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Irregular wave shift

These parameters are used to specify how the pseudo-random phases of individual wave components in wave spectra are to be calculated and offer a range of options to control the time-history of wave elevation. Selecting either of these parameters would impact the time-history of wave elevation without altering the wave spectrum.

  • Maximum wave height: Pseudo-random phases of individual wave components are selected so that a certain maximum wave height target is actually experienced during the dynamic simulation. The maximum wave height target can be further specified through the data located in the maximum wave height frame and the reference point frame.

    • Hmax/Hs ratio: The ratio of the maximum wave height over the significant wave height. This therefore allows specifying the target maximum wave height to be experienced during the dynamic simulation.

    Note

    In case no value is specified here, the Hmax/Hs ratio that is defined in the irregular wave component is used. Setting any value for this parameter would supersede previous values specified in the irregular wave component.

    • Duration: The duration over which the target maximum wave height will be looked for. Setting the value to its default "-" value would be set to 12500 x max (simulation final time, 8 x wave peak period).

    Note

    • The default duration is generally sufficient to ensure that the target maximum wave height will actually be reached. High Hmax/Hs ratio may however require longer search durations that can be defined here.
    • If double peaks wave spectra are defined, the first peak period is considered to set the default duration.
    • If several spectra are defined, the highest peak period is chosen.
    • Percentage above: The upper bound accuracy value to achieve the target maximum wave height. This value must be positive. Setting PERCENTAGE_UP equal to 5 means that the wave height must be at most 5% higher than the target value. The default value for PERCENTAGE_UP is 100. This means that the maximum wave height could be as a maximum 100% larger than the target value.

    The target wave height Hmax shall be comprised between an lower bound value and an upper bound value, as follows:

    • Percentage below: The lower bound accuracy value to achieve the target maximum wave height. This value must be positive. Setting PERCENTAGE_DOWN equal to 1 means that the wave height must be at most 1% lower than the target value. The default value for PERCENTAGE_DOWN is 1.333 which means that the maximum wave height would be as a minimum 1.333% lower than the target value.

      Warning

      Setting PERCENTAGE_DOWN to 0 results in the default value being used. Very low values like 0.01 should therefore be used instead of 0 to ensure that the maximum wave height will actually be larger than the target value.

    • Reference point: The location of the point where the target maximum height will occur. The location is defined through a model component and a node belonging to that model component.

    Note

    • The reference position is the position of the node that is recorded at the end of the quasi-static analysis.
    • If the node belongs to a floater component, the reference node will be set to the floater's center of motion.
    • The default reference node is the centre of motion of the first floater component included in the analysis. In case no floater component is included in the analysis, the default reference node is set to the origin of the global coordinates system.

  • Maximum wave height with stream function: This option is to define an embedded wave. The parameters of the option above (*Maximum wave height") still apply to define the maximum wave height characteristics. Around the time of maximum wave height a stream function is used instead of the irregular wave. The user must define the stram function order. Height and period can also be defined if the user does not wish to use the default height and period found at the time of maximum irregular wave height.

  • Maximum downwards velocity: Pseudo-random phases of individual wave components are selected so that a certain maximum downwards speed is actually experienced during the dynamic simulation. The maximum downwards velocity target can be further specified through the data located in the maximum downwards velocity frame.

    • Duration: The duration over which the maximum downwards velocity will be looked for. Setting the value to its default "-" value would be set to 12500 x max (simulation final time, 8 x wave peak period).

    • Reference point: The location of the point where the target maximum height will occur. The location is defined through a model component and a node belonging to that model component.

    Note

    • The reference position is the position of the node that is recorded at the end of the quasi-static analysis.
    • If the node belongs to a floater component, the reference node will be set to the floater's center of motion.
    • The default reference node is the centre of motion of the first floater component included in the analysis. In case no floater component is included in the analysis, the default reference node is set to the origin of the global coordinates system.

  • Spatial shift: This option is to be used with existing wave data taken from a previous simulation in the wave.out file. Selecting this option updates the individual wave components found in the wave.out file by applying a spatial shift so that the time-history is generated at the location of a selected node instead of being generated at the origin of the model.

  • Time shift: This option is to be used with existing wave data taken from a previous simulation in the wave.out file. Selecting this option updates the individual wave components found in the wave.out file by applying a time shift so that the time origin of the time-history is moved as desired.