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Drill String
Failure
Handbook
This handbook contains
data shetts for the most
common drill string
failure mechanisms
J W Martin
Version 1 dated October 1995
Main CD
Contents
DRILL STRING FAILURE
HANDBOOK
By
J W MARTIN
INTRODUCTION
This Handbook contains Data Sheets for the most
common drill string failure mechanisms.
For each failure mechanism advice is given on identifying
features; contributory factors (drilling operations,
conditions, equipment design, etc.); immediate actions to
take to try to prevent a recurrence; recommended
further actions/assessments.
It is intended that this Handbook is used in conjunction
with the 'Drill String Failure Data Base', being set-up
within DEAP, to ensure the correct classification of the
failure mechanism/s within the data base.
In the context of these Guidelines "failure" should
include any incident that leads to a loss of drilling time
(i.e. a twist-off or a wash-out). However, it is
recommended that 'inspection rejects' or 'scrapping of
drill equipment items' should also be considered.
FAILURE MODE DATA SHEET 1.
TENSILE OVERLOAD
1. Description of the Failure Mode
Tensile overload, as the name indicates, occurs when a stress is applied to a downhole component which exceeds the
tensile strength of the material. Most commonly this mode is observed in the drill pipe body, as this is the "weak link" in
the drill string in terms of tensile strength.
2. Typical Characteristics of the Failure/s to Look Out For
A tensile overload fracture is characterised by a reduction in the pipe cross-sectional area immediately adjacent to the
point of failure, known as 'necking'. In addition, there will probably be a thinning of the drill pipe wall, this being more
pronounced at the point of failure. The fracture surfaces will often be at an angle of approximately 45o to the pipe axis,
as final fracture in thin walled tubes occurs by a 'shear' mechanism.
3. Contributory Factors to the Failure Mode to Look Out For
Tensile overload fractures will often occur when the drillstring becomes stuck and too large an over-pull is applied to the
drill string. Failure will be preceded by plastic deformation of the drill string, such that an apparent reduction in
hookload may be observed.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. The remainder of the string should be checked to see whether the lengths have increased. In particular, the threads
should be checked for stretch. Any stretched components should be laid down for further detailed inspection prior to
scrapping or downgrading.
. The drilling records just prior to failure should be checked to determine what loads were applied to the drill string that
resulted in failure. Clearly where excessive loads are identified future operations should take care to prevent a
recurrence.
5. Recommendations for Further Actions and Assessments, if any
. The failed component should be checked to establish whether it meets the specification with respect to mechanical
properties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation of
the whole string.
. When estimating the acceptable loading (hookload + overpull) on a drillstring, any reductions in wall thickness, e.g.
due to wear, should be considered and the safety factors in API 7 applied.
. If there may be a requirement for additional overpull/hookload in excess of the capability of the drill string, consider
using a higher strength or higher weight drill pipe.
FAILURE MODE DATA SHEET 2.
BRITTLE FRACTURE
1. Description of the Failure Mode
Brittle fracture occurs very rapidly with little associated plastic deformation and indicates a deficient material, i.e.
inadequate toughness.
2. Typical Characteristics of the Failure/s to Look Out For
Brittle fracture will occur in materials that have inadequate 'toughness'. There will be little or no plastic deformation
associated with the failure, which is likely to have a 'crystalline' appearance. The fracture will often occur at stress-
raisers such as regions of mechanical damage, threaded regions, pre-existing cracks, changes of section, etc. It can easily
be confused with fatigue, such that toughness tests (Charpy tests) are often necessary to confirm that the material is
brittle. Fracture surfaces are usually normal (at right angles) to the applied stress.
3. Contributory Factors to the Failure Mode to Look Out For
Brittle fracture will occur very rapidly and there is normally little or no warning that failure is about to occur. Brittle
materials are usually quite strong. However, they are very sensitive to stress raisers (low toughness equates to high 'notch
sensitivity') such as at mechanical damage, sharp changes in section, etc. Hence failures can occur at loads well below
the theoretical load capacity of the item. Failures will often occur under shock load conditions, e.g. during jarring
operations, during handling top-side if the drilling component strikes/is struck by another item. Steel toughness is often
inferior at lower temperatures, therefore brittle fracture is more likely at lower temperatures (e.g. top-side in colder
climes).
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. The history of other components in the string should be checked, to determine if they may be from the same 'batch' as
the faulty material. Any suspect material should be laid down for further evaluation.
. The drilling records just prior to failure should be checked to determine if loads were applied to the drill string that
could have resulted in the failure. Clearly if excessive loads are identified, future operations should take care to prevent a
recurrence.
5. Recommendations for Further Actions and Assessments, if any
. The failed component should be checked to establish if it was brittle, this will necessitate undertaking Charpy
(toughness) tests.
. The manufacturer/supplier should be contacted in instances where the relevant specifications are not met, to ensure
there is no recurrence.
FAILURE MODE DATA SHEET 3.
TORSIONAL OVERLOAD
1. Description of the Failure Mode
Torsional overload is synonymous with tensile overload; in this case it occurs when a stress is applied to the downhole
component which exceeds the torsional strength of the material. Most commonly, torsion-associated failures are
observed in tool-joints, as these are the "weak links" in the drill string in terms of torsional strength. Torsional failures
can occur in the drill pipe body, but these are relatively rare. For tool-joint failures refer to Failure Datasheet 12.
2. Typical Characteristics of the Failure/s to Look Out For
A torsional overload within the drill pipe body is characterised by a spiral fracture path at an angle of approximately 45o
to the stress axis, giving a fracture such as shown in the Figure above.
3. Contributory Factors to the Failure Mode to Look Out For
Torsional overload fractures are often the result of a torque applied to the drillstring that is greater than its torsional
strength. This may occur when the drillstring becomes stuck, e.g. if the bottom section of the drillstring becomes stuck
and the upper section keeps rotating.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. The remainder of the string, should be checked to see whether there has been any torsional yielding. Any such pipes
should be laid down for further inspection prior to clearance for re-use, scrapping or downgrading.
. The drilling records just prior to failure should be checked to determine if torques were applied to the drill string that
could have resulted in the failure. Clearly where excessive loads are identified future operations should take care to
prevent a recurrence.
