| Failure
Analysis
Root
Cause Failure Analysis is Essential for Failure Frequency Reduction
in Wells With Artificial Lift. Cost-effective failure management begins with prevention, and the time to stop the next failure is now-prior to an incident! Simply fishing and hanging the well on after a sucker rod failure will not prevent failure recurrence. In fact, most failures continue with increasing frequency until the entire rod string must be pulled and replaced. Achievable failure frequency reductions require accurate failure root cause analysis and the implementation of corrective action measures to prevent failure recurrence. A database capable of querying the well "servicing" history is needed to track and identify failure trends. Once a failure trend is identified, remedial measures should be implemented during well servicing operations to prevent premature rod string failures. The database failure history should include information on the failure type, location, depth, root cause, and the corrective action measures implemented. Sucker rods can be caused to fail prematurely. Understanding the effects of seemingly minor damage to rod strings, and knowing how that damage can produce catastrophic failures, is very important for production personnel. Sucker rod failure analysis is challenging and you need to be able to look past the obvious and seek clues from the not so obvious. All production personnel should have adequate knowledge and training in failure root cause analysis. Understanding how to identify failures and their contributing factors allows us an understanding of what is required to correct the root cause of the failure. Every step that can be taken to eliminate premature sucker rod failures must be taken. On-going training programs concerning sucker rods should include formal and informal forums that advocate following the recommendations of manufacturers for artificial lift design, care & handling, storage & transportation, running & rerunning, and makeup & breakout procedures. A variety of training schools are currently available and, with advanced notice, most can be tailored to meet the specific needs of production personnel. Failure
Mechanisms Fatigue failures are progressive and begin as small stress cracks that grow under the action of cyclic stresses. The stresses associated with this failure have a maximum value that is less than the tensile strength of the sucker rod steel. Since the applied load is distributed nearly equally over the full cross-sectional area of the rod string, any damage that reduces the cross-sectional area will increase the load or stress at that point and is a stress raiser. A small stress fatigue crack forms at the base of the stress raiser and propagates perpendicular to the line of stress, or axis of the rod body. As the stress fatigue crack gradually advances, the mating fracture surfaces opposite the advancing crack front try to separate under load and these surfaces become smooth and polished from chafing. As the fatigue crack progresses, it reduces the effective cross-sectional area of the sucker rod until not enough metal remains to support the load, and the sucker rod simply fractures in two. The fracture surfaces of a typical fatigue failure have a fatigue portion, tensile portion, and final shear tear. Fatigue failures are initiated by a multitude of stress raisers. Stress raisers are visible or microscopic discontinuities that cause an increase in local stress on the rod string during load. Typical visible stress raisers on sucker rods, pony rods and couplings are bends, corrosion, cracks, mechanical damage, threads, and wear or any combination of the preceding. This increased stress effect is the most critical when the discontinuity on the rod string is transverse (normal) to the principle tensile stress. In determining the stress raiser of a fatigue failure, the fatigue portion opposite the final shear tear (extrusion/protrusion) must be carefully cleaned and thoroughly examined. Fatigue failures have visible or macroscopic identifying characteristics on the fracture surface, which help to identify the location of the stress raiser. Ratchet marks and beach marks are arguably two of the most important features in fatigue failure identification. Ratchet marks are lines that result from the intersection and connection of multiple stress fatigue cracks while beach marks indicate the successive position of the advancing fatigue crack. Ratchet marks are parallel to the overall direction of crack growth and lead to the initiation point of the failure. Beach marks are elliptical, or semi-elliptical rings radiating outward from the fracture origin and indicate successive positions of the advancing stress fatigue crack growth.
