Numéro : 2819 - Year : 2025
Importance of measurement to the stability assessment of operational vessels
Alain COTA - Société d’Exploitation des Procédés Alain Cota – Paris
Initially, every vessel is designed to be stable, with its typical loadings in the face of the most severe conditions at sea that it may encounter, in an intact state or realistically damaged condition.
For normal operations, the Master has access to the required GM curves, established at the design stage. These curves indicate that stability is sufficient to withstand the hypothetically worst external conditions and any conceivable damage.
Realistically these curves established at the outset are often unusable, especially following damage, and the captain often lacks sufficient knowledge of the situation to replace them with a calculation. The stability curve GZ(θ), the basis of this calculation, can be constructed from four parameters: fore and aft drafts, heel, and GM value. Importantly however, all these parameters are accessible for measurement, even GM through an analysis of the vessel's rolling motion. By adding the roll amplitude, which is also measurable, all stability parameters can be obtained, and in particular, the values of the roll/wind torque can be monitored relative to their acceptable values. Performed continuously, these measurements make it possible to track changes in stability following progressive flooding. A combination of weather forecast on an emergency course to steer, makes it possible to identify a safe return to port.
This report specifies the measurement and processing conditions to obtain reliable results for heel, roll amplitude, and the roll period, the basis for measuring GM. It also specifies the method for converting the roll period to GM, in accordance with IMO recommendations.
The period measurement is applied in the same way to the pitching motion, which makes it possible to visualize the risk of resonance, parametric roll, or loss of stability in a following sea.
A final application of real-time measurement of the period of slow oscillations is mentioned (since ATMA is also aeronautical) for the recognition of phugoid oscillations in aircraft instability.
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