Under inspection18 November 2004
Application of an ROV helped reduce operating cost and minimise downtime at dams operated by Chelan County Public Utility District, explains Bruce Caldwell
OVER the past forty years Remote Operated Vehicles (ROV) have become a well-proven part of the basic toolkits for getting work done underwater in a wide variety of industrial, scientific, military and law enforcement activities. In many cases the impetus to adopt and evolve this technology was to reduce the risks and costs of using divers to perform these tasks. In other cases, the ROV is asked to do things that divers were never able to do. The arena of hydroelectric dams is not new to the use of ROVs, but the realisation of their full potential is still in its infancy compared to many other industries. This article describes an innovative application of the ROV’s capabilities to dramatically reduce operating costs and minimise downtimes for one public utility district in operating its dams.
Chelan County Public Utility District (Chelan) operates two hydroelectric dams on the Columbia river in mid-state Washington, US. One of these, Rocky Reach dam, has 11 vertical-axis Kaplan turbines capable of generating over 1200MW. The first seven units are rated at 140,000hp and have six blades. The other four units have recently been upgraded from vertical propeller type turbines to six-bladed Kaplans and are now rated at 177,000hp. The other is the Rock Island dam, which has 18 Bulb-type turbines of various capacities that spin on a horizontal axis and are capable of generating a combined peak of 660MW. An article in the June 2002 issue of International Water Power & Dam Construction – ‘Large, Low Head Hydro Rehab’ – describes a major turbine refurbishment programme at Rocky Reach and gives further details of the significant improvements made to both the operating efficiency of the generating units and to the migrating fish survivability aspects of the new turbine designs.
Cavitation is a destructive process that can significantly shorten the life of a turbine runner and will reduce the hydrodynamic
efficiency of the blades. It happens when the high-speed movement of water around the rotating machinery causes a localised and very intense low-pressure region that causes the water to instantly
vaporise and form a gas bubble. When these bubbles violently
collapse, the shock wave has the tendency to tear out a chunk of the nearby material. This usually happens on a fairly small scale to begin with and the accumulated effects have the appearance of roughened patches in the otherwise polished blade surface (see Figure 1). The repair consists of removing the cavitation-damaged material and then welding on additional metal to replace it. The surface is then ground back to the original contour. If the damage is allowed to continue and if the runners are driven particularly hard, cavitation pitting can produce sizable voids in the blades. The location of the damage tends to be on the downstream side of the blades and most commonly, very close to the blade tips. Although cavitation is the most common damage seen, the inspections also look for any other signs of trouble, such as evidence of blade strikes to the liner.
Mitigating the effects of cavitation pitting of hydroelectric turbine runner blades requires regular inspections. This is traditionally accomplished by dewatering an entire generating unit to below the turbine. Typically this can take a unit offline for two to four days significantly affecting availability and revenues. Chelan, working closely with Deep Ocean Engineering, Inc. (DOE), has led the way to achieving substantial savings through the utilisation of ROV technology to perform the inspections without dewatering and reduce unit outages to less than four hours.
When Chelan first contacted DOE to discuss the feasibility of performing the desired inspections with an ROV, it was determined that the application called for one of the more powerful models in order to achieve the long penetrations involved in reaching the Kaplan turbines from downstream. The dimensional constraints imposed by the Bulb-type turbine inspections of the Rock Island dam further limited the choices to an HD2+2. This vehicle, in its standard form has four thrusters for forward motion, a lateral thruster and a single vertical thruster. With the long, upward excursion needed to reach the Kaplan turbine blades in mind, an extra vertical thruster was added in the position normally occupied by the camera and its tilt mechanism. Because of the need to inspect all of the areas in the confined spaces around the runners, a special pan-and-tilt mechanism was installed forward of the normal camera position. Two additional cameras were added to complete the suite of video capabilities. A pair of lasers mounted parallel to the primary camera and spaced exactly ten centimeters apart gave the ability to scale an object in view at various distances.
A trailer was used to house and transport all of the ROV system including the vehicle, the control consoles, the umbilical, transformer, tools, safety equipment and spares. The trailer is driven to the work site; the vehicle slides out of the back door onto a wheeled cart; the main power lead is plugged into an outlet on the dam and the system is ready for its pre-dive checkout in preparation for the dive (figure 2). The control consoles are secured to a bench in their operating positions with all the connections already in place. The trailer is thus rapidly converted from a storage and transport unit to become the ROV control center (figure 5). Heating and air conditioning are built-in for year-round operations.
This ROV system was delivered in 1998 and was successful in completing the desired inspections right from the initial deployment. The main tasks that the ROV was used for were cavitation inspections, trash rack inspections and to survey erosion in the spillway at Rocky Reach. The savings realised in these early operations paid back the acquisition costs within the first year. In 2000 and 2001 the savings were over three times the investment to date, which included another round of upgrades. Even with these exemplary results, there was room for further operational improvements that would enhance the return on investment even more.