The
remaining examples are fatigue failures on: casehardened sucker rods;
normalized and tempered sucker rods; and quenched and tempered sucker
rods. The example on the far left is a torsional fatigue failure from
a progressing cavity pump. Ratchet marks found in the large fatigue
portion, and originating from the surface of the rod body, completely
encircle the fracture surface with the small tensile tear portion shown
slightly off middle-center. The second rod body on the left is a casehardened
fatigue failure. The case encircling the rod body diameter carries the
load for this high tensile strength sucker rod and if you penetrate
the case, you effectively destroy the load-carrying capability of this
type of manufactured sucker rod. The stress fatigue crack advances around
the case and progresses across the rod body. A fatigue failure on a
casehardened sucker rod generally exhibits a small fatigue portion and
a large tensile tear. The third rod body from the left is typical in
appearance for most fatigue failures. Typical fatigue failures have
a fatigue portion, tensile portion and final shear tear. The width of
the fatigue portion is an indication of the loading involved with the
fracture. Mechanical damage can prevent or hinder failure analysis by
destroying the visual clues and identifying characteristics normally
found on a fatigue fracture surface. Care must be exercised when handling
the fracture halves. It is very important to resist the temptation to
fit the mating fracture surfaces together since this almost always destroys
(smears) microscopic features. To avoid mechanical damage, fracture
surfaces should never actually touch during fracture-surface matching.
Design
and Operation Failures Commercially available computer design programs allow the system designer to optimize production equipment at the least expense for the well conditions existing at the time of the design. After the initial design and installation of the rod string, periodic dynamometer surveys should be utilized to confirm that equipment load parameters are within those considered acceptable. A good initial design may become a poor design if well conditions change. Changes in the fluid volume, fluid level, stroke length, strokes per minute or pump size severely impact the total artificial lift system. Changes in fluid corrosiveness can affect the fatigue endurance life of sucker rods and may lead to premature failures. When one of the preceding conditions change, the design of the artificial lift system must be re-evaluated.
The second rod body from the left is a flexing fatigue failure. Flexing fatigue failures occur from the motion of the rod string having a constant lateral or side movement during the pumping cycle. Stress fatigue cracks due to flexing will concentrate along the area of the rod where the greatest bending stresses occurred. The fine, transverse, stress fatigue cracks will be on one half of the circumference of the rod body, closely spaced near the rod upsets and gradually spreading apart, moving toward the middle of the rod body. Most flexing fatigue failures occur above the connection in the transition zone of the rod body-between the rigid coupling and upset area and the more flexible rod body. Flexing fatigue failures will not show permanent bends since this problem occurs while the rod string is in motion. The example on the far right is a unidirectional bending fatigue failure. This type of failure generally has two tips protruding above the fracture surface. These distinct failure characteristics indicate a double shear-lip tear. Double shear-lip tears are the direct result of unidirectional bending stresses, with fractures occurring under compressive rod loads. Compressive rod loads may be the result of large bore pumps with small diameter sucker rods or multiple tapers in shallow wells. The second rod body sample on the right is a stress fatigue failure. Stress fatigue failures occur on highly stressed sucker rods as a result of worn out sucker rods, overloads, or extremely high rod loads for short periods of time. Stress fatigue failures have closely spaced, fine, transverse stress fatigue cracks that completely encircle the circumference of the rod body. The stress fatigue cracks will be on the wrench square and over the entire length of the rod body. With very old sucker rods, stress fatigue cracks and failure may occur within normal everyday operating loads.
Mechanical
Failures Bent
Rod Failures Straightening the raw bar stock is the first step in the process of manufacturing sucker rods. Cold straightening the bar deforms the grain structure below its recrystallization temperature, putting a strain in the bar that is accompanied by a work hardening effect. During the manufacturing process, the function of heat treatment is to stress-relieve the residual and induced stresses caused by bar rolling, bar straightening processes and from forging the rod upsets. Heat treatment changes the metallurgical structure of the forged ends to match that of the rod body and also controls the mechanical properties of the sucker rod. Any rod body bend created after heat treatment causes work hardening, which creates an area of hardness different than the surrounding surfaces. This condition is referred to as a "hard spot" and is a stress raiser to load. Mechanical processing, such as passing the finished sucker rod through a system of rollers, will attempt to remove the bend so it appears to be straight. However, reconditioning processes are not capable of stress relieving bent sucker rods. A bent sucker rod is permanently damaged and should not be used because all bent sucker rods will eventually fail.
