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Field guide

Why Sucker Rods Part: Corrosion, Fatigue, Pound and Buckling

Why sucker rods part: fatigue, corrosion pitting, loose connections, overload, fluid pound and buckling, how to read a break, and how to prevent the next one.
By SRABS Engineering ·
Article / What causes sucker rods to part?

What causes sucker rods to part?

Most sucker rods part from fatigue: a crack that grows a little with every stroke until the remaining steel can't carry the load. Corrosion pits, nicks, loose connections, fluid pound and buckling are what start those cracks and speed them up. Pure overload, a rod pulled in two in one go, is rare. 124

A rod string carries a load that changes on every stroke, thousands of times a day. Steel tolerates that for years when the surface is clean, the connections are tight and the loads stay where the string was designed for. Fatigue failures happen in sucker-rod steel at loads below its yield strength, 4 so a rod doesn't need to be overloaded to break. It needs a starting point for a crack and enough cycles.

Field experience going back decades points the same way. One short-course paper names four principal causes of rod-string failure: corrosion, improper joint make-up, operating and running procedures, and string design. 2 Another calls corrosion the greatest cause of sucker rod failure. 1 The rest of this guide takes them in the order a failure usually develops.

Article / Fatigue: how most rods actually break

Fatigue: how most rods actually break

A fatigue break starts at a stress raiser (a pit, a nick, a thread root) and grows across the rod under cyclic load. It is the end state of most rod failures, whatever started them. 4

Cyclic loading is the normal life of a rod string: tension on the upstroke, less on the downstroke. The patent literature on rod-pump diagnostics lists what makes those cycles more damaging: changes in fluid volume and fluid level, corrosive fluids, and changes in stroke length or speed. 4 Anything that raises the load range, or adds bending on top of it, shortens the life of the string.

Fatigue is why a parted rod is rarely a surprise in hindsight. The crack was growing for weeks or months before the string let go.

Article / Corrosion: the starting point for most fatigue cracks

Corrosion: the starting point for most fatigue cracks

Corrosion is the single biggest cause of sucker rod failure, mostly because pits become the origin of fatigue cracks. Chloride and CO₂ in produced fluid are typical culprits. 15

A corroded rod doesn't usually fail by thinning away. It fails because a pit concentrates stress, and a fatigue crack starts at the bottom of it. A published failure analysis of parted rods found pitting from chloride ions and carbon dioxide that "acted as a crack origin from which the fatigue crack initiated." 5 Mechanical damage does the same job: the rod-pump diagnostics patent notes that shock loading accelerates fatigue cracking from both corrosion pitting and mechanical damage. 4 Nicks and dents from handling are pits you put there yourself.

Article / Connections: loose couplings and pin breaks

Connections: loose couplings and pin breaks

Most pin and coupling failures trace back to loose couplings, and a loose coupling is almost always a make-up problem. Correct pin prestress, set by the circumferential displacement method, is the accepted way to prevent it. 3

A rod connection works like a bolted joint: the pin is stretched when the coupling is made up, and that prestress keeps the pin shoulder and coupling face clamped together under load. A short-course paper on rod make-up puts it plainly: "Most sucker rod pin and coupling failures are considered to be related to loose couplings," and correct pin prestress "offers the best way to prevent loose couplings." 3 Make-up by circumferential displacement, measured from hand-tight, is the method the industry settled on. 3 Use the displacement values from API RP 11BR and the rod manufacturer.

When the joint loosens, the pin starts to see the full cyclic load instead of a steady clamp. It becomes the weak link, and it breaks at the thread or the shoulder.

Article / Overload, fluid pound and buckling

Overload, fluid pound and buckling

Three operating conditions break rods even when the steel and the make-up are right: a stuck pump that overloads the string, fluid pound that shocks it on every stroke, and buckling that bends the bottom of it into the tubing. 4

  • Overload. When the load exceeds the string's tensile strength (a stuck pump is the usual cause), the rod necks down at its weakest section and pulls apart. The diagnostics patent calls this rare. 4
  • Fluid pound. When a pumped-off plunger hits the fluid on the downstroke, the jolt sends shock waves through the string. The same patent calls fluid pound "the most detrimental cause of sucker-rod flexing" and ties it to overload, coupling and pin failures, and faster fatigue cracking. 4 See the fluid pound guide.
  • Buckling. On the downstroke the bottom of the string can go into compression. Past a critical load the rods bow and wrap into a helix against the tubing wall, so every stroke adds bending stress and rubs couplings against the pipe. See the sucker rod buckling guide and sinker bars and rod guides.
Article / Reading a parted rod: what the break tells you

Reading a parted rod: what the break tells you

The break face and where it sits in the string usually tell you which cause to fix. Bag the broken pieces before they're cleaned or thrown back on the rack.

What you seeMost likely causeFix to look at
Rod body necked down and stretched at the breakTensile overload, often a stuck pump 4Pump condition, unseating loads
Flat crack face, often with a pit or nick at the originFatigue started by corrosion or mechanical damage 45Corrosion program, rod handling
Pin broken at the thread or shoulder, coupling face markedLoose connection from improper make-up 3Make-up by circumferential displacement
Repeated breaks or wear in the bottom of the stringDownstroke compression, buckling, fluid pound 4Pump-off control, string design, anti-buckling
Rod or coupling worn thin on one sideRod-on-tubing contact from buckling or a deviated holeGuides, buckling control
FIG. R · reading a parted rod · detail views, schematic, not to scale
  1. DETAIL R1123
    R1 Fatigue break

    A flat face that grew across the rod over many strokes.

