The thruster itself could only be removed from the crane barge by dismantling the unit in a dry space and taking it out from beneath the barge, either underwater or in dry dock. Going to dry dock was not an option as the nearest suitable place was in South Africa and this would have taken the repairs outside of the time frame available. Also, whilst offshore, there are detailed safety and technical procedures that must be met in order to access the thruster from within the engine room, as the thruster was located outside the vessel under the water line.

Antwerp based Hydrex was called and proposed the construction of a large mobdock (mobile mini dry dock), along with all the auxiliaries needed for the work, that could be placed on the underside of the crane barge to cover the area where the retractable thruster was housed. This mobdock was one of the largest Hydrex has ever made, measuring some 9m by 6m by 2mand weighing over 25 tons.

It was built in Belgium under the supervision of Hydrex then transported to Gabon along with all other needed equipment to carry out the work. Everything was in place at the beginning of the 30 day period and an operations base was set up on a working pontoon next to the crane barge.

The thruster was first retracted into its housing within the main body of the offshore unit and the mobdock was brought into position covering the entire area under the hull of the crane barge where the retractable thruster was now housed. The mobdock was secured in place, made watertight and all water was then pumped out of it creating a dry space within the area that held the retractable thruster.

Now access could be gained to the thruster from within the crane barge. The two main access plates above the thruster in the engine room were removed and two hoists (already built into the thruster unit structure) held the thruster in place whilst the auxiliaries were disconnected and the drive and steering gears dismounted. The water sensitive parts of the thruster were sealed off and once all this was done the chamber housing the thruster unit was sealed off again and flooded.

At this point the mobdock was removed and thruster unit was lowered onto a specially designed support unit under the barge. After being safely secured onto the support unit, it was lifted onto the pontoon by the main crane of the barge. The thruster was then transported to shore where Hydrex engineers dismantled it under the supervision of two manufacturer’s representatives and carried out all the needed repairs.

After completion, the reverse procedure was followed and the thruster reinstalled into the crane barge. Commissioning and testing were carried out satisfactorily and Hydrex’s work was done with the offshore unit fully operational once again. All work was done within the time period available. For the owners, there was a major saving in time and money as the offshore unit did not have to move to drydock.

MJ Information No: 22612