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Mud Using Downhole Drilling

Report #3: Market Research, Initial Design Concepts, and Final Design

Members:

Avinash Cuddapah

Vivek Ghosh Travis Sanders Carlos Venegas

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Contents Abstract ......................................................................................................................................................... 5

Introduction .................................................................................................................................................. 6

Nomenclature ............................................................................................................................................... 7

Scope ........................................................................................................................................................... 10

Overview ................................................................................................................................................. 10

Goals ....................................................................................................................................................... 11

Market Research ......................................................................................................................................... 12

Project Background ................................................................................................................................. 12

Coil Tubing .............................................................................................................................................. 12

Drilling Fluid ............................................................................................................................................ 14

Types of Mud Motors.............................................................................................................................. 16

Positive Displacement Motor .............................................................................................................. 16

Downhole Turbine .............................................................................................................................. 16

Electric Motor Drill .............................................................................................................................. 16

Standards ................................................................................................................................................ 17

Materials and Components......................................................................................................................... 18

Power Section ......................................................................................................................................... 19

Transmission Section .............................................................................................................................. 20

Bearing Section ....................................................................................................................................... 21

Design and Analysis ..................................................................................................................................... 22

Mud Motor Type Selection ..................................................................................................................... 22

Room for Improvements ......................................................................................................................... 24

Elastomer Stator ..................................................................................................................................... 25

Stator Tube.............................................................................................................................................. 28

Rotors ...................................................................................................................................................... 29

Transmission Section .............................................................................................................................. 30

Bearing Section ....................................................................................................................................... 33

Calculations ................................................................................................................................................. 37

Testing ......................................................................................................................................................... 39

Budget ......................................................................................................................................................... 40

Timeline....................................................................................................................................................... 41

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Resources .................................................................................................................................................... 42

Team Members ....................................................................................................................................... 42

Faculty Advisors ...................................................................................................................................... 43

Industry Advisors .................................................................................................................................... 43

References .................................................................................................................................................. 44

Books ....................................................................................................................................................... 44

Websites ................................................................................................................................................. 44

Web-links ............................................................................................................................................ 44

Appendix ..................................................................................................................................................... 45

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List of Figures Figure 1: Cross-Section of Rotor (Light Gray) and Stator (Red) assembly .................................................. 10

Figure 2: Coil Tubing Equipment ................................................................................................................. 13

Figure 3: Drilling Fluid ................................................................................................................................. 14

Figure 4: Coil Tubing Units running currently in the world in 2009 ............................................................ 15

Figure 5: 2.375 API REG Box connection………………………………………………………………………………………………. 18

Figure 6: Rotor with chrome coating……………………………………………………………………………………………………. 19

Figure 7: Stator Tube and Elastomer……………………………………………………………………………………………………. 20

Figure 8: Positive Displacement Motor Components……………………………………………………………….…………… 21

Figure 9: Typical rotor stator combination (on right) and even-walled stator and rotor (on left) ............. 27

Figure 10: Elastomer Fit Change with Temperature ................................................................................... 27

Figure 11: Hysteresis (Courtesy of Dyna-Drill) ............................................................................................ 28

Figure 12: Pro Engineer Rotor………………………………………………………………………………………………………………. 29

Figure 13: Pressure Force on Rotor ............................................................................................................ 30

Figure 14: Transmission Weight on Bit ....................................................................................................... 32

Figure 15: Transmission Housing- O.D. = 2.875” ........................................................................................ 32

Figure 16: Torsion Analysis on Flex Shaft .................................................................................................... 33

Figure 17: Bearing Assembly ....................................................................................................................... 35

Figure 18: Axial Bearings ............................................................................................................................. 35

Figure 19: Bearing Von Mises ..................................................................................................................... 35

Figure 20: Bearing Mandrel Displacement ................................................................................................. 36

Figure 21: Output Variables based on Differential Pressure and Efficiency ............................................... 37

Figure 22: Dynamometer used for Testing ................................................................................................. 39

List of Tables Table 1: Mud Motor Type ........................................................................................................................... 17

Table 2: Design Options .............................................................................................................................. 23

Table 3: Elastomer Types ............................................................................................................................ 26

Table 4: Material comparison for stator tube and all external parts ......................................................... 29

Table 5: Drive Shaft Selection ..................................................................................................................... 32

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Abstract

The following paper lays out the workings and components of a typical mud motor used

in the industry today, as well as the background on the mud motor. Team MUDD is determined

there is a better solution for providing the drill bit power through means of transferring fluid

energy into mechanical energy. By the end of this project Team MUDD is certain they will

produce a high quality product to be used in the coil tubing industry worldwide. This project will

help the students prepare for a professional career, where good teamwork is essential.

