This document discusses drill string failure analysis and prevention. It identifies different types of drill string failures such as tensile, torsional, fatigue, and corrosion-related failures. It explains the factors that influence drill string component life such as design, inspection, operation, and environment. It also describes various prevention measures that can be taken to reduce drill string failures and extend component life, such as selecting appropriate components, inspection programs, operation procedures, and corrosion protection methods.

1. DRILL STRING FAILURE ANALYSIS &
PREVENTION

2. OBJECTIVES
ON COMPLETION OF THIS MODULE WE WILL BE ABLE TO:
- IDENTIFY DIFFERENT TYPES OF DRILL STRING FAILURES
- UNDERSTAND THE FACTORS THAT INFLUENCE THE LIFE OF DRILL STRING COMPONENTS
- DESCRIBE THE PREVENTION MEASURES TO PREVENT DS FAILURE AND EXTEND 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 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.

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)
LOCATION?
A) TUBE BODY, TOOL JOINT OR THREADS
B) ANY DRILL STRING COMPONENT

6. TYPES OF FAILURES
MECHANISMS WHICH CAN CAUSE FAILURES:
• TENSION
• TORSION
• SULFIDE STRESS CRACKING
• FATIGUE
• OTHER CAUSES

7. DS FAILURE MECHANISMS
GROUP 1 MECHANISMS (OVERLOAD FAILURES): ACTS ONLY IF STRESSES IN A
COMPONENT EXCEEDS SOME FAIRLY HIGH STRESS THRESHOLD
• TENSION
• TORSION
• COLLAPSE PRESSURE
• BURST PRESSURE
• COMBINED TENSION AND TORSION
• COMBINED TENSION AND COLLAPSE

8. DS FAILURE MECHANISMS
GROUP 2 MECHANISMS: CAN OCCURS AT LOW STRESS LEVEL
• FATIGUE
• SPLIT BOX
• SULFIDE STRESS CRACKING (CORROSION FAILURE)
• STRESS CORROSION CRACKING (CORROSION FAILURE)

9. FAILURE STUDY

10. OVERLOAD AND FATIGUE
OVERLOAD:
• A CONDITION IN WHICH THE BULK STRESS IN A COMPONENT EXCEEDS YIELD STRENGTH AT THE
WEAKEST POINT IN THE 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
• 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
HOW DO YOU RECOGNIZE A TENSILE FAILURE?

12. TENSILE FAILURES
APPEARANCE : JAGGED AND NECKED DOWN
• ORIENTATION: 45 DEG TO PIPE AXIS
• PIN STRETCHED DUE EXCESS TENSION
AND/OR HIGH MAKE UP TORQUE

13. RESPONDING TO TENSILE FAILURES
• 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 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. TORSIONAL FAILURE
• 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.

15. TORSIONAL FAILURE
• TORSIONAL STRESS LIMIT IS EXCEEDED.
• FAILURES OCCUR IN FORM OF STRETCHED PIN OR BELLED BOX (SWELLING).
• TORSIONAL FAILURES USUALLY OCCUR IN THE TOOL JOINT.

16. RESPONDING TO TORSIONAL FAILURES
• 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.
• 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

18. COMBINATION OF TENSION/TORSION
• THESE FAILURES ARE MOST LIKELY TO HAPPEN WHILE FISHING OR PULLING ON STUCK PIPE.

19. BURST AND COLLAPSE FAILURES
• 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.

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.

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 DIES
• MINIMIZING ROTATING HOURS (USE DOWN-HOLE MOTORS)
• RUN A “CASING FRIENDLY” HARDBANDING MATERIAL ON TOOL JOINTS

22. WELD RELATED FAILURES
• WITH THE OBVIOUS EXCEPTION OF TOOL JOINT TO TUBE WELDS, WELDED COMPONENTS IN THE
DRILL STRING SHOULD BE AVOIDED.
• 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
• 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 AND AT THE SAME TIME IT IS BENT OR
BUCKLED.
• FATIGUE MAY RESULT FROM EXCESSIVE VIBRATION

25. STRESS CONCENTRATORS
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.
• 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 FAILURE OCCURS.

27. RECOGNIZING FATIGUE FAILURES
• 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, 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. RECOGNIZING FATIGUE FAULT
SHAPE AND APPEARANCE:
• FLAT PLANAR SHAPE. MAYBE ACCOMPANIED BY RAGGED AREA WHERE COMPONENT PARTED IN
TENSION
LOCATION
• BHA CONNECTIONS…NEAR LAST ENGAGED THREAD ROOTS
ORIENTATION
• PERPENDICULAR TO THE PIPE AXIS

29. RECOGNIZING FATIGUE FAILURES

30. RECOGNIZING STRESS CONCENTRATORS
SLIP CUTS
UPSETS

31. RECOGNIZING STRESS CONCENTRATORS
• CYCLIC LOADING CAUSES VERY SMALL CRACKS.
• WITH REPEATED CYCLES, THE CRACKS GROW.
• FATIGUE IS CUMULATIVE.
• FATIGUE CRACKS OCCUR IN A 90 DEGREE PLANE TO AXIS
OF PIPE.

32. PREVENTION OF FATIGUE FAILURES
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
• 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).

33. COROSSION
CORROSION OCCURS DUE TO ELECTROCHEMICAL REACTIONS WITH
CORROSIVE AGENTS.
CORROSION RATE INCREASES WHEN:
• HIGHER TEMPERATURE. RATES DOUBLE FOR EACH 31°C.
• HIGHER FLOW RATE, ESPECIALLY IF ABRASIVE SOLIDS PRESENT.
• 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

34. COROSSION
CORROSION REDUCES THE WALL THICKNESS OF TUBULAR.
THERE ARE THREE PATTERNS OF CORROSION;
• UNIFORM WALL THICKNESS REDUCTION
• LOCALIZED PATTERNS OF METAL LOSS
• PITTING

35. 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.

36. SULFIDE STRESS CRACKING

37. SULFIDE STRESS CRACKING
• OCCURS IN H2S ENVIRONMENT
• FE++ + H S⇒FES + 2H+
• ELEMENTAL HYDROGEN (H +) MIGRATES INTO STEEL AND COLLECTS AT HIGH
STRESS POINTS
• ELEMENTAL HYDROGEN COMBINES TO FORM MOLECULAR HYDROGEN (H 2)
CAUSING A CRACK.

38. PREVENTING COROSSION
CORROSIVE ATTENTION USUALLY FALLS INTO ONE OR MORE OF THE AREAS
BELOW:
• OXYGEN
• PH
• CO2 AND CHLORIDES
• HYDROGEN SULFIDE
• BARRIERS AND INHIBITORS

39. PREVENTING FAILURES
• KEEP H2S OUT OF THE MUD SYSTEM BY:
I) DRILLING OVERBALANCED
II) KEEPING PH HIGH
III) USING H2S SCAVENGERS
IV) USING AN OIL BASED MUD
• CONTROL THE METALLURGY
• USE A DIFFERENT GRADE PIPE

40. 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

41. 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)
• VISUAL

42. 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
- SEVERITY OF THE DRILLING CONDITIONS
- SAFETY AND ENVIRONMENTAL IMPACT OF A FAILURE
- COST IMPACT OF A FAILURE
- RISK TOLERANCE OF MANAGEMENT