Home > Model Components > Waves > Irregular Waves
Irregular wave types
Each irregular wave is defined as the sum of individual regular waves components. The instantaneous wave elevation at a given point is :
The parameter to be defined given by the user are :
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Number of components: This sets the number of wave harmonic components used to model the wave spectrum. This number of components must be at least 100 (defaults to 200) to ensure accurate modeling of the wave energy distribution along frequencies.
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Heading: wave heading (0 degrees is a wave propagating along the X axis from the negative value to the positive value of X, otherwise known as the 'to' direction).
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Random procedure trigger: Phases associated with each irregular wave component are pseudo-random. Phases are generated with a random number generator which requires a certain random procedure trigger (also named "seed") to be user-specified. The trigger value must be an odd integer value with 5 digits (the default value is 12345). Two dynamic simulations based on the same trigger value would generate similar phase series.
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Required Hmax/Hs ratio: to define a minimum wave height to be reached during the simulation. Prior to the dynamic simulation, the spectrum is developed during a long period of time and a specific procedure is conducted to identify a wave crest to trough elevation higher or equal to the requested Hmax/Hs ratio. Some margins (upper and lower tolerances) may be accepted for this identification. Once the criterion is achieved, the associated time is used to shift all wave components in order to ensure that the identified event occurs at t = 2.Tp + 3(TFIN-2.*Tp)/4, where TFIN is the simulation duration. If the ratio value is set to zero, no target wave height is looked for.
Warning
This specific feature shall be seen as an help to optimize simulations durations during a screening process since it allows imposing a "not too high" or "not too low" extreme wave within the simulated time window. Nevertheless, the representativity of the extreme output results shall be questioned before drawing final conclusions. Moreover, the irregular wave generation being an random process, a higher wave height may even happen during the simulations for two main reasons: a higher "trough to crest" wave height, ignored by the identification process, may occur or an second higher "crest to trough" wave train may be generated just after the identified peak.
Note
It is possible to switch the identification process from "crest to trough" to "trough to crest". This option is not supported by DeepLinesGUI but may be directly selected through keyword *TIMEDRIFT (parameter IOP)
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Period range for automatic generation: you may optionally define upper and lower cut-off wave periods that will be used when extracting the wave components from the spectrum. Specifying such boundaries may be useful for instance to prevent having wave periods outside of the period range of your RAO data.
Note
The default upper cut-off wave period is set to 2 x Tp and the default lower cut-off wave period is set to 0.4 x Tp, where Tp denotes the wave spectrum peak period. Using the default cut-off values combined with the default number of wave harmonic components (200) provides a representation of the wave spectrum that is accurate enough.
Warning
Special care must be paid to the proper discretisation of the wave spectrum in case the upper and lower cut-off wave periods are set manually especially when the upper cutoff period is significantly higher than the wave peak period (for example Tmax = 100s combined with Tp = 10s). In this case, the default number of individual wave components (200) may be too low to properly represent the distribution of energy about the peak period and should be increased. It is therefore recommended that the number of individual wave components is set in accordance with the upper and lower cut-off wave periods that are specified.
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Directional spreading data: These data allow you to model a directional spread spectrum ( Theory available ).
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Number of directions: The number of discrete wave directions to be used within the angle range. This value must be strictly higher than 2 to properly account for the spreading.
Note
The total number of individual wave components that will be used in the analysis equals the number of discrete directions multiplied by the number of components that was specified above. It is therefore recommended to update the default number of wave components from 200 to a lower value so as to keep the total number of components within reasonable limits and therefore avoid any significant increase in simulation time.
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Exponent of the cosine function: This is the spreading exponent of the cosn function.
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Maximum angle range (deg.): This sets the maximum angle range that is to be covered about the principal direction.
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JONSWAP or PIERSON MOSKOWITZ wave spectrum: Significant wave height (Hs) and Peak period (Tp) (or Tz : zero up-crossing period, Tm : mean period) and peak coefficient must be defined. Note that for a Pierson Moskowitz spectrum, gamma=1. (See theory for spectrum definition).
User defined wave elevation: The time history of wave elevation at the center of the coordinates system (0,0) must be defined in an ASCII file. The first column must contain the different times and the second one the elevation.
The solver carries out a discrete Fourier transform of the original data-set to extract all the individual wave components composing the signal. This step is necessary to derive the wave kinematics over the whole water column. The number of components defaults to half the size of the original signal. For example, this means 500 wave components when the original signal included 1000 time steps. (Uusing all the components would increase the CPU requirements to compute the Morison forces along the different lines. Moreover most of these components will be allotted very short periods, well below any actual wave period. You are then not obliged to use all these components in your analysis, and may specify the exact number of components to be used plus lower and upper cut-off frequencies). However, at the end of the simulation, you should check that the wave elevation computed by DeepLines actually matches the one you have specified.
Tip
As the simulation starts, information about the wave components may be found from the .LIS file by searching for the word 'component' for instance.
User defined wave spectrum: An external file is used to import the spectrum. The external ASCII wave file must be a list of Frequency (Hz), spectral wave amplitude (in meter). The Lower cut-off wave frequency and Upper cut-off wave frequencies must be defined.
Ochi-Hubble wave spectrum: The Ochi-Hubble spectrum is a so-called 'two peak' spectrum that combines a swell with a wind-driven sea-state. Significant wave height (Hs), Peak period (Tp) and Peak coefficient must be defined for both the swell and wind-driven seas.
An Ochi-Hubble spectrum is composed of high and low-frequency sea states represented by modified Bretschneider spectra with peak enhancement factors. The total significant wave height is the square root of the sum of the squares of the two wave heights. (See keywords for spectrum definition).
Triangle type spectrum: Used to define more realistic West-African sea states : multi peak spectrum. The different peaks are fitted by a linear model. The user will give general parameters such as Hsi, Tpi (i=1,nbpeak) and the number of peak to be considered. (See keywords for spectrum definition)
Wallops wave spectrum: Used to define more realistic West-African sea states with a WALLOPS spectral distribution (a wind sea peak). The user will give general parameters such as Significant wave height (Hs), Peak period (Tp). (See keywords for spectrum definition)
Gaussian wave spectrum: The spectrum is characterized through its significant wave height (Hs), peak period (Tp) and dimensionless shape parameter (s) coefficient.
Torsethaugen wave spectrum: The spectrum is a double peak model developped on measured spectra in Norwegian waters.
See the theory manual for more guidance on these theories.
Wave Elevation TimeTrace
The elevation of the wave as a function time can be used as input through an external ASCII file. It is possible to define the position in the global frame where the wave elevation must be imposed.
Wave elevation versus time
Directly using this option in a wave set or environment set is easily accessible.
Maximum Wave Height
It is possible to select a target for maximum wave height together with the admissible error on that target. The wave used in the dynamic run will then have the resulting maximum wave appear at ¾ of the simulation. It is possible to require that the maximum wave height is the maximum wave height over the whole simulation by selecting the option “No higher maximum” in the “Wave shift” panel of the “Calculations parameters” of a dynamic simulation.
Note
For low Hm/Hs ratio, DeepLines may be unable to find both the required ratio while not having a higher maximum in the rest of the dynamic simulation. In such case, the simulation is stopped before launching the dynamic. It is therefore advised to check that it is possible to generate the wave by creating first a dynamic analysis on the required time with only the wave spectrum.
Option "No higher maximum” for wave generation
