For model testing involving ocean waves in a model basin have to be generated to represent their realistic characteristics such as shape, height, period and spectrum. In general, it is not possible to model all parameters at the same time. The generation of 'deterministic' wave trains according to required parameters at a given time and position in the basin consists of the following steps: (i) Definition of the wave train at the position where the ship encounters it at a given time, (ii) upstream transformation of the wave train to the position of the wave generator, (iii) calculation of control signals using adequate transfer functions of the wave generator, and (vi) performance of model tests, confirmation of the compliance with the target wave parameters. For steep waves (height-to-length-ratio > 0.05) non-linear phenomena have to be taken into account, for example: (i) dispersion depending on wave steepness, (ii) Lagrangian mass transport and Eulerian wave induced (reverse) current, (iii) non-harmonic (asymmetrical) shape, (vi) wave-wave interaction. As an example of the application of deterministic wave sequences including non-linear phenomena two capsizing tests have been carried out using a Ro-Ro-vessel (model scale 1:34). The first test refers to the quasi-static loss of transverse stability (associated with an excessive righting arm reduction) in the wave crest. This mode occurs typically in following waves with low encounter frequencies. The ship can capsize, when it experiences a critically reduced righting arm for a sufficient period of time, while the wave crest overtakes the ship slowly and the ship is surging or surf-riding periodically. In irregular waves, one encountered wave of critical length wave of critical length and steepness is sufficient to cause capsizing. The second test refers to parametric excitation: The roll angle increases dangerously due to non-linear disturbances, which can cause roll motions at half the frequency of the exciting wave. The following factors contribute to this phenomenon: (i) encounter period close to one-half of the natural roll period, (ii) large wave height, (iii) wave length comparable to the ship length, (vi) wide, flat sterns in combination with pronounced bow flare. While for forward moving ships the response up to capsizing is based on wave frequency motions, a moored object in waves shows combined low-frequency and wave-frequency motions. The wave-frequency motions are related to the wave elevation and wave spectrum, while the low-frequency motions are related to the wave groups and group spectrum. Different wave realizations (wave seeds) lead to different wave group spectra - even for the same wave spectrum, in particular in the area of the resonance frequency of the floater. Therefore, generating 'neutral' or particularly 'dangerous' wave groups in a model basin is not an obvious task. Ocean waves are often considered as long-crested in model tests. In reality, however, they are short-crested, especially under hurricane conditions. Short-crested waves can be interpreted as a summation of regular wave components of different frequency, amplitude and phase (seed), where each component is additionally assigned a direction. To simulate these conditions in a model basin multi-flap wave makers are required.


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    Title :

    Generation of extreme wave sequences - From capsizing ships to offshore structures in hurricanes


    Additional title:

    Erzeugung extremer Wellenzüge - Vom Kentern eines Schiffs bis zu Offshore-Plattformen in Stürmen


    Contributors:

    Published in:

    Publication date :

    2008


    Size :

    9 Seiten, 13 Bilder, 7 Quellen



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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