Development of Empirical Method to Calculate Natural Gas Pipelines Rupture Exposure Radius

Abstract

Saudi Aramco gas pipeline location classification are designed with a similar approach to the American Society of Mechanical Engineers (ASME) B31.8, which segments the pipeline length and counts the population for each segment. For the segments width, ASME utilizes a fixed distance, i.e., 400 m, while Saudi Aramco uses the pipeline Rupture Exposure Radius (RER), a consequence modeling driven distance similar to ASME’s Potential Impact Radius (PIR). The design factors (i.e., wall thickness requirements) are selected based on the population density within the defined segments, while also affecting the number of segments and emergency isolation valves required along the pipeline. Previously, Saudi Aramco pipelines safety standards set two default RER values to be used in the pipeline design based on conservative estimates. Based on the pipeline diameter, the RER is set at 1,000m or 2,000m for less than 24″ pipeline and greater than or equal to 24″ in size, respectively. Saudi Aramco standard defined RER by modelling the downwind dispersion distance at ground level in case of a pipeline full bore rupture to the limit of ½ the lower flammable limit (LFL) of the released vapor cloud, which was shown to be smaller than the standardized values. As sweet gas pipeline systems are hugely expanding to accommodate the increase in domestic demand in the Kingdom of Saudi Arabia, an efficient method for calculating RER was developed and introduced to the standard. For future pipelines, lower RER distances resulted in more flexibility in route selection, lower pipeline location class, and hence thinner wall thicknesses, less emergency isolation valves required, and longer span between sectionalizing valves, which all translate to cost savings. Existing pipelines currently require less upgrades when encountering urban development in their route, have less number of High Consequence Areas (HCAs) and better repair prioritization. By statistically analyzing and modeling the Saudi Aramco gas pipeline network, this paper discusses the development of an empirical formula that is representative and less conservative for estimating pipelines flammable gas cloud dispersion ½ LFL. The resulted calculation method had been developed utilizing consequence modeling software, and is expressed as a simple formula as a function of the pipeline pressure and diameter. The established method is currently adopted by Saudi Aramco pipeline safety standards, and resulted in a reduction of 74% of the average pipelines RER, with a standard deviation of 4 meters from the consequence modeling results, and minor diversion in consequence distances when compared to international standards calculation methods such as ASME PIR.