Question: Why should the high consequence area extend from the beginning of the first circle to the end of the last circle containing an identified site? For identified sites close to the pipeline, this creates HCAs that appear unreasonably long.
Answer: Studies of the effect of pipeline ruptures have shown that the area affected is typically elliptical in shape, with the long axis of the ellipse parallel to the pipeline. This is likely caused by the effect of escaping gas jetting along the pipeline right-of-way. The HCA definition extends from the beginning of the first circle to the end of the last (rather than from center-to-center) to account for this effect.