Sucker Rod

Key Takeaways

• API sucker rod specifications (API Spec 11B) standardize the rod dimensions, material properties, threading, and coupling requirements that allow interchangeable use of rods and couplings from different manufacturers in rod string assemblies: standard rod diameters are 5/8, 3/4, 7/8, 1, 1-1/8, and 1-1/4 inches, with the rod diameter (and hence cross-sectional area and axial load capacity) selected based on the buoyed weight of the rod string below that point plus the polished rod load contribution from friction and pump differential pressure; API pin threads are 2.375-pitch modified Acme threads with specific shoulder makeup requirements; Class T (Norris-type) and Class SM (full-shoulder) couplings are the two main API coupling designs, with full-shoulder couplings preferred in sour service because the load is shared across a larger contact area, reducing stress concentration; rod makeup torque (applied by a power tong during installation) is critical for fatigue life -- undermade couplings allow relative motion under load (fretting and fatigue initiation at the thread root), while overmade couplings may yield the pin thread, both leading to premature coupling failure; modern coupling installation practice uses calibrated hydraulic torque wrenches or rotary shoulder make-up torque specifications from the manufacturer to achieve consistent, verified makeup.
• Sucker rod fatigue life is the primary design constraint for deep, high-fluid-level, or corrosive well applications, and is quantified using the modified Goodman diagram (API RP 11L): the Goodman diagram plots the allowable fatigue stress range (the difference between maximum and minimum stress during the pumping cycle) as a function of the mean stress, with the safe operating region bounded by the Goodman line (linear decrease of allowable stress range as mean stress increases toward the tensile yield strength); the maximum stress in a rod string occurs at the top of the string during the upstroke (buoyed rod weight + fluid load + acceleration load + friction load) and the minimum stress occurs at the bottom during the downstroke (may be compressive in deep wells with heavy rod strings, leading to buckling risk); the API RP 11L rod string design method computes peak and minimum polished rod loads at the surface, divides the rod string into sections of different diameter (tapered string) to maintain stress levels within the Goodman allowable throughout the string length, and specifies the number of rods of each grade and size from bottom to top; high-strength grades (HY, grade D) allow deeper wells or larger pump sizes for the same string weight; in sour gas (H2S) service, hydrogen embrittlement reduces the fatigue allowable substantially, requiring the use of corrosion-resistant grade K rods or corrosion inhibitor injection to prevent sulfide stress cracking at the thread roots.
• Fiberglass sucker rods are used in corrosive environments (CO2 and H2S service) and in deviated wells where steel rod buckling and tubing wear are severe problems: fiberglass rods (glass-fiber reinforced epoxy composites) have approximately one-third the density of steel (0.075 vs 0.283 lb/in^3), dramatically reducing the buoyed rod string weight and hence the polished rod load and power consumption in deep wells; the lower density also reduces the buckling force during the downstroke in deviated wells, allowing fiberglass rods to be used in wellbore inclinations up to 60 degrees where steel rods would cause continuous tubing wear; fiberglass rods are corrosion-immune, eliminating H2S and CO2 fatigue-corrosion failures that limit steel rod life to months in severe sour/sweet corrosion service; disadvantages of fiberglass include lower compressive strength (cannot be subjected to significant compression without buckling), lower modulus of elasticity (requires longer stroke lengths to achieve the same pump displacement as a steel string), and higher initial cost (typically 3 to 5 times the steel rod cost per unit length); hybrid strings (fiberglass upper section, steel lower section and pump barrel) combine the weight advantage of fiberglass with the compression resistance and pump serviceability of steel and are the most common configuration in severely corrosive deep wells in the Permian Basin, Williston Basin, and Midcontinent US.
• Continuous sucker rods (also called coil rods or FlexRods) eliminate the conventional 25-foot rod-coupling-rod assembly and replace it with a single continuous fiberglass or high-strength steel rod spooled on a reel and run into the well in one piece: elimination of threaded couplings removes the primary fatigue initiation sites (where 80 to 90 percent of conventional rod failures originate), substantially increasing fatigue life particularly in deviated and horizontal well applications; continuous steel rods (Marathon LTR and similar products) are manufactured in coiled form from cold-drawn high-strength steel and have achieved run lives of 5 to 10 years in applications where conventional rods lasted 6 to 18 months; continuous fiberglass rods (Weatherford FlexRod, Endurance Rod) are used in CO2-flood and sour-service EOR applications where corrosion-fatigue of steel is prohibitive; the installation of continuous rods requires a reel-mounted injector unit that can feed the rod into the well at a controlled rate (unlike conventional rods, which are stabbed in one at a time with a workover rig), and requires a dedicated surface pumping unit capable of handling the different spring constant and rod weight distribution of the continuous string; the higher capital cost of continuous rod equipment ($50,000 to $150,000 per well for reel, injector, and pump unit upgrade) is justified in high-workover-frequency wells where each conventional rod failure costs $20,000 to $80,000 in fishing, pulling, and rig time.
• Rod string dynamics (the behavior of the rod string as a flexible elastic system subjected to alternating loads at the surface and at the pump) differ significantly from the static analysis assumed in the basic API RP 11L design method and must be understood to optimize pumping unit operation: the rod string is a longitudinal wave system in which the cyclic polished rod motion at the surface propagates as a stress wave down the rod string at the speed of sound in steel (approximately 16,700 ft/s for a steel rod string), reflecting from the pump barrel and arriving back at the surface with a time delay of 2L/v (where L is the rod string length and v is the acoustic velocity); for deep wells (>6,000 ft), the wave travel time becomes comparable to the pump stroke period, creating wave interference effects (resonance at critical pumping speeds) that can amplify or reduce the load at the pump plunger dramatically compared to the surface measurement; the dynamometer card (a plot of polished rod load versus polished rod position recorded at surface) reflects the surface boundary condition, not the pump behavior, and must be transformed using a wave equation model (Gibbs method, Fourier transform method) to produce the pump card (load-position diagram at the pump) that directly shows whether the pump is properly anchored, whether the pump is hitting fluid or gas locking, and what the actual pump displacement is; interpretation of dynamometer cards and pump cards using computer-aided wave equation modeling (implemented in commercial software such as Echometer TM and Lufkin XSPOC) is the diagnostic method for detecting pump problems (gas interference, fluid pound, worn traveling valve, stuck pump plunger) without pulling the rod string.