Designing the mud motor will also allow students to strengthen their knowledge of dynamics,

fluid mechanics, and strength of materials. Also, this project should help the team members in

gaining contacts in the industry as well as a general understanding of downhole drilling tools.

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Introduction

A mud motor is used in the downhole drilling industry in order to supply rotational

power and torque to the drill bit. Mud motors are a necessity in coil tubing drilling, considering

the coil tubing that takes the place of drill piping cannot be rotated. Over the past few decades,

coil tubing units have been on the rise as new ways to drill more efficiently are used. Coil tubing

units are typically much faster and cheaper to run than conventional drilling using drill pipe. The

mud motor supplies rotational power to the drill bit through means of the drilling mud being

pumped through the coil tubing and through the mud motor. The mud motor design has

remained unchanged for over fifty years. There is a need for a better performing and reliable

coil tubing mud motor.

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Nomenclature a, b = empirical indices

a1 = WOB exponent

a2 = speed exponent

A = cross-sectional area

A = wave velocity

Ac = cross-sectional area of the cavity

Arn = area of the rotor nozzle

Ap = area of the piston

Af = cross-sectional area of the shaft

Ap = effective pump-off area

b = bit

BF = buoyancy factor

C = constant

Cj = discharge coefficient

Cr= mud correction factor

Db = diameter of the bit

Dh = diameter of the housing

Dr = rotor diameter

Ds = diameter of the shaft

e = eccentricity of the motor

E = Young's modulus

Ei = energy input

Es = specific energy

f = final condition

Fef = formation constant

Fhyd = hydraulic thrust

Fx = axial force

Fy = tangential force

Fs = side force

g = gravitational constant

h = height of the vane

HT = hydraulic thrust

HN = rock impact hardness number

HP = horsepower, hp

HHP = hydraulic horsepower, hp

HHPbit = hydraulic horsepower at bit, hp

i = winding/ratio configuration

k1 = housing/shaft wear coefficient

k2 = material property coefficient

k3 = housing, shaft, pitch wear coefficient

kq = coefficient, 9.48

ks = coefficient, 1.8x10*

K = constant

Kb = formation hardness, teeth, bearing, mud coefficient

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Kf = formation drillability factor, ft/hr

Ki = winding ratio coefficient

Kii = winding ratio coefficient

Krc = pressure drop coefficient

Kx = constant (5,252)

Ky = constant, 0.01

Kz = constant, 0.028

l = stroke length

L = tool length

m = maximum

mdot = mass flowrate. Ibm/sec

MHP = mechanical horsepower

nb = number of blows per minute

ns = number of stages

N = rotary speed, rpm

Nr = runaway speed in rpm

NT = net thrust

Ph = pitch of the housing

Ps = pitch of the shaft

Ps = stall pressure

PR = pressure ratio

Q = flowrate

Qi = inlet losses that can be neglected

Qm, = bypass flowrate through the rotor nozzle, gpm

Qr = reference flowrate

Qs = the leakage between the running clearance between the seals

Qt = geometrical or theoretical displacement per minute

r = clearance, ft

rbar = median blade radius

rrs = radius of exposed roller section

ROP = rate of penetration

Rbp = bypass flowrate

s = turbulence coefficient

SHN = shore hardness

t = time

t1 = thickness of the elastomer of the housing

t2 = metal thickness of the housing casing

T = torque

Tas = actual torque measured at the stall condition

Tf = torque on the cutting face of the bit, ft-lbf

Ts = torque due to side force, ft-lbf

w = width of the exposed roller section

wu = mass of the U-joints in fluid, lb

wr = mass of the rotor in air, lb

Wp = pump-off force, lb

WOB = weight on bit, klb

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Wrs = width of the exposed roller section

= flow ratio

ß = index of flow time

ße = exit angle, °

m = mud weight, lbm/gal

= overall efficiency

h = hydraulic efficiency

m = mechanical efficiency

s = operating stall efficiency

v = volumetric efficiency

= bit to formation friction factor

= density, Ibm/ft^

i = incident stress

m = maximum stress

r = reflected stress

pb= pressure drop across the bit, psi

pm = pressure drop across motor, psi

pmax maximum pressure drop across motor, psi

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Scope

Overview

A mud motor consists of a few main sections that each serves a specific purpose in the

working of the mud motor. The main sections in the mud motor are the power section which

creates the power to drive the drill bit from the fluid being pumped down hole from the mud

pump on the surface. The next section is the transmission section where the eccentric rotation

of the power section is converted to concentric rotation for the drive shaft. The last section is

the bearing section, which is where everything is kept functioning and rotating properly and

smoothly.

