Ensuring Safe Facility Blowdown and Lessons Learnt from Incident Investigation

Abstract

A facility blowdown in the oil and gas industry is a safety critical operation; required to ensure the safe shut-down of processing facilities during a plant emergency. In a full facility blowdown operation, the entire plant is isolated into a number of segments and then depressurized into the facility’s flare system. The outcome of a blowdown event is dependent on the performance of a number of safety critical control system and final elements in the plant; these are designed to ensure the safe and successful depressurization of the plant without any conspicuous incidents. Blowdown events are inherently fast transient processes, reliant on a large number of valve opening and closing operations with rapid reduction in pressure and temperature. It is critical to ensure that the process is depressurized quickly enough (cf. API 521) but whilst respecting constraints such as minimum design metal temperatures, accoustic and flow induced vibration limits in flare tailpipes and back-pressure constraints throughout the flare system. Blowdown incident investigations typically involve a critical review of the plant historian data and require dynamic simulation studies to adequately assess the event so as to determine whether the blowdown operation proceeded as planned, that no design constraints were violated and to glean any process safety lessons that can be learnt. In this paper, with reference to a number of recent events on oil and gas facilities, we explain how to carry out a comprehensive blowdown investigation. We discuss a number of case studies from facilities in different parts of the world, including those that have shown operational deviations such as non-closure/partial closure emergecy shut down valves, delays in opening or non-opening of BDVs, and mal-functioning check valves. Some of these deviations caused flare capacity issues in the plant, minimum design metal temperature violation risks to process drums, unexpected pressurization of process drums, flow reversal into low pressure segments causing near miss events, and other violations of recommended practice. We explain how these studies have helped understand the current state of the plant’s safety systems and have been used by operating companies to identify design and operational changes critical for safe plant operation. Finally we discuss how these studies provided input to plant maintenance personnel in order to help prioritize maintenance activities and to make prudent maintenance investment.

Airborne Assessment of Methane Emissions from Offshore Platforms in the U.S. Gulf of Mexico

“…While it is premature to identify the process­(es) responsible, it is unlikely that we observed a rare phenomenon. High emissions may be released from nonroutine venting during a blowdown for equipment depressurization in maintenance operations associated with turnarounds or shut-downs . However, if these events are uncorrelated between platforms, it is unlikely that such infrequent practices were present on all three sites (SI Appendix S8).…”

“…High emissions may be released from nonroutine venting during a blowdown for equipment depressurization in maintenance operations associated with turnarounds or shut-downs. 35 However, if these events are uncorrelated between platforms, it is unlikely that such infrequent practices were present on all three sites (SI Appendix S8). Although there is some probability that flights sampled a rare high emission spike from another source of nonroutine venting, the probability of observing infrequent high emission pulses from multiple facilities on multiple days is low: we estimate less than 4% (SI Appendix S8).…”