ships these days cost millions of dollars to build and then occasionally
like everything else on the planet they need maintenance - have you ever
wondered how it is done or more precisely where does that happen? that
happens in drydock where the ship is taken out of the water
to access those parts which are usually underwater. Drydock is a structured area
where in construction, repairs, and maintenance of merchant vessels and
boats are carried out. This unique construction or arrangement allows the
water to be filled up in an area also known as a lock, so that vessels can be
manoeuvred in and out of that area. As per safety of life at sea or SOLAS
requirements, all merchant vessels require a complete survey of the hull in
a dry dock twice within five year period and an intermediate survey within not
more than thirty six months. This includes maintenance of hull, propeller,
rudder etc and other parts which are immersed in water and are normally
inaccessible by staff when the ship is sailing
there are different types of drydock facilities available all over the world
out of which graving dock and floating dock are most commonly used. Graving dock
is normally constructed on land near a coastal waters with a rectangular solid
concrete construction with blocks walls and gates. The vessel is shifted inside
the dry dock and rested on the blocks. After the ship is in the required
position, the gate is closed and water is removed. A floating dock is in
the form of U structure which is mainly used in Salvage to carry ships
that have met with an accident and is damaged to an extent that has made them
unable to sail further to a coastal dock. However, now many regular seagoing
vessels small and mid-sized are also drydocking in a floating dock. Several
U type floating docks can be joined to carry a large vessel. Once the type of
dry dock is selected by the manager of the ship, the next process is to prepare
the ship to enter the drydock and then place the ship's keel carefully on the
blocks provided in the drydock floor
When the ship touches the blocks, there is a reaction at the point of contact
which raises the centre of the gravity G and reduces the metacentric height G M
hence adequate initial metacentric height is required to compensate the
same. While entering the dock the vessel needs to be upright which means that
there should be no port or starboard list. When the ship touches the blocks if
the point of contact of the ship and keel blocks is outside the centerline of
the vessel, it may force the vessel to tip over. A moderate trim aft is usually
kept when making the ship's keel sit on the keel block. As the water level in the
dock lowers, the slight trim allow ascending of stern and bow in tandem
rather than simultaneously which will reduce the load and pressure on a hull
and keel of the vessel. The floor of the dry dock is lined with keel blocks which
are also arranged such that they can bear the weight of the ship. The interval of
time from when the stern takes the blocks to the moment when the entire
ship's weight is borne by the block is called critical period. So what happens
during this critical period and Why? When the ship's Stern just touches the
keel blocks part of the ship's weight is being borne by the keel blocks. The
contact between the stern and the keel blocks creates a normal reaction or up
thrust. The magnitude of this upward normal reaction increases as the water
level in the dry dock reduces, hence it is very crucial to maintain sufficient
positive metacentric height before docking, lacking which the ship may heal
over to either side, or even slip off the keel blocks
and capsized
the maximum pressure that can be exerted on the block's
is a function of the material used for the blocks. This value being a constant
the minimum block area required for each block is calculated. If you observe the
nature of the weight curve, it is usually high at the mid ship region and
decreases at the forward and aft region. The pressure exerted on keel blocks is
called the block pressure and the average block pressure is the total
weight of the ship divided by total bearing area. After all plan ba has been
completed the dock is flooded and prepared for person departure. The most
important thing at this time is to ensure all the openings done for repairs
are shut and hull is intact. Also, a complete stability check of the ship is
to be carried out to ensure that acceptable metacentric height
GM is achieved once the ship is cleared of the blocks. The vessel is then towed
out of the dry dock with the help of thugs and shipyard personal. Once the
vessel reaches a safe Anchorage area the ship undergoes sea trials to ensure all
machinery including those undergone repair are working fine. On completion of
sea trials the vessel is commissioned back to the service. Dry docking is one
operation which allows the ship's crew to learn those areas of the ship which
cannot be explored when the ship is sailing. It also helps the ship manager
to assess the condition of the ship's hull and those machineries which are not
accessible when the ship is in water. The world's largest Rada is presently in
India which is 662 meters in length and 65 meters in breadth.