Surface
Damage Failures
Connection
Failures
Mid-length coupling fractures, with ratchet marks leading from the outside, indicate another type of failure. The stress fatigue crack starts from the outside coupling surface, progressing inward toward the threads, then around the coupling wall to a tensile fracture. Mid-length fractures indicate coupling failures from mechanical damage to the coupling surface, exceeding the stress fatigue endurance limit of the material, or a manufacturing defect. Most mid-length coupling fractures are the result of mechanical damage or overload. Mid-length coupling fractures due to overload have a small fatigue portion and large tensile tear portion. This failure is common with high strength sucker rods and Class SM couplings. Use Class T couplings to avoid mid-length coupling failures with high tensile strength sucker rods.
Polished
rod pin failures generally occur due to the installation of sucker rod
couplings. Polished rod pins have a 9A thread taper between the straight-threaded
section and the shoulder. Sucker rod couplings have a 30A starting thread
and a deep recess that doesn't engage all the polished rod pin threads.
Polished rod couplings have a 9A starting thread and a profile designed
to properly fit the polished rod pin. The shallow recess to the first
thread easily distinguishes polished rod couplings from sucker rod couplings
and allows every polished rod pin thread to be engaged.
Corrosion
Failures All downhole environments are corrosive to some degree. Some corrosive fluids may be considered non-corrosive if the corrosion penetration rate, recorded as mils of thickness lost per year (mpy), is low enough that it will not cause problems. However, most producing wells are plagued by corrosion problems and no currently manufactured sucker rod can successfully withstand the effects of this corrosion alone. While corrosion cannot be completely eliminated it is possible to control its reaction. All grades of sucker rods must be adequately protected through the use of effective chemical inhibition programs (reference current editions of API Specification 11BR and NACE Standard RPO195). Some sucker rod grades, due to different combinations of alloying elements, microstructures and hardness levels, are capable of longer service life in inhibited corrosive wells than other grades of either low or high tensile strength. Why do new sucker rods seem to corrode faster than older rods in the same string? Two sucker rods with the same chemical analysis will form a galvanic corrosion cell if the physical condition of one is different from the other. Physical differences in a sucker rod may be caused from poor care and handling practices (i.e. surface damage resulting in bruises, nicks, bends) and/or corrosion deposits. Since new sucker rods go into the well without corrosion deposits, they often corrode preferentially in relation to rods that are coated with corrosion deposits. Corrosion on steel starts very aggressively but often slows down as soon as an obstructive surface film of corrosion deposit (scale) is formed upon the metal surface. For example, CO2 generates iron carbonate scale as a by-product of its corrosion. This scale coats the sucker rod and retards the corrosion penetration rate, which tends to slow down corrosion. However, if this deposit is continuously cracked by a bending movement or removed by abrasion, aggressive local corrosion continues on the area with the scale removed, and results in deep corrosion pitting. Can high tensile strength sucker rods be used in a corrosive environment? Generally soft rods tolerate corrosion better than hard rods and, as a rule of thumb, you should always use the softest rod that will handle the load. However, if load requirements dictate the use of high tensile strength rods then it is important to protect the rods with an effective surface film of corrosion inhibitor. In most cases, if you can adequately protect downhole equipment from corrosion, you should be able to adequately protect high tensile strength rods from corrosion by increasing the application frequency of the corrosion-inhibitor program. In other words, if you effectively batch treat once a week with 40 parts per million (ppm) of corrosion inhibitor for D class rods, you will need to batch treat twice weekly with 40 ppm of corrosion inhibitor for high tensile strength rods. Treatment volumes vary and are dependent upon many factors too numerous to list. Always consult with a corrosion control specialist prior to the installation of every rod string, especially when stress corrosion fatigue is suspected as the failure root cause.
Acid
Corrosion Chloride
Corrosion CO2
Corrosion
Dissimilar
Metals Corrosion H2S
Corrosion
Microbiologically
Influenced Corrosion (MIC)
Figure
23 shows several examples of microbiologically influenced corrosion
(bacteria) on sucker rod bodies. Oxygen
Enhanced Corrosion
Scale
Corrosion Stray
Current Corrosion Manufacturing
Defects
Figure 30 is an example of a mill defect and a machining defect. The lower example is a failure due to a large, internal, nonmetallic inclusion in the pin. The fracture began internally and is brittle or granular in appearance. A crack initiation site may or may not be visible and a fatigue portion may not be present on the fatigue fracture surface. The upper example is from rolling the pin threads twice. Rolling twice has flattened the pin thread crest and will not be capable of achieving the correct friction load required for makeup.