. Tool-joints within the string should be checked for cracks at the thread roots (using MPI).
. Tool-joints within the string should also be checked for pin stretch and box belling.
5. Recommendations for Further Actions and Assessments, if any
. The failed component should be checked to establish whether it meets the specification with respect to mechanical
properties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation of
the whole string.
FAILURE MODE DATA SHEET 4.
FATIGUE
Overload Fatigue
1. Description of the Failure Mode
Fatigue failure is the result of repeated (cyclic) stressing at a load level less than the material strength. It is a three stage process comprising initiation, fatigue
propagation followed by final fracture when the remaining material cross sectional area can no longer support the applied load. Corrosion fatigue is a related
mechanism in which fatigue failure occurs prematurely as a result of the simultaneous effect of corrosion (refer to Datasheet 10).
2. Typical Characteristics of the Failure/s to Look Out For
Fatigue fractures most commonly occur within the threaded region of BHA tool-joints [Figure A] (often in the last engaged thread). Other common areas
include the end of the internal upset of drill pipe [Figure B] and changes in section of drilling equipment. These are all regions of stress concentration within
the drill string. A fatigue fracture is characterised by a flat relatively featureless area, sometimes with surface markings caused by crack arrest (known as
'beach markings') and an area of final fracture (tensile tearing and/or shear). The fracture surfaces will often suffer post-failure damage as the result of mud
wash-out through cracks prior to final fracture and/or rubbing damage after fracture. This can obliterate the fracture surfaces, making positive identification
of fatigue difficult.
3. Contributory Factors to the Failure Mode to Look Out For
Fatigue failures are often the result of rotating the drill string within a curved section of hole (i.e. a dog-leg) and/or vibration. In addition, fatigue failures of
the pin connection can result from insufficient make-up torque, causing excessive fatigue loading of the pin. Poor drill string component design features, such
as sharp changes in section, thread run-outs, etc. can result in premature fatigue failure. Corrosion fatigue will be promoted by inadequate corrosion control of
drilling muds, particularly in the case of water-based drilling muds.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Other components in the string close to the failure location (e.g. the BHA if that is where failure occurred) should be checked for cracks in the threaded
regions, at changes of section and/or in the taper section. Any components in which cracks are identified should be laid down for further inspection and
scrapping or renovation.
. The drilling records prior to failure should be checked to ensure that sufficient make-up torque was used and determine if failure was associated with a
severe dog-leg and/or vibration. In addition, the previous history of the component should be checked to determine when the last inspection occurred and
previous usage. If insufficient make-up torque is indicated, steps should be taken to ensure sufficient make-up torque in the future. If severe vibration is
indicated, steps should be taken to prevent a recurrence.
. Ensure that the threaded regions of drill string components (especially in the BHA) have stress-relief groove and bore-back features to reduce the risk of
fatigue failure.
5. Recommendations for Further Actions and Assessments, if any
. The failed component should be checked to establish that the failure was indeed the result of fatigue, whether the material meets the specification with
respect to mechanical properties and if there were any contributory causes (mechanical damage, stress corrosion cracking, etc. - see other datasheets as
appropriate).
. The inspection programme for the drill string should be reviewed to ensure that the inspection intervals are optimised to prevent further failures without
invoking excessive inspection (refer to BPX Guidelines - Inspection Frequency Guidelines for BHAs, DCB/9/95).
. The likelihood of severe vibration should be assessed and actions taken to prevent a recurrence if necessary.
. Check the design of drill string components to ensure there are no sharp changes in section that can promote fatigue failures.
. Where corrosion fatigue occurs within the upset region, consider the use of internally plastic coated drill pipe.
FAILURE MODE DATA SHEET 5.
MECHANICAL DAMAGE
1. Description of the Failure Mode
'Mechanical damage' occurs as the result of the drillstring component being struck by or
pressed/rubbed against another physical body, e.g. handling equipment, junk in hole. Mechanical
damage will often contribute to premature failure by other mechanisms such as fatigue, overload,
stress corrosion cracking. Refer to other datasheets as appropriate.
2. Typical Characteristics of the Failure/s to Look Out For
Mechanical damage can take a number of forms and there are a number of causes. Typical forms of
damage are tong marks; slip marks; cuts and scores due to rubbing against junk in hole, hard
formations, etc.; impact marks ('dents').
3. Contributory Factors to the Failure Mode to Look Out For
Poor handling procedures, e.g. impact against handling equipment, incorrect lifting procedures, poor
storage.
Junk left in the hole.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Check other components in the string for similar damage. Any damaged components should be laid
down for further inspection prior to clearance for further use, downgrading or scrapping according to
the criteria of API RP7G.
. Ensure good 'house keeping' and care/handling procedures are invoked.
5. Recommendations for Further Actions and Assessments, if any
. The failed component should be checked to establish whether mechanical damage was the main cause
of failure or whether there were other contributory causes that need to be addressed.
FAILURE MODE DATA SHEET 6.
HEATCHECKING
1. Description of the Failure Mode
Heatchecking results from frictional heating causing the temperature of a drill string component to increase
significantly. Three types of heatchecking have been identified: Type I in which frictional heating followed by rapid
quenching results in a "crazed" pattern of quench cracks on the surface; Type II in which the bulk properties of the
component (metallurgical and mechanical) are changed by frictional heating; Type III in which the loss in strength
resulting from frictional heating leads to plastic deformation and probably failure.
2. Typical Characteristics of the Failure/s to Look Out For
Type I - a characteristic 'craze' cracking of the surface (see photograph); signs of wear in affected region; in more
advanced cases deeper longitudinal cracks in affected region.
Type II - significant wear in affected region; possible 'blueing' (oxide film) in the affected region.
Type III - gross plastic deformation in the affected region leading to significant 'bottlenecking' or significant reduction
of cross-sectional area; significant wear in affected region; possible 'blueing' (oxide film) in the affected region.
