fse on ship stability

FREE SURFACE EFFECT ON SHIP STABILITY

Submitted by

G. Vinod kumar M100 420 CE

Over View of the presentation

LAW OF FLOATING BODIES

A floating object has the property of buoyancy A floating body displaces a volume of water equal in weight to

the weight of the body. A body immersed (or floating) in water will be buoyed up by a

force equal to the weight of the water displaced.

STABILITY REFERENCE POINTS

CL

M

G

B

K

etacenter

ravity

uoyancy

eel

CENTER OF BUOYANCY (B)

The centroid of the underwater volume of the ship is the location where the resultant buoyant force acts.

B

WATERLINERESERVE BUOYANCY

B1

B “B” FOLLOWS THE WATERLINE

B

WLWL

B

WL

B

WL

B

WL

B

CENTER OF GRAVITY (G)

Point at which all weights could be concentrated. Center of gravity of a system of weights is found by taking

moments about an assumed center of gravity, moments are summed and divided by the total weight of the system

G moves towards the weight addition. G moves away from the weight removal.

GGGGGGG1

KG1

KGo

G

META CENTER (M)

The intersection of vertical lines through the center of buoyancy of a floating body when it is at equilibrium and when it is floating at an angle.

The location of the metacenter is an indication of the stability of a floating body.

the metacenter will change positions in the vertical plane when the ship's displacement changes.

“M” MOVES OPPOSITE OF “B”

LINEAR MEASUREMENTS IN STABILITY

CL

M

G

B

K

GM

KG

BM

KM

METACENTRIC HEIGHT

HEIGHT OF METACENTER

METACENTRIC RADIUS

HEIGHT OF GRAVITY

OVERALL STABILITY

The horizontal distance between the positions of the ship’s displacement vector and the buoyant force vector help determine stability.

The relationship changes when a ship is heeled by an external moment.

External forces :- wind, wavesExternal moment:- It can be caused by wind pushing on one side of the vessel and

water resisting the motion on the other side. Each distributed force can be resolved into a resultant force

vector. The wind acts above the waterline and the water resistance acts below the waterline.

Continued…..

The two forces create couple because they are equal in magnitude, opposite in direction, and not aligned.

The couple causes rotation or heeling. The vessel will continue to rotate until it returns to Static

Equilibrium.Internal forces :- The resultant weight , The resultant buoyancy Internal Forces create a Righting Moment to counter the

Upsetting Moment of the External Forces.

Internal Righting Moment

The perpendicular distance between the Weight and the Buoyancy Force vectors is defined as the RIGHTING ARM (GZ).

The moment created by the resultant Weight and the resultant Force of Buoyancy is defined as the RIGHTING MOMENT (RM). It may be calculated by

Where: RM is the internal righting moment of the ship s is displacement of the ship in Fb is the magnitude of the resultant buoyant force GZ is the righting arm

RM GZ GZ F b

M

ZG

B B1

CL

OVERALLSTABILITY

RM = GZ x Wf

FINAL DISPLACEMENT

CURVE OF STATICAL STABILITY

Shows the Heeling Angle () versus the righting arm (GZ). Assumes the vessel is heeled over quasi- statically in calm water.

Continued…..

Predictions made by the Curves of Intact Statical Stability are not accurate for dynamic seaways because additional external forces and momentum are not included in the analysis.

Since stability is a function of displacement, there is a different curve for each displacement and KG! These are called the Cross Curves.

RANGE OF STABILITY

The range of angles for which there exists a positive righting moment.

The greater the range of stability, the less likely the ship will capsize.

If the ship is heeled to any angle in the range of stability, the ship will exhibit an internal righting moment that will right the ship if the external moment ceases

DYNAMICAL STABILITY The work done by quasi-statically rolling the ship through its

range of stability to the capsizing angle. It can be calculated by the the product of the ship’s displacement

with the area under the Curve of Intact Statical Stability. Does not account for the actual dynamics, because it neglects

the impact of waves and momentum.

The largest Static Moment the ship can produce. Calculated by multiplying the displacement of the vessel times

the maximum Righting Arm The larger the Maximum Righting Moment, the less likely the

vessel is to capsize. The angle of inclination where the maximum Righting Arm occurs Is called angle of maximum righting arm.

MAXIMUM RIGHTING MOMENT

Continued…..

MEASURE OF STIFINESS The initial slope of the intact statical stability curve indicates the

rate at which a righting arm is developed as the ship is heeled over. This slope is GM.

