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Chapter 4
Ship Structure
and
Structural Analysis
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Contents
1- Loading
2- Structural Elements3- Structural Analysis
4- Structural Failure
5- optimization
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1- Loading
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2- Structural Elements
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Panmax Bulk Carrier
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Longitudinal Structural Components
Starting from the keel to the deck:
Keel
- Large center-plane girder- Runs longitudinally along the bottom of the ship
Longitudinals
- Girders running parallel to the keel along the bottom- It provides longitudinal strength
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Longitudinal Structural Components (contd)
Deck Girder
- Longitudinal member of the deck frame (deck longitudinal)
Stringer- Girders running along the sides of the ship
- Typically smaller than a longitudinal
- Provides longitudinal strength
.Primary role of longitudinal members :
Resist the longitudinal bending stress due to sagging and hogging
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Transverse Structural Components
Floor
- Deep frame running from the keel to the turn of the bilge
Frame
- A transverse member running from keel to deck
- Resists hydrostatic pressure, waves, impact, etc.
- Frames may be attached to the floors (Frame would be the
part above the floor)
Starting from the keel to the deck:
Deck Beams
- Transverse member of the deck frame
Primary role of transverse members : to resist the hydrostatic loads
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Plating
- Thin pieces closing in the top, bottom and side of structure
- Contributes significantly to longitudinal hull strength
- Resists the hydrostatic pressure load (or side impact)
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LONGITUDINAL
MEMBERS
TRANSVERSE
MEMBERS
FLOOR
LONGITUDINAL
STRINGERS
DECK
GIRDERS
PLATING
KEEL
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LONGITUDINAL
MEMBERS
TRANSVERSE
MEMBERS
FLOOR
LONGITUDINAL
STRINGERS
DECK
GIRDERS
PLATING
KEEL
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The ships strength can be increased by:
- Adding more members
- increasing the size & thickness of plating and structural pieces
All this will increase cost, reduce space utilization, and
allow less mission equipment to be added
Optimization
Longitudinal Framing System
Transverse Framing System
Combination of Framing System
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Longitudinal Framing System
A typical wave length in the ocean is 300 ft. Ships of this lengthor greater are likely to experience considerable longitudinal
bending stress
Ship that are longer than 300ft (long ship) tend to have a
greater number of longitudinal members than transverse
members
Longitudinal Framing System :
- Longitudinals spaced frequently but shallower
- Frames are spaced widely
Primary role of longitudinal members :to resist the
longitudinal bending stress due to sagging and hogging
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Transverse Framing System
Ships shorter than 300ft and submersibles
Transverse Framing System:
- Longitudinals are spaced widely but deep.
- Frames are spaced closely and continuously
Transverse members: frame, floor, deck beam, platings
Primary role of transverse members : to resist the hydrostatic loads
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Combined Framing System
Combination of longitudinal and transverse framing system Typical combination :
- Longitudinals and stringers with shallow frame
- Deep frame every 3rd or 4th frame
Optimization of the structural arrangement for the expected
loading to minimize the cost
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Double Bottoms
Resists:
- Upward pressure
- bending stresses
- bottom damage by grounding and underwater shock
The double bottom provides a space for storing:
- fuel oil
- ballast water & fresh water
Smooth inner bottom which make it easier to arrange cargo &
equipment and clean the cargo hold
Two watertight bottoms with a void space
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Watertight Bulkheads
Primary role
- Stiffening the ship
-Reducing the effect of damage
The careful positioning the bulkheads allows the ship to fulfill
the damage stability criteria
The bulkheads are often stiffened by steel members in the
vertical and horizontal directions
Large bulkhead which splits the the hull into separate sections
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Bulk Carrier
C i dditi
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Common corrosion additions
Long.bhd 2
Deck, external surfaceInternals in
upper portionof WBT
Stringer in WBT
Stiffenersin WBT
Deck and Sheerstrake in WBT
Sideshell in WBT
Webplate in WBT
Long girders in WBT Bottom and bilge
Faceplate in WBT
Stiffenersin WBT
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Hatch covers
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3- Structural Analysis
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Longitudinal Bending Stress
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Sagging
Hogging
BendingMoment
BowStern Keel : tension
Weather deck : compression
Bending
Moment
BowStern
Keel : compression
Weather deck : tension
Longitudinal Bending Stress
Logitudinal Bending Stress
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Sagging & Hogging on Waves
Sagging condition
Hogging condition
TroughCrest
Trough Crest
Crest
Trough
Logitudinal Bending Stress
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Distributed Forces
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Distributed Forces
Distributed Buoyancy
- Buoyant forces can be considered as adistributed force.
2 LT/ft
barge
50 ft
100LT50ftft
2LTFB
uniformly
distributed
force
Distributed Forces
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Distributed Forces
Distributed Weight
-Weight of ship can be presented as adistributed force.
- Case I : Uniformly distributed weight
2 LT/ft
barge
2 LT/ft
50 ft
B
s
F
100LT50ftft
2LT
Distributed Forces
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Distributed Weight
2 LT/ft
barge
1 LT/ft
50 ft
B
s
F100LT
100LT10ftft
1LT10ft
ft
2LT10ft
ft
4LT10ft
ft
2LT10ft
ft
1LT
- Case II :Non-uniformly distributed weight
2 LT/ft
4 LT/ft
2 LT/ft1 LT/ft
Distributed Forces
10ft
Shear Stress
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-Shear stress present at points P, Q, R, S & T due to unbalanced forces
at top and bottom.
