ORIGINAL ARTICLE

Design and Analysis of an Active Helical Drive Downhole Tractor

Yujia LI1 • Qingyou LIU2• Yonghua CHEN3

• Tao REN1

Received: 18 January 2016 / Revised: 11 May 2016 / Accepted: 22 August 2016 / Published online: 16 March 2017

� Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2017

Abstract During oil-gas well drilling and completion,

downhole tools and apparatus should be conveyed to the

destination to complete a series of downhole works.

Downhole tractors have been used to convey tools in

complex wellbores, however a very large tractive force is

needed to carry more downhole tools to accomplish works

with high efficiency. A novel serial active helical drive

downhole tractor which has significantly improved per-

formance compared with previous work is proposed. All

previously reported helical drive downhole tractors need

stators to balance the torque generated by the rotator. By

contrast, the proposed serial downhole tractor does not

need a stator; several rotator-driven units should only be

connected to one another to achieve a tractive force mul-

tifold higher than that was previously reported. As a result,

the length of a single unit is shortened, and the motion

flexibility of the downhole tractor is increased. The major

performance indicators, namely, gear ratio, velocity, and

tractive force, are analyzed. Experimental results show that

the maximum tractive force of a single-unit prototype with

a length of 900 mm is 165.3 kg or 1620 N. The analysis

and experimental results show that the proposed design has

considerable potential for downhole works.

Keywords Serial � Active � Helical � Downhole � Tractor �Wellbore

1 Introduction

Unconventional or low-permeability oil and gas produc-

tivity can be greatly enhanced by drilling horizontal,

deviated, and extended-reach wells [1–3]. During well

drilling and completion, downhole tools and apparatus used

for well logging, workover, flushing, fishing, and perfo-

rating should be conveyed to the destination. The tradi-

tional downhole conveyance technology is not applicable

to these complex wellbores [4]. In response to the chal-

lenges presented by complex well trajectories, downhole

tractors have been used since the 1990s to convey evalu-

ation, remediation, and intervention tools [5]. Using

downhole tractors costs less because they do not necessi-

tate the use of drilling rigs. In addition, downhole tractors

can be positioned precisely to conduct cutting and perfo-

rating. Fig. 1 shows the principle of a downhole tractor

operating in an oil or gas well.

Various in-pipe robot topologies have been developed

since the 1950s [6]. They can be classified into eight dif-

ferent types according to their driving mechanisms [7–13],

i.e., pig, wheeled, caterpillar, wall press, walking, inch-

worm, helical, and snake. These types of in-pipe robots

have their own advantages in specific working spaces and

environments. In the 1970s, the high maintenance

requirements of the pipeline, tank container, and wellbore

used in harsh environments, such as oil, natural gas, shale

gas, and nuclear industry, spurred the development of in-

Supported by Sichuan Provincial Science and Technology Program of

China (Grant Nos. 2013GZ0150, 2014GZ0121), and Research Project

of Key Laboratory of Fluid and Power Machinery of Ministry of

Education, Xihua University, China.

& Qingyou LIU

liuqy666@yahoo.com

1 School of Mechatronic Engineering, Southwest Petroleum

University, Chengdu 610500, China

2 Key Laboratory of Fluid and Power Machinery of Ministry of

Education, Xihua University, Chengdu 610039, China

3 Department of Mechanical Engineering, The University of

Hong Kong, Hong Kong, China

123

Chin. J. Mech. Eng. (2017) 30:428–437

DOI 10.1007/s10033-017-0076-6

pipe robots. Owing to the special functional and environ-

mental restrictive conditions, namely, high pressure, high

temperature, strong corrosion, small diameter, and large

payloads in wellbores, only two of the aforementioned

types of in-pipe driving mechanisms are used in downhole

tractors, namely, wheel-type continuous driving and inch-

worm reciprocating driving [4].

In 1993, Statoil and Welltec became the first companies

to research on cable-driven downhole tractor based on

wheel-type continuous driving. Their product consists of a

motor, hydraulic system, driving wheel unit, controlling

unit, and pressure compensation system [14]. The motor

drives hydraulic pumps to generate a certain pressure force

and friction force. GE Oil and Gas Company developed a

downhole tractor driven by only motors and mechanical

transmissions [15]. With a body length of 7.46 m, it could

log while tractoring and accommodate payloads of up to

2675 N. Aker Solutions also devised a wheel-driven

downhole tractor, which has a pulling force of 2225 N per

drive section and a capability for open-hole logging. This

type of downhole tractor can work in small wellbores.

