Once statistics have been calculated to quantify spectral parameters (\(H_{m0}\) and \(T_p\) or \(T_e\)), the DLC generator tool provides functionality to realize sea states into surface elevation time series.

These time series can then be used in tank testing, or dynamic simulations of your WEC device.

DLC’s

DLC’s are defined in IEC TS 62600-2, Table 7 - Design load cases for WECs. There are other environmental conditions beyond the wave conditions generated through the tool that must be considered when creating the full DLC for each design condition.

Sea State Selections

Any point on a contour line or the spectral parameters returned from a Univariate analysis can be used to satisfy the \(H_{m0}\) requirements for the DLC’s.

IEC TS 62600-2 Section 6.2.2.4 specifies that the sea states that would cause the most onerous loads to be considered in testing.

Wave Parameter Selections

When realizing sea states the tool groups the realizations into the three unique wave conditions that are required for the Design Load Cases (\(H_{m1}\), \(H_{m50}\), and \(H_{rated}\)). The wave parameters selected apply to all sea states selected for the given wave condition.

Wave Type

Irregular waves will be generated. Young, Ian. (2017). Regular, Irregular Waves and the Wave Spectrum.

Spectrum

  1. JONSWAP - Applicable to a developing sea state in a fetch limited situation. This spectrum is often used for extreme event analysis.
  2. Bretschneider - Also called ISCC or Modified Pierson Moskowitz, this spectrum is a two parameter spectrum that has less constraints than Perios Moskowitz.

More information on the spectrums can be found in IEC TS 62600-2 Annex C.

Spectral density is calculated using MHKiT function mhkit.wave.resource.jonswap_spectrum.

Gamma

Gamma only applies to the JONSWAP spectrum, as the Bretschneider spectrum is equivalent to a JONSWAP spectrum with a gamma of 1.

Realization Length

The default value of 3 hours is typical for storm sea state realization. This is the total length of the time series being generated.

IEC TS 62600-2 Section 7.3.6 specifies requirements around realization length for testing.

Sample Rate

Sample rate, in \(Hz\) is simply the temporal resolution of the surface elevation time series that will be generated.

Use your best judgement, along with the capabilities of your dynamic modeling or tank testing to determine what sample rate is needed. Typically, 20 \(Hz\) should be sufficient.

Very long or very short period values (\(T_e\) or \(T_p\)) may benefit from adjusting the sample rate.

Random Seed

The random seed initializes the random number generator used to create the random phases for the surface elevation time series.

IEC TS 62600-2 Section 7.3.6, has requirements around load case modeling and simulation. Changing the random seed, then downloading the new surface elevation time series may help satisfy these requirements.

Sea State Realization

Surface elevation time series are generated from your selected wave parameters and sea states using the inverse Fourier transform of the complex amplitude spectrum.

NumPy’s numpy.fft.irfft is used to perform the inverse Fourier transform.

\(H_{rated}\) Period Range

The tool also allows the developer to input their \(H_{rated}\) value, then use the IEC TS 62600-2 Section 6.2.2.3 Formula (3) to calculate the associated peak period range for the \(H_{rated}\) spectral parameters.

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