1. SUMMARY
Stability and Trim

2. 3 Summary 3.1 Introduction to Stability and Trim 3.2 Intact Stability
3.4 Damage Stability
3.5 Load Lines
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3. Introduction to Stability and Trim
3.1
Introduction to Stability
and Trim
3.1 Introduction to Stability and Trim
“Stability is the ability to return to the original position of rest when disturbing forces are removed”
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4. Stability
3.1
Introduction to Stability
and Trim
Stability
When a vessel is inclined at a small angle from its position of rest;
it returns to original position - positive stability
it remains in displaced position - neutral equilibrium
it moves further away from original position - negative stability
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5. 3.1 Stability Stability (continued…)
Introduction to Stability
and Trim
Stability (continued…)
When a vessel is inclined:
Inclination in transverse direction is the “heel” or “list”
Inclination in longitudinal direction is the “trim”
Weight of the vessel acts through “Centre of Gravity” (G)
Buoyancy force acts vertically through “Centre of Buoyancy” (B)
A vessel placed in water will
settle until buoyancy equals weight
rotate until B and G are in the same vertical line
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6. 3.1 Metacentre Metacentre Introduction to Stability and Trim
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7. Longitudinal stability
3.1
Introduction to Stability
and Trim
Longitudinal Stability
Where
IL = 2nd moment of area about a transverse axis through the centre of floatation F
= Volume displacement = L I BM
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8. Trim
3.1
Introduction to Stability
and Trim
Trim
Trim is the difference between draught aft and draught forward
A ship trims about centre of floatation F (i.e. centroid of waterplane)
Moment to change trim 1 cm - this is the moment which must be imposed on a ship in order that the trim will be changed by 1 cm. It is plotted against draughts and used for calculating trim
MCT 1cm =
Where
GML = Longitudinal Metacentric Height
= mass displacement of the vessel in tonnes
L = length between perpendiculars
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9. 3.1 Trim Trim (continued…) Moment to change trim 1cm (MCT 1cm) = Where
Introduction to Stability
and Trim
Trim (continued…)
Moment to change trim 1cm (MCT 1cm) =
Where
GML = Longitudinal Metacentric Height
= mass displacement of the vessel in tonnes
L = length between perpendiculars
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10. 3.2 Intact Stability 3.2 Intact Stability Intact Stability
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11. Tranverse Intact Stability
3.2
Intact Stability
Tranverse Intact Stability
Vertical distance between G and M is the Metacentric Height GM.
G and B1 form a couple called the Righting Lever GZ (can be called KN at keel level). GZ = GM sinθ
Moment of the couple is GZ called Moment of Intact Stability
Position of M is determined by
Where IT = transverse moment of inertia
V = Volume displacement
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12. 3.2 Characteristics of GM Characteristics of GM Heeling moment
Intact Stability
Characteristics of GM
Heeling moment
Angle of loll
Righting moment
GM -ve = unstable GM 0 = neutral equilibrium GM +ve = stable
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13. 3.2 Characteristics of GM Characteristics of GM
Intact Stability
Characteristics of GM
A large GM value causes quick and violent rolling, “stiff” ship
A small GM, easier to incline, slow rolling, “tender” ship
[Typical values of GM : Passenger liners 0.6 – 1.2 m, Cargo vessels 0.3 – 1.0 m, Tugs 0.4 – 0.5 m, Sailing vessels 0.75 – 1.0 m]
For large angle stability GM is not valid, righting lever GZ is used
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14. Loss of GM depends on size of free surface not quantity of fluid!
Free surface effect
3.2
Intact Stability
Free surface effect
Virtual centre of gravity of the ballast water in the inclined position is m
The centre of gravity of the ship raised to GO
Loss of GM is:
Where
= moment of inertia of free surface
=density of fluid in tank
= density of seawater
V = volume displacement of the ship
Loss of GM depends on size of free surface not quantity of fluid!
