Inspection of Laxiwa Hydropower Station Case Background

In May 2019, the surface of the water cushion pond of Laxiwa Hydropower Station was repaired.in order to understand the erosion situation of the water cushion pond after long-term flood discharge, check the surface repair area of the water cushion pond, and search for potential safety hazards, CHASING M2 was used to conduct surface inspection of the underwater part of the water cushion pond.

Date: June 8, 2020 - June 9, 2020

Location: Xiwa Town, Guide County, Hainan Tibetan Autonomous Prefecture, Qinghai Province.

Description of the site:

The outlet of Laxiwa Power station has been closed during the test. Due to the water-holding effect of the second dam, the overall water flow in the water cushion pond is gentle and basically in a state of static water, with visibility of about 0.5 meters and water depth of 20-30 meters.

Inspection preparation:

• 1. Personnel preparation
• 2. 5 Surveying And Mapping Engineers
• 3. 1 Underwater Robot Control Engineer  
• 4. Instruments And Equipment
• 5. CHASING M2 (optional 200WH battery. 200m cables)
• 6. External LED Lights
• 7. Phone and Pad (both run on Android)

Scheme of Inspection

• 1. Mark a position every 5 meters on the two dams, and mark 8 positions at 5 meters, 15 meters,25 meters, 35 meters, 45 meters, 55 meters, 65 meters and 75 meters respectively as the starting point of the 8 test sections. And named one, two, three... Eight.

• 2. Starting from the detection section of 5 meters, each section begins the recording time, and after the detection recording is completed, the recording is finished and the recording time is recorded, that is, the detection video of this section is recorded.
• 3. M2 dive to the position of each section, which is the lens in the second dam descend and start to record video, descend to 2 began after crossing the water cushion pond dam bottom until the dam at the bottom of the platform and the water cushion pond bottom junction, which is to rise and vertical to the overflow tank top position detection of dam body platform, the entire section length is about 200 meters.

• 4. Submit underwater video images of a total of 8 detection sections.

Inspection Process of Water Cushion Pond

• 1. Connecting Process of M2. In order to ensure run time,M2 was equipped with an external LED light during this work, and the battery was removed and replaced with 200WH.

• 2. Mark the starting and inspection position of the second dam according to the plan, and prepare for operation.

• 3. Execute inspection scheme (CHASING M2 has 8 Vectored Thruster layout which allows OMNI movement in all directions, with this, it can accurately realize high-definition shooting at any Angle without missing details)

Inspection & Analysis

• A. The images taken by M2 show that a small amount of dents were repaired at the bottom of the water cushion pond, a small amount of shallow scouring, cracks, reinforcement and a small amount of silt and debris were deposited on the bottom and side walls of the water cushion pond, without obvious quality defects.

M2 captured the image of the dents left behind in the repair area of the water cushion Pond platform.

Erosion image of expansion joints in the repair area of water cushion pond platform.

M2 captured the steel head reserved for the construction of the water cushion pond platform.

Reinforcement bars not cleaned after the surface construction of the water cushion pond platform.

There is a small amount of silt on the surface of the repaired area.

• B. A small amount of silt deposition on the floor and side walls of the water cushion pond, and the side walls were scour without obvious defects.

There is a small amount of silt on the floor and side walls of the water cushion pond, and scour on the walls.

• C. A small amount of shallow concrete surface was scour off the vertical plane of the platform below the dam, and there was no obvious problem.

Vertical scour image of the platform below the dam.

• D. M2 shows that there are a few cracks in the vertical concrete of the second dam, The cracks are located at a depth of -6.8 meters in the fifth detection section.

Vertical scour image of the platform below the dam.

• E. M2 shows that there are a few cracks in the vertical concrete of the gap between the dam platform and the water cushion pond platform, The cracks are located at a depth of -23.2 meters in the seventh detection section.

Image of scour gap between the dam platform and the water cushion pond platform.

Inspection Conclusion

CHASING M2 successfully completed the recording task of underwater image in the repair area of the water cushion pond, including detecting the quality defects in the repair area of the floor of the water cushion pond and close-up video of the area with exposed reinforcement and flushing pits.

The underwater video image analysis results are basically consistent with the underwater topographic map and underwater multi-beam sonar scanning image results, which can determine the current specific situation of the water cushion pond, and make relevant decisions - the shallow surface scouring area at the bottom of the water cushion pond can be temporarily not repaired, and some serious erosion gaps can be repaired in appropriate time.

It is suggested that the underwater quality of the water cushion pond should be tested regularly to grasp the scouring situation of the water cushion pond under the high-speed water flow, and the defect repair should be arrange a period according to the specific situation of the water damage.

Inspection Summary

1. Traditional inspection is mainly conducted by divers or large ROV-leased underwater robots. Manual patrol inspection has great safety hazards and large ROV-leased costs. M2 is a light industrial small size underwater robot, safe, economical and easy to operate, improving work efficiency and safety.

2. CHASING M2 ROV is equipped with 4K anti-shaking lens to provide clear image data for underwater patrol and assist the accurate analysis of the results of topographic sonar mapping.

3. When users observe a section of the rock mass, they need to keep the machine moving smoothly from left to right or at ±90°. M2 has 8 Vectored thrusters layout which allows OMNI movement in all directions.

4. CHASING M2 is a platform design and supports multiple mounts. External LED lights and laser calipers are mounted during the dam inspection, which significantly improves the operating efficiency.

5. CHASING M2 can be equipped with 200WH battery and support battery removal and replacement to extend operating time.