Wave basin model tests are performed to determine the motion behaviour of a vessel or floating construction. They are performed for a predetermined loading condition, often being a limiting operational condition with challenging motion behaviour. To represent the motion behaviour for this operational condition, accurate modelling of the predetermined weight distribution (mass, centre of gravity and inertia properties) is of crucial importance.
How to determine weight distribution? – ‘Resonic-F’ has the answer
Jan de Boer,
j.d.boer@
marin.nl
Until now the calibration of the weight distribution was a cumbersome process without a direct measurement of the final as-built weight distribution. Today we have found a new way to accurately measure the weight distribution using the fully ballasted and outfitted test model.
Resonic, a German company, came up with a new machine concept to measure the mass properties, the so-called Resonic-F. This machine is suitable for objects between 20 - 2000 kg covering the complete range of models used for seakeeping model tests.
The Resonic-F consists of a table floating on air bearings, which is kept in position by springs. This table can move freely in 5 degrees of freedom. The model is placed on the table and is randomly oscillated in all five directions. From the motions of the ta- ble, the as-built model mass, centre of gravity, moments and products of inertia are determined. The uncertainty of these values is less than 0.5%. This was validated with objects with well known mass properties.
The Resonic-F has now been in use for a year and is fully integrated in our test procedures. Each model used for seakeep- ing tests is positioned on the table twice. This is carried out once, while empty, in order to order to determine the inertia of the bare
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hull model. Based on this measurement the positions of ballast weights are determined in order to realise the specified weight distribution. Finally, the model is positioned on the table again to measure the realised weight distribution. This will always slightly deviate from the specified distribution.
We can now provide clients with the as- built values of the model during the tests. The as-built values include the moments of inertia and also the products of inertia - valuable data for further design and engineering simulations.
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