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Sucker Rod

Failure AnalysisA Special Report from Norris

A NORRISA ______ D O V E R

)C O M P A N Y

2 © 2000-2007 Norris/A Dover Company All rights reserved

Failure Mechanisms........................................................ 1

Design and Operating Failures ....................................... 5

Mechanical Failures ........................................................ 8

Bent Rod Failures ............................................................ 8

Surface Damage Failures ............................................... 9

Connection Failures ........................................................ 10

Corrosion-fatigue Failures................................................ 14

Acid Corrosion ....................................................... 15

Chloride Corrosion ............................................... 16

CO2 Corrosion ...................................................... 16

Dissimilar Metals Corrosion .................................. 17

H2S Corrosion ...................................................... 17

Microbiologically Influenced Corrosion (MIC) ....... 18

Acid Producing Bacteria ............................. 18

Sulfate Reducer Bacteria ........................... 19

Oxygen (02) Enhanced Corrosion ........................ 19

Stray Current Corrosion ....................................... 20

Under-Deposit Corrosion....................................... 20

Manufacturing Defects ..................................................... 20

Failure Analysis Request Form........................................ 23

A NORRISA DOVER) COMPANY


Root Cause Failure Analysis is Essentialfor Failure Frequency Reduction in Wellswith Artificial Lift.

Most failures associated with artificiallift systems can be attributed to one of threeprimary downhole components—subsurfacepump, sucker rod or tubing string. A subsur-face pump, sucker rod or tubing failure is de-fined as any catastrophic event requiring ser-vicing personnel to pull or change-out one ormore of these components. By this definition,the failure frequency rate is the total number ofcomponent failures occurring per well, peryear. Marginally producing wells with high fail-ure frequency rates are often classified as"problem" wells and effective failure manage-ment practices can mean the difference be-tween operating and plugging these wells.Failure management includes preventing,identifying, implementing and recording the"real" root cause of each failure and is centralto overall cost-effective asset management.For the purpose of this photo essay, we willdeal only with sucker rod failures.

Cost-effective failure management be-gins with prevention, and the time to stop thenext failure is now—prior to an incident! Sim-ply fishing and hanging the well on after asucker rod failure will not prevent failure recur-rence. In fact, most failures continue with in-creasing frequency until the entire rod stringmust be pulled and replaced. Achievable fail-ure frequency reductions require accurate fail-ure root cause analysis and the implementa-tion of corrective action measures to preventfailure recurrence. A database capable ofquerying the well "servicing" history is neededto track and identify failure trends. Once a fail-ure trend is identified, remedial measuresshould be implemented during well servicingoperations to prevent premature rod string fail-

ures. The database failure history should in-clude information on the failure type, location,depth, root cause and the corrective actionmeasures implemented.

Sucker rods can be caused to fail pre-maturely. Understanding the effects of seem-ingly minor damage to rod strings, and know-ing how that damage can produce catastro-phic failures, is very important for productionpersonnel. Sucker rod failure analysis is chal-lenging and you need to be able to look pastthe obvious and seek clues from the not soobvious. All production personnel should haveadequate knowledge and training in failureroot cause analysis. Understanding how toidentify failures and their contributing factorsallows us an understanding of what is requiredto correct the root cause of the failure. Everystep that can be taken to eliminate prematuresucker rod failures must be taken. On-goingtraining programs concerning sucker rodsshould include formal and informal forums thatadvocate following the recommendations ofmanufacturers for artificial lift design, storage& transportation, care & handling, running &rerunning and makeup and breakout proce-dures. A variety of training schools are cur-rently available and, with advanced notice,most can be tailored to meet the specificneeds of production personnel.

Failure Mechanisms

All sucker rod, pony rod and couplingfailures are either tensile or fatigue failures.Tensile failures occur when the applied loadexceeds the tensile strength of the rod. Theload will concentrate at some point in the rodstring, create a necked-down appearancearound the circumference of the rod, and frac-ture occurs where the cross-section is re-duced. This rare failure mechanism only oc-curs when pulling too much load on the rodstring—such as attempting to unseat a stuckpump. To avoid tensile failures, the maximumweight indicator pull for a rod string in "like

A NORRISA DOVER) COMPANY


new" condition should never exceed 90% ofthe yield strength for the known size andgrade of the smallest diameter sucker rod.For unknown sucker rod conditions, sizes orgrades a sufficient de-rating factor should beapplied to the maximum weight pulled. Allother sucker rod, pony rod and coupling fail-ures are fatigue failures.

Fatigue failures are progressive andbegin as small stress cracks that grow underthe action of cyclic stresses. The stresses as-sociated with this failure have a maximumvalue that is less than the tensile strength ofthe sucker rod steel. Since the applied load isdistributed nearly equally over the full cross-sectional area of the rod string, any damagethat reduces the cross-sectional area will in-crease the load or stress at that point and is astress raiser. A small stress fatigue crackforms at the base of the stress raiser andpropagates perpendicular to the line of stress,or axis of the rod body. As the stress fatiguecrack gradually advances, the mating fracturesurfaces opposite the advancing crack front tryto separate under load and these surfaces be-come smooth and polished from chafing. Asthe fatigue crack progresses, it reduces theeffective cross-sectional area of the sucker roduntil not enough metal remains to support theload, and the sucker rod simply fractures intwo. The fracture surfaces of a typical fatiguefailure have a fatigue portion, tensile tear por-tion and final shear tear.

Fatigue failures are initiated by a multi-tude of stress raisers. Stress raisers are visi-ble or microscopic discontinuities that causean increase in local stress on the rod stringduring load. Typical visible stress raisers onsucker rods, pony rods, and couplings arebends, corrosion, cracks, mechanical damage,threads and wear or any combination of thepreceding. This increased stress effect is themost critical when the discontinuity on the rodstring is transverse (normal) to the principletensile stress. In determining the origin of astress raiser in a fatigue failure, the fatigueportion opposite the final shear tear

(extrusion / protrusion) must be carefullycleaned and thoroughly examined. Fatiguefailures have visible or macroscopic identifyingcharacteristics on the fracture surface, whichhelp to identify the location of the stress raiser.Ratchet marks and beach marks are arguablytwo of the most important features in fatiguefailure identification. Ratchet marks are linesthat result from the intersection and connec-tion of multiple stress fatigue cracks whilebeach marks indicate the successive positionof the advancing fatigue crack. Ratchet marksare parallel to the overall direction of crackgrowth and lead to the initiation point of thefailure. Beach marks are elliptical or semi-elliptical rings radiating outward from the frac-ture origin and indicate successive positions ofthe advancing stress fatigue crack growth.

