Bending Strength Ratio (BSR) is the ratio of the box section modulus to the pin section modulus which in simpler terms, is an indication of the relative stiffness of the box to the pin for a given connection. This is usually calculated for Drill Collars or other Bottom Hole Assembly (BHA) components and does not apply to connections on normal weight drill pipe. BSR is used as an approximate indicator of the suitability of a given connection with the given outside and inside diameters. If a connection has a pin and box that are out of balance, the stress level is increased and fatigue accelerates in the weaker member. 

API RP 7G-2, NS-2™ Standard and Standard DS-1^® Volume 3 offers guidelines for the range of acceptable BSRs.

API RP 7G-2 range of acceptable BSRs:

a) BHA smaller than 152mm (6 inches): 1.90 to 2.50

b) BHA 152mm to 203mm (6 inches to 8 inches): 2.25 to 2.75

c) BHA larger than 203mm (8 inches): 2.50 to 3.20

NS-2™ Standard range of acceptable BSRs:

a) BHA less than 6 inches: 1.80 to 2.50

b) BHA 6 to 8 inches: 2.25 to 2.75

c) BHA larger than 8 inches: 2.50 to 3.20

Standard DS-1^® Volume 3 range of acceptable BSRs:

a) BHA less than 6 inches: 1.80 to 2.50 

b) BHA 6 to below 8 inches: 2.25 to 2.75

c) BHA 8 inches and larger: 2.50 to 3.20 

In theory, higher BSRs should cause accelerated pin failure and low BSRs should cause accelerated box failures. A balanced BSR should provide maximum connection life. However, staying within recommended BSR guidelines does not eliminate connection fatigue failures nor does exceeding the recommended ranges always lead to fatigue failures.

BSR ranges are rough guidelines established by historical experience and should not be considered inviolable. Customers might require a specific OD and ID for the given connection which might result in the BSR being out of the acceptable range.

For more information about the various standards mentioned, please click on the link below:

API Standards

NS-2™ Standard

Standard DS-1^®