3. Contributory Factors to the Failure Mode to Look Out For
A number of drilling conditions are likely to increase the risk of heatchecking, i.e. mudstone (>50%)/sandstone (<50%)
lithologies; RPM >125; stuck pipe; reaming; large drill pipe/small hole ratios; lost mud circulation or low annular
drilling fluid velocities; hole wash-outs; high dog-leg severity/hole tortuosity; increases in friction.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Other components in the string close to the 'heatchecked' component (e.g. the BHA if that is where failure occurred)
should be checked for signs of heatchecking. Any components in which heatchecking is suspected should be laid down
for further inspection prior to clearance for further use, scrapping or renovation.
. To minimise the likelihood of 'heatchecking' in future operations refer to BPX Guidelines (Guidelines for Heatchecking
Minimisation - ESR.ER.071). Actions include: minimise losses in mud circulation; minimise RPM (<100 if possible);
maximise mud annular fluid flow rate (>100fps if possible); minimise hole tortuosity and dog-leg severity; consider
minimising use of large pipe sizes in small holes.
5. Recommendations for Further Actions and Assessments, if any
. Refer to BPX Guidelines (Guidelines for Heatchecking Minimisation - ESR.ER.071) for detailed recommendations on
dealing with heatchecking regarding identification; minimising problems; refurbishment.
. Undertake further failure analysis (including metallography, hardness checks, mechanical properties, etc.) to confirm
presence of heatchecking, particularly in the case of Types II and III heatchecking.
FAILURE MODE DATA SHEET 7.
GALLING
1. Description of the Failure Mode
Galling occurs most commonly in the threaded regions of drill string components. It results from two metallic surfaces
sliding against each other with insufficient lubrication and/or excessive contact loads. Microscopic asperities 'weld'
together (bond) at the sliding faces under the very high local forces. In the extreme this can lead to total fusing of the
two surfaces. Alternatively, the sliding forces fracture the 'welds' leading to material transference from one surface to the
other.
2. Typical Characteristics of the Failure/s to Look Out For
For threaded connections galling damage is characterised by the 'bonding' of the two components making break-out
difficult (or even impossible!). Complete threads can be 'stripped' in the connection (see Figure A).
For two surfaces rubbing together the transfer of material from one component to the other leads to a rough surface
showing sign of 'tearing' (see Figure B).
3. Contributory Factors to the Failure Mode to Look Out For
Galling of threads is the result of poor lubrication (e.g. insufficient or incorrect thread dope); thread damage prior to
make-up (e.g. mechanical damage causing 'high spots'); overtorquing (e.g. downhole make-up); poor make-up
procedures (e.g. 'forced make-up').
Galling of surfaces can also result from high contact forces.
Non-magnetic components are particularly prone to galling due to the high chromium/nickel content.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Check other components in the string for galling damage. Any galled components should be laid down for further
inspection prior to clearance for further use, downgrading or scrapping.
. Check the correct thread compound is being used and is being applied correctly.
. Check that the proper joint make-up procedures are being adhered to, including the correct make-up torque and speed.
5. Recommendations for Further Actions and Assessments, if any
. The galled components should be checked to establish whether the correct surface treatments have been used, e.g.
phosphating treatment or copper plating for ferromagnetic components, shot peening and oxalation treatment/copper
plating or equivalent for non-magnetic components.
. Ensure that new tool-joints are subjected to the appropriate 'breaking-in' procedure.
FAILURE MODE DATA SHEET 8.
WEAR
1. Description of the Failure Mode
Wear damage is the removal of material from the surfaces of components caused by rubbing (typically
against casing, formation rock, junk in hole, casing shoes, etc.). By itself, wear rarely results in
failures, more likely it will be the precursor for failure by another mechanism, e.g. heatchecking,
overload, fatigue.
2. Typical Characteristics of the Failure/s to Look Out For
Worn areas have characteristic surface damage ranging from mild polishing-type material attrition
through to severe material wastage and surface roughening (e.g. gouging). In extreme cases,
heatchecking can result (See Failure Datasheet No. 6).
3. Contributory Factors to the Failure Mode to Look Out For
A number of factors can exacerbate wear. These include, poor hole stability (leading to ledges, etc.);
junk in hole; hard rock formations; poor clearance; poor lubrication.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Where a failure occurs as the result of wear, the remainder of the string should be checked to see
whether similar levels of wear have occurred to other components, especially those close to the point
of failure. Any worn components should be laid down for detailed inspection prior to consideration for
re-use, downgrading or scrapping.
. For the more severe types of wear, the source of the wear should be identified and the necessary
actions taken, e.g. prevent junk-in-hole, consider use of drill pipe protectors.
5. Recommendations for Further Actions and Assessments, if any
. The mild polishing-type wear should be identified during normal inspection and action taken to
prevent over-worn items being re-used. The criteria for acceptable amounts of wear (acceptable
minimum ODs) are contained in API RP7G.
. Prime areas for wear are large OD components, e.g. tool-joints, HWDP centre section, drill collars,
etc. These areas should be hardbanded in line with the BPX Guidelines (Drillstring Hardbanding BPX
Drilling DCB Specification, DCB/20/94).
FAILURE MODE DATA SHEET 9.
EROSION
1. Description of the Failure Mode
Erosion is the wastage of material due to mechanical removal of the surface material by a flowing
environment. Such wastage is more extreme when solids (e.g. sand) are present in the environment.
Erosion alone will only rarely cause drillstring failures. However, erosion damage, in the form of mud
wash-out, is often associated with other failure mechanisms, e.g. fatigue.
2. Typical Characteristics of the Failure/s to Look Out For
Eroded surfaces will often have a characteristic 'scalloped' appearance, due to the 'eddy currents'
caused in the very turbulent flow (Figure A). The material surface itself will be smooth and clean. The
most common form of erosion in drill string components is the result of drilling mud washing out
through a very small gap, such as a through-wall crack (Figure B).
3. Contributory Factors to the Failure Mode to Look Out For
Erosion is often the result of either a through-wall crack or a poorly tightened connection such that the
shoulder seal is not sufficiently 'energised'. The drilling mud will flow through the resultant small gap
resulting in rapid material wastage. In addition, a drop in the mud pump pressure may be observed.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. If the erosion damage is the result of a through wall crack, establish the cause of cracking and go to
the necessary Failure Data Sheet, e.g. fatigue, stress corrosion cracking.
