Docks

Docks Meaning and Conceptual Learning

Docks Overview

Docks are enclosed area used for berthing of ship to keep them at uniform level for loading and unloading of cargo or renovation.

They are classified as :

1. Graving or dry dock

2. Floating dock

3. Marine Railways

4. Liff Docks

• Graving or Dry Docks :

It is a long excavated chamber which is having sidewalls, semicircular end walls, and floor. As it is having two ends, the open end is provided with a gate and acts as an entrance to the clock. The side walls formed with the series of steps known as altar courses to receive the ends of shores which support the vessel in a vertical position while being docked and such altar courses of granite to withstand heavy wear. Culvert provides for filling and emptying the dock.

• Size of Dock : 

Size depends on the size of largest ship it has to be a dry dock. Dry dock to handle modern big ships having 1000ft. in length, with an entrance width of 60ft. to 100ft. The ratio of length to breadth of modern ocean lines are about 9.5:1.

• Forces on Dry Docks:

1. Weight of ship along the centerline of the dock floor when the dock is empty.

2. Weight of water on floor when a dock is flooded.

3. Upward pressure under the floor when emptied.

4. Earth and hydrostatic pressure.

5. The load imposed by shores on the inside face of the sidewall.

6. Surcharge on sidewalls due to crones.

Dock empty: Floor subjected to heavy uplift than the weight of the floor itself. The weight of ship resting on empty docks adds concentrated load along the center line and in this case, heavy reinforcement floor section may become necessary if the soil is soft or yielding or if the intensity is high as 75 tons to 90 tons per running foot. Generally assume that as 5/8 of such load caused by keel block and 3/8 on bilge block

Dock Filled with Water :

This condition imposed the greatest load on the foundation. The inverted arch action is absent under such conditions or loading.

Note : 

i. To design Floor thickness an inverted arch of about 6ft. the thickness and 1/4 rise adding up to an actual Floor thickness of 16ft. is required to satisfy the condition.

ii. Also, it is necessary to design the sidewall and floor independent sections.

Keel and Bilge Block :

Keel block consists of hardwood blocks. Block is made up of 2 or 3 blocks placed one above the other which is the place at 4ft. centers long. The height of the block is about 4ft. to 4 and a half ft. to give enough clearance to workmen.

Bilge blocks consist of two or more thickness of timber and fixed on both sides of keel blocks but at longer internal apart upper part is of wedge-shaped to give level seating.

• Floating Dry Dock :

The floating vessel can lift the ship out of the water and return it above water by means of its own buoyancy. As it is a hollow structure of concrete or steel consisting of 2 side walls and floor with ends open.

Types of floating docks :

i. Rigid type and non-self docking.

ii. Self-docking type.

iii. Self-docking offshore type.

Rigid type: In this side wall fixed to the pontoon or bottom section. The floor portion is divided into the number of chambers to berth listing ships by partial unballasting of the chamber.

Self Docking type: It is a system in which docks are divided into sections longitudinally and action which is capable of being lifted while the remaining are docks for purposed cleaning, painting, and repairing.

Offshore type: The main point is that it has no sidewall on the waterside and has L shape crossection. This typed dock is convenient in a sheltered situations and adaptable for being attached to River quays.

Advantages of Floating Dry Dock :

i. Cheaper in initial and working costs.

ii. It has good mobility so transfer from one place to another.

iii. Take half time to construct compared to construct graving dock.

iv. It has no elaborate entrance or gate arrangement.

Disadvantages of Floating Dry Dock :

i. Durability is less which has a life up to 50 years. 

ii. Maintenance required regularly.

iii. The maneuvering and towing of floating Dry Dock required great skill.

Marine Railways

It is an inclined Railway extending from shore wall into the water as well as foreshore to enable the ship to be drawn up clear out of water.

Parts :

i. Cradle - which moves up and down made of steel and provided with keel and Bilge block to receive the ship.

ii. Inclined track.

iii. Track supported on foundation or pile foundation.

• Cradle mounted on a system of rollers which move on iron tracks carried by long timbers and which are supported on cross ties or other foundation.
• The ways consist of heavy rails sealed too long sleeper supported on cross ties and laid at an inclination of 1 in 15 to 1 in 20.
• Rocking device provided to receive safety powls under cradle and placed in the center of ways to keep cradle from slipping back if hauling cable breaks.
• The use of this type of dock is economical but limited to a vessel, not more than 5000 tons. Now it is becoming more popular. 

Lift Dry Docks

This are constructed as platform capable of being lowered and raised from the water and it is being carried by hydraulic power. As this modern ship grown in tonnage ancient method of dry docks is now discontinued.