Flammable Mists from Accidental Hydrocarbon Releases Offshore

Offshore oil and gas installations store and process very large inventories of different hydrocarbon mixtures. The crude oil/natural gas is pumped from the seabed and separated by distillation at different pressures and temperatures. Therefore the installations need to heat and pressurise the crude, which requires further non-process hydrocarbons for fuel and to act as lubrication and heat transfer fluids.

2014.04.09 - Flammable Mists from Accidental Hydrocarbon Releases Offshore

An accidental release of any of these fluids is hazardous because of their inherent flammability. Gaseous and two-phase releases will have at least some flash fraction that will disperse through the installation and if ignited can form either a flash-fire or, depending on the degree of congestion, a vapour cloud explosion. Higher flashpoint fluids, such as lubrication oils, may form pool fires or, if atomised, flash-fires or explosions. In addition, all hydrocarbons may flash back to the source and form a jet fire.

Any of these hazards can damage plant work around the release and lead to a domino effect. Because of the inherent financial restraints of operating offshore, the cost increases with the size of the installation and the distance between equipment is kept to a minimum, as is the distance between plant work and occupied buildings.

The nature of the offshore gas and oil industry requires the facilities to be geographically isolated and requires the workforce to remain within the vicinity of the plant work. For all these reasons, hydrocarbon releases are closely monitored by HSE OffShore Division who maintain a database of all reported releases since 1993 for the use of estimating failure rates and identifying trends.

UK HSE released recently a report that investigates offshore hydrocarbon releases in an attempt to identify whether mists of hydrocarbon droplets were formed, and if these presented a flammability hazard. Scientific literature has been searched for the present state of knowledge of the physics of two-phase mist generation. An empirical correlation has been used to model the reported hydrocarbon releases for a five year period and estimate whether flammable mists could have been generated.

The main finding of this report are summarized below:

  • The combustion behaviour of fuel mists have been summarised and the burning properties of droplets sizes less than 30 µm have been highlighted as particularly hazardous.
  • The combustion behaviour of fuel mists have been summarised and the burning properties of droplets sizes less than 30 µm have been highlighted as particularly hazardous.
  • Of ignited releases, 78 % of non-jet fires occurred as explosions or flash-fires rather than pool fires demonstrating that realistic release conditions favour significant airborne dispersion over rainout.
  • An analysis of ignited and un-ignited releases found that 45% (96% of liquid releases) occurred under conditions that could have formed droplets within the flammable size range.
  • Demonstration has been made that industry is aware of the current lack of knowledge of aerosol generation and steps are being taken to resolve this but the focus remains on single component, superheated gases rather than multi-component process fluids and high flashpoint hydrocarbons.

Source: UK HSE

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