CE707 Lec09 Dry Dock

8/20/2019 CE707 Lec09 Dry Dock

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Dry Dock

Prof. Manasa R. BeheraDept. of Civil Engineering, IIT BOMBAY

Dry docks are structures that allow complete dry

access to vessel for maintenance, overhaul, and repairs

or for new construction and launching.

- Basin dry dock

- Marine railways

- Floating dry dock

- Vertical lifts

Types & Characteristics

Desirable characteristics and features for efficient operation of a dry dock

- Adequate space in and around the dry dock for ease of personnel and material

movement with the vessel in the dock

- Fast and efficient access is needed to and from the dock

- Adequate light and ventilation

- Support facilities (civil, electrical and mechanical services)

- Material handling systems such as cranes

- Proper blocking system to support the weight of the ship



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Selection of appropriate type of dry dock

- The dimensions, weight characteristics and general features of the vessels to be

serviced by the dry dock.

- Conditions at the site including available land & water area, proximity to

navigational channel, tides, currents, topography and soil conditions.

- Purpose : new construction – long term / short term vessel repair.

- Short term and long term goals of the shipyard and potential for future

extension.

- Financing – based the investors interest.

- Basin dry docks are rarely damaged by overloading and can accommodate over

1,000,000 DWT vessels.

- Marine railways can’t take heavy loads and can take up to 8000 long tons.

- The largest floating dry docks have a lifting capacity of 81,000 long tons.

- Vertical lifts should have sufficient lifting strength.

Selection of appropriate type of dry dock

- The dimensions, weight characteristics and general features of the vessels to be

serviced by the dry dock.

- Conditions at the site including available land & water area, proximity to

navigational channel, tides, currents, topography and soil conditions.

- Purpose : new construction – long term / short term vessel repair.

- Short term and long term goals of the shipyard and potential for future

extension.

- Financing – based the investors interest.

- Basin dry docks are rarely damaged by overloading and can accommodate over

1,000,000 DWT vessels.

- Marine railways can’t take heavy loads and can take up to 8000 long tons.

- The largest floating dry docks have a lifting capacity of 81,000 long tons.

- Vertical lifts should have sufficient lifting strength.



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Basin dry dock

- A basin dry dock is simply an excavation or depression in the earth with one end

in free communication with the sea. The sea ward end is locked or sealed off

with a gate for dry docking.

General Features and Characteristics

- The floor

- Side walls

- Head wall

- Dock gate

There are 3 types:

- Mass gravity

- Ground anchored

- Underdrain or Pressure relieved Basic features of a basin dry dock

Basin dry dock

Operation

- Once the ship and the docking blocks have been properly prepared, the basin is

filled with water.

- The water is filled until the water level inside is equal to the level in the harbour.

- Then, the gate is opened, and the ship is moved into the dock.

- Warping into the dock usually is accomplished by using head lines and spring

lines on capstans or winches.

- The dock gate is closed and alignment of vessels is done as per the docking

blocks.

- The ship is properly positioned before energizing the pumps.

- The water level is lowered and the ship settles gently on the blocks.

- During dewatering the ship’s position is carefully monitored and adjusted.

- Similarly, in the reverse order the undocking is carried out.



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Basin dry dock

Operation

- Unlike the floating dock, the floor of the basin is not adjustable.

- A vessel docked with a substantial trim will result in an extremely high load on

making initial contact with the blocking.

- This could make the ship unstable.

- An acceptable trim usually is one foot for every 100 feet of ship length.

Longitudinal section of a ship during critical period

Moment to Change Trim 1 cm (MCTC)

Basin dry dock

Operation

- Energy consumption for pumping water is inversely proportional to the ship size.

- Because of deep dock floor, the ventilation and natural light are somewhat

restricted and adversely affect the working condition.

- The movement of crane are quite comfortable in this case.

Dimensions

- Length, width and depth depends on the type of ships to be docked and use.

- The effective length of a basin is the minimum horizontal distance measured

along the centerline between the head wall and the dock gate.

- The effective length should be at least 10 to 15 feet longer than LOA.

- For propeller and shaft work, an additional 25 to 100 feet should be provided.



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Basin dry dock

Dimensions…

- The entrance width is the clear distance between the permanent dock fenders or

wall structure at the dock entrance.

- The width at the entrance should be at least 6 to 10 feet wider than the design

ship.

Basin dry dock

Minimum clearances for construction



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Floating dry dock

Floating dry docks are floating structures with sufficient dimensions, strength,

displacement and stability to life a vessel from the water.

The major components are

– pontoons and

– wing walls

Floating dry dock

Pontoon is the prime supporting body, which must displace the weight of the vessel

and the dry dock. It also must withstand the transverse bending caused by the ship’s

weight along the centerline opposed by the water pressure from beneath.

The wings walls provide stability when the pontoons are submerged, as well as

space for equipment on the wing deck. Part of the wing is also used for ballast

water.

Size: - Range up to 100,000 long tons lift capacity

- length > 1000 feet and width > 200 feet

Material: - wood, iron, steel, RCC or combination

Advantages: Floating dry docks are the most flexible type of repair facility available.

They can accommodate vessels of any size. Can be readily relocated if needed.



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Floating dry dock

Floating dry dock

Principles of Operation:

It works on the basis of Archimedes principle – displacing a volume of water equal

in weight to its own weight and loads.

- Water is added to the ballast tanks, sinking the dock until the desired water

depth is attained.

- The vessel is then placed in or removed form the dry dock.

- Water is then evacuated from the ballast tank and the dry dock rises from the

water until the pontoon deck is above the water level.

The capacity may be limited by its buoyancy, its static stability and strength of the

structure.



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Floating dry dock

Ship-dock stability:

The intact stability must be assessed for all phases during a docking operation.

Floating dry dock

Vessel load distribution:

The intact stability must be assessed for all phases during a docking operation.



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Floating dry dock

Design considerations:

- The area of the pontoon deck must provide sufficient working space under and

around the vessel.

- The wing deck must accommodate the equipment to be mounted on.

- The pontoon must be deep enough to provide required lift capacity.

- The wing wall must provide the required inertia and buoyancy for conditions of

minimum stability and full submergence.

- Transverse strength of the pontoon must adequately support the concentrated

vessel loading and distribute uniformly to the underneath buoyancy.

- Longitudinally, shear forces and bending moments must be investigated to keep

the stress and deflection within allowable limits.

- Design guidelines are provided by Indian Register of Shipping, American Bureau

of Shipping, Lloyd’s Register of Shipping.

Thank you

CE707 Lec09 Dry Dock

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