For wells with a high failure frequency rate, it could point to inadequate protection during handling and storage of the sucker rod string's components. All failures with sucker rods, pony rods, and couplings point to tensile or fatigue problems with the string.
Tensile failures happen when the load applied to the rig exceeds the rated strength of the rods. With tensile failures, the load will concentrate at a specific point in the string, causing a necked-down visual indication around the circumference of the rod. This only occurs when pulling an excessive load on the total string. To avoid these failures, the load should never exceed 90% of the rated amount for a new sucker rod.
Fatigue failures are more common in sucker rod strings and occur for several different reasons. They can start as small stress fractures or corrosion build up during use and usually progress quickly. This is due to the distribution of load along the entire cross-section. Pumpers can identify these types of failures with visual inspections for beach or ratchet marks before a catastrophic failure occurs.
Types of fatigue failures include:
Casehardened failure
Torsional failure
Surface Damage Failures
Connection Pre-load Failures
To prevent fatigue failure frequency rates, every sucker rod string's components need persistent protection before installation. For the best results, whenever you pull the string, you'll need to use caps that seal the threaded parts.
