Oil mist hazards on dual fuel diesel engines

Offshore Information Sheet No. 8/2009

(Issued November 2009)



This information sheet brings attention to the oil mist hazards on dual fuel diesel engines that require high pressure seal oil to operate on process gas.  Oil mists are hazardous particularly in the vicinity of hot surfaces as oil has a relatively low auto ignition temperature (AIT) of 230oC.1


An incident occurred on a FPSO that resulted in the generation of a large oil mist from the diesel engine in the engine room. The engine room is classed as a non hazardous space; hence electrical and other equipment will present a potential ignition source to hydrocarbon vapour. The oil mist surrounded the engine and was detected by a person visiting the engine room; it was not detected by the fixed detection systems, or by the engine protection systems.

The diesel engine is supplied by 350bar process gas, through a pipe in pipe arrangement, fitted with gas detection. The design of the gas supply arrangement is supported by suitable safety studies.

To control the injection of the HP gas a seal oil system is used. This is supplied at a higher pressure than the gas, 370bar. The seal oil supply system consists of a hydraulic pump unit that is supplied with oil from the diesel engine sump. The pump discharges to the engine at 370bar via standard pipe work and flexible hose connection to the engine pipework. The pump unit and pipe work are adjacent to the engine.

The oil mist was generated through a pinhole in the seal oil pipe, caused by wear due to pipe to pipe contact as a result of failed pipe supports. Oil ejected through a small hole at these high pressures generates a mist with a high vapour component that has the potential to generate an explosion with a very high overpressure. The combination of high pressure, large reservoir of oil (sump) and hot surfaces (ignition source) has serious consequences as a very large mist/vapour cloud could have been formed and ignited.  

During the investigation into this incident, it was apparent that no safety studies had been carried out that demonstrate the risks associated with operation of the 370bar seal oil system had been reduced to as low as reasonably practicable (ALARP).


Duty holders should ensure that they have carried out a suitable assessment of risks associated with operating the seal oil system prior to operating, or continuing to operate, the dual fuel engines on gas. The risk assessments should include a survey of the effectiveness of the detection system and its sensitivity to oil mist detection. Serious consideration should also be given to installing dedicated oil mist detectors.


1. HSE Offshore Information Sheet No. 10/2008 Fire and explosion hazards in offshore gas turbines . Back

This information sheet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.

Updated 2020-11-13