1. Structure and principle
PDM Drill is composed by Bypass valve assembly, motor assembly, Cardan shaft assembly and transmission shaft assembly. The drill transforms energy from the high pressure fluid into mechanical energy through the stator and the rotor. When the high pressure fluid flows into the inner hole of the drilling tool, the bypass valve is switched off. The fluid is then sealed in the chamber formed by the stator and rotor. Due to the pressure difference the rotor is forced to roll along the spiral channels of the stator. Therefore, the rotor is rotating around its own axis while revolving around an axis parallel to the central line of the stator. This is the so-called planetary transmission principle of the PDM drill. Due to the reverse spirals used by the rotor and the stator, the rotor has a counterclockwise rotation around the rotor axis and a clockwise rotation, which drives the drill bit, around its own axis. The output torque of the drilling equipment is proportional to the pressure drop of the high pressure fluid running through the motor; the output speed is proportional to the output volume.
1.1 Bypass valve assembly
Bypass valve is composed by valve body, valve core, valve cover, spring and O-shape circle. The bypass valve is used to link the liquid inside and outside the drill string: when there is no circulation, the spring keeps the valve core at its original position and the bypass channel is open; when the mud displacement reaches a certain level, the pressure force the valve core to move and the bypass channel is closed, and the mud flows into the motor. When the pump is stopped, the spring will push the valve core to its original position and make the bypass channel open, which then made the liquid inside the drill string connected to the outside liquid.
1.2 Motor assembly:
The motor is multi-stage displacement motor, which is composed by the stator and rotor. The stator comprises a high quality steel alloy shell and rubber liner. The rubber liner is a left-spiral face type chamber, which is oil resistant, abrasion resistant and heat durable (the safe working temperature of the stator is -29~120ºC,-29~150ºC); The rotor is made of heat treated no-stress steel alloy with a hard chromium layer, this can effectively prevent corrosion and abrasion. The rotor and the stator form a series of sealed cavities which are isolated from each other. When the driving liquid goes into the motor, the energy of the liquid is transformed in to mechanical energy, this will form a kinetic moment and force the planetary motion of the rotor inside the stator.
1.3 Cardan shaft assembly
The Cardan shaft assembly is composed by the shaft shell and the shaft. The upper and lower ends of the shell are connected to the stator of the motor and the shell of the transmission shaft respectively. The upper and lower ends of the shaft are connected to rotor of the motor and the transmission shaft. The Cardan shaft assembly is used to transmit the planetary motion of the rotor into the rotation of the transmission shaft. The Cardan shaft assembly is made via special process, which makes the driving smoother, the speed being constant and the vibration smaller. This structure completes the transformation between energy and motion and the transmission of the energy.
1.4 Transmission shaft assembly:
Transmission shaft assembly is one of the major components of the PDM drill. The upper end of the shell is connected to the shell of the Cardan shaft, the flow deflector cap of the transmission shaft is connected to the Cardan shaft, and the lower end is connected to the drill bit. The axial load caused by drilling pressure is bore by several thrust ball bearings. Radial bearings, which are made of sintering hard alloy, are mounted on both ends of the transmission shaft to bear the radial load caused by the deflection torque.
Most of the mud discharged by the motor is drained through the drill hole in the transmission, which is useful to cool and clean the drill bit. Other part of the mud is drain through the radial and thrust ball bearings, which can cool and lubricate the bearing system.
