signaling the VFD54controlling the bowl drive motor24.
Conveyor speed is varied by the control system PM by signaling the VFD56controlling the conveyor drive motor34.
Slurry feed rate is varied by the control system PM by signaling the VFD46controlling the slurry feed pump44.
Pond depth is varied by the control system PM by signaling the control motor29or linear motor29′.

The parameters are adjusted to achieve the optimum as established by one of the two above equations using an “evolutionary operation” approach. A flow diagram showing steps in the operation in accordance with the present invention is shown inFIG. 7. This entails using a set of operating parameters as a starting point, for example, the rotational speed of bowl12, the rotational speed of conveyor18and speed of feed pump44are initially set at the speeds used in the centrifuge's last use. The control system PM calculates the value of an optimization algorithm, such as the algorithm above. The control system PM measures the performance using at least one sensor. If the performance is not optimal, as defined by the algorithm, then the control system will select a parameter to change, for example, one of the rotational speed of bowl12, the rotational speed of conveyor18and speed of feed pump44. The control system checks that making a small change to the selected operational parameter is within constraints. For example, if the bearings33are not able to cope with the small increase in speed of the conveyor18, then the control system would not increase the speed of the conveyor18and move on to making a small change in another parameter, such as the bowl speed12, which again would be checked to be within constraints. The small change to the parameter would then be made by the control system PM. The control system then monitors the performance for improvement toward optimum performance. If the performance improves, then this new set of operating parameters would become the new starting point. If not, then the parameter that was changed would be changed by a small amount in a different, preferably opposite direction and the performance measured again. By automatically repeating this process over and over with every parameter, the centrifuge is made to continually seek out optimal performance. Preferably, each change is made every fifteen minutes, although it is envisaged that a change made every two minutes, five minutes, twenty minutes or thirty minutes or any other reasonable time interval is feasible, preferably allowing at least a portion of the feed slurry separation to be separated by the centrifuge10under the new changed parameter before the process is repeated. The time interval is preferably programmed into control system PM so that the process is repeated automatically.

When the control system PM checks that making a small change to the selected operational parameter is within constraints and if the parameter would not be within constraints, then another parameter is picked by the control system. This is preferably defined by a predetermined list programmed into the control system PM of parameters and the control system moves on to the next parameter in the predetermined list.

The parameters are constrained by the following constraints:

1. Maximum allowable % moisture on cuttings (as determined by regional regulations or customer preference). If the % moisture on cuttings discharged from the centrifuge is too high, then the centrifuge control system PM adjusts any or all of the following parameters:

a. reduce the feed pump44speed in order to reduce the solids load in the bowl12;

b. reduce the differential speed between the conveyor18and bowl12preferably, by reducing the conveyor drive motor34speed in order to increase the retention time of the solids in the bowl12;

c. reduce the pond depth of the feed slurry11in the bowl12by activating gate position motor29,29′ to retract the gate20a,20a′ to allow more fluid to be returned to the active mud system;

2. Maximum allowable torque on conveyor drive motor34or bowl drive motor24. If the torque is too high, then the centrifuge control system PM adjusts any or all of the following parameters:

a. reduce the feed pump44speed in order to reduce the solids load in the bowl12;

b. increase the differential speed between the conveyor18and bowl12by increasing the conveyor drive motor24speed to push the solids out faster and therefore have a shallower solids bed dragging against the conveyor18;

3. Maximum allowable barite loss rate (determined by the customer's barite loss tolerance). If the barite loss rate is too high, then the centrifuge computer would adjust any or all of the following parameters:

a. reduce the feed pump44speed in order to reduce the amount of barite processed by the centrifuge.

The following data may be obtained for use in the control system PM. At least one, preferably several and most preferably all of the following will be required and measured values sent to the control system. The data includes feed slurry data, flow rate data, retrieved solids data, retrieved fluid data and centrifuge apparatus data. Preferably, the data is retrieved in real time, taken every few minutes, although certain of the data may take a relatively long time to obtain, taken every few hours.

