New Sucker-Rod Connection Designed for High-Load Applications

Abstract

Abstract The present paper describes the premium connection (PC) sucker rods field experience in various applications and shows the results collected until now. Ay the moment this paper was written, 11 wells were working with this sucker rods under different loads ratings. Introduction Sucker rods connection-related failures represent one of the main limitations of beam pumping applications today. More demanding field operative conditions are pushing connections to their limits, consequently becoming the weakest link of the system. API Sucker Rods Specifications (11B) haven't changed much since the 70's. Poor stress distribution, along with the tendency to loosen from API connection design, is the main cause of stress concentration points, which will eventually lead to failure. After several attempts to come out with a solution for this problem, a new premium connection (PC) was developed. The lab first showed that it is capable of working above the sucker rod body capacity and field tests are now showing its reality. Through this revolutionary development, current conventional pumping application limits are expanded and thereby adding a new solution to the artificial lift systems users. Development 1- Limitations of API connection design Although API 11B sucker rod thread design has worked in the field for decades and it's manufacturing is quite simple, it has some limitations that should be mentioned:· Loosening tendency of the connection due to a gap between threads. This gap allows movement between coupling and pin threads when rods suffer compression and/or shocks. (See figure 1)· Finite Elements Analysis (FEA) verifies permanent deformations in thread during well and make-up operations. These deformations make the appearance of cracks and propagation easier, and they produce difficulties in displacement repetition for make-up. (See figure 2)· Non uniform stress distribution in thread profile generates stress concentration areas and therefore makes cracks propagation easier (also verified with FEA). (See figure 3)· High reliance with the make-up operation due to the fact that an over-torque (increase in the stress level) as well as a loosen connection (stress concentration during well operation) are harmful for the connection and produces failures in sucker rod pins and couplings. In addition to the aforementioned limitations, internal corrosion as well as thread deformations cause failures in the connection. However, the difference is that these cannot be solved with the thread design since they depend on the material handling and storage. All of these analyses, in combination with the field experience, show that the conventional connection resistance is inferior to the rod body resistance. Therefore, the connection is the weakest point in a sucker rod string. In order to solve this, high strength (HS) sucker rods were first developed. With them, all the material capacity is increased since a connection with higher capacity is produced. After this, sucker rods with reinforced connections were developed (7/8" rod body with 1" pin and 3/4" rod body with 7/8" pin) but they faced the physical limitation that most used tubing are 2 3/8" or 2 7/8" and in this pipes the 7/8" or the 1" connection cannot be increased due to the limited space. Therefore, the weak point in the sucker rod string was always located in the connection.