BHA integrity is important to successful drilling operations. Fatigue failure is the primary cause of twistoffs. There has been no method to predict and monitor fatigue life of BHA components while drilling. Current preventative methods include predefining the collar life based on past experience and tracking the cumulative pumping hours. When total hours reach the predefined life, the collar is junked. This method is inaccurate because the pumping hours do not accurately reflect the fatigue life of a collar. Regular inspections by nondestructive testing (NDT) to detect any cracks should reveral the initiation of fatigue failure. However, a collar can still consume its fatigue life, initiate a crack, and finally twist off while drilling, causing significant loss of time and money.
Fatigue damage consists of two stages, crack initiation and crack propagation, with crack initiation accounting for most of the total life. Rotating bending is the driving force for fatigue cracking. It induces cyclic stresses and strains at the stress risers, which are the fatigue-critical features. Fatigue data can be presented in the form of M-N curves, where M is the applied bending moment and N is the total life in number of cycles. The BHA bending moment distribution can be computed with a BHA analysis. Given a bending moment, the cyclic stresses and strains at the most critical feature of the most critical BHA component are typically determined with finite element analysis. The life of the most critical feature can then be calculated with the stress-life or strain-life curve of the collar material. This governs the life of the entire BHA.
A fatigue management workflow has been created for planning and execution monitoring phases. During planning, the engineer can calculate the expected fatigue life of the BHA and optimize for the expected duration of the job. The result can be used to select reliable components with sufficient fatigue life for the job. While drilling, this method enables engineer to continuously monitor the consumed fatigue life of any BHA component and make the decision to replace the tool before a failure occurs downhole. After the job, the consumed life can be recorded in the maintenance system to track the remaining life and decide what preventive maintenance is required.
This innovative workflow has been implemented to manage BHA integrity for medium to high dogleg severity (DLS). The approach to managing fatigue life enables risk mitigation for BHA integrity, pushing kickoffs deeper, shortening the curved sections, and increasing reservoir exposure, including reentry applications. Field cases have proven that the solution provides engineers with a new tool to reduce fatigue failures by predicting expected fatigue life consumed on a job, selecting correct tools, and proactively managing the drilling risks.