Installation of 250T Bulkhead at 50 Meters Depth and Rehabilitation of Hydro Power dam Intake

DCN Diving was awarded by SHGC with an EPIC (Engineering, Procurement, Installation, and Commissioning) Project in North-Africa for the re-commissioning of a water intake unit in a hydro power
generation dam. Due to a lack of power in this region it was of major importance that the re-commissioning of this intake, was carried out in a timely manner.

The intake has two major gates, which is a control gate (controls the water flow) and a maintenance gate (to perform maintenance). Due to an unfortunate event where the maintenance gate fell out of the gantry crane, it impacted with the top sealing area of the maintenance gate and caused the lintel beam be ripped
out of the concrete.

In order to rehabilitate the intake a method was designed to de-water the intake and carry out work in the dry. The concept was to place a steel bulkhead in front of the intake, seal it by de-activating the hydraulic wheels, closing the valves and pumping water out from the intake. Once the intake area is dry, rehabilitation is possible. Final repair method was to install a lintel beam and grout it into the old concrete.

DCN Diving designed a steel bulkhead of 18 meters high and 13 meters wide, which has 4 supporting wheels, 7 hydraulically operated valves and is deployed by two winches in front of the intake.

The bulkhead was manufactured in the Netherlands. It consists of 18 sections and has a total dry weight of 260.000 kg and 95.000 kg when it is completely submerged due integrated water-tight compartments. The bulkhead underwent a full Factory Acceptance Test (FAT), which included dimensional check (after assembly), NDT (on the weld connections), pressure testing (on the watertight compartments), compression testing of the D-Fenders.

Prior to operations, a HAZID Meeting was organized with the Client to assess all the operations and to ensure all risks were mitigated and that contingencies were in place.

After completion and approval of the FAT by the client and the technical consultant from Lahmeyer the individual bulkhead sections were transported to East Africa in 40 ft. open top containers.

At the project site the bulkhead sections were prepared by attaching rubber gaskets and d-fenders.

Both winch frames and winches were installed on the dam crest. Once both winches were installed, the sheave blocks were connected to the winch wires.

In order to assemble the bulkhead and to keep the bulkhead section rows in
the correct angle, two guide frames were installed as per design. Installation of the guide frames was carried out by drilling holes with a hydraulic core drill and install the anchors. For the installation of the anchors, injection mortar was utilized. After the injection mortar was cured according to the supplier’s
specifications, the guide frames were ‘rigged up’ in such a way it was lifted in
the correct angle, which made installation efficient.

Once preparations on the single bulkhead sections were completed, the sections were individually transported to the dam crest. Once arrived on the dam crest, the two individual sections were hinged together. Vertical bolts and hinge pins were installed.

After all the preparations were completed (including installing the hydraulic power packs and functiontesting the supporting wheels), the first section-row was launched and attached to the sheave blocks shackles. After successful completion, the crane’s rigging was detached from the bulkhead.

The four next row sections were without any valves or supporting wheels and were deployed and bolted to the section(s) below. With every lift, it was important that the pre-deployment bulkhead checklists were completed in order to ensure all the necessary bolts, gaskets and plugs were installed.

Three row sections had hydraulically operated valves (6 in total). Once all the hydraulic connections were tested, the bulkhead row sections were deployed with the valves in ‘open’ position.

The baskets (as displayed in photo 13) were installed to prevent any debris blocking the valves during closing them. It also prevents entrapment of the diver in the event of a pressure difference between up- and downstream of the installed bulkhead in front of the intake.

The last section row consisted of the top two supporting wheels and a top valve. The top valve is utilized to allow the air pocket to escape from the top. Once the bulkhead is just below the surface, venting of the bulkhead is necessary, otherwise a big air pocket stays entrapped at the top end of the bulkhead which may result in potential risks for safe deployment and the strength of the winches and frames.

On the top of the bulkhead the manifold of the hydraulic valves were installed and all the hydraulic hoses were neatly bundled in order to prevent entanglement during deployment. All the hydraulic hoses (from
the valves and supporting wheels) ran at one side to the top of the dam crest, where they were properly coiled.

After the full bulkhead assembly was completed and all the hydraulic hoses were properly connected, the bulkhead was deployed just below the surface. Afterwards both guide frames were removed.

Prior to full deployment of the bulkhead a flow-check was conducted in order to ensure the flow in front of the intake was within the approved acceptance criteria (<0.5 m/s). After the flow check was deemed approved, the deployment was appropriate to start. Two measuring tapes were installed
on both east- and westside of the bulkhead. These measuring tapes
were utilized to determine if the bulkhead was deployed at the correct
position. At approximately every 5 meters a dive survey was conducted in order to ensure if there weren’t any irregularities and what the approximate distance was from the inner d-fender till the trash screen of the intake. Once the bulkhead was in the correct position, the supporting wheels were deactivated’ and all the valves were closed simultaneously. After that the
intake was dewatered due to a small leak underneath the intake.

After cleaning of the intake, preparations could be made for installation of the lintel beam. Prior to entering the intake a gas analysis was taken in order to
ensure no hazardous gasses were present in the area. In order to reach the repair area a scaffolding was installed inside the intake. Once the work platform was ready, the surface of the repair area was cleaned with HP waterjet and afterwards chipping of the old concrete could start. The old concrete was removed in order to reveal the old anchors and to properly install the new lintel beam.

After completion of the removal of the concrete and further cleaning with
HP waterjet, the Lintel Beam could be initially fixed. Initial fixation was carried
out by tack welding the lintel beam at lateral positions and to several revealed old anchors.

After the initial fixation, a dry-test was conducted. This was carried out by lowering the maintenance gate in front of the lintel beam sealing area. After positioning of the maintenance gate, the sealing area was checked by the client and consultant and deemed approved. After the approval of the dry-test, final
welding of the lateral positions and fixing of new anchors into the concrete was
completed. Last activity prior to grouting was fixing of the formwork. Once this was completed the grout pump and hoses were set-up and a test was conducted with water. The grout pump was installed on top of the dam crest. The distance to bridge between the grout pump and inlet of the formwork was approximately 70 meters, therefore it was important that the configuration was properly tested prior to grouting operations.

Grouting of the lintel beam was conducted in the early hours due to temperature build up at the project location later in the day (temperatures can raise up until 50 degrees celsius).

The recommended curing time before removing the formwork (as per supplier recommendation) is 48hrs. In the meantime grout samples were tested and the results were deemed approved to conduct the final wet test (lowering of the maintenance gate and open the valves). Once the formwork was removed the state of the grout looked perfect, no surface cracks were visible. After full completion of the lintel beam installation, all remaining equipment was removed from the intake.

The wet test was conducted by fully lowering the maintenance gate and opening the 6 front valves of the bulkhead. Once this was carried out, a dive intervention was conducted in order to open the top valve (to relieve the air pocket from top side bulkhead) and once the wheels were activated, a dive survey was conducted in order to verify if the wheels were in proper ‘working position’ and that the bulkhead was away from the wall. When this was the case the bulkhead was fully recovered to just below the surface. Once this was done, the guide frames were re-installed (repositioning of the guide frames were necessary due to changing of water level in the reservoir). Remaining activity was to dismantle and properly store the bulkhead and remove all remaining equipment.

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