Drainage Systems on Offshore Platform: Overview, Open Drain System and Low Point Drains
1. About Drainage Systems on Offshore Platform
1) Offshore platform drainage and waste disposal systems shall be designed in accordance with API RP 14E “Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems” and all national, state and local codes and regulations. The design of all offshore platform drainage and waste disposal systems shall be subject to approval by Company.
2) Segregation of wastes, oil recovery sludge disposal, biological and chemical treating and other disposal problems shall be considered in the design of the drainage systems. A separate collection system shall be provided to collect, store and dispose of refined oils drained from equipment.
3) Open drain systems and closed drain systemsshall be completely independent of one another.
4) Sizing of sanitary drains shall be based on Uniform Plumbing Code fixture unit values. The minimum size for any single branch into a header shall be 4” NPS. The minimum size for any header containing the flow of two or more branch lines shall be 6” NPS. All sanitary drain piping shall slope a minimum of 1/4” per foot of pipe run.
A. Open Drain System
1) EPC Contractor shall be responsible for overall design of open drain systems to ensure that water will drain properly and not stand or pond at any location. Responsibility includes the design, sizing, layout and quantity of deck drains and troughs; sizing and routing of drain lines, branches and mains; and locating cleanouts and flushing tees.
2) The minimum size for any single drain line shall be 3” NPS, except that drain lines less than 8’-0” in length may be reduced to 2” NPS. The minimum size for any single branch line shall be 4” NPS. The minimum size for any main containing the flow of two or more branch lines shall be 6” NPS.
3) Hydrocarbon open drain systems shall not be installed within enclosed areas (such as a building) unless the hydrocarbon open drain systems are piped to a safe area before making the connection to a gravity drain system.
4) If necessary, pipe, hubs, and funnels shall be provided to drain oil and chemicals to suitable drain piping.
5) Removable bar grating shall be provided on the top of open type drains to provide access for cleaning. Individual collection drains shall be designed to prevent small debris and trash from entering the platform drain piping.
6) Changes in direction on open drain headers shall be made with a 45o lateral (“wye” type tee) and a 45o elbow complete with a flushing tee. Drain and branch lines shall enter open drain headers at a 45o angle from the top or side utilizing a 45o lateral, and a 45o elbow if needed. Laterals shall be constructed flush with the inside of the main pipe, i.e. no internal projection.
7) The ends of all headers shall be terminated with blind flanges to facilitate cleaning and unplugging.
8) Liquid seals shall be provided in accordance with the Technical Requirements, and shall be provided with either a drain valve or a plugged drain connection. As a minimum, liquid seals shall be provided on each deck drain main tying into the main drain downcomer, and on branch piping from skids located inside buildings. Containment curbs shall be provided under all liquid seals.
9) Spillover (overflow) pipes from vessels and tanks shall discharge into an open drain splash hub to allow observation that the vessel or tank is being overfilled.
10)Where specified, EPC Contractor shall provide a drilling drain/wash down system which shall be completely independent of other open or closed drain systems. The system shall be provided with cleanouts, flushing tees and/or tie-in points for jetting of the piping. Access shall be provided to all cleanouts.
B . Low Point Drains
1) Drain connections shall be provided at all piping low points. Drain connections shall be adequately sized to drain the piping within a reasonable period of time under gravity conditions, i.e. not pressurized. As a minimum, drains shall be 3/4” NPS on lines 10” NPS and smaller, and 1” NPS on lines 12” NPS and above.EPC Contractor shall note that P&ID’s typically specify only those drains required for process purposes, and are by no means complete relative to low point drains. The absence of drain connections on the P&ID’s shall not constitute a waiver of the requirement to provide valved drains at all piping low points.
2) Drain connections shall be provided with permanent block valves, including plugs or blinds, unless specified otherwise by the Technical Requirements.
3) Drain connections in services rated at ANSI Class 600 or less shall be equipped with a single block valve.
4) Drain connections in services rated at ANSI Class 900 or higher, as well as services with lower ratings but operating in hazardous areas shall be equipped with double block and bleed valves, if drain is not isolated by two block valves that are upstream and downstream of connection.
5) In liquid services where the liquid is pentane or heavier, with check valves located in vertical lines, a valved drain shall be installed above the seat of check valves, i.e. downstream of the valve to allow drainage.
6) Drain valves, if practicable, shall be located so that discharge can be observed.
7) Drain valves that are commonly operated shall be piped from the block valve to a splash hub for observation.
8) If approved by Company, small drain lines may be disconnected from valves and the valves plugged when the drain lines are not in service.