NITROGEN CYLINDER RUPTURED

 SOURCE: https://s3.eu-west-1.amazonaws.com/resources.stepchangeinsafety.net/downloads/SF-18-24.pdf

Nitrogen cylinder ruptured
What happened
A high pressure Nitrogen cylinder in a quad of 12 cylinders located on a drill rig floor ruptured without warning. The cylinders were charged to 2,400psi and were not connected to anything else at the time of the incident. The rack was destroyed and the other cylinders in the quad were propelled by the blast up to 15m away.

Why did it happen
The base of the cylinder that ruptured was heavily corroded. Other cylinders in the quad had evidence of similar but less severe corrosion.
The cylinders were 9 years into their 10-year hydrotest cycle, and had been visually inspected by the supplier in accordance with applicable industry guidance, when they were refilled some three months before the incident, before being loaded out to the rig. However, the severe corrosion that caused the rupture was not spotted during this inspection;
The cylinder that ruptured was located in the middle of the quad and therefore not easy to properly inspect without disassembly of the quad.
Calculations showed that the fatal blast zone for a single cylinder of this size pressurised to 2,400psi would be around 1.5 metres. The severity of the blast caused the rack to be destroyed and the other cylinders to be propelled across the deck.
Lessons learned
• Are the cylinders on your facility or vessel certified and in good condition?
• Are you able to see the condition of all cylinders in every rack?
• Are all cylinders located on a free draining base to minimise corrosion?
• Do you know what level of visual inspection of cylinders/quads is provided by your supplier?
• Are all cylinders on your facility stored and located in accordance with the relevant industry regulations?

LUBE OIL TANK FLAMMABLE ATMOSPHERE PROBABLY IGNITED BY STATIC DISCHARGE

 

Description of Incident

During start-up of the ancillary lube oil system, an overpressure event occurred, causing lube oil tank deformation and a minor loss of lube oil to the deck. There was no activation of the fixed fire & gas systems and no further escalation potential. Following a detailed investigation, it was determined that the most likely cause of the overpressure was a combustion event in the lube oil tank.

 SOURCE: https://www.stepchangeinsafety.net/alerts-moments/high-value-learning-lube-oil-tank-damage-due-to-overpressure/

The compression train has wet seals with ancillary seal and lube oil systems. Lube oil is circulated throughout the compression train and stored in an atmospheric lube oil reservoir tank. The storage tank is located on the deck below the compressors, with a pressure rating of 0.04 bar. The seal and lube oil systems are connected via a common atmospheric vent line. At the time of the incident, the seal oil system was not running.

Air was present in the lube oil tank as it would have been drawn in through the atmospheric vent when the lube oil pumps were started. A flammable atmosphere was created through low levels of hydrocarbon gas migrating into the lube oil tank and/or the formation of flammable mist of lube oil. The most likely ignition source was found to be static discharge from the lube oil.

There have been other static related incidents in industry, most commonly in the marine sector. Static charges can build up in some mineral oils due to turbulent fluid flow. Various studies have shown that, even with earthing / bonding of equipment, static discharge can still occur.

Good Practice Guidance

• Consider looking at your operations to see if you have risk of static accumulation in your seal / lube oil.
• Consider using anti-static additives which can help prevent static discharge.
• Consider looking at your system to see if you can reduce the risk of static accumulation through equipment like anti-static filters.
• Consider holistic risk assessments during plant reinstatement, including all pressure sources and interconnections between systems.
• Consider inerting lube oil / seal oil tanks prior to start up.

Human factors during hydraulic torqueing of bolted joints

 SOURCE: https://www.stepchangeinsafety.net/alerts-moments/high-value-learning-hydraulic-over-torqueing-of-bolted-joint-equipment/

 Description of Incident

Two incidents have occurred during the hydraulic torqueing of bolted joints on assets operated by two different operators in the UKCS. In one instance, the event caused significant damage to the bolted joint; in the other event, the over torqueing value was identified before serious damage could occur.

In both instances the hydraulic torqueing equipment was wrongly set to a torque level higher than the bolted joint design value.

Common issues in both incidents include:

• The torqueing value was entered incorrectly (~1000psi higher than design)
• Both gauge faces involved contained both Bar and PSI scales
• Both torqueing sets were in a position where reading the gauge could be problematic due to the low angle
• No verification of equipment settings was carried out prior to torqueing equipment being operated

Good Practice Guidance

• Consider whether the gauge on the equipment you use is required to carry both Bar and PSI. Could it cause confusion?
• Consider whether your processes are sufficiently robust to prevent a similar incident occurring
• If you contract out this activity, consider how you assure yourself on the competence of the operators provided