Today risk based inspection (RBI) has become very popular as it allows the inspection budget to be used more efficiently compared to conventional inspection procedures. The key parts of a risk based inspection programme for aged pipelines is the determination of all relevant risk driving effects for each specific pipeline, the definition of acceptance criteria and the assessment of the residual lifetime for the pipeline. These tasks require a high level of co-operation with the pipeline operator and the RBI-experts.
This paper describes some of the major aspects of the methodology developed to estimate the risk to offshore pipelines. Based on the assessment of the risk an optimised inspection strategy can be developed.
The complete RBI-process typically starts by gathering all relevant data used in design phase (including construction phase) and operation phase. In most cases a variety of risk assessment methods will be used, depending on the quality of existing pipeline data. When insufficient data is available a simple qualitative assessment with an index procedure can be used. With more detailed data available the method becomes a semi-quantitative approach.
This paper presents the results of a general risk assessment of 22 aged offshore pipelines. Although only basic data has been available this was sufficient to work out reliable results within a semi-quantitative approach. Depending on varying wall thickness, diameter, etc. a pipeline was divided into several different segments. The assessment reveals different governing threats and failure modes for pipelines and risers, like ageing, free span or corrosion. In a final step the results of the risk assessment are used for the optimisation of the inspection intervals within the proposed risk based inspection framework.