The National Offshore Petroleum Safety and Environmental Management Authority (NOPESMA) recently issued a safety alert following an incident involving the failure of an instrument tubing line connected to a wellhead gas lift line.
This resulted in the release of a significant volume of hydrocarbon gas from the production annulus over a time period of 3.5 hours, elevating the risk on the facility during the release. Although initiation of the surface shutdown also closed the shutdown valve fitted to the gas lift line, this did not isolate the flow of hydrocarbon gas from the production annulus as the failed instrument line had been located on the gas lift line between the wellhead production annulus and gas lift line shutdown valve.
The alert said there were a number of implications as a result of the failure, and noted that locating the fitting for the instrument tubing line between the production annulus wing and gas lift shutdown valve created the potential for an unrestricted pathway from the failed instrument tubing line to the full volume of the production annulus.
This configuration is contrary to international guidelines and standards, and this left the manually operated wing valve open to the production annulus to allow daily readings of the pressure in the production annulus.
This resulted in an unrestricted pathway from the production annulus to the failed instrument tubing line as the shutdown valve was not able to safely isolate the gas inventory of the production annulus.
The alert said there were a number of lessons as a result of the incident, and said the following points should be considered:
- The risk for an uncontrolled gas release of a significant volume of hydrocarbon gas from the production annulus of gas lift production wells requires that effective barrier controls are in place.
- Valve fittings, instrumentation and small bore tubing fitted to wells that may be subject to movement due to thermal change from production or wave motion have the potential to be damaged or suffer an integrity failure which may lead to an unrestricted pathway for release of hydrocarbon gas.
- Damage or loss of integrity to valve fittings, instrumentation and small bore tubing fitted to wells may compromise the integrity of the secondary barrier envelope and could lead to a situation of relying on a single barrier envelope to control the well reservoir.
- Maintaining well integrity with a two barrier philosophy is considered good industry practice.
- Gas lift line shutdown valves (SDV) should be fitted as close as practicable to the wellhead and the SDV should be activated with the same signal as the surface safety valve to effect isolation of the gas lift line and any small bore take-offs.
- Keeping abreast of relevant industry literature: the risks of significant gas lift gas release had been identified previously. Refer to publications referenced below.