Offshore gas dispersion assessments

Critical analysis to ensure safe operations


A jack-up drilling rig operating over a fixed platform was planning to carry out a well test as part of a well programme. The customer needed to know if there was a danger from hydrocarbon gas being vented from the well test package during the operation, or in an emergency situation. 

DNV GL solution

DNV GL’s validated consequence modelling was used to model the gas dispersion scenarios. A range of weather conditions were considered from a light breeze to the highest wind speeds in which the rig could operate. The maximum extent of lower flammability limit (LFL) and 50% LFL gas concentrations were modelled to determine the maximum potential extent of flammable gas and the potential for gas detectors on the fixed platform to be triggered, or for ignition to occur.

Outcome and benefits

Incorrectly planned gas venting could cause a fire or explosion resulting in injury, loss of life or damage to the asset and loss of production. The dispersion assessment resulted in minor, but critical design changes to avoid this situation. It also increased awareness of the consequences of gas dispersion scenarios for the well test package operator, the rig owner and the platform operator.