Failure Analysis

Design and Operation Failures

Sucker rod failure prevention begins with design. It is possible for poorly designed rod strings to contribute to other component failures in the artificial lift system, such as rod cut tubing resulting from compressive rod loads. Designing the artificial lift system is a compromise between the amount of work to be done and the expense of doing this work over a cost-effective period of time. Numerous combinations of depths, tubing sizes, fluid volumes, pump sizes and configurations, unit sizes and geometries, stroke lengths, pumping speeds and rod tapers are available to the system designer. Sucker rod size and grade selection is dependent upon many factors including predicted maximum stresses, stress ranges, and operating environments.

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.

(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 of (Figure 2), the Class SM coupling second from left in Figure 2, and the rod body on the left of Figure 3 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 of Figure 2 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.

The second rod body from the left in Figure 3 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 in (Figure 3) 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 in (Figure 3) 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.

(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 of (Figure 5) 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 in (Figure 5) 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.