Drill String Failure Handbook

Drill StringFailureHandbookThis handbook containsdata shetts for the mostcommon drill stringfailure mechanisms

J W Martin

Version 1 dated October 1995

Main CDContents

DRILL STRING FAILUREHANDBOOK

ByJ W MARTIN

INTRODUCTION

This Handbook contains Data Sheets for the mostcommon drill string failure mechanisms.

For each failure mechanism advice is given on identifyingfeatures; contributory factors (drilling operations,conditions, equipment design, etc.); immediate actions totake to try to prevent a recurrence; recommendedfurther actions/assessments.

It is intended that this Handbook is used in conjunctionwith the 'Drill String Failure Data Base', being set-upwithin DEAP, to ensure the correct classification of thefailure mechanism/s within the data base.

In the context of these Guidelines "failure" shouldinclude any incident that leads to a loss of drilling time(i.e. a twist-off or a wash-out). However, it isrecommended that 'inspection rejects' or 'scrapping ofdrill 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 thetensile strength of the material. Most commonly this mode is observed in the drill pipe body, as this is the "weak link" inthe 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 thepoint of failure, known as 'necking'. In addition, there will probably be a thinning of the drill pipe wall, this being morepronounced 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 thedrill string. Failure will be preceded by plastic deformation of the drill string, such that an apparent reduction inhookload 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 threadsshould be checked for stretch. Any stretched components should be laid down for further detailed inspection prior toscrapping or downgrading.. The drilling records just prior to failure should be checked to determine what loads were applied to the drill string thatresulted in failure. Clearly where excessive loads are identified future operations should take care to prevent arecurrence.

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 mechanicalproperties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation ofthe 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, considerusing a higher strength or higher weight drill pipe.


BRITTLE FRACTURE


Brittle fracture occurs very rapidly with little associated plastic deformation and indicates a deficient material, i.e.inadequate toughness.


Brittle fracture will occur in materials that have inadequate 'toughness'. There will be little or no plastic deformationassociated 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 easilybe confused with fatigue, such that toughness tests (Charpy tests) are often necessary to confirm that the material isbrittle. Fracture surfaces are usually normal (at right angles) to the applied stress.


Brittle fracture will occur very rapidly and there is normally little or no warning that failure is about to occur. Brittlematerials are usually quite strong. However, they are very sensitive to stress raisers (low toughness equates to high 'notchsensitivity') such as at mechanical damage, sharp changes in section, etc. Hence failures can occur at loads well belowthe theoretical load capacity of the item. Failures will often occur under shock load conditions, e.g. during jarringoperations, during handling top-side if the drilling component strikes/is struck by another item. Steel toughness is ofteninferior at lower temperatures, therefore brittle fracture is more likely at lower temperatures (e.g. top-side in colderclimes).


. The history of other components in the string should be checked, to determine if they may be from the same 'batch' asthe 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 thatcould have resulted in the failure. Clearly if excessive loads are identified, future operations should take care to prevent arecurrence.


. 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 ensurethere is no recurrence.


TORSIONAL OVERLOAD


Torsional overload is synonymous with tensile overload; in this case it occurs when a stress is applied to the downholecomponent which exceeds the torsional strength of the material. Most commonly, torsion-associated failures areobserved in tool-joints, as these are the "weak links" in the drill string in terms of torsional strength. Torsional failurescan occur in the drill pipe body, but these are relatively rare. For tool-joint failures refer to Failure Datasheet 12.


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.


Torsional overload fractures are often the result of a torque applied to the drillstring that is greater than its torsionalstrength. This may occur when the drillstring becomes stuck, e.g. if the bottom section of the drillstring becomes stuckand the upper section keeps rotating.


. The remainder of the string, should be checked to see whether there has been any torsional yielding. Any such pipesshould 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 thatcould have resulted in the failure. Clearly where excessive loads are identified future operations should take care toprevent 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.


. The failed component should be checked to establish whether it meets the specification with respect to mechanicalproperties. If a rogue pipe is identified, the remainder of the string should be quarantined prior to detailed evaluation ofthe whole string.


FATIGUEOverload Fatigue


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, fatiguepropagation followed by final fracture when the remaining material cross sectional area can no longer support the applied load. Corrosion fatigue is a relatedmechanism in which fatigue failure occurs prematurely as a result of the simultaneous effect of corrosion (refer to Datasheet 10).


Fatigue fractures most commonly occur within the threaded region of BHA tool-joints [Figure A] (often in the last engaged thread). Other common areasinclude 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 withinthe 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 mudwash-out through cracks prior to final fracture and/or rubbing damage after fracture. This can obliterate the fracture surfaces, making positive identificationof fatigue difficult.


