Many users of drilling tools focus on drilling hours to evaluate tool performance; specifically, how long a roller cone segment bearing will last in a reaming application. The length of drilling time a roller cone bearing lasts is an important measure, but it can vary widely, even for the same roller cone segment, depending on the tool configuration and drilling parameters.
This is why Inrock focuses on the concept of Cone Revolutions. Bearing performance is typically the key driver to the life of a roller cone cutting segment. Whether that be a roller ball roller bearing or a friction bearing, they are both machines containing moving parts that have a finite life. A bearing will only rotate around a journal so many times before parts wear and finally fail. The number of times these parts rotate before failure is the concept of Cone Revolutions.
The number of Cone Revolutions a cutting segment should last is a number that can be provided by the manufacturer. The number of Cone Revolutions of course depends on certain assumptions relating to proper operation of the tool, but the number is critical to understand when approaching a project. This number can be used to build an HDD reaming program and evaluate tool performance. However, in order to use this number during drilling operations, the driller must also know how to calculate Cone Revolutions.
Fortunately, the math is relatively simple. The general equation to determine Cone Revolutions is as follows:
Cone Revolutions = Cutter Ratio x Drill Pipe RPM x Joint Duration (minutes) where
Cutter Ratio = Reamer Diameter divided by Segment Diameter [note: segment diameter is the actual diameter of the segment, not the diameter of a tricone bit equivalent]
Drill Pipe RPM = the revolutions per minute (RPM) of the drill pipe for the duration of a joint
Joint Duration = the length of time to drill the joint in minutes
For example, if you are using a 30” reamer which is using cutter segments that are 12.1” in Diameter, the Cutter Ratio is 2.47. If you turn the drill pipe at 30 RPM and it takes 60 minutes to drill the joint, the cutter will have experienced 4,446 Cone Revolutions during that joint.
Now if you are using the same 12.1” diameter cutter on a 42” reamer, rotating the drill pipe at 30 RPM for 60 minutes, the number of Cone Revolutions is 6,246 a 40% increase vs the 30” reamer. This is because the cutter ratio is now 3.47 vs. 2.47.
However, recall that the number of Cone Revolutions a cutter will last is generally fixed. So, with everything else being equal, a 48” reamer will experience “less life” than a 30” reamer simply because the cones are turning faster at the larger tool diameter. The concept of Cone Revolutions is critical to planning a successful HDD reaming program. To learn more, please contact your sales representative at Inrock.