Thursday, February 3, 2011

Fundamental of floater naval architecture considerations

Ship or floater ( jackup or semi-submersible ) stability is a complicated aspect of naval architecture which has existed in some form or another for years. Historically, floaters stability calculations relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship/floater model basin allows much more complex analysis.

When a ship, jackup or semi-submersible hull is designed, stability calculations are performed for the intact and damaged states of the vessel. Floaters are usually designed to slightly exceed the stability requirements (refer below), as they are usually tested for this by a classification society. Jack-ups are also considered in the stability calculation as the rig will at times move to other drillsite locations either by wet tow, in terms of staggered field tows or long ocean towage.

Intact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.

Damaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.

The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height (GMt). This assumes the floater remains completely stationary and upright or with slight heel or trim. However, once the ship or floater is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.

Stability is also lost due to flooding when, for example, an empty tank is holed and filled with seawater. The lost buoyancy of the tank results in that section of the ship lowers into the water slightly. This creates a list unless the tank is on the centerline of the vessel.

In stability calculations, when a tank is holed, its contents are assumed to be lost and replaced by seawater. If these contents are lighter than seawater, (light oil for example) then buoyancy is lost and the section lowers slightly in the water accordingly.

An Inclining test is performed on a ship or floater to determine its stability and the coordinates of its center of gravity. The test is applied to newly-constructed floaters greater than 24m in length. Inclining test procedures are specified by the International Maritime Organization and other international associations.

The weight of a floater can be readily determined by reading draughts and comparing with the known properties. The metacentric height (GM), which dominates stability, can be estimated from the design, but an accurate value must be determined by an inclining test. During the construction of the rig, weight control report updating the current design lightship weight of the vessel is done progressively from various inputs, eg, the steel weights, equipment weights,etc. from various design engineering disciplines.

The inclining test is usually done inshore in calm weather, in still water, and free of mooring restraints to achieve accuracy. The GM position is determined by moving weights transversely to produce a known overturning moment in the range of 1-4 degrees if possible. Knowing the restoring properties (buoyancy) of the rig or vessel from its dimensions and floating position and measuring the equilibrium angle of the weighted vessel, the GM can be calculated. Usually this kind of test takes less than two days, however for jackups, may take slightly longer as it involves spudding down the legs at the quayside after completing the test.

For U.S. flagged vessels, blueprints and stability calculations are checked against the U.S. Code of Federal Regulations (CFR) and SOLAS conventions. Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. The extent of damage required to design for is included in the regulations. The assumed hole is calculated as fractions of the length and breadth of the vessel, and is to be placed in the area of the ship where it would cause the most damage to vessel stability.

NA Lecture Notes

MARIN, the Maritime Research Institute Netherlands, is one of the leading institutes in the world for hydrodynamic research and maritime technology. The services incorporate a unique combination of simulation, model testing, full-scale measurements and training programmes. MARIN provides services to the shipbuilding and offshore industry and governments. Today MARIN disposes of the following 7 test facilities: Shallow water basin, Deep water basin, High speed basin, Offshore basin, Seakeeping and Manoeuvring basin, Vacuum tank and Cavitation tunnel.


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Naval Architecture

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Kim Whye said...