VFD Decanting centrifuge

A brief review of decanter centrifuge is beneficial prior to a discussion of control objectives and methods. Decanting bowl centrifuges are well known in the industry for separating fine solids out of drilling mud. A decanter centrifuge is designed to process a physical mixture of two constituents: a liquid and a solid mechanically separating one from the other. This type of decanter centrifuge features a rotating cylindrical section called the bowl, supported by bearings with a spiral screw conveyor disposed inside of the bowl. The conveyor is supported by a second set of bearing internal to the bowl. In the particular case considered here the conveyor rotates in the same direction as the bowl but at a slightly slower speed. The feed mud is a mixture of a liquid having relatively fine solids entrained therein. This feed mud enters the bowl and the high centrifugal forces generated from the rotational speed of the bowl cause the solids to separate from the liquid due to the dramatically increased “g” forces on the solids. These solids sediment to the inner wall of the rotating bowl and the scraping and transported to the solids discard end of the bowl by the scraping and transport of the screw conveyor. The liquid which has the solids being removed is volumetrically displaced above the sedimenting solids and follows the helix of the conveyor to the liquid discharge end.

Drilling mud decanter centrifuge
Figure 1. Decanter centrifuge for waterbased drilling fluid

There are many different models available, which conform to this same basic design. They differ by size, horsepower required to power them and the details of how the conveyor and bowl are powered. A general design which is discussed here includes a bowl which is belt driven from an explosion proof electric motor. The conveyor in this particular type is powered through a gearbox, which establisheds the differential speed of the conveyor to the bowl. The gearbox is powered through a belt drive and explosion proof electric motor. Some of the parameters which are involved in the operation of any such decanting bowl centrifuge are: torque on the conveyor, conveyor speed, bowl torque, bowl speed, mud feed rate, fluid properties, dilution and or polymer injection. Changing any of these parameters will change the operational result of the machine. For example, increase in flow rate will decrease the residence time in the bowl and in general raise the size of the smallest solids just captured before the fluid exits the bowl. In general higher bowl speed will generate higher “g” force on the solids, which settle according to Stoke’s law and capture smaller solids. There are many cases where the solids react to the dynamics of the rotating bowl and conveyor. In some cases excessive bowl speed will cause shearing forces, which will increase the size of the smallest solid captured. The two fluid streams and solids discard stream (sludge) can best be described by their respective particle size distribution. This is a graph of a sample taken from that particular fluid stream while the machine was operating in a steady state manner. Various particle size distribution graphs will be discussed later to show particular operational changes and resulting solids discard changes. The density of the solids being extracted is obviously important since the gravitational forces will cause greater accelerations on barite than on drilled solids

Centrifuge rotafing assembly details
Figure 2. Centrifuge rotafing assembly details

One example would be a decanter centrifuge which is set up to operate with a cut point of 10 microns on drilled solids with conventional water based mud, This decanter centrifuge could be installed to dry the solids discard from a desilter and desander. The incentive to do this operationally is that the desilter ordinarily is sized to process in excess of 100°/0of the rig circulation rate. The discard from the desilter will be normally very wet when the cyclones are operating optimally in a spray discard. The decanter centrifuge can take this drilled solids concentrate and recover the majority of the water. Once again on our example the decanter centrifuge will recover solids slightly smaller than the desilter has extracted. This same centrifuge could not have much of an impact on the active mud system if the majority of the solids were finer than the same 10 microns. This is the kind of situation, which makes variable speed centrifuges attractive. A simple increase in bowl speed would lower the cut point of this machine so that it could have a completely loaded conveyor regardless of the feed fluid. Control of these changes in operational parameters is the new frontier which is being explored.

Decanter centrifuge Processes

Drilling mud centrifuge
Drilling mud centrifuge

Decanting bowl centrifuges are still needed to recover drilling fluid weight material in some situations. In many circumstances a barite recovery decanter centrifuge will return the barite to the active system while the high speed centrifuge will discard the fine solids from the liquid out of the barite recover centrifuge. Decanter centrifuges often operate in parallel, to discard drill solids when one or both had been used earlier in the well as barite recovery machines, Sometimes this is in conjunction with the rest of the rigs solids control equipment, Decanter centrifuge are often used on oil mud systems to process the total circulation rate in combination with fine screen shakers. In many cases oil mud systems are used over and over or even rented to the operator, At the end of some oil mud jobs the decanter centrifuges are used to strip as much of the solids as possible from the mud system so that the next job with the same mud system will not require massive dilution. Many times a very dry discard is necessary for oil mud processes due to the disposal requirements of the sludge for environmental considerations. Dewatering has become more and more popular where a decanter centrifuge will discard all solids by using a polymer injection system to flocculate the solids to the extent that all solids will be discarded and clear water will be recovered for drilling. In general the most common case would be for fine drilled solids extraction to control plastic viscosity, reduce gels, and create a better filter cake on the well bore.