Clayton T. Hendricks is Manager International Sales and Technical Services of Sucker Rod Operations, for Norris/A Dover Resources Company, located in Tulsa, Oklahoma USA. Hendricks has been with Norris for 20 years and has served in various engineering/technical capacities. In 1991 Hendricks was awarded the position of Manager of Technical Services, prior to the addition of Manager of International Sales in 1994. Hendricks graduated in 1980 with a BS in Mechanical Engineering from Oklahoma State University. He currently is a member of SPE, NACE, and is a task group member of API Committee 1, Subcommittee 11, Field Operating Equipment. Russell
D. Stevens is Technical Services Coordinator of Norris Sucker Rod Operations
in Midland/Odessa, Texas. Stevens has been with Norris for 17 years
and has served in various sales/technical capacities. Previous experience
includes 10 years with Norris/O'Bannon. During this tenure he became
District Sales Manager for the bottom hole pump product group. In 1993
he became involved in Norris sucker rod sales until 1996 when he was
promoted to Technical Services Coordinator. Stevens is a member of SPE,
NACE, and ASM. He serves as an alternate task group member of API Committee
1, Subcommittee 11, Field Operating Equipment.
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Figure
1 is an example of tensile and fatigue failure mechanisms. The two examples
on the right are tensile failures. A tensile failure is characterized
by a reduction in the diameter of the cross-sectional area at the point
of fracture. Typical tensile failures have cup-cone fracture halves.
The second example from the right is typical in appearance for tensile
failures. Fractures from tensile failures rupture, or shear, on 45A
angles to the stresses applied. A good example of the shear is the characteristic
cup-cone fracture surfaces of a typical tensile failure. The rod body
on the right is an excellent example of needing to look past the obvious
for the not so obvious. A fatigue failure is primarily responsible for
this failure even though fracture occurred while trying to unseat a
stuck pump. Visual examination of the fracture surface reveals a small,
semi-elliptical, stress fatigue crack. This sucker rod had pre- existing,
transverse stress fatigue cracks, from in-service stresses. One of the
stress fatigue cracks opened during the straight, steady load applied
in attempting to unseat the pump, and fracture occurred. The tensile
failure is secondary and results in the unusual appearance of the fracture
surface-with the small fatigue portion, large tensile portion and unusually
large 45A double shear tears.
Figures
2 and 3 are examples of design and operationally induced mechanical
failures. Wear, flexing fatigue, unidirectional bending fatigue, and
stress-fatigue failures indicate compressive rod loads, deviated wells,
fluid pound, gas interference, highly stressed sucker rods, improperly
anchored tubing, pumps tagging bottom, sticking pump plungers, unanchored
tubing, or some combination of the preceding.
Wear
causes rod failures by reducing the cross-section of the metal, exposing
new surface metal to corrosion, and causes joint failures from impact
and shoulder damage. The Class T coupling on the left, the Class SM
coupling second from left, and the rod body on the left are all examples
of wear. Wear on the sucker rod string is defined as the progressive
removal of surface metal by contact with the tubing. Wear that is equal
in length, width, and depth usually suggests a deviated or crooked well
bore. Angled wear patterns indicate rod strings that are aggressively
contacting the tubing at an angle, usually as a result of fluid pound
or unanchored (improperly anchored) tubing. The middle rod body represents
corrosion-abrasion wear. Wear also removes corrosion inhibiting films
and exposes new surface metals to corrosive well fluids-which accelerate
the rate of corrosion. The Class T coupling on the far right has a work-hardened
ridge from tubing-slap wear. Tubing-slap wear is the result of the rod
string "stacking out"-probably as a result of fluid pound, gas interference
or pump tagging. The work-hardened material doesn't wear as fast as
the softer material on either side of the work-hardened area, and it
leaves a ridge of material as the rest of the coupling wears.
Figure
4 is an example of coupling-to-tubing slap. Coupling-to-tubing slap
is the result of extremely aggressive angle contact to the tubing by
the rod string. This aggressive contact is the direct result of severe
fluid pound, unanchored (or improperly anchored) tubing, sticking (or
stuck) pump plungers, or any combination of the preceding.
