Improvement Plan Using Fatigue Analysis Minimizes Jar Twistoff
This paper presents the study, resulting recommendations, and a proposed change in standard bottomhole-assembly configurations to reduce service-quality-compromising incidents and productive time lost from jar twistoffs.
Jar twistoff is a well-known service quality issue for some applications, especially in relatively large borehole sizes such as 22 and 16 in. The jar is one of the weakest points in the drillstring because of moving parts, which include complex shapes, resulting in stress concentration. The complete paper presents results of a study focusing on placing the jar in the bottomhole assembly (BHA) to reduce bending moments and bending stresses on the jar connections. As a result, the risk of twistoffs during drilling is minimized, while the effectiveness of the jar is maintained if a stuck event occurs.
Jars traditionally are placed between drill collars in the applications described in this study.