Topic- 13 Stability Considerations

      1. Crane Outreach

When using cranes and other lifting gear such as A frames that are barge mounted, it must be noted that the weight of the lifted load acts at the point of suspension – not at the base of the crane. The overturning moment on the barge, tending to cause it to capsize, is the product of the weight of the lifted load, and the (horizontal) distance of the point of suspension from the centre of buoyancy.

The greatest uplift or detachment force, acts at the point of attachment (of the crane to the barge) furthest from the point of suspension. This is the force tending to turn the crane over and the moment of this force is the product of the weight of the lifted load, and the (horizontal) distance of the point of suspension from the point of uplift.

 2. Free surface effect

Fluids such as fuel and water can adversely affect the stability of a moving vessel. A shallow covering of water over a large enclosed deck can cause a significant problem. 150 mm of fresh water covering a 24 m by 6 m deck weighs 21.6 tonne, and as the vessel rolls this weight will be transferred outboard to the down side of the roll. Sloshing is another phenomenon, which can greatly amplify the destabilizing effect of a large free surface of fluid. The effect of sloshing is worst if the movement of fluid coincides with the movement of the vessel. Baffles are used to break up the free surface within a tank and to prevent sloshing.

3. Shifting Cargo

Securing arrangements should be of such design that they are strong enough to prevent any cargo movement during transit. Maritime Rule part 24B gives prescribed requirements for stowage and securing of all cargoes.

       4. Loading and Discharge

It is vital that stability is considered during all phases of barge operations, including loading and discharge. The stability conditions during loading and discharge are often quite different from those when fully loaded. High loads, moving loads, and off–centerline loading plans all need special consideration. A low initial GM value, a combined KG that is close to or below the required minimum and small righting areas all mean that the loaded barge will have poor recovery characteristics when rolling in a seaway.