Figure 1: Cross-Section of Rotor (Light Gray) and Stator (Red) assembly

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Goals

By the end of this design project Team MUDD plans to design and build a more powerful

and efficient mud motor used in coil tubing drilling applications. Also, our team believes we can

reduce the overall length of the mud motor as well as the cost to build the mud motor. Another

team goal of ours is to increase the reliability of the current mud motors. By the end of this

senior design project our team will have made major improvements to the current mud motor

as well as test our prototype using a dynamometer. Team MUDD plans to research new

innovative designs for a mud motor to revolutionize the coil tubing industry worldwide.

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Market Research

Project Background

A mud motor, also referred to as a drilling motor, is a type of Progressive Cavity Positive

Displacement pump or PCPD pump. This type of pump is used to transfer fluid through a

sequence of distinct cavities, as a rotor is turned by the fluid. This flow rate ̇ is produced

through a mud motor is proportional to the rotational rate of the rotor, but bi-directionally. In

the case of well drilling, the PCPD pump acts as a motor when the fluid is pumped through its

interior. These PCPD pumps are used in applications for pumping viscous or shear sensitive

materials through tubing or piping. For downhole drilling, the fluid being pumped is drilling

fluid, commonly known as mud in the industry. In the case of coil tubing applications,

compressed air or water with a suspension additive are commonly used as the drilling fluid,

though there are other drilling fluids that can be used.

Coil Tubing

Coil tubing drilling was first introduced in the early 1990’s. Coil tubing is metal piping

that typically ranges from one inch to three and a quarter inches in O.D., outside diameter. The

most typical O.D. sizes of coil tubing is 2”, 2 3/8”, and 2 7/8”. In 1997 BP, British Petroleum,

successfully field tested a 2 3/8” BHA, bottom hole assembly, in Alaska. Coil tubing comes on a

giant spool after it is produced. This is beneficial when it comes to downhole drilling

considering using conventional drilling methods the drill pipe must be assembled and

disassembled when tripping in or out of the hole. Coil tubing is one long continuous string so

that it can simply be ran in or out of the hole without any connections, except for the tools on

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Figure 2: Coil Tubing Equipment

the end of the coil tubing string. The bottom of the coil tubing string where the mud motor, drill

bit, and other drilling tools are located is known as the Bottom Hole Assembly (BHA). In coil

tubing drilling applications it is not wise to rotate the drill string considering it can buckle

helically in the hole creating massive failure. In traditional drilling, the drill pipe can be rotated

to drive the drill bit at the bottom of the hole. This is done by a rotary table or top drive at the

surface of the well. In coil tubing drilling, this rotational power supplied to the drill bit is

supplied by the mud motor, in this case, this drilling is known as slide drilling. Coil tubing drilling

has its advantages to conventional drill pipe drilling. One advantage it has over drill pipe is that

fluid can still be pumped down hole while tripping in or out of the hole, unlike drill pipe. Also,

not having to remove the piping section by section makes it a safer environment for the rig

hands. Coil tubing rigs are faster to assemble and disassemble than

conventional rigs using drill pipe. Another advantage of CTD versus

conventional drill pipe is the faster trip times. Typical coil tubing

drilling trip time are 150 ft/s or more at times. Also, fewer personnel

are required to be onsite for both rig-up and drilling applications.

The majority of these advantages are advantages on their own, but

they are also cost effective as well.

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Figure 3: Drilling Fluid

Drilling Fluid

Drilling fluid, known as mud in the industry, can be tailored to the environment of the

well it is going into. There are three main categories for drilling fluids: water-based, non-

aqueous or oil based, and gaseous. Water based fluid is distilled to prevent oxidation of the

inner components of the mud motor. It is one of the easiest to filter, clean, and circulate

through the entire drill string. The cost of this fluid also makes this a very cost effective

material to work with. Companies such as NOV and Dyna-Drill use water for testing in their

facilities. When mixed with different additives, this fluid can also act as a suspension agent for

the cuttings, creating an equalization of pressure around the housing of the mud motor. Typical

drilling mud densities range from 8.33 ppg (pounds per gallon) to 17 ppg. The viscosity of the

drilling mud is determined by means of a marsh funnel test. The typical mud viscosities range

from 26 seconds to 48 seconds according to the marsh funnel tests. Oil based mud works very

well in more extreme conditions where temperature, pressure, and speed must be precise.

