Free Span Full Fatigue

[robb4]

Hi guys,

Regarding free span analysis according DNV RP F105 I've the following question:

Is there a minimum gap ratio (free span clearance/External Pipeline Diameter) that lead to VIV onset (i.e. below that limit no VIV occurs)?

I found in a document that with gaps ratio below 0.3 no VIV occurs, but I don't have a clear reference on that.

Thank you for your help.

 

[BigInch]

Not entirely sure about that, but attacking from logic alone ... Span to diameter ratio is essentially making a proportionality to the dynamic frequency of the pipe. As the ratio decreases, the frequency decreases too. Longer tubes in musical instruments produce lower notes for that reason. You can expect to have more vibration at lower frequencies as the ratio reduces, since lower current velocities will be able to vibrate them with ease. The limiting ratios would occur where the actual vibration frequency/natural frequency of a thin wire = 1; where you would usually want to stay outside a range of 0.70 to 1.40 to guarantee small amplitudes. The other practical limit of course is the structural integrity of the wire, or the pipe in this case, making such a long span with a minimum diameter.

Everything can vibrate, what you want to avoid is vibration at or near resonant, or "natural" frequency, so stay out of the range.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[robb4]

Hi Big",
maybe there is a misunderstanding because with free span clearance/External Pipeline Diameter I meant a different thing (see attached).
Anyway I agree with you that a very important thing is the other ratio (free span length/external diameter).
Thank you

 

[BigInch]

I don't think there is a minimum clearance, as passing velocities would tend to increase with reduced area of flow underneath the pipe, and so would drag. Drag effects might become more important than frequency concerns.

Then again, the current as you get closer to the bottom could tend to decrease too, due to roughness and boundary layer effects. There must be practical limits there, as soon you would start getting a higher velocity over the top of the pipe than under it and the votex shedding frequency would start breaking up. I would guess something like that when you get closer than 2 x the pipe diameter to the bottom, bottom effects would start to become more significant. You could try to do some "hand work" based on simple streamline analysis for a cylinder near a wall, or if you're versed in CFD .. even better.

From "BigInch's Extremely simple theory of everything."

 

[robb4]

Finally, I found a reference paper for the minimum gap ratio below that no VIV occurs. It is " 1997 OMAE, Volume V, Pipeline Technology, ASME 1997 (Mark, Vitali, Velley) – The multispan project: design guideline for free spanning pipelines". In this paper it is stated "In general it can be assumed that vortex shedding only occurs if the gap ratio e/D between the pipe and the sea bottom is greater than 0,3”.
But in DNV RP F105 there is no mention of this general statement
I would use that reference for my scope.

 

[BigInch]

Sounds reasonable. By that distance, the flow above must be much higher than below, possibly being close to stagnated.

From "BigInch's Extremely simple theory of everything."

 

[LSThill]

DNV Service Specifications, Standards and Recommended Practices - Valid as of January 2012

http://exchange.dnv.com/publishing/Codes/ToC_edition.asp