1. Liquid density measured preferably at at least one of the following:

a. of the feed slurry at the input to centrifuge10, preferably using differential pressure measurement along a part of the feed pipe. First and second pressure sensors100,101are located along the feed pipe42, spaced a few meters apart. The differential pressure readings taken from the pressure sensors100,101sent back to a measurement system102, such as a computer, which may be a part of the control system PM or separate. The measurement system102calculates the pressure differential and density of the slurry can thus be derived. Other factors may also be measured to obtain the density of the feed slurry.

b. separated liquid output from centrifuge10in the liquid discharge pipe105. The method set out above may be used in a liquid discharge pipe, using a first and second pressure sensors104,106located along the discharge pipe105, spaced a few meters apart. The differential pressure readings taken from the pressure sensors104,106sent back to a measurement system107, such as a computer, which may be a part of the control system PM or separate. The measurement system107calculates the pressure differential and density of the slurry can thus be derived. Other factors may also be measured to obtain the density of the feed slurry.

c. the holding tank into which the centrifuge10discharges. Preferably the liquid is put into a holding tank110of an active mud system of a rig, the measurement advantageously made in the holding tank, preferably using first and second vertically spaced pressure sensors108,109differential pressure is measured in the tank in a similar method to that stated above to calculate density.

The density of the solids output may also be obtained using a solids density sensor or by weighing the tank122into which the solids are discharged, sensing a volume and calculating the density therefrom.

2. Feed slurry viscosity may be sampled and measured manually with a Marsh Funnel or by a viscosity sensor and the measured result sent to the control system. The feed slurry viscosity is measured preferably at at least one of the following:

a. input to centrifuge

b. liquid output from centrifuge

c. the tank that the centrifuge is discharging into

3. Flow rate, i) mass flow rate and/or ii) volume flow rate preferably of the feed slurry flow rate in conduit42and advantageously separated liquid flow rate in discharge pipe105.

i) Mass flow rate is measured using a Coriolis mass flow meter111,112. Each Coriolis mass flow meter111,112preferably uses an inlet and an outlet arm which vibrate in synchronous when there is no flow of slurry feed/liquid, but vibrate out of synchronous when there is a flow of slurry feed/liquid. This phase shift in vibration produces a signal indicative of mass flow through the pipe. Each Coriolis meter is in communication with the control system.
ii) Volumetric flow rate is measured with an ultrasonic flow meter or paddle wheel112,114, which are in communication with the control system.
4. low gravity drilled solids content of feed slurry % by volume and mass [feed slurry data] measured using a low gravity drilled solids sensor or sampled and analyzed manually at preferably the slurry feed input to centrifuge10and/or advantageously the output from centrifuge10.
5. low gravity commercial solids content of feed slurry % by volume and mass, measured using a low gravity commercial solids sensor or sampled and analyzed manually at preferably the slurry feed input to centrifuge10and/or advantageously the output from centrifuge10
6. high gravity commercial solids content % by volume and mass, measured using a high gravity commercial solids sensor or sampled and analyzed manually at at preferably the slurry feed input to centrifuge10and/or advantageously the output from centrifuge10.
7. % of oil or water on the discharged cuttings by wet or dry calculation measured using a near infrared (NIR) moisture meter121at solids output from centrifuge. The solids are discharged through a discharge pipe120into a solids collection box122or hopper of a solids conveying system. An NIR moisture meter121measures the moisture content of the solids and sends a signal representative of the moisture content reading back to the control system PM.
8. Salt content by volume and mass, measured using a salt content sensor or sampled and analyzed manually at preferably the input to centrifuge10and advantageously, the liquid output from centrifuge10.
9. Particle size analysis, measured using a particle size sensor or sampled and analyzed manually at preferably the slurry feed input to centrifuge and advantageously the liquid output from centrifuge10.
10. Temperature, measured using a thermometer or other temperature sensor preferably at at least one of the following:

a. rotating assembly bearings

c. VFD control cabinet

d. ambient air

f. drilling mud input to centrifuge

11. Vibration frequency and amplitude, measured using an accelerometer64a,64b,64c,64dor other suitable device preferably at at least one of the following:

12. Rotational speed of the bowl12and conveyor18measured using a bowl rotational speed sensor135for the bowl12a conveyor rotational speed sensor130for the conveyor18.

