1 summary 3.stability and trim. 2 title of the presentation - dd/mm/yy summary 3.1 introduction to...

1

SUMMARY

3. Stability and Trim

2Title of the presentation - DD/MM/YY

SummarySummary

3.1 Introduction to Stability and Trim

3.2 Intact Stability

3.4 Damage Stability

3.5 Load Lines

3Stability and Trim


Introduction to Stability and TrimIntroduction to Stability and Trim

3.1 Introduction to Stability and Trim

3.1 Introduction to Stabilityand Trim

“Stability is the ability to return to the original position of rest when disturbing forces are

removed”


StabilityStability

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



StabilityStability

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



MetacentreMetacentre

Metacentre

Metacentre (M)

W 1 L1

BB1



Longitudinal stabilityLongitudinal stability

Longitudinal Stability

F

B B1

WL

WL1

M L

Where

IL = 2nd moment of area about a transverse axis through the centre of floatation F

= Volume displacement

LLI

BM



TrimTrim

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

L

GML

100



TrimTrim

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

L

GML

100



Intact Stability Intact Stability

3.2 Intact Stability

3.2Intact Stability

G Z


Tranverse Intact Stability Tranverse 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

M

G Z

OW 1 L1

K

BB1

N

.

.

. .

.

TTI

BM• Position of M is determined by

Where IT = transverse moment of inertia

V = Volume displacement

3.2Intact Stability


Characteristics of GM Characteristics of GM

Characteristics of GM

Heeling moment Angle of loll Righting moment

MG

W 1 L1

BB1

. .

.M

G Z

W 1 L1

BB1

.

. .

.M

GZ

W 1 L1

B B1

.

. .

.

GM -ve = unstable GM 0 = neutral equilibrium GM +ve = stable

3.2Intact Stability


Characteristics of GMCharacteristics of GM

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

3.2Intact Stability


Free surface effect Free surface effect

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

i

M

G

W 1 L1BB1

.

. .

..G O

.m

. .bb1

Loss of GM depends on size of free surface not quantity of fluid!

S

TO

iGG

TS

3.2Intact Stability


Hydrostatic Curves Hydrostatic Curves

Hydrostatic Curves

3.2Intact Stability


GZ Curve GZ Curve

GZ Curve

3.2Intact Stability


Cross curves of stability Cross curves of stability

Cross curves of stabilityK

N C

urv

es

3.2Intact Stability


IMO Res A.749(18) + BV Rules IMO Res A.749(18) + BV Rules

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°) 0.055 m.rd

» Area (0-40°) 0.090 m.rd (or angle of flooding if < 40°)

» Area (30-40°) 0.030 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):

3.2Intact Stability


Trim and Stability booklet Trim and Stability booklet

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

3.2Intact Stability


Inclining experiment Inclining experiment

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

3.2Intact Stability


Damage Stability Damage Stability

3.3 Damage Stability

3.3Damage 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.

3.3Damage 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

3.3Damage Stability


Applicable regulations for damage stabilityApplicable regulations for 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

3.3Damage Stability


Deterministic ApproachDeterministic Approach

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

3.3Damage Stability


Example of Deterministic ApproachExample of Deterministic Approach

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

» longitudinal extent - 1/3 L2/3 or 14.5 m

» 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

» vertical extent – B/15 or 2 m

3.3Damage 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 0.0175 m. rd within the range of 20°

» unprotected openings not immersed

» emergency source of power capable of operating

3.3Damage 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

3.3Damage Stability


Probabilistic ApproachProbabilistic Approach

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

3.3Damage 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.)

3.3Damage Stability


Ro-Ro Stability Ro-Ro 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)

3.3Damage Stability


Load LinesLoad Lines

3.4 Load Lines

3.4Load Lines


History of Load LinesHistory 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

3.4Load Lines


DefinitionsDefinitions

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

3.4Load Lines


DefinitionsDefinitions

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

3.4Load Lines


Freeboard CalculationsFreeboard Calculations

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

3.4Load Lines


Freeboard CalculationsFreeboard Calculations

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

3.4Load Lines


B V

W NA

W

S

T

F

TF

Load Line MarksLoad Line Marks

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

Seasonal allowances

Deck line

Load Line mark

3.4Load Lines


Conditions of AssignmentConditions of Assignment

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

3.4Load Lines



Conditions of Assignment (continued…)

Position requirements

11

22

2

AP FP

0.25L

3.4Load Lines




• 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

3.4Load Lines




• 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

3.4Load Lines

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