However, it cannot be used for high payloads and large

wellbores.

Another widely used downhole tractor is of the inch-

worm reciprocating driving type. Reciprocating driving

uses locking devices and imitates the motion of an inch-

worm with an anchor–extend–release–re-anchor movement

[4]. Smart Completions Ltd. has designed an electro-hy-

draulic fully bi-directional crawler-type tractor named

SmarTract Robotic Downhole Tractor [16], which is 10 m

long and has a maximum pulling force of 6664 N and a

speed of up to 549 m/h. Omega Completion Technology

and WWT have also developed downhole tractors of this

kind [17]. Their tractors have both high speed and large

pushing or pulling force. Moreover, their downhole tractors

are applicable to a wide range of pipe diameters. Never-

theless, their motion is discontinuous; thus, they cannot

perform logging and tractoring simultaneously.

In recent years, many studies have focused on helical in-

pipe robots, which are driven by only one motor and have a

simple structure and control system [18–23]. Although

these typical helical in-pipe robots possess increased

motion stability, to our knowledge, they are all passively

driven, that is, the driving wheel rotates passively relying

on the friction force exerted by the pipe wall on the wheels.

As a result, the mechanical transmission efficiency of these

in-pipe robots is low, and their tractive force is insufficient

for drag testing or repairing downhole tools. Accordingly,

we proposed in our previous work [24] a novel active

helical drive in-pipe robot with a compound planetary gear

system to increase traction significantly. However, this

helical drive in-pipe robot is composed of at least two

units, namely, a rotator and a stator, according to the laws

of action and reaction [25]. This traditional type of helical

drive structure increases the length of the robot and con-

sequently reduces its motion flexibility.

To realize a larger payload, better motion flexibility, and

simultaneous logging and tractoring, we propose a novel

active helical drive downhole tractor with a serial structure.

Unlike traditional helical in-pipe robots, this novel down-

hole tractor does not require a stator to balance the torque

generated by the rotator. Several rotator-driven units only

need to be connected to one another to obtain a tractive

force multifold higher than those of previous in-pipe

robots.

2 Concept and Design of the Serial Active HelicalDrive Downhole Tractor

2.1 Overview on the Structure

Fig. 2 shows two different structures of active helical-drive

downhole tractors. Fig. 2(a) shows the traditional structure,

which consists of a rotator and a stator. The driving motor

is installed on the stator and provides the driving force to

the tractor. The centering wheels installed on the stators

play a significant role in centering and guiding. The driving

wheels and wellbore comprise a compound planetary gear

system, in which the sun gear is driven directly by the

motor. As the rotator, the driving wheels, are in contact

with the wellbore and produce a forward driving force. In

this configuration, the stator is an indispensable part to

balance the torque generated by the rotator.

Fig. 2(b) shows a novel serial structure with rotators only.

The driving motor is installed on the rotator and rotates

together with the planet carrier. Owing to this significant

improvement, a stator is no longer needed to balance the

Fig. 1 Principle of a downhole tractor operating in an oil or gas well

Design and Analysis of an Active Helical Drive Downhole Tractor 429

123

torque; thus, the length of a single unit can be shortened,

and a more flexible motion can be realized. The driving

wheels can be used not only as a driving unit but also as a

centering unit. Thus, the tractor can be centered inside the

wellbore, and the motion stability of the downhole tractor

can be consequently enhanced. This tractor can perform

logging while tractoring.

