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#73104 - 04/02/19 09:24 AM Subsea Modal Analysis
Ltorrado Offline
Member

Registered: 10/19/10
Posts: 35
Loc: Metairie, LA
I am currently analyzing a subsea pipeline and would like to know if CAESAR II can be used to run a modal analysis to check VIV.

Will the modal analysis consider that it will vibrate in a water submerged environment instead of air? Or is the modal analysis only applicable to piping in an air environment?

Thank you.

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#73108 - 04/02/19 03:44 PM Re: Subsea Modal Analysis [Re: Ltorrado]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 818
Loc: Louisiana, US
I would believe that CAESAR assumes a vacuum, for which air is a good representation.

While you can still use CAESAR to estimate natural frequency in a vacuum, you'd need to correct this value to meet that of water.

Here's a link that describes what you need.

https://www.comsol.com/blogs/natural-frequencies-immersed-beams/

I've also attached a pdf of the article, in case the link becomes broken.


Attachments
Natural Frequencies of Immersed Beams.pdf (89 downloads)


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#73133 - 04/04/19 01:44 PM Re: Subsea Modal Analysis [Re: Michael_Fletcher]
Ltorrado Offline
Member

Registered: 10/19/10
Posts: 35
Loc: Metairie, LA
Hi Michael,

Thanks for your reply and the attachment. Good stuff.

I just think that my situation is a bit different than just a cantilevered thin beam. Modal analysis of a custom large OD pipeline system with friction and suspended masses is already an "inexact science" and using the CAESAR module + applying that equation would make it even more inexact.

I would think CAESAR has a solution or suggested methodology for this since they boast subsea analysis capabilities.

Richard or Dave, do you guys have any input on this?

I submitted a technical query to Hexagon, I am just looking for an answer ASAP since client is requesting the VIV check.

Thank you.

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#73137 - 04/05/19 12:44 PM Re: Subsea Modal Analysis [Re: Ltorrado]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2378
Loc: Houston, TX, USA
In calculating system natural frequencies and modes of vibration, CAESAR II does not consider the environment. The calculated system response assumes the piping is is NOT surrounded by water.
(Not to hijack this thread, but ...
Other (temperature-related) conditions may also come into play here - 1) the operating modulus of elasticity, try running Eh rather than Ea (Ea is the code's "reference modulus of elasticity" or E at 70F. Ea is specified for expansion stress range calculations. 2) the "tuning-a-guitar-string effect". The frequency response of a system is sensitive to any pre-tension in the system. Our analysis is based on a totally relaxed (otherwise unloaded) system.
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Dave Diehl

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#73139 - 04/08/19 07:58 AM Re: Subsea Modal Analysis [Re: Dave Diehl]
Ltorrado Offline
Member

Registered: 10/19/10
Posts: 35
Loc: Metairie, LA
Thank you for your response, Dave.

If that's the case, I will have to defer to the subsea company for the VIV assessment. Too bad because I was excited to try to do this.

Interesting what you mention about those two points. I will make it a point to use Eh or E for your 1) point.

But can you please expound a bit more on your 2) point? Does this mean that the modal analysis module of CAESAR is not reliable for systems that have any degree of stress within them, be it from temperature or sustained loads? This seems like a very limiting requirement. It makes me a bit alarmed about modal analyses I've ran for loaded systems like hot reciprocating compressor lines which as you know are rigidly clamped down.

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#73140 - 04/08/19 09:45 AM Re: Subsea Modal Analysis [Re: Ltorrado]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2378
Loc: Houston, TX, USA
I do not know the magnitude of this "guitar string effect" so I cannot say it is significant in piping systems. I do not believe is generally considered.
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Dave Diehl

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#73143 - 04/09/19 06:07 AM Re: Subsea Modal Analysis [Re: Ltorrado]
engineer001ch Offline
Member

Registered: 12/21/18
Posts: 18
Loc: China
It's easy. Just add uniform mass of water displaced by the pipe. It called "added mass" method

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#73150 - 04/09/19 12:23 PM Re: Subsea Modal Analysis [Re: Ltorrado]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 818
Loc: Louisiana, US
Adding mass will increase natural frequency by a ratio of the square root of masses.

How much mass does one add for it to be accurate? The amount of mass the pipe has to displace? While not terrible, how do we quantify that?

With regards to tension, it only serves to increase natural frequency.

As motive force is applied, it bends the pipe to the tune of Sb=My/I.

However, if you imagine if instead of bending a volume, you're actually compressing one side and tensioning the other Sa=+/-F/A. And further, if you think of Hooke's law applying... F=kx, applying tension means it takes more energy to bend the pipe, and therefore the natural frequency will increase.

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#73162 - 04/11/19 03:34 PM Re: Subsea Modal Analysis [Re: Ltorrado]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 818
Loc: Louisiana, US
Correction: I misspoke - I meant to say that adding mass will exacerbate the natural frequency, i.e. decrease it.

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