Cities and sewerage agencies often collect wastewater from large areas that must be conveyed to distant treatment plants. Handling this enormous volume requires large-diameter trunk lines.

It is not unusual to find trunk lines more than 100 inches in diameter. Because these lines carry such large flows, they often run well above the half-pipe and at more than 10 feet per second. This often prevents the use of standard camera crawlers for CCTV inspection.

Most inspection equipment manufacturers offer crawlers designed for large-diameter pipes. As water level increases, these crawlers have remote-controlled elevators, or scissor lifts, that raise the camera 1 to 3 feet. Usually, that is enough to keep the camera above water. But once the level exceeds the height to which a camera can be raised, an alternative method of inspection is needed. This is where floating platforms come into play.

Riding the surface

A floating platform (float) is a boat or pontoon on which a camera is placed. The float with camera is then pulled through the pipeline. This allows the camera to traverse the pipeline even when the water level is more than 3 feet high.

One disadvantage of this method is that the bottom of the pipe — the area below the water line — is not visible. However, with experience, an operator can learn to read turbulence in the line and determine where problems exist. Conversely, the floating platform is highly effective for inspecting the upper portion — or crown — of the pipe. The information from that alone can make this type of inspection worthwhile.

The Sanitation Districts of Los Angeles County, for example, operate a vast sewer system of large-diameter reinforced concrete pipe (RCP) lines. The primary concern is the crown of the pipe, where hydrogen sulfide eats away at the cement and exposes the aggregate and rebar. CCTV inspections record the deterioration progress so that the districts can set mitigation priorities. In most cases, floating platforms are the only way to look at these pipes because they carry high volumes 24 hours per day.

Although some manufacturers fabricate floats, many contractors and agencies choose to construct their own. Any style is fine, as long as it is buoyant enough to support the weight of the camera.

Since a mainline camera is being used, all of the attributes of a standard pipeline inspection hold true. Operators can still scan and zoom in on points of interest while recording the inspection. The result is a high-quality inspection of the pipe crowns. In the largest lines, a standard inspection camera may not provide sufficient light. In this case, auxiliary lighting may need to be part of the custom float design.

The process of floating a pipeline can be quite involved. Most likely, the job will require additional personnel, and setups may take much longer than for a standard inspection. On the other hand, it may also be possible to inspect longer reaches where only the length of the cable is the limiting factor.

Working downstream

There are two methods for inspecting pipelines with floating platforms. The first is quicker to perform, but also carries more risk. Here, the CCTV vehicle sets up at the upstream manhole. The float is then tethered from the rear as it travels downstream through the pipeline. A small parachute can be placed in front of the float, with 20 to 30 feet of rope, to give the float added propulsion.

­Some float crews use only the CCTV cable as a tether, but that can be very risky. If the cable or connector breaks, the camera and float could be lost. A safer approach (although more cumbersome) is to use an additional rope or cable attached to a winch as a safety line.

In this method, the propulsion for the float comes exclusively from the flow in the line. If the flow is slow or stagnant, this method may not work well. On the other hand, if there is a significant flow, holding back the float to get good data can be a real challenge.

Movement of the float is controlled with the cable winch. The operator inspecting the line must communicate the platform’s speed and movement to the person controlling the winch. This requires a team effort to produce a quality inspection.

As the float travels down the pipe, the cable or cables will become heavy, and the float will slow down. At some point, it may need additional propulsion. One effective way to increase velocity is to attach more parachutes.

If prefabricated parachutes are not available, they can be made out of tarps, heavy-duty garbage bags or even 1-gallon plastic bottles. These items must be connected at all corners to a rope, which is then tied around the cable and allowed to float down the line. Once the parachute reaches the float, it engages and provides added power. More than one parachute may be needed, especially on runs longer than 1,500 feet.

For retrieval, heavy flow in the line makes it nearly impossible to pull the float and camera backward. For this reason, the float needs to be removed at the downstream manhole. A pole with a hook on the end, or a rope with a grappling hook, can be used to retrieve the float once the parachutes are visible.

It is important not to let the float travel past the manhole, as that makes retrieval difficult. Once the hook grasps the rope and parachute, it can be pulled to the surface. This may require a lot of muscle, and the camera can become dislodged or damaged. Use of a confined-space tripod can minimize this effort considerably. Once the float and camera have been removed, the cables are disconnected and pulled back with the winch.

Inserting the float into the water can be tricky, especially in the beginning. Once the float hits the water, it wants to take off, and it may have a tendency to flip over. Patience, practice and experience simplify the process over time. It is critical to use a safety tension release line to keep tension off of the main cable during insertion. This is another argument in favor of a backup safety line.

Removing the float can easily damage the camera. As the flow pulls against the float, it becomes difficult to manage. Since there is so much pull on it, only brute force can remove the float from the water flow. As the crew pulls up on the cables, the camera will tend to hit the crown of the downstream pipe. Every effort should be made to flip the float over and expose its underside — rather than the camera — to contact with the pipe.

Two-end tether

A second and perhaps more widely used method is to tether the float on both ends. This means the float is connected to the CCTV vehicle at the upstream manhole and is also connected at the downstream manhole. This method provides the most security and stability.

In this case, a cable is first strung from manhole to manhole. This can be done by tying the end of the line to a plastic 1-gallon milk jug one-third filled with water and letting it float downstream. Once the line reaches the downstream manhole, it can be retrieved with a hook on a pole or a rope with a grappling hook.

Sometimes it is easier to float a light line first, such as fishing line, and then use that to pull the heavier line through. Either way, the line must be strong enough to pull the float and to provide enough strength if the other line breaks.

Obviously, this method requires someone at each manhole. While the winch lets out cable at the CCTV vehicle, cable is reeled in at the downstream manhole. Personnel on each end must be in continuous radio contact. Having the float tethered on both ends not only provides additional security — it also provides the needed propulsion. For this reason, this method can be used where the water flow is nearly stagnant.

As metropolitan populations continue to grow, so will the need for large-diameter trunk lines. Because diverting flow from these lines for inspection is cost-prohibitive, proper assessment requires some type of floating CCTV inspection. Eventually, more technologically advanced floats will be developed, but the basic concept of placing a camera on a floating platform will be around for decades to come.

Jim Aanderud is owner of Innerline Engineering, a video pipeline inspection company based in Corona, Calif.