2.2 Structural Design of a Single Driving Unit

Fig. 3 shows the structure of the proposed downhole trac-

tor. It consists of a motor, controlling PCB, planetary gear

system, and other transmission mechanisms. Motor 1 is the

motor driving the sun gear directly. The sun gear is in mesh

with the first planetary gear set. The second planetary gear

set is connected to the first planetary gear set by three

telescopic universal joints. Motor 2 is used to drive the ball

screw, which is connected to the linear slider, and to push

or pull the connecting rod by moving in the axial direction

so as to auto-control the contraction and expansion of the

driving wheels. The moving direction is marked by arrows

in Fig. 3. A spring is used to allow the downhole tractor to

adapt to small changes in pipe diameters and prevent the

deflection of the tractor from the wellbore because of

impurities or the localized deformation in the pipeline. The

driving wheels press firmly onto the internal wall of the

wellbore and tilt at an angle when in operation. The pipe

wall is engaged as the internal ring gear of the planetary

gear system. Then, a tractive force is generated. Motor 1 is

fixed on the body of the tractor and rotates together with

the planet carrier. Therefore, all the forces generated are

the internal forces of the compound planetary gear system.

A stator is unnecessary to balance any external torque.

3 Gear Ratio, Velocity, and Force Analysis

3.1 Description and Definition of Helical Angle

Figs. 4(a), (b), and (c) show the driving unit viewed from the

front, right, and top, respectively. Three important angles,

i.e., a, c, and u, are defined and shown in Fig. 4. Angle a is

the helical angle of the driving wheels, that is, the angle

between the following two geometric features [24]:

(1) The intersecting line between the wheel plane and

the plane tangent to the circumference of the pipe

wall at the contact point of the wheel and the pipe

wall;

(2) The pipe cross-section through the contact point of

the wheel and the pipe wall.

The helical angle can be calculated as follows [26]:

tan a ¼ tan c sinu: ð1Þ

3.2 Gear Ratio and Velocity

Gear ratio is the most important factor in a compound

planetary gear system as it decides the relationship between

the parameters of the driving motor and the moving speed

of the tractor. Fig. 5 shows the kinematic diagram of the

gear transmission with serial two-unit structure. The body

Fig. 2 Two different structures of active helical drive in-pipe robots

1st Planetary gear set

2nd Planetary gear set

Telescopic universal joint Connecting rod

Ball screw

Embedded springBody(motor1 and controlling PCB)

Motor 2

Linear slider

Planetary carrier

Fig. 3 Structural design of serial active helical drive downhole

tractor (single unit)

430 Yujia LI et al.

123

of the driving motor rotates together with the planetary

carrier. Fig. 6 presents the simplified gear transmission

diagram of the single driving unit. Symbols S, H, 1, 2, and

P in Fig. 6 represent the sun gear(connected to the output

shaft of the motor), the planetary carrier, first planetary

gear set, second planetary gear set, and pipe wall, respec-

tively. The pipe wall can be regarded as the internal ring

gear of the compound planetary gear system. Thus, the

angular velocity of the pipe wall is zero. The gear ratio iHsPbetween the sun gear and the pipe wall (internal ring gear)

can be calculated by

iHsP ¼ xs � xH

0� xH

¼ 1� isH; ð2Þ

where xs and xH are the absolute angular velocities of the

sun gear and planetary carrier relative to the pipe, respec-

tively. Thus, the gear ratio isH between the sun gear and

planetary carrier is

isH ¼ 1� � z1zP

zsz2

� �; ð3Þ

where z1, z2, zs, and zP are the numbers of the respective

gear teeth. In this study, the diameter of the pipe, DP,

should replace zP. Parameter z2 should be replaced with the

pitch diameter of the driving wheel, D2. Thus,

isH ¼ 1þ z1DP

zsD2

¼ xs

xH

: ð4Þ

If z1 = 30, zs = 20, DP = 105 mm, and D2 = 36 mm,

then the gear ratio between the sun gear and the planetary

carrier is 5.375. Thus, we can obtain the relative angular

velocity between the sun gear and the motor body (planetary

carrier) based on the basic parameter of the driving motor.

xs � xH ¼ isH � 1ð ÞxH ¼ 2pn60

; ð5Þ

where n is the rotation speed of the driving motor. If the

rotation speed is known, then the absolute angular velocity

of the planetary carrier can be obtained.

Finally, the relationship between the moving speed, v, of

the tractor and xH can be established as follows:

v ¼ xH � rP � tan a; ð6Þ

where rP is the radius of the pipe. If the rotation speed is

the rated speed of 170 r/min and the helical angle, a, is themaximum value of 18�, then the moving speed is

69.38 mm/s.