2. Specifications and technical parameters:
2.1 Specifications and parameters of borehole drilling drills
|Recommended borehole size mm||118-152||118-152||118-152||118-152||149-200||213-251|
|Input flow rateL/S||4-8||6-10||6-10||8-12||9-14||16-24|
|Load pressure drop Mpa||3.2||3.2|| 3.2||3.2||3.2||3.2|
|Output torque N.m||600||900||1000||1000||1400||3500|
| Speed r/min||180||180||150||180||180||120|
|Output power KW||6-11||8-16||10-16||10-16||15-23||39-55|
| Recommended drilling pressure T||2||2.5||2.5||2.5||3.0||7.5|
|Length of the drilling tool mm||3800||3800||4350||4350||4500||7300|
|Weight of the drilling tool Kg||165||180||180||200||280||900|
|Thread connection(upper)||2 3/8REG||2 7/8REG||2 7/8REG||2 7/8REG||3 1/2REG||4 1/2REG|
|Thread connection(lower)||2 3/8REG||2 7/8REG||2 7/8REG||2 7/8REG||3 1/2REG||4 1/2REG|
|Diameter of the drilling toolmm||90||95||95||100||127||165|
2.2 Specifications and parameters of borehole repairing drills
|Recommended borehole size mm||60-76||89-114||114-152||118-152||118-152||120-150||146-200|
|Input flow rateL/S||2-3||3-4||6-8||6-8||8-10||9-12||10-12|
|Load pressure drop Mpa||2.2||2.4||2.8||2.8||2.8||3.2||3.2|
|Output torque N.m||180||300||900||1000||1000||1400||1700|
| Speed r/min||250||240||180||150||200||180||180|
|Output power KW||4-10||6-11||8-14||10-16||10-16||12-18||15-22|
| Recommended drilling pressure T||0.5||0.8||1.5||1.5||2.5||3.0||3.5|
|Length of the drilling tool mm||2080||2700||3000||3200||3500||4200||4300|
|Weight of the drilling tool Kg||30||85||110||130||160||185||260|
|Thread connection(upper)||1.900TBG||2 3/8TBG||2 7/8TBG||2 7/8TBG||2 7/8TBG||2 7/8TBG||3 1/2TBG|
|Thread connection(lower)||1.900TBG||2 3/8TBG||2 7/8TBG||2 7/8REG||2 7/8REG||2 7/8REG||3 1/2REG|
|Diameter of the drilling toolmm||54||73||90||95||100||100||120|
Stators with 150ºC durable rubber liners can be customized upon request. (The torque between such rotor and stator pair is 90 ~ 110 NM)
2.3 Drills can be made into drills with certain angle for horizontal directional borehole.
User Guide for directional PDM drill:
2.3.1 Angle building rate
Angle building rate is concerned by directional PDM drill users. Due to the various structure types of directional PDM drills, users usually choose various drill combinations, together with the influence caused by different strata structure, it is not appropriate to simply provide the angle building rate.
In respect of the directional PDM drill structure, the angle building rate can be affected by:
1) Generally, the bigger the bending angle, the greater the angle building rate.
2) The use of bent Cardan shaft shell can make a greater angle building rate than the bent connection.
3) Stabilizer and back-up plate is useful to increase angle building rate.
For directional PDM drills with asymmetric stabilizers, it is necessary to make the bent Cardan shaft shell/the angle of the bent connector and the asymmetric stabilizer to be co-planar. The misalignment will deplete the angle building rate and make the borehole location difficult to control.
In the drilling process, the angle building of the directional PDM drill is represented by the variation of the deviation angle or the location of the borehole. For the angle building process, the bend of the directional PDM drill and the deviation angle have to be co-planar. When the angle building rate is insufficient, attention shall be paid to the variation of the borehole location, to ensure the co-planar condition of the bend and the deviation angel. For direction torsion, the bend of the directional PDM drill and the direction angle have to be co-planar.
2.3.2 Complex Drilling
When drilling using the directional PDM drills, complex drilling can be implemented by using the rotary plate. The complex drilling is mainly used for the continuous control of directional drilling, which uses a PDM kit. By using a single kit of drilling tools, angle building, holding and dropping can be completed. Complex drilling can also stack the rotating speeds of the rotary plate and the drill, increase the speed of the drill bit, and consequently increase the mechanical rotating speed.
The directional PDM drill is with bent connector/ shell and stabilizer. Due to the bigger drift of the drill bit, there is a very big lateral force, which can cause downhole accident. Therefore, when using drill combination with big angle and shift distance, the use of rotary plate should be avoided.
The recommended bend angle for complex drilling is below 1° (Max 1.5°); the maximum rotating speed of the rotary plate is 60r/min; the drilling speed should be less than the 15% of the recommended speed. Exceeding these limitations will cause severe damage to the drill and may cause downhole accident.
