One of the most common uses for commercial underwater remote-operated vehicles (ROVs) is in conducting inspections of water tanks, ship hulls, and submerged infrastructure such as bridge components or dams. A critical element of these inspections is measuring the thickness of metal components like hulls or girders. How do ROVs conduct this type of measurement?
On the Aquabotix Endura line of high-performance commercial ROVs, we offer the Cygnus NDT Metal Thickness gauge as an optional accessory. These gauges use ultrasound technology to measure the thickness of metal objects underwater. By emitting an ultrasonic beam into the surface of the metal and analyzing the return sound, the Cygnus NDT can measure metal of thicknesses up to 10”, even through coatings such as paint up to 0.787” thick.
The Cygnus NDT is extremely easy to use. The included CygLink software allows the ROV operator to visualize the tool’s measurements remotely on the video feed. To make things even easier, the optional Cygnus Probe Handler automatically aligns the probe to the wall or item being measured, with 15 degrees of movement, even if the operator has not perfectly approached the measurement subject.
Tools like the Cygnus greatly enhance the utility of our Endura ROVs. As ROVs take on more responsible roles in things like underwater inspections, the need for tools such as the Cygnus will continue to grow.
There are almost 80,000 dams in the US Army Corps of Engineers National Inventory of Dams (NID), and that isn’t even a comprehensive list; the NID only covers dams which reach a minimum height and which hold back a minimum volume of water. There are many thousands of other dams, on private and government land, which are used for irrigation, flood control, aquaculture, drinking water, industrial use, and a myriad of other applications. Although most of these dams are extremely useful, they can also pose a serious danger to people and objects downstream of the dam. For obvious reasons, dam failure can have catastrophic consequences.
Accordingly, it is critically important that dams be inspected on a regular basis.
The federal government, via the Federal Emergency Management Agency (FEMA), provides guidelines for inspections on dams under federal control. These guidelines suggest that dams should receive an informal “eyes-on” inspection as needed or following any significant incident at a dam (such as a flood, an earthquake, or vandalism), as well as a more thorough intermediate inspection of the dam and all related structures on an annual basis. Federal guidelines also suggest a formal full inspection of the dam be carried out at least every five years, with a full special inspection to be conducted when there is a major event such as a large flood or a major earthquake. Around 3,200 dams in the United States are owned outright by the federal government, and another 5,200 or so are not owned by the government but are located on government land. Although there are some complications about the regulation of those non-federally-owned dams, all told about 10% of the largest American dams fall under the federal guidelines.
The remainder of the dams in the United States are regulated by the state governments, with the notable exception of Alabama which leaves dams essentially unregulated. The state laws and regulations are a fairly diverse patchwork; some states require inspections to be paid for by the dam owners, while other states put that expense on the taxpayer. A few states like Texas do not have a formal inspection schedule at all (although inspections are strongly suggested) while most states have stringent schedules and extensive systems of classifications for which kinds of dams must be inspected and how often.
In general, however, in most places, dams are supposed to be inspected at intervals ranging from annually to every five years. The size of the dam’s impoundment (the volume of water the dam is holding back) and the population of the area in the dam’s potential flood area are the major factors determining how often and how stringent such inspections must be. Again depending on the location, inspections can be carried out by divers or by surface inspection, usually with at least some underwater work being required. Clearly, there is an astonishingly large amount of inspection work called for in the dam safety regulations.
ROVs play a major role in accomplishing the difficult task of inspecting dams while leaving them in service. Many dams are simply not able to be “turned off” so that an inspection can take place. Even when dams are equipped with the ability to dewater areas for inspection or repair, such dewatering is time-consuming and expensive. ROVs and human divers are able to work in areas of the dam without having to go through the dewatering process, saving large amounts of money.
One area where ROVs have saved enormous sums for dam owners is in assessing the need for cleaning of trashracks, reservoirs, and head ponds.
Rather than adhering to a calendar schedule for the dredging of such ancillary dam components, an ROV inspection using sonar can inexpensively check whether an area needs to be dredged at all.
Of even more importance than saving money is saving lives. At many dams, particularly hydropower dams, there are areas where it is very dangerous for a human diver to enter the water. High water flows and high pressure differentials can be life-threatening conditions. The use of an ROV allows dive teams to conduct preliminary safety assessments, measuring flow, depth, and water temperature before ever putting a human being into the water. ROVs have also been used to check safety concerns such as the open or closed status of a head gate, where a gap of a few inches in a nominally “closed” gate could pose a lethal risk to a diver in the area.
At large dams with deep water areas, ROVs have the ability to work deeper than human divers without needing mixed-gas equipment. (Divers going below 30 meters require decompression tanks and other specialized equipment that make a dive more expensive.) By using divers for shallow work at a dam, and ROVs for the deeper dives, a cleaning or inspection process can be accomplished at much less expense. In addition, ROVs can conduct survey work using tools such as multibeam sonar more efficiently than human dive teams. As dam infrastructure ages, inspections of components like downstream draft tube slabs require a great deal of precise surveying work to assess the condition of the slab.
As in many other areas of underwater work, ROVs are capable of making a large contribution to the jobs being done by human divers.
ROVs make underwater work safer, more effective, and less expensive.