3.3 Force Analysis

3.3.1 Driving Force Analysis

Analyzing the driving force of the downhole tractor is of

significance because the driving force affects the mechan-

ical structure design and real application scope [27]. The

force diagram of a driving wheel is shown in Fig. 7.

(a) Front view (b) Right view

(c) Top view

φ

γ

α

Fig. 4 Definition of angles a, c, and u

Fig. 5 Kinematic diagram of the gear transmission

Fig. 6 Simplified gear transmission diagram of the single driving

unit

Design and Analysis of an Active Helical Drive Downhole Tractor 431

123

Three forces act on the wheel to ensure its steady state

of motion. The equilibrium equations for the forces acting

on the driving wheel are as follows:

FT �W � sin a ¼ 0;Ff �W � cos a ¼ 0;

�ð7Þ

where W is the pulling force exerted by the load on each

wheel, Ff is the sliding friction force exerted by the internal

pipe wall on the driving wheel, and FT represents the tor-

que of the driving wheel. If the rated rotation speed, n, and

rated power, P, of the motor are known, then the rated

torque can be calculated. Thus, FT can be obtained as

follows:

FT ¼ 9550P � is2

3n � D2/2; ð8Þ

where is2 is defined as the gear ratio between the sun gear

and the second planetary gear set and is related to the pipe.

Given that the first and second planetary gear sets have the

same angular velocity, is2 can be replaced with is1. The gear

ratio between the sun gear and the first planetary gear set in

relation to the carrier is as follows:

iHs1¼ xs � xH

x1 � xH

¼ � z1

zs: ð9Þ

The angular velocity, x1, of the first planetary gear set

can be obtained by substituting Eqs. (2), (4), and (5) into

Eq. (9). Subsequently, the gear ratio is2 can be calculated

as follows:

is2 ¼xs

x1

: ð10Þ

The maximum pulling force of the entire tractor can be

obtained by substitute Eqs. (8) and (10) into Eq. (7). Fig. 8

shows the relationship between a, P, and 3 W.

3W ¼ 3FT

sin a¼ 19 100P z1DP þ zsD2ð Þ

nD2 sin a zsD2 � zsDPð Þ : ð11Þ

3.3.2 Torque-Balance Analysis

A torque-balance analysis is conducted to compare the

novel serial active helical drive structure with the tradi-

tional one. As shown in Fig. 9(a), the motor body, which is

connected to the centering wheels, is the stator. The planet

carrier is used as a rotator, and it rotates in relation to the

stator. When the gears are in mesh and rotate, the sun gear

is subjected to an anti-torque. The motor body bears the

anti-torque. Given that the motor body and the centering

wheels are connected, the pipe exerts a torque to balance

the anti-torque.

By contrast, in the novel serial active helical drive

structure (Fig. 9(b)), the motor body is the rotator. It rotates

together with the planetary carrier. Different from the tra-

ditional structure, the planetary carrier and motor body in

the novel structure bear the anti-torque. Thus, the outside

environment should not provide a torque, and a stator is not

necessary in this structure. This design simplifies the

structure, enhances motion flexibility, and improves the

strength of the parts.

4 Experiments

4.1 Prototype

A prototype based on the design in Fig. 10(a) is con-

structed, as shown in Fig. 10(b). The outer diameter of the

tractor body is 74 mm. The length of the entire body of the

tractor is 900 mm. DC Motor 1 (200 W, 170 r/min) is used

to provide power to the entire tractor system. DC Motor 2

can auto-control the contraction and expansion of the

driving wheels.

The centering unit, also called testing unit, is fabricated

to test the tractive force of a single tractor, as shown in

Fig. 11. Each wheel arm is equipped with a force sensor.

When the centering unit is placed into the pipe, the

pressing force between the centering wheels and internal

pipe wall can be monitored and adaptively adjusted

automatically.

FfFT

Wα

Fig. 7 Force diagram of a driving wheel

Fig. 8 Relationship between a, P, and 3 W

432 Yujia LI et al.

123

4.2 Control System

As shown in Fig. 12, the downhole tractor is driven by two

DC motors, which are equipped with encoders. An MCU

controls the driving module via the PWM; thus, the current

of the motors can be controlled. A DC power supply pro-

vides power to the driving module and MCU. At the same

time, the MCU communicates with other modules by using

RF (Radio Frequency) technology. Owing to the short

transmission distance of an RF signal in metal pipelines, a

special design is proposed. The RF signal is coupled to a

DC power supply through the capacitor to form a system

similar to a power line carrier system. Using this structure,

we only need to connect the multiple downhole tractors and

control them altogether.

