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DRY DOCKING-2 (Special Drydocks and Emergencies)

Q. Describe a floating drydock?
Ans. A floating dry-dock is a type of pontoon for dry docking ships, with ‘U’ shaped cross-section. It has floodable DB tanks and side tanks as buoyancy chambers. The wall tanks are used to give the dry-dock stability contributing towards transverse water plane area), when the floor or deck is below the surface of the water. This is because the stability, which is directed to a large extent by BMT, is a function of water plane area. The walls provide water plane, away from the centerline of the composite unit i.e. the ship & the DB underneath. When valves are opened, the chambers fill with water, causing the dry-dock to float low in the water. The deck becomes submerged and this allows the ship to be moved into position inside. When the water is pumped out of the chambers, the dry-dock rises and the ship is lifted out of the water on the rising deck. The advantage of floating dry-docks is because of their versatility in respect of size, location, trim, etc. They are more economical in construction as compared to conventional dry docks.

Q. Can a floating dry-dock have a varying length?
Ans. A typical floating dry-dock involves multiple U shaped transverse sections. One or more of such units can longitudinally be added to accommodate a longer ship. These sections can be combined to handle ships of various lengths and the sections themselves, can come in different dimensions. Each section contains its own equipment for emptying the ballast and to provide the required services, and the addition of a bow section can facilitate the towing of the dry-dock once assembled. Smaller dry-dock unit may be self sufficient in terms of bow section allowing an attachment & steering facility at stern.

Q. What is the critical period in case of a floating dry-dock?
Ans. The floating dry-dock can be trimmed to take the ship all over, almost at the same time or a trim very slightly less than the ship’s trim. This would mean that the up-thrust at critical instant is very small; whether the ship has a small trim, large trim or even ‘head trim’. Thus, the critical period is minimized in case of a floating dry-dock. There however, is another critical period of concern. After taking blocks all over and additional support of bilge blocks, the ship with the dry-dock, becomes composite unit, with a common stability. BMT of the composite unit will depend on the water plane area. This area is low when the water level is between the keel of the ship & tank top of the double bottom. This period with low WPA for the composite unit, is the additional (main) critical period for a floating dry-dock.

Q. What are the different types of dry-docks?
Ans. There are mainly three types of dry-docks. The first type is a graving dock. A graving dock is a channel cut out of a basin or river in which, a caisson is placed in front of the opening and water is pumped out. As the water is pumped out, the ship is lowered onto the blocks. This type of dock can handle, the largest of vessels.

The second type of dock is the marine railway. This type of dock has blocks mounted on a skid and the skid is lowered into the water on rails. The ship is positioned and chains or line haul up the skid, with the vessel on the blocks. In the past horses were used to power these railways. These railways typically do not have as much lifting power as a graving dock or a floating dry-dock. The marine railway consists of a cradle of wood or steel with rollers on which the ship may be hauled out of the water along a fixed inclined track leading up the bank of a waterway. The advantages of a marine railway lie in the economy of the original construction and the relative low cost of maintenance. A marine railway is ideal for ships upto 5000 t.

The third type of drydock is a ‘floating drydock’. This type of dock is U-Shaped and can be sunk to allow the vessel to enter and be positioned.  When the ship is in position, the water in the ballast tanks of the dock is pumped out and the dock rises to meet the ship. Then the ship is risen out of the water as the dock is de-ballasted. This type of dock can typically lift heavier ships than the railway but not as heavy as a graving dock. The benefits of this dock are that it can be towed anywhere in the world and that it can be ballasted to lift a damaged ship with an excessive list or trim.

Another type of dry-docking system is the mechanical lift dock, somewhat similar in action to the floating dry-dock. The vessel, after taking up on the keel and bilge blocks in the dock, is bodily lifted clear of the water. The mechanical platform dock has much more flexibility than other types and has increased greatly in size and use.

Q. What conditions must be fulfilled for allowing an ‘In water survey’?
Ans. An In water survey may be accepted in lieu of the intermediate docking between special surveys.

Q. How can a ship with bow damage be drydocked?
Ans.  The ‘straight forward dry-docking’ itself, is a critical operation. A ship with the bow damage is definitely more complex than normal dry-docking. Definitely, an expert’s guidance would be needed. The location, size and type of damage are very crucial in deciding how complex the dry-docking would be.