Conventional Machines

A standard decanter centrifuge is described as a machine with a fixed electric or hydraulic bowl speed, which can be changed only by changing the drive sheave combinations. In order to change the drive sheave combination it is necessary to take the machine out of service and remove the existing sheaves and replace them with the appropriate combination, A standard decanter centrifuge could have a fixed back drive (no back drive electric motor) where the input to the sun wheel of the gearbox is held stationary, With such a fixed back drive the differential of the conveyor is dependent on the bowl speed where the outside of the planetary gearbox rotates with the bowl and the input to the gearbox is held. In cases where the sun wheel of the gearbox is driven by a motor the decanter centrifuge effectively has three conveyor speeds; one with the input sun wheel fixed, with the motor driving the sun wheel at speed in the same direction as the bowl and one where the motor is driving the sun wheel in the opposite direction with respect to the bowl. Henceforth a standard decanter centrifuge will mean a decanter centrifuge with a main drive motor for the bowl and a back drive motor for the gearbox and therefore conveyor. Therefore a standard decanter centrifuge by this definition is understood to operate continuously at a fixed bowl speed and constant conveyor differential speed regardless of the properties of the fluid it is processing or intended objectives.

These conventional decanter centrifuges have processing limitations based on either the gearbox torque rating, main drive motor capacity, relative size and mechanical integrity of the rotating assembly. The gearbox being a high cost component is protected by a clutch or shear pin type torque limiting device. When the decanter centrifuge is being operated beyond its capacity the solids are separating out faster in the rotating bowl than the conveyor can transport them to the discard ports. When the decanter centrifuge starts to plug more and more surface area of solids is being sheared by the conveyor. The fiction goes up as the mass of separated solids accumulates in the conveyor and or the separated solids become dryer and require more torque to move them in the bowl. The increase in Election and transport of solids will increase the torque on the conveyor and driving gearbox. When the torque reaches the preset torque limit the input sun wheel will be released from its restraint and the conveyor will attain the speed of the bowl almost instantly. Normally a safety interlock system will be keyed when the torque device releases and the power to all motors and pump will be stopped instantly. The separated solids that have caused the safety shutdown will have to be cleaned out before the clutch or torque limiting device can be reset and the decanter centrifuge put back in operation. In most cases this amounts to a service call to the decanter centrifuge rental company and a number of hours of downtime getting the decanter centrifuge back in service. Processing feed rates higher than designed can lead to main drive (bowl drive) motor failure. The current on the bowl drive motor is a function of the mass rate into the bowl, which must be accelerated to the bowl speed. Bearings and drive components have finite lives and proper. Maintenance as in all mechanical devices is very important for a dependable service life.

In all decanting bowl centrifuges the speed of the conveyor in rpm, with respect to the rotating bowl, is defined as the differential speed. The general formula for finding the differential speed for our example is:

Diff = {[(drive sheave ratio)*main drive motor rpm] – [(drive sheave ratio)*back drive motor rpm]}/gearbox reduction

This expression would be for clockwise rotation of the bowl from the bowl drive end and back drive motor positive rotation when it turns the input to the gearbox in the same direction as the bowl is turning, For one set of possible configurations the differential speed of the conveyor in revolutions per minute would be;

Diff = {[(10.6”PD/5.6”PD)*1750]-[(10.6”PD/13.9”PD)*(-1750)]}/57.12

Diff81.34 rpm for 3312 rpm bowl and -1750 for back drive motor

centrifuge conveyor differential speed
Centrifuge conveyor differential speed

Conventional centrifuges are installed to operate in a range of different drilling fluid weights and solids concentrations, Normally the installation is based on knowledge of the best compromise settings for the area and expected drilling conditions, Often the decanter centrifuge installation is a best compromise between the capacity and limitations of the decanter centrifuge and the expected range of fluids to be processed. The flow rate is established by the pump, which is used to feed the decanter centrifuge. There are two basic types of feed pumps, centrifugal and positive displacement moyno type pumps. In most cases the centrifugal pumps are constant speed and therefore the flow rate was a function of the centrifugal pump curve, Feed from the centrifugal is often established by a valve placed in the feed line causing a restriction to the flow. Centrifugal pumps have a parabolic relationship between the pressure and flow. The moyno type pumps flow rate is determined by the speed at which the pump is driven so they normally have a mechanical variable speed drive. In all cases once the flow rate is established it is normally not varied regardless of the changes in fluid properties or objectives of the process.