Figure 1 is an example of fatigue andtensile failure mechanisms. The two exam-ples on the right are tensile failures. A tensilefailure is characterized by a reduction in thediameter of the cross-sectional area at thepoint of fracture. Typical tensile failures havecup-cone fracture halves. The second exam-ple from the right is typical in appearance fortensile failures. Fractures from tensile failuresrupture, or shear, on 45° angles to the stressesapplied. A good example of the shear is thecharacteristic cup-con fracture surfaces of atypical tensile failure. The rod body on theright is an excellent example of needing tolook past the obvious for the not so obvious.A stress fatigue crack is primarily responsiblefor this failure even though fracture occurredwhile trying to unseat the pump. Visual exami-


nation of the fracture surface reveals a small,semi-elliptical, stress fatigue crack. Thissucker rod had preexisting, transverse stressfatigue cracks, from in-service stresses. Oneof the stress fatigue cracks opened during thestraight, steady load applied in attempting tounseat the pump, and facture occurred. Thetensile failure is secondary and results in theunusual appearance of the facture surface—with the small fatigue portion, large tensile por-tion and unusually large 45° double shear-liptears.

The remaining examples are fatiguefailures on: casehardened sucker rods; nor-malized and tempered sucker rods; andquench and tempered sucker rods. The ex-ample on the far left is a torsional fatigue fail-ure from a progressing cavity pump. Ratchetmarks found in the large fatigue portion, andoriginating from the surface of the rod body,completely encircle the fracture surface withthe small tensile tear portion shown slightly offmiddle-center. The second rod body on theleft is a casehardened fatigue failure. Thecase encircling the rod body diameter carriesthe load for this high tensile strength suckerrod and if you penetrate the case, you effec-tively destroy the load-carrying capability ofthis type of manufactured sucker rod. Thestress fatigue crack advances around the caseand progresses across the rod body until com-plete fracture occurs. A fatigue failure on acasehardened sucker rod generally exhibits asmall fatigue portion and a large tensile tear(unless lightly loaded). The third rod bodyfrom the left is typical in appearance for mostfatigue failures. Typical fatigue failures have afatigue portion, tensile portion and final sheartear. The width of the fatigue portion is an in-dication of the loading involved with the frac-ture. Mechanical damage can prevent or hin-der failure analysis by destroying the visualclues and identifying characteristics normallyfound on a fatigue fracture surface. Care mustbe exercised when handling the fracturehalves. It is very important to resist the temp-tation to fit the mating fracture surfaces to

gether since this almost always destroys(smears) microscopic features that aid in de-termining the failure cause. To avoid me-chanical damage, fracture surfaces shouldnever actually touch during fracture-surfacematching.

Design and Operating FailuresSucker rod failure prevention begins

with design. It is possible for poorly designedrod strings to contribute to other componentfailures in the artificial lift system, such as rodcut tubing resulting from compressive rodloads. Designing the artificial lift system is acompromise between the amount of work tobe done and the expense of doing this workover a cost-effective period of time. Numer-ous combinations of depths, tubing sizes, fluidvolumes, pump sizes and configurations,pumping unit sizes and geometries, strokelengths, pumping speeds and rod string tapersare available to the system designer. Suckerrod size and grade selection is dependentupon many factors including predicted maxi-mum stresses, stress ranges, and operatingenvironments.

Commercially available computer de-sign programs allow the system designer tooptimize production equipment at the least ex-pense for the well conditions existing at thetime of the design. However, after the initialdesign and installation of the rod string, peri-odic dynamometer surveys should be utilizedto confirm that equipment load parameters arewithin those considered acceptable. A goodinitial design may become a poor design if wellconditions change. Changes in the fluid vol-ume, fluid level, stroke length, strokes per min-ute or pump size severely impact the total arti-ficial lift system. Changes in fluid corrosive-ness can affect the fatigue endurance life ofsucker rods and may lead to premature fail-ures. When one of the preceding conditionschange, the design of the artificial lift systemmust be re-evaluated.


Figure 3

Figures 2 and 3 are examples of designand operationally induced mechanical failures.Wear, unidirectional bending fatigue andstress-fatigue failures indicate compressiverod loads, deviated well bores, fluid pound,gas interference, highly stressed sucker rods,unanchored or improperly anchored tubing orsome combination of the preceding.

Abrasive-wear causes rod string fail-ures by reducing the cross-section of themetal, exposing new surface metal to corro-sion and causes sucker rod connection fail-ures from impact and shoulder damage. In

figure 2, the Class T coupling on the left andthe Class SM coupling in the middle are exam-ples of abrasive-wear. In figure 3, the rodbody on the left is an example of abrasive-wear. Abrasive-wear on the rod string is de-fined as the progressive removal of the sur-face metal by contact with the tubing string.Abrasive-wear that is equal in length, widthand depth usually suggest a deviated orcrooked well bore. Angled abrasive-wear pat-terns indicate rod strings that are aggressivelycontacting the tubing at an angle, usually as aresult of fluid pound, gas interference, severepump tagging, or unanchored or improperlyanchored tubing. The middle rod body in fig-ure 3 represents corrosion-abrasion. Abra-sive-wear also removes corrosion inhibitingfilms and exposes new surface metals to cor-rosive fluids—which accelerate the rate of cor-rosion. The Class T coupling on the far rightin figure 2 has a work-hardened ridge fromtubing-slap. Tubing-slap is the result of therod string "stacking out"—probably as a resultof severe fluid pound, gas interference orpump tagging. The work-hardened materialdoesn't wear as fast as the softer material oneither side of the work-hardened area and itleaves a ridge of material as the rest of thesofter material wears.

