GTI reports progress in developing tow-tension monitoring/data logging tool

Pipeline & Gas Journal, May, 2003 by Maximillian Kieba, J., Christopher J. Ziolkowski

A tow-tension monitoring tool (TTMT) is being developed by Gas Technology Institute (GTI). Developers seek to provide a means of directly monitoring and date-time logging the tensile load on PE pipe as it is pulled underground during HDD installations.

The work is sponsored by GTI and KeySpan Energy. Developers expect to help prevent instances where PE pipe is overstressed or breaks when the tensile load exceeds the safe maximum limits. The logging capability will allow a utility company to have an archived record that documents proper pipe installation. This is important to prevent cases where the pipe fails prematurely at some future date due to improper installation even though it did not break during installation.

The TTMT is a new technology. The device takes a live reading of the load and transmits the information to a receiver at the drill rig. In addition, the information is logged into memory with a time/date stamp for an archived record of the events.

Although HDD is used to install a variety of pipe sizes and materials, the TTMT prototype is being developed for testing on 3- to 6-inch PE pipe.

Stationary Receiver

The TTMT being developed does not require the receives to be carried alongside the drill path. Instead, the receiver remains stationary at the drill rig.

The TTMT transmitter (Figure 1) is a self-contained pod approximately 4 inches in diameter and 20 inches long that is installed between the pipe end and the backreamer. Shackles on each end of the load cell allow coupling to most swivels used by different HDD companies. During the pull, a load cell in the pod measures the tensile load at the pipe end. A 10,000-pound capacity load cell is used in the pod.

[FIGURE 1 OMITTED]

Based on calculations using ASTM F1804-97 ("Standard Practice for Determining Allowable Tensile Load for PE Gas Pipe during Pull-In Installation"), this covers the safe allowable tensile loads for all 3-inch and 4-inch PE pipe as well as most of 6-inch pipe (down to 13.5 SDR which yields a safe ATL of 9,344 pounds while 11.5 SDR yields 10,816 pounds). A larger load cell could be used to cover all 6-inch PE pipe SDRs, but some resolution would be sacrificed when pulling pipes with the smaller diameters.

With the current load cell, each bit of resolution corresponds to 47 pounds. The TTMT pod digitizes the load cell data with an on-hoard microprocessor and transmits the data by inducing an RF signal through the drill stem to the receiver above ground. The pod also logs the data with a time/date stamp onto an on-board Dallas I-Button device. After the pull, the I-Button can be removed and its information downloaded to obtain a histogram of the pull's events.

The TTMT receiver consists of an antenna that picks up the induced signal and electronics that amplify and condition the data. The receiver is designed not to hinder the drill stem as it exits from the ground and is automatically loaded back on the drill rig. A tone decoder is used in the conditioning electronics to extract the signal from surrounding noise. A handheld computer connected to the electronics displays the serial number of the transmitter's I-Button and the tensile load data.

The tensile load data is displayed as a percentage of the allowable tensile load limit. Alarms are displayed when the values reach the maximum allowable tensile load. The only calculations required of the operator are entering the pipe's diameter and SDR, values on the handheld computer before the pull is started. The receiver electronics automatically convert and display the necessary, information based on this entered data. The operator's display is shown in Figure 2.

[FIGURE 2 OMITTED]

"The TTMT transmission range was tested in a lab with a series of drill stems linked together. Them was no significant difference in the signal-to-noise ratio when the transmitter and receiver were 10 feet apart (one drill stem) or when they were 40 feet apart (four drill stems). Although certainly not indicative of the noise that will be experienced in the field, noise was generated by dragging a chain along the drill stem as well as hitting the drill stem with a large wrench. In both cases, additional noise was seen on the receive end, but the signal was still picked up.

To test the accuracy of the load cell, electronics and the strength of the overall unit, the TTMT was tested on an Instron servohydraulic tensile/compression machine. The test was set-up to pull the TTMT from 0 to 8,500 pounds, holding at each 1,000-pound increment for 60 seconds to ensure the electronics and Instron captured their respective readings when the unit was in a hold state. Between each increment the unit was pulled at a rate of about 56 pounds per second. On average, the difference between the TTMT and Instron readings was no greater than 1.6%. The test was repeated a second time, will the average difference between readings being 1.2%.

A separate test as run to test the overall strength of die device. Settings were set similar to the accuracy test, but the TTMT was pulled up to 15,000 pounds. With the first test, the rod ends coupling the TTMT to the end shackles broke at about 11,000 pounds. When replaced with higher-capacity rod ends, the unit withstood loads up to 15,000 pounds. Since the load cell was pulled above its rated capacity, another test was performed to check its accuracy. The load cell was fine, supporting the case that although it shouldn't be pulled at greater than 10,000 pounds for too long, it can still withstand momentary spikes during the pull.