3.1 Requirement for drilling liquid
PDM drill can work effectively with any type of mud, including oil-based mud, emulsion mud, clay mud and clear water.
The viscosity and gravity have small influence on the drilling tool, but have direct influence on the pressure of the whole system. If the pressure under recommended discharge is bigger than the rated pump pressure, the mud discharge or the pressure drop through the drilling tool and drill tip has to be reduced.
The impurity in the mud will affect the performance of the drilling tool and increase the abrasion speed of the bearing and motor stator, therefore the impurity content in the mud has to be confined less than 1%.
Each type has its own input flow rate range. Generally the optimum rate should be the median value of the range.
3.2 Requirement of the mud pressure
When the drill is in air, the discharge remains constant and the pressure drop of the mud remains constant. When the pressure of the drill increases, the pressure of the circulating mud increases, and so does the pump pressure. The operator can use the following equation to control the operation:
Drill pump pressure= Circulation pump pressure + load pressure drop.
Circulation pump press is the pump pressure when the drill has no contact with the bottom, sometimes referred to as off-bottom pump pressure. Note that the off-bottom pump pressure is not a constant, and it changes with the depth and the characteristics of the mud. In practice, it is usually approximated by the off-bottom pump pressure when connecting single pipe.
When the drill pump pressure reaches the recommended maximum pressure, the drilling tool will provide optimum torque. When the pressure is beyond the rated value, the motor may brake, at this time the drill pressure should be reduced to avoid damage to the drilling tool.
The torque is proportional to the pressure drop of the mud flowing through the motor, and the rotating speed is proportional to the input flow rate. When the input remains constant, the torque increases while the rotating speed is invariant. The speed variation is usually less than 10% from empty load to full load.
All the connection threads between each part of the PDM drill are already glued and tightened according to rated torque before they leave the factory. There is no need to tighten again before use.
4.1 Above ground check
Lift the drilling tool and put it into the slip in the rotary plate using the lift sub and make the bypass valve on top of the rotary plate, mount the safety clamp, remove the lift sub, check the flexibility of the bypass valve (use a stick to press the core and release, the core should get back to its original position, try 3-5 times), then move the bypass hole to a position under the rotary plate and start the pump. Start the motor and check if the driving connector is rotating. Stop the pump, the valve should be reset and the mud should discharge from the bypass hole.
4.2 Put the drilling to into the hole
4.2.1 When putting the drilling to into the hole, the dropping speed must be strictly controlled to prevent tripping and damage to the drilling tool.
4.2.2 When in the deep hole or high temperature zone and flowing sand zone, the mud should circulating regularly to cool the drilling tool and protect the rubber in the stator.
4.2.3 When approaching the bottom, the speed needs to be reduced.
4.2.4 Free fall of the drill and leaving the drill at the bottom should be avoided.
4.3.1 Clean the bottom and circulating pump pressure before drilling.
4.3.2 Increasing the drilling pressure slowly when started drilling. When drilling normally, the operator can control the operation using the following equation: Drill pump pressure= Circulation pump pressure + load pressure drop.
4.3.3 The drilling speed is preferable to be slow when started to drill.
4.3.4 The torque of the drilling to is proportional to the pressure drop of the motor, therefore increase the pump pressure can increase torque.
4.3.5 Drilling evenly can ensure a smooth hole and an accurate direction.
4.4 Check the drilling tool when pulling out the drilling tool.
4.4.1 Clean the bypass valve with clear water, move the valve core with a stick to ensure its flexibility.
4.4.2 Use the tong to fix the drilling tool, turn the driving connector clockwise, inject water into the bypass valve and inject some mineral oil into the motor.
