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GTI reports progress in developing tow-tension monitoring/data logging tool. (Horizontal Directional Drilling).

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.


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.


"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.

Recent Pull Testing

Two in-ground pull tests have been performed. The initial test consisted of installing three runs of plastic pipe on GTI property. These runs were of two- four-, and six-inch diameter plastic (straight pipe) and were 120 feet long. Tension loads were successfully measured but were low due to the length and diameters involved. Some minor intrusion of water into the tow pod was observed. A presentation was made shortly after the first test at the AGA Operations Conference in May 2002.

A second field test on a 500-foot installation (coiled pipe) at KeySpan Energy was performed. Load data was successfully transmitted to the drilling machine during the first half of the run. Loads 200 percent of expected (around 21,000 pounds) were observed in the middle of the run and data transmission ceased shortly afterward. The pod remained intact, so there was a concern that the load cell was damaged. When the pod had dried out for several days, it resumed functioning.

When the pod was taken back to GTI, it was tested again on the tensile machine. The load cell didn't appear to be damaged as the 1-2% difference from the earlier tensile tests was repeated. In addition, tests at loads of 15,000, 18,000 and 21,000 pounds were performed. After each, the lower load tensile tests were repeated with the same success.

Since the load cell was intact, there was a concern that the readings prior to loss of transmission may not have been erroneous. The drill rig operator claimed to have only seen readings of around 2,000 "lbs" on the rig hydraulic display at the point where the 21,000-pound TTMT readings were noticed. In talks with the manufacturer of the drill rig, it was determined that the gauges on the drill rig are measured in psi. Several factors (amount of drilling fluid, drill stem torque) prevent an exact correlation between psi and tensile pounds-force on the pipe end. However, the engineer's best estimate was a 9 to 1 ratio of pounds-force to psi. Given this, the 2,000 "lbs" hydraulic gauge reading seen by the operator was indeed very close to 21,000 pounds seen by the TTMT display.

Moreover, alter the KeySpan test when the pipe end emerged and the drill rig was turned off, the pipe quickly retreated back into the hole, This suggests there was indeed stress on the pipe.

Given all the factors above, the best guess for the cause of the loss of data is that the overload had compromised the seals of the tow pod enough to allow drilling fluid to enter the interior.

Current Status

GTI has prepared an invention disclosure covering the novel transmission method that allows the data to be delivered to the drilling machine. A provisional patent has been filed.

Pipeline Equipment Specialists Limited in the UK has entered into a non-disclosure agreement with GTI and is reviewing the design of the tow pod. PESL has provided consultation with GTI throughout the duration of the project and expressed interest in helping commercialize this technology. Pending the results of their own tests on the device, they will he willing to assist in improving the mechanical design to better protect the electronics

The subject of real-time monitoring of pull-in operations is of high interest to the industry, as evidenced by the number of papers on the subject at the most recent Underground Construction Technology Conference in Houston, sponsored by Oildom Publishing Co. There is a high probability that this technology will be commercially deployed in some form.


Allouche, E.N. and ME. Baumert, "Real-time monitoring of HDD Installations," Proceedings. Underground Construction Technology Conference, Houston, TX, January 200.2

Anon. ASTM F 1804-97: Standard Practice for Determining Allowable Tensile Load for Polyethylene (PE) Gas Pipe During Pull-In Installation. Published November 1997.

Anon., Historical Development of Horizontal Directional Drilling Trenchless Technology Center.

Authors: Maximillian J. Kieba is an electrical engineer in GTI's Electronics and Telecommunications R&D. He joined GTI in 1999 and is the principal investor for a project co-funded by the DOE to develop an obstacle detection sensor for use with pipe installations performed by horizontal directional drilling (HDD). This sensor, installed in the drill bit used for the pre-boring stage, utilizes a differential soil impedance measurement technique that will be sensitive to the presence of plastic and ceramic, as well as metallic obstacles. The sensor will provide range and direction data for obstacles near the HDD head. Prior to joining GTI, be performed Y2K testing for South Carolina Electric and Gas in Columbia, SC. He received a BS degree in Electrical Engineering from the University of Pennsylvania in 1999.

Christopher J. Ziolkowski is Associate Director, Electronics & Telecom Research at GTI. He holds a BS degree in electrical engineering from Michigan State University and has been with GTI for 20 years. He directs and manages the activities of project managers who perform research in the areas of electronics and telecommunications. Technical focus areas include sensor design, automated data acquisition, capture of field data using PDAs, wireless telemetry, data communications and security and SCADA interoperability. Prior to joining the Institute of Gas Technology in 1982, he worked in the R&D labs of Miner Enterprises. He is the recipient of the American Gas Association 2002 Gas Industry Research Award.
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Title Annotation:Gas Technology Institute
Comment:GTI reports progress in developing tow-tension monitoring/data logging tool. (Horizontal Directional Drilling).(Gas Technology Institute)
Author:Kieba, Maximillian, J.; Ziolkowski, Christopher J.
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:May 1, 2003
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