the anatomy of the ship

Chalmers University of Technology

Department of Shipping and Marine TechnologyDivision of Ship Design and Engineering

Ship Structures Advanced Course 2007, p. 1 (31)Associate Professor Jonas RingsbergLECTURE 1

Contents - Lecture 1

• General introduction to ship structures and loads.– Introduction.– The anatomy of the ship: structural functions of elements.

• The hull girder.• Primary strength elements.• Secondary strength elements.• Bottom structures.• Deck structures.• Side structures.• Transverse bulkheads.• Fore ship and stern.• Details.

– Different types of loads that act on the ship.




Introduction

• Cargo carrying ships are designed with optimum transport work as the main design goal.– The main dimensions of the ship, the general arrangement etc are

determined with this purpose and safety and environmental requirements as boundary conditions.

• The structural design is made after the general arrangement.

• The weight of the ship hull structure and the building costs are very important for the total ship capital cost and future earning capacity.– Optimisation of the hull structure is usually made within the

classification rules, i.e. without endangering the safety.




Introduction

• The design solution is more or less based on experiences.

• The classification rulebooks are descriptions of “survivors”.




The anatomy of the ship

Global structural effects

Local structural effects

• Beam theory(bending and twisting)

• Classification rules• FE analysis

• Plate theory andeffective breadth ofstiffened plating

• Buckling• FE analysis

SSAC 2007

SSBC 2006




The anatomy of the ship Shell behaviour – effective breadth




The anatomy of the shipStiffened plate – effective breadth




The anatomy of the ship

• Skin plate of deck, sides and bottom: – Geometric boundaries to the ship hull girder.– Skin plates are supported by longitudinal or transverse stiffeners in

order to make them stiff to carry planeloads without buckling and also to carry lateral load without too large stresses and deflections.

• Transverse bulkhead: – Maintains the shape of the hull girder. It also subdivides the tanks in

different dedicated spaces, i.e. cargo holds or ballast tanks.• Bottom plate:

– Takes lateral loads, e.g. water pressure. These loads are distributed and transferred into the ship structure via longitudinal stiffeners to the structural members of the hull girder.










The anatomy of the shipThe hull girder

• The hull girder is the highest level.– All local differences between external and internal loads will in the end

be seen as shear forces and bending moments in the hull girder.– The hull girder can be seen as one large beam, where the side shell is

web plates and the deck and the bottom structures are flanges.





• The design of the hull girder is normally driven by the vertical bending moment.

• The longitudinally oriented stresses are derived from the total bending moment.

– The longitudinal strength of the girder is determined by its section modulus.

– All structural elements with continuous reach in the longitudinal direction contribute to the stiffness and strength of the hull girder.

• Deck and bottom structures have the most significant influence on the section modulus as they are far from the neutral axis.





Dry cargo ship with longitudinal framing

Bulk ship with single skin

Tanker with single skin




The anatomy of the shipPrimary strength elements

• Objective with primary strength elements: – To maintain the shape of the hull girder on the local level.

• Deck transverse or deck beam.• Web, main frame, vertical web side.• Stringer.• Longitudinal girder.• Floor or bottom structure.




The anatomy of the shipSecondary strength elements

• Objective with secondary strength elements:– To support the plates to maintain their shape and to limit the spacing in

order to keep plate stresses within reasonable values.• Stiffeners.• Beams.• Longitudinals.• Stays.• Tripping brackets.




• The purpose of stiffeners is to prevent the plate areas of a ship from distorting under the influence of the shearing loads, bending moments, and local loads.

The anatomy of the shipSecondary strength elements – stiffeners




The anatomy of the shipSecondary strength elements – stiffeners




The anatomy of the shipBottom structures

• Today, most commercial ships are built with double bottom.– Beneficial from a strength point of view as the external bottom and the

inner bottom plating are effectively utilised as flanges to the primary structural members (girders and floors).

– From environmental point of view, double bottom is preferred instead of single bottom in order to prevent oil spill at e.g. grounding.




The anatomy of the shipBottom structures

• The main (local) loads comes from lateral pressure, e.g.:

– external hydrostatic load, and/or– load/pressure from cargo.

• Bottom longitudinalscontribute a lot to the global hull girder section modulus.

• Floors reach between longitudinal bulkheads and side shells.

• Bottom girder (CL-and side girders):– The CL-girder supports the ship in the

dry docking condition.– Girders redistribute loads on one floor

to neighbouring floors in the case of very local cargo loads.




The anatomy of the shipDeck structures

• The total area of the deck plating and longitudinal members of the deck are mainly determined by the explicit contribution to the hull girder section modulus.

• The worst stresses of the hull girder will appear in the “shear strake”, i.e. the plate that connects the side shell and the deck plate.




The anatomy of the shipDeck structures

• Large container ships may have difficulties to achieve enough material in the deck to get section modulus.– This is due to the requirement of vertical load handling with very large

hatch openings.

• Local lateral load on Ro-Ro decks may be quite severe due to wheel loads from e.g. trucks.




The anatomy of the shipSide structures

• When the hull girder is exposed to vertical loading, the side structures are the shear elements of the structure.– The plate thickness is needed to maintain reasonable shear stresses.– The side structure must also be able to withstand external hydrostatic

and wave pressure loads.– Torsion rigidity is enhanced by double skin structure compared with

single skin structure.




The anatomy of the shipTransverse bulkheads

• The function of a transverse bulkhead can vary:– Subdivision into several tanks (tank bulkhead).– Strength bulkhead: main function is to maintain the shape of the hull girder.– Swash bulkhead to reduce problem with waves in cargo holds.– Collision bulkhead (usually placed 0.05L - 0.08L aft of FP).– Aft peak bulkhead.– Engine room bulkhead.

Single skin Double skin

Shear flow in single skin structure




The anatomy of the shipFore ship and stern

• Fore ship– Is subjected to large pressure from

the waves and from the pitching motion of the ship. The ship must be strengthened against these loads.

– Slamming loads may also occur.– Ice strengthening may require

frames between the original frames, intermediate frames, in addition to a thicker plate.

• Stern– Is determined by stiffness criteria in

order to avoid vibration from the engine room and the propeller.

– Web frames are deeper around the engine and the floors are usually located on each frame to increase stiffness.




The anatomy of the shipFore ship and stern




The anatomy of the shipDetails

• To avoid structural damage, all details need to be considered.

• Details may fail due to overload but more frequently die to fatigue.

• Continuity and smooth notches are the keys to low stress concentrations in the hull girder and in all members of the ship that are subjected to dynamic loads.

– Dynamic loads may come from waves or from propulsion and engine.




Different types of loadsthat act on the ship

• The deterministic load conditions given by the classification societies are to be seen as useful for assessment rather than real load cases.

• Static loads– Still water loads:

• Global (loads that affect the hull girder).

• Local (loads that affect the local structures like plates, stiffeners and frames)

– Frequency

• Dynamic loads (causes both local and global loads)

– Wave induced loads:• Variation in wave induced pressure

due to waves and the ship motions.• Variation in the inertia forces caused

by the motion of the ship in waves.

• Vibration– Global vibration:

• The hull girder performs modal vibration like a beam the first ten modes being vertical hull girder bending modes.

– Local vibrations:• From engine and propellers.








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Global static loads and wave influence:









Stress distribution and deflection simulation using the finite element method.





• Strength criteria:– Collapse (buckling, yielding, brittle fracture).– Fatigue.

the anatomy of the ship

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