Clean-to-Inspect (hydrovac cleaning, followed by a CCTV inspection) is the method typically used to assess the condition of our sewer and stormwater assets. However, an Inspect-to-Clean method is also useful to gain information about the asset before evidence is washed away, and can reduce the unnecessary expense of cleaning pipelines that don’t need pre-cleaning, especially in large diameter pipelines and siphons. This article will discuss the two methods, and the benefits of each.
Why We Inspect
The NASSCO Pipeline Assessment and Certification Program (PACP) coding system requires that the CCTV operator record various O&M observations and Structural defects which will help the Owner assess pipe condition, and also help to determine proper inspection and maintenance frequencies for those assets over time. We typically clean before inspecting to ensure we have a clear unobstructed view of the pipe to obtain high-quality CCTV video, and also so we don’t waste time trying to inspect when the line segment has so much debris that we cannot get the camera tractor through the pipe. On the other hand, evidence of issues such as grease, debris, roots, and infiltration areas can be washed away by pre-cleaning activities. It is useful to record this information and make a plan to repair these situations, reducing future maintenance costs over time. At a minimum, those problem lines should be placed on a hot-spot work order list until a permanent repair can be made.
In smaller diameter pipelines, typically between 6-in. and 21-in. diameter, pre-cleaning is usually recommended in order to prevent wasted field assessment efforts as those lines are small enough that grease or debris may easily stop a tractor. Pre-cleaning is recommended for these smaller diameter lines to remove debris that may block the CCTV tractor progress, and to also remove cobwebs that may foul the camera lens. If excessive debris or roots are evident during the cleaning process, then heavy cleaning may be performed, as needed.
For siphons and pipeline segments that are 24-in. and larger, a good approach is to inspect first, and clean only as needed. Large diameter pipe is extremely expensive to contract out for cleaning if you don’t already know that the pipe needs cleaning. Contractors will always bid on the high side of debris levels to cover themselves in the event significant debris is found.
The depth of flow in large sewers will determine the CCTV platform that should be used; either a tractor or floating pontoon raft. Night deployments can also be performed to take advantage of lower flow levels. Siphons can be inspected while in service with sonar. The sonar is placed in a protective cage and pulled through the siphon to obtain a 360-degree scan of the siphon interior.
Floating pontoon rafts are readily available that can be used to transport the CCTV camera on top of the water. It is good practice to add sonar when performing a CCTV inspection of large sewers to obtain valuable information of the debris quantity and potential defects under the water line. The pontoon will float from upstream to downstream with sufficient water level and flow rate. For sluggish flow velocities, a tag line can be also be threaded through the pipe and then used to pull the raft through the pipe segments with a winch.
If there is little water in the pipeline and a tractor must be used, but debris is present, a reversal can be attempted from the downstream manhole. However, if debris is readily visible down the pipe, then a reverse should not be attempted, and the call for cleaning must be made to prevent further wasted effort and cost.
The Power of Sonar
Sonar allows us to ‘see’ underwater; to measure any debris that is present, and to determine if there are any large defects under the water line. Sonar is particularly useful in siphons and in large sewers; to first determine if cleaning is needed, and then also used post-cleaning to prove that the asset was cleaned to the Owners specifications.
In a typical contract that we perform on large diameter sewers where debris levels are unknown, the condition assessment will proceed first. Multi-sensor platforms are used that, in one inspection pass, will collect CCTV (and optional Laser) above the water line, and sonar below the water line. Any combination of sensors can be used: CCTV/Laser/Sonar, CCTV and sonar, CCTV/Laser, or sonar only. Laser is used to measure the internal dimensions of the pipeline and create a 3D profile of the pipe interior. This information is useful to determine the extent of any corrosion that has occurred, if rehabilitation is needed and when, and then can also be used to design the best rehabilitation method for the asset.
The sonar profiling scans provide data that allow us to measure the depth of the water in the sewers, and the amount of debris under the water line. Debris graphs are produced from the sonar scans that pinpoint the location of debris, and the quantity of debris. This information can then be used to direct cleaning efforts exactly where needed, which will greatly reduce the cleaning costs for a line segment. It also allows for more accurate and cost-effective bidding because Contractors know exactly how much debris is in each pipe segment, and have general information on overall flow depth, so that they can bid the appropriate equipment and disposal fees necessary to get the job done.
Post-cleaning sonar should then be performed in conjunction with the final CCTV inspection to verify that the pipelines were cleaned to the Owners specifications.
The Clean-to-Inspect and Inspect-to-Clean methods are both acceptable ways to maintain our assets, and each has their advantages. The method to be used depends on the last cleaning date, the diameter of the assets, the current internal condition of the assets, and consent decree or SSMP requirements regarding annual target footages for cleaning and inspection.
Neither method, however, replaces the requirement to perform regularly scheduled proactive cleaning of our system assets. Regular cleaning and PACP-coded CCTV inspections will help the Owner ascertain the unique needs in all areas of their system.
The information from each inspection can then be used to adjust cleaning and inspection frequencies over time, increasing the frequency for critical assets and decreasing the frequency for assets of low concern, which will maximize limited maintenance budgets.