For Discussion Purposes:
Common practice both in onshore installation and offshore facilities is to provide a single relief header for a given backpressure level to be imposed on relief valves. There appear to be no unique circumstances in this design to warrant a departure from a typical installation of relief headers. This would indicate that the current design should be left alone.
If the decision is made to revise the design (including the revision of flowsheets, piping drawings and isometrics) with a new design basis, then a new relief scrubber for the new relief line is required. The purpose of this scrubber is to remove liquid hydrocarbons from the relief system to avoid polluting the offshore environment. The references noted below include the recommended practice from API for surface safety design and its intent is to avoid undesirable events such as the release of hydrocarbon liquids into the water.
- API RP 14C: Recommended Practice for Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms.
- API Standard 521. Pressure-relieving and Depressuring Systems.
- 30 CFR Part 250 - Oil and Gas and Sulphur Operations in the Outer Continental Shelf.
The recommendation is that the relief header should not be split since it would not add a level of protection for the platform commensurate with the added cost. If the header is split and the relief is vented, then the regulations indicate that a vent scrubber is required to avoid putting liquid hydrocarbon into the sea. This requires a new relief scrubber and the associated removal of liquid hydrocarbons to the drains.
There is no problem of using two separate inlets or outlets for the dual relief valves. Many other operators rarely consider the added cost of an extra vessel nozzle to offset the possible increased safety (through redundancy) of a separate nozzle.
It is NOT recommended to route the vent line outlet below the water surface. This would result in an accumulation of liquid hydrocarbon (oil slick) on the water surface. In addition, a process upset would result in a release of large amounts of flammable gas near marine operations and the platform.
Finally, it is highly recommended that a complete review of the safety systems be performed in the as-built condition and then revised to completely protect the platform. If the relief system is modified in an isolated area, it may no longer be protected in the way it was originally intended to function. This may have an undesirable impact on the project since some long lead safety equipment may need to be added to complete the required two levels of protection.