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#1 tomwhite

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Posted 26 July 2010 - 08:27 AM

Hi All,

I am actually going through some opertaional problem we are having on one of the FPSO.

The water injection pump shut off head is higher than the design pressure of the flowline in the subsea.I like to clarify with you that do we need to consdier hydrostataic head in the flow line or not?

For example

The water injection pump dishcarge pressure is 100 barg at the inlet of the riser( just above the sea water surface)

The depth of the sea water bed is 200 meters which means at the bottom of the sea bed the water head euaivalent roughly is 20 bar.

The water injection flow line goes down to the sea bed to inject the water in the well which the total pressure acting at the bottom of the bed in the flow line is 100 + 20 = 120 barg. this is the pressure insdie the flow line pipe.

outside pressure on the surface of the flow line pipe at the bottom of the seam bed is 20 bar which sea water exerts 20 bar pressure on the outer surface of the flow line pipe.

the differential pressure will be 120 - 20 = 100 barg. Is this statement is correct? can we use this logic that inside pressure - outisde pressure = is the actaul pressure for which the flow line has to be deisgned.


the deisgn pressure of the flowline is 110 barg ,if i do not consider the differential pressure and if i consider only the insdie pressure in the flowline then we are exceeding the deisgn pressure of the flowline.

Usullay what is deisgn cretria in flow line design presusre rtaing. do we need to consider differential pressure or actuall presusre in the flolwine.
If i am degsning the flowline then definitely i will cosnider inside pressure only and i will make usre that design pressure is higher than the atcual inside pressure.

but the opertaional problem now is design pressure is lower then the actual inside pressure.in this case can i use the logic that we only have to consider the differential presure and not the actual presusre.

please give your valuable feedback and advises onthis.

i am typing to fats with and hence will have type errors.please ognore them.

#2 ankur2061

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Posted 26 July 2010 - 09:17 AM

tomwhite,

Aren't you worrying a tad too much for the design conditions for the line. Why don't you check the pressure rating for the pipe? In your case, if the pipe is 900# rated then your actual design pressure or MAWP is much higher then the design pressure that you have fixed based on the process design i.e. 110 barg

As far as your question for design pressure, the actual design pressure has to consider the surrounding pressure which means that if your line is operating at 120 barg and the surrounding pressure is 20 barg then the actual pressure exerted on the pipe wall is the difference of the pressure inside the pipe minus the surrounding pressure. Thus if the pressure inside the pipe at a depth of 200 m is 120 barg and the surrounding pressure is 20 barg then the pipe wall is actually seeing a pressure of 120-20 = 100 barg.

Hope this helps.

Regards,
Ankur.

#3 Zauberberg

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Posted 26 July 2010 - 09:27 AM

A common sense is following the same logic Ankur has presented in his reply, and that was your tought as well. You mentioned certain operating problems, but we couldn't grasp from your post what was all about?

#4 tomwhite

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Posted 26 July 2010 - 09:33 AM

Ankur,

Thanks for the reply. The logic applies that the pipe sees only the differential pressure but however the stress acting insdie the pipe would be high due to 120 barg even though outisde pressure is 20 barg.

lets say if the pipe has some hole on the pipe surface then the diffreential pressure of 100 barg applicable very well at the leak point.at the lekaing point the presure would be only 100 barg. but i feel when there is no leak and insdie pressure is 120 barg and outside pressure is 20 barg still the inner suracfce of the pipe would see high stress equalivalent to 120 barg in this case.

i have discussed on this with many of my freinds but different people have different opinion.

#5 ankur2061

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Posted 26 July 2010 - 10:42 AM

tomwhite,

Stress acts on both sides. So your contention that the pipe wall is seeing pressure only from inside is not correct. Does seem very simple to me.

Regards,
Ankur.

#6 tomwhite

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Posted 26 July 2010 - 11:46 PM

A common sense is following the same logic Ankur has presented in his reply, and that was your tought as well. You mentioned certain operating problems, but we couldn't grasp from your post what was all about?