4.4.3 Pay attention to the pulling out speed to prevent.
4.4.4 Measure the intervals between bearings, if the distance exceeded the allowed maximum deviation, maintenance or exchange is required,
5. Trouble shooting
Table 3 Trouble shooting
loss sudden rise
|Motor speed loss|
|Lift Screw Drill for 0.5m, check circulating pressure, slowly enhance drilling weight .If gauge pressure rises accordingly in normal condition, the problem is speed loss.|
|Motor drive shaft stuck and bit nozzle plugged|
|Lift bit away from the bottom. If gauge pressure is still too high, only lift out the drill for checking or to change the bit.|
|Gauge pressure slowly increasing (not means increase of the normal pressure loss with the increase of drilling depth||Bit nozzle plugged|
|Lift bit away from the bottom, again check the pressure. If the pressure is still over circulating pressure, try to improve circulating flow rate or to move the drill pipe up and down. If it is not solved, only lift out the drill for repair or change.|
|Bit wear out|
|Go on operating for careful watch. If still no footage rate, lift out the drill for change.|
|Formation change||Lift the drill a little bit .If the pressure is same with circulating pressure, it may go on working.|
|Gauge pressure slowly drops|
|Fluctuation of circulating pressure loss||Check fluid flow rate|
|Drill pipe damaged|
|Lift the drill a little bit. If the gauge pressure is still lower than circulating pressure, lift it out for examination.|
No footage rate
|Motor speed loss||Gauge pressure increases, lift the drill away from the bottom, check circulating pressure, to increase drilling weight gradually.|
|By-pass valve in OPEN position|
|Gauge pressure is too low, lift the drill a bit and start and stop the pump twice. If it is not solved, lift it out to check or change bypass valve.|
|Cardan shaft damaged||Often with pressure fluctuation, lift the drill a bit, less fluctuation range. Only lift it out for checking and changing.|
|Bit wear out||Change new bit|
The service life of the drilling tool can be affected greatly by the maintenance in addition to the careful design, precise manufacturing and appropriate use. Careful wellhead maintenance for the drilling toll is required. For drilling tools with its inter-bearing internal exceeded maximum deviation,repairing is required.
6.1 Disassemble the drilling tool
6.1.1 After use, if faulty is diagnosed by technicians and the drilling tool cannot be used, the drilling tool needs to be sent to the maintenance department for check and repair.
6.1.2 Be familiar with the structure before disassemble the drilling tool.
6.1.3 The disassembling of drilling tool comprises the shell and the inner transmission connectors. Because the joints (Cardan shaft shell and motor transmission shaft shell, Cardan shaft and rotor, transmission and the flow deflector cap) are glued with anaerobic adhesive, the joints need to be heated up to 250-300ºC and disassembled quickly when disassembling, rather than use big torque to force off. The residual adhesive in the threads needs to be removed.
6.1.4 Before disassembling, record has to be made (serial no., operating company, footage, operating time, temperature, intervals between axes, reason for maintenance, etc.)
6.2 Maintenance of the bypass valve assembly
6.2.1 Clean and check every component. If there is any groove, damage or craze of coating, the component needs to be changed. Change all the O-shape circles.
6.2.2 Change the springs after 300hrs in use.
6.2.3 Spread butter on the surface of the components. After fitting, ensure the flexibility of the valve core.
6.3 Maintenance of the motor assembly
6.3.1 Take out the rotor, clear the inner chamber of the stator and the thread.
6.3.2 Check the rubber on the stator (unglued, flake), change the stator if necessary.
6.3.3 Check the coating of the rotor, change the rotor if necessary.
6.3.4 After checking and cleaning, spread butter on the rotor surface. Put the rotor back to the stator. Conduct the loss and rotation moment experiment with the motor tilted to 30 degrees to check if the motor needs to be replaced.
6.4 Maintenance of the transmission shaft assembly
6.4.1 If the distance between the bearing s exceeded the allowed maximum deviation, or there is obvious groove, pitting, cracked steel balls, the bearing unit needs to be replaced.
6.4.2 New and old steel balls cannot be used together. Steel balls without sifting cannot be used either.
6.4.3 Check the inner and outer circle of the upper and lower radical bearings, if there is coating separation, alloy block damage or the diameter reduced by 1mm, the radical bearings need to be replaced.
6.4.4 Check the transmission shaft, replace if there is any crack.
6.5 Assemble of the drilling tool
After the bypass valve assembly, motor, Cardan shaft, transmission shaft are checked and assembled, the shells and inner transmission component should be connected, the threads should then be cleaned, brushed with LETAIR 227 anaerobic adhesive and tighten with appropriate torque.