A steep initial slope indicates the rapid development of a righting arm and the vessel is said to be stiff. Stiff vessels have short roll periods and react strongly to external heeling moments.

A small initial slope indicates the slower development of a righting arm and the vessel is said to be tender. Tender vessel have longer roll periods and react sluggishly to external heeling moments.

FREE SURFACE EFFECT

A free surface is fluid that is allowed to move freely, such as water in a partially filled tank. As the ship lists, the fluid in the tank moves.

The fluid movement acts like a weight shift, causing the center of gravity of the fluid to move which causes the ship's center of gravity to shift in both the vertical and horizontal directions

The effect of the vertical shift is negligible at small angles and is discounted, but the horizontal (transverse) shift of the center of gravity causes a decrease in the righting arm.

The distance the center of gravity would have to rise to cause a reduction in the righting arm equivalent to that caused by the actual transverse shift is called the Free Surface Correction (FSC). he righting arm (GZ).

The position of this new center of gravity is called the "virtual" center of gravity (Gv) and for GM effective meta center.

FREE SURFACE EFFECT

G

B

M

G

B

M

B1

G2G

B

M

B1

G2G

B

M

B1

G2

B1

G2G

B

M

B1

G2

G

B

M

B1

G2

ZZ2

Z3G3

B

M

B1

G

G3

GG3

FREE SURFACE CORRECTION

GG3 =B3 x L

12 x 35 x Wf

B = BREADTH OF COMPTL = LENGTH OF COMPTWf = SHIP'S DISPLACEMENT

FREE SURFACE CORRECTION

= MOMENT OFINERTIA

=SHIP'S DISPLACED VOLUME

STATIC EFFECTS

Virtual rise in center of gravity. Smaller range of stability. Smaller righting arm. Small angle at which maximum righting arm occurs. exaggerated list and trim if the ship is listing or trimming.

DYNAMIC EFFECTS

It has nothing to do with the dynamic effects of the water rushing back and forth.

This effect is also detrimental but is not described by the free surface correction.

EXAMPLES Fire engine with out baffles. Baffles are a good way to minimize the dynamic effects of free

surface.

CASE STUDY

MAIN PARTICULARSLENGTH O.A. 97.054 m

LENGTH W.L 96.839 m

LENGTH B.P. 96.839 m

BEADTH MLD. 33.3 m

DRAFT 3.7 m

DISPLACEMENT 11080 T

GENERAL PARTICULARSNAME OF SHIP SURYA PRATHAMA

TYPE PONTOON

TONNAGEGROSS TONNAGE 18347.72 T

NET TONNAGE 1570.23T

MODELLING OF THE SHIP

PLAN AND PROFILE VIEW

ISOMETRIC VIEW

TANK PLAN

LOADING CONDITIONS

10 % ARRIVALLight Weight = 3474 TTotal weight =11076.848T DISP = 11076.910 T LCG = -0.468 m LCB = -0.467 m LCF = -1.149 m TPC = 31.083 T MTC = 222.587 m-t TRIM = -0.001 m MEAN DRAFT = 3.699 m DRAFT FP = 3.700 m DRAFT AP = 3.699 m

VCG = 2.655 m GMT UNCORR. = 24.854 m FREE SURF. = 0.779 m GMT CORR. = 24.076 m

FSM =8623.68T-m

Intact stability curveCOND: 10000000002 10% ArrivalDISPL= 11076.91 LCG= -0.47 VCG= 2.66

0 15 30 45 60 75 90

0.0

7.5

15.0

22.5

30.0

GZ IN METRES

ANGLE IN DEGREES -> Starboard

GZ

GMT

Light Weight = 3474 TTotal weight =11068T DISP = 11068.746 T LCG = -0.469 m LCB = -0.469 m LCF = -1.149 m TPC = 31.083 T MTC = 222.587 m-t TRIM = -0.001 m MEAN DRAFT = 3.697 m DRAFT FP = 3.697m DRAFT AP = 3.696 m

VCG = 2.509 m GMT UNCORR. = 25.017 m FREE SURF. = 0.591 m GMT CORR. = 24.426 m

FSM =6539.36T-m

100% Fully Loaded Departure

Intact stability curveCOND: 10000000001 100% Fully Loaded DepartureDISPL= 11068.75 LCG= -0.47 VCG= 2.51

0 15 30 45 60 75 90

-7.5

0.0

7.5

15.0

22.5

GZ IN METRES

ANGLE IN DEGREES -> Starboard

GZ

GMT