Onboard offshore platforms the wear imposed on pipes used for the transport of gases can be severe. The transport pipes potentially contain highly flammable and explosive hydrocarbon gases and hence the dangers that can be created by a leaking joint are serious. In these offshore installations, to minimise the danger created by a hydrocarbon gas leak, companies and government agencies proposing to commission oil or gas platforms often complete a safety study. This study includesd etailso f safetyp roceduresa nd the potentiale ffectsc reatedb y leaks. At present the methods used are based on the assumption that the leaks are all from more well known circular cross-sectional jets. This project was initiated to study leaking flange joints which are a typical source for gas leaks. Many leak geometries and a range of pipeline pressures up to a maximum of 410kPa were studied to consider their effect on the resulting gas leak jet. Laser light sheet imaging was used to obtain flow visualisation images, with total pressure probe measurements providing valuable time-averaged data. This alloweda two-dimensionasl liceo f the emergingle akj et to be studiedi n detail with two very differenta nalyticatl echniques. A steady core was observed at the point of exit of each gas jet studied. The core was shown to be a region in which the shear layers, initiated at the jet boundaries, grow towards a merging point on the jet centreline. At this location, the velocity decay rate was found to increase as the jet entered into a second flow regionWithin this region, the rate of velocity decay was shown to be similar to that typically observed in a planar jet. This is much slower than would develop in a roundj et. At the downstreamli mit of the experimentaml easurementsa, t hird flow region was observed. The centreline velocity within this region showed no tendencyt o a lineard ecayr ate as would be foundi n a round cross-sectionje t. Significant differences were noted between the spreading angle of a round jet and those included in the present study. A wide range of spreading angles were observed, which suggests that it is unrealistic for all flange joint leaks to be represented by the round jet assumption. More specifically, because of the effect of spreading angle, the round jet assumption is likely to show discrepancies in predicting the magnitude of important parameters, such as the gas concentration levelsa nde ntrainmenrta tes. As safetys tudiesa re at presentb asedo n circularc ross-sectiojne t data, the findings of this report indicate that the validity of those safety studies is probably being compromised