For the case in discussed here, where the conveyor is moving slightly slower than the bowl, an important point to consider is that the torque load on the back drive and gear box is opposite in direction to the rotation of the bowl. Essentially this means that the back drive is braking the conveyor. The back drive is preventing the conveyor from being accelerated to the bowl speed, working against the drag from the accumulating solids. In the situation where there is a back drive motor in place, driving the gearbox input sun wheel, that motor is acting as a brake. The slip on the back drive motor will be opposite in direction from normal, In conjunction with the volume rate of solids fed to the decanter centrifuge, the dryer the solids discard the higher the friction and once again the higher the torque load on the gearbox and back drive electric motor. With a standard decanter centrifuge, the increase in load to the conveyor is reacted out with a simple increase in electric motor amps. This correlation continues until the torque on the safety clutch is exceeded or the current limits of the back drive electric motor are exceeded. In the case of the standard decanter centrifuge the dynamics of the loading and breaking free of the conveyor are absorbed in the real time loading of the rig generator.

The decanter centrifuge conveyor capacity is determined by both conveyor speed and bowl speed. A typical application in water based solids control is maximum solids discard where high fluid process rates dictate short residence time in the bowl. The solids that are extracted are a very small percentage of the solids being processed but the impact is dramatic because of the sequential nature of the rig circulation rate. In oil based mud-stripping situations low pump rates will result in high solids extraction and a wide range of size of solids removed.

Conveyor loading as a function of feed rate
Conveyor loading as a function of feed rate

The decanter centrifuge is normally used in conjunction with the rig solids control equipment and therefore is operated by the derrick man with recommendations from the mud engineer. This usually amounts to minimal supervision and less than precise operational control. Roughnecks do their job well but optimization of solids control equipment may be beyond the scope of their position. The operation of the decanter centrifuge must be straightforward and minimal monitoring should be anticipated. A visual check once an hour more for leaks and safety considerations is typical while the derrick man is completing his appointed rounds, Standard decanter centrifuges are designed for very simple operation. There is a start button for the main drive and a start button for the back drive (if one exists) and possibly a start button for the pump Often the control of the pump is simply mounted on the pump its self During such operation the discard of the decanter centrifuge is a function of how the fluid going to the decanter centrifuge reacts to the operating settings of the decanter centrifuge and the pump. For optimum operation the opposite needs to be true, the decanter centrifuge needs to react to the fluid it is processing. The type of rock being drilled, rate of penetration and status of the rest of the solids control equipment would determine the real loading of the decanter centrifuge. Improved performance could be obtained by just varying the speed of the bowl and conveyor. For example after two circulations of oil mud stripping the volume of solids from two decanter centrifuges has decreased to less than half of what it was initially. Initially the bowl speed for this example was 2200 rpm and most of the solids being discarded was slightly smaller than sand sized particles. By increasing the bowl speed to 2500 rpm the sludge discard rate would increase to the prior rate and finer solids would be extracted. Processing the fluid over and over at higher and higher rates has the drawback of fracturing solids in the pumps and decanter centrifuge due to the high shearing forces involved but this process will extract more solids faster than constant operational bowl speeds.

Semiautomatic Operation

An improvement on this arrangement in a hydraulically driven system would be a system similar to the Viscotherm decanter centrifuge drive package which is available on a number of oilfield decanter centrifuges. This back drive system is comprised of a high torque, low speed hydraulic motor driven by a hydraulic pump unit. The hydraulic motor housing is connected to the decanter centrifuge bowl and its rotor is directly keyed to the scroll conveyor. The system is designed so that the hydraulic motor will always attempt to supply a constant torque to the scroll by varying the oil pressure. Consequently, when the solids load rises or falls the differential speed increases or decreases simultaneously. The decanter centrifuge will therefore operate at its optimum differential speed at all times without danger of plugging as long as the bowl speed limits are not exceeded. Cake consistency and dryness will be best obtained when operating at the design capacity of the decanter centrifuge.

There are various electrical driven analogies to the Viscotherm system most of which operate by varying the back drive speed and some control over the decanter centrifuge feed pump as a function of conveyor loading. These systems avoid the cold climate limitations of hydraulic driven systems, have less maintenance and have quicker response. There have been both variable speed mechanical drives and electric variable speed drives, dc and ac such as variable frequency drives that are so popular in manufacturing and process industries. All of these systems monitor the conveyor loading and vary the conveyor speed accordingly. In general the objective is to insure that overload could be avoided by the decanter centrifuge sensing its own conveyor loading in a real time manner. Still this system is far from a complete solution since it can only control one facet of the overall operation and has only one response. For example if the back drive speed is at its maximum and that is the only parameter of the machine that can be varied and solids input is still increasing then the system will be forced to shut down in a similar fashion to the standard decanter centrifuge. By adding pump control to the monitored back drive speed it is possible to endure a much wider range fluids being processed but the sludge discard was still to a great degree beyond control.

