Irregular Wave.

If Fatigue Simulations (FLS) or Ultimate Limit State (ULS ) analysis should be performed, the irregular wave gives the best prediction of "the reality". The wave is defined similar to the regular waves using the WaveData command. However, some additional parameters are required for an irregular wave specification:

• Hs and Tp (instead of Height and Period for a regular file)
• "Random Seed" parameter, (instead of phase )
• Wave energy spectrum (f ex Jonwap or PM)
• Number of frequency components and the frequency range (T_low, T_high).
• Spectrum integration method (equal dOmega, Constant area, etc.

In the actual example, the following data are used:

• Hs = 12m, Tp=15s
• Wave is ramped up over a distance of 200m
• Jonswap spectrum is used with fixed gamma parameter=4.1
• 30 frequencies ranging from T=3s to T=20 s are used.
• Frequencies are distributed with highest density around Tp (type=2)

The "Wave Ramp" function, (see Fig 1), is defined similar to regular wave. In the example, four (4) discrete points are used to ramp up the wave to level 1.0, (over a distance of 200m). The points are given in the sequence described in the figure, from left towards right., (the first point has the lowest X coordinate).

Figure 1 : Ramping up the wave. Definition of "envelope" curve in space by discrete points (X1,Fac1, X2/fac2 etc.).

Figure 2: Plot of actual elevation vs time. At the first seconds (left) the elevation is almost zero due to the ramp function.

Figure 3 : Snapshot at Time=0. The sea is flat (but "something "is coming from the left)

Figure 4 : Snapshot at Time=10. The sea is gradually increasing , (the flat sea has moved to the right)

Figure 5 : Snapshot at Time=20. The sea is ramped up to normal (full) irregular wave

Usfos input files to the actual example are found in the following two files:

=> Usfos Control File
=> Model File