    1. 1origin, often a pit or nick at the surface
    2. 2beach marks: the crack front, stroke after stroke
    3. 3final fast fracture: rough, where the rest let go
  2. DETAIL R2CYCLIC LOADproduced fluid · Cl⁻ · CO₂ROD BODY12
    R2 Corrosion pit origin

    Section at the rod surface: the pit concentrates stress.

    1. 1corrosion pit
    2. 2fatigue crack growing from the bottom of the pit
  3. DETAIL R3RODCOUPLING12
    R3 Pin break, loose connection

    The shoulder stands off the coupling face; the pin carries the full cyclic load.

    1. 1gap: pin shoulder off the coupling face
    2. 2break at the first thread root
  4. DETAIL R4TENSION12
    R4 Tensile overload

    Necked down and stretched before it parted. Rare.

    1. 1neck: the rod thinned before it broke
    2. 2cup and cone at the break

Log every failure with its depth, rod size, break type and photos. Patterns across a field, like the same depth or the same taper, matter more than any single break.

Article / How to prevent the next parted rod

How to prevent the next parted rod

Rod failures are cumulative, so prevention is a stack of small disciplines: keep the steel clean, make the connections up right, design the string for today's well, and take the shock and compression out of the downstroke. 123

  1. Run a corrosion program. Corrosion is the leading cause, 1 and pits are where fatigue starts. 5 Match inhibition to the produced fluid, and look at failed rods for pitting.
  2. Handle rods like the fatigue parts they are. Avoid nicks, dents and bending on the rack and in the derrick; mechanical damage is a crack starter. 4
  3. Make up every connection by circumferential displacement. It is the accepted way to reach correct pin prestress. 3
  4. Re-check the string design when the well changes. String design is one of the four principal causes. 2 A taper chosen at completion may be wrong after the fluid level, water cut or pumping speed has changed.
  5. Stop fluid pound. Match pumping to inflow with pump-off control or speed control; see pump-off controller vs. AI well control.
  6. Take the compression out of the bottom of the string. Sinker bars and guides manage buckling; a pump that keeps the string in tension removes it.
Article / Where SRABS fits

Where SRABS fits

Two of the causes above are what SRABS is built around: compression at the bottom of the string, and fluid pound.

The SRABS Pump is a drop-in anti-buckling rod pump that keeps the rod string in tension where it would otherwise buckle, so the bottom of the string never goes into compression. SRABS Brain reads dynacards, sets pump speed and prevents fluid pound across a whole rod-pumped fleet. Corrosion, handling and make-up stay with your rod program; no pump fixes those. For the mechanism see how it works, and to estimate what fewer rod failures would mean for your fleet, run the ROI calculator. Its default 100-well scenario is modelled, not a field result.

Article / The bottom line

The bottom line

Rods part from fatigue, and fatigue needs a starting point. Corrosion pits, nicks, loose connections, pound and buckling all provide one. Read every break, fix the cause it points to, and take the shock and compression out of the downstroke.

If you're pulling rods from the bottom of the string again and again, test that on one well.

Loan us a well →
Article / FAQ

FAQ

What is the most common cause of sucker rod failure?

Corrosion. It rarely eats through a rod on its own; it leaves pits, and fatigue cracks start at the bottom of those pits and grow with every stroke until the rod parts.

Why do sucker rods break at the pin?

Usually because the connection was loose. When make-up doesn't put enough prestress in the pin, the joint stops clamping and the pin carries the full cyclic load until it breaks at the thread or shoulder.

How do I make up sucker rods correctly?

By circumferential displacement measured from hand-tight, not by torque alone. Take the displacement values from API RP 11BR and the rod manufacturer's tables for your rod grade and size.

Can fluid pound break sucker rods?

Yes. The impact on the downstroke sends shock waves through the string, flexes the rods and speeds up fatigue cracking, and it is linked to coupling and pin failures as well as overload.

Does rod buckling cause rod parts?

It contributes. Compression at the bottom of the string bends rods into a helix against the tubing, adding bending stress and wear, so failures cluster in the bottom of the string.

What does a necked-down rod break mean?

Tensile overload: the rod was pulled past its strength and stretched before it parted. A stuck pump is the usual cause, and it is far rarer than a fatigue break.

Article / Sources

Sources

  1. Morris, R. A., "Sucker Rod Failure Analysis," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/sucker-rod-failure-analysis
  2. Hardy, A. A., "Common Cause of Failure in Sucker Rod Strings," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/common-cause-failure-sucker-rod-strings
  3. Morris, R. A., "Sucker Rod Make-Up," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/sucker-rod-make
  4. US Patent 4,509,901, "Method and apparatus for detecting problems in sucker-rod well pumps" (FMC Corporation, 1985). https://patents.google.com/patent/US4509901
  5. Elshawesh, F., Elhoud, A., Elagdel, E., "Tests pinpoint sucker-rod failures," Oil & Gas Journal, 26 May 1997. https://www.ogj.com/home/article/17242034/tests-pinpoint-suckerrod-failures