As part of its regular servicing schedule, the Saipem 7000 needed to have work carried out on one of its thrusters and to replace or overhaul a large number of valves as well as renewal of cathodic protection anodes.

Antwerp based Hydrex was called in to do a number of jobs, key amongst them being to ensure one of the thruster units was secured in a dry, watertight and atmospheric environment so that inspections and maintenance could be carried out.

The Saipem 7000 has retractable thrusters located in its floaters.

When the vessel is in deep water the retractable thrusters are extended from beneath the floater, enabling the vessel to correct its position with Dynamic Positioning (DP).

In shallow water such as in a port, the thruster is retracted into the thruster container and housed within a compartment within the floater itself. Using special circular mobdocks the compartment can be sealed off from beneath the floater, water evacuated from the thruster garage and then inspections and maintenance can take place on the thruster, within the 'dry' thruster container itself.

Hydrex installed a special ring-shaped mobdock designed especially for the Saipem 7000.

Using the crane on the Hydrex workboat, the mobdock is lowered underneath the floater. From the top of the floater to the opening of the thruster container underwater is over 10m, so once the crane had lowered the mobdock to the depth of the thruster entrance (directly underneath the floater), divers used a block and tackle system installed within the thruster container to pull the mobdock into place to create a watertight environment in the thruster housing room above it.

The mobdock is around 10m in diameter with a number of separated ballast/buoyancy tanks to control weight by manoeuvring under water. It has its own pump system to evacuate the water but due to this being rather slow, taking some 12 hours to pump water out of the tunnel, a Hydrex pump unit was installed, which enabled the job to be done in just over one hour.

Once the mobdock was fully secured, the thruster technicians were then able to access the now dry thruster container from inside the floater and do their work. In addition to this, over 30 overboards and intakes were blanked in order for valves to be overhauled or replaced, including opening each of the eight sea chest grids in order to access and blank off pipes within each of them whist valves were overhauled or replaced. All these tasks were completed with the minimum of delay.

MJ Information No: 21707