Risk Based Inspection for Offshore Pipeline (English Ver.)

In the pipeline industry much effort has been taken to ensure safety. Therefore, in-depth research has been carried out with respect to allowable failure probabilities. Since also the consequences of failure play a more important role, risk based approaches are becoming more common. They can be used during design as well as during operation. The focus is here on the operational phase regarding risk based inspection. 

The implementation of a risk based inspection (RBI) procedure starts with the determination of the relevant failure modes that should be regarded (Figure 1). 

After identification of the relevant failure modes, the corresponding probability and consequence have to be estimated. The probability of failure can be estimated by using different methods, such as: 

• Qualitative methods
• Semi-quantitative methods
• Quantitative methods

Qualitative methods are based on few essential data and lead to a rough estimation of the failure probability. Semi-quantitative methods use more information and some calculations are carried out, which results into a more accurate failure probability. The quantitative methods consider fully probabilistic approaches and lead to an accurate determination of the existing failure probability. However, in engineering praxis the data required for the fully quantitative approach is typically not available. 

Therefore, in the following the semi-quantitative approach is used. This approach gives a more detailed failure probability than the pure qualitative approach and is normally applicable. Details of the complete risk assessment, i.e. by also taking into account the consequences, are given below. 

As the risk is not constant along the pipeline route, a segmentation of the pipeline is carried out. After estimating the risk related to each segment, an appropriate inspection strategy has to be developed. The inspection effort and interval should be determined taking into account the current and the future risk of the segment regarded.

The combination of remaining life time and index procedure is able to cover all relevant failure modes. When only using the remaining life time approach threats like impact damage, which is the reason for 30% of all pipeline damages, are not covered. The proposed procedure fits well within the framework suggested in current codes like DNV RP F116 [4] and has been successfully applied to different offshore pipelines.

Source:

• http://www.pm-pipeliner.safan.com/mag/pploctdec13/t30.pdf