13. Torque at gearbox input and gearbox output measured using an input torque sensor140and output torque sensor.

14. The level of the slurry fluid11in the bowl12, known as pond depth measured preferably using an ultrasonic distance measuring sensor150. The ultrasonic distance measuring sensor150is arranged outside of the bowl12aimed at the fluid level in the bowl12through the holes20′ in flange19″ of the flanged shaft19forming the end plate of the bowl12. Alternatively or additionally, the position of the adjustable gate20a,20a′ is sensed with sensor155,155′ from which the pond depth can be calculated, as the end of the gate20c,31. The measurements are sent to the control system PM.

Each of the sensors is preferably controlled by the control system. The control system takes readings from each sensor at predetermined time intervals or continuously. The predetermined time intervals may be at regular time intervals or irregular time intervals. If any of the data is obtained from a manual analysis, the obtained figure may be input to the control system PM. Preferably, the time intervals are such that up-to-date readings can be made from the small change made. The small incremental changes are most preferably made every fifteen minutes and thus readings are preferably taken immediately before the next change is made, for example between ten and fifteen minutes after the change such that the control system can accurately determine if an improvement has been made towards optimum performance to establish in which direction a further change should be made.

Referring toFIG. 8, there is shown a first embodiment of a flow system in accordance with the present invention in a first stage of operation wherein feed slurry parameters are measured.FIG. 9shows the feed system shown inFIG. 8in a second stage of operation wherein liquid discharge parameters are measured.

It will be appreciated that this flow system ofFIGS. 8 and 9may be incorporated into the system shown inFIG. 2, replacing certain parts of that system. The flow system200comprises a feed tank199and a plurality of two-way flow valves201to206in a plurality of conduits207to211and213to223. A parameter measuring apparatus212, such as a multi-parameter measuring apparatus is arranged in the conduit to measure at least one parameter of the feed slurry.

The feed slurry is drawn from feed tank199, (like tank40ofFIG. 2) through conduit207using either a feed pump240or the head of the feed slurry in the feed tank199and flow controlled through a flow control valve (not shown) which can vary the feed rate of the feed slurry. The feed tank199usually contains in the order of 50 to 200 barrels (7,900 to 31,800 liters) of solids laden drilling fluid. The feed tank199is fed solids laden drilling fluid from at least one further solids laden drilling fluid processor (not shown), such as shale shaker, mud cleaner, hydrocyclone, degasser, settling tank, etc. The at least one further solids laden drilling fluid processor removes certain solids, gases or liquids from the solids laden drilling fluid returned from the well. The feed tank199may thus act as a buffer to facilitate containing the solids laden drilling fluid between the centrifuge10and the further solids laden drilling fluid processor due to inter alia varying speeds at which the various processors process the solids laden drilling fluid. During drilling, the drill bit (not shown) may pass through different formations strata. The solids laden drilling fluid flowing from the drill bit into the feed tank199may thus contain very different solids and have very different properties such as viscosity, and commercially added solid and liquids. Thus, as the drill bit passes from one stratum to another the solids laden drilling fluid may change from a solids laden drilling fluid having a first set of properties to solids laden drilling fluid having a second set of properties. However, this change is not seen as a sudden change by the centrifuge10, as the solids laden drilling fluid having a second set of properties mixes with the solids laden drilling fluid having a first set of properties in the feed tank199. Mixing in the feed tank199may be induced with an impeller198driven by motor197.

The feed tank199may form part of a mud tank system (not shown) comprising a series of tanks. The mud tank system may comprise a first tank fed with underflow of screened solids laden drilling mud from a shale shaker. The first tank may comprise a sand trap, such that sand settles therein on a pan. The sand is tapped off after sufficient build up. The screened solids laden drilling mud is then pumped from the first tank through a degasser to remove at least a portion of any gas which may be present in the screened solids laden drilling mud and flows into a second tank. The screened and degassed solids laden drilling mud is pumped from the second tank through a hydrocyclone to further remove sand particles. The screened, degassed and hydrocycloned solids laden drilling mud flows into a feed tank, such as feed tank199for further processing with the centrifuge10. The first tank may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The feed tank199may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank may comprise an impeller to agitate to inhibit solids from settling. The inventors observed that the impellers in the second tank and the feed tank199tend to mix incoming flow with the solids laden drilling mud already in the respective second tank and feed tank199.

