Dr. Siv K. Ali's lecture discusses drill string accessories. Key points include: 1) Drill collars provide weight on the bit for efficient drilling and keep the drill string in tension to reduce bending stresses. Stabilizers centralize the bottom hole assembly and increase bit life. 2) Roller reamers replace stabilizers in abrasive formations, consisting of blades with embedded rollers to smooth the wellbore. Drilling jars deliver sharp blows to free stuck pipe. 3) Subs are used to adapt parts of the drill string with different threads or designs, and may perform special functions like providing shoulders for elevators or reducing shocks.

1. Siv K. Ali
Drilling Engineering
Siver.Kais@knu.edu.iq
Department of Petroleum Engineering - Knowledge University
Drilling Engineering I
Lecture-3: Drill String Accessories
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2. 2
Drill StringAccessories
5- Drill String Accessories

3. Drill Collars
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4. 4
Drill Collars
The functions of drill collars are:
To provide enough weight on bit for efficient drilling.
To keep the drill string in tension, thereby reducing bending stresses and failures.
To provide stiffness in the BHA for directional control.
To minimize the stability problems from vibrations, wobbling and jumping.

5. Drill Collars
• Drill pipe will tend to buckle when run in compression. Repeated buckling will
eventually lead to early drill pipe failure by fatigue.
• Since elastic members can only buckle in compression, fatigue failure of pipe can be
eliminated by maintaining drill pipe in tension.
• Research and field experience proved that buckling will not occur if weight on bit is
maintained below the buoyed weight of the collars.
• In practice weight on bit should not exceed 85% of the buoyed weight on the collars.
• This practice also place the neutral point at the drill collars.
• The drill collars are the first section of the drill string to be designed. The length and size
of the collars will affect the grade, weight and dimensions of the drill pipe to be u
1s
1 ed.

6. Drill Collars
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7. Drill StringAccessories
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8. Drill StringAccessories
Stabilizers
• Stabilizers are tools placed above
the drill bit and along the bottom
hole assembly (BHA).
• Stabilizers consist of a length of
pipe with blades on the external
surface.
• These blades may be either
straight or spiral and there are
numerous designs of stabilizers.
• The blades can either be fixed on
to the body of the pipe, or
mounted on a rubber sleeve. 4

9. 9
Drill StringAccessories
Stabilizers, cont.
Functions:
1. Centralize and provide extra stiffness to the BHA.
2. Allow higher WOB since the string remains concentric.
3. Increase bit life by reducing wobble (i.e. all three cones loaded
equally).
4. Control hole deviation and dogleg severity.
5. Prevent differential sticking.

10. Drill StringAccessories
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0

11. Drill StringAccessories
Roller Reamers
• Wellbores are not always as smooth as they need to be which is critical for
tripping in and out of the hole, conducting wireline open hole logging, and
running casing.
• Reamer is a tool used in drilling to smooth the wall of a well, enlarge the hole to
the specified size, help stabilize the bit, and straighten the wellbore if kinks or
doglegs are encountered.
• Roller reamers are used to replace near bit and string stabilizers in bottom hole
assemblies where abrasive formations are encountered.
• A roller reamer consists of stabilizer blades with rollers embedded into surface of
the blade.
• The rollers may be made from high grade carburized steel or have
tungsten carbide 7

12. Drill StringAccessories
Roller Reamers, cont.
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2

13. Drill StringAccessories
1
3

14. Drill StringAccessories
Drilling Jars
• The purpose of these tools is to deliver a sharp blow to
free the pipe if it becomes stuck in the hole.
• A jar is basically a sliding mandrel that
allows a brief and sudden axial acceleration of
the drill string above the jar.
• Hydraulicjars are activated by a straight pull and give an
upward blow.
• Mechanical jars are preset to operate when a given
compression load is applied and give a downward blow.
• Jars are usually positioned at the top of the drill collars. 10

