a brief introduction - gard · a brief introduction ... gravity pulls the ship’s mass towards the...
TRANSCRIPT
Gunnar Hjort 2013-05-29
Damage stability and watertight doors
A brief introduction
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
What’s in this lecture?
Basic hydrostatic stability explained
Basic floatability and damage stability
Importance of closed watertight doors
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© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Basic hydrostatic stability
Archimedes Newton
Archimedes: Buoyancy is equal to the weight of the displaced liquid
Newton: Gravity pulls the ship’s mass towards the Earth’s centre
Hydrostatic stability is obtained when the buoyancy equalises the weight of the vessel. The ship floats in a stable position.
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
How is buoyancy created? Water pressure increases with depth,
about1atmosphere per 10 meters
The buoyancy force is the sum of the vertical forces from the water pressure acting on the ship
The pressure acting on the flat bottom at 5 m draught in seawater is about 5.1 tonnes/square metre
The location of G and B are given in 3 dimensions; longitudinal, transverse and vertical position
B
G
This force is equal to the weight of the water displaced by the ship To simplify calculations we imagine weight and buoyancy concentrated in a centre of gravity (G) and a centre of buoyancy (B).
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
What is “stability”? “Positive stability is the vessel’s ability to roll back to the initial position after being
exposed to a heeling moment” (IMO definition)
B will move since the shape of the underwater body will change when the ship heels. This creates a righting moment
B
G
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
What causes a capsize? A ship will capsize if the sum of heeling moment(s) become greater than the righting
moment
The ship is stable if the heeling and righting moments are in balance
A moment is a force multiplied by its distance to a reference point In this illustration a chosen reference point is indicated as “K”
Note that stability will improve if G is lowered towards K
Weight and displacement are equal, opposed forces, so stability for each heeling angle Θ can be determined by the difference between heeling and righting arms
Intact and damage stability requirements are normally based on how the net righting arm, referred to as the righting lever, varies when the ship heels
G
K
y
a
W
Θ
BΘ
Righting lever
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Damage stability and watertight doors
2013-05-29
Intact and damage stability criteria The righting lever can be calculated and presented as a function of heeling angle
when the location of B and G are known.
These curves are often referred to as “GZ curves”
Note: The curve will not be correct if unprotected openings to volumes assumed to provide buoyancy become immersed. The part beyond the flooding angle is disregarded.
Righ
ting
leve
r (GZ
)
Heel
Range
Value
Heel at equilibrium
Area (Energy) Common regulatory parameters are: - Static heel for a given heeling moment - Range of positive stability - Minimum obtained righting lever - Potential righting energy
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Free surface of liquids The righting lever will be affected if there are slack tanks in the ship.
The margin for avoiding capsize is reduced
If the ship starts to heel the centre of gravity of the tank contents will be free to move and G for the whole ship will move as a result
BΘ
G G’Stability margin is reduced
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Why do ships sink - occasionally? A ship will sink if its total weight becomes greater than the available buoyancy;
Inflooded water
Buoyancy lost to compensate added water
New waterlineResidual (reserve) buoyancy
(Added weight)
Flooding limited by watertight bulkheads
Watertight bulkheads are required to limit the spread of water inside the ship
- Water enters the ship, increasing the total weight*
- The ship must sink deeper into the water to compensate the added weight
- Residual buoyancy is lost
* Not entirely true…
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Damage stability and watertight doors
2013-05-29
Staying afloat – Floatability or floodability The sinking stops if the water level inside the ship becomes equal to the
sea level outside.
• If the water in the flooded
space can communicate freely with the sea it is no longer part of the ship; so
• The volume of the flooded space is no longer part of the buoyancy
This shows a parallel sinking. In real life trim changes and transverse stability are also vital factors for survival
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Damage stability – Flooding of spaces Filling a tank from the outside
will increase the ship’s weight
The ship will sink deeper to provide more buoyancy; and
B
G
B’
G’
The centre of gravity shifts as weight increases
The ship will heel, trying to balance weight and buoyancy
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Damage stability – Prevention of capsize If a space is open to the sea the buoyancy of that space and its contents will eventually be lost.
The centre of buoyancy and the centre of gravity* will shift towards the undamaged side
In this situation the buoyancy will not be able to prevent a capsize if equilibrium cannot be found at a larger angle
(* If the tank was not empty)
B’’
G
The volume of tanks and spaces must be limited with watertight bulkheads to improve stability
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Maintaining watertight integrity In order to ensure sufficient floatability and stability after damage it is vital to
prevent water propagating further through the buoyant parts of the ship.
Damage stability was not a factor in
this famous example
All watertight doors were closed immediately after impact
The ship sank due to progressive flooding as water could spill over the transverse bulkheads
A watertight bulkhead deck would have delayed (but possibly not prevented) the sinking
The double bottom was better subdivided than shown here. Unfortunately, the damage was above the tank top
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Flooding through a watertight door – Simplified example
A2A1 v0
H0
00 2gHu =
Slide 14
Theoretical velocity u according to Bernoulli
Mean velocity for a mean head of water h _
0
_hFuu c ••=
where Fc represents flow resistance in the opening
The following example shows an estimate of the amount of water that may pass through an open watertight door
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Flow using some typical values
Slide 15
Door size 800*2000 mm => Cross-section is 1.6 m2
(Mean cross-section while closing is 0.8 m2)
Head of water in the damaged compartment at centre of the door : 4 m
Flow resistance for the opening (roughly) Fc=0.6
An Olympic size swimming pool contains at least 2500 m3 of water. At this flowrate it could be filled in about 5 minutes
Note: In real life the head of water will vary with trim and heel and the water level in the neighbouring compartment(s)
H
Clear opening 1,60 m2Head of water 4,00 m2Flow resistance 0,60 (-)
Ideal velocity uo 8,9 m/sMean velocity u1 5,3 m/sFlowrate 8,5 m3/s
Reaction 10 sDelay for alarm 10 sDoor travel time 40 s
Sum 340 m3
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
Summing up
Slide 16
• Keeping watertight doors closed might be vital to survival
Thank you for your attention!
© Det Norske Veritas AS. All rights reserved.
Damage stability and watertight doors
2013-05-29
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