a brief introduction - gard · a brief introduction ... gravity pulls the ship’s mass towards the...

Gunnar Hjort 2013-05-29

Damage stability and watertight doors

A brief introduction

© Det Norske Veritas AS. All rights reserved.


2013-05-29

What’s in this lecture?

Basic hydrostatic stability explained

Basic floatability and damage stability

Importance of closed watertight doors

2



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.



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).



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



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



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



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



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…



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



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



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



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



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



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



2013-05-29

Summing up

Slide 16

• Keeping watertight doors closed might be vital to survival

Thank you for your attention!



2013-05-29

Safeguarding life, property and the environment

www.dnv.com

17

a brief introduction - gard · a brief introduction ... gravity pulls the ship’s mass towards the...

Documents