Dock - Hull Work
Works carried out in drydock
It is not ideal to generalize the works which are to be carried on all
ships in the drydock as different ship owners have their own arrangements
and plans for their ships when they are in the drydock. So, in this section,
we are going to name some of the typical works that are done to vessels
after entering the drydock.
Once the water has been removed from the basin, the hull structure is
inspected thoroughly for:
Damages and distortions
Most hull damage can be seen readily. Cracked
and broken members are obvious faults. Likely locations for cracks or
breaks are in areas of high stress or where the structure undergoes a
sudden change in shape. The turn of the bilge is the prime location for
breaks of this type. The harder the turn, the more chance that damage
has been done. Bent frames are particularly susceptible to breakage under
bilge stringers, especially when the stringers are substantially thicker
than the plating or when there are large diameter fastenings in the stringers.
All attachments such as bilge keels etc.
should also be inspected for corrosion and cracks.
The underwater portion of the hull structure accumulates marine growth,
commonly known as “Marine biofouling”. Marine organisms attach
themselves to some metals and alloys more readily than they do to others.
Steels, titanium and aluminum will foul readily, and they can be detrimental,
resulting in unwanted excess drag on vessels in sea water. This requires
a lot more horsepower and more horsepower requires more fuel, raising
the operating cost ultimately.
To remove these undesirable marine growths, the hull areas are therefore
water blasted or grit blasted to clean off to the bare metal.
High- pressure water jetting
Using tubular lances which shoots pressurized water ranging from 150 –
500 bars. The lower pressure being sufficient to remove the growth and
the higher pressure being used to scrub the hull of the bare metal. However,
there are pros and cons for this type of cleaning.
a) Good quality of surface cleanliness is achieved.
b) No grit residual or clean up required after completion.
c) Dustless atmosphere improves condition for coating applications.
a) Wet environment is generated and produces mist, aiding first corrosion
b) Compatible with only surface tolerant coatings rather than the more
sophisticated high quality coatings.
c) Large amount of fresh water is needed in the process.
d) Limited to only external maintenance tasks.
e) Requires ‘hands on’ operator time.
f) Safety limitations with double operators in close proximity.
g) Expensive and less efficient.
Utilizes a jet of abrasive at about 5 to 7 bar pressure. Like water jetting,
it has its ups and downs too.
a) Correct equipment and suitable abrasive media gives a high quality
b) No ‘wet problems’.
c) Operators with suitable personal protective equipments can work longer
d) High production means more tasks can be accomplished in a shorter period
e) Safe as operator only uses one nozzle.
f) Low investment, higher efficiency.
a) Abrasive required with subsequent wastage. A portion of it will be
contaminated and it is non recyclable.
b) Dusty environment.
c) Dust emission encroaches other working discipline in the vicinity.
d) Dust containment equipment needed in certain areas.
Using scrapers and wire brushes or by using
power discs or power driven
wire brushes. This is usually done to all damaged areas showing signs
of rust or blistering.
In cases of large areas, blast cleaning may be preferred. Any marine growth
blast cleaning is also removed using this method.
In addition to blasting operation, any protective
gratings over seawater intakes or thruster
units should be inspected and removed for cleaning, if required. Intakes
inspected for any signs of corrosion or damages. While overboard discharges
plugged using drains, to allow dirty waste to clear the ships side. After
cleaning, damaged portion of the shell plating, if required, are cut out
Then, the hull is touched up with at least
two coats of suitable primer using either brush
or airless spray. It should be noted that to avoid painting on areas which
are damp or experiencing condensation from bottom tanks. To end this,
bottom tanks should be emptied as soon as docking has been completed.
Coatings should be applied in accordance
with the manufacturer’s instruction and
unnecessary thinning of paints should be avoided. If mixing of paints
It is often a good practice to let the paint stand for 15 to 30 minutes
following a thorough mixing period.
The hull is coated to the recommended thickness
using airless spray units, and it should be done during rain free periods
or when the hull is completely dry. To achieve the correct thickness of
the coating, the numbers of spray units are allocated to a fix amount
of paint. In between each coatings, a designated drying time specified
by the paint manufacturers must be allowed to provide a better finish.
The bottom area given a final coat with an anti-fouling paint which comes
in two types:
Ablative Antifouling Paint
This paint wears down, much like a bar of soap, as the vessel moves through
the water. As a result, fresh layers of biocide are constantly being exposed
throughout the boating season. This type of paint works well in high marine
growth areas and continues to work even with multiple haul-outs, just
as long as any of the biocide remains. Because of the way it works, putting
on 2 or 3 coats of paint initially is a good idea.
Another plus is that ablative paint can be
applied over most other antifouling paints. The downside is that because
these paints are relatively soft, the bottom paint will be removed with
each brush stroke each time the bottom or hull is cleanse. Also, it wears
away quickly on high drag areas such as rudders or other bottom appendages.
Hard Antifouling Paint
This paint starts leaching out biocide upon contact with the water to
prevent marine growth. However, after a period of time (say 6 months to
a year), the paint starts to run out of "ammo" and becomes much
less effective than it was when first applied.
In addition, hard antifouling paint will
build up. Because the medium that holds the biocide does not wear off,
it needs to be periodically, physically removed to prevent excessive paint
build-up on the bottom. Some hard antifouling paints have Teflon added
to further reduce surface friction.
The painting is preferably to be carried out in warm dry weather and generally
not under direct sunlight. During this period it should be ensured that
there are no trickles of any discharge running down the shipside till
the paintwork has cured.
protection anodes and equipment
If the ship is fitted with sacrificial anodes their size and attachments
are checked. Usually
such anodes are replaced every two years.
If an impressed current system is provided, then the anodes and their
are inspected for proper attachments to the hull. The inert protective
shield is checked for
damage and deterioration and other equipment check for correct functioning.
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