For decades, the industry has used Bending Strength Ratio (BSR) as a guideline for fatigue design considerations in drill collars and other stiff body BHA components. While the use of BSR as a design tool is standard practice, it has limited value as a predictor of connection fatigue. For example, two dissimilar connections (type, size, or both) can have the same BSR but display dramatically different fatigue performance. Connection Fatigue Index (CFI) is a new design approach that considers the dimensional, material, and operational parameters that directly impact fatigue performance. Using finite element analysis simulation, specific connections are modeled under cyclic loading conditions to produce CFI values. By accounting for variables that impact fatigue, such as connection features, material properties, stress relief features, applied make-up torque, and loading conditions, CFI offers a more accurate and robust tool for BHA design.
The CFI for each connection type is calculated for common box ODs, pin IDs and dog leg severities and presented in tabular format. The designer need only look in the tables for the largest CFI value (of available options) to know which connection type and size has the best fatigue performance, and how the planned trajectory impacts fatigue in the connection. Additionally, since the values are quantitatively comparable, CFI allows the designer to easily determine how much better one option is relative to another (i.e. an option with a CFI value of 10 has twice the fatigue resistance of an option with a CFI value of 5.) CFI tables exist for the most common connection types and sizes, and separate tables will exist for connections with stress relief features and those operated in especially corrosive environments.
This paper describes the need for a technology like CFI, the advantages CFI presents over currently available technology, the basic methodology used to determine CFI, and how and when to use CFI.
Dramatic increases in rig spread costs and activity levels, coupled with the technical challenges of today's wells, have magnified the risk of connection fatigue failure in BHA components. Addressing fatigue concerns at the design stage is a low-cost or no-cost method for mitigating the risk of BHA connection failures while operating and increasing the service life of the BHA component.
An effective design tool should be accurate, technically-based, and robust, offering the user the ability to compare alternatives and select the best fit for a given project. To create a design tool for a mechanism as complex as fatigue requires computing power that was not available at the time BSR was introduced. As such, BSR is a ratio of box section modulus to the pin section modulus, a relatively simple hand calculation.
Based on empirical data, a connection with a BSR of 2.5 should be "balanced", with equal fatigue loading between the box and pin. Initially a BSR acceptance range of 2.25–2.75 was established. Subsequent acceptance ranges have been adopted which index the BSR range to the connection OD as shown below.