Based on API & TH Hill DS1

1. CASING
Drill String Failure Prevention

2. Objectives
On completion of this module you will be able to:
Indentify different types of Drill String failure
Understand the factors that influence the life of Drill String
components
Describe the prevention measures to prevent DS failure and
extend its lifeextend its life

3. Introduction
Premature and unexpected failures of drill strings cause
great losses in time and material.
Reducing drill string failures will improve rig operating
performance and reduce expenses

4. The “ADIOS”* Elements
Attributes: These are the metallurgical properties and dimensions
that are built into each drill string component at manufacturing.
Design: Drill string design is selecting components and configuring
assemblies to accomplish the drilling objective.
Inspection: Drill string components, unless new, have been exposed
to handling damage and an unknown amount of cumulative fatigueto handling damage and an unknown amount of cumulative fatigue
damage.
Operation: The Drilling operation presents many opportunities to
overload and misuse the drill String.
Surroundings: The chemical and mechanical environment
surrounding the drill String can have major effect on failure
probability.
* TH Hill

5. What is a Drill String Failure?
What is a Drill String Failure?
a.When a component cannot perform its function
b.Complete separation (parting)
c. Leak (washout)
UTC
c. Leak (washout)
Location?
a.Tube body, Tool Joint or Threads
b.Any drillString component

6. Failure Types
Mechanisms which can cause failures:
Tension
Torsion
UTC
Torsion
Sulfide Stress Cracking
Fatigue
Other Causes

7. Group 1 Mechanisms (Overload failures):
Acts only if stresses in a component exceeds some
fairly high stress threshold
DS Failure Mechanisms
UTC
Tension
Torsion
Collapse Pressure
Burst Pressure
Combined Tension and Torsion
Combined Tension and Collapse

8. Group 2 Mechanisms:
Can occurs at low stress level
Fatigue
Split Box
DS Failure Mechanisms
UTC
Split Box
Sulfide Stress Cracking (Corrosion failure)
Stress Corrosion Cracking (Corrosion failure)

9. Failure Study
Fatigue
Torsion
SSC/SCC
FailureMechanism
0% 20% 40% 60% 80%
% of All Failures
Tension
Other
FailureMechanism

10. Overload:
A condition in which the bulk stress in a component
exceeds yield strength at the weakest point in the
component.
Overload and Fatigue
UTC
component.
Fatigue
Damage that accumulates when a component undergoes
cyclic stress. At some point, cumulative damage results in
the formation of a fatigue crack which can grow under
continuing stress cycles until failure occurs.

11. Tensile failures occur when the tensile load exceeds the capacity
of the weakest component in the drill String.
Occasionally the pin will fail if the connection was made up
beyond recommended torque.
Tensile Failures
UTC
How do you recognize a Tensile failure?

12. Tensile Failure
Appearance : Jagged and
Necked down
Orientation: 45 deg to pipe
axis
UTC
Pin stretched due excess tension
and/or high make up torque
Box do not fail in tension

13. Select drill pipe that is capable of carrying the
anticipated loads plus a Margin of Over-pull plus a
design factor.
Use a marking system that shows tube weight and
grade. Check pin markings to make sure that the
Responding to Tensile Failures
UTC
grade. Check pin markings to make sure that the
weight and grade are correct.
Make sure that the rig weight indicator is calibrated
properly and does not exceed the allowable tensile
load.

14. API Standard tool joints are 80% as strong in torsion as
the tube to which they are attached.
Therefore in all cases, torsional failures will occur in
tool joints.
Torsional Failures
UTC
tool joints.

15. Torsional Failures
Torsional stress limit is exceeded.
Failures occur in form of stretched pin or belled box (swelling).
Torsional failures usually occur in the tool joint.
UTC

16. Select tool joint ID and OD so that the maximum makeup torque
exceeds the maximum anticipated torsion.
Check tool joints to ensure that they meet with all the dimensional
requirements.
Make sure torque application device is working and calibrated properly.
Responding to Torsional Failures
UTC
Make sure torque application device is working and calibrated properly.
Use API tool joint compound with a FF between 0.95 and 1.05 or
compensate the applied torque accordingly.
Make up connections to recommended torque.

17. Increase of Make Up Torque
UTC

18. Combination of Tension/Torsion
These failures are most likely to happen while fishing or
pulling on stuck pipe.

19. Drill pipe tubes may burst or collapse if pressure loading exceeds
capacity.
Burst is more likely to happen when pipe is high in the hole
Collapse is most likely to happen deep in hole, evacuated for drill
String testing.
Burst and Collapse Failures
UTC
String testing.

20. Wear
If during drilling significant wear is expected then
tools can be run to measure wall thickness reduction.
Collapse and burst pressures will be determined by
the thinnest part of the wall, tensile strength by the
remaining cross sectional area.remaining cross sectional area.
Determined by
minimum wall
thickness
Burst strength
Tensile strength
determined by
remaining area.

21. Wear Prevention
Reducing side force by minimizing DLS (especially high up in the
hole) and using drillpipe protectors.
Using drilling fluids containing solids (weighted)
Always using sharp tong diesAlways using sharp tong dies
Minimizing rotating hours (use down-hole motors)
Run a “casing friendly” hardbanding material on tool joints

22. With the obvious exception of tool joint to tube welds, welded
components in the drill string should be avoided.
Weld Related Failures
Welding alters the mechanical properties unless the component is re-heat
treated.