Most rheologically engineered fluids are oil based and range in viscosity depending on the

formations in the well. Gaseous fluids, such as nitrogen, can also be used when the there is a

need for a small diameter drilling string.

Mud has very specific task in a well, it has the task of

providing pressure for the mud motor, lubricating open bearings,

cooling the drill bit, and equalizing pressure through the drill

string. As the mud is pumped into the drilling string it is turned

into mechanical energy by the mud motor. Once the mud reaches

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the drill bit, it returns to the surface where it is collected, filtered and re-circulated in to the

drilling string again.

Figure 4: Coil Tubing Units running currently in the world in 2009

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Types of Mud Motors

Positive Displacement Motor

This type of mud motor is the most widely used mud motor in the industry. This type of

motor operates off the Moineau theory. This theory operates of displacing fluid to make

mechanical energy. For the mud motor, it is converting fluid energy to mechanical energy in the

form of rotational speed and torque.

Downhole Turbine

The turbine motor operates using a power section consisting of a series of rotors and

stators that rotate concentrically. With this concentric rotation there is no need for a

transmission section like in the positive displacement motor. The stator deflects the fluid

against the rotor forcing it to turn.

Electric Motor Drill

The electrodrill motor operates off electrical lines that are ran down hole. This works in

conventional drilling wells, but it is still not used widely in the industry. For coil tubing drilling

applications the electrodrill motor cannot be used due to the overall OD of the coil tubing.

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Table 1: Mud Motor Type

Standards

For the team project there are a few standards set by the industry that must be

followed. The most important standard is that the OD, outside diameter, of the mud motor

must be 2 7/8”. The second major standard that must be followed is the connection placed in

the top sub of the mud motor that connects the top of the mud motor to the BHA. This

connection must adhere to the API specifications for the 2 3/8” threaded connection. The bit

box, which is the lower connection of the mud motor placed in the end of the bearing mandrel

that connects to the drill bit, also has the same threaded connection cut into it. These are the

standards that our team must adhere to for this design project.

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Figure 5: 2.375 API REG Box connection

Materials and Components

The mud motor itself is made up of many individual parts that are assembled together.

To better understand how a mud motor works, the first step is to know the parts that make up

the mud motor and what they do to make the mud motor work properly. The best way to

describe the parts of the mud motor is to discuss the parts in order from the top portion of the

mud motor all the way down to the drill bit. The top portion of the mud motor is the interface

between the drill collar, which is what adds weight to the drill bit for drilling, and the power

section of the mud motor.

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Power Section

The power section typically consists of a steel rotor with chrome coating added to it, a

stator that is made out of an elastomer, and a stator tube, that is made out of steel. This stator

tube is also the outside of the mud motor. The steel rotor is what converts the pressure and

flow from the mud to torque and rotational speed for the drill bit. The mud motor’s power

section uses different configurations for the rotor and stator to provide optimum performance

for different applications. These applications range from high torque with low speed to low

torque with high speed, depending on the needs of the company running the drilling rig. When

the number of lobes in the rotor and stator increase, or the length of these two components

increases, the return is a higher horsepower. The pitch of the helical cuts in the rotor and stator

also play a role in the speed and torque of the mud motor. When the rotor is inside the stator,

the lobes of the rotor and stator form cavities for which the mud to flow through the cavities.

The fluid will pass through these cavities forcing the rotor to turn creating mechanical energy

for the drill bit. The rotor always has one less lobe than the stator to allow one chamber to be

completely filled while the one on the opposite

side is completely empty. The continuous flow of

the mud through the mud motor is what forces the

rotor to turn inside the stator. Typical output

rotational speed for a drill bit using a mud motor

can range from 60 rpm to over 100 rpm.

Figure 6: Rotor with chrome coating

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Figure 7: Stator Tube and Elastomer

Transmission Section

The adjustable bend housing is what typically follows the power section of the mud

motor. The adjustable bend housing of the mud motor is usually oriented from zero degrees to

four degrees, depending on the application. For straight downhole drilling, a bend of zero

degrees is used considering the hole is drilled in a straight direction. Anything above a zero

degree bend is used for directional drilling. The angle is used to direct the drill string in a

specific direction based on the desired destination. The adjustable bend housing also provides

lateral force to the bit to aid in directional drilling. The next part of the mud motor is the

universal joint or flex shaft. The universal joint or flex shaft is cased in the adjustable bend

housing or transmission housing, if it is a straight vertical mud motor. The transmission section

takes the eccentric rotation of the rotor and converts it to concentric rotation for the drill bit in

the bottom of the hole. The universal joint is connected to the drive shaft, or bearing shaft. The

drive shaft is what rotates on the inside of the bearing section. The flow diverter is also housed

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in the transmission section. The flow diverter takes the mud that is being pumped through the

mud motor and transmits it to the center of the bearing shaft so that it can then travel through

the drill bit, cooling the drill bit and removing cuttings from the bottom of the hole and taking

them to the surface to be filtered out.

