Bending Strength Ratio BSR In Drilling String BHA

Bending Strength Ratio Definition

In General, Bending Strength ratio BSR is a ratio of the relative stiffness of the box to the pin for a given connection. The predominant consideration, especially in selecting larger Bottom Hole Assembly BHA connections is (BSR).  In Drill String Design If we select a connection with either a pin or box that is out of balance with the other member, we tend to increase the stress level and accelerate fatigue in the weaker member. In this article we will discuss its recommended ranges. Also you can download bending stress strength ratio calculator.

Bending Stress Ration Ranges

The traditional target BSR is 2.5 and acceptable Bending Strength Ratio‘s ranges center on this point. However, BSR ranges are rough guidelines established by “experience” and should not be used as strict operating limits such as (for example) drill pipe tube tensile capacity.  Staying within recommended BSR guidelines does not eliminate connection fatigue failures, nor does exceeding the recommended ranges always lead to fatigue failures.

High & Low Bending Strength Ratio

In theory, high bending stress ratio ’s should cause accelerated pin failure and low BSR’s should cause accelerated box failures.  A balanced BSR should provide maximum connection life.  However, field experience shows that larger OD Drill collars sizes (8 inch and up) suffer predominantly from box fatigue cracks even when they operate at or near the ideal BSR of 2.5.

This indicates that higher BSR Bending Strength Ratio ’s may be appropriate for these sizes.  On the other hand, 4.3/4 inch collars with bending stress ratio ‘s as low as 1.8 are widely used but rarely experience box fatigue cracks.  Therefore, the suggested BSR ranges in Table 5.1 are probably better.  In every case however, experience under given conditions should be a major determinant in BSR selection.

Drill Collar OD	Traditional BSR Range	Suggested BSR Range
Less than 6 inches	2.25 – 2.75	1.8 – 2.5
6 – 8 inches	2.25 – 2.75	2.25 – 2.75
More than 8 inches	2.25 – 2.75	2.5 – 3.2

One frequently overlooked connection is the one between the top drill collar and the bottom joint of Heavy Weight Drill Pipe HWDP. If a straight (non-bottleneck) crossover sub is used and the collar OD is larger than the HWDP tool joint OD, the resulting Bending Strength of that one connection will be exceedingly high.  Pin failures in the bottom joint of HWDP are not uncommon and this is the probable reason.   The problem is helped by using a bottleneck sub to smooth the change in cross section.

Bending Strength Ratio Calculator Spreadsheet

Download Bending Strength Ratio Calculator From Spreadsheet website

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