When you operate top drive, you may need to know critical RPM that you can go. If you rotate pipe more than the critical RPM, it will create a lot of vibration that can cause failure in your drilling equipment such as drill pipe, TDS, etc.
In order to find out how much critical RPM, you may need high-tech simulation but sometimes you don’t really have that information supplied from town. So you really need to be able to roughly estimate how much critical RPM is ( at least you get a idea for this limitation). This formula below shows you how to estimate the critical RPM and it has accuracy of 15% roughly.
Critical RPM = 33,055 x (OD2 + ID2) 1/2 ÷ (L)2
Where;
OD = drill pipe outside diameter in inch
ID = drill pipe inside diameter in inch
L = length of one joint of drill pipe in feet
Example: Determine critical RPM from these following information
L = length of one joint of drill pipe = 32 ft
OD = drill pipe outside diameter = 4.0 in.
ID = drill pipe inside diameter = 3.5 in.
Critical RPM = 33,055 x (42+ 3.52)1/2 ÷ (32)2
Critical RPM = 172 RPM
Please remember this is ONLY estimation of the critical RPM. If you have your service companies or you have specific programs to determine it, please use the value from those programs because it should consider many parameters than this simple formula. USE IT IN CASE OF YOU HAVE NOTHING AVAILABLE TO CALCULATE THE CRITICAL RPM.
Please find the Excel for calculating the critical RPM.
Ref book: Formulas and Calculations for drill, production and workover by Norton J. Lapeyrouse
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