Bearing Section

The bearing assembly follows the flow diverter down the line in the BHA, bottom hole

assembly. This bearing assembly transmits bit loads to the drill string. The bearing shaft, which

is housed in the bearing assembly, delivers torque, rotation, and mud, or drilling fluid, to the

drill bit. There are two different types of bearing assemblies. The first is the sealed bearing

assembly, where the lubrication for the bearings is housed in the bearing assembly and does

not come in contact with the drilling mud. Also, the sealed bearing assembly ensures that the

bit hydraulic is maximized. Considering the bearings are in a sealed assembly, this extends the

life of the bearings. The sealed bearing assembly is also pressure balanced to ensure its working

in the field. The second type of bearing assembly is the mud lube bearing assembly. In this type

of bearing assembly the drilling mud used for the power for the drill bit is also used to lubricate

the bearings housed in the bearing assembly. This mud lubricated bearing assembly allows for

high operating temperatures, torques, and large loads of weight on the bit.

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Design and Analysis

The following sections present team MUDD’s initial design concepts and required

calculations to aid in improving the current mud motor design. The many possible design

improvements proposed in the following sections came from the team’s extensive market

research and meeting with industry and faculty advisors. Through our extensive research our

team believes these improvements will be very beneficial to the mud motor manufacturing

companies, as well as improve the use of the mud motor in the wellbore.

Mud Motor Type Selection

From our teams thorough market research our team has selected to design and improve

the PDM (Positive Displacement Motor). There were many reasons for our selection of the PDM

over the other two major mud motor types. For CTD (Coil Tubing Drilling) the rigs have limited

vertical space to assemble the BHA (Bottom Hole Assembly), this means that the shorter the

length of any tools in the BHA the better. The down hole turbine motor must be very long

considering its operating principle operates on fluid moving through the vanes of the motor

producing high speed. In order to increase the torque in the turbine gear reducers must be

used in the motor increasing the length considerably, which is not suitable for CTD. The

Electrodrill motor was another type of mud motor researched by our team. After researching

the Electrodrill motor, it was not chosen due to the issue of running electrical lines through the

coil tubing to power the motor. Considering the internal diameter of the coil tubing it is not

beneficial to run electrical lines through it reducing the flow area for the drilling mud. After

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discussing the pros and cons of the different types of mud motors, our team found it best

suited to the industry to decide to improve the PDM.

Table 2: Design Options

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Figure 8: Positive Displacement Motor Components

Room for Improvements

The overall working principle of the PDM is very useful for drilling, due to its torque

produced over a relatively short length and 80%-90% efficiency. Though, it also has its weak

points that leave room for improvements to be made. The typical length of a mud 2 7/8” mud

motor is 10-14 feet. Our team believes that for the coil tubing mud motor it would be very

beneficial to reduce the overall length of the mud motor while maintaining the outputs of the

longer motors. Our team believes that through our extensive research and combined abilities

that we can reduce the overall length of the mud motor by 40-50%. The typical run times of

mud motors is another area that has room for improvements. The longer the run time of the

mud motor the longer the drillstring can remain in the hole without having to trip out and

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replace the mud motor. Another area that needs improvement is the maximum operating

temperature of the current PDMs. The current maximum operating temperature of mud

motors is 500 degrees Fahrenheit. With deeper wells being drilled now, it would be beneficial

to have a mud motor that can handle temperatures up to 700 degrees Fahrenheit. Our team

will accomplish these improvements through improvements made to the individual parts

housed in the mud motor as well as selecting other high quality components. The following

sections breaks down the mud motor into the components and discusses the improvements

our team plans to make for our mud motor to be superior to what is available today.

Elastomer Stator

The major weak point in the PDM is the problems associated with the Elastomer stator.

One of the typical problems of the elastomer is called chucking, this is where pieces of the

elastomer will rip off losing the seal and start the complete breakdown of the elastomer. The

following picture shows a stator that has undergone severe chunking that our team saw at our

NOV (National Oilwell Varco) facility tour and discussion. Another problem that is seen in the

elastomer is hysteresis; this is what can lead to chunking as well. The following images show the

progression of hysteresis in the elastomer. Hysteresis is caused by high heat in the wells that

causes the elastomer to swell and gaps in the thicker portions of the elastomer form reducing

the performance of the mud motor. Elastomers are composed of the following types of

materials. The materials used are shown in the following table. The table shows the elastomers

that are currently used in the market today. After the teams extensive research into other

elastomers that are on the market, but not currently used in the elastomer for the mud motor

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resulted in no further possible implementations for a new elastomer. The one major elastomer

that the team researched was viton. Viton is an elastomer that is used in the oil industry for o-

rings.