We also develop a software to control the downhole

tractor by computers. Fig. 13 shows the interface and

function of this control software. The contraction and

expansion of the driving and centering wheels can be

controlled to enable the wheels to press onto the internal

wall of wellbore firmly. The rotation speed can be changed

by dragging the speed adjustment bar. The force of each

testing arm and the current of the driving motor are dis-

played on this software interface. The values of the tractive

force can be collected by the force sensor and shown on the

computers.

Fig. 9 Torque-balance comparison of two active helical drive

structures

(a) Design model

(b) Prototype of two driving units

1st Planetary gear set

2nd Planetary gear set

Body ( Motor 1 andControlling PCB )

Motor 2Ball screwLinear slider

1st Planetary gear set2nd Planetary gear set

Controlling PCB

Motor 2Ball screwLinear slider

Body

Motor

1

Fig. 10 Design and prototyping of the proposed tractor

Centering wheel

Motor 2

Linear slider and ball screw

Spring

Force sensor

Fig. 11 Prototype of the centering unit

Design and Analysis of an Active Helical Drive Downhole Tractor 433

123

4.3 Experimental Scenario and Results

A seamless steel tube with an outer diameter of 127 mm

and a length of 9 m is used in the experiment.

Fig. 14(a) presents a diagram of the experimental design.

The downhole tractor is placed into a seamless steel tube.

A power line communication system is used to provide

power to the tractor and communicate with the computers

at the same time. A hoisting cable, which passes through

two fixed pulleys, is tied to the tractor and connected to the

force sensor at the other end. An air cylinder and piston are

used for tightening the hoisting cable. When the tractor

moves forward, a tractive force acts on the force sensor,

and the instrument displays this tractive force.

Fig. 14(b) shows the actual experimental environment and

devices.

We gradually increase the air pressure in the air cylinder

when the tractor is moving forward in the pipe. As the

pressure and hoisting cable tension increase, the pulling

force acting on the tractor increases gradually. When the

driving wheels of the tractor skid and can no longer move

forward, the pulling force becomes the maximum tractive

force displayed on the instrument and recorded on the

computers. In the experiment, we continuously recorded

the tractive force for 53 s. Finally, the maximum tractive

force of the downhole tractor is 165.3 kg or 1620 N, as

shown in Fig. 15. This tractive force is much larger than

the 490 N tractive force of the active helical drive in-pipe

robot in our previous work [24]. Furthermore, the novel

single-unit downhole tractor is much shorter than the

existing original downhole tractors. However, its tractive

force per unit length is much larger than the original ones.

The driving voltage is 64 V in the experiment. The

current value of the driving motor is collected and shown in

Fig. 16, which shows that the current slightly fluctuates.

The possible reasons for the current fluctuation are as

follows:

Fig. 12 Structure of the downhole tractor control system

Fig. 13 Interface of the downhole tractor control software

(a) Schematic of the experiment

(b) Actual experimental environment and devices

Air cylinder

Traction display

Tractor in pipe

Power

Air bottle

Camara

Power

Air cylinderForce sensor

Traction display

Downhole tractor

PulleyPulley

Pipe

Fig. 14 Experimental environment and devices

434 Yujia LI et al.

123

(1) The center of gravity of the tractor leads to different

compression or elongation magnitudes of the three

driving arms. The deviation in manufacturing the

downhole tractor results in an asymmetric size.

These factors lead to pipe eccentricity and conse-

quently produce current fluctuation.

(2) The localized deformation and impurity accumula-

tion in the pipeline result in the driving wheels

skidding in relation to the pipe wall. As a result, the

current instantaneously decreases.

(3) The periodic revolution of the planetary gear system

causes small-scale fluctuations in the current of the

driving motor.

