Pipeline Stability - Effect of Wave Period

[JT100]

When analyzing pipeline stability using morisons equations, the stability generally decreases as the wave period is increased. Is this due to to a longer period carrying a larger volume of water and hence generating a larger force on the pipeline? As I would assume that a lower period would also generate larger velocities which would decrease stability? Are either of these correct or is it not this simple? Any advice appreciated.

 

[BigInch]

Higher period = lower velocities, but for a longer duration.

Shorter period = faster velocities, but for a shorter duration. You also get more impulses during any given time.

The overall response you get is dependent on two relationships

1.) the ratio of the pipeline's natural frequency to the wave period, basically responsible for larger deflection amplitudes, as those forces are sustained for longer times, larger deflection movements can be produced, but at lower frequencies.

2.) the ratio of the pipeline's natural frequency to the vortex shedding frequency generated by the drag of the water flowing around the pipeline, but usually at higher frequencies and primarily responsible for higher vibration modes.

You want to try to keep the pipeline's natural resonance frequency outside the affecting range of both of those and perhaps some of the prime harmonics of those frequencies as well.







We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[JT100]

Thanks Big Inch, so given a set of metocean data is it down to engineering judgement as to what period you would use. For example, when considering pipeline/spoolpiece stability and given the maximum wave and a range of periods? It is my experience that the maximum wave be used in conjunction with a maximum or associated period, however metocean reports can also present an associated lower/central/upper period. I would generally use the central period, but am curious about anyone elses thoughts since a small change in period used can have a large effect on the stability (based on a 2D analysis using morrisons equations).

 

[BigInch]

No. You have to check over the whole range. Maybe if you had a lot of experience at it, much more then me; you might immediately know what would be the critical combination, but I don't, so I have to check the whole range. Anything near the natural resonance frequencies will be a kill. Offhand I'd say that you want to be outside the 0.7 to 1.3 frequency ratio, don't know what your code says about it.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky