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[essanna]

Hi,

I am a Process Safety Engineer and am trying to design drainage for a FPSO such that the deluge firewater can be drained from the plated decks of the FPSO to the Main Deck scuppers. I have difficulties in determining the maximum / allowable water velocity that may flow through the drain pipe in the event of deluge. See attached sketch.

Option 1: Use self-venting gravity flow as per typical process open drain pipe sizing. That means use of Froude Number limited to 0.3 to prevent air entrainment and pulsations within the drain pipes. That translates to a large number of drain boxes that are impractical to be installed.

Option 2: NORSOK P-01 gives guidance as follows: "Drainage of deluge water from drain boxes through vertical lines shall be sized on basis of 50 % of the available head (assuming the pipe running full of liquid) and not Froude number.". Using this basis, the calculated velocities through the drain pipes are way too high (> 7m/s) and I do not believe plausible.

Option 3: If I simply consider basis of 50% of available head (based on the drain box 50% liquid filled) and use equation: hpg = 0.5pv^2 where
h = height of water in drain box
p = water density
g = gravitational constant
v = velocity
This gives a velocity of 1.7m/s for a 6in drain pipe.

I have provided my calculations in the attached file.

Given that deluge is not a regular event on the facility, it seems excessive to design drainage pipes for self-venting (Option 1).

Questions:
1. Does anyone have any guidance or references for drain design for firewater deluge?
2. Is Froude Number a necessary consideration in drainage design for firewater deluge? Some vibrations in drainage pipe are surely acceptable in a deluge scenario?

Thanks!

Attached Files

•  Deluge Drain.xlsx   53.46KB   73 downloads