The incentive to develop an automated completely variable speed decanter centrifuge came from many different sources each seeking a perceived benefit, First of all variable bowl speed could make it easier to vary the feed rate so that the decanter centrifuge could discard larger volumes of solids by sequential pass process as discussed earlier. The basic overall goal is to keep the decanter centrifuge loaded constantly regardless of feed mud properties. Once a microprocessor is accepted many different benefits of logic can be attained, microprocessors can maintain memory for service assistance and job histories. Many other benefits other than the control of the decanter centrifuge are available, by including a flow meter it is possible to anticipate accurately the length of time to run a decanter centrifuge to attain an expected mud weight drop. Of course a microprocessor will make the logic available to monitor the machine and control it beyond the abilities of the most educated human operator since it can make more decisions faster and extrapolate to determine future options more accurately.

An automated decanter centrifuge can vary the main drive for feed streams that would exceed the limit for back drive variation only. In addition automation of existing technology and mechanical systems will gain improved performance without increased power requirements or larger footprints.

Variable Frequency Drive (VFD Decanter Centrifuge)

VFD centrifuge component shematic
VFD centrifuge component shematic

Variable frequency drive is the electrical foundation for the logic necessary to completely control an oilfield decanter centrifuge. Variable frequency drives are compatible with more sophisticated logic components such as PLC’S (Programmable Logic Controllers) and SLC’S (Small Logic Controller’s), The SLC is a microprocessor controller which can communicate with the variable frequency drive over a single cable and monitor hundreds of functions at a loop speed slightly faster than twenty thousandths of a second. Variable frequency drive makes it possible to use ac motor technology in variable speed and torque situations, VFD are offered by many manufacturers and has been on the market for over ten years but the technology has been advancing quite rapidly recently, Such functions as sensor-less control where encoders are not necessary for speed reference within certain limits. These drives are indeed complex devices but the results are rewarding. The drives are capable of generating a broad range of output frequencies to change the motor’s synchronous speeds as required. It is not possible to vary the current so the voltage must be modulated within safe limits. If a drive’s output voltage were fixed while its frequency was changed then motor overheating problems would quickly develop, For example, if the output frequency to the motor were to be reduced from 60 Hz to 30 Hz (50%.) then the stator current would have to double, thus overheating the motor, If the frequency were increased from 60 Hz to 120 HA then the current would be halved and the torque would suffer. To prevent overheating at 30 Hz, the current must be reduced. To provide adequate torque at 120 Hz the current must be increased. However, if the stator applied voltage were reduced 50% while the frequency is being decreased 500%, then the ratio of voltage to frequency would remain constant. The stator current would not be affected, and assuming that the motors cooling is not dramatically changed by the speed then the motor would not overheat. The torque would not be affected and the motor would perform properly at reduced speed. This is the method that variable frequency drives use, change the voltage with the frequency to maintain proper current and torque. Every AC motor has a ratio of voltage to frequency, known as its volts per hertz ratio. As long as voltage and frequency are held in this relationship, the motor will function properly. A normal induction motor’s volts per hertz ratio can be obtained from the motor nameplate information, A motor nameplate indicating 460 volts and 60 Hz has a volts per hertz ratio of 460/60 or 7.6 volts per hertz. Please note that a motor rated for 380 volts at 50 hertz also of 7.6 volts per hertz. This ratio shows that for each 1 hertz change in frequency the voltage must make the corresponding change by 7.6 volts. There are minor exceptions to these relationships. At very low frequencies it is necessary to increase the voltage in order to maintain the torque. The logic to determine when this boost must be used and to what degree is part of the software included in the drive from the manufacturer. The logic residing in the drive controls most facets of motor operation. The drive will bring the motor up to speed according to the acceleration ramp that is selected by the user. This means smooth low current startups eliminating the need for fluid clutches, couplings and high current components. The VFDS will exactly and precisely execute the commands from the SLC without question over the single cable communication link. The VFD has hundreds of parameters that are set by the user but this can be done easily by laptop once and not changed for the foreseeable future. The drives can be replaced when a failure does occur as individual components of a packaged system. Impedance matching and line reactors were not necessary with normal cable lengths involved in drilling settings. An isolation transformer was used and is recommended but it is not a power filter it just offers a capacitance effect for spikes and very short power variations. The VFDS are sensitive to power variations and this is a cause for major concern, design considerations to compensate for this especially in a drilling environment are mandatory.