The method devised for inspecting the Kaplan turbines of the Rocky Reach dam from downstream made use of the one of the four stop logs (that would normally be used to close off the draft tube outlet) as a platform to carry the ROV down to the opening of the draft tube (figure 7). A simple aluminium stage was fabricated to give the ROV a measure of protection from the turbulence of the tailrace during the descent of the launch phase and the ascent of the recovery phase. This stage also provides a fairlead for guiding the ROV umbilical from the long vertical drop into the draft tube. It is secured to the top of a stop log. Since the stop logs are designed to stack two high and two wide for each generating unit, placing the ROV stage on top of the lower stop log positions the jumping off point of the vehicle at near mid-height in the draft tube opening. The ROV trailer is set up about thirty feet above the water on the tailrace deck next to the gantry crane used to raise and lower the stop logs. Once the stop log reaches the bottom of its travel, the vehicle is then flown off of the stage and down to the bottom of the draft tube floor. From there the pilot sets the vehicle down on the bottom and uses the sonar to confirm the location and heading, and proceeds up the draft tube keeping in contact with the floor. Following the floor the pilot will see a steady increase in the depth readings, and then the depth will start to decrease as the tunnel gradually transitions from horizontal to vertical. Once the vehicle reaches the height of the runner, the inspection can be completed.
The method of inspecting the Bulb-type turbines at Rock Island dam differs fundamentally from the Kaplan by accessing the turbines from the upstream side through the head gate slots. The trailer is set up on the intake deck and the water level in the head gate slot is close enough to deck level to allow a very simple launch using a small mobile crane and a simple hook on the ROV’s lift point. Using the stop log downstream of the turbine stops the flow through the unit. The wicket gates are opened to their fullest extent and the turbine runner is ideally stopped in a position so that the gaps between the wicket gates and the gaps between the turbine blades line up. The ROV is piloted from the bottom of the head gate slot into the scroll case and through the wicket gates and turbine blades into the draft tube. Since the area of most interest on these horizontal axis turbines is also on the downstream side of each of the blade tips, the vehicle needs to pull enough umbilical down below the runner for manoeuvering around the full circumference of the runner and this is accomplished by driving some distance down the draft tube and doing a U-turn to come back to do the inspection.
The long horizontal penetration and vertical ascent up the draft tube for the Kaplan inspections were a challenge even with the added vertical thruster. Working inside a dam with significant vertical flows is an unusual set of conditions rarely encountered by ROVs in other environments. While all ROVs need to ascend and descend efficiently, pulling their umbilical upward while pushing their largest cross-section through the water against a downward flowing current is not what they are typically designed to do. The wicket gates are closed to minimise the flow, which is easily done from the control room, but they are often not able to stop the flow completely. In most cases, it was necessary to set head gates in order to reduce flow sufficiently for the inspection to be carried out. This adds considerably to the time that each unit is offline for inspection and to the labour costs of each cycle. If the ROV were able to perform the Kaplan inspections consistently using only the flow control available from the wicket gates alone, cost savings would improve significantly.
Another area where the capabilities of the HD2+2 were being stretched was in the amount of equipment that it was carrying, or the payload. With three cameras, a pan-and-tilt mechanism, sonar, lasers, a manipulator and the extra vertical thruster, it was necessary to provide added flotation. All of these externally mounted items added considerably to the drag through the water. The inherent stability of an ROV, which comes from having a high center of buoyancy and a low center of gravity, tends to suffer as the weight of the onboard equipment pushes the designed payload limits. Stability makes an ROV easier to pilot and greatly enhances an ROV’s efficiency as an inspection platform.
After several years of successful, cost-saving operations, Chelan and DOE entered into discussions about designing a custom ROV that would specifically address the enhancements that operational experience showed would improve the performance. DOE designed and manufactured a custom ROV for this application that has substantially improved performance for both types of turbine inspections (see Figures 8 and 9). It features four vectored thrusters for maneuvrability in the horizontal plane, and three vertical thrusters to overcome downward currents in the long upstream penetration of Rocky Reach’s Kaplan units. The hydrodynamics of the vehicle were emphasised throughout the design process, but in particular the drag experienced by ascending through a downward current was given special attention. A Smart Zoom Color Camera and an ROS Navigator low light camera are mounted on a Sidus Pan-and-Tilt.
This arrangement provided a greater range of camera views as compared to the previous arrangement. A rear-facing camera and another camera that can be positioned as needed provide a highly versatile set of tools for visualizing the condition of the turbine blades. Parallel lasers mounted (note the red dots on the cavitation pitted patch shown in Figure 1) on the Pan-and-Tilt provide the ability to accurately scale pits or other damage through the imagery provided by the cameras and recorded to VHS videotape. A Fiber Optic Gyro (FOG) is installed to give the pilot an accurate heading despite the adverse magnetic environment that prevents typical flux gate compasses from being any use at all. This heading data along with vehicle pitch and roll, and camera pan-and-tilt position, plus water temperature are displayed and recorded as a programmable overlay on the video with DOE’s On-Screen Display (OSD). An Imagenex 881A multi-frequency sonar for navigation and obstacle avoidance and a DOE Manipulator round out the list of standard accessories integrated on this vehicle.
The powerful vertical thruster array combined with the vehicle’s refined hydrodynamics have proven capable of performing routine inspections without the time consuming installation of head gates, even where the wicket gates allow substantial flow through the turbine. This has allowed the ROV crew to complete back-to-back unit inspections within the four-hour outage window originally intended for a single inspection. Overall savings are being realised from the lower cost of performing the inspections and the improved revenues from reducing the time the generating units are offline. In addition, the versatility of the ROV has enabled various other underwater tasks in and around the dam and lake areas that would otherwise have been done at higher cost, and at greater risk by divers.
Chelan has successfully taken a bold initiative to utilise an ROV in a previously untried application in the hydroelectric industry, and has proven that there are significant rewards to be realised by doing so. Using this approach for both Kaplan and bulb-type turbines should have great potential at other sites. There are, no doubt, many other potential tasks that can benefit from the advances made in ROV technologies and techniques from this and other industries.
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