The second rod body from the left andthe first rod body from the right in figure 3 areunidirectional bending fatigue failures. Unidi-rectional bending fatigue failures occur fromthe motion of the rod string having a constantlateral or side movement during the pumpingcycle. Stress fatigue cracks will concentratealong the area of the sucker rod where thegreatest bending stresses occurred. The fine,transverse, stress fatigue cracks will be onone half of the circumference of the rod body,closely spaced near the rod upsets and gradu-ally spreading apart moving toward the middleof the rod body. Most unidirectional bendingfatigue failures occur above the connection in


the transition zone of the rod body—betweenthe rigid coupling and upset area and the moreflexible rod body. Unidirectional bending fa-tigue failures will not show permanent bendssince this problem occurs while the rod stringis in motion. The example on the far right is aunidirectional bending fatigue failure. Thistype of failure generally has two tips protrudingabove the fracture surface. These distinct fail-ure characteristics indicate a double shear-liptear. Double shear-lip tears are the direct re-sult of unidirectional bending stresses, withfractures occurring under compressive rodloads. Compressive rod loads may be the re-sult of large bore pumps with small diametersucker rods or multiple tapers in shallow wells.

The second rod body failure on the rightin figure 3 is a stress-fatigue failure. Stress-fatigue failures occur on highly stressedsucker rods as a result of worn out suckerrods, overloads or extremely high rod loads forshort periods of time. Stress-fatigue failureshave closely spaced, fine, transverse stressfatigue cracks that completely encircle the cir-cumference of the rod body. The stress fatigue cracks will be on the wrench square andover the entire length of the rod body. Withvery old, worn-out sucker rods, stress fatiguecracks and failure may occur within normaleveryday operating loads.

Figure 4 is an example of coupling-to-tubing slap. Coupling-to-tubing slap is the re-sult of extremely aggressive angle contact tothe tubing by the rod string. This aggressivecontact is the direct result of severe fluidpound, unanchored (or improperly anchored)tubing, sticking (or stuck) pump plungers, orany combination of the preceding.


Figure 5 is an example of rod guide re-lated damage. The example on the left is areconditioned, high tensile strength suckerrod. Turbulent fluid flow, associated withshort, blunt-end injection molded rod guides,allowed crevice-corrosion in the critical washarea around the end of the guide. Prior to in-specting, the mold-on rod guides were re-moved from the rod body for reconditioning.Class 1 reconditioned sucker rods cannothave discontinuities greater than 20 mils(0.020") per API Specification 11BR. Thecrevice corrosion was under the 20 mils al-lowed for a Class 1 reconditioned sucker rod.However, the notch sensitivity (discontinuityintolerance) of a high tensile strength sucker


rod is high. In other words, small pits can bedetrimental to the high tensile stresses associ-ated with the high strength sucker rod and re-conditioned high strength sucker rods shouldbe de-rated for load. The example in the mid-dle is an erosion-corrosion failure resultingfrom short, blunt-end, field-applied rod guidesin small tubing with high fluid velocities. Ero-sion-corrosion pits will be "fluid cut" withsmooth bottoms. Pit shape characteristicsmay include sharp edges and steep sides ifaccompanied by CO2 or broad smooth pitswith beveled edges if accompanied by H2S.The example on the right is abrasion-wearfrom a field-applied rod guide moving up anddown on the rod body during the pumping cy-cle. Generally speaking, mold-on rod guidesprovide better laminar flow, a minimum ofthree to four times more bonding and retentionand are more cost-effective than are field-applied rod guides.

Mechanical FailuresMechanical failures account for a large

percentage of the total number of all rod stringfailures. Mechanical failures include everytype except corrosion-fatigue failures andmanufacturing defects. Mechanical damageto the rod string contributes to a stress raiserwhich will cause sucker rod failures. The timeto failure will be influenced by many variables,of which maximum stress, operating environ-ment, orientation of the damage, sucker rodchemistry, sucker rod heat treatment type,stress range and type of damage will be of themost important. Mechanical damage can becaused by inept artificial lift design, impropercare & handling procedures, careless makeup& breakout procedures, out-of-date operatingpractices, or any combination of these ele-ments.

Bent Rod FailuresBending fatigue failures account for a

significant number of all mechanical failures.It is a fact that all bent sucker rods eventuallyfail. New sucker rods are manufactured to abody straightness of no less than 1/16 inch inany twelve inches of rod body length. Sucker

rods within this tolerance of straightness willroll easily on a rack with five level supports.Any degree of bend greater than this willcause an increase in local stress at the pointof the bend during applied load. When thebent rod body is pulled straight during load,the ultimate strength of the material is quicklyreached. The cycle of continually exceedingthe ultimate material strength is repeated dur-ing the pumping cycle and causes stress fa-tigue cracks on the concave side of the bend.These stress fatigue cracks progress acrossthe bar, during load, until not enough metalremains in the bar to support the load, andfracture occurs.

Straightening the raw bar stock is thefirst step in the process of manufacturingsucker rods. Cold straightening the bar de-forms the grain structure below its recrystalli-zation temperature, putting a strain in the barthat is accompanied by a work hardening ef-fect. During the manufacturing process, thefunction of heat treatment is to stress-relievethe residual and induced stresses caused bybar rolling, bar straightening processes andfrom forging the rod upsets. Heat treatmentchanges the metallurgical structure of theforged ends to match that of the rod body andalso controls the mechanical properties of thesucker rod. Any rod body bend created afterheat treatment causes work hardening, whichcreates an area of hardness different than thesurrounding surfaces. This condition is re-ferred to as a "hard spot" and is a stress raiser


to load. Mechanical processing, such aspassing the finished bent sucker rod through asystem of rollers, will attempt to remove thebend so the rod body appears to be straight.However, reconditioning processes are notcapable of stress relieving bent sucker rods.A bent sucker rod is permanently damagedand should not be used because all bentsucker rods will eventually fail.