The first stage of operation of the flow system is shown inFIG. 8in which feed slurry parameters are measured. The feed slurry is prevented from flowing from conduit207through conduit220,221by closed two-way flow valve201and is allowed to flow through open two-way flow valve202into conduit209and is prevented from flowing through conduit210aby closed two-way flow valve205and allowed to flow through line210through parameter measuring apparatus212, such as a multi-parameter measuring apparatus which preferably carries out at least one of the following measurements: mass flow rate; volume flow rate; velocity; viscosity; density; and temperature of the flow of feed slurry across the multi-parameter measuring apparatus. The feed slurry proceeds through conduit211, prevented from flowing into conduit213by a closed two-way flow valve204and allowed through conduit214through open two-way flow valve203, into conduit215then 216 into centrifuge10through a feed tube16(seeFIG. 1). The feed slurry is prevented from returning through conduit221by closed two-way flow valve201. The centrifuge10separates solids from the liquid as disclosed above with regard toFIGS. 1 and 2. The liquid phase is discharged through liquid discharge outlet conduit218, prevented from flowing through conduit210aby closed two-way flow valve205and allowed to flow through open two-way flow valve206to a return tank or return line (not shown) of the active mud system for re-circulating in the well. The solids phase is discharged through port230into conduit217.

The second stage of operation of the flow system is shown inFIG. 9in which liquid discharge parameters are measured. The feed slurry is prevented from flowing from conduit207into conduits210by closed two-way flow valve202and allowed to flow through conduit220, through open two-way flow valve201into conduit221, through conduit216into centrifuge10through a feed tube16(seeFIG. 1). The feed slurry is prevented from returning through conduit215by closed two-way flow valve203. The liquid discharge phase is discharged through liquid discharge outlet conduit218, prevented from flowing into the return tank and allowed to flow through open two-way flow valve205into conduit210ainto conduit210and through parameter measuring apparatus212, such as a multi-parameter measuring apparatus which preferably carries out at least one of the following measurements: mass flow rate; volume flow rate; velocity; viscosity; density; and temperature of the flow of liquid discharge phase across the multi-parameter measuring apparatus. The liquid discharge phase continues through conduit211, prevented from flowing through conduit214by closed two-way flow valve203and allowed to flow through conduit213through open two-way flow valve204into the return tank of the active mud system for re-circulating in the well. The solids phase is discharged through port230into conduit217.

Preferably, the parameter measuring apparatus212measures mass flow rate using a coriolis meter. The coriolis meter which preferably uses an inlet and an outlet arm or tube which vibrate in synchronous when there is no flow of slurry feed/liquid, but vibrate out of synchronous when there is a flow of slurry feed/liquid. This phase shift in vibration produces a signal indicative of mass flow through the pipe. Preferably, the coriolis meter is arranged such that the flow of solids laden fluid falls vertically therethrough or at such an angle that solids would not settle on the inner pipe wall of the coriolis meter. The parameter measuring apparatus212advantageously also measures volume flow rate, preferably with an ultrasonic flow meter or paddle wheel and produces a signal indicative and/or proportional to the volume flow rate. The parameter measuring apparatus212advantageously also measures velocity, preferably with an ultrasonic flow meter or paddle wheel and produces a signal indicative and/or proportional to the velocity across the parameter measuring apparatus. The parameter measuring apparatus212advantageously also measures the temperature, preferably with a temperature sensor and produces a signal indicative and/or proportional to the temperature in the fluid flowing across the meter. The parameter measuring apparatus212advantageously also measures the density, preferably using differential pressure measurement along a part of the feed pipe. First and second pressure sensors (not shown) are located along the conduit42, preferably at either side of the parameter measuring apparatus212, thus spaced apart. The differential pressure readings taken from the pressure sensors are sent back as signals to control system PM, which calculates the pressure differential and density of the slurry can thus be derived.

