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Dry 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.


Hull
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.

Corrosion

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.

Advantages:
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.

Disadvantages:
a) Wet environment is generated and produces mist, aiding first corrosion effects.
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.

Shot blasting
Utilizes a jet of abrasive at about 5 to 7 bar pressure. Like water jetting, it has its ups and downs too.

Advantages:
a) Correct equipment and suitable abrasive media gives a high quality surface finish.
b) No ‘wet problems’.
c) Operators with suitable personal protective equipments can work longer periods.
d) High production means more tasks can be accomplished in a shorter period of time.
e) Safe as operator only uses one nozzle.
f) Low investment, higher efficiency.

Disadvantages:
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.

Manually

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 left after
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 should be
inspected for any signs of corrosion or damages. While overboard discharges should be
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 and renewed.

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 is required,
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.


Cathodic 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 reference electrodes
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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