‘Dry Docking of Large Vessels’ – An Insight

(Image Courtesy: Youtube)

Dry-docking is when a vessel is taken to the service yard and brought to dry land so that submerged portions of the hull can be cleaned and inspected. This work is both preventative as well as a regulatory requirement within the industry. As per the industry practices, ships are dry docked once in 4 to 5 years. Once the vessels are put back into service, they are more eco-friendly and fuel-efficient.

Arrival and Entering the Dry-dock

After completing all necessary preparatory work, the vessel arrives at the shipyard. On completion of port formalities, the vessel proceeds to the dry-dock with assistance from tug boats and shipyard personnel. Prior to this, the blocks are arranged by the shipyards as per each vessel’s specific docking plan.

The water is drained from the dry-dock (generally overnight) over the course of about eight to ten hours, depending on the size of the vessel. Before the vessel is rested on the block, specialised dive teams ensure the block structures are in the correct location.

Docking Vitals

  • The time from when “Stern touches the blocks” to when “full ship is on the blocks” is the critical period.
  • During the critical period, the vessel’s GM reduces. This is because vessel’s Gravitational centre ‘G’ moves upwards when Stern touches the blocks.
  • It is required and a good practice to have the least trim while docking so that the critical time is minimal.

Safe Stability Conditions

While docking the ship, it is slightly trimmed by the stern so that the stern block touches the stern frame first. This is necessary to steady the ship by locating the stern on the block. The stern side is chosen since the stern frame is very strong and can resist the strong single reaction occurring at the instant the stern block touches the stern frame at the coffin plate or bottom of the stern frame.

Almost all modern ships have aft machinery space and when floating in light condition, they trim very badly by stern. This heavy stern trim has to be corrected by filling the forepeak tank sufficiently to result in a slight trim by stern. Hence it is not correct to state that the ship should be completely empty to make it light. This was true for older ships which had mid ship machinery space and in light condition they floated on an even keel.

Hence all modern ships with aft machinery have their forepeak tanks filled sufficiently to result in slight stern trim. In some cases, the forward ballast tanks may also be filled up to correct the trim to give slight aft trim.

Requirements for Docking

Mooring Bollards – A large ship needs a strong bollard setup to tie up to, in order to remain safely docked during winds that may be encountered in each port. For long ships, this may include the addition of ‘dolphins’ – like a small dock with bollards on, extending further out than the main dock.

Harbour Depth – Ships that are particularly wide, or extend further than normal down into the water, may require the harbour to be dredged (the removal of sand, mud and rocks from the bottom of the harbour). This can be used to make the harbour deeper, allowing larger ships to dock.

Dock Master’s Role

The dock master has a huge responsibility of calculating the stresses on the dock as well as the ship’s structure. Any miscalculation can lead to serious accidents resulting in huge damages. The dock master is trained for block arrangement and stability during dry docking.

From the docking plan, dock master should have information on –  Hull structure so that he can arrange the blocks to support the ship’s hull; locations of transducers for log and echo sounders so that these do not come beneath the blocks; location of sea chests and drain plugs for the same reason.

Based on the docking plan provided by the ship, the dock master prepares his own docking plan for the ship.

(Image Courtey: workbargebrokers.com)

Vessel on the Blocks

Before the dock master starts to remove dock water, a diver will make an underwater inspection. The diver will ensure that the echo sounder and log sensors are clear and not sitting under the blocks. He will also ensure physically that the vessel’s centerline is in line with the blocks.

After the diver has made his inspection, the dock master will start pumping out dock water. Once on shore power, the dock master will continue to dry the dock. Once the dock is dry and the ship is sitting on the blocks, deballasting is done by gravity.

Best Practices

The key to perfect docking is practice. One has to understand how the vessel is going to react to the helm, the wind and any currents. There are also many different docking situations, each of requires different handling. As ships keep getting larger, the infrastructure needed to support their operation also needs to grow. Continued cooperation between shipping lines and the ports they visit will help to ensure a successful future for both parties.

(References: DiESELSHiP, TEEKAY, MySeaTime)

Sea News, April 2