Table 3: Elastomer Types

The one specification of viton that lead the team to wanting to implement this material

into the stator of the mud motor was the high temperature limit of viton. Viton temperature

limit is 300 degrees Celsius. The team is unable to use viton as the elastomer stator in the mud

motor is due to its inability to handle the wear of the elastomer from the rotor rotating

eccentrically inside it. After discussing this idea with the team’s industry advisors, they told us

that this was a good idea that we researched, but would not be beneficial to use in a mud

motor. After much further research of the elastomers that could be implemented into the

teams mud motor, the team has elected to go with the HSN-38 compound that is currently ran

in some mud motors on the market today. This not set in stone until it has been manufactured,

so the team will still research more elastomer materials with the help of the team’s industry

advisor Edmee Files (the chemist who formerly worked for Dyna-Drill with the elastomer

research).

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Figure 5: Typical rotor stator combination (on right) and even-walled stator and rotor (on left)

The figure on top illustrates the difference between and even-walled stator (on the right) and a

typical circular walled stator (on the left). The even-walled stator is typically twice the cost to

manufacture versus the circular walled stator. Also, the even walled stator typically produces one and a

half times the torque, with the same configuration of the power section. Though this is beneficial for the

ROP, (Rate of Penetration) the cost of the even-walled stator does not justify the extra ROP. After the

research of these values as well as weighing the pros and cons of both, the team has elected to use the

circular walled stator to help reduce the overall cost of the mud motor.

Figure 6: Elastomer Fit Change with Temperature

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Figure 7: Hysteresis (Courtesy of Dyna-Drill)

Stator Tube

The stator is made of a hard composite material or plastic and is placed in the steel tube

jacket. The rotor can be made of steel or composite material and coated with an even thickness

of a soft and durable polyurethane. The urethane offers increased wear resistance and

mechanical properties over conventional Elastomer and the even wall thickness offers. The

tube connects with the Elastomer by use of adhesives. The Adhesive system is proprietary.

Adhesives can be either solvent or water based systems. The adhesive system is typically

designed to co-cure with the rubber material and the metal stator tube. It is critical whenever

evaluating new adhesives to determine the bond strength after the oil has aged in with water

and any drilling fluid used in the field. Bond failures can occur when the fluid used for aging

attacks the bond but not the rubber. The sole purpose of the stator tube is to house the stator,

accept some of the weight on bit force, and be the outside of the mud motor in the power

section. After looking into different steel alloys to use for the stator tube, the team has decided

to use 4340 steel as the stator tube as well as all the other outside components on the mud

motor. The reasons behind choosing this steel alloy over others are mainly based on the fact of

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Figure 8: Pro Engineer Rotor

ease of machining, and the cost of this steel alloy. This steel alloy will accept all the forces

acting on the mud motor while in the wellbore drilling.

Table 4: Material comparison for stator tube and all external parts

Rotors

A small improvement that can be done on the rotor is placing a hole through the middle

of it. However, this would cause the rotor to be lighter and have less torque. The advantage of

placing a hole in the middle of the rotor is to reduce axial and radial stresses. The normal rotor

is composed of one less lobe than the stator. The typical setup is having 4 lobes on the rotor

and 5 lobes on the stator; this is known as 4/5. There are also 5/6 set ups and 7/8 set ups. The

increase of the number of lobes would increase the amount of torque. However the more lobes

there are, the closer the rotor becomes to looking cylindrical which is bad for differential

pressure. The best choice in this set up would be a 7/8 type rotor-stator since this would

provide higher torque. After the team’s extensive market research, the team concluded that

there are not many variations in the material choices for the rotor. Approximately 95% of the

rotors manufactured for mud motors are made from stainless steel, due to its coefficient of

thermal expansion and the wear resistance of the stainless steel. 17-4 SS is the material of

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choice for the stainless steel rotor. Once the stainless steel rotor has been manufactured, it is

then dipped in chromium to apply a thin coating of chrome on it. The chrome coating applied to

the rotor helps reduce the wear on the elastomer stator due to the eccentric rotation and

heavy pressure of the rotor rotating inside the stator. On the following pages shows the FEA

analysis ran on the rotor as well as the elastomer stator.