15. Drill StringAccessories
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16. Drill StringAccessories
Subs (Substitutes)
• Sub is a short, threaded piece of pipe used to adapt parts of the drilling string
that cannot otherwise be screwed together because of differences in thread size
or design.
• A sub (a substitute) may also perform a special function.
• Lifting subs are used with drill collars to provide a shoulder to fit the drill pipe
elevators;
• A Kelly saver sub is placed between the drill pipe and the Kelly to prevent
excessive thread wear of the Kelly and drill pipe threads;
• A bent sub is used when drilling a directional hole;
• Shock subs are used to reduce the effect of bit bounce and isolate it from the drill
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17. Drill StringDesign
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18. Drill StringDesign
Drill String Design
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19. 19
Drill StringDesign
Introduction
• Drill string design involves thedetermination of thelengths,weight and grade of the
drill pipe whichcanbe usedduring drilling, coring,or any other operation.
• Drill string design depends onseveral factors, including:
1. Holedepth
2. Holesize
3. Mudweight
4. Desiredsafetyfactorintensionand/ormarginofoverpull
5. Lengthandweightofdrillcollars,And
6. Desireddrillpipesizeandinspectionclass.

20. Drill StringDesign
Introduction
• Thefollowing design criteria will be usedto selecta suitable drill string:
A. Tension
B. Collapse
C. ShockLoading
D. Torsion
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21. Drill StringDesign
A.Tension
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22. Drill StringDesign
A. Tension
• Prior to driving any equation, it should be observed that
only submerged weights are considered, since all
immersed bodies suffer from lifting or buoyancy forces.
• Buoyancy force reduces the total weight of
the body andits magnitude is depend on
fluid density.
• Referring this figure, the total weight , P, carried by the top
joint of drill pipe at JJ is given by:
P = (weight of drill pipe in mud) + (weight of drill collars in mud)
• Note: weight of drill bit and other accessories is
normally included within the weight of drill collars.
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23. Drill StringDesign
A.Tension,cont.
• The total weight, P:
𝑷 = 𝑳𝒅𝒑× 𝑾𝒅𝒑 + 𝑳𝒅𝒄× 𝑾𝒅𝒄 × 𝑩𝑭 equ.1
• The buoyancy factor:
𝑩𝑭 = 𝟏−
𝝆𝒎
= 𝟏−
𝜸𝒎
𝝆𝒔 𝜸𝒔
equ.2
Where:
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24. Drill StringDesign
B. Collapse
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25. 25
Drill StringDesign
B. Collapse
• Collapse pressure may be defined as the
external pressure required to cause yielding
of drill pipe or casing.
• Cases:
– In normal drilling operations, the mud columns inside and outside the drill pipe are both equal in
height and are of the same density. This results in zero differential pressure across the pipe body
and in turn of zero collapse pressure on the drill pipe.
– In some cases, as in DST operation, the drill pipe is run partially full to reduce the HP exerted
against the formation pressure. This is done to encourage formation fluids to flow into the well bore.
Once the well flows, the collapsing effects are small, as the drill pipe is now full of fluid.
– Thus, maximum differential pressure Δp across the drill pipe exists prior to the opening of the
DST
tool, and can be calculated as follows:

26. Drill StringDesign
B. Collapse,cont.
144
∆𝑃= 𝐿𝜌1
−
𝐿−𝑌 ×𝜌2
144
equ.
8
• Wher
e
• When densities expressed in
ppg:
∆𝑃 = −
𝐿𝜌1 𝐿−𝑌 ×𝜌2
19.251 19.251
equ.
9
L
Y
L-Y
Drill pipe
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Mud
Surface

27. Drill StringDesign
C. Shockloading
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28. 28
Drill StringDesign
C. Shockloading
• Shockloading ariseswhenever slipsare setonmoving drill pipe and cancontribute
to parting of pipe inmarginal design.
• It canbe shownthat theadditional tensile forces, 𝐹𝑠,generated by shockloading is
calculatedfrom:
𝐹
𝑠= 3200𝑊𝑑𝑝 equ.13 (refer to H.Rabia,page227 for theequation derivation)
Where:
𝑊𝑑𝑝 = weight of drill pipe per unit length

29. Drill StringDesign
D. Torsion
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30. Drill StringDesign
D. Torsion
• It canbe shownthat thedrill pipe torsional yield strengthwhensubjectedto pure
torsion isgiven by:
𝑸 = 𝟎.𝟎𝟗𝟔𝟏𝟔𝟕𝑱𝒀𝒎
𝑫
equ.14
• Where
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