Bending Strength Ratio (BSR) is a time-honored technique for choosing BHA connections, and is often the basis for the popularity of some connections over others. However, experience and some isolated research indicates that it might not be accurate in predicting the best connection. This paper attempts to use Finite Element Analysis (FEA) coupled with fatigue-life modeling to develop a systematic picture of the effectiveness of BSR as a design criteria.
The modeling process used has been developed previously (Ellis et al 2008, for example). Each connection and collar size studied is modeled in detail using Finite Element Analysis (FEA) techniques, and the resulting stresses and strains can be used to calculate the number of cycles to failure for a given set of parameters. These results are not accurate for predicting actual component failure, but they are helpful for comparison between options to choose the best designs. This work represents a fairly-complete cross section of the BHA connections in regular use, studied for their relative performance.
A challenge of directly modeling fatigue lives is finding a way to compare the results for global patterns. Since different collar sizes will have different applied stress levels at different curvatures, a method for normalizing the results is developed here. This allows an easy global comparison between the different connections.
The outcome of this in-depth study of connection fatigue-modeling results is a surprise: BSR is not helpful for design, and there seems to be no clear pattern for ID and OD sizing. Instead, the drill string designer should choose the right connections at the time of manufacturing, and use a normalized comparison method to determine the relative risk in each operation.