CE707 Lec09 Dry Dock
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Transcript of 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