Sizing of overboard water lines

Question

Sizing of overboard water lines

aimless (Chemical)

(OP)

17 Jul 07 15:31

Help, I'm new to offshore work. What is the sizing criteria for overboard lines? I'm assuming they have to at least be self-venting but is there other criteria or a minimum size for structural reasons? I get blank stares when I ask this question. Apparently we've just copied the same sizes for a few years.

Thanks

MikeHalloran (Mechanical) · Answer 1 · 2007-07-17 16:50:57

<blank stare>

...

Uh, carrying what kind of water, from where, to where? Etc.

Mike Halloran
Pembroke Pines, FL, USA

aimless (Chemical) · Answer 2 · 2007-07-17 17:04:38

It's mostly injection water from relief valves, also outflow from the sewage treatment unit. They run into a vertical pipe and then to the deep blue sea. It's the vertical pipe portion I need help with. Right now, the lines seem generously sized, but of course I've been asked if that size can be reduced. I'm assuming that there is a minumum size that can realistically be supported. Most projects I looked at for reference were much larger than the current facility so I know those sizes will work, but is it overkill? I'm used to plants on solid ground - no breakers there, except maybe in hurricanes.

Ussuri (Civil/Environmental) · Answer 3 · 2007-07-18 03:46:21

I dont quite understand your post, but will have a go.

You have a vertical pipe attached to something above water (topside, installation vessel, FPSO...) and the pipe passes through the air/water interface and discharges?

I guess from a structural perspective you will have some kind of hang off arrangement which supports the pipe. This may be a top hangoff or multiple supports along its length. Alternatively it may be held in place with guides.

So this will be subject wave loading, possible impact from vessels etc

MikeHalloran (Mechanical) · Answer 4 · 2007-07-18 11:14:59

Downsizing a pipe carrying water and nutrients into the sea will cause it to become clogged with critters sooner.

Understand, the critters _will_ come, and they _will_ clog the pipe, completely. You're just deciding on the maintenance schedule.

Mike Halloran
Pembroke Pines, FL, USA

WmacG (Structural) · Answer 5 · 2007-07-18 18:59:16

Structuraly, the lines should be sized for bending stress based on the span between their supports and loading (wave, wind, current). Vortex induced vibration may control for smaller diameters (6"&8"). 8" pipe is not uncommon for 15' spans near the waterline.

Check the lines at the supports since they are continuous and your monent is highest there. The lines may be cantilevered at the bottom and may be overstressed at the last support.

Marine growth near the waterline will significantly increase the diameter and loads, but not the strength and stiffness.

Corrosion should also be accounted for.

This is only for sewage discharge lines, etc, not pressurized lines.

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Sizing of overboard water lines