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 12 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