Calculating an equivalent ID

Softdrill NL   


For certain tools, for example mud motors and MWD tools, the internal diameter (ID) may not be known or applicable.  However, for the Torque & Drag model, a Tool ID value is required.

Based on an (estimated) relative stiffness, an equivalent ID can be calculated using the Moment of Inertia (I) formula:



An example


To calculate the equivalent ID of a 9 1/2" mud motor that has a relative stiffness of 95% of that of a 9 1/2" x 3" drill collar, the following calculation is made:


1. Calculate the Moment of Inertia for the drill collar:



2. Assuming the mud motor has a 95% relative stiffness, calculate its Moment of Inertia:



3. From the Moment of Inertia formula, back-calculate the equivalent ID of the mud motor:


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