Inspection and monitoring of underwater structures has always been a challenge for power plants with submerged cooling system structures, mainly due to those structures' inherent inaccessibility. Commercial divers and remotely operated vehicles (ROVs) work well for inspecting specific areas, but it's hard to assemble the point measurements that they make into a big picture of an asset's health.
High-flow conditions further complicate the task. Turbulence often dictates shutting down not only the generating unit being inspected but also units with adjacent intake structures and pumping stations. As a result, manual inspection methods incur three different kinds of costs: lost generation sales, the labor costs of the inspection itself, and the time costs of completing the regulatory paperwork and performing the administrative procedures needed to shut down and restart one or more units following the inspection.
Naturally, inspections of underwater structures may be done manually on a routine basis, and only during planned outages. But where more frequent inspection is warranted, any noninvasive technique able to examine structures under full operational load would provide more-useful data at a great cost savings. Scanning sonar is one such technique.
Easy as XYZ
Any sonar system transmits a pulse of sound and measures the time it takes to receive an echo from a target. Multiply this time by the speed of sound in the medium, and you know the distance to the target. Scanning sonar systems repeat this pulse-echo process several times as the transducer head is rotated through preset increments, effectively scanning a line.
Scanning sonar can work in either imaging or profile mode. In the former case, the images delivered can be used to make subjective assessments of the integrity and fouling of bridge piers, cooling water intakes, and dam walls (Figure 1). But because the images are two-dimensional, they cannot be used to identify the location of debris or damage in the third dimension, or its extent. In order to quantify the volume of debris or the extent of scour, measurements must incorporate survey points that are known in three dimensions so that volumetric calculations can be made.
1. Scanning sonar in imaging mode. This mosaic was made from imaging scans showing possible debris locations. The extent of the debris in the third dimension (toward and away from the viewer) cannot be determined from this type of survey. Source: ASI Group Ltd.
A scanning sonar system configured for profiling instead of imaging (Figure 2) can obtain the third set of coordinates, producing something akin to a relief map. Setting up the sonar at several locations enables collection of enough survey points to cover the desired area. If the process is properly done, it takes much longer to do the setups than the scans.
2. Scanning sonar in profiling mode. A bathymetric plot made by interpolating several profiling scans perpendicular to the intake face. Enough scans at different locations were done to provide 1,200 data points. Source: ASI Group Ltd.
A big benefit of using dual-axis scanning sonar is that it allows the position of the scanning head to be fixed at one or more known locations. Realize that if the head moves due to vibration or an insecure mount, the resulting measurements will be skewed unless they are extensively post-processed to correct for the positional errors. Well-designed dual-axis scanning sonar systems calibrate the readings to assist with post-processing by incorporating attitude sensors for measuring pitch, roll, and possibly yaw or heading.
Dual-axis scanning sonar systems such as the one whose work is shown in Figure 3 also include a precision actuator. The system's primary axis is the one around which the transducer rotates. The secondary axis, provided by the actuator, is used to rotate the body of the sonar perpendicular to the primary axis. With the head rotation on both axes known, the location of the point (return echo) can be determined in Cartesian coordinates. Making multiple scans from different angles allows generation of "point cloud" representations.
The system's on-board attitude sensors enable correlation of measurements to the actual mounting angle of the scanning head. Based on feedback from the sensors, the scanning process can be halted if the system moves due to vibration and then be restarted after it stabilizes. Calculations of the speed of sound can be made more accurate by optional water depth, temperature, and conductivity sensors.
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