Rotary Shouldered Connections (RSCs) are subjected to harsh downhole environments and aggressive fluids that can cause pitting damage. Pitting, especially in the thread roots, increases the likelihood of fatigue crack initiation and propagation. Currently, no industry standard provides quantitative analysis of the effects of pitting on the fatigue life of RSCs. This often leads to deployment of a conservative acceptance criterion of disallowing pitting in any thread roots; resulting in frequent repair and recuts of RSCs, and at times reducing the service life of the components.
This paper details the analysis performed and methods adopted to assess the effects of pitting on fatigue life of RSCs. Six API connections commonly used on small, medium, and large size drill pipe and bottom hole assembly (BHA) components were modeled. Finite Element Analysis (FEA) was used to examine stresses in different thread roots of these connections. Connections with and without stress relief features were analyzed. A methodology was developed to integrate the effects of stress concentration caused by a pit into the stresses obtained from FEA analysis.
It was found that the stresses in many thread roots, even with a pit, were lower than the stresses in the last engaged threads. This result was used to establish new pitting acceptance criteria allowing pitting in some thread roots as long as the stresses were lower than the maximum stress found in the last engaged threads.
This study provides a scientific approach towards assessment of pitting damage. The study shows that a significant number of threads could be allowed to have pitting without compromising the overall fatigue life. Based on these results, universal inspection criteria were developed covering all API connections to provide well-defined, consistent and user-friendly method of inspection. These new acceptance criteria will reduce the cost of frequent machining, repair and replacement of components.