. If the erosion damage is the result of poor connection make-up check the make-up procedures,
make-up torque, etc.
5. Recommendations for Further Actions and Assessments, if any
None required
FAILURE MODE DATA SHEET 10.
CORROSION
1. Description of the Failure Mode
Corrosion can be defined as metal wastage resulting from the reaction of the metal with its
environment. For drillstrings there are two main types of corrosion, i.e. 'general corrosion' which is the
wastage of the entire component at about an 'even' rate or 'localised corrosion' which is the wastage of
small regions of the component leading to 'pits' (known as "pitting"). Of these two mechanisms the
more destructive is pitting, because pitting corrosion rates can be much more rapid than general
corrosion rates. In addition pits act as 'stress raisers', which can promote other failure mechanisms
such as stress corrosion cracking, corrosion-fatigue, fatigue, brittle fracture.
2. Typical Characteristics of the Failure/s to Look Out For
Metal wastage can be either general [Figure A] or localised ('pitting') [Figure B]. There will often be a
degree of surface roughening and the presence of corrosion products ('rust') in the corroded areas.
3. Contributory Factors to the Failure Mode to Look Out For
Corrosion will normally be more prevalent in water based drilling muds than in oil based muds. The
drilling mud chemistry will affect its corrosivity. For example, lower (more acidic) pH levels or air
entrainment will increase mud corrosivity.
Pitting/localised corrosion is often exacerbated under deposits, e.g. underneath mud deposits left on
drill string components after retrieval.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. Take steps to improve the corrosion control of drilling muds. For guidance refer to the BPX
Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling,
DCB/28/93/BR).
. Clean mud deposits from recovered drill string components, this is particularly important for non-
magnetic austenitic stainless steel components.
5. Recommendations for Further Actions and Assessments, if any
Use internally plastic coated drill pipe.
FAILURE MODE DATA SHEET 11.
STRESS CORROSION CRACKING
1. Description of the Failure Mode
Stress corrosion cracking (SCC) is a mechano-environmental mechanism, whereby the conjoint action of an environment and stress
results in cracking of metallic materials. For drill string components there are two common sources of SCC, i.e. for high strength
steels, such as used for ferromagnetic drill string components, there is the possibility of sulphide stress cracking in the presence of
hydrogen sulphide; for austenitic stainless steels, such as used for non-magnetic drill string components, there is the possibility of
chloride stress corrosion cracking in aqueous solutions containing chlorides (e.g. drilling muds, brines, etc.).
2. Typical Characteristics of the Failure/s to Look Out For
SCC alone does not often result in failure of drill string components, rather it is the precursor for failures due to fatigue, overload,
etc. Visually SCC cracks can be difficult to discern, as they can be very fine. Therefore, it is normally necessary to undertake
inspection (e.g. MPI, dye penetrant inspection) and, in the extreme, metallographic examination to confirm the presence of SCC.
SCC will often result in a network of cracks on the material surface (see Figure A above). However, such networks are not always
discernible. SCC cracks are often associated with stress raisers, e.g. at the bottom of threads and changes of section, associated with
corrosion pits.
3. Contributory Factors to the Failure Mode to Look Out For
Sulphide stress cracking (SSC) of high strength ferromagnetic steels is associated with the presence of hydrogen sulphide. Therefore,
this type of cracking is normally associated with drilling sour wells.
Chloride stress corrosion cracking (CSC) of non-magnetic austenitic materials is associated with the presence of chloride,
temperatures in excess of about 50oC and an oxidising species (e.g. oxygen and/or hydrogen sulphide). There will often (but not
always) be corrosion pitting associated with the cracks.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. In the case of SSC consider the use of a lower strength drill pipe that is not prone to this type of cracking (e.g. X95 drill pipe). Take
steps to improve the corrosion control of drilling muds, e.g. use of sulphide scavengers, minimum pH of 11. For guidance refer to the
BPX Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR).
. In the case of CSC ensure that mud deposits are always cleaned from the component on retrieval. Take steps to improve the
corrosion control of drilling muds, e.g. consider use of oxygen scavenger for water-based muds. For guidance refer to the BPX
Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR).
5. Recommendations for Further Actions and Assessments, if any
. The presence of stress corrosion cracking (SCC) is very difficult to confirm visually. Therefore, further failure analysis (especially
metallography) should be undertaken to confirm presence of SCC (see Figures B & C).
. In the case of non-magnetic austenitic materials, to help avoid CSC only low carbon materials should be used (0.05% C max.). In
addition, the bore of components exposed to drilling mud should be hammer or shot peened; threaded regions should be cold rolled or
shot peened.
FAILURE MODE DATA SHEET 12.
OVER TORQUE OF TOOL-JOINTS
1. Description of the Failure Mode
Over-torque of tool-joints will result in a number of possible failure mechanisms. The main failure mechanisms are:
tensile failure of the pin connection in the last engaged thread; belling or splitting of the box connection; stretching of
the pin connection.
2. Typical Characteristics of the Failure/s to Look Out For
Tensile failure of the pin will often result in a 'cup and cone' failure [Figure A] or a mixed failure of 'cup and cone' and
shear [Figure B].
Detection of box belling or pin stretch will often require the use of a straight edge (e.g. steel rule) or thread profile gauge
respectively.
3. Contributory Factors to the Failure Mode to Look Out For
Overtorquing of tool-joints can occur either during make-up (e.g. using too high a make-up torque; loss of torque
capacity due to wear/corrosion) or as a result of 'downhole torque' (i.e. tool-joints being tightened during drilling).
Downhole torque may occur when the drillstring becomes stuck, e.g. if the bottom section of the drillstring becomes
stuck and the upper section keeps rotating, but more commonly it occurs during drilling as a result of resistance to bit
rotation.
4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ?
. If torsional damage to tool-joints is detected then all the tool-joints within the string should be checked for cracks at the
thread roots (using MPI) and pin stretch/box belling. Any suspect pipes should be quarantined for detailed inspection
prior to clearance for re-use, scrapping or downgrading.
. The make-up torque should be checked to ensure that it conforms with the recommended values (see API RP7G for
guidance).
. Worn/corroded tool-joints should be checked to ensure that they conform with the required minimum OD (see API
RP7G for guidance).