Figure 6 is an example of bending fa-tigue failures. Bending fatigue failures can beidentified by the angled fracture surface, whichwill be at some angle other than 90° to theaxis of the rod body. The example on the leftillustrates a fracture caused by a long radiusbend, or gradual sweeping bow in the rodbody (top example in Figure 7). The fracturesurface is normal in appearance, but has aslight angle when compared to the axis of therod body. The middle example is a short ra-dius bend (bottom example in Figure 7). Thefracture surface is at a greater angle to theaxis of the rod body with a small fatigue por-tion and a large tensile tear portion. The ex-ample on the right is the result of a cork-screwed sucker rod. Notice how convolutedthe fracture surface is in appearance. As ageneral rule, the greater the bend in the rodbody, the more convoluted the fracture sur-faces appear. In operation, the time for therod to fracture is greatly shortened. Poor care& handling procedures usually cause bentsucker rods.

Surface Damage FailuresEverything possible should be done to

prevent mechanical surface damage to suckerrods, pony rods and couplings. Surface dam-

age increases stress during applied loads, po-tentially causing rod string failures. The typeof damage, and its orientation, contributes tothis increased stress effect. The orientation ofthe damage contributes to higher stresses withtransverse damage having increased stressesover those associated with longitudinal dam-age. A sharp nick will create a higher stressconcentration and would be more detrimentalto load than a shallow, broad-based depres-sion. Sucker rods with indications of surfacedamage must not be used and must be re-placed. Care should be used to avoid allmetal-to-metal contact that might result indents, nicks or scratches. To prevent potentialsucker rod damage, place strips of wood be-tween metal storage racks and between eachlayer of sucker rods so metal-to-metal contactcan be avoided. Use sucker rods for whatthey were designed for – to lift a load. Neveruse sucker rods as a walkway or workbench.Keep metal tools not intended for use onsucker rods and all other metal objects awayfrom the sucker rods. Make sure the tool youuse is intended for the purpose and ensurethat it is in proper working order.

Figure 8 is an example of various sur-face damage failures. The example on the leftshows a slight depression from a wrench, tool,or other metal object. The second examplefrom the left is damage from a pipe wrench

1=211==1;Figure7


used in applying field-installed rod guides.The second example from the right has asmall longitudinal scratch from metal-to-metalcontact; possibly by allowing sucker rods torun down other rods in a rod bundle during in-stallation. The example on the right exhibitstransverse surface damage.

Figure 9 is an example of surface dam-age caused by sucker rod elevators. The bot-tom example is damage from worn or mis-aligned elevator seats. After an extended pe-riod of service, the elevator seats become soworn and damaged that they develop an ovalshape rather than a round shape. As the ovalshape grows, the tangency ring of the rod up-set to the elevator seat face is lowered in thefront half of the seat. As the seat continues towear, the seating position of the rod upset ismoved forward of the elevator trunnion center-line. This causes an offset in the hook loadand tilts the elevator body forward. When theelevator lifts the rod string load, the hook loadwill bend the sucker rod centerline to coincidewith the elevator trunnion centerline. As therod string weight increases, the hook load willbend every sucker rod engaged by this eleva-

tor. Bent sucker rod failures that occur belowthe surface upset bead may be from bad ele-vator seats. The top example is damagecaused by the elevator latches. This type ofdamage normally occurs as a result of pickingup or laying down in doubles. Never pick upor lay down anything more than one singlesucker rod. Anything else causes the elevatorlatches to act as a fulcrum and allows bendingstresses to concentrate in the transition zoneof the rod body and the forged upset.

Connection Failures

The API sucker rod connection is de-signed as a rotary shouldered, friction loadedconnection. Since the fatigue endurance ofthe sucker rod connection is low when sub-jected to cyclic loads, it is necessary to limitcyclic loading with pin preload. If the pin pre-load is greater than the applied load, the loadin the connection remains constant and no fa-tigue occurs from cyclic loads. The frictionload that develops between the pin shoulderface and the coupling shoulder face helps lockthe connection together to prevent it fromcoming loose downhole. However, if the pre-load is less than the applied load, the pinshoulder face and the coupling shoulder facewill separate under load during the cyclic mo-tion of the pumping unit. Once these facesseparate the connection is cyclically loadedand will result in a loss of displacement, orloss of tightness, failure. Loss of displacementfailures may result from improper lubrication,inadequate makeup, over-torque, tubing-slap


Figure 12

wear, or any combination of these elements.

Figure 10 is an example of pin failuresdue to a loss of displacement. The sample onthe right is typical in appearance for a loss ofdisplacement pin failure. Insufficient makeup,or the loss of tightness, caused the pin shoul-der face and the coupling shoulder face toseparate. When these faces separate, abending moment is added to the tensile load inthe pin. The threaded section of the pin isheld rigid while the rest of the pin flexes. Themotion of the rod string causes stress fatiguecracking to initiate in the first fully formedthread root above the stress relief or undercut.Small stress fatigue cracks begin to formalong the thread root during applied load andeventually consolidate into a major stress fa-tigue crack. The fracture surface of a typicalloss of displacement pin failure has a smallfatigue portion covering approximately one-third of the fracture surface with the tensiletear portion and final shear tear covering theremaining fracture surface. The examples onthe left and in the middle will occur as a resultof stress loading when stress-raising factorssuch as corrosion or mechanical damage ispresent on the surface of the stress relief orpin undercut.

Figure 11 is another example of twotypes of pin failures. The sample on the left istypical in appearance for a loss of displace-ment pin failure. However, this pin fracturewas caused by the hydraulic rod tongs duringmakeup as is evidenced by the stair-steppedtensile tear. It is not uncommon for pin frac-tures to occur at makeup, if the pin has a pre-existing stress fatigue crack, due to the hightorque required with large diameter Class Dand all sizes of high tensile strength suckerrods. The sample on the right is an exampleof excessive torque on a soft pin. The fracturesurface has a large fatigue portion, with multi-ple ratchet marks in the pin-thread root, and asmall tensile portion.