Figure
5 is an example of rod guide related damage. The example on the left
is a reconditioned, high tensile strength sucker rod. Turbulent fluid
flow, associated with short, blunt-end injection molded rod guides,
allowed crevice corrosion in the critical wash area around the end of
the guide. Prior to inspecting the mold-on rod guides were removed from
the rod body for reconditioning. Class 1 reconditioned sucker rods cannot
have discontinuities greater than 20 mils (0.020") per API Specification
11BR. The crevice corrosion was under the 20 mils allowed for a Class
1 reconditioned sucker rod. However, the notch sensitivity (discontinuity
intolerance) of a high tensile strength sucker rod is high. In other
words, small pits can be detrimental to the high tensile stresses associated
with the high strength sucker rod and reconditioned high strength sucker
rods should be de-rated for load. The example in the middle is an erosion/corrosion
failure resulting from short, blunt-end, field applied rod guides in
small tubing with high fluid velocities. Erosion/corrosion pits will
be "fluid cut" with very smooth bottoms. Pit shape characteristics may
include sharp edges and steep sides if accompanied by CO2 or broad smooth
pits with beveled edges if accompanied by H2S. The example on the right
is abrasion wear from a field applied guide moving up and down on the
rod body during the pumping cycle. Generally speaking, mold-on rod guides
provide better laminar flow, a minimum of three to four times more bonding
and holding power and are more cost-effective than are field applied
rod guides. 
Figure
6 (with inset of Figure 7) is an example of bending fatigue failures.
Bending fatigue failures can be identified by the angled fracture surface,
which will be at some angle other than 90A to the axis of the rod body.
The example on the left illustrates a fracture caused by a long radius
bend, or gradual bow in the rod body (left example in Figure 7). The
fracture surface is normal in appearance, but has a slight angle when
compared to the axis of the rod body. 
The
middle example is a short radius bend (right example in Figure 7). The
fracture surface is at a greater angle to the axis of the rod body with
a small fatigue portion and a large tensile tear portion. The example
on the right is the result of a corkscrewed sucker rod. Notice how convoluted
the fracture surface is in appearance. As a general rule, the greater
the bend in the rod body, the more convoluted the fracture surfaces
appear. In operation, the time for the rod to fracture is greatly shortened.
Poor care and handling procedures usually cause bent rods.
Figure
8 is an example of various surface damage failures. The example on the
left shows a slight depression from a wrench, tool, or other metal object.
The second example from the left is damage from a pipe wrench used in
applying field-installed rod guides. The second example from the right
has a small longitudinal scratch, through metal-to-metal contact, by
allowing sucker rods to run down other rods in a rod bundle during installation.
The example on the right exhibits transverse surface damage.
Figure
9 is an example of surface damage caused by sucker rod elevators. The
bottom example is damage from worn or misaligned elevator seats. After
an extended period of service, the elevator seats become so worn that
they develop an oval shape rather than a round shape. As the oval shape
grows, the tangency ring of the rod upset to the elevator seat face
is lowered in the front half of the seat. As the seat continues to wear
the seating position of the rod upset is moved forward of the elevator
trunnion centerline. This causes an offset in the hook load and tilts
the elevator body forward. When the elevator lifts the rod string load,
the hook load will bend the sucker rod centerline to coincide with the
elevator trunnion centerline. As the rod string weight increases, the
hook load will bend every sucker rod engaged by this elevator. Bent
sucker rod failures that occur below the surface upset bead may be from
bad elevator seats. The top example is damage caused by the elevator
latches. This type of damage normally occurs as a result of picking
up or laying down in doubles. Never pick up or lay down anything more
than one single sucker rod. Anything else causes the elevator latches
to act as a fulcrum and allows bending stresses to concentrate in the
transition zone of the rod body and the forged upset.
