Figures 21 and 22 : Installation of the steel shafts using the floating crane ( sheerleg ) Taklift 7 of SMIT / Boskalis at the wet dock
After completion of steel shaft installation and confirmation of the required positional accuracy of the shafts on each caisson , the bottom 4m of the shafts was concreted as shown in the figure below . This concreting was achieved by means of 8 tremie pipes installed within the shaft cells at a height of 27m above the caisson top slab elevation . Concrete pumps with a boom length of 56m were used .
FINAL LAUNCHING AND SUBMERGING
Final launching and submerging operations included towing , ballasting and de-ballasting , motion analysis during installation and controlling stability during towing and installation .
The caisson ballasting system consisted of valves and pumps to fill clusters of cells to ballast and sink the caissons in a controlled manner onto the prepared gravel layer on the seabed . These ballasting operations were based on the following safe system of work :
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Valves were remotely operated from a support vessel ( DSV );
During ballasting operations no persons were allowed on the Caisson ;
The tanks could withstand the pressure of a complete 25m water column above if empty ;
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The ballast sequence was reversible ( up to a defined point ).
It was calculated that a bollard pull of approx . 120t for towage of each caisson was required , divided between two tugs to provide adequate manoeuvrability and steering .
The following marine equipment requirements were estimated :
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Towing Tug 1B 89 t BP AHT ( Anchor Handling Tug )
Towing Tug 2B 89 t BP AHT ( Anchor Handling Tug )
Trailing Tug 3B 50-70 t BP “ Open ” stern Trailing Tug 4B 50-70 t BP “ Open ” stern
DSV ( Diving Support Vessel ) Equipped with ROV ( Remote Operating Vessel )
When the weather forecast was favourable , the vessels proceeded to commence the tow-out operation .
Tugs were connected to the tow points and after the caissons disconnected from the mooring lines , they were towed to the installation site . Submerging operations included :
a ) Hook-up into pre-laid mooring system .
b ) Positioning of caisson at the installation site . The caissons were kept in position , by means of the tug ’ s winches .
c ) Lowering the caisson on the seabed .
Tugs were used to hold the caissons in position during ballasting down to the seabed . Just before touchdown , the caissons were accurately positioned within installation tolerances ( less than 200mm ).
The tugs were connected until the last ballast sequence was finalized and the correct position was confirmed by means of an ROV inspection . Ballasting of the caissons was controlled from the DSV support vessel .
3 / 2018