Published on November 6, 2007
Ship Structures: Ship Structures Slide2: 6.3 Ship Structure 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 Slide3: Longitudinal Structural Components (cont’d) 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 Slide4: 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 Slide5: 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) Slide6: LONGITUDINAL MEMBERS TRANSVERSE MEMBERS FLOOR LONGITUDINAL DECK BEAM FRAME STRINGERS DECK GIRDERS DECK PLATING PLATING KEEL Slide7: LONGITUDINAL MEMBERS TRANSVERSE MEMBERS FLOOR LONGITUDINAL DECK BEAM FRAME STRINGERS DECK GIRDERS DECK PLATING PLATING KEEL Slide8: The ship’s 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 Longitudinal Framing System Transverse Framing System Combination of Framing System Slide9: Longitudinal Framing System A typical wave length in the ocean is 300 ft. Ships of this length or 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 Slide10: 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 Slide11: 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 Slide13: 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 Slide14: 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 Slide15: Modes of Structural Failure 1. Tensile or Compressive Yield 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) Slow plastic deformation due to an applied stress greater than yield stress Slide16: 2. Buckling Examples: Deck buckling: by sagging or hogging, loading on deck Side plate buckling: by waves, shock, groundings column bucking: by excessive axial loading Substantial dimension changes and sudden loss of stiffness caused by the compression of long column or plate Slide17: 3. Fatigue Failure Endurance limit- stress below which will not fail from fatigue 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 The failure of a material from repeated application of stress such as from vibration Slide18: 4. Brittle Fracture Depends on: - Material (low toughness & high carbon material) - Temperature (operating below transition temperature) - Geometry (weak points: sharp corners, edges) - Type/Rate of Loading (tensile/impact loadings are worse) A sudden catastrophic failure with little or no plastic deformation Slide19: 5. Creep Example : piano wires Creep is not usually a concern in ship structures The slow plastic deformation of material due to continuously applied stresses that are below its yield stress.