S1 Geometry&Stability

8/10/2019 S1 Geometry&Stability

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GEOMETRY OF SHIPS AND SHIP-SHAPEDDEEPWATER FLOATING SYSTEMS

Principles of Naval Architecture, Ed. Lewis, SNAMEhttp://wetlands.simplyaquatics.com/d/14881-1/chap_1.pdf


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Ships and ship-shaped offshore platforms have been key to deepwater field

developments.

Tankers move oil from its source to the refinery.

Seismic surveys are done by specially outfitted ships for this purpose.

Exploratory drilling makes use of drill-ships, which may be called as

ship-shaped drilling rigs.

Production and processing equipment may be placed on ship- shaped

or barge-shaped structures called FPSOs (floating, production, storage, and

Offloading) units.

Floating ship-shaped offshore structures serve the functions of storage of

crude oil and their offloading into shuttle tankers.

Processed oil in platforms may be stored in floating ships or

barge-shaped structures called FSOs (floating, storage, and offloading units),

to be offloaded into shuttle tankers.


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It is imperative that design of ship-shaped offshore platforms will require

knowledge of ship geometry, ship stability, ship structural design and

motion of platforms in waves (without speed).

The exterior of ships hull is a curved surface which must be represented

precisely so that all internals can be accommodated and the hull can be

built. The shape of the outer surface of the ships hull is defined by Lines

Drawing or simply the Lines . It consists of orthographic projections of

the intersection of hull form with 3 mutually perpendicular (chosen

suitably) planes, all drawn to a suitable scale.

Lines define geometry of Moulded Surface, which is the surface inside

the skin (i.e. shell plating) of the ship.


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Aft perpendicularLocation.


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Sectional Area Curve


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Bonjean curves


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Form Coefficients

= Immersed volume L = Length between perpendicularsB = Beam T = Draft

AM= Area of midship section A WP = Area of waterplaneLWL= Length of waterline D = Depth of midship section

Then

= ; =

; =

; =

; =

The block coefficient ( C B)is the ratio of the immersed hull volume at aparticular draft to that of a rectangular prism of the same length, breadth,and draft as the ship.

The midship section coefficient ( C M ) is the ratio of the area of the immersedmidship section ( AM ) at a particular draft to that of a rectangle of thesame draft and breadth as the ship.

The waterplane coefficient ( C WP ) is the ratio of the area of the waterplane( AWP ) to that of a rectangle of the same length and breadth as the ship.


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The longitudinal prismatic coefficient ( C P ) is the ratio of the immersedvolume to the volume of a prism with length equal to the ships and cross -section area identical to the midship section.

The vertical prismatic coefficient ( C VP ) is the ratio of the immersed hullvolume to the volume of a prism having a length equal to the ships draft anda cross section identical to that of the waterplane.

Typical dimensional ratios

Ship type L /B B /T T /DGeneral Cargo 6.3 to 6.8 2.1 to 2.8 0.66 to 0.74

Tankers 7.1 to 7.25 2.4 to 2.6 0.76 to 0.78

VLCC 6.4 to 6.5 2.4 to 2.6 0.75 to 0.78

Weight displacement of ship = ( is density of water) = 1025 ton/m 3 (sea water, SW); 1000 ton/m 3 (fresh water, FW)


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Hydrostatic Curves or Curves of Form


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Two major conditions of loading are referenced in dealing with

commercial vessels:

Ligh t sh ip , L igh tweigh t , or Ligh t Disp lacement The ship with all

items of outfit, equipment, and machinery, including boiler

water and lubricating oil in sumps, but without cargo, provisions, stores,

crew, or fuel.

Ful ly L oaded Lightship plus cargo, fuel, stores, etc., to settle theship to her load line. Also loaded , load , or full-load displacement. For

ships designed to carry different classes of cargo, full-load conditions

may be tabulated for each type of cargo.

The trim and stability booklet will normally tabulate stability data for

ballasted and partly loaded conditions, and for end of voyage and

intermediate conditions with varying amounts of fuel and stores

consumed.


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Deadweight ( DWT ) is the load carried by a ship. It is the difference between

the lightship displacement and total displacement of the ship at any time.

Maximum or load deadweight is the carrying capacity of a ship measured in

2,240-pound long tons, and is the difference between the lightweight and

fully loaded displacements. Deadweight includes fuel, provisions,

munitions, crew and effects, cargo, or any other weight carried. For a

merchant ship, cargo deadweight , paying deadweight , or payload is thepart of the deadweight that is cargo and therefore earning income.

=

= Deadweigh t coefficient; = Weight displacement at full load

DWT = Deadweight


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Tons per cm of immersion: TP cm = AWP / 100

=

=

= + = +

= =

=100

= + = + = =

Tp = Draft, T = Draft variable, K: Keel, B: Cnetre of buoyancy,M: Transverse Metacentre, M L: Longitudinal metacentre,

G: Centre of gravity, BM = Transverse metacentric radius,

BML = Longitudinal metacentric radius, GM = Transverse metacentric height,

GM L = Longitudinal metacentric height.

For rectangular waterplane:IT = LB 3/12, I L = BL 3/12


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(a) and (c) are 180 deg apart.


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Ships are inclined by various external forces:


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Ships are inclined by various external forces:Wave action,Wind,Collision,Grounding,Shifting of onboard weights, and

Addition or removal of weight.

Heel Noncyclic, transient inclinations caused by forces that may be

removed or reversed quickly. Such forces include wind pressure, centrifugal

force in high-speed turns, large movable weights, etc.

Lis t A permanent, or long-term inclination, caused by forces such as

grounding or offcenter weight that are not likely to be removed suddenly.

Rol l When an inclining force is suddenly removed, a ship does not simply

return to its upright position, but inclines to the opposite side and oscillates,

or rolls, about its equilibrium position for some time before coming to rest.

Rolling is cyclic in nature and is induced or aggravated by short duration,

repetitive or cyclic forces, such as wave forces.


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GZ = GM sin

Righting moment = W GZ


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Inclining ExperimentThe measurable effects of off-centerplane weight are used to

determine height of center of gravity in an inclining experiment. Byshifting a known weight ( w ) a specified distance ( d ) athwartship, the

movement of the center of gravity can be determined.

Righting moment = . . sin

Heeling moment = cos

Equating RM and HM; tan

W GZ W GM

wd

wd GM W

KG KM GM KB BM GM


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A ships afloat stability can be impaired or otherwise changed by any of the

following:

Addition, removal, or shift of weight, changing KG,

Change in the shape of the submerged hull from grounding or hull

damage changing KM ,

Free surface effect of loose liquids ( FS ), causing a virtual rise of G,

Free communication with the sea ( FC ), causing a virtual rise of G, or

Any combination of the above.

The first three conditions affect stability of the intact ship as well. Only free

communication with the sea is predicated on damage to the hull.

GM = KM KG FS FC


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Evaluation of Large Angle and Dynamical Stability


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Pressure on ship due to beam winds: P


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The term seakindly is used to describe a ship whose metacentric height

is great enough to give adequate stability, but not large enough to cause

excessive stiffness.

The natural rolling period is a function of weight and buoyancy

distribution and can be expressed as a function of GM and transverse

radius of gyration ( k ):

T R = natural rolling period, seconds

k = transverse radius of gyration of the ship mass

0.4 to 0.5 times the beam


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S1 Geometry&Stability

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