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Inspection de barrages

Dam Survey:

The main goal is to survey the submerged concrete wall in order to detect holes or potential cracks, and also to carefully inspect the key elements of the dam, i.e. the sluice gates and water inlet.

The basic way to inspect these types of building consists of removing all the water from the dam and survey the concrete walls visually, but it rapidly appears to be more cost effective and ecological to do it without emptying, especially since ROV availability, providing that ROV position was available and recorded simultaneously.

The ROV positioning is a critical need because when a default in the concrete wall is encountered, it is essential to locate it in order to evaluate the impact on the whole building and to be able to survey this specific point several months or years after.

In 2003, PLSM decided to address this particular application using an AQUA-METRE R300 underwater positioning system configured as follow.

The basic system includes three main components

The MiniPointer R300 and its hydrophone attached to the ROV with perfectly know 3D offsets from the reference survey point (about 20 cm in front of the main camera),

– The Base is tilted backwards and hanging from a cable taut between the riversides, it is coupled with a radio-modem buoy (UHF 869MHz band), which allows control of the Base from the ROV & NAV control unit. The Base is immerged at about 5 metres depth, the radio-modem being enclosed in a sealed container on a buoy.

– The onshore NAV PC running AQUA-CAD and Intellicad softwares, connected to a PC USB Interface Unit that communicate with the Base and interfaces it with the computer (PC) as well as the video overlay.

The Nav PC sends a command to the Base to request an acoustic interrogation of the Pointer.

The Base executes the command, measures the distance and two angles (3D interferometric scheme) and sends the result back to the Nav PC through the radio datalink.

The Nav PC transforms the local coordinates to global coordinate system associated with the building, refreshes the Autocad document and video overlay.

Local to absolute coordinate system calibration:

The transformation from local acoustic to absolute cartographic coordinate system is defined by measuring the acoustic coordinates of two reference points named pivot and lever, the absolute coordinates of these two reference points being known and identified in the Autocad drawing.

Then, the calibration just consists of moving the ROV to the reference point and to pick the position in the Autocad drawing and wait for acoustic measurement completion. The reference point are often chosen along the seal between two concrete blocks which position is well known (critical vertical lines to be surveyed).

The calibration lasts about 20 minutes and is valid until the next Base translation.

Base on « T » frame

Inside the ROV&NAV room

–Accuracy (including calibration errors):

       - distance: +/-10 cm

       - angles: +/-0.2 degree

– Standard deviation:   - distance: +/- 2 cm

                               - angles: +/- 0.08 degree

– Range:  150 metres for acoustic, >1 km for the radio-modem,

– Measurement rate:  > one second per point

– Autonomy:    -30 hours for transponder,

                     - 5 days for the Base

              (sleeping mode during the night).

Result example:

The following results come from a dam survey that took place in the South of France.

This was a single arch type dam about 70 metres high and 300 metres long.

SASA Company was in charge of the ROV survey and an expert from EDF (Electricité De France, the French electric supplier), the customer, managed the survey.

The ROV type was a SUPER ACHILLE from COMEX, a 100 kg class ROV well suited for video inspection.

The lightweight AQUA-METRE system was first appreciate when starting the day with the Base set up 40 metres lower down…The survey lasted two full days, more than 30 vertical lines were inspected leading to more than 2000 3D coordinates recorded and displayed in the Intellicad drawing of the dam.

It is important to notice that the positioning system was set up and operated by a single technician during the whole survey.

Final CAD drawing issued from the dam survey

Dam survey using the Aqua-Metre R300 system demonstrated the benefit of accurate, lightweight and easy to set up system. Only one technician was necessary to operate it.