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…loads

Ocean and Naval Architectural Design

Loads & the ship girder

  Loads

  Loads on ships at sea are difficult to predict. Indeed, we cannot determine them in advance with complete certainty. Probabilistic methods are useful for predicting loads of ship structures.

  At this stage, we can look at the main loads on the ship’s hull in simple terms. Likewise, we can look at the ship structure in its simplest form: as a ship hull girder – a watertight box beam.

  Loads

Loads, ship girder, steel

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  Loads

  These concepts will help us understand the fundamentals of loads, structural strength and structural arrangements.

  This also provides a useful introduction to the main material used in ships and floating offshore platforms: steel.

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Loads, ship girder, steel

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Loads, ship girder, steel

  Loads

  Weight   Buoyancy   Hydrostatic pressure   Wave   Inertial   Slamming   Drydocking & launching   Vibration   Sloshing

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Loads, ship girder, steel

  Loads

  Weight loads   Structural elements   Machinery   Cargo   Outfit   Equipment   Fuel   Etc.

  gΣmi = weight

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Loads, ship girder, steel

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  Weight loads

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Loads, ship girder, steel

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  Buoyancy loads   ρgV = buoyancy

  ρgΣAi×Δxi ~ buoyancy distribution follows the curve of cross sectional areas along the length…

  Bonjean curves

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Loads, ship girder, steel

  Loads

  Buoyancy loads

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  Loads

Loads, ship girder, steel

  Loads

  Distribution of weight & buoyancy loads in calm water.

  Not in balance throughout the length of the ship. (The total weight must equal the total buoyancy.)

  Resulting load distribution causes the overall structure – the hull girder – to bend.

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Loads, ship girder, steel

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  Consider the case illustrated below:   Weight per unit length distribution follows the

‘lumpy’ distribution of masses (e.g. machinery, cargo, etc.

  Buoyancy distribution follows the volume of the under water hull over the length of the ship.

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Loads, ship girder, steel

  Loads

  Consider the case illustrated below:   There is more buoyancy than weight in the

midship region.   There is more weight than buoyancy at the stern

and the bow.   This loading scenario will cause the hull to HOG.

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Loads, ship girder, steel

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  Exaggerated view of a ship hull in the hogging condition.   The hull girder bends elastically in response to

the distribution of weight and buoyancy loads.

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Loads, ship girder, steel

  Loads

  Exaggerated view of a ship hull in the hogging condition.   The hull girder bends elastically in response to

the distribution of weight and buoyancy loads.

Where are the maximum stresses?

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Loads, ship girder, steel

  Loads

  In response to the still water weight and buoyancy loads, the vessel will bend as illustrated above.

  In the case discussed, the girder is bending longitudinally in the vertical plane. This is generally the primary loading condition (although not the most onerous loading case).

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Loads, ship girder, steel

  Loads

  There are also dynamic loads that have to be accounted for:

  Wave loads ~ cause longitudinal bending in the vertical (hogging and sagging) & horizontal planes, torsional bending and transverse bending.

  Inertial loads ~ ships in motion experience accelerations, giving rise to inertial loads (mi×ai). E.g. consider the inertial loads of an engine on its supporting structure.

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Loads, ship girder, steel

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  There are also dynamic loads that have to be accounted for:

  Slamming and shipping (green water) loads ~ cause very high pressure loads on the structure at the bow.

  Sloshing and hydrostatic pressure loads in cargo tanks

  Vibration loads set up by machinery.

  Launching and drydocking loads.

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Loads, ship girder, steel

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  Back to longitudinal bending in the vertical plane

  The structure has to resist the bending loads and do so while maintaining watertight integrity.

  Only structural elements that run longitudinally throughout (most of the length of) the ship contribute to the hull girder strength that resists this bending loading.

  These structural elements must be continuous in order to transfer the loads from one element to the next; otherwise, stress concentrations will occur.

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Loads, ship girder, steel

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  Buoyancy loads in “sagging” wave condition.   Wavelength equal to ship length, crests at bow &

stern & trough at midships   How to find buoyancy load distribution?

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Loads, ship girder, steel

  Loads

  Buoyancy loads in “hogging” wave condition.   Wavelength equal to ship length, troughs at bow &

stern & crest at midships

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Loads, ship girder, steel

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  How to find buoyancy load distribution?   Bonjeans.

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Loads, ship girder, steel

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  Buoyancy loads in “hogging” wave condition.

  References

  Zubaly, R. 1996. Chapter 7: Ship Strength in Applied Naval Architecture, Society of Naval Architects & Marine Engineers.

  Rawson, K.J & Tupper, E.C. Strength. 1983. Chapter 6: The Ship Girder in Basic Ship Theory, Longman Scientific & Technical.

  Van Dokkum, K. 2003. Ship Knowledge, A Modern Encyclopedia, Dokmar.

Loads, ship girder, steel

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…ship girder

Ocean and Naval Architectural Design

Loads & ship girder