API 11B Sucker Rod

Sucker Rod Grades and Their Use

API 11B classifies sucker rods by grade, and each grade suits a different load and corrosion environment. Grade C is a carbon steel with a minimum tensile strength of 620 MPa and a minimum yield strength of 414 MPa, used in shallow, lightly loaded, non-corrosive wells. Grade D is a carbon-manganese steel with a minimum tensile strength of 793 MPa and a minimum yield strength of 586 MPa, giving higher strength and fatigue resistance, and is the general workhorse for medium to deep wells. Grade K is a carbon-manganese steel with lower strength than D but much better resistance to hydrogen sulfide, used specifically in sour service. Grade KD combines higher strength with sour service resistance. Grades HL and HY are high strength grades for heavy loads in deep wells. Selecting the wrong grade, for example using a high strength grade in a sour well, leads to early failure through sulfide stress cracking, so the grade must be selected from both the load calculation and the produced fluid analysis.

Rod String Design and Taper

A sucker rod string is rarely a single size. Because the load on the rod decreases from the top of the string to the pump, the string is normally tapered: larger rods at the top where the load is greatest, and smaller rods near the pump, so that the rod weight and the resulting stress are distributed efficiently. A common design in a medium depth well might be a 1 inch upper section, a 7/8 inch middle section and a 3/4 inch lower section. Each change of size requires a crossover coupling. The string design should be calculated against the API 11B modified Goodman or equivalent fatigue diagram, which accounts for the alternating stress from the pumping cycle, not just the static load. Taper design, rod size selection and the pumping speed all interact, so the design should be confirmed with a rod pumping analysis before ordering.