Figure
10 is an example of pin failures due to a loss of displacement. The
sample on the right is typical in appearance for a loss of displacement
pin failure. Insufficient makeup, or the loss of tightness, caused the
pin shoulder face and the coupling shoulder face to separate. When these
faces separate, a bending moment is added to the tensile load in the
pin. The threaded section of the pin is held rigid while the rest of
the pin flexes. The motion of the rod string causes stress fatigue cracking
to start in the first fully formed thread root above the undercut. Small
stress fatigue cracks begin along the thread root and consolidate into
a major stress fatigue crack. The fracture surface of a typical loss
of displacement pin failure has a small fatigue portion covering approximately
one-third of the fracture surface with the tensile tear portion and
final shear tear covering the remaining fracture surface. The examples
on the left and in the middle will occur as a result of stress loading
when stress-raising factors such as corrosion or mechanical damage is
present on the surface of the pin undercut.
Figure
11 is another example of two types of pin failures. The sample on the
left is typical in appearance for a loss of displacement pin failure.
However, this pin fracture was caused by the hydraulic rod tongs during
makeup as is evidenced by the stair-stepped tensile tear. It is not
uncommon for pin fractures to occur at makeup, if the pin has a pre-existing
stress fatigue crack due to the high torque required during joint makeup,
with large diameter Class D and all sizes of high tensile strength sucker
rods. The sample on the right is an example of excessive torque on a
soft pin. The fracture surface has a large fatigue portion, with multiple
ratchet marks in the pin-thread root, and a small tensile portion.
Figure
12 is an example of a loss of displacement coupling failure. The fracture
initiated in the coupling thread-root opposite the first fully formed
pin starting thread. One-third/two-third fracture halves, in length,
with ratchet marks originating in the thread root indicate a loss of
displacement coupling failure. The fracture surface of a typical loss
of displacement coupling failure has a small fatigue portion and a large
tensile tear portion. Loss of displacement coupling failures are primarily
associated with Class D sucker rods and high tensile strength sucker
rods.
Figure
13 is an example of thread galling in the sucker rod connection. Thread
galling is mechanical damage to the sucker rod and/or coupling threads.
Thread galling is the result of damaged or contaminated threads causing
the interference between the threads to be great enough to rip and tear
the thread surfaces. The threads weld together during makeup and strip
apart at breakout and the connection is damaged and destroyed beyond
use. Hard stabbing damage to the leading thread, and contaminated threads
are the primary causes of thread galling. Cleaning the threads prior
to makeup, properly lubricating the threads and following careful makeup
procedures will prevent thread galling.
Figure
14 is an example of wrench square failures. Wrench square failures are
extremely rare and seldom occur unless from mechanical damage, corrosion
or manufacturing defects. The example on the left is a wrench square
failure from severe mechanical damage. A loose or sloppy backup on the
hydraulic rod tongs has rounded the wrench square corner. The stress
fatigue crack began in the corner of the wrench square and progressed
to final rupture or fracture. The example on the right is a wrench square
failure from a manufacturing defect. The failure initiated in the die
stamp mark and is an example of an excessive die stamp depth failure.
Die stamp markings can become notches that serve as stress raisers if
the depth of the die stamping, during the forging process, is not controlled
and kept within API Specification 11B, Allowable Tolerances.
Figure
15 is an example of the damage that occurs as a result of severely over-tightening
the sucker rod connection. The example shown is an over-tightened coupling
that has flared out or bulged near the contact face. Slim-hole couplings
are more susceptible to this type of over-tightened damage than are
full sized couplings. Over-tightened full size couplings on Class D
and high strength sucker rods generally exhibit slight bulges and have
the concentric deformation ridge of material on the coupling shoulder
face from the impression of the pin shoulder face. Over-tightening with
hydraulic rod tongs will twist off soft pins resulting in a tensile
failure appearance. The pin undercut will neck down and fracture occurs
rapidly. With Class D sucker rods, an indication of over-tightening
is the concentric deformation ridge of material on the pin shoulder
face from the impression of the coupling shoulder face. Over-tightening
on normalized and tempered high tensile strength sucker rods will begin
to pull the threads out of the coupling.
Figure
16 is an example of impact cracks on couplings. The practice of "warming
up," or hammering, on couplings in order to loosen them should not be
allowed. This example shows how impact damage to a Class T coupling
causes stress fatigue cracks around the impact points and accelerated
localized corrosion. Hammering on Class SM couplings causes stress fatigue
cracks in the hard spray surface and results in a coupling failure due
to stress/corrosion fatigue.
