This enables their specialists to do repairs or even change thrusters whilst a vessel is carrying out port activities such as loading or unloading, without all the added time and problems connected to going into dry dock.

The first mobdocks for thruster tunnels were initially large, specially shaped, metal structures that fitted to the curved hull around the thruster tunnel. In many cases, this is now no longer needed. Hydrex has broken new ground in the last year and come up with a 'flexible mobdock' allowing diver-technicians to work within the thruster tunnel once the water has been removed. The cost of producing heavy metal structures is usually no longer necessary, which in turn means time saved on the repairs.

Rene Huybreghts, general manager of Hydrex said, 'The flexible mobdocks are the latest result of a fast service we have been looking to achieve for years. We have already done a number of jobs using them last year and now they are into full operation with several jobs done this month. They open up a whole new dimension to provide better service in the field of underwater ship repairs.'

One of the jobs, done in Portsmouth, UK, was the removal and replacement of a bow thruster gearbox. A flexible mobdock was installed and all water was removed from the tunnel. In this 'dry environment' underwater, the propeller blades were first removed from the hub.

The gearbox and electromotor were then dismounted and disconnected from the structure holding it in place. The thruster tunnel was flooded with water and the thruster was moved to the end of the tunnel and hoisted to the surface, along with the propeller blades, where it was sent for repairs.

A blind flange had been installed within the tunnel where the thruster had been fitted, which then allowed the vessel to sail on schedule. Several weeks later the gearbox had been renovated and was ready for installation. It was taken into the thruster tunnel, along with the four detached propeller blades.

The propeller blades were fitted with new seals, the gearbox refilled with oil and then air vented on the propeller hub, monitored by the diver. After completing all mechanical and electrical connections the bow thruster was tested and found working perfectly.

MJInformation No: 21955