How to Avoid Losing Drill String, Breaking Thread Joint
Now, you ask, what all this has to do with my drill rig. I ask, “Do you know what the torque value for your drill pipe thread is? Is it 31⁄2 API thread, or a 27⁄8 IF thread? How do I achieve the proper torque for my drill pipe thread joints?” In order for the drill string to work as one, like any thread connection, the proper torque should be applied to the threads at each tool joint. If the tool joints are not in proper tension you are at risk of breaking a thread joint and/or losing a drill string.
The hydraulic rotary top drive drill rig is a very powerful, efficient piece of equipment with a varying amount of rotary torque dependent upon the hydraulic components ordered in the rotation circuit. The torque of a hydraulic motor measured in inch-pounds (in-lb), is equal to pressure (PSI) times the motor displacement (in³/rev) divided by two times pi (6.282). The hydraulic motors are installed in a rotation gearbox that reduces the speed but increases the mechanical advantage. In the case of most Schramm gearboxes, that ratio is 3.5:1. All hydraulic components are given an operating efficiency dependent on the design. Under most operating conditions these motors are around 92 percent efficient. Divide the result by 12 to determine foot-pounds of torque.
(3,500 psi x 29.2 in³/rev) ÷ 6.282
= 16,268 x the gear ratio (3.5:1)
= 56,938 x component efficiency (92%)
= 52,382 in-lb / 12 in/ft
= 4,365 ft-lb
In the example above a rotation gearbox with a single 29.2 cubic inch motor will deliver 4,365 foot-pounds of torque at 3,500 psig. If the gearbox is of the two motor designs, and the volume of the motor remains the same, the torque value would double or equate to 8,730 foot pounds. If the gearbox is of the four motor design, then the torque would increase to a maximum of 17,460 foot-pounds.
The next question should be, “How tight do I make my thread joint?” And where do I find this information? Drill pipe thread joint torque values can be found in the tool tech section of the Baker Oil Tools handbook, or any other quality drilling tool manual. Here is a sample of the more commonly used thread joint torque values.
If you are drilling with drill pipe that is equipped with 27⁄8 IF thread joints, you would need 5,900 foot-pounds of torque to properly tighten this joint. So, what do you do if your drill rig makes up to 8,730 foot-pounds (104,764 in-lbs) of torque? You would have to limit the hydraulic pressure to achieve the proper make-up torque and not over tighten the joint. This is the purpose of the torque-limiting option on your Schramm drill rig. Using the attached drill pipe torque calculator we can determine the correct pressure setting for the torque limiter should be 2,365 psig. If your maximum torque capability is 17,460 foot pounds (209528 in-lbs) the pressure setting would be 1,180 psig.
Now what if you know your drill pipe recommended make-up torque but you don’t know your drill rig’s maximum torque capability? You must refer to your rig-specific hydraulic diagram for these details.
Having the torque limiter turned to the on position to make-up joints and drill the borehole will ensure proper tension of all the pipe joints in your drill string. By design the torque limiter circuit is only engaged in the clockwise rotation direction. Following this drilling practice will leave you sufficient torque for breaking joints loose when removing your drill string from the borehole.
After achieving borehole completion, increasing the make-up torque at the gearbox tool joint will result in the joint breaking at the table reliably. Typically, the tool joint is set 200 to 300 psi above the drill pipe torque for the drilling process to accomplish this. Optional dual torque limiters
can make this job easier. One set for the make-up torque at the first joint and the other set for drill pipe torque for drilling.
Proper torque of drill pipe joints makes drilling safer, increases drill pipe life and helps to achieve a straight borehole.
Steve Hanley is a veteran service technician with Schramm. For more Tech Topics columns, visit www.nationaldriller.com/techtopics.