Session: 06-16-03 Wave Mechanics, Modeling and Wave Effects III

Paper Number: 79880

79880 - Generation of Controlled Irregular Wave Crest Statistics in a Numerical Wavetank Using HOS-NWT Solver 

In order to assess the reliability of offshore structures facing extreme environmental conditions, sea keeping tests are performed at model scale in experimental or numerical wavetanks. The wave conditions used for such tests are defined using the notion of sea state. The latter consists in a power density spectrum associated with a certain duration, typically 3hours at full scale. To generate the sea state, the vast majority of industrial practices follow the stochastisc approach. Several realizations of long irregular wave sequences are generated in the tank. They are built in the Fourier space, using the spectrum of interest and random phases. The resulting wave field needs to contain the events leading to the extreme responses of the structure. In a wave tank environment, the waves are generated with a wave maker located at the beginning of the domain. The qualification of the wave field is then performed further in the tank, at the structure location. It mainly relies on the analysis of the mean spectrum (over all the realizations) and the ensemble crest height distribution (considering all the realizations) [1]. However, depending on the wave conditions, the wave propagation can be affected by nonlinear mechasims and dissipation phenomena which lead to i) deviations of the mean spectrum from its target shape at the location of interest and ii) a spatial evolution of the crest heigth distribution. To face those issues, the most advanced wave generation procedures usually focus on the quality of the spectrum, iterating on the wave maker inputs [2]. However depending on the target location, the crest height statistics differ [2, 3]. In particular, in unidirectionnal configurations, the distributions obtained at target locations far from the wave maker (more than 20 peak wavelength) enlight the presence of a significant number of extreme events that are likely to strongly affect the response of the tested structure. Note that the usual target locations used in the industry ar relatively close to the wave maker (less than 10 peak wavelength). In this area, the probability of occurrence of extreme events is smaller. On those grounds, the present study proposes a procedure which offers the possibility to generate closer to the wave maker the same distributions that are measured when targeting the sea state further in the tank. The framework is limited to non breaking unidirectional waves. The procedure is numerically tested with the nonlinear potential wave solver HOS-NWT [4]. A typical design sea state is first accurately generated at 52 wavelength from the wave maker. Then, using the free surface elevation measured at 48 wavelength as input for the wave maker, the sea state is generated at 4 wavelengths from the wave maker. The associated statistics are similar to the one obtained at 52 wavelengths, validating the procedure. The study opens the way to an accurate control of the wave statistics at any location in wave tank environment.

 

References

[1] NWT Preparation Workgroup. (2019). Year 1 Report. Technical Report. JIP on Reproducible

CFD Modeling Practice for Offshore Applications.

 

[2] Canard, M., Ducrozet, G., & Bouscasse, B. (2020, August). Generation of 3-hr Long-Crested

Waves of Extreme Sea States With HOS-NWT Solver. In International Conference on OffshoreMechanics and Arctic Engineering (Vol. 84386, p. V06BT06A064). American Society of

Mechanical Engineers.

 

[3] Onorato, M., Cavaleri, L., Fouques, S., Gramstad, O., Janssen, P. A., Monbaliu, J., ... & Trulsen,

K. (2009). Statistical properties of mechanically generated surface gravity waves: a laboratory

experiment in a three-dimensional wave basin.

 

[4] Ducrozet, G., Bonnefoy, F., Le Touzé, D., & Ferrant, P. (2012). A modified high-order spectral

method for wavemaker modeling in a numerical wave tank. European Journal of

Mechanics-B/Fluids, 34, 19-34.

Presenting Author: Maxime Canard LHEEA

Authors:

Maxime Canard LHEEA
Guillaume Ducrozet LHEEA
Benjamin Bouscasse LHEEA