Figure 9: Pressure Force on Rotor

Transmission Section

The one main purpose of the transmission section is to convert the eccentric rotation of

the rotor to concentric rotation for the drill bit. If this eccentric rotation was not converted to

concentric rotation the bore hole would not be uniform in diameter and not be a smooth bore.

There are two main types of transmission sections; the first is the universal joint. The universal

joint is very similar to what is seen in cars, it is two rods connected together that allow for

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smooth rotation that has an offset for both shafts. The second type of transmission section is

the flex shaft. The flex shaft does the same thing for the mud motor as the universal joint

except that the flex shaft is a ductile shaft that is allowed to flex to convert the eccentric

rotation to concentric rotation. The team has studied these two types of transmission sections

and has determined for the project the flex shaft is the best route for the team to take. The flex

shaft allows the team to accomplish the goals of both reducing the overall length and cost of

the mud motor as well as simplifying the design of the mud motor. On the universal joint, at

both ends of the transmission section, there are ball bearings to aid in the conversion of the

rotation. Where these ball bearings are housed there is a sleeve that allows these ball bearings

to stay lubricated. This makes the universal joint a much more complex part in the mud motor,

whereas the flex shaft is simply one piece, and keeps the design simple, which goes a long way

in the oil industry. The following table lays out the team’s decision making process for the

transmission section.

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Figure 10: Transmission Weight on Bit

Table 5: Drive Shaft Selection

Figure 11: Transmission Housing- O.D. = 2.875”

Through much research and deliberation amongst the team, a decision was made to use

titanium for the flex shaft. If the team was to use 4340 steel alloy the flex shaft would need to

be approximately twice as long as the titanium flex shaft, due to the ductility as well as other

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properties of the materials. The downside to using titanium for the flex shaft is the level of

ability to machine the titanium. Also, threads made from titanium are not very strong and

typically break at the connections. Due to this nature of titanium, the team has decided to use

4340 steel and slip fit the threaded connection to the titanium flex shaft. The titanium flex shaft

will have to be outsourced for machining due to the nature of this metal while machining. The

following pages illustrate the FEA analysis ran on the flex shaft that the team is creating using

Titanium Beta Alloy.

Figure 12: Torsion Analysis on Flex Shaft

Bearing Section

For the bearing section of the mud motor there are also two main types to choose from.

The first type of bearing assembly is the sealed bearing assembly that was previously discussed

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in this report. The second type of bearing assembly is the mud lubricated bearing assembly. The

main difference between these two bearing assemblies is the fact that the sealed assembly

contains its own lubrication to aid in the rotation of the bearing mandrel and axial forces in the

bearing assembly, and the mud lubricated bearing assembly uses the drilling mud to both cool

and lubricate the bearings housed in the bearing assembly. The bearing assembly is a very

critical part of the mud motor and companies typically treat their designs of this section as top

secret data. Our team narrowed down the two best bearing sections; one sealed bearing

assembly and one mud lubricated bearing assembly. The best sealed bearing assembly on the

market is the one designed by Dr. Kalsi from Kalsi Engineering. The best mud lubricated bearing

assembly on the marked is the one designed by Dr. Gunther of Ashmin. After much research

and weighing of the pros and cons, the team believes the Ashmin mud lubricated bearing

assembly is the best suited bearing assembly to aid in our improvements to the mud motor.

The Ashmin bearing assembly is much shorter than the Kalsi sealed bearing assembly. Also, the

mud lubricated bearing assembly our team has chosen is one of the shortest and simplest

bearing assemblies on the market. We believe by using this bearing assembly our team will be

able to achieve our overall goals of this senior design project. Though, once the team has

acquired this bearing assembly, the team will be able to analyze the bearing assembly more in

depth and determine if any improvements can be made to the bearing assembly. Also, the

team is still playing with the idea of designing our own bearing assembly, due to the fact that as

this project progresses and the team sees more and more bearing assemblies, the team is

gaining valuable knowledge about the bearing assembly. This is due to the fact that no

company will give us their dimensions for the bearing assembly, so this is the one section in the

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mud motor that is hardest to learn key values for manufacturing. The team will research further

in the summer whether or not the team will be able to design and build a custom bearing

assembly that can be called team MUDD’s bearing assembly. One solution that the team

believes has potential is to purchase both the axial thrust bearings and radial bearings, while

designing and manufacturing the rest of the components in the bearing assembly.

