A New Fatigue Management Workflow for BHA Integrity Risk Reduction

Abstract

Abstract Our industry continues drilling more challenging wells - deeper and higher dogleg severity (DLS). Bottom Hole Assemblies (BHAs) have also become more complex and required to sustain higher loads. The current design and engineering practices still rely on prevention mechanical overload which based on calculated maximum stress versus component material yield strength. While fatigue failure is well known and recognized as the primary cause of component twist-offs downhole, but there still limited approach to prevent it during planning phases such as BHA design, execution monitoring, and evaluation. The industry still relies on inspection and traditional cumulative pumping hours tracking as a preventive action against fatigue failure. 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 S-N curves, where S is the applied bending stress and N is the total life in a number of cycles. An advancement in BHA modeling with the capability of finer detailed finite element modeling of drilling tool component allows for accurate calculation of BHA bending moment distribution. Given a bending moment, the cyclic stresses and strains at the most critical feature of the most critical BHA component can be determined. The life of the most critical component can then be calculated with the stress-life or strain-life curve of the collar material. This governs the life of the entire BHA. This paper will present the development of new approach on fatigue management workflow which includes bending moment & stress analysis based BHA design, fatigue life prediction and sensitivity analysis during planning and execution monitoring on consumed fatigue life, including job tracking to component maintenance system. The paper will also discuss the accelerated cumulative fatigue due to shock & vibration. A fatigue management workflow has been created for planning, execution monitoring, and post-job evaluation 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 the 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. The new modeling approach enables the drilling engineer to optimize performance and managing BHA integrity risk.