By Robert Romocki
The 285-foot-high Lower Baker Dam, which creates Lake Shannon on the Baker River in northwest Washington, was completed in 1925. The dam was built by Puget Sound Power & Light Company (now Puget Sound Energy or PSE) to provide electricity for the cement industry in the town of Concrete, Wash. At that time, the concrete arch dam impounded water for a powerhouse with a generating capacity of about 80 MW.
|This crack in the concrete apron at the Lower Baker project was discovered using a remotely operated vehicle.|
In 2013, we added a 30-MW powerhouse to contain Unit 4. Design-build delivery of this new powerhouse was spawned by a need to improve flow releases for better movement of migratory salmon.1 Lower Baker Dam and hydro plant is part of the combined Baker River facilities, along with the 90-MW Upper Baker hydro project and dam, which were completed in 1959.
As part of PSE’s dam safety program, we conduct periodic inspections of critical components to ensure they are functioning as designed and to intercept any developing problems before they become a major issue affecting the integrity of the structure. Typically, these inspections occur on a five-year basis unless there is an indication that a problem exists. The previous detailed inspection of the Lower Baker intake structure had been completed in 2009, so by 2014 it was time to conduct another close examination of the structure.
In the past, the complete dewatering of the structure was followed by a visual inspection of the intake tower, spillway gates and trashracks, conducted by a climbing team. Obviously, significant measures need to be taken to allow these personnel to safely inspect the facility. In addition, the previous inspection revealed that leakage from the intake gates limited visibility for the climbing team.
Remotely operated vehicles (ROV) offer benefits in terms of:
- Adaptability (quick response time, limited labor required and access to remote locations);
- Documentation (record of the complete inspection, improved imaging and GPS locations); and
- Cost (labor, mobilization, setup and investigation).
|During the August inspection, a small debris pile was located in the bottom of the intake slot.|
For these reasons, PSE decided that for this round of inspections they would use an ROV to evaluate the condition of the concrete intake, Gates 1 and 2 and the trashracks at the Lower Baker head gate building, without dewatering the unit.
Recent advances in ROV technology would also allow the use of computer imaging techniques in the event that visibility from the cameras was not clear enough for the inspection.
Continuing improvements in the technology make ROVs valuable for nondestructive testing and sample collection as well.
The Lower Baker intake structure consists of an upper vertical intake (105 feet in elevation), a short horizontal shaft (where the intake gates are located), and a lower vertical shaft (148 feet in elevation) that connects to the main pressure tunnel. Four vertical, 4-foot-diameter air duct shafts extend down from the gate house to the ceiling of the horizontal shaft just downstream from the intake gates. Access to the intake areas to be inspected is either through one of the air duct shafts or through the gate slot opening that allows the intake gates to be removed.
Performing the work
We hired Global Diving & Salvage Inc., based in Seattle, to perform the evaluation.2 Global Diving & Salvage personnel inspected the vertical section of the penstock and the intake gates with a submersible ROV in August and September 2014. They used a Pro 3 GTO XT ROV system supplied by VideoRay, with a crash frame. One reason this ROV was chosen was that it fit into the access points (air duct vent or gate slot opening) in the gate house. The inspections were recorded on DVDs.
During the August inspection, the ROV was lowered through the gate slot opening and the gates were observed open and closed. Points of interest were noted and logged, and snapshot locations were noted on the drawings, along with the time of recording on the DVD. Particular points of interest noted during the August inspection were:
- Small debris piles in the bottom of the intake slot;
- A deformed seal/gasket on the penstock fill valves above Gate 1;
- Several concrete spalls at construction joints;
- Cracks (including a crack in the floor of the intake structure from trashrack to gate slot);
- Some “rust nodules” and minor paint failures on the intake gates; and
- Two deficiencies with the Intake Gate 2 stem guides (one stem guide missing and a second stem guide missing a bolt with a fractured base plate).
|Watch our exclusive video on how drones can aid dam inspections. http://bit.ly/2cwK1DO|
Comparison of results of previous inspections with the results of this ROV inspection of the vertical shaft identified that the deteriorating concrete patches, gaps at construction joints, concrete spalling at construction joints, and cracking had not significantly changed and therefore only continued monitoring will be required. Repair/replacement for the stem guides on Intake Gate 2 may have an impact on proper seating of the intake gates, resulting in a better seal, so PSE engineers are developing a detailed repair plan for this work.
The second inspection, in September, was required to provide an ROV capable of diving to the bottom of the vertical section of the penstock. The original ROV used did not have enough power to overcome the buoyancy of the umbilical/tether cord that provides the power and video feeds for the vehicle at the greater depth below the intake horizontal section. To inspect the lower 148 feet of the Lower Baker intake shaft, a larger ROV that could still fit within the access points was required. For this inspection, the air shaft was utilized for ROV entry.
In the future, a Falcon ROV supplied by Saab Seaeye will be used to get the benefit of sonar navigation and extra power to pull the tether around bends.
Since this inspection, PSE has completed ROV inspections of the dam upstream face for both the Upper and Lower Baker sites.
Cost for this type of work will vary based on the size of the ROV and the detail required on the inspection. Computer imaging (using BlueView software from Teledyne) and processing also add cost but can provide detailed information and allow precise comparison of features over time. The use of the ROV in this case also allowed the inspection without dewatering the power tunnel, which saves time and cost in completing the inspection.
|The remotely operated vehicle was lowered for inspection via the gate slot.|
Through this and other diver underwater inspections, PSE has discovered several important keys to success when using ROVs at dams and hydropower facilities. These are:
- Know the capabilities and limitations of the ROV
- Know where the ROV is located at all times
- Use a drawing to record findings during the inspection
- Identify findings verbally and document where in the DVD recording the item of interest is, to prevent looking through endless hours of recordings
- Include measurements, orientation and other important descriptions on the recordings and in your notes
- Be aware of features that may entangle the ROV umbilical cord that will require a diver to free the unit.
1Nigus, Monty, et al, “No Ordinary Fish Tale: Design-Build of a New Powerhouse for Lower Baker,” Hydro Review, Vol. 33, No. 7, September 2014,.
2Langen, Mike, and Jeff Martin, “Renovating a Landmark: Updating the Outlet Works at Cheesman Dam,” Hydro Review, Vol. 30, No. 4, June 2011,.
Rob Romocki is chief dam safety engineer with Puget Sound Energy.