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Table of Contents

SEA Files

In Bladed, sea states can be specified by means of a text file with the extension .SEA, also referred to as SEA files. The SEA file format is outlined in detail below. The file content is essentially a list of monochromatic, unidirectional wave components, which are superimposed to create a linear, multi-directional, polychromatic (irregular) wave field within the Bladed code using the linear Airy wave theory.

The option to use SEA files can be found in the Tide and SEA file tab under the Sea State menu option. Here the user can define the sea state based on a pre-processed SEA file. A SEA file can be generated by clicking the Generate SEA File button.

The description of a Bladed simulation sea state as a list of constituent frequency components allows known surface elevation time series to be reproduced from tank or numerical data by conducting a Fourier analysis on the data whilst maintaining phase information. Equally, SEA files can be rapidly created for measured, site-specific wave spectra, or, as may be expected, from known, parameterised spectral shapes.

The SEA file format can be edited using standard text editing tools. It is therefore possible to write or even modify existing SEA files externally to Bladed such that constant energy spectra or constrained waves can be simulated using SEA files. While the user is free to select the range and resolution of frequencies used to define the power spectral density of the sea state the user should verify that the required statistics/properties of the input wave spectra are satisfied for the free surface elevation time-history created (for example, spectral shape, \(H_\text{max}\) , and so on). It would also be important to make sure that the possible frequency responses of the structure will be covered such that no important frequencies of the structure response are omitted.

SEA file format

The SEA file contains a list of wave components that make up a single sea state. A set of header lines contain parameter-value pairs separated by a :. All of the header lines must appear in the SEA file, and in the order described below unless specified otherwise.

source: Used to specify the source of the file. Can contain arbitrary text written by the user. Text content not read by Bladed.

identifier: Reserved for a sea state reference number. May be left blank. Not read by Bladed.

duration: The duration or repeat time of the SEA file, for irregular waves (equal to the reciprocal of the frequency step). For regular waves (single wave component), this may be left blank. Typically a duration of a least 1800s is required in an irregular wave simulation but the precise duration to be used will be dependent on both the individual machine in question and the purpose of the simulation.

funit: The units used for the frequency values. May be 'Hz' or 'rad/s'.

dunit: The units used for the direction values. May be 'deg' or 'rad'. Phases must always be supplied in radians.

dconv: The direction convention used (Cartesian or Nautical). May be set to 'cart' or 'naut'. Direction values are the directions waves are coming from. SEA file north is aligned with Bladed negative global X. Nautical angles are measured clockwise from North, Cartesian angles anticlockwise from Global positive X.

seed: The seed used for the generation of any randomly determined amplitudes, phases or directions. Not read by Bladed. Typically set to '1' for regular waves, although the value is not important.

amp method: Used to specify whether the wave component amplitudes have been set deterministically or randomly. The value is for the user's reference only and is not read by Bladed.

phase method: Used to specify whether the wave component phases have been set deterministically or randomly. The value is for the user's reference only and is not read by Bladed.

dir method: Used to specify whether the wave component directions have been set deterministically or randomly. The value is for the user's reference only and is not read by Bladed.

components: Indicates that the list of wave components starts on the line below.

The wave components are listed as the frequency, amplitude, direction and phase of each component, as comma separated values. When a SEA file is generated, the four properties are defined as follows:

  • Frequency \(f\): the reciprocal of the period of the wave. The number of frequency components generated depends both on the "duration" variable \(D\), and on the frequency cut off values selected. The frequency resolution \(df = 1/D\), so the longer the duration, the smaller the frequency steps between adjacent frequency components. Harmonic components with frequencies that lie outside of the frequency range indicated in the user interface will be omitted. If 'Automatic Frequency Range' is selected (this is the default option), 0.05% of spectral energy at each end of the spectrum will be omitted so that only 0.1% of energy is lost overall. Alternatively, it is up to the user to specify the range of frequencies to be used. If the user enters their own values, they need to ensure that the most important features of the spectrum are retained, both in terms of frequency range, and energy distribution. In some cases for instance, it may be acceptable to cut off high frequency 'tails'.

  • Amplitude \(H/2\): With the deterministic amplitude method, the amplitude is determined by the spectral density. Random amplitudes are calculated according to a Rayleigh distribution with the scale parameter proportional to the spectral density.

  • Direction \(\mu_w\): Assigned randomly according to the directional distribution about the mean direction. More details on the directional distributions are given in the theory manual.

  • Phase \(\phi\): A uniformly distributed random variable.

If regular waves are chosen rather than a JONSWAP spectrum, then there is only one component in the SEA file.

