Please help - Free falling water in a pipeline riser and potential for subsequent errosion/damage

[Glenhoy]

Hi, can any one please help/provide guidance on the following: -

I have a carbon steel pipeline riser and section of pipeline offshore West Africa that I want to fill up with filtered seawater however I am concerned that the falling velocity of the water and the pressure may cause damage to the riser/pipeline and possible errosion to the valve.

The riser is approximately 20in in diameter which drops vertically for about 2,140m (~7,000 ft) in a gradual curve until is has a step out distance of about 1,375m (~4,500 feet) where it levels off along the seabed and transitions into a 24" pipeline. The 24" pipeline section then runs along the seabed (with slight minor profile changes no greater than 75m/250 feet) for a distance of approximately 20km (~12.5 miles) where it terminates at a closed ball valve.

Before I can tie-in the riser I have to fill it up with water. The plan would be to keep the valve at the end of the pipeline closed and pump in water (stopping periodically to bleed off the pressure of the trapped/compressed air) until the pipeline section (20km / 12.5 miles) and riser (which has a total length of approimately 3,650m / 12,000 feet) is completely full of water. The rate at which I would be injecting the water would be approximately 3.5 cu.m/min (925 US gpm).

Based on these specific I am worried that the velocity of the falling water due to acceleration and pressure could cause damage to the pipeline and/or errosion to the valve.

I did consider using a pig in front of the water (which would be pumped out at a later date) however believe that the head pressure of water behind the pig would gradually become to great whereupon it would accelerate out of control and could potentially run into the closed valve again with the potential for damage. If you could please provide me with any assistance/thoughts on this matter it would be very much be appreciated.

Many thanks for everyones thoughts on this.
Regards
G

 

[BigInch]

I don't think that erosion will be a problem for initial filling. If you think it will be, then add wall thickness until you are comfortable. I don't think it is necessary. I imagine you have plenty of wall thickness to prevent collapse from extreme external pressures. In any case, once there is some water at the bottom, any additional water entering will be somewhat cushioned. Of perhaps more worry is how you will ensure that any air in the pipeline will be forced out the end if the valve is closed.

What if...
you put the filter at the end of the pipeline, opened the valve and reverse-filled the pipeline through the filter and back up to the riser. You might have better control of the fill rate and air would more easily be displaced upward. You may have a considerable pressure drop across that valve, so I might even consider using two valves. If ball valves, use them only for on/off service, open one then the other, one then the other, letting in some reasonable amount of water each time, and don't try to throttle the entry flow, leaving them at partial percent open positions as that might indeed cut the valve up internally. Just an idea.

I hate Windows 8!!!!

 

[LittleInch]

Wow, this is one hell of a riser. I find it difficult to see it going in a gentle curve over that sort of depth, but I'll work with what you provide.

Given the huge length of flat or nearly flat pipeline, there will be no velocity reaching the valve and for a one off fill, negligible erosion.

The real issue as BI says is that you will end up with a lot of entrained air.

You often see subsea lines filled from the end point and you can normally do it a few ways, either a line to the surface to allow a pump on the boat to fill it in a controlled manner and inject chemicals etc, fit a control valve to manage your inlet flow rate to a reasonable figure or pressurise the line to a pressure equal to the head at the end (in your case circa 200 bar(!) and gently bleed off the air from the inlet end. The latter you could do the other way around and bleed air off either subsea or via a hose to the surface once you pressurise the riser to 200 barg, run a pig and pump water in behind it, gradually releasing air, but keeping a solid water column behind the pig

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MortenA]

I would try to see if you could get in contact with similar pipelines e.g. in Norway or the medgas pipeline from Algiere to Spain. They may share their experience with you.

Best regards, Morteb

 

[rconner]

You may be interested in the reply of bimr mentioning "terminal velocity" (that may not be as extreme as some might think in normal situations) in the thread last year at

http://www.eng-tips.com/viewthread.cfm?qid=326191.

[As to potential for any other sorts of damage and at least semi-control of air removal etc., while I'm not sure it's necessary a crazy thought might be to have a small pipe inserted in the riser, fill slowly from the bottom up, then withdraw the filler pipe.]