The flow system200incorporates a control system PM, such as the control system PM used in the embodiment inFIG. 2. The parameter measuring apparatus212is in communication with the control system PM, such that the control system receives signals from the measured parameters therefrom, i.e. at least one and preferably all of: mass flow rate, volume flow rate, velocity, density and temperature. The parameter measuring apparatus212is preferably hard wired to the control system PM, advantageously with a data bus link. Alternatively or additionally, the parameter measuring apparatus212is wirelessly linked to the control system PM, using a data transfer protocol such as Wi-Fi, blue-tooth or the like. The control system PM also may activate the parameter measuring apparatus212when measured parameter readings are required. Each of the two-way flow valve201to206has a valve position sensor231to236in communication with the control system PM. The valve position sensors231to236each send a signal to the control system PM indicative of the position of the valve: open, closed and preferably a signal to indicate if there is a problem with the associated two-way flow valve201to206. Each two-way flow valve201to206also has an actuator (not shown), such as a stepper motor which is also linked to the control system PM, such that the control system PM controls the actuator to toggle between the two-way flow valve201to206between an open and closed position. The actuator and valve position sensors231to236are preferably hard wired to the control system PM, advantageously with a data bus link, preferably using a protocol such as TCP. Alternatively or additionally, the parameter measuring apparatus212is wirelessly linked to the control system PM, using a wireless data transfer protocol such as Wi-Fi, blue-tooth or the like.

The control system PM activates the two-way flow valve201to206to toggle between open and closed positions to alternate flow of feed slurry and liquid discharge phase through the parameter measuring apparatus212. A short period of time will need to be left before logging data whilst toggling, due to residual liquid and solids in pipe work across the parameter measuring apparatus212and in lines210,211and214. Thus, a short period of flushing time is required to flush through feed slurry when measuring the liquid discharge parameters and of liquid discharge when measuring the feed slurry parameters. For the avoidance of doubt, flushing is carried out by continuing operation of the centrifuge, not a separate flushing step of the pipework only. Furthermore, flow conditions through the centrifuge10may be upset slightly at each toggle. Thus, a period of time is allowed before logging parameter measurements with the parameter measuring apparatus212. The upset period may be longer or shorter than the flushing period, which can both be assessed by continuously taking measurements using the parameter measuring apparatus212, but only logging the parameter measurements once the control system PM detects the flow when stable. This period may be in the order of one to two minutes. The control system PM preferably toggles every five to sixty minutes and most preferably between ten minutes and thirty minutes.

The conduits207to211and213to223may each be a solid walled pipe or a flexible hose or a combination thereof.

Referring toFIG. 10, there is shown a second embodiment of a flow system in accordance with the present invention in a first stage of operation wherein feed slurry parameters are measured.FIG. 11shows the feed system shown inFIG. 10in a second stage of operation wherein liquid discharge parameters are measured.

It will be appreciated that this flow system ofFIGS. 10 and 11may be incorporated into the system shown inFIG. 2, replacing certain parts of that system. The flow system300comprises a feed tank299and a plurality of three-way flow valves301to303in a plurality of conduits304to311and313to316. A parameter measuring apparatus312, such as a multi-parameter measuring apparatus is arranged in the conduit to measure at least one parameter of the feed slurry.

The feed slurry is drawn from feed tank299, (like tank40ofFIG. 2) through conduit304using either a feed pump340or the head of the feed slurry in the feed tank299and flow controlled through a flow control valve (not shown) which can vary the feed rate of the feed slurry. The feed tank299usually contains in the order of 50 to 200 barrels (7,900 to 31,800 liters) of solids laden drilling fluid. The feed tank299is fed solids laden drilling fluid from at least one further solids laden drilling fluid processor (not shown), such as shale shaker, mud cleaner, hydrocyclone, degasser, settling tank, etc. The at least one further solids laden drilling fluid processor removes certain solids, gases or liquids from the solids laden drilling fluid returned from the well. The feed tank299may thus act as a buffer to facilitate containing the solids laden drilling fluid between the centrifuge10and the further solids laden drilling fluid processor due to inter alia varying speeds at which the various processors process the solids laden drilling fluid. During drilling, the drill bit (not shown) may pass through different formations strata. The solids laden drilling fluid flowing from the drill bit into the feed tank299may thus contain very different solids and have very different properties such as viscosity, and commercially added solid and liquids. Thus, as the drill bit passes from one stratum to another the solids laden drilling fluid may change from a solids laden drilling fluid having a first set of properties to solids laden drilling fluid having a second set of properties. However, this change is not seen as a sudden change by the centrifuge10, as the solids laden drilling fluid having a second set of properties mixes with the solids laden drilling fluid having a first set of properties in the feed tank299. Mixing in the feed tank299may be induced with an impeller298driven by motor297.

