Failure Analysis

Connection Failures

The API sucker rod connection is designed as a shouldered, friction loaded connection. Since the fatigue endurance of the sucker rod connection is low when subjected to cyclic loads it is necessary to limit the cyclic loads with pin preload. If the pin preload is greater than the applied load the load in the connection remains constant and no fatigue occurs from cyclic loads. The friction load that develops between the pin shoulder face and the coupling shoulder face, helps lock the connection together to prevent it from coming unscrewed downhole. However, if the preload is less than the applied load, the pin shoulder face and the coupling shoulder face will separate during the cyclic motion of the pumping unit. Once these faces separate the connection is cyclically loaded and will result in a loss of displacement, or loss of tightness, failure. Loss of displacement failures may result from improper lubrication, inadequate makeup, over-torque, tubing-slap wear, or any combination of the these elements.

Figure 10 is an example of pin failures due to a loss of displacement. The sample on the right in figure 10 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 in Figure 11 is typical in appearance for a loss of displacement pin failure. However, the 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.

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.

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 of Figure 14 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 of Figure 11 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 of Figure 17 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.

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.