- Load diagramcan be drawn by summing up the distributed
force vertically. 4 LT/ft
2 LT/ft
1 LT/ft2 LT/ft 2 LT/ft
1 LT/ft
1LT/ft2LT/ft
1LT/ft
O P Q R S T
Shear Stress
Load DiagramO P Q R S T
P
Shear Force at point P
Shear Stress
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Shear Stress
How to Reduce Shear Stress of ship
To change the underwater hull shape so that buoyancy
distribution matches that of weight distribution.
- The step like shape is very inefficient with regard to
the resistance.
- Since the loading condition changes every time, this method
is not feasible.
To concentrate the ship hull strength in an area where largeshear stress exists . This can be done by
- using higher strength material
- increasing the cross sectional area of the structure.
Logitudinal Bending Stress
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Logitudinal Bending Stress
Longitudinal Bending Moment and Stress
Uneven load distribution will produce a longitudinal
Bending Moment.Bending Moment
- Buoyant force concentrates at bow and stern.
- Weight concentrates at middle of ship.
The longitudinal bending moment will create a significant
stress in the structure calledbending stress.
Logitudinal Bending Stress
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Quantifying Bending Stress
Compression
Tension
Sagging condition
Neutral Axis
y
A
B
A
B
I
M y
Bending Stress :M: Bending Moment
I : 2nd Moment of area of the cross section
y : Vertical distance from the neutral axis
: tensile (+) or compressive(-) stress
Logitudinal Bending Stress
y
Longitudinal Bending Stress
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Quantifying Bending Stress
Hogging conditiony
Compression
Tension
Neutral Axis
AB
A
B
Neutral Axis : geometric centroid of the cross section ortransition between compression and tension
Longitudinal Bending Stress
Longitudinal Bending Stress
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Example :Bending Stress of Ship Hull
Ship could be at sagging condition even in calm water .
Generally, bending moments are largest at the midship area.
NeutralAxis
BowStern
A
B
Deck
Keel
B
A
Deck : Compression
Keel : Tension
Tickness
Longitudinal Bending Stress
crosssection
Longitudinal Bending Stress
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Example :Bending Stress of Ship Hull
Neutral Axis
BowStern
A
B
Deck
Keel
B
A
Tickness
Longitudinal Bending Stress
crosssection
y
Keel
This ship has lager bending
stress at keel than deck.
N.A.
Longitudinal Bending Stress
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Reducing the Effect of Bending stress
Bending moment are largest at midship of a ship.
Ship will experience the greatest bending stress at the deck
and keel.
The bending stress can be reduced by using:
- higher strength steel
- larger cross sectional area of longitudinal structural elements
Longitudinal Bending Stress
Logitudinal Bending Stress
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Hull Structure Interaction
Bending stress at the superstructure is large because of its
distance from the neutral axis.
In Sagging or Hogging condition, severe shear stresses between
deck of hull and bottom of the superstructure will be created.
This shear stresses will cause crack in area of sharp corners
where the hull and superstructure connect.
This stress can be reduced Expansion Joint
Logitudinal Bending Stress
Longitudinal Bending Stress
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Compression orTension on deck
Expansion Joint
By using Expansion Joint, the super structure will beallowed to flex along with the hull.
Compression orTension on bottom
Longitudinal Bending Stress
Example : Bending Stress
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Example : Bending Stress
Solid Beam
I-Beam
bh
b=ftm
h=1ft
b
h
43
12
1
12
1f tbhI
(1212 I
0.6h
0.3b
12
)6.0)(3.0(2
3hbI
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Torsion
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Zone and Local Strength
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Liquid Natural Gas (LNG) Ship
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Liquid Natural Gas (LNG) Ship
Global finite element model
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4- Structural Failure
Modes of Structural Failure
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1. Tensile or Compressive Yield
Slow plastic deformation of a structural component due to an
applied stress greater than yield stress
To avoid the yield, Safety factors are considered for ship
constructions.
Safety factor = 2 or 3
(Maximum stress on ship hull will be 1/2 or 1/3 of yield
stress.)
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2. Buckling
Substantial dimension changes and sudden loss of stiffness
caused by the compression of long column or plate
Buckling load on ship : cargo, waves, impact loads, etc.
Ex :
Deck buckling : by sagging or hogging, loading on deck
Side plate buckling : by waves, shock, groundings
column bucking : by excessive axial loading
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Buckling
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3. Fatigue Failure
The failure of a material from repeated application of stress
such as from vibration
Endurance limit : stress below which will not fail from fatigue
Fatigue failure is effected by- material composition (impurities, carbon contents,
internal defects)
- surface finish
- environments (corrosion, salinities, sulfites, moisture,..)- geometry (sharp corners, discontinuities)
- workmanship (welding, fit-up)
The fatigue generally create cracks on the ship hull.
Hull Structure
Fatigue
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Fatigue damages are caused by dynamic
loading
Fatigue
Fatigue Crack
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Fatigue Crack
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Fatigue Crack
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4. Brittle Fracture
A sudden catastrophic failure with little or no plastic deformation Brittle fracture depends on
- Material Low toughness & high carbon material
- Temperature Material operating below its transition temperature
- Geometry Weak point for crack : sharp corners, edges
- Type/Rate of Loading Tensile/impact loadings are worse
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5. Creep
The slow plastic deformation of material due to continuously
applied stresses that are below its yield stress.
Example : piano wires
Creep is not usually a concern in ship structures.
Structural Monitoring system
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4- Optimization
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Questions?
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