Zauberberg/Ankur,

Here is the pratcical problem

The operating parameters of main water injection pump is 200 M3/Hr @ 1665 M head. At minimum flow conditions the operating parameter of one pump is 50 M3/Hr @ 1900 M head. Suction of the WI main pumps gets ~ 8 barg suction pressure from WI booster pumps. Sea water specific gravity is 1.025. FPSO has two main WI pumps in parallel and both will stay online simultaneously. At the WI main pump skid discharge pressure estimated 1900/10 * 14.22 * 1.025 = 2769 + 8 barg (8 * 14.5) suction pressure = 2885 PSIG. High – High pressure trip set point of the main WI pump is at 190 barg (190 * 14.5) = 2755 PSIG. The water depth of filed is ~ 1360 M and the main water injection pump elevation is about 40 M from the bottom of the vessel. The vessel minimum draft is 10 M. Total hydrostatic head works out 1390~1400 M. Sea water specific gravity is 1.025. Total hydrostatic pressure of WI riser at sub sea estimated 1400/10 * 14.22 * 1.025 = 2040 PSIG. With the data supplied now FPSO facility water injection flexible riser is having a pressure limitation inside the riser (MAWP) of 4500 PSI (subsea level). With the data we have, worst case scenario (mechanical failures of well sub sea choke, sub sea well valves, pressure switches, recycle PV or human failure to maneuver valves during start up of pumps etc) may bring the WI flexible riser (for short pressure shock or prolonged high pressure) to 2885 + 2040 = 4925 PSIG.


HERE WE ARE CONSIDERING THE HYDRSTATAIC HEAD ALSO IN COMPARASION TO PRESSURE RATING OF THE RISER.THE QUESTION IS DO WE NEED TO TAKE INTO ACCOUNT THE HEAD ACTING AT THE BOTTOM OF RISER?

#7 Zauberberg

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Posted 27 July 2010 - 06:39 AM

Can you upload a sketch of this system? It is very difficult to follow your line of thoughts, especially when you use different pressure units.

Also, you say that the PAHH (at which location/height?) value is set @ 190 barg while having 2885 psig discharge pressure (>198 barg) of the Injection pump system. This means the pump will never operate unless the flowrate is such that the discharge pressure falls below 190 barg. Or, with 8 barg suction pressure and 190 barg as a PAHH value, the pumps can develop up to ~1810m head, with the flow obviously being somewhere between 50 and 200 m3/hr. It is really difficult to grasp what and where is the problem so please take a couple of minutes and do prepare a sketch.

Our Inspection Engineer says that you should look into the records and see what was the test pressure used for this flowline prior to installation. As for the opeerating pressure concern, it is the same issue you have raised in your previous post: there is 1,400m head inside but also outside the pipe.

#8 herrani

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Posted 29 July 2010 - 01:10 AM

Hi all

I am not entirely sure if I got the problem right, but here are my two cents:

Min. flow case

P @ pump inlet = 8 bar g
Pump head = 1900 m seawater = 9.8*1025*1900/100000 = 191 bar
P @ pump outlet = 199 bar g

Normal flow case

P @ pump inlet = 8 bar g
Pump head = 1665 m seawater = 9.8*1025*1665/100000 = 167 bar
P @ pump outlet = 175 bar g

Hydrostatic pressure

Pump elevation above sea level = 40 m = 9.8*1025*40/100000 = 4 bar
Pump elevation below sea level = 1360 m = 9.8*1025*1360/100000 = 137 bar

Pump operation

During normal pump operation the pump outlet pressure is lower than 190 bar g (PAHH), which means that the pumps will operate normally. If for some reason the pressure rises up to 190 bar g at pump outlet (because of lower flowrates) the pumps will trip. The minimum operating case will never be reached. I assume that there will be some kind of special procedure for start-up that includes and override of that trip, or a lower inlet pressure.

Hydrostatic pressure

Assuming the worst case scenario (pump outlet pressure = 190 bar g), then the pressure at sea level inside the pipe will be 194 bar g. At the bottom of the sea, the pressure in the pipe will also be 194 bar g, and not 194 + 137 = 331 bar g. The reason for this is the definition of gauge pressure: actual pressure - atmospheric pressure. Once you are at 1360 m of depth, your atmospheric pressure will be 137 + 1 = 138 bar.

In my opinion, your line should have a design pressure of 194 bar g. In order to verify this, you should check the pressure rating of the line as ankur suggested (900#?), and the test pressure. Both should be available from the line list or similar documentation. How long has this system been in operation? Do you have any reason to suspect leaks, or other problems in the line? If not, this means that our thoughts are correct.




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