As a general trend the complexity of drilling operations and bottom hole assemblies (BHAs) is increasing. It has become common to rotary drill directional wells with higher dogleg severity (DLS), more aggressive parameters are being used to improve drilling performance, and hole-opening while drilling tools are being used more frequently. The implication of these trends is an increased risk of BHA and drillstring failures due to higher component stresses accelerating fatigue rates. On other hand, with increasing sensitivity to rig spread cost and operations efficiency, there is an increased focus on reducing operational nonproductive time (NPT). Drillstring failures can cause significant NPT as the drilling operation needs to stop and time must be spent fishing to recover a failed BHA component downhole or, potentially, the well must be abandoned and the hole sidetracked to continue drilling. Therefore, addressing fatigue failure concerns at the BHA design stage is critical; mitigating the risk of BHA and drillstring failures is less costly than recovering from drillstring and BHA failure while drilling. Proper design can also increase the service life of the BHA components.

Conventionally, an assessment of the BHA component fatigue failure risk has not been a standard automated part of the BHA design process during the well design stage, but rather mitigated via the inspection of tubulars as per industry standards. A new automated workflow for modeling BHA static loads has been developed and used to deliver a more comprehensive solution for BHA design. The workflow is implemented within a well design software application that enables a drilling engineer to efficiently evaluate the effect of different borehole curvature magnitudes at a range of wellbore inclinations, to understand the risk associated with the plan and deviation from the plan in the execution phase. The modeling results help a drilling engineer to ensure that from the planning stage the BHA components bending moment and bending stress levels remain below the recommended thresholds at the execution and provide theoretically infinite component life. Using this information a drilling engineer can optimize the BHA or other parameters to reduce the risk, and formulate a better plan & contingency.

This modeling approach has been evaluated through several case studies which demonstrate how it can help to optimize BHA design and minimize or avoid fatigue failures. The case studies presented in this paper are some examples of downhole BHA component failures that could have been avoided through early planning phase identification of the risk.

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