Small Logic Controller, SLC

The small logic controller can be programmed to directly address the programmable functions resident in the drives. The particular SLC which was used is programmed in a graphic interface language called ladder logic. The speed and control of the drives is impressive, not only speed up and slow down but less obvious functions such as resetting faults. The SLC in addition to holding the program to optimize the operation of the decanter centrifuge can display hundreds of different parameters to whatever kind of display is in use. The SLC will accept information not only from the drives but also from other outside sensors such as a flow meter and integrate this information into the decisions it makes to exercise control over the drives.

In the case under study here the SLC can constantly monitor parameters from the drives such as back drive current, main drive current and speeds as well as the flow rate and bearing temperature. The SLC processing speed is utilized to control events that happen in elapsed times of less than a tenth of a second. The wntrifuge will not react this fast of course but in situations of controlling the variations of incoming power processing speed is the only possible means for adequate control.

Some of the parameters that the SLC will control are:

  1. The pump speed and therefore the pump rate to the decanter centrifuge are controlled based on the operational mode selected.
  2. If events are occurring too quickly for pump adjustment to be effective then the SLC will turn the pump off at a predetermined limit and after the decanter centrifuge has cleared its self the pump will be turned on again to resume operation. The setting of the limits which determine the priority of decisions and the way the SLC responds can be done at any time but usually is done only once at the start of the job based on the mud system to be used and the requirements of the customer.
  3. If necessary and depending on the mode of operation selected the SLC will modulate the speed of the conveyor, the differential speed. If the torque on the back drive motor is above another preset limit then the SLC will command the back drive motor to increase the conveyor differential. Refer to figure 3 to see that often this speed adjustment to increase the differential speed of the conveyor means to slow down the back drive motor, When the back drive motor is running in the same direction as the bowl differential speed increases are obtained by slowing down the speed of the back drive motor.
  4. Depending on the mode of operation selected the SLC will adjust the speed of the bowl. Usually this is to either compensate for torque on the conveyor, excessive or to increase it, or to compensate for flow rate changes which have exceeded the current limits set to protect the main drive motor.
  5. The SLC will determine the shut down procedure to be used as well as the start up sequence to use, There are a number of different shut down sequences depending on the reason causing the shut down. A normal shut down due to operator pressing the kill button will turn off the pump instantly, slowly decelerate the bowl and run the back drive to a clean out speed and direction for a sufficient period of time to remove the solids from the bowl that would collect while the bowl is slowing down, Elapsed times and exact shut down sequences depend on perceived inertial loads of the rotating bowl.
  6. The SLC will initiate real time operating decisions. For example in brown out control the SLC will determine the best brown out survival sequence to select based on an extrapolation of the brown out evidence very quickly.
  7. The SLC will sense other operational difficulties and generate alarms as well as taking the best compromise choices for continued operation, An example is a hot bearing alarm and associated machine shut down, There are many other finctions which are monitored and controlled in a similar manner.

Besides selecting the best operational choices to select continuously the SLC can report information to a control console on the decanter centrifuge so that the operator can observe the operation. At a glance the derrick manor other concerned party can see the loading of the decanter centrifuge from the currents that are displayed and the overall operational status from alarm lights or messages. The SLC will accept interrupts and inputs from the operator depending on the mode of operation, However, the SLC is also the system watchdog, it will not tolerate inputs or requests that it has been instructed to avoid. If the operator attempts to select a differential speed that the SLC knows is below the lowest acceptable speed for the particular loading and limits that have been set it will disregard the new input. For example if the maximum bowl speed it will accept from the console is 3400 rpm and the operator attempts to exceed this ceiling then a warning indicator and or message will come on and the SLC will not carry out the requested change. Once again all of these operating limits are input or changed as required at rig up and they can be changed by password authorization anytime.

Since the SLC is reviewing the incoming data stream from the drives very quickly it can draw curves and extrapolate trends much faster than an operator can react. The SLC is powered through a Universal Power Source so that if and when the power is interrupted the logic for the operation of the decanter centrifuge will last for an additional 30 minutes. After that period of time batteries in the drives and SLC will maintain the program memory and logic. The battery in the SLC will maintain its memory so that all functions, programs and limits will survive no matter what happens to rig power.

The SLC will follow the command of whatever program is selected and loaded for the current application. This means that different programs can be loaded in about ten minutes for different operational situations. For example if there is not a VFD pump available for a particular job then simply install the program for the conventionally powered pump, This program will differ in that the logic will turn the pump on and off based on limits instead of varying the output flow rate in order to control the rate of feed to the decanter centrifuge. This program is often used in oil mud situations where low flow rates are required and a moyno type pump is necessary. The flow rate for the moyno pump is selected at the pump by the pumps mechanical speed control and then the SLC will turn the pump on and off based on real time events when the loading becomes excessive etc. Programs for multiple automated decanter centrifuge operation and links for rig instrumentation are a disk away all made possible by the versatility of the SLC and its microprocessor.

