# Windage area modeling in Autohydro for MODUs

Discussion in 'Software' started by babri, Dec 23, 2013.

1. Joined: Sep 2009
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### babriJunior Member

Hi experts,
I am going to use Autohydro for stability calculation for a jack-up. I have a question about windage area modeling for MODUs which shall be calculated in all directions.

I can use "axis" command to control wind direction. Is this command accurate enough to model wainadge area in any direction? Or I must create many models (one model per wind direction) for wind overturning moment calculation?

I hope you experts help me

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### immaculateJunior Member

Hi Babri,

I'm also working with Jack-ups using autohydro.

usually i only create one model to be used for all wind directions.

In Wind Heeling Moment Calculations, typically i calculate wind critical axes in steps of 10 degs from -90 to 90 deg.

Namely -90, -80, .... , 0, 10, 20 ..., 90 (Total 180 deg, considering symmetrical about centreline)

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### babriJunior Member

Thank you immaculate.
I'm going to prepare a stability booklet for BV and I hope they accept this method.
Another question is "How do you consider SHAPE COEFFICIENT for different sail components in jack-up modeling?"
For example, shape coefficient for deckhouse to be 1.1 and for truss legs to be 0.6. Can I modify permeability of components to define shape coefficients? Does Autohydro consider shielding effect of different components?

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### immaculateJunior Member

Typically in the wind heeling calculations, i prefer to do this on a separate spreadsheet before inputting the final wind heeling moments using the HMMT command in autohydro run file.

So, the different components of the jackup can be broken down and calculated separately for the corresponding shape coefficients, and height coefficients for the legs in particular.

Autohydro will not help to calculate these moments for you as it doesn't know how to differentiate which components should have which coefficients.

Also do note Permeability values are not related to the Cs and Ch coefficients, changing them will not give you the correct result.

Setting of Permeability values in the Run file is only relevant for damage stability / residual stability calculations.

In my context, autohydro only provides a platform to check the allowable VCGs based on HMMT which i provide.

It is not able to consider shielding unless I have factored shielding into my wind heeling moment calculations.

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### babriJunior Member

There is a way in autohydro to define the height coefficient: Defining wind pressure gradient in height. For example, using following command for wind speed=70kn:
wind (pressure) 0.0809@0,0.089@15.3,0.097@30.5,0.1051@46,0.1108@61,...

(see GHS help: http://www.ghsport.com/support/QA016.HTM)

I hoped to be able to define shape coefficient for sail components as well, and let the software calculate wind heeling moment. (for example by changing the "effectiveness" of the sail components in model maker)
Anyway, shielding effect cannot be modeled. In manual calculation, we need to have one calculation sheet per wind direction. Am I right?

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### vipul29New Member

wind overturning moment and heeling moment for jack up rig

hello,
I am doing stability analysis for jack up rig in autohydro. I wanted to know if the software is capable of calculating the wind overturning moment and heeling moment for different angle of heel and wind angle at a given draft for the vessel. if yes, can i get the set of commands to be used for the same?

Also, can i get a allowable/max vcg for different drafts at a series of wind angle from 15* to 345*

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