5. Recommendations for Further Actions and Assessments, if any
. If additional torque capacity is required in excess of the capability of the drill string consideration should be given to
using high torque tool-joints or high coefficient of friction thread compounds.

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Bp drill string failure

  • 1. Drill String Failure Handbook This handbook contains data shetts for the most common drill string failure mechanisms J W Martin Version 1 dated October 1995 Main CD Contents
  • 2. DRILL STRING FAILURE HANDBOOK By J W MARTIN INTRODUCTION This Handbook contains Data Sheets for the most common drill string failure mechanisms. For each failure mechanism advice is given on identifying features; contributory factors (drilling operations, conditions, equipment design, etc.); immediate actions to take to try to prevent a recurrence; recommended further actions/assessments. It is intended that this Handbook is used in conjunction with the 'Drill String Failure Data Base', being set-up within DEAP, to ensure the correct classification of the failure mechanism/s within the data base. In the context of these Guidelines "failure" should include any incident that leads to a loss of drilling time (i.e. a twist-off or a wash-out). However, it is recommended that 'inspection rejects' or 'scrapping of drill equipment items' should also be considered.
  • 3. FAILURE MODE DATA SHEET 1. TENSILE OVERLOAD 1. Description of the Failure Mode Tensile overload, as the name indicates, occurs when a stress is applied to a downhole component which exceeds the tensile strength of the material. Most commonly this mode is observed in the drill pipe body, as this is the "weak link" in the drill string in terms of tensile strength. 2. Typical Characteristics of the Failure/s to Look Out For A tensile overload fracture is characterised by a reduction in the pipe cross-sectional area immediately adjacent to the point of failure, known as 'necking'. In addition, there will probably be a thinning of the drill pipe wall, this being more pronounced at the point of failure. The fracture surfaces will often be at an angle of approximately 45o to the pipe axis, as final fracture in thin walled tubes occurs by a 'shear' mechanism. 3. Contributory Factors to the Failure Mode to Look Out For Tensile overload fractures will often occur when the drillstring becomes stuck and too large an over-pull is applied to the drill string. Failure will be preceded by plastic deformation of the drill string, such that an apparent reduction in hookload may be observed. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . The remainder of the string should be checked to see whether the lengths have increased. In particular, the threads should be checked for stretch. Any stretched components should be laid down for further detailed inspection prior to scrapping or downgrading. . The drilling records just prior to failure should be checked to determine what loads were applied to the drill string that resulted in failure. Clearly where excessive loads are identified future operations should take care to prevent a recurrence. 5. Recommendations for Further Actions and Assessments, if any . The failed component should be checked to establish whether it meets the specification with respect to mechanical properties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation of the whole string. . When estimating the acceptable loading (hookload + overpull) on a drillstring, any reductions in wall thickness, e.g. due to wear, should be considered and the safety factors in API 7 applied. . If there may be a requirement for additional overpull/hookload in excess of the capability of the drill string, consider using a higher strength or higher weight drill pipe.
  • 4. FAILURE MODE DATA SHEET 2. BRITTLE FRACTURE 1. Description of the Failure Mode Brittle fracture occurs very rapidly with little associated plastic deformation and indicates a deficient material, i.e. inadequate toughness. 2. Typical Characteristics of the Failure/s to Look Out For Brittle fracture will occur in materials that have inadequate 'toughness'. There will be little or no plastic deformation associated with the failure, which is likely to have a 'crystalline' appearance. The fracture will often occur at stress- raisers such as regions of mechanical damage, threaded regions, pre-existing cracks, changes of section, etc. It can easily be confused with fatigue, such that toughness tests (Charpy tests) are often necessary to confirm that the material is brittle. Fracture surfaces are usually normal (at right angles) to the applied stress. 3. Contributory Factors to the Failure Mode to Look Out For Brittle fracture will occur very rapidly and there is normally little or no warning that failure is about to occur. Brittle materials are usually quite strong. However, they are very sensitive to stress raisers (low toughness equates to high 'notch sensitivity') such as at mechanical damage, sharp changes in section, etc. Hence failures can occur at loads well below the theoretical load capacity of the item. Failures will often occur under shock load conditions, e.g. during jarring operations, during handling top-side if the drilling component strikes/is struck by another item. Steel toughness is often inferior at lower temperatures, therefore brittle fracture is more likely at lower temperatures (e.g. top-side in colder climes). 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . The history of other components in the string should be checked, to determine if they may be from the same 'batch' as the faulty material. Any suspect material should be laid down for further evaluation. . The drilling records just prior to failure should be checked to determine if loads were applied to the drill string that could have resulted in the failure. Clearly if excessive loads are identified, future operations should take care to prevent a recurrence. 5. Recommendations for Further Actions and Assessments, if any . The failed component should be checked to establish if it was brittle, this will necessitate undertaking Charpy (toughness) tests. . The manufacturer/supplier should be contacted in instances where the relevant specifications are not met, to ensure there is no recurrence.
  • 5. FAILURE MODE DATA SHEET 3. TORSIONAL OVERLOAD 1. Description of the Failure Mode Torsional overload is synonymous with tensile overload; in this case it occurs when a stress is applied to the downhole component which exceeds the torsional strength of the material. Most commonly, torsion-associated failures are observed in tool-joints, as these are the "weak links" in the drill string in terms of torsional strength. Torsional failures can occur in the drill pipe body, but these are relatively rare. For tool-joint failures refer to Failure Datasheet 12. 2. Typical Characteristics of the Failure/s to Look Out For A torsional overload within the drill pipe body is characterised by a spiral fracture path at an angle of approximately 45o to the stress axis, giving a fracture such as shown in the Figure above. 3. Contributory Factors to the Failure Mode to Look Out For Torsional overload fractures are often the result of a torque applied to the drillstring that is greater than its torsional strength. This may occur when the drillstring becomes stuck, e.g. if the bottom section of the drillstring becomes stuck and the upper section keeps rotating. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . The remainder of the string, should be checked to see whether there has been any torsional yielding. Any such pipes should be laid down for further inspection prior to clearance for re-use, scrapping or downgrading. . The drilling records just prior to failure should be checked to determine if torques were applied to the drill string that could have resulted in the failure. Clearly where excessive loads are identified future operations should take care to prevent a recurrence. . Tool-joints within the string should be checked for cracks at the thread roots (using MPI). . Tool-joints within the string should also be checked for pin stretch and box belling. 5. Recommendations for Further Actions and Assessments, if any . The failed component should be checked to establish whether it meets the specification with respect to mechanical properties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation of the whole string.
