moses time domain analysis

Hi Georgina,

What are the other options which change the seeds?

What are the values of seeds to be used? I checked Moses files and found TB=18000 is used with all -t_reinforce options.

Best regards,

Ahmed 

Hi Ahemed,

Thank you for your question.  Yes, -t_reinforce is the option for setting the seed.

I have also seen people use the "-time tobserv delta_time ttra_set ncycle" to control when the maximum environment occurs.

I do not understand your question.  The default is TB = 18,000.  Maybe we need to define what is "seed".

From Wikipedia, "A random seed is a number used to initialize a pseudorandom number generator."  MOSES is using the TB, seed, to pick the phases of the frequency components.  We try to model a random sea.  The phases are what we are trying to make random.

I hope this helps.

georgina

Don't forget that if using -t_reinforce, TB must be a high number (i.e. larger that the simulation time), since at this point all the Fourier components add up and creates a huge wave - if I recall correctly.

Don't forget that if using -t_reinforce, TB must be a high number (i.e. larger that the simulation time), since at this point all the Fourier components add up and creates a huge wave - if I recall correctly.

Hi,

based on mp_moor, i tried to input different values of t_reinforce,TB. 

T_REINFORCE = T_REINFORCE IS NOT ADDED IN &ENV
MAX = 100.14 99.48 99.40 99.73 152.48 179.99 180.15 148.42
PRED_MAX = 105.33 106.46 106.25 104.72 164.01 198.05 198.02 158.44
MAX_MEAN = 13.84 18.00 17.84 13.32 31.64 48.54 48.40 27.85
PRED.MAX_MEAN = 19.02 24.98 24.69 18.32 43.17 66.61 66.28 37.87

T_REINFORCE = 0
MAX = 99.93 99.55 99.57 99.74 145.62 169.80 170.59 145.36
PRED_MAX = 104.91 106.14 106.10 104.55 155.10 184.34 185.36 154.79
MAX_MEAN = 13.28 17.45 17.42 12.93 25.65 39.64 40.26 25.53
PRED.MAX_MEAN = 18.26 24.04 23.95 17.74 35.12 54.18 55.03 34.96

T_REINFORCE = 5
MAX = 101.92 102.27 102.29 101.55 145.57 172.38 172.24 145.25
PRED_MAX = 107.75 110.13 110.14 107.23 154.68 187.38 187.12 154.30
MAX_MEAN = 15.49 20.37 20.34 14.97 24.83 40.89 40.56 24.65
PRED.MAX_MEAN = 21.32 28.23 28.19 20.65 33.94 55.88 55.44 33.70

T_REINFORCE = -5
MAX = 101.92 102.27 102.29 101.55 145.57 172.38 172.24 145.25
PRED_MAX = 107.75 110.13 110.14 107.23 154.68 187.38 187.12 154.30
MAX_MEAN = 15.49 20.37 20.34 14.97 24.83 40.89 40.56 24.65
PRED.MAX_MEAN = 21.32 28.23 28.19 20.65 33.94 55.88 55.44 33.70

T_REINFORCE = 10800
MAX = 94.31 91.20 91.58 94.46 136.09 155.83 155.66 136.16
PRED_MAX = 97.56 95.25 95.81 97.71 141.51 164.88 164.57 141.67
MAX_MEAN = 8.87 11.04 11.36 8.80 15.06 24.31 23.95 15.30
PRED.MAX_MEAN = 12.12 15.09 15.59 12.05 20.47 33.35 32.86 20.80

My question is to follow guideline from BV 493-NR, i need n=5 and i believe the option T_REINFORCE 5 is the correct way to do it? Since moses cant produce result of Tk of each seed (elementary wave), how can i calculate the value for 

Tm : Mean of the n maximum values of Tk? Is the given Max_mean Value = Tm?  I hope you understand my question.

Hi Naim,

I'm having some difficulty understanding your question. What do you mean by "Tk"?

As far as I comprehend, it seems like you are attempting to run an analysis for 5 different seed values. The mean tension would be the average of the 5 tensions from these analyses. To clarify:

seed1 - Pred.Max. = T1, seed2 - Pred.Max. = T2, ..., seed5 - Pred.Max. = T5.
Then Tm = (T1 + T2 + ... + T5) / 5.

Now, in MOSES (or any other software), you will need to run the simulation for 5 different seeds, as the software will not perform this automatically for you. Though you can utilize loops and other programming features in MOSES to calculate the average, I suggest running 5 separate analyses and then finding the average outside of MOSES, perhaps using an Excel spreadsheet or similar tool.

Regarding changing the seed in MOSES, as Georgina mentioned, the most common option is to use the -t_reinforce command. By default, -t_reinforce is set to 12000 in MOSES, which is greater than 10800 (3 hours), as this is the expected duration of the storm for which the user will run the simulation. It's essential to set -t_reinforce greater than your simulation time. For example, if your simulation time is 3600 seconds (1 hour), you should ensure that -t_reinforce tb > 3600 seconds. MOSES synthesizes an irregular sea based on the provided spectra (and Hs), and you want to avoid analyzing the system when all the components are adding up. You can refer to the MOSES theory manual (Environmental Series Section) to understand how MOSES performs this synthesis.

In your message, I noticed that you are using TB = 5. This is not correct for the reasons mentioned above. Although you might not see the reinforced sea at t=5, it's because the default ramp time in MOSES is set to 20 seconds.

To plot the wave elevation, you can use the PRCPOST menu under the POINTS command as shown in the below screenshot. Take a look at what happens at t=600 seconds when -t_reinforce is set to 600.

The wave height at 600 seconds is much higher as all the Fourier components have added up at that point. However, this is not representative of Hmax, and therefore your floating body should not be subjected to this sea when your Hs is only 1m.

For your problem, if you are using a simulation time of 3600 seconds, you can use -t_reinforces such as 7000, 9000, 11000, 22000, 50000, and basically any value greater than 3600. Plotting all these time series (in prcpost menu) will allow you to observe their differences.

I hope this explanation clarifies things for you. Please feel free to ask if you have any further questions.

Thanks,

Rahul Kanotra

Dear Rahul,

Understand. So i just need to run the analysis 5 times for 3hrs (10800s) by changing the t_reinforce. Is there any rule for setting up the value? Can i assume like this..

seed 1: t_reinforce=12800

seed 2: t_reinforce=12800*2=25600

seed 3: t_reinforce=12800*3=38,400

seed 4: t_reinforce=12800*4=51,200

seed 5: t_reinforce=12800*5=64,000

Based on BV rule Tk define as

MOORING.zip

Thank you for your explanation. 

Hi Naim,

Those are valid choices since the difference between the consecutive "-t_reinforce" is > 10800 (your simulation period), and all the "-t_reinforce" values are greater than 10800. These are the two things you need to keep in mind to ensure that you are not analyzing the repeating pattern. You can always plot the wave and check it visually. I hope this helps.

Thanks,

Rahul

Dear Rahul, Thanks for the help.