Hazardous Open Drain Systems in Oil & Gas Facilities

This technical preprint provides a comprehensive, end-to-end guide on Hazardous Open Drain (HOD) systems in onshore and offshore oil and gas facilities. Designed for process, safety, piping, HSE, and operations engineers, as well as engineering students, the document bridges the gap between fragmented standards and practical plant design.

Key Topics Covered:

• Hazard Fundamentals & Architecture: Analysis of flammable risks, vapour/liquid pathways, the three-tier drain segregation philosophy (HOD, NHOD, and closed drains), and the seven layers of protection.

• Components & Interfaces: Detailed breakdown of collection points, liquid seals (U-traps/seal pots), branch lines, headers, vents, and receiving elements (atmospheric tanks and open-bottom caissons / skim piles).

• Hydraulic Theory & Sizing: Reproducible methods for open-channel flow (Manning equation), Froude number checks, hydraulic grade lines (HGL), surcharge criteria, and backflow prevention between fire areas.

• Seal Dynamics & Evaporation: Evaluation of U-seal responses to sudden pressure steps, anti-siphon vent requirements, evaporation bounds across various climates, and depth budgets incorporating floating unit motions (heel and heave).

• Safety, Standards & Operations: Mapping of key regulations and standards (e.g., Norwegian facilities regulations, NORSOK, ISO 13702, API, OSHA/EPA rules), area classification aspects, maintenance routines, and a review checklist.

• Failure Modes & Lessons Learned: Analysis of historical public incidents (BSEE, CSB) and a structured hazard register for liquid seals and inlets.

• Worked Examples: 15 step-by-step calculations covering header capacity, deluge scenarios, seal height, caisson hydrostatics, Stokes law oil-retention checks, and HGL profiles.