Operational Modes

The conventional program which has been used the most recently has three basic modes of operation:

  1. Cut Point – Decanter centrifuge parameters are varied to maintain a specific bowl speed. This can be a high bowl speed for a fine cut point where the decanter centrifuge is extracting very small sized solids or it can be a relatively low speed for barite recovery for example.
  2. Maximum Solids Discard – Here the SLC monitors the decanter centrifuge operation and his its logic force the system to a maximum conveyor loading for as high a flow rate as possible. Normally this extracts larger solids first in the sequential circulation rate based integrative process.
  3. Manual – Here the operator selects a particular speed or speed range for the bowl and then the SLC modulates the other parameters to maintain a maximum conveyor loading for that particular bowl speed.

The pump can be either operated as a variable flow controlled pump or a constant flow rate pump in either Cut Point or Manual operation. The SLC recognizes when the decanter centrifuge has stabilized or has stabilized enough for further adjustment. In many cases especially in water based mud situations the time for the decanter centrifuge to stabilize after a change is over ten minutes. This is long reaction time is caused by the complex events that are going on inside of the bowl at the conveyor, bowl interface and the degree to which the solids adhere to each other. The SLC will amplify the parameters which indicate stabilization and make decisions accordingly. The speed at which reactions are taken are affected by the limits and setup increments. If bowl speed changes will be in increments of 100 rpm then major variations of feed mud can be accepted without external demonstration of high currents or torques but finer adjustments will not go past decanter centrifuge tuned optimum operational points for best possible speed based on a steady stream of incoming fluid, Dilution control and polymer addition control are additional functions that can be adjusted by the SLC. Dilution is the attempt to modify the viscosity of the fluid carrying the suspended solids so that the solids will gravitate to the wall of the decanter centrifuge quicker. In oil mud situations a dilution of two gallons per minute will have a dramatic effect in increasing the solids discard rate. Other additions such as water to water wet the solids can be added at the pump and the discard rate from oil mud will be multiplied dramatically.

In some cases it is beneficial to run the decanter centrifuge at very low feed rates to attempt to discard all of the solids possible in one pass through the machine, This is beneficial in drilling situations after fishing jobs or whenever there has been a number of slug additions to the mud system which have raised the mud weight above recommended. The result is similar to dilution of the mud system with oil in that the decanter centrifuge will discard as much of the fine solids as possible with the barite that it sees. In this situation the decanter centrifuge is discarding the maximum amount of drilled solids possible to give the greatest benefit to the mud system. Oil mud sludge is far less resistant to transport, the solids stick to themselves much less. Consistently higher back drive currents can be earned with oil base mud operations without danger of a regenerative plugging situation which can happen with water based muds, Once again selecting the proper limits for the mud system in use will avoid possible plugging situations. One variable which this system does not have direct control over is the liquid level in the bowl, The liquid level in the bowl affects the length of the beach in the conical section of the bowl and by that the cut point of the device. A constant liquid level was maintained in the bowl for this development for best overall operation and cut point considerations, If the liquid level in the bowl is changed mechanically the SLC’ will react to lower liquid levels just like it reacts to large solids which create high fiction in proportion to their concentration. The SLC will compensate for these frictional increases by modifying conveyor differential, bowl speed and pump rate.

Particle Size Distributions – Results

These graphs show the effectiveness of the decanter centrifuge at extracting solids depending on the sizes of the solids that are being processed. In some cases the discard will show larger solids than appeared in the feed, This is because of the related rates of the feed and discard, use ten to one as a possible ratio of feed to sludge. With a tenth of the large solids present in the feed the particle size analyzer will overlook small concentrations of that particular size. The particle size analysis were taken on a Laser Particle Size Counter 9064. Occasionally the samples especially the sludge samples were screened prior to the test being taken but the maximum volume of solids that was screened out was always less than three percent. Over one hundred distributions have been taken.

