4 replies to this topic

[heldream]

Dear friends
I am involved in one project which deals with some modification on offshore production platforms to
produce 1000 MMSCFD multi phase Gas stream.
The question is about Depressurizing of platform where We need to depressurizie two new piping section with
the follwoing charecteristics:
1)8" pipe made from inconel 625 and 18 mm thickness,volume is 1.41 m3
2)16" pipe made from inconel 625 and 32 mm thickness,volume is 6.05 m3
the design pressure of two section is around 290 barg and we are going to calculate the Blowdown Flowrate
and size of Blowdown orifices in fire case and also in cold blowdown case.
Maximum individual blowdown rates for other sections have been calculated on the emergency depressuring
(fire wetted) from system design pressure 139 barg to 7 barg in 15 minutes.But due to installation of
HIPPS system, design pressue of two abovementioned pipe sections are 290 barg which is also the maximum expected
pressure (wellhead shut-in pressure).
There are some questions as follows:
a) Whether piping can be treated the same way as vessel?
b ) What should be the criteria for depressurizing of 8" and 16" pipe sections? Should we follow the
normal depressurizing criteria (i.e depressuring from system design pressure 290 barg to 7 barg in 15 minutes)
or we can use 50% design pressure instead of 7 barg.
c) what is the affect of thickness on determination of the final pressure after depressurizing time(15 min).For 16" section the thickness is around 32 mm.
Are we allowed to consider 50% design pressure instead of 7 barg.what about 8" section with thickness of 18 mm?
d)If in any case the blowdown rate exceed the existing platform flare capacity,what solution or recommendation
shall be applied?
Thanks in advance

Edited by heldream, 09 July 2011 - 06:09 AM.

[Erwin APRIANDI]

Dear heldream,

a)Whether piping can be treated the same way as vessel?
answer: If we are doing a calculation of a system we assumed the piping as an additional vessel volume that we take into consideration, therefore for calculation of piping depressuring we can assume the same.

b)What should be the criteria for depressurizing of 8" and 16" pipe sections? Should we follow the
normal depressurizing criteria (i.e depressuring from system design pressure 290 barg to 7 barg in 15 minutes) or we can use 50% design pressure instead of 7 barg.
answer: I would suggest you to read on API 521 before doing any calculation, on API 521 Fifth Edition. API allow depressuring to 50% for vessels with a wall thickness of approximately 25,4 mm (1 in) or more in which you can apply for your 16" pipe. But if your flare system is still capable to have a depressing up to 100 psig in 15 minute for the 16" pipe, I would suggest you go for 100 psig, which reduce the posible risk of fire escalation due to inventory inside the piping.

c)What is the affect of thickness on determination of the final pressure after depressurizing time(15 min).For 16" section the thickness is around 32 mm.
Are we allowed to consider 50% design pressure instead of 7 barg.what about 8" section with thickness of 18 mm?
answer: Please refer to API STD 521, and for less than 1" thickness I don't recommend you to do this.

d)If in any case the blowdown rate exceed the existing platform flare capacity,what solution or recommendation shall be applied?
answer: I have applied staggered blowdown for my previous project, so in case of fire in one area, we will open the BDV in sequence starting from the nearest BDV to are which fire is detected and end to the most fartest. However for this aplication you have to consider that actually after 15 minute of blowdown opening you will still have inventory coming out from the BDV, so this flow should be consider. Since after 15 minute of blowdown you will not close the valve right?

Hope it help heldream

Edited by erwin.apriandi, 12 July 2011 - 02:36 AM.

[fallah]

