Open and Closed Drains: Onshore Sewers vs Offshore Drain Systems

Two kinds of drain

Closed drains collect liquids drained from pressurised equipment and piping during maintenance or operation: vessel drains, pump casings, instrument drains. They flow in closed piping to a closed drain drum, from which liquid is pumped back to the process and vapour goes to flare.

Open drains collect liquid at atmospheric pressure: rainwater, firewater, washdown, and small spills from equipment areas, through drain boxes, funnels and deck drains.

Why they must stay separate

If a closed drain is connected to an open drain, gas or high-pressure liquid can reach open funnels and escape in the process area, creating a fire or toxic hazard. For the same reason, hazardous-area open drains (which may carry oil) are separated from non-hazardous ones, so that gas can’t travel through a drain line into a safe area such as accommodation.

Onshore

• Closed drains go to a closed drain drum or slop system and back to the process.
• Open drains become the plant’s oily water sewer network, mostly underground, with sealed catch basins (liquid seals) to stop vapour travelling through the sewer, and vented manholes.
• Clean rainwater from non-process areas goes to a separate storm water system.
• Oily water is treated (separators, flotation) before discharge.
• Underground sewer lines need careful slopes and good joints to avoid leaks to soil.

Offshore

• Closed drains go to a closed drain drum on the platform, with liquid returned to the process.
• Open drains from decks are collected by deck drain boxes and gutters into a hazardous open drain tank or caisson and a non-hazardous one.
• Drain piping runs through congested modules on the underside of decks, so slopes are hard to achieve and routing is planned with the structural layout.
• On floating units, vessel motion affects drainage, so larger slopes or collection tanks with level control are used.
• Water is treated before discharge overboard to meet oil-in-water limits.

Item	Onshore	Offshore
Closed drains	Closed drain drum, back to process	Closed drain drum, back to process
Open drains	Underground oily water sewer with liquid seals	Deck drains to hazardous and non-hazardous tanks or caissons
Main layout challenge	Slopes and sealing underground	Slopes under congested decks, vessel motion

What to take back to your desk

On the P&ID, follow every drain to where it ends. If a closed drain ends in an open funnel or a hazardous drain ties into a non-hazardous one, flag it immediately.