The different types of situations can be due to the following:

1. Damage to an empty compartment.

2. Damage involving general cargo and hold accessible from inside.

3. Damage involving bulk cargo and hold not accessible from inside.

4. Damage symmetrical about centre line.

5. Damage not symmetrical causing list.

6. Damage above or below the water line.

7. Damage to hull being well defined, allowing a temporary patch-up to prevent free flow of water.

8. Damage irregular, where the hull plates do not allow temporary patch-up.

9. Damage causing excessive trim.

10. Damage to the side of a compartment versus damage to the bottom.

The prime focus should be to maintain the intactness of the hull by providing a temporary patch by welding a thin sheet around the damage or covering the damage by a repair kit that may be available or a cement box and then pumping out water from the compartment so that there is no free flow of water or adding of water mass.

Q.  No, the damage is such that the temporary repair cannot be done. Or let us put it this way that the port does not have facility to carry out such repairs.
Ans. I will look in to the possibility of off loading the cargo ashore or shifting to holds aft to cause the required trim aft. The Dock Master is consulted regarding the range of trim that is acceptable. Any possibility of changing declivity, arrangement of extra blocks, etc may also be considered.

Q.  What if the possibility of off loading, shifting, etc. is ruled out?
Ans. I will enquire about the availability of a floating dry dock. A floating dry-dock is versatile in respect of trim of the vessel & can allow a vessel to dock with the forward trim. Thus, a floating dry dock taking a trim close to that of the ship’s, would be best.

Q.  What additional precautions may be taken?
Ans. As has been said earlier, the complexity will depend on various factors. In case the intactness cannot be maintained, the measures must be taken to ensure that the vessel remains upright throughout the critical period. Probably, the vessel may list as the dock is pumped out hence the calculations must ensure that she remains upright during critical period. It would be best if the critical period and hence the change of trim required to sit all over is minimum. This is because once the critical period is over, the ship can be adequately supported. Sometimes, fabrication of a frame (cradle) to support a portion of hull may be an option for a ship of smaller size.

Q.  Can a ship be dry-docked with a head trim?
Ans. In a floating dry-dock it is definitely possible. A floating dry-dock can trim itself as required to suit the trim of vessel to be dry-docked. The vessels in floating dry-dock normally take blocks all over, with a very small critical period and hence trim change. In graving dry-dock a vessel can be dry-docked with head trim in following two circumstances:

a. Where the declivity of blocks can be altered in a reverse way so that the shoreward slope of blocks is made slightly more than the trim of ship and hence in spite of the head trim ship still takes the stern block first.

b. Where it is intended that the vessel will take the blocks forward, a suitable location in the fore part is selected, with due strengthening, reinforcing, etc. the caution observed here being that the change of trim should be as small as possible. This is because P-critical is directly proportional to the change of trim before sitting all over.

Q.   What if the size of dry dock is enough but the weight of ship does not permit?
Ans. The water in dry-dock is pumped little more than beyond the critical period so that ship is adequately supported all over on blocks (plus on sides or along additional line of blocks). If the ship’s draft is say 2.8 m with ship supported in above manner, the displacement for 2.8 m is the buoyancy shared by the water in dry-dock. (The actual displacement – displacement at 2.8m) is the weight that will come on the blocks. The necessary / emergency maintenance can be done in this phase.

Q.   While refloating the dry-dock, vessel starts listing. What action must be taken?
Ans. To start with, all the items of the ‘checklist prior floodingmust be attended. The listing can occur due to one of the following reasons:

1. One or more of the bottom plugs not in place.

2. Departure load condition (ballast) not same as arrival load condition.

3. Some valve on shipside is not holding.

4. A new breach of hull (rare).

A very careful and continuous check must be kept on the list. Although, the support of buoyancy continuously increases as the dock is being flooded but at no stage the listing moment should be large enough to cause capsizing in the available stability. Thus, if the filling of tank is causing a high listing moment / FSC and the flooding of dry-dock is rather slow, the ship may capsize. The ship may also go to a large list owing to low GM. In either case, sharp localised stress is likely to come on the hull from the blocks. It is therefore advisable to suspend flooding just before critical period starts, check all the soundings and water-tightness of hull and valves. In a situation as above, since the vessel is showing list, it is occurring in the critical period. It will be of great concern to know, whether the water level is close to sitting allover stage or free floating stage. And normally the closer stage should be approached. In suspending of the flooding & reversing the process & rectification of fault would involve the danger of reducing buoyancy support in a situation of increase of listing moment. Careful and cautious watch on developing list is very important.

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