Figure 12 is an example of a loss of dis-placement coupling failure. The fracture initi-ated in the coupling-thread root opposite thefirst fully-formed pin starting-thread. One-third/two-third fracture halves, in length, withratchet marks originating in the thread-root in-dicate a loss of displacement coupling failure.The fracture-surface of a typical loss of dis-placement coupling failure has a small fatigueportion and a large tensile tear portion. Lossof displacement coupling failures are primarilyassociated with Class D sucker rods and hightensile strength sucker rods.


Mid-length coupling fractures, withratchet marks leading from the outside, indi-cate another type of failure. The stress fatiguecrack starts from the outside coupling surface,progressing inward toward the threads, thenaround the coupling wall to a tensile fracture.Mid-length fractures indicate coupling failuresfrom mechanical damage to the coupling sur-face, exceeding the stress fatigue endurancelimit of the material, or a manufacturing defect.Most mid-length coupling fractures are the re-sult of mechanical damage or overload. Mid-length coupling fractures due to overload havea small fatigue portion and large tensile tearportion. This failure is common with highstrength sucker rods and Class SM couplings.Use Class T couplings with high strengthsucker rods to avoid mid-length coupling fail-ures.

Figure 13 is an example of thread gall-ing in the sucker rod connection. Thread gall-ing is mechanical damage to the pin and/orcoupling threads. Thread galling is the resultof damaged or contaminated threads causingthe interference between the threads to begreat enough to rip and tear the thread sur-

faces. The threads weld together duringmakeup and strip apart at breakout and theconnection is damaged and destroyed beyonduse. Hard stabbing damage to the leadingthread and contaminated threads are the pri-mary causes of thread galling. Cleaning thethreads prior to makeup, properly lubricatingthe threads and following careful makeup pro-cedures will prevent most problems withthread galling.

Figure 14 is an example of a wrench

square failure. Wrench square failures are ex-tremely rare and seldom occur unless frommechanical damage, corrosion or manufactur-ing defects. The example shown is a wrenchsquare failure from severe mechanical dam-age. Loose or sloppy backups on the hydrau-lic rod tongs has rounded the wrench squarecorner. The stress fatigue crack began in thecorner of the wrench square and progressedto final rupture or fracture.

Figure 15 is an example of the damagethat occurs as a result of severely over-tightening the sucker rod connection. The ex-ample shown is an ever-tightened couplingthat has flared out or bulged near the contactface. Slimhole couplings are more susceptibleto this type of over-tightened damage than arefullsize couplings. Over-tightened fullsize cou-


Figure 15

plings on Class D and high strength suckerrods generally exhibit slight bulges and havethe concentric deformation ridge of material onthe coupling should face from the impressionof the pin shoulder face. Over-tightening withhydraulic rod tongs will twist off soft pins re-sulting in a tensile failure appearance. The pinundercut will neck down and fracture occursrapidly. With Class D sucker rods, an indica-tion of over-tightening is the concentric defor-mation ridge of material on the pin shoulderface from the impression of the couplingshoulder face. Over-tightening on normalizedand tempered high tensile strength sucker rod

will begin to pull the threads out of the cou-

pling.

Figure 16 is an example of impactcracks on couplings. The practice of "warmingup", or hammering, on couplings in order toloosen them should not be allowed. This ex-ample shows how impact damage to a Class Tcoupling causes stress fatigue cracks aroundthe impact points and accelerated localizedcorrosion. Hammering on Class SM(spraymetal) couplings cause stress fatiguecracks in the hard spray surface and results incoupling failures due to corrosion-fatigue.

Figure 17 is an example of polished rodfailures. The majority of all polished rod fail-ures occur either in the body, just below thepolished rod clamp, or in the pin. Polished rodbody failures below the polished rod clamp re-sult from the addition of bending stresses.These bending stresses may be imposed bypumping units out of alignment, carrier barsthat do not set level, worn carrier bars, mis-aligned load cells, or incorrect polished rodclamp installation. The polished rod failure onthe left is an example of a polished rod clampon the sprayed portion of a spraymetal pol-ished rod. Spraymetal polished rods have an


unsprayed portion for polished rod clampplacement. Never put a polished rod clamp onthe sprayed portion of a spraymetal polishedrod. The polished rod failure on the right hassmall, longitudinal scratches caused from mis-handling.

Polished rod pin failures generally oc-cur due to the installation of sucker rod cou-plings. Polished rod pins have a 9° thread ta-per between the straight-threaded section andthe shoulder. Sucker rod couplings have a30° starting thread and a deep recess thatdoesn't engage all the polished rod pinthreads. Polished rod couplings have a 9°starting threads and a profile designed toproperly fit the polished rod pin. The shallowrecess to the first thread easily distinguishespolished rod couplings from sucker rod cou-plings and allows every polished rod pinthread to be engaged.

Corrosion-fatigue Failures

Corrosion is one of the greatest prob-lems encountered with produced fluids andaccounts for about two-thirds of all sucker rodfailures. Corrosion is the destructive result ofan electrochemical reaction between the steelused in making sucker rods and the operatingenvironment to which it is subjected. Simplyput, corrosion is nature's way of reverting aman-made material of a higher energy state(steel), back to its basic condition (native ore)as it is found in nature. The elemental iron insteel combines with moisture or acids, to formother compounds such as iron oxide, sulfide,carbonate, etc. Some form and concentrationof water is present in all wells considered cor-rosive and most contain considerable quanti-ties of dissolved impurities and gases. For in-stance, carbon dioxide (CO2) and hydrogensulfide (H2S) acid gases, common in mostwells, are highly soluble and readily dissolve inwater—which tends to lower its pH. The cor-rosivity of the water is a function of the amountof these two gases that are held in solution.All waters with low pH values are consideredcorrosive to steel, with lower values represent-

ing greater acidity, or corrosiveness.