The feed tank299may form part of a mud tank system290comprising a series of tanks. The mud tank system may comprise a first tank291fed with underflow of screened solids laden drilling mud from a shale shaker292. The first tank291may comprise a sand trap293, such that sand settles therein on a pan. The sand is tapped off after sufficient build up. The screened solids laden drilling mud is then pumped from the first tank291through a degasser294to remove at least a portion of any gas which may be present in the screened solids laden drilling mud and flows into a second tank295. The screened and degassed solids laden drilling mud is pumped from the second tank295through a hydrocyclone296to further remove sand particles. The screened, degassed and hydrocycloned solids laden drilling mud flows into the feed tank299for further processing with the centrifuge10. The first tank291may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank295may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The feed tank299may be in the order of 20 to 200 barrels (3200 to 32,000 liters). The second tank295may comprise an impeller to agitate to inhibit solids from settling. The inventors observed that the impeller298in the feed tank299tend to mix incoming flow with the solids laden drilling mud already in the respective second tank295and feed tank299.

The first stage of operation of the flow system is shown inFIG. 10in which feed slurry parameters are measured. The feed slurry is prevented from flowing from conduit304through conduit314by orientation of three-way flow valve301and is allowed to flow through conduits305and307and is prevented from flowing through conduit306by orientation of three-way flow valve302. The feed slurry flows from conduit307through parameter measuring apparatus312, such as a multi-parameter measuring apparatus which preferably carries out at least one of the following measurements: mass flow rate; volume flow rate; velocity; viscosity; density; and temperature of the flow of feed slurry across the multi-parameter measuring apparatus. The feed slurry proceeds through three-way flow valve302into conduit309and is prevented from flowing into return tank by orientation of the three-way flow valve. The feed slurry flows from conduit309into conduit310and is prevented from returning in conduit314by the orientation of three-way flow valve301. The feed slurry flows from conduit310into centrifuge10through a feed tube16(seeFIG. 1). The centrifuge10separates solids from the liquid as disclosed above with regard toFIGS. 1 and 2. The liquid phase is discharged through liquid discharge outlet conduit313, prevented from flowing through conduit306by orientation of three-way flow valve303and allowed to flow through three-way flow valve303to the return tank or return line (not shown) of the active mud system for re-circulating in the well. The solids phase is discharged through port330into conduit311.

The second stage of operation of the flow system is shown inFIG. 11in which liquid discharge parameters are measured. The feed slurry is prevented from flowing from conduit305by orientation of three-way flow valve301and allowed to flow through conduits314into conduit310and into centrifuge10through a feed tube16(seeFIG. 1). The feed slurry is prevented from returning through conduit309by orientation of three-way flow valve302. The liquid discharge phase is discharged through liquid discharge outlet conduit313, prevented from flowing into the return tank and allowed to flow through three-way flow valve303into conduit306into conduit307and through parameter measuring apparatus312, such as a multi-parameter measuring apparatus which preferably carries out at least one of the following measurements: mass flow rate; volume flow rate; velocity; viscosity; density; and temperature of the flow of liquid discharge phase across the multi-parameter measuring apparatus. The liquid discharge phase continues through conduit317, prevented from flowing through conduit309by orientation of three-way flow valve302and allowed to flow into the return tank of the active mud system for re-circulating in the well. The solids phase is discharged through port330into conduit311.

