A third of the 1.3 million miles of natural-gas pipeline in the United States are "unpiggable" because their diameters vary, or the pipe has sharp bends. Yet as these pipes age and corrode, the need to inspect them becomes more urgent.
Recently, PIG designs have been improved at Carnegie Mellon University, where Explorer II, a 66-pound, eight-foot-long wireless robot was developed that looks like a series of sausage links (Figure 3). It twists and turns with ease, and because it has a drive train, it also allows operators to precisely control where it starts and stops, instead of being propelled by the oil or gas flow, as is the case with previously discussed PIGs. In addition, the use of permanent magnets slows down the movement of the previous PIG designs. In contrast, Explorer II replaces permanent magnets with a compact electromagnetic coil, and thereby eliminates the reduction of the speed.
On some pipelines it's easier to use remote visual inspection equipment to assess the condition of the pipe. Robotic crawlers of all shapes and sizes have been developed to navigate pipes. Typically, the video signal so obtained is fed to a truck, where an operator reviews the images and controls the robot. A recent advance is to use special paints that change color if leakage occurs at pipe joints, and this color change then can be detected automatically through remote inspection, including the use of drones.
Controlling the PIGs
ILI PIGs go where people can't, but controlling them requires good process control because most depend solely on the pipe's pressurized fluid for propulsion, and therefore it is difficult to stop the ILI at specific points. Good speed control requires the measurement of velocity, drag, pressure drop and flow. On in-compressible oil service, it's relatively easy to make these measurements to control speed. On incompressible gas service, it is not.
What we would need for automatic velocity control is a cascade loop, where the master controller is velocity, and the slave is a flow controller through a bypass valve (BPV) in the nose of the ILI (Figure 4). This valve provides a path for the process fluid to pass through the core of the ILI, so that if the PIG speeds up, the BPV opens up further, and if it slows, it closes a bit. What makes such a cascade loop interesting is that the flow sensor element and the control valve in this loop are the same. The flow is measured by the differential pressure across the BPV (or the whole rig), while the control valve whose opening is being manipulated is the BPV itself.
Speed control via a bypass valve
In the coming articles in this series, I will elaborate on PIG control and other control systems serving pipeline safety and then go into fracking and oil-sand safety automation.