In the experiment, the average current is 2.5 A, and the

voltage is 64 V; thus, the power is 160 W. The rotation

speed of Motor 1 is 170 r/min. The helical angle of the

driving wheels is 15�. The teeth numbers of the sun gear,

first planetary gear set, and second planetary gear set are

20, 30, and 50, respectively. The inner diameter of the pipe

and the pitch diameter of the second planetary gear set are

105 mm and 36 mm, respectively. The maximum pulling

force (also called tractive force) of the tractor, which can

be calculated by Eq. (11), is 5413.18 N. The experimental

tractive force is 1620 N. The efficiency of the entire system

is approximately 30%.

The efficiency of the motor is approximately 86%, and

the transmission efficiency of the planetary gear reducer on

the motor is 65% [28]. The transmission efficiency of the

planetary gear is approximately 90%, and the transmission

efficiency of the two-stage planetary gear system is 81%.

Thus, the total efficiency of the entire system is 45.3%. In

addition, circuit and friction losses contribute to efficiency

reduction. The final estimated efficiency is approximate to

the calculated value. Therefore, the given theoretical

analysis is accurate.

5 Discussion

However, the design of the downhole tractor may be fur-

ther optimized. Given that the serial driving units are

rigidly connected, a push-and-pull effect will be generated

in case a power loss occurs. Another serious problem is the

possibility of the tractor being stuck in the pipe. A possible

solution to this problem is to replace the telescopic cross

shaft universal joint with a telescopic constant-velocity

universal joint to realize a constant-velocity transmission

between the driving shaft and the driven shaft. A telescopic

connection can compensate for the processing and assem-

bly errors. In addition, an overrunning clutch can be

installed on either side of the universal joint shaft. When

deformation and impurities exist in the wellbore, this

structure can protect the driving wheels from motion

interference and improve transmission efficiency. Further-

more, a flexible connection can be considered between

every two driving units to eliminate motion interference.

All the aforementioned possible solutions will be consid-

ered in our future work.

6 Conclusions

(1) A serial active helical drive downhole tractor is

proposed and prototyped. Unlike traditional helical

in-pipe robots, this novel downhole tractor does not

require a stator to balance the torque generated by

the rotator. Several rotator-driven units only need to

be connected to one another to obtain a tractive force

multifold higher than those of previous in-pipe

robots. Additionally, the length of a single unit can

be shortened, and a more flexible motion can be

realized.

(2) The driving wheels can be used not only as a driving

unit but also as a centering unit. Thus, the tractor can

be centered inside the wellbore, and the motion

stability of the downhole tractor can be consequently

enhanced. This tractor can perform logging while

tractoring.

(3) The gear ratio of the downhole tractor system is

calculated, and the velocity and force are analyzed.

Fig. 15 Tractive force recorded continuously

Fig. 16 Current fluctuation graph

Design and Analysis of an Active Helical Drive Downhole Tractor 435

123

Experiments are conducted to evaluate the tractive

force. The final estimated efficiency is approximate

to the calculated value. Therefore, the given theo-

retical analysis is accurate.

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Yujia LI, born in 1990, is currently a PhD candidate at School of

Mechatronic Engineering, Southwest Petroleum University, China.

She received her bachelor degree from Southwest Petroleum Univer-

sity, China, in 2012. Her research interests include the oil and gas

equipment, in-pipe inspection robot and the downhole tractor. E-mail:

yujiali321@foxmail.com.

Qingyou LIU, born in 1965, is currently a professor at Xihua

University, China. He received his PhD degree from Southwest

Petroleum University, China, in 1997. His research interests include

the oil and gas equipment, oil and gas wells engineering mechanics

and in-pipe inspection robot. E-mail: liuqy666@yahoo.com.

436 Yujia LI et al.

123

Yonghua CHEN, born in 1963, is currently a professor at University

of Hong Kong, China. He received his PhD degree from University

of Liverpool, England, in 1990. His research interests include

engineering design, rapid prototyping and robotic machining, haptic

modeling and computer aided medical surgeries. E-mail:

yhchen@hku.hk.

Tao REN, born in 1988, is currently a PhD candidate at School of

Mechatronic Engineering, Southwest Petroleum University, China.

He received his bachelor degree and master degree from Southwest

Petroleum University, China, in 2011 and 2014, respectively. His

research interests include the oil and gas in-pipe inspection robot and

the downhole tractor. E-mail: rtswpu@qq.com

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