  • 6. FAILURE MODE DATA SHEET 4. FATIGUE Overload Fatigue 1. Description of the Failure Mode Fatigue failure is the result of repeated (cyclic) stressing at a load level less than the material strength. It is a three stage process comprising initiation, fatigue propagation followed by final fracture when the remaining material cross sectional area can no longer support the applied load. Corrosion fatigue is a related mechanism in which fatigue failure occurs prematurely as a result of the simultaneous effect of corrosion (refer to Datasheet 10). 2. Typical Characteristics of the Failure/s to Look Out For Fatigue fractures most commonly occur within the threaded region of BHA tool-joints [Figure A] (often in the last engaged thread). Other common areas include the end of the internal upset of drill pipe [Figure B] and changes in section of drilling equipment. These are all regions of stress concentration within the drill string. A fatigue fracture is characterised by a flat relatively featureless area, sometimes with surface markings caused by crack arrest (known as 'beach markings') and an area of final fracture (tensile tearing and/or shear). The fracture surfaces will often suffer post-failure damage as the result of mud wash-out through cracks prior to final fracture and/or rubbing damage after fracture. This can obliterate the fracture surfaces, making positive identification of fatigue difficult. 3. Contributory Factors to the Failure Mode to Look Out For Fatigue failures are often the result of rotating the drill string within a curved section of hole (i.e. a dog-leg) and/or vibration. In addition, fatigue failures of the pin connection can result from insufficient make-up torque, causing excessive fatigue loading of the pin. Poor drill string component design features, such as sharp changes in section, thread run-outs, etc. can result in premature fatigue failure. Corrosion fatigue will be promoted by inadequate corrosion control of drilling muds, particularly in the case of water-based drilling muds. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Other components in the string close to the failure location (e.g. the BHA if that is where failure occurred) should be checked for cracks in the threaded regions, at changes of section and/or in the taper section. Any components in which cracks are identified should be laid down for further inspection and scrapping or renovation. . The drilling records prior to failure should be checked to ensure that sufficient make-up torque was used and determine if failure was associated with a severe dog-leg and/or vibration. In addition, the previous history of the component should be checked to determine when the last inspection occurred and previous usage. If insufficient make-up torque is indicated, steps should be taken to ensure sufficient make-up torque in the future. If severe vibration is indicated, steps should be taken to prevent a recurrence. . Ensure that the threaded regions of drill string components (especially in the BHA) have stress-relief groove and bore-back features to reduce the risk of fatigue failure. 5. Recommendations for Further Actions and Assessments, if any . The failed component should be checked to establish that the failure was indeed the result of fatigue, whether the material meets the specification with respect to mechanical properties and if there were any contributory causes (mechanical damage, stress corrosion cracking, etc. - see other datasheets as appropriate). . The inspection programme for the drill string should be reviewed to ensure that the inspection intervals are optimised to prevent further failures without invoking excessive inspection (refer to BPX Guidelines - Inspection Frequency Guidelines for BHAs, DCB/9/95). . The likelihood of severe vibration should be assessed and actions taken to prevent a recurrence if necessary. . Check the design of drill string components to ensure there are no sharp changes in section that can promote fatigue failures. . Where corrosion fatigue occurs within the upset region, consider the use of internally plastic coated drill pipe.
  • 7. FAILURE MODE DATA SHEET 5. MECHANICAL DAMAGE 1. Description of the Failure Mode 'Mechanical damage' occurs as the result of the drillstring component being struck by or pressed/rubbed against another physical body, e.g. handling equipment, junk in hole. Mechanical damage will often contribute to premature failure by other mechanisms such as fatigue, overload, stress corrosion cracking. Refer to other datasheets as appropriate. 2. Typical Characteristics of the Failure/s to Look Out For Mechanical damage can take a number of forms and there are a number of causes. Typical forms of damage are tong marks; slip marks; cuts and scores due to rubbing against junk in hole, hard formations, etc.; impact marks ('dents'). 3. Contributory Factors to the Failure Mode to Look Out For Poor handling procedures, e.g. impact against handling equipment, incorrect lifting procedures, poor storage. Junk left in the hole. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Check other components in the string for similar damage. Any damaged components should be laid down for further inspection prior to clearance for further use, downgrading or scrapping according to the criteria of API RP7G. . Ensure good 'house keeping' and care/handling procedures are invoked. 5. Recommendations for Further Actions and Assessments, if any . The failed component should be checked to establish whether mechanical damage was the main cause of failure or whether there were other contributory causes that need to be addressed.
  • 8. FAILURE MODE DATA SHEET 6. HEATCHECKING 1. Description of the Failure Mode Heatchecking results from frictional heating causing the temperature of a drill string component to increase significantly. Three types of heatchecking have been identified: Type I in which frictional heating followed by rapid quenching results in a "crazed" pattern of quench cracks on the surface; Type II in which the bulk properties of the component (metallurgical and mechanical) are changed by frictional heating; Type III in which the loss in strength resulting from frictional heating leads to plastic deformation and probably failure. 2. Typical Characteristics of the Failure/s to Look Out For Type I - a characteristic 'craze' cracking of the surface (see photograph); signs of wear in affected region; in more advanced cases deeper longitudinal cracks in affected region. Type II - significant wear in affected region; possible 'blueing' (oxide film) in the affected region. Type III - gross plastic deformation in the affected region leading to significant 'bottlenecking' or significant reduction of cross-sectional area; significant wear in affected region; possible 'blueing' (oxide film) in the affected region. 3. Contributory Factors to the Failure Mode to Look Out For A number of drilling conditions are likely to increase the risk of heatchecking, i.e. mudstone (>50%)/sandstone (<50%) lithologies; RPM >125; stuck pipe; reaming; large drill pipe/small hole ratios; lost mud circulation or low annular drilling fluid velocities; hole wash-outs; high dog-leg severity/hole tortuosity; increases in friction. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Other components in the string close to the 'heatchecked' component (e.g. the BHA if that is where failure occurred) should be checked for signs of heatchecking. Any components in which heatchecking is suspected should be laid down for further inspection prior to clearance for further use, scrapping or renovation. . To minimise the likelihood of 'heatchecking' in future operations refer to BPX Guidelines (Guidelines for Heatchecking Minimisation - ESR.ER.071). Actions include: minimise losses in mud circulation; minimise RPM (<100 if possible); maximise mud annular fluid flow rate (>100fps if possible); minimise hole tortuosity and dog-leg severity; consider minimising use of large pipe sizes in small holes. 5. Recommendations for Further Actions and Assessments, if any . Refer to BPX Guidelines (Guidelines for Heatchecking Minimisation - ESR.ER.071) for detailed recommendations on dealing with heatchecking regarding identification; minimising problems; refurbishment. . Undertake further failure analysis (including metallography, hardness checks, mechanical properties, etc.) to confirm presence of heatchecking, particularly in the case of Types II and III heatchecking.
