First Deployment - Planning Confirmation

Posted by Aaron Gavey at January 25. 2012

Hi Nortek,

We are due to deploy our AWAC for the first time down in Jersey, CHannel Islands.

We have set it up in the frame and aim to complete compass calibration shortly.

We are looking at deployment in between 6-8m charted water depth. We have a large tidal range at nearly 12m, so I am working on a max water depth of 20m plus 10%margin.

My deployment planning settings will be:

Current

Profile Interval - 600s

Number of Cells - 22

Cell size - 1m

Fregquency - 600khz

Wave

Number of samples - 1024

Sampling rate - 1Hz

Interval - 1024 s

I appreciate your comments on the above configuration, and whether it is suitable for said water depth.

Kind Regards

Aaron

Port of Jersey

CHannel Islands.

Re: First Deployment - Planning Confirmation

Posted by Jonas Røstad at January 26. 2012

Dear Aaron,

Your deployment planning is not changed much from the default planning in our software. The standard settings are selected by Nortek based on years of experience.

Your changes are cell size, and wave interval.

Cell size

Choosing 2m cells gives velocity accuracy of 0.7cm/s VER, 2.2cm/s HOR, while 1m cells gives velocity accuracy of 1.5cm/s VER, 4.5cm/s HOR. These numbers are really the standard deviation of your measurements. The AWAC (and all other profilers) measure the Doppler-shifted frequency directly, and the quality of these measurements depends on the number of ensonified particles in a measuring cell (hence the increased standard deviation with smaller cells) and the number of seconds used to average over for each profile. Longer averaging time improves the accuracy. However, in a tidal stream, where the mean currents are high this is probably not an issue.

Wave interval

The cost for this is power and memory usage. If this is acceptable you should have no worries. When using my copy of the AWAC AST sw (1.42) my option of wave interval is 1200s not 1024s. Please check that you have an updated copy of the software.

I think your choice of adding a 10% margin is a good one.

Other criteria of success are proper compass calibration, pressure sensor offset adjustment (please be sure you have an updates version of the software, and to be sure that the tilt values are kept low.

Please tell us if there are uncertainties.

Best regards

Jonas Røstad

Re: First Deployment - Planning Confirmation

Posted by Aaron Gavey at January 26. 2012

Jonas,

Thanks for your prompt reply.

I have checked and I am running AWAC AST v1.42. I got my wires crossed with number of samples/intervals

Below are updated planning settings based on your comments

Current

Profile Interval - 600s

(Average interval 60s)

Number of Cells - 11 (halved as cell size is now 2m - (water depth of 22m...))

Cell size - 2m

Frequency - 600khz

Wave

Number of samples - 1024

Sampling rate - 1Hz

Interval - 1200s (it allows me 1200,1800,2400 etc) I may have to increase this depending on battery utilization.

By decreasing the wave interval, will conflict with the current measurements?

We have the AWAC gimbled in a frame to be deployed on seabed - is 600s compass update rate acceptable?

Thanks again for any feedback, I want to be confident before we deploy!

Aaron

Re: First Deployment - Planning Confirmation

Posted by Jonas Røstad at January 30. 2012

Dear Aaron,

If there is a conflict between wave measurement and current measurement the waves are prioritized. Since a wave burst in this example is 1024 samples at 1Hz its duration is 1024 seconds. This mean that current profile number 2 (after 600 s) is not carried out. The result of this setup is one current profile and one wave burst repeated every 1200 seconds.

If you increase the wave interval to 2400 sec, you will get 3 current profiles and one wave burst every 2400 seconds and consuming only half the power.

I assume that your frame will not be moving so 600s compass update rate is OK.

Please tell me if there are uncertainties.

Best regards

Jonas

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First Deployment - Planning Confirmation