Geohazards including landmass movement, subsidence and floods poses significant threats to oil and gas pipelines, especially when the lines cross through hilly or mountainous terrain. Identifying potential slope hazards and monitoring slopes and is an integral part of pipeline/asset integrity programs. Pipelines that are above ground are most at risk, though subterranean (below ground) lines aren’t completely free of such threats. If an area near a pipeline has an inordinate amount of rainfall in a short time, flooding is an obvious threat. Abundant precipitation can loosen earth and trigger a landslide that could penetrate a pipeline. Even erosion in rocky areas could trigger a landslide that damages or breaches pipes.

“Not being aware of subsidence threats can be potentially catastrophic to a company’s assets.”

slope movementTo protect critical assets, operators use geotechnical instrumentation to monitor such movement. Not fully understanding subsidence threats can be potentially catastrophic to a company’s assets.

The difficulty with monitoring such activity is that some lines can be in remote and treacherous areas not easily accessed by company personnel. Personnel can take occasional measurements on site, perhaps, if it is just short-term monitoring. However, areas suspected of being unstable may require long-term monitoring. Companies may consider areas known for past subsidence or landslides riskier and want to monitor them more closely.

The instrumentation to measure such movement has been available for decades. But the ability to transmit that information easily and reliably to a remote location hasn’t. This meant personnel had to be on site to see and record the data.

Ground movement can be a threat to pipelines, especially if they’re above ground. Now, telecommunications technology can be used in conjunction with slope monitoring instrumentation, making it easier for operators to monitor slope movement from afar.

Groundwater measurements
There are two major measurements that can determine movement: groundwater levels and displacement.

The most common way to do measure groundwater is with piezometers. According to a paper titled “Instrumentation Practice for Slope Monitoring” by William F. Kane and Timothy J. Beck, two vibrating-wire piezometers are necessary to get accurate groundwater measurements. One measures atmospheric pressure and the other measures downhole pressure.

Kane and Beck wrote that using piezometers is useful for areas where groundwater levels fluctuate quickly and need to be monitored more frequently.

The state of California’s Office of Geotechnical Services writes that a portable levelogger may also be used. Leveloggers are portable and can download information remotely.

There are a variety of instruments available to monitor displacement. Kane and Beck wrote that inclinometers, tiltmeters, extensometers and time domain reflectometry can be used individually or in some combination. Stationary inclinometers and tiltmeters can be used to measure changes in tilt angle, acceleration of detected movement and the movement’s direction, according to Kane and Beck.

The state of California wrote that extensometers, tilt meters, crack meters, and beam sensors are can be used to monitor displacements on the ground surface, underground openings and rock masses. The state also said that common strain gauges and elasto-magnetic force sensors can be used in ground to test load-bearing areas.

Data transmission
Monitoring slope movementApplus+ RTD uses instruments that can be equipped with sensors that transmit the readings to a datalogger. Dataloggers have a limit on the number of sensors that can be connected. Through the use of a multiplexer one can increase the number of sensors transmitting to a given system. A typical setup could have 60-80 sensors running through multiplexers to a central datalogger. The data logger is powered by a 12V AGM battery which is charged daily through the use of a solar panel array.

Data communication can be achieved through several different methods, the most common are through cellular or direct satellite communication. Cellular communications are preferred when available as the transmission and communication is typically more robust. According to Martin Derby of Applus+ RTD, in very remote areas data may be transmitted via a satellite modem. This type of communication is performed during monitoring of pipelines for subsidence during long wall mining operations for coal. This method allows for autonomous delivery of data (at prescribed intervals), which then allows the owners, operators and engineers the review the data in real-time.

Oil and gas companies would prefer to prevent a breach to their assets, if at all possible, than have to respond to one. Geotechnical instrumentation coupled with current communication technology enables them to prepare for such situations and to do so without having to be on site continuously.