Dear friends
I am involved in one project which deals with some modification on offshore production platforms to
produce 1000 MMSCFD multi phase Gas stream.
The question is about Depressurizing of platform where We need to depressurizie two new piping section with
the follwoing charecteristics:
1)8" pipe made from inconel 625 and 18 mm thickness,volume is 1.41 m3
2)16" pipe made from inconel 625 and 32 mm thickness,volume is 6.05 m3
the design pressure of two section is around 290 barg and we are going to calculate the Blowdown Flowrate
and size of Blowdown orifices in fire case and also in cold blowdown case.
Maximum individual blowdown rates for other sections have been calculated on the emergency depressuring
(fire wetted) from system design pressure 139 barg to 7 barg in 15 minutes.But due to installation of
HIPPS system, design pressue of two abovementioned pipe sections are 290 barg which is also the maximum expected
pressure (wellhead shut-in pressure).
There are some questions as follows:
a) Whether piping can be treated the same way as vessel?
Yes.
b ) What should be the criteria for depressurizing of 8" and 16" pipe sections? Should we follow the
normal depressurizing criteria (i.e depressuring from system design pressure 290 barg to 7 barg in 15 minutes)
For vessel/piping leak,yes.
or we can use 50% design pressure instead of 7 barg.
You should use this criteria for emergency depressuring.
c) what is the affect of thickness on determination of the final pressure after depressurizing time(15 min).For 16" section the thickness is around 32 mm.
As a general rule,for vessel with 25 mm wall thickness or greater,the time of depressurizing would be 15 minutes.For vessel with wall thickness smaller than 25 mm,the time of depressurizing is equal to 15 mm minus 3 minutes for each 5 mm decrease in thickness.
Are we allowed to consider 50% design pressure instead of 7 barg.what about 8" section with thickness of 18 mm?
Yes,you should use that.For depressuring of 8" section refer to previous answer.
d)If in any case the blowdown rate exceed the existing platform flare capacity,what solution or recommendation
shall be applied?
How much exceeded? From how many BDVs in mentioned case? IMO,It could be judged after having detailed information.
Thanks in advance

Edited by fallah, 12 July 2011 - 05:46 AM.

[heldream]

Dear Mr. Fallah,
many thanks for the response,
you have mentioned: "For vessel with wall thickness smaller than 25 mm,the time of depressurizing is equal to 15 mm minus 3 minutes for each 5 mm decrease in thickness."
would you please address a standard or any other engineering practices for this criteria?

Dear friends
I am involved in one project which deals with some modification on offshore production platforms to
produce 1000 MMSCFD multi phase Gas stream.
The question is about Depressurizing of platform where We need to depressurizie two new piping section with
the follwoing charecteristics:
1)8" pipe made from inconel 625 and 18 mm thickness,volume is 1.41 m3
2)16" pipe made from inconel 625 and 32 mm thickness,volume is 6.05 m3
the design pressure of two section is around 290 barg and we are going to calculate the Blowdown Flowrate
and size of Blowdown orifices in fire case and also in cold blowdown case.
Maximum individual blowdown rates for other sections have been calculated on the emergency depressuring
(fire wetted) from system design pressure 139 barg to 7 barg in 15 minutes.But due to installation of
HIPPS system, design pressue of two abovementioned pipe sections are 290 barg which is also the maximum expected
pressure (wellhead shut-in pressure).
There are some questions as follows:
a) Whether piping can be treated the same way as vessel?
Yes.
b ) What should be the criteria for depressurizing of 8" and 16" pipe sections? Should we follow the
normal depressurizing criteria (i.e depressuring from system design pressure 290 barg to 7 barg in 15 minutes)
For vessel/piping leak,yes.
or we can use 50% design pressure instead of 7 barg.
You should use this criteria for emergency depressuring.
c) what is the affect of thickness on determination of the final pressure after depressurizing time(15 min).For 16" section the thickness is around 32 mm.
As a general rule,for vessel with 25 mm wall thickness or greater,the time of depressurizing would be 15 minutes.For vessel with wall thickness smaller than 25 mm,the time of depressurizing is equal to 15 mm minus 3 minutes for each 5 mm decrease in thickness.
Are we allowed to consider 50% design pressure instead of 7 barg.what about 8" section with thickness of 18 mm?
Yes,you should use that.For depressuring of 8" section refer to previous answer.
d)If in any case the blowdown rate exceed the existing platform flare capacity,what solution or recommendation
shall be applied?
How much exceeded? From how many BDVs in mentioned case? IMO,It could be judged after having detailed information.
Thanks in advance

[fallah]

Dear Mr. Fallah,
many thanks for the response,
you have mentioned: "For vessel with wall thickness smaller than 25 mm,the time of depressurizing is equal to 15 mm minus 3 minutes for each 5 mm decrease in thickness."
would you please address a standard or any other engineering practices for this criteria?

heldream,

It is included in one of Total Co. Specs has been used in the design of gas refinery projects in my country.

Fallah