Thank you to our Marine Operations deparment for sharing this great summary about the Spirit of Tasmania dry docking procedure.  Enjoy...

Spirit of Tasmania vessels dry dock at Captain Cook graving dock, Garden Island, Sydney.

Dry docks are used to construct, maintain and repair ships. A dry dock is a basin that is flooded to allow a ship to be floated in and then drained of water to allow the ship to come to rest on wooden blocks.

Before the ship arrives, a docking plan is sent to the dry dock Dockmaster (who is in charge of the docking and undocking processes). This plan shows the recommended locations of the wooden support blocks. Keel blocks are set along the keel line (centre line) of the vessel and provide the main support when the vessel is docked. Bilge blocks are exactly the same as keel blocks, but they are placed either side of the keel blocks to give more support and to stop the ship from tipping sideways. It is important to put the blocks in the right locations or else they could damage the hull or be in the way of items like sensors and tank drains.

When the ship arrives, it is carefully moved into position using markers on the dock side. When the ship is in position, the ships mooring lines are put out and the engines are shut down. The caisson, which is basically a floating wedge-shaped door, is moved into position at the dock entrance. Once there, the caisson is allowed to fill with water and slowly sink in place. When the caisson is down, the dry dock is sealed from the harbour and large pumps can then pump water out from the dock.

As the water is drained out, the Dockmaster closely monitors the activity and keeps the ship aligned by issuing orders to gently move the vessel using the four mooring lines.

After some hours the vessel is resting on the support blocks. When all is checked the remaining water is pumped out quickly until the dock floor is dry. The mooring lines are brought back aboard the ship and work can now commence.

Water hoses are connected to the ship's systems to provide cooling water for the running generators, domestic and technical water use, and to keep the fire main pressurised for emergencies. Cooling water outlets, normally open to the sea, are also connected up to hoses so staff and equipment working on the dock floor don't get soaked and painting can occur. The sewage discharge outlets are also connected to hoses so sewage water can be disposed of.

Essentially, the work done on Spirit of Tasmania falls into two groups. Firstly, there is maintenance and inspection needed on equipment and other items that cannot be done when the vessel is in the water. Painting the underwater hull area or removing propeller blades for inspection and overhaul are examples of these jobs. The second group is work that cannot be done or is difficult to do when the ship is on the run; while at dock there may be a couple of straight weeks where the work can progress uninterrupted. Major maintenance of the main engines and re-tiling of the main galley are examples of this type of work. This sounds relatively simple but a terrific amount of planning and preparation is needed months before the vessel enters dry dock.

At the end of the docking period everything is checked and double checked. Equipment, tools and rubbish are removed from the dock floor and preparations are made to flood the dock. After a final inspection water is slowly allowed to re-enter the dock. Just before the vessel is lifted off the blocks by the water, the water is stopped. This is called check height.

Ship's staff and dockyard staff do another set of inspections and checks to ensure that the ship is watertight and ready to float. At this time, the hoses are disconnected and the ship becomes its normal self. Flooding recommences until the water reaches harbour height. The caisson is pressurised with air, floats up and is moved away from the entrance. Main engines, rudders, and side thrusters are started up and checked, and then the Spirit slowly extricates herself from the dry dock. Once she is clear and starts on her way back to her normal routine, the Master gives three loud blasts on the ship's horn as a farewell until next time.