Improvement Plan Using Fatigue Analysis Minimizes Jar Twistoff
Authors: Carpenter Chris 
Published: December 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 210723, “Service Quality Improvement Plan Using FEA Fatigue Analysis Minimizes Jar Twistoff in Challenging Applications,” by Chaouki Boufama, SPE, Baozhong Yang, and Sultan Alfaraedhi, Schlumberger, et al. The paper has not been peer reviewed.

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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.

Introduction

Jars traditionally are placed between drill collars in the applications described in this study. These applications include 22- and 16-in. vertical holes with simple BHA designs, either rotary or with positive displacement motors. In cases of lateral vibration that creates high bending stress, this design is likely to cause the jar to experience extreme stress levels. The design concept of placing the jar between other components with lower weight and stiffness appears to be the most logical approach. This design would have to be validated with finite element analysis (FEA) dynamic modeling simulations to confirm lower stress at the jar and that a new weak point has not been introduced in the string. Impact and impulse also must be evaluated to confirm the effectiveness of the jar in the case of a stuck-pipe condition.

The study is based on the work flow shown in Fig. 1 of the complete paper.

Statement of Theory and Definitions

A more in-depth analysis was performed on a 22-in. vertical drilling application with nonhomogeneous formations. A downhole real-time recording tool was added to the drillstring above the BHA to evaluate and verify the magnitude of vibration and bending moments acting on the BHA during drilling operations. The 22-in. vertical section was selected because it was determined to be the most challenging section. The following key observations were made during the drilling run when the circulation was lost to the formation:

- Significant lateral vibration levels

- Chaotic whirling motions

- High bending moment

Mechanical specific energy (MSE) and drilling parameters were controlled during the run to alleviate vibration. Vibration severity could have been influenced by the absence of the mud column as well as the borehole quality because of formation washout and an oversized hole. However, this was never verified using a caliper log.

The string entered a state of chaotic whirl as the loss of circulation fluid began. The parameters, and thus the MSE, were reduced, but the severe vibrations persisted.

Even though the run did not cause any jar-component failure or crack, multiple previous incidents showed similar failure modes in the same downhole conditions. These failures, which included the fractured face of each of the parted components, were analyzed and evaluated (Fig. 1). The slanted fracture indicated a high combined stress-related fracture. In addition, beach marks present near the fracture area also indicated fatigue-induced damage.