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 ofthe pin connection can result from insufficient make-up torque, causing excessive fatigue loading of the pin. Poor drill string component design features, suchas sharp changes in section, thread run-outs, etc. can result in premature fatigue failure. Corrosion fatigue will be promoted by inadequate corrosion control ofdrilling muds, particularly in the case of water-based drilling muds.


. 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 threadedregions, at changes of section and/or in the taper section. Any components in which cracks are identified should be laid down for further inspection andscrapping 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 asevere dog-leg and/or vibration. In addition, the previous history of the component should be checked to determine when the last inspection occurred andprevious usage. If insufficient make-up torque is indicated, steps should be taken to ensure sufficient make-up torque in the future. If severe vibration isindicated, 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 offatigue failure.


. The failed component should be checked to establish that the failure was indeed the result of fatigue, whether the material meets the specification withrespect to mechanical properties and if there were any contributory causes (mechanical damage, stress corrosion cracking, etc. - see other datasheets asappropriate).. The inspection programme for the drill string should be reviewed to ensure that the inspection intervals are optimised to prevent further failures withoutinvoking 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.


MECHANICAL DAMAGE


'Mechanical damage' occurs as the result of the drillstring component being struck by orpressed/rubbed against another physical body, e.g. handling equipment, junk in hole. Mechanicaldamage will often contribute to premature failure by other mechanisms such as fatigue, overload,stress corrosion cracking. Refer to other datasheets as appropriate.


Mechanical damage can take a number of forms and there are a number of causes. Typical forms ofdamage are tong marks; slip marks; cuts and scores due to rubbing against junk in hole, hardformations, etc.; impact marks ('dents').


Poor handling procedures, e.g. impact against handling equipment, incorrect lifting procedures, poorstorage.Junk left in the hole.


. Check other components in the string for similar damage. Any damaged components should be laiddown for further inspection prior to clearance for further use, downgrading or scrapping according tothe criteria of API RP7G.. Ensure good 'house keeping' and care/handling procedures are invoked.


. The failed component should be checked to establish whether mechanical damage was the main causeof failure or whether there were other contributory causes that need to be addressed.


HEATCHECKING


Heatchecking results from frictional heating causing the temperature of a drill string component to increasesignificantly. Three types of heatchecking have been identified: Type I in which frictional heating followed by rapidquenching results in a "crazed" pattern of quench cracks on the surface; Type II in which the bulk properties of thecomponent (metallurgical and mechanical) are changed by frictional heating; Type III in which the loss in strengthresulting from frictional heating leads to plastic deformation and probably failure.


Type I - a characteristic 'craze' cracking of the surface (see photograph); signs of wear in affected region; in moreadvanced 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 reductionof cross-sectional area; significant wear in affected region; possible 'blueing' (oxide film) in the affected region.


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 annulardrilling fluid velocities; hole wash-outs; high dog-leg severity/hole tortuosity; increases in friction.


. 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 downfor 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 HeatcheckingMinimisation - 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; considerminimising use of large pipe sizes in small holes.


. Refer to BPX Guidelines (Guidelines for Heatchecking Minimisation - ESR.ER.071) for detailed recommendations ondealing with heatchecking regarding identification; minimising problems; refurbishment.. Undertake further failure analysis (including metallography, hardness checks, mechanical properties, etc.) to confirmpresence of heatchecking, particularly in the case of Types II and III heatchecking.


GALLING


Galling occurs most commonly in the threaded regions of drill string components. It results from two metallic surfacessliding 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 thetwo surfaces. Alternatively, the sliding forces fracture the 'welds' leading to material transference from one surface to theother.


For threaded connections galling damage is characterised by the 'bonding' of the two components making break-outdifficult (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 surfaceshowing sign of 'tearing' (see Figure B).


Galling of threads is the result of poor lubrication (e.g. insufficient or incorrect thread dope); thread damage prior tomake-up (e.g. mechanical damage causing 'high spots'); overtorquing (e.g. downhole make-up); poor make-upprocedures (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.


. Check other components in the string for galling damage. Any galled components should be laid down for furtherinspection 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.


. 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/copperplating or equivalent for non-magnetic components.. Ensure that new tool-joints are subjected to the appropriate 'breaking-in' procedure.


WEAR


Wear damage is the removal of material from the surfaces of components caused by rubbing (typicallyagainst casing, formation rock, junk in hole, casing shoes, etc.). By itself, wear rarely results infailures, more likely it will be the precursor for failure by another mechanism, e.g. heatchecking,overload, fatigue.


Worn areas have characteristic surface damage ranging from mild polishing-type material attritionthrough to severe material wastage and surface roughening (e.g. gouging). In extreme cases,heatchecking can result (See Failure Datasheet No. 6).


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.