Figure
17 is an example of polished rod failures. The majority of all polished
rod failures occur either in the body, just below the polished rod clamp,
or in the pin. Polished rod body failures below the polished rod clamp
result from the addition of bending stresses. These bending stresses
may be imposed by pumping units out of alignment, carrier bars that
do not set level, worn carrier bars, misaligned load cells, or incorrect
polished rod clamp installation. The polished rod failure on the left
is an example of a polished rod clamp on the sprayed portion of a spraymetal
polished rod. Spraymetal polished rods have an unsprayed portion for
polished rod clamp placement. Never put a polished rod clamp on the
sprayed portion of a spraymetal polished rod. The polished rod failure
on the right has small, longitudinal scratches caused from mishandling.
Figure
18 is an example of stress-corrosion fatigue from CO2 corrosion. The
size of the pit, as far as when it becomes detrimental to the rod, depends
on two factors-material type and hardness. Class K sucker rods may develop
deeper and larger pits than a Class D sucker rod before it becomes detrimental
to the rods. Class D sucker rods may develop deeper and larger pits
than a high tensile strength rod before it becomes detrimental to the
rods. Softer materials with lower rod stress tolerate larger pits than
do harder materials with higher rod stress. Therefore, small pits can
be detrimental to higher tensile strength sucker rods as opposed to
a softer rod that does not have as much rod stress.
CO2
combines with water to form carbonic acid which lowers the pH of the
water. Carbonic acid is very aggressive to steel and results in large
areas of rapid metal loss that can completely erode sucker rods and
couplings. The corrosion severity increases with increasing CO2 partial
pressure and temperature. CO2 corrosion pits are round based, deep with
steep walls and sharp edges. The pitting is usually interconnected in
long lines but will occasionally be singular and isolated. The pit bases
will be filled with iron carbonate scale, a loosely adhering gray deposit
generated from CO2.
Figures
19 and 20 show typical examples of CO2 corrosion. Figure 19 is an example
of CO2 corrosion on couplings and Figure 20 is an example of CO2 corrosion
on rod bodies. 
H2S
pitting is round based, deep with steep walls and beveled edges. It
is usually small, random, and scattered over the entire surface of the
rod. A second corrodent generated by H2S is iron sulfide scale. The
surfaces of both the sucker rod and the pit will be covered with the
tightly adhering black scale. Iron sulfide scale is highly insoluble
and cathodic to steel which tends to accelerate corrosion penetration
rates. A third corroding mechanism is hydrogen embrittlement, which
causes the fracture surface to have a brittle or granular appearance.
A crack initiation point may or may not be visible and a fatigue portion
may not be present on the fracture surface. The shear tear of a hydrogen
embrittlement failure is immediate during fracture due to the absorption
of hydrogen and the loss of ductility in the steel. Although a relatively
weak acid, any measurable trace amount of H2S is considered justification
for chemical inhibition programs when any measurable trace amount of
water is also present.
Figure
21 and 22 are examples of H2S corrosion. The three rod body samples
on the left are examples of localized corrosion (pitting) and the two
rod body samples on the right are examples of general thinning corrosion
from under- scale deposit corrosion. The sample in Figure 22 is an example
of a pin failure due to hydrogen embrittlement.
MIC
has the same basic pit shape characteristics of H2S, often with multiple
stress cracks in the pit base, tunneling around the pit edge and/or
unusual anomalies (i.e. shiny splotches) on the rod surface. Bacteria
are very aggressive and all sucker rod grades corrode rapidly in downhole
environments containing bacteria. Sulfate reducers (SRB's), those that
produce H2S, probably cause more problems to downhole artificial lift
equipment than do any other bacteria type. Multiple cracking in the
pit bases results from the hydrogen sulfide by-product of the bacterial
lifestyle, which corrode and embrittle the surface of the steel under
the colony.
