Learning from Incidents - Oily Water Drainage System

Abstract

Among the various drainages in oil and gas plant, oily water drainage system is critical with high risk considering its hydro carbon content and accumulation of explosive vapours due to improper functioning of any components in the system. This paper presents some of the shortcomings in the design/arrangement/maintenance of the different elements, learnings from an explosion incident and the improvements to avoid accidents. The drainage systems in ADNOC Gas Processing plants are Clean Water Sewer, Sanitary Sewer, Oily Water Sewer (Accidentally Oil Contaminated / Continuously Oil Contaminated) and Chemical Drains. One of the unforeseen hazards has caused an explosion in an existing oily water sump inside the process area. The sump was of concrete construction and the cover slab was blown up resulting in considerable damages to the surrounding pipes/support. There was no injuries to personal. The root cause analysis of the incident was done and a study carried out for a safety review of oily water drainage system at all plants. A study was carried out to identify possible hazards/design deficiencies of the oily water drainage facilities and recommend rectification measures. The study identified that the lines are placed in slope & mainly "run dry" after use, liquid seal not available in some manholes causing HC vapour upstream movement, the manholes/sumps are closed, vent pipes size is small and some are blocked. These lead to accumulation/formation of hydrocarbon mixture inside closed sump, ignition by overheating of the pump installed over the pit and subsequent explosion. Recommendations for existing system include regular flushing of the lines with water to ensure transport of oily effluent & maintain liquid seal, regular removal of the floating hydrocarbon liquid from pits using vacuum truck, open up the manholes and use grating covers, provide self-skimming bucket where feasible, etc. Additionally, for new facilities it is recommended to lay the pipes horizontally to ensure liquid seal, provide turn-up elbows to reduce hydrocarbons accumulation in manholes, provide oil skimming/baffle wall in oil/water sumps, use classified equipment, carryout proper maintenance, etc. The sump was reconstructed with the new design, which is now functioning well and the recommendations are being implemented in existing facilities and in all new projects. The proposed improvements for the existing system as well as adopting the recommendations in future new drainage system can ensure the prevention of possible explosions and thereby reducing the related hazards to plant facilities/operations. Sharing of the related information among others having similar facilities increases the awareness about such unseen sources of incidents in a drainage system and related pro-active mitigating measures.