All downhole environments are corro-sive to some degree. Some corrosive fluidsmay be considered non-corrosive if the corro-sion penetration rate, recorded as mils ofthickness lost per year (mpy), is low enoughthat it will not cause problems. However, mostproducing wells are plagued by corrosionproblems and no currently manufacturedsucker rod can successfully withstand the ef-fects of this corrosion alone. While corrosioncannot be completely eliminated, it is possibleto control its reaction. All grades of suckerrods must be adequately protected throughthe use of effective chemical inhibition pro-grams (reference current editions of API Rec-ommended Practice 11BR and NACE Stan-dard RP0195). Some sucker rod grades, dueto different combinations of alloying elements,microstructures and hardness levels, are ca-pable of longer service life in inhibited corro-sive wells than other grades of either low orhigh tensile strength.

Why do new sucker rods seem to cor-rode faster than older rods in the same rodstring? Two sucker rods with the same chemi-cal composition will form a galvanic corrosioncell if the physical condition of one is differentfrom the other. Physical differences in asucker rod may be caused from poor care &handling practices (i.e. surface damage result-ing in depressions, nicks, scratches, bends,etc) and/or corrosion deposits (iron oxide, car-bonate and sulfide scale, etc). Since newsucker rods go into the well without corrosiondeposits, they often corrode preferentially inrelation to sucker rods that are coated withcorrosion deposits. Corrosion on steel startsvery aggressively but often slows down assoon as an obstructive surface film of corro-sion deposit (scale) is formed upon the metalsurface. For example, CO2 generates ironcarbonate scale as a by-product of its corro-sion. This scale coats the sucker rod and re-tards the corrosion penetration rate—which


tends to slow down corrosion. However, if thisdeposit is continuously cracked and removedby a bending movement or by abrasion, ag-gressive local corrosion continues on the areaof the sucker rod with the scale removed—which results in deep corrosion pitting.

Can high tensile strength sucker rodsbe used in a corrosive environment? Gener-ally soft rods tolerate corrosion better thanhard rods and, as a rule of thumb, you shouldalways use the softest rod that will handle theload. However, if load requirements dictatethe use of high tensile strength sucker rodsthen it is important to protect the rod stringwith an effective surface film of corrosion in-hibitor. In most cases, if you can adequatelyprotect downhole equipment from corrosion,you should be able to adequately protect hightensile strength sucker rods from corrosion byincreasing the application frequency of thecorrosion-inhibitor program. In other words, ifyou effectively batch treat once a week with 40parts per million (ppm) of corrosion inhibitor forClass D sucker rods, you will need to batch

treat twice weekly with 40 ppm of corrosioninhibitor for high tensile strength sucker rods.Treatment volumes vary and are dependentupon many factors too numerous to list here.Always consult with a corrosion control spe-cialist prior to the installation of every rodstring, especially when corrosion-fatigue issuspected as a prior failure cause.

Figure 18 is an example of corrosion-fatigue from CO2 acid gas corrosion. The sizeof the pit, as far as when it becomes detrimen-tal to the sucker rod, depends on three fac-tors—load, material type and hardness. ClassK sucker rods may develop deeper and largerpits than a Class D sucker rod before it be-comes detrimental to the sucker rod. Class Dsucker rods may develop deeper and largerpits than a high tensile strength sucker rod be-fore it becomes detrimental to the sucker rod.Softer materials with lower rod stress toleratelarger pits than do harder materials with higherrod stress. Therefore, small pits can be detri-mental to higher tensile strength sucker rodsas opposed to a softer sucker rod that doesnot have as much rod stress.


Figure 19 is an example of acid corrosion. Service companies use acids for wellstimulation and cleanout work. All acid work should have an effective inhibitor mixed with


the acid prior to injection into the well. Spentacids are still corrosive to steel and the wellshould be "flushed" long enough to recover allspent acid. In rare instances, some producedwaters contain organic acids that have formeddownhole, such as acetic, hydrochloric andsulfuric acids. Corrosion from acid is a gen-eral thinning of metal, leaving the surface withthe appearance of sharp, feathery or web-likeresidual metal nodules. Metallic scale will notbe formed in the pits.

Chloride Corrosion

Figure 20 is an example of chloride cor-rosion. Chlorides contribute to the likelihoodof an increase in corrosion related sucker rodfailures. The corrosivity of water increases asthe concentration of chlorides increase. Cor-rosion inhibitors have more difficulty reachingand protecting the steel surface of sucker rodsin wells with high concentrations of chlorides.Corrosion, from water with high concentrationsof chlorides, has a tendency to be more ag-gressive to carbon steel sucker rods than toalloy steel sucker rods. Chloride corrosiontends to evenly pit the entire surface area ofthe carbon steel sucker rod with shallow, flat-bottomed, irregular shaped pits. Pit shapecharacteristics include steep walls and sharp

pit edges.

CO2 Corrosion


CO2 acid gas corrosion combines withwater to form carbonic acid—which lowers thepH of the water. Carbonic acid is very aggres-sive to steel and results in large areas of rapidmetal loss that can completely erode suckerrods and couplings. The corrosion severityincreases with increasing CO2 partial pressureand temperature. CO2 corrosion pits areround based, deep with steep walls and sharppit-edges. The pitting is usually intercon-nected in long lines but will occasionally besingular and isolated. The pit bases will befilled with iron carbonate scale, a loosely ad-hering gray deposit that is a by-product of CO2acid gas corrosion.

Figures 21 and 22 show typical exam-ples of CO2 acid gas corrosion. Figure 19 isan example of CO2 acid gas corrosion on acoupling and Figure 20 is an example of CO2


acid gas corrosion on rod bodies.

Dissimilar Metals Corrosion

An extremely rare failure, dissimilarmetals corrosion may result when joining twometals with differences in solution potentialstogether in the same solution. One metal hasa marked tendency to corrode in preference tothe other, and under certain fluid conditions,the less noble metal corrodes at a higher rate.Dissimilar metals corrosion is usually greatestnear the joining of the two metals. Since mostsucker rod materials are compatible, this fail-ure is seldom seen in the rod string.