For this situation the conveyor differential and bowl speed have been increased to maintain a back drive current limit of no more than 6,2 and a maximum main drive current of 43 amps. The sludge rate out at this high flow rate would be approximately the same as discarding 13 gallons of whole mud per minute for a mud weight of 9.8 with no oil or chlorides. In general the benefit of automation on water based muds is based on increased discard rates of sludge compared to standard decanter centrifuges. Increased discard rates relates to less whole mud jetted to the reserve pit or whatever for disposal in addition to chemical costs savings and improved mud properties. The sludge rate out of the decanter centrifuge is a function of the speed of the bowl, the conveyor differential, the feed rate, the solids concentration, distribution of the sizes of the suspended solids and the viscosity of the fluid. Once the limit for the feed rate from the pump has been attained then the bowl speed to keep the conveyor loaded within limits is attained. The high sludge rate is important in surface and upper hole drilling. With approximately 50°/0 solids in the sludge this is still only 5% of the solids generated by a 17 ‘/2 hole with minor washout at 3 feet per minute. Often two or three decanter centrifuges are necessary due to this high volume of solids to be extracted depending on the formations penetrated and the effectiveness of the rig shaker screens. In long term operation the decanter centrifuge will slowly adjust its self and as time goes on you will see a change in the appearance of the sludge.

As the decanter centrifuge runs for over three circulations on the order of six hours the discard from the decanter centrifuge may change in appearance depending on drilling rate. The sludge rate will continue to be in excess of two gallons per minute but the bowl speed will be increased so that finer and finer solids are being discarded. Due to the surface area of the solids involved and the corresponding volume of water necessary to wet this surface area the discard will have a much wetter appearance and will no longer feel gritty but feel more like grease to the touch. In slow drilling hard spots this effect is more obvious where the rate at which solids are being generated is not balanced by the rate of solids being discarded. With standard Decanter centrifuges the appearance would be a drop in sludge rate to less than one half a gallon per minute and that would be only 30 to 35% solids. Tie same effect can be seen at the desliter cyclones. When the solids are in a range that the desilter will extract then there will be a spray discharge of significant volume carrying these solids out but when the solids are smaller than the cut point for the cyclone then the discard from the cyclone will dry up and the continued operation of the pump to feed the cyclone will just act to continue to grind the solids smaller and smaller to the extent that dilution will be necessary eventually. It is important to recognize that the vector of solids being circulated around is constantly getting being offset to the left. The solids are being broken and fractured constantly by the shearing at the bit and by all of the mud handling equipment so a concert effort to control this fine solids generation needs to be made.

Water based operation at reduced flowrate
Water based operation at reduced flowrate

The same water based mud here is pumped at a reduced rate of 140 gallons per minute with the pump in the manual mode. The solids being discarded are coarser but dryer with almost the same median of the solids for the liquid being returned to the active mud system. The main drive current has fallen substantially and the back drive current has gone up due to the dryer discard and associated increased friction.

Sludge particle size influences
Sludge particle size influences

Situations like this show the benefit from modulating the parameters based on real data instead of extrapolating from generated math models. The decanter centrifuge is removing more fine solids at a lower bowl speed with very high conveyor back drive loading. In some situations small changes in speeds affect the braking of solids at the feed bowl interface where the solids are accelerated to the speed of the bowl. Oil base solids are much easier to deal with and separate out more efficiently. A possible contradiction to this is when the decanter centrifuge is processing oil mud which is stored and is not at the usual temperature and viscosity that would be found on a drilling setting. Lowered temperatures will give conventional oil muds viscosity in excess of 300 seconds per quart. Separation of solids from a liquid with this degree of increased viscosity is comparable to the bb dropping in jello as compared to falling free in water or alcohol. The only technique which was found to be help fill other than heating the oil mud was to dilute it with additional oil to change the oil water ratio.

Oil mud at 2600 RPM
Oil mud at 2600 RPM

This feed rate was fixed for these tests, the pump was in manual mode so it was controlled only by the upper bound limit of the SLC. The main drive and back drive currents are shown for comparison purposes. This is an example of the drilling stripping operation where as much solids as possible were extracted with as fine a cut point as possible. Days later a similar situation developed.

Oil mud at 2800 RPM bowl speed
Oil mud at 2800 RPM bowl speed

The solids’ in the feed were slightly higher and the SLC selected a higher bowl speed but the effectiveness of the decanter centrifuge was about the same with the same median point of about 1,65 microns, The sludge here was slightly dryer and the back drive current went up accordingly.

Oil mud feed distributions after a short time interval
Oil mud feed distributions after a short time interval

This shows a slug of oil mud coming around even thought the mud weight is approximately the same the concentration of calcium carbonate is evident. The decanter centrifuge went to a lower bowl speed and the discard rate was maintained over 2 gallons per minute without excessive changes in conveyor differential or changing feed rate.

