LabVIEW Real-Time: Real-Time Control for Pipe Coupling with Power Tongs

Author(s):
Steve Conquergood - Advanced Measurements Inc

Industry:
Oil and Gas/ Refining/ Chemicals

Products:
LabVIEW

The Challenge:
Controlling torque when assembling oil pipes.

The Solution:
Using the real-time capabilities of LabVIEW Real-Time to provide deterministic control.

A common real-time task in the oil field industry is controlling torque when assembling oil pipes. The central oil pipe on an oil rig is composed of smaller sections of pipe called "joints." These joints are connected and assembled on the drill rig using a hydraulically driven tool called a power tong. Because joints are threaded together, users must apply the correct torque to each joint so that it will not leak or loosen over time. Over-torquing damages the pipe connections and results in loss of the pipe sections as well as valuable rig time to remove and replace the segments. With LabVIEW Real-Time,users can deterministically control torque in the pipe joints.

Torque and Joint Connections
The rig operators must achieve a specified torque for each joint connection. Depending on pipe size, this value ranges from 2,400 to 12,000 ft-lb of torque; the allowable tolerance ranges are from ±20 ft-lb to ±100 ft-lb of torque. Hundreds of connections per day are made on a single operating rig, so users cannot tolerate failures because of the high costs involved. For this particular application, two sections of pipe are spun together at 10 to 30 RPM. The torque during this portion of the operation typically rises at a rate of500 to 2,000 ft-lb per second because of the tightening of the pipe threads. When a metal "shoulder" on each of the segments makes contact, the torque jumps upward sharply, rising at rates up to 20,000 ft-lb per second. This is where the control system must determine the correct time to open the solenoid for the hydraulic motor on the power tong, so that correct final torque is achieved. The hydraulic system will have a latency of15 to 20 ms between solenoid activation and stopping rotation at the final torque value.

A control system for joint coupling has historically been built using a combination of dedicated hardware circuits for control, PC-based software such as LabVIEW for data reporting and storage, and an operator display screen for monitoring both quality and productivity. General-purpose operating systems have not allowed deterministic control with timing in the range of1 to 10 ms, because of the variety of other tasks they must accomplish, such as networking and file I/O.

LabVIEW for Reliable Control
The real-time version of LabVIEW, LabVIEW Real-Time,now gives us the opportunity to build complex applications using very high level graphical languages, and know that robust, reliable control will be achieved during every operation. The job of the LabVIEW Real-Time software is to calculate the correct time when the solenoid must be opened, so that the final torque value is achieved within the specified range. This requires a software loop operating deterministically at 1 to 2 ms per iteration. In this control loop, we measure the input signal, compute the final torque to be achieved, and generate a control output signal if required. The computation portion of the control loop must cope with widely varying ramp rates, varying signal to noise ratios, and other variations in the overall system. The factors that make LabVIEW Real-Time so well suited for this application are as follows:

• Deterministic timing of software operations down to 1 millisecond resolution
• Simple integration of data acquisition hardware - inputs and outputs
• Complex software operations easily implemented using graphical programming methods
• Easy merging of the real-time control portion of the software, with routine operations such as data storage, display and reporting.

For more information, contact:

Steve Conquergood

Advanced Measurements Inc.

6205 10th St.S.E.

Calgary,AB, T2H 2Z9,Canada

Tel: (403) 571-7273

Fax: (403) 571-7279

E-mail: info@advmeas.com

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