H25 acid gas corrosion is round based,deep the beveled pit-edges. It is usuallysmall, random and scattered over the entiresurface of the sucker rod. A second corrodentgenerated by H25 acid gas corrosion is ironsulfide scale. The surfaces of both the suckerrod and the pit will be covered with the tightlyadhering black scale. Iron sulfide scale ishighly insoluble and cathodic to steel—whichtends to accelerate corrosion penetrationrates. A third corroding mechanism is hydro-gen embrittlement, which causes the fracturesurface to have a brittle or granular appear-ance. A crack initiation point may or may notbe visible and a fatigue portion may or maynot be present on the fracture surface. Theshear tear of a hydrogen embrittlement failureis immediate during fracture due to the ab-sorption of hydrogen and the loss of ductility inthe steel. Although a relatively week acid(when compared with CO2 acid gas), anymeasurable trace amount of H25 acid gas isconsidered justification for chemical inhibitionprograms when any measurable trace amountof water (H2O) is also present.

H2SCorrosion


Figure 27

Figures 23 and 24 are examples of H2Sacid gas corrosion. The three rod body sam-ples on the left are examples of localized cor-rosion (pitting) and the two rod body sampleson the right are examples of general thinningcorrosion from scale-deposit corrosion. Thesample in Figure 24 is an example of a pin fail-ure due to hydrogen embrittlement.

should be monitored continuously by sam-pling, identifying and counting the bacteria.The extinction dilution technique is commonlyused to culture bacteria for an estimation ofthe number of bacteria present in the well.Bactericide or biocide should be used on allsuspect fluids to control bacteria populations.

Acid Producing Bacteria

Figure 25 shows several examples mi-crobiologically influenced corrosion (MIC) onrod bodies. Some amount of microscopic lifeform is present in essentially every producingwell. Of primary concern to the rod string arethe single celled organisms capable of living inall sorts of conditions and multiplying with in-credible speed—commonly referred to as bac-teria or "bugs". Bacteria are classified accord-ing to their oxygen (02) requirements: aerobic(requires 02), anaerobic (does not require 02)and facultative (either). Some bacteria gener-ate H2S, produce organic acids or enzymes,oxidize soluble iron in produced water, or anycombination of the preceding.

Microbiologically Influenced Corrosion

ing bacteria (APB) on a rod body and Figure27 is an example of APB on a coupling. Corrosion pitting due to APB's have the same basic pit shape characteristics of CO2 acid gascorrosion. Corrosion pitting from APB has acavernous appearing pitedges and the pitgrainy. The pit will not contain scale deposits.


MIC is very aggressive and all sucker rodgrades corrode rapidly in downhole environ-ments containing bacteria. Suspect fluids


Figure29

Figure30

Figure 28 is an example of sulfate re-ducer bacteria (SRB) on a rod body. SRB's,those that produce H2S, probably cause moreproblems to downhole equipment than do anyother type of bacteria. Corrosion due to SRBhave the same basic pit shape characteristicsof H2S acid gas corrosion, often with multiplestress cracks in the pit-base, tunneling aroundthe pit-edges (pits-within-pits), pit clusteringand/or unusual anomalies (i.e. shiny splotcheson the rod surface). The multiple cracking inthe pit-base results from the hydrogen sulfideby-product of the bacterial lifestyle—which

Oxygen (02) Enhanced Corrosion

02 enhanced corrosion will be mostprevalent on couplings, with a few instancesfound on rod upsets. 02 enhanced corrosionis rarely seen on the rod body. In fact, aggres-sive 02 enhanced corrosion can erode cou-plings without harming the sucker rods on ei-ther side. The rate of 02 enhanced corrosionis directly proportional to the dissolved 02 con-centration, chloride content of the producedwater and/or presence of other acid gases.Dissolved 02 can cause severe corrosion atextremely low concentrations and erode largeamounts of metal. Pitting is usually shallow,flat-bottomed and broad-based with the ten-dency of one pit to combine with another. Pitshape characteristics may include sharpedges and steep sides if accompanied by CO2or broad, smooth craters with beveled pit-edges if accompanied by H2S. Corrosion

Sulfate Reducer Bacteria (SRB) corrodes and embrittles the surface of thesteel under the colony.


rates increase with increased concentrationsof dissolved 02.

Figures 29 and 30 are examples of 02enhanced corrosion. The coupling on the leftis an example of the effects of 02 enhancedCO2 acid gas corrosion. The slimhole cou-pling in the middle and the fullsize coupling onthe right are examples of the effects of 02 en-hanced H2S acid gas corrosion. The suckerrod samples in Figure 30 show the effects of02 enhanced CO2 acid gas corrosion near theupset (left) and on the rod body (right).

Stray Current CorrosionRarely seen in most wells, stray current

corrosion refers to the induced, or stray, elec-trical currents that flow to or from the rodstring. Stray current corrosion can be causedby grounding electrical equipment to the well-head, casing or from nearby cathodic protec-tion systems (pipelines). Arcs originating fromthe rod string leave a deep, irregular shapedpit with smooth sides, sharp pit-edges and asmall cone in the base of the pit. Arcs origi-nating from the tubing leave deep pits withsmooth sides and sharp edges that are ran-dom in dimension and irregular in shape.Stray current corrosion pits are usually singu-lar and isolated in a row down one side of thesucker rod near the upsets.

Under-deposit Corrosion

Scales such as barium sulfate, calciumcarbonate, calcium sulfate, iron carbonate,iron oxide (rust), iron sulfide, and strontiumsulfate should be prevented from forming onsucker rods. Although scale on a sucker rodslows down the corrosion penetration rate, italso reduces the effectiveness of chemical in-hibitors. Severe localized corrosion, in theform of pitting, results anytime the scale iscracked by a bending movement or removedby abrasion.

Manufacturing DefectsFailures due to a manufacturing defect

are rare and seldom occur. Manufacturing de-fects are easily recognized and it is importantthat you understand what these defects looklike if you are to file accurate claims for war-ranty reimbursement. No manufacturer is ex-cluded from the possibility of defects in mate-rial or workmanship and the following exam-ples include defects from several differentmanufacturers.