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15. 3.2 Hydrostatic Curves Hydrostatic Curves Intact Stability
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16. 3.2 GZ Curve GZ Curve Intact Stability
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17. Cross curves of stability
3.2
Intact Stability
Cross curves of stability
KN Curves
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18. 3.2 IMO Res A.749(18) + BV Rules IMO Res A.749(18) + BV Rules
Intact Stability
IMO Res A.749(18) + BV Rules
Intact Stability for All Types of Ships over 24m
Design criteria (based on GZ curve):
GM  0.15 M
Area (0-30°)  m.rd
Area (0-40°)  m.rd (or angle of flooding if < 40°)
Area (30-40°)  m.rd
GZ (30°)  0.20 m
 (GZMAX)  25°
Requirements on free surface effects, timber deck cargoes, icing, and ship types (incld. cargo, fishing, offshore, MODU, container)
Severe wind and rolling criterion (weather criterion):
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19. Trim and Stability booklet
3.2
Intact Stability
Trim and Stability booklet
Contains data as specified in IMO Res.749(18) + BV Rules
General description of the ship
Instructions on the use of the booklet
General arrangement and capacity plans
Position of the draught marks
Hydrostatic curves, Cross curves
Tank sounding tables
Lightship data from the inclining test - approval details
Standard loading conditions
Intact stability results
Information on loading restrictions, openings, crossflooding fittings
Any other guidance
Table of contents and index
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20. 3.2 Inclining experiment Inclining experiment
Intact Stability
Inclining experiment
To determine lightship weight and locate G to calculate GM
Consists of a draught survey and shifting of weights, requirements outlined in IMO Res. 749(18) and BV Rules
Experiment performed for new ships, weighing test acceptable for sister vessels
Displacement determined with draught mark readings aft, midship and fwd
The incline employs eight distinct weight movements, weights transversally shifted.
Resultant tangents after each weight movement plotted on graph
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21. 3.3 Damage Stability 3.3 Damage Stability Damage Stability
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22. 3.3 Damage Stability Damage Stability (continued…)
When a vessel is damaged, aim is to:
minimise loss of transverse and longitudinal stability
minimise damage to cargo
minimise loss of reserve buoyancy
Ideally, sustain continuous flooding without loss of stability, consequential sinking is due to loss of reserve buoyancy (“foundering”)
At design stage, define to what extent vessel can withstand damage, normally defined by regulations
“Damage Stability” - calculations of draft, trim, heel and stability following a damage to one or more compartments of a ship.
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23. 3.3 Damage Stability Damage Stability (continued…)
There are two methods:
Added weight method i.e. displacement of ship in undamaged condition + weight of water admitted, or
Lost buoyancy method i.e. volume displacement of undamaged ship - flooded volume
Flooded compartments do not fill up 100%. Ratio of floodable volume to total volume is called permeability. Typical permeability values:
Machinery spaces : 0.85
Tanks : 0 or 0.95
Void / Other spaces except tanks : 0.95
Cargo holds : 0.95 / 0.70
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24. Applicable regulations for damage stability
3.3
Damage Stability
Applicable regulations for damage stability
Two approaches to calculations in regulations:
Deterministic (Oil, Chemical & Gas Tankers, Passenger Vessels and for Reduced Freeboard)
Probabilistic (Cargo Ships, Bulk Carriers, Container Vessels, Ro-Ro,...)
The following regulations apply for main ship types:
Passenger vessels : SOLAS 90, Res A.265
Oil Tankers : MARPOL, ILLC
Chemical Tankers : IBC CODE, ILLC
Gas Tankers : IGC CODE, ILLC
Dry Cargo : SOLAS 92, Res A.320
BV Rules - SDS notation
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25. Deterministic Approach
3.3
Damage Stability
Deterministic Approach
Based on standard dimensions of damage extending anywhere along the ship’s length or between transverse bulkheads
Consequence of such a standard damage is the creation of “group of damage cases”, number of cases and compartments involved depends on ship’s dimensions and internal subdivision
For each loading condition, each damage case is considered, and criteria applied
Different deterministic methods in damage stability developed depending on ship type, freeboard reduction, kind of cargo carried
Future - this approach likely to be replaced by probabilistic approach
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26. Example of Deterministic Approach
3.3
Damage Stability
Example of Deterministic Approach – Gas Tankers - IGC Code
Extent of damage assumptions (also depends on tanker type and length)
Side damage:
longitudinal extent - 1/3 L2/3 or 14.5 m
transverse extent - B/5 or 11.5 m
vertical extent - no limitation
Bottom damage:
For 0.3L from FP
transverse extent - B/6 or 10 m
vertical extent – B/15 or 2 m
Any other part
longitudinal extent - 1/3 L2/3 or 5 m
