In order to reduce the risks associated with the occurrence of free spans, offshore pipelines are subject to external survey on a periodic basis. The true impact of this strategy on safety and reliability is generally unknown.

This paper describes a structural reliability-based assessment that was carried out to investigate the probability of failure of an offshore pipeline due to the presence of free spans and develop a survey strategy that ensures a quantified and acceptable level of reliability.

The probabilities of failure of free spans of varying lengths are evaluated using a structural reliability based approach. Then, the probability of occurrence of free spans as a function of time is evaluated in order to quantify the overall time-dependent reliability of the pipeline. Finally, the influence of the survey frequency is investigated. The outcome of the study is a recommended strategy for future surveys of the pipeline.

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