Figure 31 is an example of mill defects.Mill defects occur along one side of the rodbody and these discontinuities normally havelongitudinally tapered bottoms and sharpedges with indications of the longitudinal seamin the base of the discontinuity. The exampleon the far left and the rod body third from theleft are examples of a sliver. (In the examplethird from the left, the protrusion folded againstthe fracture surface during "fishing"). The rodbody second from the right is an example of ascab. A sliver is a small loose or torn segmentand a scab is a large loose or torn segment ofmaterial longitudinally rolled into the surface ofthe bar. One end of the sliver or scab is nor-mally metallurgically bonded into the rod bodywhile the remaining end is rolled into the sur-face and physically attached. Fatigue failures,which result from slivers or scabs, will have apiece of loose material protruding over the fa-tigue portion of the fracture surface. The rodbody second from the left is an example ofrolled-in-scale. Rolled-in-scale is a surfacediscontinuity caused when scale (metal oxide),formed during a prior heat, has not been re-


moved prior to bar rolling. The rod body onthe far right is an example of a rolling lap.Rolling laps are longitudinal surface disconti-nuities that have the appearance of a seamfrom rolling, with sharp corners folded overand rolled into the bar surface without metal-lurgical bonding.

Figure 32 is an example of forging de-fects. The fracture begins internally below aforging crack in the upset area and is brittle orgranular in appearance. A crack initiation sitemay or may not be visible and a fatigue por-tion may or may not be present on the fatiguefracture surface. The examples on the left andin the middle occur as a result of low forgingtemperatures. The example on the left is froma cold-shut and the example in the middle isfrom a forging crack. The fracture on the rightis a failure caused by a subsurface longitudi-nal seam located near the end of the raw bar.During the forging process the orientation of

this discontinuity was changed transversely.

Figure 33 is an example of incipientgrain boundary melting, an extremely raremanufacturing defect. This condition iscaused by forging the upset end of the rod attoo high a temperature for the steel. Unfortu-nately, no inspection exists that will catch thisbefore the rod is shipped. Fortunately, thesebrittle pins usually snap off during makeup.No crack initiation point is visible and no fa-tigue portion will be present on the fracturesurfaces. Optical pyrometers on forging


equipment have virtually eliminated this prob-lem.

Figure 34 is an example of processingdefects. The lower example is a casehard-ened sucker rod and the upper example is acoupling that has been processed through agrinding operation to reduce the diameter. In


both examples, a difference in the materialhardness has resulted in preferential corrosionattack.

Figure 35 is an example of a mill defectand a machining defect. The lower example isa failure due to a large, internal, nonmetallicinclusion in the pin. The fracture began inter-nally and is brittle or granular in appearance.A crack initiation site may or may not be visi-ble and a fatigue portion may or may not bepresent on the fracture surfaces. The upperexample is a "ran-twice" defect from rolling thepin threads twice. Rolling the threads twicehas flattened the pin thread-crest and will notbe capable of achieving the correct frictionload required for makeup.

Your initial investment in sucker rods issubstantial. Moreover, the costs related to re-placing damaged sucker rods generally out-weighs the original cost of the new rod string.Protecting your investment and getting themaximum service life out of your sucker rodsjust makes good sense. It is important to diag-nose rod failures accurately and to implementcorrective action measures to prevent futurefailure occurrences. This photo essay is in-tended for use as a reference guide in suckerrod failure analysis. It explains how rod fail-ures occur and expounds methods for identify-ing the characteristics of the failure mecha

nisms. Where sucker rod failures are con-cerned, there are no absolutes and no two fail-ures look exactly alike in appearance. But, byrecognizing the visual clues and identifyingcharacteristics of the different failures, correc-tive action measures can be taken to preventsucker rod failures, thus allowing the operatorto produce marginally profitable wells morecost effectively.

FAILURE ANALYSIS REQUESTWhen requesting a failure analysis, please fill out this form as accurately and completely as possible. This information and theaccompanying samples will serve as the basis for determining the failure cause.

Company:

Lease & Well Number:

Office Telephone:

Cellular Telephone:

Contact: ___

Location: ___

Office Facsimile:

Email Address:

SamplesInclude the following:

1. Coupling Failures: Include both halves of the coupling or the entire coupling, depending upon the type of failure.2. Pin and Upset Failures: Include the pin end with the broken pin and the coupling with the broken pin stub or the pin end with the

galled pin threads and the coupling with the galled threads.

a. All failures should include the stamped end for identification of the sample. (Only one end is stamped for identification andtraceability.)

3. Sucker Rod Body and Sinker Bar Body / Elevator Neck Failures: Include approximately 18" on either side of the failure. If thefailure is within 18" of the pin end, include the pin end and 18" of the sucker rod body / sinker bar body on the other side of the failure.

a. All failures should include the stamped end for identification of the sample. (Only one end is stamped for identification andtraceability.)

4. Rod Guide Failures: Include the entire rod guide or pieces of the rod guide, depending upon the type of failure. If the rod guide is stillon the rod body but has cracked, moved, or is starting to break apart, cut the rod body leaving 6" of rod body on either side of the rodguide.

DataYou will be contacted if further information is required to complete our analysis.

Application:Install Date:

Manufacturer:

Type / Class:

Heat Code:

Failure Date:

Size:

Grade:

Failure Depth (FS):

L e n g t h :

Product:Manufacturing Date:

Additional Comments

ShipmentsPlease contact the service center in advance to advise us that you will be shipping a sample for analysis.

InternationalNorrisPO Box 1496(4801 West 49th Street (74107))Tulsa, OK 74101-1496Attn: Technical ServiceTelephone: (918) 445-7600Facsimile: (918) 445-7632

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Information and/or Samples Furnished By:

Company: Location: Contact:

Office Telephone:

Email Address:

Office Facsimile: Cellular:

Subsequent to analysis, Norris will email, mail, or fax you a report of the findings. Questions and/or concerns should be directed tothe service center that received your shipment.

Form106A,Rev.B1/08/2007

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