transverse extent - B/6 or 5 m
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27. Example of Deterministic Approach
3.3
Damage Stability
Example of Deterministic Approach – Gas Tankers - IGC Code (continued)
Local side damages anywhere in cargo area - extent inboard 760 mm
Survival requirements at final equilibrium after flooding :
righting lever curve range  20°
GZ max  0.1 m within the range of 20°
area under the curve  m. rd within the range of 20°
unprotected openings not immersed
emergency source of power capable of operating
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28. Example of Deterministic Approach
3.3
Damage Stability
Example of Deterministic Approach - Passenger Ships - SOLAS 90
Subdivision factor and floodable length (margin line-now outdated)
Extent of damage assumptions
longitudinal extent - 3m + 3%L or 11m for subdivision factor > 0.33
transverse extent - B/5
vertical no limit
flooding 2 or 3 adjacent compartments, depending on subdivision
Survival requirements:
area under GZ curve  0.015mrd
positive residual GZ range 15° beyond angle of equilibrium
residual GZ(m)= (heeling moment/displacement) + 4 , min. value > 0.1 m
Additional survival requirements for ships carrying more than 400 passengers
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29. Probabilistic Approach
3.3
Damage Stability
Probabilistic Approach
Ship safety in damaged condition based on probability of survival after collision - referred to as the attained subdivision index A
Calculations performed for a limited number of draughts and GM values, to draw a min. GM curve where the attained subdivision index A achieves the minimum required level of safety R
For cargo ships, each case of damage is not required to comply with applicable criteria, but the attained index A. This is the sum of the contribution of all damage cases, equal to or greater than R
This method applies to “cargo ships” (SOLAS definition) > 80 m, and for which no deterministic methods apply
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30. Example of Deterministic Approach
3.3
Damage Stability
Example of a Probabilistic Approach - Cargo ships
SOLAS Ch II-1 & BV Rules
Attained subdivision index A : A = Σ pisi
pi : probability of compartment or group of compartments i flooded
si : probability of survival after flooding of compartments i
Extent of damage limited vertically by the deck defining the subdivision length Ls - used for determination of R factor
Calculations made for two draughts with even keel - Summer load line draught and partial load line draught
“si” depends on loading condition, “pi” does not
Main stability information supplied to the master:
Curve of min. GM v Draught or allowable KG v Draught
Damage control plan (or Loading instrument with same info.)
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31. 3.3 Ro-Ro Stability Ro-Ro Stability
Damage Stability
Ro-Ro Stability
Ro-Ro vessels are vulnerable if water enters vehicle decks, there is rapid loss of stability due to free surface effect on a large area
Until 1990,stability in damaged condition was as for vessel with conventional dimensions. Since 1990, criteria for passenger Ro-Ro’s considers larger angle of heel, vehicle deck immersed, calculations must consider flooding, residual righting moment requirement applicable to all passenger vessels has to be met
In 1992 some flag administrations introduced A/Amax for existing passenger Ro-Ro’s, eventually implemented in SOLAS
As from July 1997, all Ro-Ro passenger vessels with more than 400 passengers to comply with two adjacent compartment flooding
Additional measures introduced for all existing Ro-Ro passenger vessels in NW Europe and Baltic sea (Stockholm agreement)
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32. 3.4 Load Lines 3.4 Load Lines Load Lines
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33. 3.4 History of Load Lines History of Load Lines
19th century, increase in trade, many overloaded ships lost, some sailors refused to go to sea
1870, Samuel Plimsoll in UK wrote a book on the disastrous effects of overloading ships. Campaigned for improving safety at sea
In 1872, a Royal Commission on Unseaworthy Ships was set up to look at evidence and recommend changes
Merchant Shipping Act of 1876 made load lines compulsory
May 1930, International Load Line Convention convened in London
1966 Convention followed - origin of the Regulations applicable today
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34. 3.4 Definitions Definitions
Load Lines
Definitions
Conditions of Assignment – Conditions to meet for assignment of a load line
Freeboard – Vertical distance downward amidships from upper edge of deck line to upper edge of the related waterline
New ship – Keel laid, or at similar stage of construction, on or after 21/7/1968
Superstructure – Decked structure on FB Dk. Extending side to side, or not inboard of shell plating more than 4% of ship’s beam
Bridge – superstructure which does not extend to either FP or AP
Forecastle – superstructure which extends from FP to fwd of AP
Full superstructure – extends as a minimum, from FP to AP
Poop - superstructure which extends from AP fwd to aft of FP
