Ship tecnic Sharif university Lecture 4

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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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