Antwerp based underwater contractors Hydrex were called in to do a number of jobs, key amongst them being to ensure that different thrusters units were secured in a dry, watertight environment so that inspections could be carried out on them.

The task given to Hydrex was to install a special ring-shaped mobdock, supplied by the crane vessel itself, onto the base of the thruster room on the bottom of the floater. This would create a watertight environment in the thrusters housing room above, enabling the surveyor to work.

Both starboard and port bow thrusters had to be checked out. In the construction of this vessel, an access shaft had to be built into the bow thrusters tunnels. However, in order for the surveyor to go into the tunnel, both ends had to be sealed with giant plugs that had been designed at the time the vessel was built. These were approximately 2.5m in diameter and consisted of the plug itself and a balance that extended out from the plug beyond the thrusters tunnel.

To accommodate this part of the plug, dredging was done at one end of the thrusters tunnel, using a Hydrex salvage/ sand pump where the water was not deep enough. The thrusters tunnel also had to be cleaned out as there was a great deal of accumulated fouling. The plug was then lowered into the water from the crane on a Hydrex workboat and eased into place underwater by divers.

The same procedure was repeated at the other end of the thrusters tunnel and all water was then pumped out. Further measures were taken to ensure there was no risk of water entering the dry space within the tunnel, after which the bow thrusters inspection was conducted.

The entire operation was repeated on the other bow thrusters. This constituted the major portion of Hydrex' work but the company was also required to open the sea chest for inspection, weld a number of anodes onto different parts of it, and blank off some overboard pipes. A class inspection of the sea chest was then carried out to the satisfaction of the inspector.

The net result of Hydrex' efforts saw the giant crane vessel pass its five year survey without setting foot in drydock

MJ Information No: 18324