Preferably, the parameter measuring apparatus312measures mass flow rate using a coriolis meter. The coriolis meter which preferably uses an inlet and an outlet arm or tube which vibrate in synchronous when there is no flow of slurry feed/liquid, but vibrate out of synchronous when there is a flow of slurry feed/liquid. This phase shift in vibration produces a signal indicative of mass flow through the pipe. Preferably, the coriolis meter is arranged such that the flow of solids laden fluid falls vertically therethrough or at such an angle that solids would not settle on the inner pipe wall of the coriolis meter. The parameter measuring apparatus312advantageously also measures volume flow rate, preferably with an ultrasonic flow meter or paddle wheel and produces a signal indicative and/or proportional to the volume flow rate. The parameter measuring apparatus312advantageously also measures velocity, preferably with an ultrasonic flow meter or paddle wheel and produces a signal indicative and/or proportional to the velocity across the parameter measuring apparatus. The parameter measuring apparatus312advantageously also measures the temperature, preferably with a temperature sensor and produces a signal indicative and/or proportional to the temperature in the fluid flowing across the meter. The parameter measuring apparatus312advantageously also measures the density, preferably using differential pressure measurement along a part of the feed pipe. First and second pressure sensors (not shown) are located along the conduit42, preferably at either side of the parameter measuring apparatus312, thus spaced apart. The differential pressure readings taken from the pressure sensors are sent back as signals to control system PM, which calculates the pressure differential and density of the slurry can thus be derived.

The flow system300incorporates a control system PM, such as the control system PM used in the embodiment inFIG. 2. The parameter measuring apparatus312is in communication with the control system PM, such that the control system receives signals from the measured parameters therefrom, i.e. at least one and preferably all of: mass flow rate, volume flow rate, velocity, density and temperature. The parameter measuring apparatus312is preferably hard wired to the control system PM, advantageously with a data bus link. Alternatively or additionally, the parameter measuring apparatus312is wirelessly linked to the control system PM, using a data transfer protocol such as Wi-Fi, blue-tooth or the like. The control system PM also may activate the parameter measuring apparatus312when measured parameter readings are required. Each of the three-way flow valves301to303has a valve position sensor331to333in communication with the control system PM. The valve position sensors331to333each send a signal to the control system PM indicative of the position of the valve: open, closed and preferably a signal to indicate if there is a problem with the associated two-way flow valve301to303. Each three-way flow valve301to303also has an actuator (not shown), such as a stepper motor which is also linked to the control system PM, such that the control system PM controls the actuator to toggle between the two-way flow valve301to303between an open and closed position. The actuator and valve position sensors331to333are preferably hard wired to the control system PM, advantageously with a data bus link, preferably using a protocol such as TCP. Alternatively or additionally, the parameter measuring apparatus312is wirelessly linked to the control system PM, using a wireless data transfer protocol such as Wi-Fi, blue-tooth or the like.

The control system PM activates the two-way flow valve201to206to toggle between open and closed positions to alternate flow of feed slurry and liquid discharge phase through the parameter measuring apparatus312. A short period of time will need to be left before logging data whilst toggling, due to residual liquid and solids in pipe work across the parameter measuring apparatus312and in lines306,307and317. Thus, a short period of flushing time is required to flush through feed slurry when measuring the liquid discharge parameters and of liquid discharge when measuring the feed slurry parameters. For the avoidance of doubt, flushing is carried out by continuing operation of the centrifuge, not a separate flushing step of the pipework only. Furthermore, flow conditions through the centrifuge10may be upset slightly at each toggle. Thus, a period of time is allowed before logging parameter measurements with the parameter measuring apparatus312. The upset period may be longer or shorter than the flushing period, which can both be assessed by continuously taking measurements using the parameter measuring apparatus312, but only logging the parameter measurements once the control system PM detects the flow when stable. This period may be in the order of one to two minutes. The control system PM preferably toggles every five to sixty minutes and most preferably between ten minutes and thirty minutes.

The conduits304to311and313to317may each be a solid walled pipe or a flexible hose or a combination thereof.

The discharged liquid phase may flow from the conduit217,311into a further centrifuge (not shown) to be further processed, particularly, but not exclusively to remove barites from drilling mud. The centrifuges may run at different speeds, such as a slow speed for the first centrifuge and a high speed for the second centrifuge. Each of the two centrifuges may use a flow system of the present invention.