To assess ageing pipeline networks an ongoing development from prescriptive standards towards probabilistic or risk-based assessment methods is observed. Pipeline operators are increasingly requiring risk assessment of pipelines in operation. Risk-based methods allow the identification of parts or system parameters critical for the integrity (safety, environment and reliability) and may have significant advantages compared with the traditional deterministic approaches.
The selection of an appropriate risk-based method can be classified from qualitative to quantitative methods. Well-known mechanical systems can be properly described and a calculation of the system reliability with probabilistic tools complemented by a determination of the consequences may be adequate for risk evaluation. However, for other cases the known uncertainties allow only a more rough investigation. Quantitative analysis requires more effort than qualitative, but provides more accurate results. Under consideration of economical boundary conditions generally a quantitative risk analysis must be focused on important subsystems. Thus, methods are required which allow an economic identification of the subsystems or parameters relevant for the risk of the whole system. Semi-quantitative methods are adequate for such analysis.
Using a step-wise semi-quantitative method the risk driving parameters and the quantification of the risk for a pipeline network can be calculated. The experiences with this method are explained as follows.