There is an increasing demand for operators of transmission pipelines to maximise the throughput of their pipeline systems. This can be achieved relatively easily with new pipelines, by using limit state design, new materials, novel fabrication techniques etc., Operators are also looking to maximise the throughput of existing transmission pipelines. Obviously, they are not able to make use of new materials etc., but limit state design concepts can be applied to show that a pipeline can be safely uprated to a higher design pressure.

The limit state of an engineering structure or component is defined as the mathematical relationship between the parameters associated with a particular failure mode at the onset of failure. To assess the impact on pipeline integrity and safety, a limit state approach incorporating probabilistic analysis has been developed. The approach addresses all credible failure modes and takes account of uncertainties in the relevant parameters for each mode.

The probabilistic approach takes the limit state approach a step further by describing the parameters as statistical distributions rather than single values. This allows failure probabilities to be computed which are a more meaningful measure of safety and allow areas of over conservatism (or under conservatism) to be identified. It is important to note that the approach is only as good as the limit states used and the data used to construct the distributions. Clearly uncertainties in both of these can exist and the absolute values of the computed probabilities must be viewed with caution. However, the strength of the approach lies in the relative values of the computed probabilities and previous ‘safe’ operation. If a system has a significant operational history with few or no failures and it can be shown that there is little change in the theoretical failure probability associated with a change in operating conditions, it can be inferred that few or no failures will occur in practice.

The paper describes the above approach in detail and outlines a study carried out to determine the effect on pipeline integrity of uprating three pipelines from a current maximum operating pressure of 70 bar to an uprated pressure of 85 bar, exceeding the current design criteria. By application of the limit state approach incorporating probabilistic analysis, it is shown that there would be an insignificant change in failure probability as a result of uprating to 85 bar, and hence that the integrity of the pipelines is unimpaired by uprating.

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