An innovative technology that is being increasingly applied to locating existing pipe networks or those installed by way of horizontal drilling is inertial or gyro locating. Using this technology, it is possible to map networks of pipes or ducts with diameters ranging from 29 mm (1.1 inches) and larger for hundreds of meters with high precision in 3D including depth. The technology does not require boots on the pavement and hence is safer than other mapping methods. It avoids traffic and other disruptions that surface-based underground detection methods such as electromagnetic or ground penetrating radar require. The output of an inertial system is a 3D model compatible with most GIS and CAD software.
Capturing the path of a pipe
The gyro mapping device includes sensors such as accelerometers, inclinometers, and odometers that measure changes in heading, pitch/inclination, roll and the distance travelled. These four elements are measured one hundred times per second (100 Hertz). The device includes memory modules and a battery enabling data to be stored in the device making it fully autonomous so that it does not require a communications cable. It can measure the path of pipes with a diameter as small as 29 millimetres as well as larger pipes and ducts. Capturing the GPS location of the starting and end points of the pipe being surveyed enables the trajectory recorded by the device to be output as real world coordinates. If it is not physically possible to measure the location of the starting and end points, there is a small radio transmitter in the device that can be detected from the surface that can be used to determine the real world coordinates for two point locations on the path.
The process of measuring the path of a pipe involves pulling or pushing the device through the pipe. For larger pipes there are different wheelsets that are used to keep the device centralized in the pipe. For complex pipes with laterals, deformations or 90 degree turns, there is a specialized invert wheelset. The device is pulled or pushed through the pipe at a speed of one to two meters per second. When the device reaches the end point it is allowed to remain stationary for a short calibration period of about 30 seconds and then it is pulled back to the starting point. This process enables the path to be captured twice, which provides a check on the trajectory recorded by the device. Once the data has been collected, the device is removed from the pipe and the data downloaded to a laptop where it is processed to generate output in different formats readable by a variety of GIS and other software.
Typically gyro mapping is conducted in decommissioned, temporarily depressurized, or newly installed pipes. To broaden the applicability of gyro locating, a recent technology development and demonstration project was sponsored by the Pipeline and Hazardous Materials Safety Administration (PHMSA) and Operations Technology Development (OTD). The project was performed by a partnership between the Gas Technology Institute (GTI), Reduct and PRISUM Technologies (Condux International). It demonstrated that the gyro device can be inserted through a vertical Hot Tap Entry (HTE) for mapping live (pressurized) gas pipe networks as small as 2 inches (5 cm), with no disruption in service to downstream gas customers.
Advantages of gyro mapping
Gyro mapping has distinct advantages compared to other technologies for mapping underground infrastructure. First of all, it can be used to map any type of utility running through a pipe or a duct. All that is required is that the pipe or duct be larger than 29 mm. Secondly the pipe or duct can be any material and any shape round, rectangular or X -shaped. Most importantly from a safety perspective, gyro mapping does not require above ground tracing like sond technology does, in other words, no boots on the ground except at the starting and end points. The device is not limited with respect to depth. It has been used to map pipes more than 50 meters deep. Critical in congested utility environments, it is not sensitive to external electromagnetic noise. It has been used to record the path of pipes lying adjacent to high voltage electric cables, railway tracks and other sources of electromagnetic noise. High frequency recording is also an important advantage of the inertial device. The device captures data 100 times per second, which translates typically into every one or two centimeters. This has important implications for a number of applications including measuring bend radii, which is a key parameter used by engineers for stress calculations on pressurized pipes, and for calculating the slope of gravity sewers.
Applications of gyro mapping
Reliable as-builts - Gyro mapping provides high accuracy 3D data including a depth profile, which allows visualization in 3D and in various 2D views. How the customer prefers to visualize the data is typically discussed and agreed to in advance. The final report provides accurate as-builts in a format that the customer has requested. In addition the final report is a very important tool as part of the contractor handover management process. Disputes between contractors and owners are not uncommon. One of the reasons is that contractors often prepare the as-builts which can represent a conflict of interest. Inertial navigation provides an independent verification of a pipe installation. For example, for horizontal drilling (HDD), the path of a pipe provided by the contractor in an as-built is based on the steering system that was used and that the contractor got from their computer while they were drilling the hole. There are technical characteristics of this data which introduce uncertainty in the as-builts submitted by the contractor. For example, a common problem is inaccurate bend radii, which are important parameters for calculating stress for pressurized pipes.
Damage prevention - Damage to underground utilities during construction is a major cause of delays in construction projects, can lead to injuries and even deaths to workers and the public, and is a significant drag of tens of billions of dollars on the economy. Construction processes for underground utilities often result in the submission of unreliable as-builts with the result that utility network owners' records cannot be relied on to provide accurate location data for underground utilities. Awareness of this problem is growing and legislation, regulations, and other measures are being implemented in a number of jurisdictions to improve the quality (accuracy and timeliness) of underground records. Gyro mapping underground utilities is an effective and efficient method for improving the quality of utility records while at the same time providing other benefits such as monitoring pipe movement as a result of soil conditions or ground movement caused by seismic events.
Stress analysis for pressurized pipes - The depth profile of a 425 meter long pipe installed using horizontal drilling (trenchless) was recorded using the inertial device. Around the 300 meter mark, there is a clear bend and to determine the stress at this point it is standard practice to calculate a bend radius. The drilling company that installed the pipe may have used seven meter or five meter drill rods. Depending on which they used they would have calculated a bend radius using the end point of the drill rods. For seven meter drill rods they would have reported a bend radius of 120, with five meter drill rods the bend radius would have been estimated at 105 meters. Because the inertial device captures the pipe path every centimetre or two, it is possible to determine that the actual bend radius is 40 meters. If the specification from the owner was that the minimum bend radius was 110 meters, it is possible to see how important an independent high frequency recording of the pipe path is.
Flow management for gravity sewers - An actual inertial recording of the sewer pipe path is shown. Green indicates where it is sloping down and red where it is sloping up. The first graph uses an interval of 10 centimetres which reveals every detail of the slope of the pipe. When a larger interval of three meters is used, the slope becomes nearly uniformly green which is generally what a contractor wouls prefer the customer to see.
The important point is that to see what is really going on, it is important to capture the highest possible point frequency, because only then is it possible to see the details required to optimize the gravity sewer network.
Maintenance planning - Typically utilities have a static maintenance policy, which means that every X years a pipe is inspected. However, there may be a reason that in certain areas it is important to monitor underground assets more closely. Perhaps the pipe is traversing several types of soil and there could be differences in subsidence between the different soil types. Or it could be an earthquake prone area where pipes are affected by ground shifting. For these areas it might be valuable to prioritize maintenance by measuring the pipe's path periodically to identify areas where there is more pipe movement than in another places. With this information it is possible to redirect maintenance dollars to those areas with the greatest or fastest growing pipe movement and delay maintenance in more stable areas where there is less movement.
This post is based on Otto Ballintijn's talk at the Canadian Underground Forum (CUF). You can listen to all the talks at CUF on the GeoIgnite CUF Youtube channel.
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