  • 9. FAILURE MODE DATA SHEET 7. GALLING 1. Description of the Failure Mode Galling occurs most commonly in the threaded regions of drill string components. It results from two metallic surfaces sliding against each other with insufficient lubrication and/or excessive contact loads. Microscopic asperities 'weld' together (bond) at the sliding faces under the very high local forces. In the extreme this can lead to total fusing of the two surfaces. Alternatively, the sliding forces fracture the 'welds' leading to material transference from one surface to the other. 2. Typical Characteristics of the Failure/s to Look Out For For threaded connections galling damage is characterised by the 'bonding' of the two components making break-out difficult (or even impossible!). Complete threads can be 'stripped' in the connection (see Figure A). For two surfaces rubbing together the transfer of material from one component to the other leads to a rough surface showing sign of 'tearing' (see Figure B). 3. Contributory Factors to the Failure Mode to Look Out For Galling of threads is the result of poor lubrication (e.g. insufficient or incorrect thread dope); thread damage prior to make-up (e.g. mechanical damage causing 'high spots'); overtorquing (e.g. downhole make-up); poor make-up procedures (e.g. 'forced make-up'). Galling of surfaces can also result from high contact forces. Non-magnetic components are particularly prone to galling due to the high chromium/nickel content. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Check other components in the string for galling damage. Any galled components should be laid down for further inspection prior to clearance for further use, downgrading or scrapping. . Check the correct thread compound is being used and is being applied correctly. . Check that the proper joint make-up procedures are being adhered to, including the correct make-up torque and speed. 5. Recommendations for Further Actions and Assessments, if any . The galled components should be checked to establish whether the correct surface treatments have been used, e.g. phosphating treatment or copper plating for ferromagnetic components, shot peening and oxalation treatment/copper plating or equivalent for non-magnetic components. . Ensure that new tool-joints are subjected to the appropriate 'breaking-in' procedure.
  • 10. FAILURE MODE DATA SHEET 8. WEAR 1. Description of the Failure Mode Wear damage is the removal of material from the surfaces of components caused by rubbing (typically against casing, formation rock, junk in hole, casing shoes, etc.). By itself, wear rarely results in failures, more likely it will be the precursor for failure by another mechanism, e.g. heatchecking, overload, fatigue. 2. Typical Characteristics of the Failure/s to Look Out For Worn areas have characteristic surface damage ranging from mild polishing-type material attrition through to severe material wastage and surface roughening (e.g. gouging). In extreme cases, heatchecking can result (See Failure Datasheet No. 6). 3. Contributory Factors to the Failure Mode to Look Out For A number of factors can exacerbate wear. These include, poor hole stability (leading to ledges, etc.); junk in hole; hard rock formations; poor clearance; poor lubrication. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Where a failure occurs as the result of wear, the remainder of the string should be checked to see whether similar levels of wear have occurred to other components, especially those close to the point of failure. Any worn components should be laid down for detailed inspection prior to consideration for re-use, downgrading or scrapping. . For the more severe types of wear, the source of the wear should be identified and the necessary actions taken, e.g. prevent junk-in-hole, consider use of drill pipe protectors. 5. Recommendations for Further Actions and Assessments, if any . The mild polishing-type wear should be identified during normal inspection and action taken to prevent over-worn items being re-used. The criteria for acceptable amounts of wear (acceptable minimum ODs) are contained in API RP7G. . Prime areas for wear are large OD components, e.g. tool-joints, HWDP centre section, drill collars, etc. These areas should be hardbanded in line with the BPX Guidelines (Drillstring Hardbanding BPX Drilling DCB Specification, DCB/20/94).