23. Group 2 Mechanism
Can occur at low stress levels:
Fatigue
Split box
UTC
Split box
Sulfide Stress Cracking
Stress Corrosion Cracking

24. Fatigue - contributing factors
Sources of Cyclic Loads
Fatigue damaged is caused by repeated
stress cycles.
Usually occurred when the string is rotated
UTC
Usually occurred when the string is rotated
and at the same time it is bent or buckled.
Fatigue may result from excessive vibration

25. Stress concentrators….The accelerators of fatigue:
Stress concentrators focus and magnify the cyclic stress at local points.
These points become the origin of fatigue cracks, which act as their
own concentrators, to speed crack growth to ultimate failure.
Stress Concentrators
UTC
Internal upsets, thread roots, slip cuts and corrosion pits are the most
common stress concentrators

26. Fatigue
Under cycle loading, microscopic damage at high stress
points…
A microscopic crack forms…
The crack grows under continuing stress cycles until a
UTC
The crack grows under continuing stress cycles until a
failure occurs.

27. A fatigue crack will be smooth and planar, unless the surface is altered
by erosion or mechanical damage.
The crack will be oriented perpendicular to the axis of the pipe or
connection.
Fatigue cracks will originate at high stress concentrators namely,
Recognizing Fatigue Failures
Fatigue cracks will originate at high stress concentrators namely,
internal upsets, slip cuts and corrosion pits.
A fatigue crack surface will clearly show mode of attack. Ratchet marks
appear when small multiple cracks join to form a large one.

28. Fatigue in connection
Shape and Appearance:
Flat planar shape. Maybe accompanied by ragged area where
component parted in tension
Location
UTC
Location
BHA Connections…Near last engaged thread roots
Orientation
Perpendicular to the pipe axis

29. Recognizing Fatigue Failures
UTC

30. Recognizing Stress Concentrators
Slip cuts
Upsets

31. Recognizing Stress Concentrators
Cyclic loading causes very
small cracks.
With repeated cycles, the
cracks grow.
UTC
Fatigue is cumulative.
Fatigue cracks occur in a 90
degree plane to axis of pipe.

32. Recognizing Fatigue Failures

33. Fatigue cannot be eliminated:
REDUCE THE NUMBER AND SEVERITY OF CYCLIC AND STRESS
CONCENTRATORS
Do not buckle Drill-pipe / Jar
Plan the trajectory with the lowest dogleg severity
Prevention of Fatigue Failures
Ensure good rig site operation practices
Check BSR and SR, stress relief features
Chose the right connection type (NC)
Follow inspection program
Consider rotating the string more slowly, by means of introducing a
mud motor (if hole cleaning and directional objectives allow).

34. Corrosion
Higher temperature. Rates double for each 31°C.
Higher flow rate, especially if abrasive solids present.
Corrosion occurs due to electrochemical reactions with corrosive agents.
Corrosion rate increases when:
Higher concentration of corrosive agents (O2, H2S, CO2).
Corrosion rate decreases when:
Reducing dissolved O2
Reducing dissolved CO2
Increasing pH to > 9
Add coatings and inhibitors

35. Corrosion
Corrosion reduces the wall thickness of tubular.
There are three patterns of corrosion;
Uniform wall thickness reduction
Localized patterns of metal lossLocalized patterns of metal loss
Pitting

36. SSC / H2S Embrittlement
Exposure of high tensile steels to partial pressures of H2S
greater than 0.05 psi at less than a threshold pressure (which
varies by steel grade) can lead to catastrophic failure.
The metal becomes brittle and will break suddenly and without
warning.
UTC
warning.

37. Sulfide Stress Cracking

38. Sulfide Stress Cracking
Occurs in H2S environment
Elemental hydrogen (H +) migrates into steel and
+++
+⇒+ HFeSSHFe 22
Elemental hydrogen (H +) migrates into steel and
collects at high stress points
Elemental hydrogen combines to form molecular
hydrogen (H 2) causing a crack.
222 HeH ⇒++

39. Preventing Corrosion
Corrosive attention usually falls into one or more of the areas below:
OXYGEN
pH
CO2 AND CHLORIDES
UTC
CO2 AND CHLORIDES
HYDROGEN SULFIDE
BARRIERS and INHIBITORS

40. Preventing SSC Failures
Keep H2S out of the mud system by:
i) drilling overbalanced
ii) keeping pH high
iii) using H2S scavengersiii) using H2S scavengers
iv) using an oil based mud
Control the Metallurgy
Use a different grade pipe

41. Why Inspect Connections/tubes?
Guarantee the integrity of our connections
Avoid lost in hole
Avoid tool damage such as flooding & washouts
To assess threads for repair
Customer requirements

42. Inspection Methods
Ultrasonic (wall thickness)
Magnetic Particle (cracks in thread roots and stress relief features)
Liquid (Dye) Penetrant (thread roots and stress relief features)
Electromagnetic (DP)Electromagnetic (DP)
Visual

43. Follow an Inspection Program
What is a good program?
There is no “Perfect” answer
DS-1 is a guide but not a policy
Areas to consider when creating a program
UTC
Severity of the drilling conditions
Safety and environmental impact of a failure
Cost impact of a failure
Risk tolerance of management

44. References
API RP 7G Drill String Design and Op Limits
API SPEC 7 Specifications for Rotary Drilling Elements
API SPEC 5D Specifications for Drill Pipe
SLB Drill String Design manualSLB Drill String Design manual
TH Hill DS-1 Drill String Design