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Diploma in Shipping Logistics

General Ship Knowledge

Unit 6

Stabilization and Stress Control

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Lessons in Unit 6

Lesson 1: Stabilization – loading, discharging and shifting weights.

Lesson 2: Stress Control, Shear Forces and Bending Moments

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Stabilization

Having completed Unit 5 we should understand by now that a key factor in determining the stability of a ship is the value of its GM or metacentric height.

Positive GM is always desirable over neutral or negative GM. It is therefore worthy to note that an increasing centre of gravity reduces GM and affects the ship’s stability.

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Movement of Centre of Gravity

The centre of gravity of a ship moves depending on where the cargo is loaded, discharged or shifted.

When cargo is loaded the CoG moves towards the cargo.

When cargo is discharged, the CoG moves away from the cargo that is loaded.

When cargo is shifted, the CoG moves parallel to the direction that the cargo moves.

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Movement of Centre of Gravity

We will assume that any weight aboard the vessel is not free to move relative to the vessel. For stability calculations, such weights form an integral part of the vessel and need no special consideration. Weights that can move relative to the vessel are treated differently. Examples of such weights are:

Suspended weights Free surface liquids in tanks Cargo that shifts due to improper stowage These weights may have serious effects on a ship’sstability.

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Suspended Weights

When a weight is lifted by the ship’s crane, the centre of gravity of that weight effectively acts from the top of the crane just at the moment when the load is lifted. This is known as the virtual centre of gravity of the load. Note that the centre of gravity does not act from the load. When this occurs, the CoG of the ship will go upwards thus reducing the GM and the effective stability of the vessel.

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Free Surface Effect

Free surface liquids in tanks will move whenever the ship heels. This movement will adversely affect the stability of the ship in the same way as does a suspended weight. Some free-flowing bulk cargoes, such as grain, can also exhibit this free-surface effect if stored in holds that are not full.

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Free Surface Effect (FSE)

Diagram showing virtual CoG of a free surface liquid

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Free Surface Effect

FSE in a full tank

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Shifting Cargo

Cargo may shift if not properly stowed and secured. We are considering cargo that moves slowly relative to the motion of the vessel in heavy seas. The effect of this condition gets progressively worse as the voyage proceeds causing the vessel to list, or its trim to change. This can be extremely difficult for the vessel and its crew because the stability of the ship is reduced when weather and sea conditions are bad, and often at a time when the crew can do little to correct the situation.

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Reduced Stability

Virtual Effect of Free Surface

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The Effective Righting Moment

RM = displacement × G'Z'

RM = displacement × GVZV

RM = displacement × (GM – GGV) sinө°

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Stresses in Ships

There are complex stresses acting on a ship’s structure resulting from the effect of cargo, stores, ballast, bunker distribution and even drydocking.

The combined weight of the vessel and any weights on board were considered to act vertically downward through the centre of gravity, G. Similarly, the upthrust due to buoyancy was considered to act vertically upwards through the centre of buoyancy, B. In reality, the combined weight is composed of several components, which act in different parts of the ship. Similarly, the buoyancy acts over all of the underwater section of the ship. If the downward and upward forces at a point are not equal then stresses will result.

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Shear Force in Ships

A shear force in a ship is most evident when a hold or tank is full and the adjacent hold or tank is empty. It is commonly measured in kilograms-force or tonnes-force.

The buoyancy upthrust acting on the two holds is approximately equal as they are both in the midsection of the ship, however, hold No. 3 has less cargo than hold No. 4. The resultant force is the shear force, which will induce a shear stress tending to shear the vessel.

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Shear Stress

Shear stress on a ship

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Bending Moments

Bending moments induce tensile and compressive stresses in a ship.

A ship with 5 hatches has holds No. 1 and 5 loaded and the others are empty. The combination of the excess gravitational forces on holds No. 1 and No. 5, and the excess buoyancy forces on holds 2, 3, and 4 induce a tensile stress in the deck, and a compressive stress in the keel resulting in hogging condition.

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Bending Moments

Hogging

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Bending Moments

Sagging