  • 11. FAILURE MODE DATA SHEET 9. EROSION 1. Description of the Failure Mode Erosion is the wastage of material due to mechanical removal of the surface material by a flowing environment. Such wastage is more extreme when solids (e.g. sand) are present in the environment. Erosion alone will only rarely cause drillstring failures. However, erosion damage, in the form of mud wash-out, is often associated with other failure mechanisms, e.g. fatigue. 2. Typical Characteristics of the Failure/s to Look Out For Eroded surfaces will often have a characteristic 'scalloped' appearance, due to the 'eddy currents' caused in the very turbulent flow (Figure A). The material surface itself will be smooth and clean. The most common form of erosion in drill string components is the result of drilling mud washing out through a very small gap, such as a through-wall crack (Figure B). 3. Contributory Factors to the Failure Mode to Look Out For Erosion is often the result of either a through-wall crack or a poorly tightened connection such that the shoulder seal is not sufficiently 'energised'. The drilling mud will flow through the resultant small gap resulting in rapid material wastage. In addition, a drop in the mud pump pressure may be observed. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . If the erosion damage is the result of a through wall crack, establish the cause of cracking and go to the necessary Failure Data Sheet, e.g. fatigue, stress corrosion cracking. . If the erosion damage is the result of poor connection make-up check the make-up procedures, make-up torque, etc. 5. Recommendations for Further Actions and Assessments, if any None required
  • 12. FAILURE MODE DATA SHEET 10. CORROSION 1. Description of the Failure Mode Corrosion can be defined as metal wastage resulting from the reaction of the metal with its environment. For drillstrings there are two main types of corrosion, i.e. 'general corrosion' which is the wastage of the entire component at about an 'even' rate or 'localised corrosion' which is the wastage of small regions of the component leading to 'pits' (known as "pitting"). Of these two mechanisms the more destructive is pitting, because pitting corrosion rates can be much more rapid than general corrosion rates. In addition pits act as 'stress raisers', which can promote other failure mechanisms such as stress corrosion cracking, corrosion-fatigue, fatigue, brittle fracture. 2. Typical Characteristics of the Failure/s to Look Out For Metal wastage can be either general [Figure A] or localised ('pitting') [Figure B]. There will often be a degree of surface roughening and the presence of corrosion products ('rust') in the corroded areas. 3. Contributory Factors to the Failure Mode to Look Out For Corrosion will normally be more prevalent in water based drilling muds than in oil based muds. The drilling mud chemistry will affect its corrosivity. For example, lower (more acidic) pH levels or air entrainment will increase mud corrosivity. Pitting/localised corrosion is often exacerbated under deposits, e.g. underneath mud deposits left on drill string components after retrieval. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . Take steps to improve the corrosion control of drilling muds. For guidance refer to the BPX Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR). . Clean mud deposits from recovered drill string components, this is particularly important for non- magnetic austenitic stainless steel components. 5. Recommendations for Further Actions and Assessments, if any Use internally plastic coated drill pipe.
  • 13. FAILURE MODE DATA SHEET 11. STRESS CORROSION CRACKING 1. Description of the Failure Mode Stress corrosion cracking (SCC) is a mechano-environmental mechanism, whereby the conjoint action of an environment and stress results in cracking of metallic materials. For drill string components there are two common sources of SCC, i.e. for high strength steels, such as used for ferromagnetic drill string components, there is the possibility of sulphide stress cracking in the presence of hydrogen sulphide; for austenitic stainless steels, such as used for non-magnetic drill string components, there is the possibility of chloride stress corrosion cracking in aqueous solutions containing chlorides (e.g. drilling muds, brines, etc.). 2. Typical Characteristics of the Failure/s to Look Out For SCC alone does not often result in failure of drill string components, rather it is the precursor for failures due to fatigue, overload, etc. Visually SCC cracks can be difficult to discern, as they can be very fine. Therefore, it is normally necessary to undertake inspection (e.g. MPI, dye penetrant inspection) and, in the extreme, metallographic examination to confirm the presence of SCC. SCC will often result in a network of cracks on the material surface (see Figure A above). However, such networks are not always discernible. SCC cracks are often associated with stress raisers, e.g. at the bottom of threads and changes of section, associated with corrosion pits. 3. Contributory Factors to the Failure Mode to Look Out For Sulphide stress cracking (SSC) of high strength ferromagnetic steels is associated with the presence of hydrogen sulphide. Therefore, this type of cracking is normally associated with drilling sour wells. Chloride stress corrosion cracking (CSC) of non-magnetic austenitic materials is associated with the presence of chloride, temperatures in excess of about 50oC and an oxidising species (e.g. oxygen and/or hydrogen sulphide). There will often (but not always) be corrosion pitting associated with the cracks. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . In the case of SSC consider the use of a lower strength drill pipe that is not prone to this type of cracking (e.g. X95 drill pipe). Take steps to improve the corrosion control of drilling muds, e.g. use of sulphide scavengers, minimum pH of 11. For guidance refer to the BPX Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR). . In the case of CSC ensure that mud deposits are always cleaned from the component on retrieval. Take steps to improve the corrosion control of drilling muds, e.g. consider use of oxygen scavenger for water-based muds. For guidance refer to the BPX Guideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR). 5. Recommendations for Further Actions and Assessments, if any . The presence of stress corrosion cracking (SCC) is very difficult to confirm visually. Therefore, further failure analysis (especially metallography) should be undertaken to confirm presence of SCC (see Figures B & C). . In the case of non-magnetic austenitic materials, to help avoid CSC only low carbon materials should be used (0.05% C max.). In addition, the bore of components exposed to drilling mud should be hammer or shot peened; threaded regions should be cold rolled or shot peened.
  • 14. FAILURE MODE DATA SHEET 12. OVER TORQUE OF TOOL-JOINTS 1. Description of the Failure Mode Over-torque of tool-joints will result in a number of possible failure mechanisms. The main failure mechanisms are: tensile failure of the pin connection in the last engaged thread; belling or splitting of the box connection; stretching of the pin connection. 2. Typical Characteristics of the Failure/s to Look Out For Tensile failure of the pin will often result in a 'cup and cone' failure [Figure A] or a mixed failure of 'cup and cone' and shear [Figure B]. Detection of box belling or pin stretch will often require the use of a straight edge (e.g. steel rule) or thread profile gauge respectively. 3. Contributory Factors to the Failure Mode to Look Out For Overtorquing of tool-joints can occur either during make-up (e.g. using too high a make-up torque; loss of torque capacity due to wear/corrosion) or as a result of 'downhole torque' (i.e. tool-joints being tightened during drilling). Downhole torque may occur when the drillstring becomes stuck, e.g. if the bottom section of the drillstring becomes stuck and the upper section keeps rotating, but more commonly it occurs during drilling as a result of resistance to bit rotation. 4. What Immediate Actions Can be Taken to Try to Prevent a Recurrence of the Failure ? . If torsional damage to tool-joints is detected then all the tool-joints within the string should be checked for cracks at the thread roots (using MPI) and pin stretch/box belling. Any suspect pipes should be quarantined for detailed inspection prior to clearance for re-use, scrapping or downgrading. . The make-up torque should be checked to ensure that it conforms with the recommended values (see API RP7G for guidance). . Worn/corroded tool-joints should be checked to ensure that they conform with the required minimum OD (see API RP7G for guidance). 5. Recommendations for Further Actions and Assessments, if any . If additional torque capacity is required in excess of the capability of the drill string consideration should be given to using high torque tool-joints or high coefficient of friction thread compounds.