In this paper, moment of momentum theorem is introduced and used to model the ratio of the rotary speed of cone to the bit. The ratio model has been integrated in a ROP simulator and the simulator can predict the ratio of both tricone and monocone bits. A series of tests measuring the ratio of a 517 IADC roller cone bit has been done. These tests were done with three types of rocks, different weight on bit (WOB) and revolution per minute (RPM). Comparisons between the predictions with the simulator and experimental results are carried out. It shows that the prediction accuracy of the tricone bit is close to 90% and that there is also a good prediction accuracy to the ratio of a monocone bit. The modeling of the ratio of rotary speed of the cone to the bit has improved the accuracy of the ROP simulator, which plays an important role in drilling optimization. Integration of the cone ratio into the modeling increases the overall bit design capabilities to a higher level by using simulators for drill bit performance analysis.

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