Continuous Sucker Rod and FRP Sucker Rod

Why Continuous Rod

In a conventional sucker rod string, the great majority of failures occur at the couplings, not in the rod body, because the connection is where the stress concentration is highest and where the thread can be damaged during make-up. A continuous sucker rod eliminates most of these connections: the rod is manufactured in a long coil and run into the well with only the two end connections, so a string that would have had several hundred couplings has only a handful. The practical benefits are fewer failures, faster running and pulling, and less well servicing cost, which is why continuous rod is popular in deviated and horizontal wells where connection fatigue is worst. The trade-offs are that continuous rod requires special handling equipment and a service rig that can handle the coil, and that a break requires cutting and rejoining rather than simply replacing a coupling.

FRP Sucker Rods

A fiberglass reinforced plastic sucker rod is made of glass fibers embedded in an epoxy resin, pultruded into a solid rod, with steel end fittings bonded or molded onto each end. Its advantages are a very low density, roughly one quarter that of steel, which significantly reduces the hanging weight of the rod string and therefore the load on the pumping unit and on the upper rods, and complete resistance to corrosion by produced fluids, since the body contains no metal. Its limitations are a much lower modulus of elasticity than steel, so the rod stretches more under load and the effective stroke at the pump is reduced, and a lower temperature limit, typically around 100 to 120 degrees Celsius depending on the resin. FRP rods are used in corrosive wells and in wells where rod weight is a constraint, most often in combination with steel rods in a mixed string, with steel at the top for load and FRP lower down for weight and corrosion resistance.