(53k) Integrated Quantitative Risk Assessment for Natural Gas Transmission Networks

This study objective is the development of an integrated quantitative risk assessment methodology in gas transmission systems, specifically the calculation of death probability per year of an unprotected person due to an accident on natural gas transmission pipelines or in compression/metering/monitoring stations. Every component of the equipment was examined for full bore and fracture rupture events that may end up as Fire Ball and/or Jet Fire accidents. Flash Fire accident was not considered at the calculations. According to incidence data, the probability of a Flash Fire occurrence is limited; thus, in its place a Jet Fire accident was considered as a worst-case scenario. This choice was also corroborated by a parametric study done using DNV Safeti v.8.23; the study concluded that jet fire has more dire consequences than a flash fire. Another key assumption in our analysis was the fact that in case of a gas release the respective rate follows a rather sharp decrease in short period of time after the failure onset. In addition, during the failure occurrence both parts of the pipe contribute to results.

Failure frequency calculation for buried pipes was based on EGIG (European Gas Pipeline Incident Data Group) statistical data and a specific methodology was developed. EGIG data take into account events from the European gas distribution networks. Above ground equipment frequencies came from IOGP (International Association of Oil & Gas Producers) tables; the latter present data coming from accidents in hydrocarbon extraction platforms in the Northern Sea (HSE, UK).

Ignition probability also plays a key role on the final illustration of the results, on account of the buried pipe ignition probability calculation that follows the IGEM approach. This approach considers a delayed ignition probability at 50% and it is coined on the basis of statistical data. For the above ground equipment the approach from Bevi Risk Assessment (RIVM, Netherlands) was used. In any case, above ground pipe accidents were considered with 100% ignition probability. Alternatively, immediate ignition probability was also examined using the CCPS methodology (Center for Chemical Process Safety, USA), in which the event conditions (pressure, hole diameter etc.) were included. If a systematic policy minimizing the ignition surfaces (Ignition Control Philosophy) were applied, the HSE (Health and Safety Executive, UK) approach would be adopted. According to this approach, the ignition probability, which has been calculated earlier, could be sharply reduced.

Death probability per accident was calculated using the dose – response function (Probit function). Dose calculation was based estimating and accounting for the thermal radiation emitted during the incident and the exposure time. Finally, the methodology presented herein has been applied on a pilot basis in compressing stations included the respective transmission pipelines. LSR (Location Specific Risk) isometric curves, which express the risk ran by a person with permanent presence at the point, are illustrated in the plot plan of the station. Furthermore, Individual Risk computation, which is based on LSR, takes into account the exposure time attributed to each member of the gas station personnel. Last step was the evaluation of the Aggregate or Societal Risk (number of mortalities per year in a specific region) based on RIVM and HSE guidelines.