. Where a failure occurs as the result of wear, the remainder of the string should be checked to seewhether similar levels of wear have occurred to other components, especially those close to the pointof failure. Any worn components should be laid down for detailed inspection prior to consideration forre-use, downgrading or scrapping.. For the more severe types of wear, the source of the wear should be identified and the necessaryactions taken, e.g. prevent junk-in-hole, consider use of drill pipe protectors.


. The mild polishing-type wear should be identified during normal inspection and action taken toprevent over-worn items being re-used. The criteria for acceptable amounts of wear (acceptableminimum 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 BPXDrilling DCB Specification, DCB/20/94).


EROSION


Erosion is the wastage of material due to mechanical removal of the surface material by a flowingenvironment. 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 mudwash-out, is often associated with other failure mechanisms, e.g. fatigue.


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. Themost common form of erosion in drill string components is the result of drilling mud washing outthrough a very small gap, such as a through-wall crack (Figure B).


Erosion is often the result of either a through-wall crack or a poorly tightened connection such that theshoulder seal is not sufficiently 'energised'. The drilling mud will flow through the resultant small gapresulting in rapid material wastage. In addition, a drop in the mud pump pressure may be observed.


. If the erosion damage is the result of a through wall crack, establish the cause of cracking and go tothe 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.


None required


CORROSION


Corrosion can be defined as metal wastage resulting from the reaction of the metal with itsenvironment. For drillstrings there are two main types of corrosion, i.e. 'general corrosion' which is thewastage of the entire component at about an 'even' rate or 'localised corrosion' which is the wastage ofsmall regions of the component leading to 'pits' (known as "pitting"). Of these two mechanisms themore destructive is pitting, because pitting corrosion rates can be much more rapid than generalcorrosion rates. In addition pits act as 'stress raisers', which can promote other failure mechanismssuch as stress corrosion cracking, corrosion-fatigue, fatigue, brittle fracture.


Metal wastage can be either general [Figure A] or localised ('pitting') [Figure B]. There will often be adegree of surface roughening and the presence of corrosion products ('rust') in the corroded areas.


Corrosion will normally be more prevalent in water based drilling muds than in oil based muds. Thedrilling mud chemistry will affect its corrosivity. For example, lower (more acidic) pH levels or airentrainment will increase mud corrosivity.Pitting/localised corrosion is often exacerbated under deposits, e.g. underneath mud deposits left ondrill string components after retrieval.


. Take steps to improve the corrosion control of drilling muds. For guidance refer to the BPXGuideline 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.


Use internally plastic coated drill pipe.


STRESS CORROSION CRACKING


Stress corrosion cracking (SCC) is a mechano-environmental mechanism, whereby the conjoint action of an environment and stressresults in cracking of metallic materials. For drill string components there are two common sources of SCC, i.e. for high strengthsteels, such as used for ferromagnetic drill string components, there is the possibility of sulphide stress cracking in the presence ofhydrogen sulphide; for austenitic stainless steels, such as used for non-magnetic drill string components, there is the possibility ofchloride stress corrosion cracking in aqueous solutions containing chlorides (e.g. drilling muds, brines, etc.).


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 undertakeinspection (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 alwaysdiscernible. SCC cracks are often associated with stress raisers, e.g. at the bottom of threads and changes of section, associated withcorrosion pits.


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 notalways) be corrosion pitting associated with the cracks.


. 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). Takesteps to improve the corrosion control of drilling muds, e.g. use of sulphide scavengers, minimum pH of 11. For guidance refer to theBPX 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 thecorrosion control of drilling muds, e.g. consider use of oxygen scavenger for water-based muds. For guidance refer to the BPXGuideline document (Corrosion Control in Drilling Muds for Extended Reach Drilling, DCB/28/93/BR).


. The presence of stress corrosion cracking (SCC) is very difficult to confirm visually. Therefore, further failure analysis (especiallymetallography) 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.). Inaddition, the bore of components exposed to drilling mud should be hammer or shot peened; threaded regions should be cold rolled orshot peened.


OVER TORQUE OF TOOL-JOINTS


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 ofthe pin connection.


Tensile failure of the pin will often result in a 'cup and cone' failure [Figure A] or a mixed failure of 'cup and cone' andshear [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 gaugerespectively.


Overtorquing of tool-joints can occur either during make-up (e.g. using too high a make-up torque; loss of torquecapacity 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 becomesstuck and the upper section keeps rotating, but more commonly it occurs during drilling as a result of resistance to bitrotation.


. If torsional damage to tool-joints is detected then all the tool-joints within the string should be checked for cracks at thethread roots (using MPI) and pin stretch/box belling. Any suspect pipes should be quarantined for detailed inspectionprior 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 forguidance).. Worn/corroded tool-joints should be checked to ensure that they conform with the required minimum OD (see APIRP7G for guidance).


. If additional torque capacity is required in excess of the capability of the drill string consideration should be given tousing high torque tool-joints or high coefficient of friction thread compounds.

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Drill String Failure Handbook