Clients

Client: RWEnpower.

Location: Pembroke Power Station.

Description of work: Cooling Water Intake Isolation, Pembroke Power Station.

In the summer of 2009, Kaymac Marine & Civil Engineering Ltd were commissioned to carry out works to isolate the cooling water intakes of the redundant oil fired power station at Pennar Gut, Pembroke, to enable renovation and reuse of the intakes and forebay area for a new 2000MW combined cycle gas turbine (CCGT) power station at the site. The works were to be carried out in three phases; the dredging out of over 2200m³ of silt and debris from inside the CW intake chamber, the installation of a 3 tier propping system to brace the internal face of the coarse screen wall, and finally, the installation of steel panels to form a cofferdam across the full length of the intake structure.

The works commenced in August 2009 and the completed works handed over to the client in February 2010.

Intake structure prior to project commencement	Diver entering CW Chamber
Dredging Dredging of silt from the cooling water intake chamber had to be carried out to allow the installation of the lower levels of the propping system, and this was carried out using divers due to the lack of access into the chamber, preventing the use of heavy plant and machinery. Entry into the chamber was via stop log openings 900mm wide and 3000mm long in the deck slab above the chamber, and to accommodate safe access a man riding basket on a crane was used to lower and raise the diver to the water. Inside the chamber, the silt was found to be5m high in places, and hydraulic dredging pumps were used by the divers to remove the silt to a designated discharge point 150m away. The upper 3m layer of silt was soft and easily removed, while the lower layers had consolidated into clay like material, which had to be broken up using an air lance prior to pumping out. The seaward face of the coarse screen wall was also dredged to expose the concrete invert using a 6" hydraulic pump and HP water jetting system to allow later installation of the cofferdam system.
Diver preparing for water	Completed propping system
Propping System Installation With the chamber cleared of silt, the 3 tier propping system could now be installed. This would prevent the deflection of the coarse screen wall due to the water pressure acting upon the completed cofferdam once installed. The upper 2 tiers were fitted from a floating pontoon rising and falling with the tide inside the chamber, while the lower 3rd tier was installed below MLWS level by the dive team working in nil visibility. In total, 48 stools were fitted to the rear curved walls of the chamber, with 21 brackets fitted to the rear of the coarse screen wall. Between the stools and brackets, 80 Megashor props were lowered into location and bolted. The props were then finally jacked out against the coarse screen wall.
Cofferdam section prepared for positioning	Cofferdam construction underway
Cofferdam Installation The cofferdam consisted of 17 panels 10m high and varying in width from 3m to 3.7m and weighing in at between 8 & 9 tonnes. On top of each panel, an extension piece was fitted in order to raise the height of the dam by a further 2.5mm to above extreme high water levels to prevent storm surges and wave action from overtopping into the chamber. The main problems facing the cofferdam installation team were the overhang of the deck soffit and the silt which continually backfilled into the previously dredged trenches. Before fitting the panels, saddle brackets and stools were installed on the 16 piers in front of the coarse screen wall at 5 levels to allow the cofferdam to bear onto the internal propping system. These were installed above water from a specially designed man riding basket on a mobile crane, while the saddle brackets and stools below water were installed by the dive team. Starting at the centre, the panels were lowered into position using a 50t crawler crane. With the panel hanging vertically from the crane, a specially designed pulling beam was lowered to the diver, which was fitted into the coarse screen guides. Clamps were then fitted to the horizontal beams on the rear of the gate panel. Using a chain pull fitted between the pulling beam and clamps, the diver pulled the bottom of the panel in until contact was made on the saddle bracket/stool assembly. The panel was then lowered off until contact was made on the invert. A similar pulling beam and clamp arrangement was then fitted above water, and with the weight off the crane, the top of the panel was pulled in until contact was made on the top saddle bracket/stool assembly. The bolts securing the panel to the stool assemblies on the piers could then be installed, and the holding down bolts from the panel into the invert fitted.
Final panel installation	With the panel installation complete, the extension piece was fitted to the top of the gate. The next panel was lowered into position and pulled in onto the sealing face using 71 20mm bolts. Once all the panels were fitted, the final end pieces were installed to seal the intake chamber. Following tide cycle testing, the chamber was pumped out, any remaining silt washed out and 30t of debris removed to provide the client with a clean and dry working area for remedial works to the structure prior to connection to the new power station infrastructure.