This incident followed the activation (opening) of a rupture disc in the system after an operational disruption which required production flaring. Parts of the disc were drawn into the flare line, causing an external gas leak from the disc holder.
With the gas leak rate assessed to have been greater than 0.1 kilograms per second, the PSA decided on 2 April 2019 to investigate the incident.
The flare system is intended to ensure safe handling of hydrocarbons in the process plant should an event occur which calls for depressurisation and emergency shutdown. This reduces the risk for and probability of escalation. Incidents which weaken or disable the flare system are therefore especially concerning.
Aasta Hansteen’s facility is designed with a closed high-pressure flare system. When flaring is required, a quick-opening valve operates so that gas can flow from the flare tip and burn. A 30-inch rupture disc is installed in parallel with the flare valve to ensure free passage for the gas if the valve fails to open as it should.
Actual and potential consequences
The incident led to a gas leak with an overall rate of more than 0.1 kg/s from two points in the flare line downstream from the high-pressure flare tank.
Production from Aasta Hansteen was shut down for 10.5 days as a result of the incident.
Equinor’s calculations show that the leak rate would have been up to 3.6 kg/s if the whole rupture disc had entered the flare line and the leak had only been limited by the gap between the flanges previously holding in the disc.
If all or part of the rupture disc had followed the gas flow into the line, it could have caused damage to or – in the worst case – a hole in the flare line downstream from the disc.
Direct and underlying causes
The direct cause of the gas leak was that the rupture disc burst in such a way that its edge was drawn into the flare line in two places. That led in turn to the loss of the seal at these points, permitting a gas leak.
The investigation has determined that a combination of the following conditions could be underlying causes of the incident:
- misaligned installation could have caused variations in clamping force around the disc’s circumference
- a pre-assembled rupture disc was chosen, although the manufacturer recommended a pre-torqued version
- the rupture disc holder is installed in a horizontal pipe, although vertical installation is recommended for large dimensions with this type of disc
- the big dimensions made it very demanding to install the disc and its holder in the line
- the project organisation failed to pay sufficient attention to the challenges of getting the disc leak-free.
The investigation has identified nonconformities related to the following conditions:
- requirements for the flare valve
- installation of the rupture disc with holder.
What happens now?
The PSA has requested that Equinor explains by 4 May 2020 how the nonconformities will be dealt with.