Figure 13: Bearing Assembly

Figure 14: Axial Bearings

Figure 15: Bearing Von Mises

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Figure 16: Bearing Mandrel Displacement

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Calculations

Figure 17: Output Variables based on Differential Pressure and Efficiency

38

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Testing

A possible testing location has been determined and contacted. One of the test

machines that are used to test a mud motor is the dynamometer. Many different fluids can be

pumped into the mud motor to test its functionality. The test site that has been determined is

one at National Oilwell Varco. However, Dyna-Drill also has a dynamometer, and has been

contacted for another possible test location. One option to help the team manufacture a

superior mud motor is to have our design built by the beginning of October, so the team can

test the mud motor several times, while breaking down the mud motor between tests to make

improvements where the team sees problems occurring. This will be very beneficial for the

team to manufacture the best mud motor possible.

Figure 18: Dynamometer used for Testing

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Budget

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Timeline

0

50

100

150

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

H

o

u

r

s

Weeks

Project Hours

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Resources

Team Members

Team MUDD is comprised of student members that bring various skills to this project.

Using all of these diverse sets of skills in combination with each other allows Team MUDD to

achieve its goals in the project. Listed below is a brief outline of each member’s skill set:

Travis Sanders (Team Leader):

Leadership qualities obtained from running a drilling company shop

Pro E/Pro-Mechanica experience

Welding, machining, and various other metal fabrication skills

Networking with the Petroleum Industry Vivek Ghosh:

Leadership qualities which include being president of SME

Networking with the Petroleum Industry through internship

Expertise in fluid mechanics, strength of materials, dynamics, and thermodynamics Avinash Cuddapah:

Experience in the industry with mechanical design

3-D Modeling, FEA, and CFD

Engineering Intern at E&C Engineers Carlos Venegas:

3-D Modeling and FEA experience

Experience in mechanical related maintenance & testing

Welding, machining, and various other metal fabrication skills Emelkin Lozano (Undergraduate Assistant):

Expert web designer

Graphic communications

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Faculty Advisors

Dr. John Eberth:

Ph.D. Biomedical Engineering, Texas A&M University, 2008

M.S. Mechanical Engineering, Clemson University 2004

B.S. Mechanical Engineering, Clarkson University 2001

Undergraduate Fluid Mechanic Professor at University of Houston Dr. Robello Samuel:

M.S. and Ph.D. Petroleum Engineering, Tulsa University (Ph.D. was on Drilling Motors)

M.S. Mechanical Engineering, Anna University

B.S. Mechanical Engineering, Madurai University

Industry Advisors

Greg Sidora & Paris Blair:

Hunting Energy

Technical Sales

Years of experience in mud motors Bill Murray

Dyna-Drill Vice President

Engineering Manager Edmee Files

Dyna-Drill

Director of Research and Development

Chemist

Elastomer research Dr. Gunther Von Gynz

Asmin LC Engineering consulting firm

Professional Engineer

Maufacturer of mud motor components National Oilwell Varco (NOV)

Many employees have helped

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References

Books

Downhole Drilling Tools – G Robello Samuel (power section geometry calculations)

Materials Science and Engineering an Introduction – William D. Callister, Jr (steel selections)

Mechanics of Materials - Gere Goodno (pressure, bending moment, shear calculations)

Thermodynamics and Heat Power – Rolle (heat transfer on rubber)

Applied Fluid Mechanics – Mott (fluid flow rates)

Machine Elements in Mechanical Design – Mott (compression, buckling calculations)

G Robello Samuel Thesis (power section horsepower calculations)

Websites

National Oilwell Varco - images

Dyna-Drill – images, specifications

Drilling Fluid Fundamentals – images

Subtech – Titanium Beta Alloy information

Tomahawk Downhole – mud motor information

Oil Online – Disposable Mud Motor – rotor images

Eastern Seals – HNBR compound information

Diracdelta – ACME threads information

Web-links

http://www.nov.com/Downhole/Drilling_Motors/Oil_Lube_Bearing_Assemblies.aspx

http://www.nov.com/Downhole/Drilling_Motors/Power_Sections.aspx

http://dynadrill.com/configurations.asp

http://www.wdcexploration.com/what_we_do/pdf/drilling_fluid_fundamentals.pdf

http://www.substech.com/dokuwiki/doku.php?id=titanium_beta_alloy_ti-10v-2fe-3al

http://www.tomahawkdh.com/features.html

http://www.oilonline.com/News/NewsArticles/LatinAmerica/articleType/ArticleView/articleId/2

3675/categoryId/1/Purpose-Built-Disposable-Motors.aspx

http://www.sealseastern.com/CompoundSpec.asp?cmpnd=7344

http://www.diracdelta.co.uk/science/source/a/c/acme%20thread/source.html

*Picture is not actual design

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Appendix