The authors show an application of a non-linear strip method and a RANSE (Reynolds-averaged Navier-Stokes Equations) solver in comparison with experiments for green water on deck of a containership model. A slender ship is assumed divided into strips of equal length. For head waves, only heave and pitch are considered and surge is neglected. Added mass and damping coefficients are expressed for various cross sections as polynomial functions of current beam-to-draught-ratio, current section area coefficient, and non-dimensional frequency parameters. These hydrodynamic coefficients are stored in look-up tables for fast interpolation at each time step for all strips at actual local immersion. The incident wave is assumed to be disturbed by the bow wave, which is calculated at every time step, and the lateral and longitudinal radiated waves, whose elevations are postulated following observations of ships at sea and models in the towing tank. Green water is assumed when the freeboard is exceeded and the undisturbed relative motion also exceeds at the local freeboard at any station where the deck taper angle is positive. 'Light' deck wetness is assumed when freeboard is exceeded, the relative motion between ship and disturbed wave elevation is upwards,and falling water lands within the forecastle deck. Given the wave conditions, vessel geometry and speed, the water motion is calculated at each time and the relative motions between deck and free surface are monitored to derive any possible occurence of deck wetness or green water. To investigate the green water phenomenon and the consequent loading experimentally, test were set up at the Hydrodynamic Laboratory of the University of Glasgow using a 1:70 model of the S-175 containership, where the incident waves, the green water pressure on the forecastle deck, and the load exerted by the green water flow on a vertical wall were measured. The simulations show that the water is not pushed backwards with reference to the earth when it is on deck. The relative velocity between water and carriage increases rapidly for low frequency and gradually reduces to zero for high frequency. The initial shape of the water surface on deck can be approximated as a trapezoid. Considering this shape as determined, two mathematical models were constructed for comparison of the green water flow on deck with experimental data: the conventional model of a dam-break and a 'dam-break model with initial velocity'. The strip method predicted the motions of the ship even in high and irregular waves much better (within 20%) than the panel method, which showed severe overpredictions. The dam break model with initial velocity predicted the green water loads fairly acccurately. The pressure on deck may be approximated by the equation proposed by Buchner (Buchner, B., 'Green water on ship-type offshore structures', Ph.D. Thesis, TU Delft), which describes the pressure as a result of gravity, deck acceleration and the rate of change of water height on deck. However, the last term may be regarded as doubtful, because it produces negative pressure spikes, which do not appear in the measurements.


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    Title :

    Green water investigation for a containership


    Additional title:

    Untersuchung von grünem Wasser bei einem Containerschiff


    Contributors:

    Published in:

    Publication date :

    2004


    Size :

    11 Seiten, 21 Bilder, 1 Tabelle, 11 Quellen



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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