Oxygen
enhanced corrosion will be most prevalent on couplings, with a few instances
found on rod upsets. Oxygen enhanced corrosion is rarely seen on the
rod body. In fact, aggressive oxygen enhanced corrosion can erode couplings
without harming the sucker rods on either side. The rate of oxygen enhanced
corrosion is directly proportional to the dissolved oxygen concentration,
chloride content of the produced water and/or presence of other acid
gases. Dissolved oxygen can cause severe corrosion at extremely low
concentrations and evaporate large amounts of metal. Pitting is usually
shallow, flat-bottomed, and broad-based with the tendency of one pit
to combine with another. Pit shape characteristics may include sharp
edges and steep sides if accompanied by CO2 or broad, smooth craters
with beveled edges if accompanied by H2S. Corrosion rates increase with
increased concentrations of dissolved oxygen.
Figures
24 and 25 are examples of oxygen enhanced corrosion. The coupling sample
on the left is an example of the effects of oxygen enhanced CO2 corrosion
(left), H2S corrosion (middle), and chloride corrosion (right) while
the rod samples in Figure 25 show the effects of oxygen enhanced CO2
corrosion near the upset (left) and CO2 corrosion on the rod body (right).
Figure
26 is an example of mill defects. Mill defects occur along one side
of the rod body and these discontinuities normally have longitudinally
tapered, sharp "V" shaped bottoms with indications of the longitudinal
seam in the base. The example on the far left is an example of a sliver.
The rod body third from the left is also an example of a sliver. When
fishing the rod failure, the sliver folded against the fracture surface.
The rod body second from right is an example of a scab. A sliver is
a small loose or torn segment and a scab is a large loose or torn segment
of material longitudinally rolled into the surface of the bar. One end
of the sliver or scab is normally metallurgically bonded into the rod
body while the remaining end is rolled into the surface and physically
attached. Fatigue failures, which result from slivers or scabs, will
have a piece of loose material protruding over the fatigue portion of
the fracture surface. The rod body second from the left is an example
of rolled-in-scale. Rolled-in-scale is a surface discontinuity caused
when scale (metal oxide), formed during a prior heat, has not been removed
prior to bar rolling. The rod body sample on the far right is an example
of a rolling lap. Rolling laps are longitudinal surface discontinuities
that have the appearance of a seam from rolling, with sharp corners
folded over and rolled into the bar surface without metallurgical bonding.
Figure
27 is an example of forging defects. The fracture begins internally
below a forging crack in the upset area and is brittle or granular in
appearance. A crack initiation site may or may not be visible and a
fatigue portion may not be present on the fatigue fracture surface.
The examples on the left and in the middle occur as a result of low
forging temperatures. The example on the left is a failure from cold-shut
and the example in the middle is a failure from a forging crack. The
fracture on the right is a failure caused by a subsurface longitudinal
seam located near the end of the raw bar. During the forging process
the orientation of this discontinuity was changed transversely.
Figure
28 is an example of incipient grain boundary melting, an extremely rare
manufacturing defect. This condition is caused by forging the upset
end of the rod at too high a temperature for the steel. Unfortunately,
no inspection exists that will catch this before the rod is shipped.
Fortunately, these brittle pins usually snap off during makeup. No crack
initiation point is visible and no fatigue portion will be present on
the fracture surface. Optical pyrometers on forging equipment will virtually
eliminate this problem.
Figure
29 is an example of processing defects. The lower example is a casehardened
sucker rod and the upper example is a coupling that has been processed
through a grinding operation to reduce the diameter. In both examples,
a difference in the material hardness has resulted in preferential corrosion
attack.
Your
initial investment in sucker rods is substantial. Moreover, the costs
related to replacing damaged sucker rods generally outweighs the original
cost of the new rod string. Protecting your investment and getting the
maximum service life out of your rod string just makes good sense. It
is important to diagnose rod failures accurately and to implement corrective
action measures to prevent future failure occurrences. This photo essay
is intended for use as a reference guide in sucker rod failure analysis.
It explains how rod failures occur and expounds methods for identifying
the characteristics of the several failure mechanisms. Where sucker
rod failures are concerned, there are no absolutes and no two fractures
look exactly alike in appearance. But, by recognizing the visual clues
and identifying characteristics of the different failure mechanisms,
corrective action measures can be taken to prevent sucker rod failures,
thus allowing the operator to produce marginally profitable wells more
cost effectively.