The following incident is a precautionary example of the potential hazards of improperly depressurizing piping to remove a hydrate. During production operations, a control room operator observed a decrease in gas lift pressure supplied through a pipeline to a remote well location. The control room operator notified the onboard platform operators of the pressure drop and the possibility of a hydrate forming inside the 3-inch gas lift piping. The control room operator and platform operators planned to isolate and bleed the pressure to remove the possible hydrate.
The control room operator from his station closed the pipeline shutdown valve (SDV) upstream of the manual flow control valve.
Additionally, the platform operators physically closed the manual isolation valve upstream of the SDV, assumed the hydrate location, and departed the pipeline.
The platform operators then started to relieve the pressure downstream of the hydrate location to atmospheric pressure without properly isolating the pipeline from the bleed point through a ball valve assembly attached to the 3-inch gas lift piping. The ball valve assembly consisted of a threadolet, threaded steel pipe nipples, two 1-inch ball valves inline, and a 90-degree elbow that pointed upward.
With approximately 900 psi trapped behind the valve, the assembly separated from the threadless, striking one of the platform operators under the left armpit area, and causing bruising and swelling. The injured offshore worker was sent for medical treatment and was later released to full duty.
The valve assembly could not be found and is suspected to have fallen overboard after striking the worker.
Hydrates can form inside a pipe from a concentration of impurities, natural gas, flow conditions, high pressure, and low temperature. At the time of the incident, the operators onboard the facility were injecting methanol at a rate of 5 quarts per day to prevent hydrate formation. A Job Safety Analysis (JSA) was not performed, so the hazards of relieving pressure to remove a hydrate were never discussed. The cause of the dislodged valve assembly is still under investigation.
BSEE recommends that operators and their contractors, where appropriate, consider doing the following:
- Isolating all energy sources, then bleeding pressure upstream and downstream of the hydrate simultaneously, maintaining the same amount of pressure so that the hydrate does not move and cause a pressure surge, which could damage associated piping.
- Verifying that JSAs identify potential hazards and mitigate those hazards for the task being performed.
- Ensuring that when changes to equipment are made, all hazards are identified and mitigated through the Management of Change process, updating facility information, and informing all personnel affected by the change(s).
- Using gas dehydration systems to remove water vapors to reduce the risk of hydrate formation inside of piping.
- Verifying current platform configurations are reflected on piping and instrumentation diagrams.
