1. Drill String and Drill Collars
Teams members:
Maricela Pérez Gómez.
Diana Ramos López.
Rosaida Suarez Sánchez.
Juan Sebastián Palacios
Reyes.

2. Objectives
At the end of this lecture YOU will be able to describe:
• Functions of Drill Pipe , Drill Collars and BHA
selection
• Grades of Drill Pipe and strength properties
• Thread types and tool-joints
• Drill collar weight and neutral point

3. Functions of the Drill String
The drill string is the mechanical linkage connecting
the drill bit on bottom to the rotary drive system on
the surface.
The drill string serves the three main following
functions :
D
1. Transmit and support axial loads - WOB P
2. Transmit and support torsional loads - Torque
3. Transmit hydraulics to clean the hole and cool
the bit. DC WOB

4. Drill String Components
The Drill String includes all
tubular equipment between the
Kelly Swivel and the bit
• Kelly
• Surface Safety Valves
• Drill Pipe
• Heavy Walled Drill Pipe
• Drill Collar
• Jars – Shock Subs – Bumper
Subs – Junk Baskets –
Accelerators etc…

5. The Kelly/Top Drive
Strictly speaking, Kelly/ Topdrive
are not components of the drill
string; however, they provide the
essential requirements for
drilling a well:
1) Transmit rotation to the
drillstring.
2) Provide access to the drilling
fluid into the drillstring.
3) Support the weight of the
string.

6. The Kelly
The Kelly is the rotating link between the rotary table
and the drill string.
• Transmits rotation and weight-on-bit to the
drillbit
• Supports the weight of the drillstring
• Connects to the swivel and allow circulation thru
pipe.
The Kelly comes in lengths ranging from 40 to 54 ft
with cross sections such as hexagonal (most
common), square or triangular.
Connected to a Kelly Saver Sub

7. Kelly Cock
The Kelly is usually provided with two
safety valves, one at the top and one at
the bottom, called Kelly cock.
The Kelly cock is used to close the
inside of the drillstring in the event of
a kick.
The upper & lower Kelly cocks
operate manually.
IBOP / DPSV/FOSV are not run in the
drill string but kept handy on the rig
floor

8. Top Drive
The top drive is basically a combined rotary table and
kelly.
It is powered by a separate motor and transmits rotation to
the drill string directly without the need for a rotary table.
Advantages over the kelly system:
1. Efficient reaming and back reaming.
2. Circulating while running in hole or pulling out of hole in
stands
3. The kelly system can only do this in singles; ie 30 ft.

9. Stabilizers
Reasons for Using Stabilizers:
1. They are used as a fundamental method of
controlling the directional behavior of most BHAs.
2. Help concentrate the weight of the BHA on the bit.
3. Minimize bending and vibrations which cause tool
joint wear and damage to BHA components such as
MWDs.
4. Reduce drilling torque by preventing collar contact
with the side of the hole and by keeping them
concentric in the hole. (FG!!)
5. Help preventing differential sticking and key seating.

10. Stabilizers

11. Drill Collars
Drill collars are used to apply weight to the bit. Their large wall
thickness gives them a greater resistance to buckling than DP. Their
large wall thickness gives them a greater resistance to buckling than
DP. The neutral point should be located at the top of the drill collars
section (never in the DP section), 75 to 85% (maximum) of the drill
collars section should be under compression (this will include HWDP
if utilised).
The lower part of the drill collar section is under compression,
therefore subject to buckling. This generates high stresses and
potential fatigue failure, particularly at the connections.
In deviated holes, to avoid sticking of the drilling assembly, the
minimum number of drill collars should be used (and these should be
of the spiral type). Heavy Weight Drill Pipe in conjunction with
undergauge stabilisers, should be used be used to substitute the drill
collar weight.

12. Drill Collars
Description
They are heavy walled metal tubes
The ends are threaded (box and pin)
Functions
To put weight on bit (WOB)
To keep the drill string from buckling
Types
Comes in many OD and ID sizes
Typically 4 ¾” to 9 ½” OD
Most commonly in lengths of 30-31 feet
Square collars where the holes tend to be
crooked
Spiral collars where there is chance of getting
stuck
Collars with elevator and slip recesses

13. Drill Collar Connections
Characteristics
• DC connections are rotary shouldered connections and can
mate the various DP connections
• The shoulder provide the only positive seal against fluid
leakage
• The lubricant is Copper based dope
• The connection is the weakest part of the entire BHA
• The DC connections go through cycles of tension-
compression and are subject to bending stresses
• Improper M/U torque, improper or insufficient lubricant,
galling can all lead to connection failure

14. Low Torque Feature
•The low torque feature
consists in removing part of
the shoulder area of the pin
and box.
•This allows for lower make
up torque maintaining
adequate shoulder loading.
•It is a common feature in
large OD connections.

15. Torsion limits for DC
Torque is rarely limited by the DC connection because it is
usually higher in the DP at surface and lower in the DC.
 If DC make-up torque >Dp make-up torque you have no routine
problems.
 BH Torque at any point should not exceed 80% of make-up
torque for the connections in the hole to avoid over tightening
connections which can lead to damage of seals.

16. Torque Limits for DC
M/U Torque as % of total torque
 API recommended make-up
torque for connections is a DC< 7 in DC>7 in
percentage of the total PAC 79.5% N/a
torsional yield of the H-90 51.1% 56.2%
connection
API NC 56.8% 62.5%

17. CONCLUSIÓN
The drill string and drill collars are very important in the drilling, the
drill collar is connected to the drill string to provide greater rigidity,
greater weight and stability to the entire drill string and thus
achieve a more efficient drilling both were designed to work
together.