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Effect of cargo including heavy lifts on the seaworthiness and

stability of the ship

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A sea worthy ship

• can take it cargo to sea without risk of danger and damage to either the ship or the cargo.

• fit in relation to its hull structure and machinery, its holds and equipment, and its manning and shipboard procedures.

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Stress and Stability 1

• ship with insufficient stability may list excessively during cargo operations

• ship with insufficient stability could be swamped by heavy seas

• ship with excessive stability the cargo could shift, and risk of structural damage

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Stress and Stability 2

• ship may become unstable during the voyage as bunkers are consumed

• ship which is stress and strained by excessive bending moments or shear force may suffer structural failure. can occur immediately but more likely a long term build up

• Incorrect calculation of drafts may lead to grounding or refused permission to sail.

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Stability basics

• Heeled, ship inclined by and external force-e.g. wind

• Listed, ship is inclined by an internal force-e.g. cargo shifting

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Stability basics

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Stability basics

• Centre of gravity (G), the point through which the total weight of the ship and all weights on board, may be considered to act vertically down.

• Centre of buoyancy (B), the point through which the total force of buoyancy may be considered to act vertically up.

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Stability basics

• Archemedes’ principle

• Metacentre (M)

• Metacentric height (GM)

• Positive stability

• Negative stability

• Righting lever (GZ)

• Moment of statical stability (GZ x Disp)

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Behavior of a ship at sea

• Rolling period, function of the GM and the ship’s beam

• large GM, STIFF

• small GM, TENDER

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Stability calculations

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Stability calculations

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Procedure for calculating the ship’s stability

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Procedure for calculating the ship’s stability

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Procedure for calculating the ship’s stability

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Procedure for calculating the ship’s stability

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Free surface effect

• Whenever a liquid can move in a tank, ther is a reduction in the ship stability.(slack tank)

• GG”-G”M

• content of tank between 5% to 95% of its max capacity

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Free surface effect

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Effect of heavy lifts

• ship use its own cargo handling equipment to load a heavy lift, the critical stage occurs when the lift is just clear of the quay

GG” = (w x Gg”) / (W + w)

G’G” = (w x gg”) / (W + w)Tangent (angle of list) = G’G”/G”M

New draft (AC) = 1/2Beam x Sin(List) (AB) + Old Draft x Cos(List) (BC)

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Effective of heavy lift

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Investigating lists

• Cargo not loaded evenly about the centreline

• Cargo shifted in a nonworking compartment

• Ballast pumping at uneven rates in pairs of port and starboard tanks

• A valve leaking on a tank not in use

• The ship grounding on the inshore side

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Strength and Stress

• loading sequence• Shear force-tend to break the material across, it is

equal and opposite to the load applied at that point• Bending moment-total moment tending to alter the

shape of the structure, it is equal to the algebraic sum of the moment all the loads acting between that point and one end of the structure-Hog&Sag

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Shear force & Bending moment

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Hog & Sag

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Hog & Sag

• Sagged : weight of the cargo is placed in the middle of the ship.

• Hogged : weight of the cargo is placed near the end of the vessel.

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Grain stability

• The free-flowing characteristics of grain reduces the stability of the ship

• grain which does not completely fill a compartment displays a free surface effect similar to a liquid in a partially filled tank

• the grain is likely to flow to one side of the compartment

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Union Purchase

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rail mounted Granty Crane

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Swinging derrick

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Heavy lift derrick

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Stuelcken Jumbo

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Crane

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