Watertight – No passage of water in any direction, under a head of water for which the surrounding structure is designed
Weathertight – In any sea conditions water will not penetrate
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35. 3.4 Definitions Definitions (continued…)
Load Lines
Definitions (continued…)
Freeboard deck – uppermost complete deck exposed to weather and sea
Type A vessel –carry liquid cargoes in bulk, cargo tanks with, small access openings closed by watertight gasketed steel covers
Type B vessel – All vessels which do not qualify as a Type A
Type B with reduced Freeboard – Based on damage stability analysis, must be over 100m in length
B-60 – FB reduced up to 60% of the total difference between the tabular Freeboard for a Type A and a Type B vessel
B-100 – FB reduced up to total difference between the tabular Freeboard for a Type A and a Type B vessel
Increased Freeboard – Greater than the minimum Type A or Type B Freeboard. Freeboards may be increased due to strength, stability, deficient hatch covers, location of shell doors
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36. Freeboard Calculations
3.4
Load Lines
Freeboard Calculations
Tabular Freeboard - This is the starting point to determine min. geometric Freeboard. Tables give values as a function of length
Corrections - Tabular values are corrected for the following:
Block coefficient - Applied when actual CB is above 0.68
Depth correction - Based on standard Depth of L/15. If actual Depth is above this value, vessel assumed to have more draft and therefore less buoyancy, therefore vessel is penalised with a greater Freeboard
Deckline correction - Not always possible to find the deckline (camber, radiused plating, tumblehome etc). The edge in this case has to be plotted as an imaginary deck edge
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37. Freeboard Calculations
3.4
Load Lines
Freeboard Calculations (continued…)
Corrections – continued…
Superstructure (always deducted) - contributes to some extent in resisting large rolling angles. Mean length (S) determined (considering any recesses, curved end bulkheads etc.). This is corrected for breadth and height to give effective length (E). Sum of all E values used in the table to find % deduction from FB
Sheer - superimpose actual sheer profile with the standard sheer profile. Depending on larger or smaller area, FB is either penalised or credited
Minimum bow height - vertical distance between the summer load waterline (including trim) and top of the exposed deck at side. To ensure that vessel has reserve buoyancy at the bow to resist pitching and minimise bow immersion in rough weather
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38. 3.4 Load Line Marks Load Line Marks
Load Lines
Load Line Marks
In case of All Seasons Freeboard (i.e. centre of the ring below lowest seasonal load line mark), only the Fresh Water Load Line need be marked
Density allowances
Deck line
Seasonal allowances
Load Line mark
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39. Conditions of Assignment
3.4
Load Lines
Conditions of Assignment
Strength and stability requirements
Strength must be to the Flag Administration. This is satisfied for classed vessels by meeting Rule requirements.
If vessel approved to a lesser scantling draught than the draught corresponding to summer load line, the Freeboard must be assigned at the scantling draught
Approved Loading Manuals must be provided, to ensure hull is not over stressed
Must meet all applicable Stability regulations and must have approved stability information on board covering all loading conditions, maximum draught within the allowable Load Line draught assigned
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40. Conditions of Assignment
3.4
Load Lines
Conditions of Assignment (continued…)
Position requirements
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41. Conditions of Assignment
3.4
Load Lines
Conditions of Assignment (continued…)
Vertical access openings - sill height requirements
Doors - superstructures fitted with permanently attached doors
Machinery casings - must be framed and have covers
Cargo ports - watertight below FB deck, strength considerations
Horizontal access openings - coaming height requirements
Must be protected by superstructure, deckhouse or companionway – equivalent strength
Hatchcovers – strength criteria, approved securing
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42. Conditions of Assignment
3.4
Load Lines
Conditions of Assignment (continued…)
Ventilators and airpipes - substantially constructed and connected, sill height requirements
Scuppers, inlets and discharges - requirements consider spaces penetrated and possibility of flooding
Side scuttles and windows - No window below Freeboard deck, side scuttle sill height requirements, deadlight requirements below Freeboard deck and 1st tier deck of superstructures
Freeing ports - necessity to quickly drain water from exposed decks, minimum size requirement
Crew protection – minimum height of bulwarks and hand rails, protected access to and from crew quarters, strength of deckhouses for accommodation and means of protection and access around deck cargo
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