Oil mud with its higher currents on the back drive cause problems which only logic can contend with regarding the impact of VFD application to decanter centrifuge automation. The torque on the back drive motor is opposite in direction from the bowl. This causes the back drive to regenerate current and at excessive rates on oil mud operation. The drives will dissipate up to10% of the rated size of the drive. It is common for oil mud operation when dry discard sludge is necessary to exceed this by three times or more. Another area of contention mentioned earlier is the brownout control problem and the unique response of the decanter centrifuge back drive failure. It should be clear that back drive motor failure in a situation where the decanter centrifuge is intentionally loaded at 85°/0 is not acceptable. One of the principle goals of automation was to achieve greater reliability so brownout failure would not be tolerated at all, Initially conservation was considered as a survival technique. Limiting the current consumption of the drives was quickly recognized as useless. Brownout is defined here as a short period of time where the output voltage from the generator or SCR is at least 50 volts below nominal.

This degree of voltage drop would normally violate the limits of a conventional variable frequency drive, When in early development a brownout resulted in back drive failure it would also result with a plugged off decanter centrifuge. Without a back drive motor the situation is completely different. The main drive motor will restart on the fly since the rotational inertia of the bowl will continue to cause the conveyor to rotate relative to the bowl, The restart current will be relatively low and the way the machine will react is very similar to a single speed three phase motors reaction on the main drive when the drive is restored. The drawback to this design is that you cannot vary the conveyor speed to maintain a desired bowl speed and cut point. Throughout the development the bowl speed, back drive speed, currents and incoming power levels were monitored.

example of parameter recording
example of parameter recording

Of course in ordinary operation the increments on the speed of the main drive and back drive motors are much smaller. They are on the order of 25 to 40 rpm therefore the imposed changes occur much more slowly. The record above has larger increments for demonstration purposes only to show the related changes. The degree to which the SLC searches for correct speeds depends on the width of the limit windows which are chosen. The limits and ranges between and overlap of limits depends on the mud system and the intended mode of operation. There are cases where the limits have not been changed over the duration of three fifteen thousand foot wells in a row. The example above also shows the deviation of the incoming power and at the same time the response of the SLC and the decanter centrifuge as a whole.

Operation Controls

Once rigged up and running all the rig crew needs to do is to press the start button for the back drive and then once it comes to speed and press the start button for the main drive, Initially the main drive would automatically start but it was determined that most operators prefer to see the back drive in operation and see the extent to which the bowl turns with the back drive to give an indication of bowl cleanout condition. It is common for the main drive to rotate at low speed just due to fiction caused by solids that have not been cleaned out of the bowl, high back drive current will initiate a rinse cycle. Once the main drive motor has come to speed by its acceleration ramp it will run for a predetermined time until the pump comes on automatically. The SLC will increase the feed rate from the pump by increasing the speed of the pump until steady state operation is achieved, The SLC constantly monitors the loading of the conveyor and maintain that level according to the mode that was selected.

VFD Decanting centrifuge
VFD Decanting centrifuge

Pressing the stop button will send a signal to the SLC which initiates a safe shutdown sequence including a dry timeout run with a rinse option during bowl deceleration, The bowl can be dynamically braked according to the limits from the SLC depending on the inertial load. The control console is intrinsic safe to meet industry safety standards, all motors and wiring is according to CSA standards, The VFD differences only required inverter duty motors which may be slightly larger in frame size but other than that the appearance is very similar to a standard decanter centrifuge.

Additional benefits to automation relate to maintenance and operational considerations. It is quite easy to diagnose worn bearings when you know the bearing temperatures. After a short orientation period the rig crew recognizes dry current levels that indicate lubrication. If the bearings are not lubricated and the temperature and current become excessive then alarms may bring attention to the situation. If not a shut down will occur shortly to avoid damage to the equipment. The SLC keeps a record of the duration of operation on every job and various operational parameters which would describe the job status. Communication to a rig instrumentation package has been accomplished through both RS232 link and hard wire output form the SLC, With either 0 to 10 volt signal or a 4 to 20 MA signal to the instrumentation. The combination of flow rate, hours in use, speeds and other data can be used for many purposes for benefit on the rig.


  1. After a development period of over two years an automated decanter centrifuge can increase the solids separated by as much as 60% over a standard decanter centrifuge over the length of a drilling job. This increase is attained by the machine automatically changing the operational speeds including but not limited to bowl speed, conveyor speed and pump rate.
  2. Major difficulties can be surmounted by force of programming including brownout survival.
  3. Different mud systems can benefit universally, with higher conveyor loading on oil mud systems and more solids separated on a continuous basis with water based systems.
  4. The rig crew accepts the new technology since it makes their job easier. The complexity stays inside the SLC the external controls and displays are simple and easy to understand.
  5. Safety is not compromised, automation does not conflict with electrical safety standards.
  6. The results exceeded the expectations of the oil company supervisors and the drilling contractor also with less down time than a standard decanter centrifuge.
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