An example of the first few lines of a SEA file is shown below.

source: testlocationA_JS4.CLM
identifier: 12
duration: 1800
funit: Hz
dunit: rad
dconv: cart
seed: 36720
amp method: det
phase method: rnd
dir method: det
components:
0.05,3.0267e-005,0,1.0709
0.050556,0.00019138,0,1.6198
0.051111,0.00026896,0,2.4932
0.051667,0.00032871,0,0.46492
...

Defining a Sea State with a SEA File

After having generated a SEA file, this can be used to describe the sea state during a simulation by ticking the checkbox Use a SEA state file in simulations in the Tide and SEA file tab. Once checked it, the Wave tab is greyed out and instead the "Define Sea State" option will be activated. Neither the constrained wave nor diffraction options are available when using a SEA file, so diffraction effects should be accounted for using the Boundary Element Method (BEM) approach to hydrodynamics and the desired sea surface elevation already contained within the SEA file. Having clicked on the Define Sea State, a new window is opened with options explained below.

  • Seastate File (.SEA): Here the path to the desired sea state file to be used during the simulation is selected

  • SEA File Contents: In the expandable box, the entire contents of the SEA file pointed to are displayed for visual inspection

  • Method for Wave-Current interaction: Various options for how the wave particle kinematics are generated and combined with the current velocities are provided to the user. Note that these options will only affect water particle kinematics used in Morison's equation and will not have an effect on any BEM applied loading. In the below description, the relative frequency refers to the orbital frequency of the wave relative to the moving current while absolute frequency would be experienced by a stationary observer when Doppler shift is applied on its relative frequency.

    • Observed freq as input, wavelength changes: Waves are assumed to be riding on the current. The component frequencies in the SEA file are interpreted as those seen by a stationary observer, so the relative wave frequencies then are calculated. Wave kinematics are calculated, and current velocities then superimposed on top. Bladed outputs (in the fixed frame of reference) and the SEA file will have the same frequencies. The amplitude of the waves is maintained from the SEA file definition. This is appropriate if the waves have been measured in the stationary frame of reference in the presence of the current (combined with that measured current).

    • No interaction, back compatibility only: This is how current and wave kinematics are combined when not using a SEA file. The wave kinematics are calculated independently of any current, then the current velocities are superimposed on the wave kinematics. This option is appropriate for testing or recreating Bladed results not produced using the SEA file and for small current speeds.

    • Doppler shift only, waves superimposed on current: Waves are assumed to be riding on the current. The component frequencies in the SEA file are interpreted as the wave's relative frequency and wave kinematics calculated from this. Current velocities are then superimposed on top. The resulting waves will therefore be Doppler shifted from the frequency values in the SEA file when Bladed results (which are in the fixed frame of reference) are plotted. Again, the amplitude of the waves is maintained from the SEA file definition. This may be appropriate if metocean wave data is recorded or generated in the absence of a current.

    • Constant frequency with height change: Waves assumed to be riding on the current and particle kinematics generated as per the "Observed freq as input, wavelength changes" case with one difference. The wave height is assumed to be that before encountering any current and therefore the wave height is adjusted appropriately to that after encountering the current. In opposing currents, the wave height will increase and in following currents the wave height will decrease. This option is unlikely to be appropriate as the "Observed freq as input, wavelength changes" option is useful when waves have been measured in the presence of a current and so the measured wave height should be suitable already.

    • Constant wavelength with height change: Waves assumed to be riding on the current and particle kinematics generated as per the "Doppler shift only, waves superimposed on current" case with one difference. The wave height is assumed to be that before encountering any current and therefore the wave height is adjusted appropriately to that after encountering the current. In opposing currents, the wave height will increase and in following currents the wave height will decrease. This is appropriate if metocean wave data is recorded or generated in the absence of currents but then combined with currents in the Bladed simulation.

  • Phase Origin: The phases defined in the SEA file are that of a cosine relative to a particular point in the horizontal plane as shown in Figure 1. By default, this is at zero of \(X_g\) and \(Z_g\) in the global coordinate system of Bladed however a different location can be chosen if desired. This may be useful if, for example, reproducing a measured wave train where surface elevation measurements have been made at a location away from the turbine.

SEA file wave phase origin

Figure 1: Schematic of a regular wave surface elevation \(\eta\) about the mean water level defined by the water depth \(d\) and tide height \(h\) at time \(t=0 \bunit{s}\). The wave phase origin at \((X_g, Z_g) = (0,0)\) results in wave crest at the global origin provided \(\phi=0\).

Last updated 06-09-2024