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Risk Based Inspection for Offshore Pipeline

Risk based inspection (RBI) has become quite popular since the inspection budget can be used much more efficiently compared to conventional inspection procedures. The RBI normally starts with the determination of all relevant risk driving effects for the specific pipeline, the definition of acceptance
criteria and the assessment of the residual lifetime for the pipeline. These tasks require a close co-operation between the pipeline operator and the RBI-experts. This paper describes some of the major aspects of the methodology developed to estimate the risk of 60 offshore pipelines. Based on this risk assessment an optimised inspection strategy is developed. Each pipeline investigated is divided in specific segments taking into account the differences in essential parameters over the pipeline length, such as wall thickness, environmental loads, or corrosion condition. Although only basic data has been available this was sufficient to work out reliable results within a semi-quantitative risk approach. 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.

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.
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.

Modelling the Risk

The combination of probability of failure (PoF) and consequence of failure (CoF) yields the risk. Several qualitative and quantitative risk assessments exist. The risk can be represented in a matrix with the columns and rows as probability and consequence respectively. This matrix can be of any dimension, however often a square matrix as e.g. 5x5 is used (as shown in Figure 2). In Figure 2 the probability of failure and the consequence of failure are categorised from negligible to serious. Negligible (Neg.) probability of failure and negligible consequence correspond to the lowest risk.

Risk acceptance criteria are the limits above which the operator will not tolerate risk for the pipeline. The highest risk is presented by the red areas with an extreme consequence and a high or serious failure probability. Such areas are unacceptable and should be avoided. The orange areas present a high risk level and indicate that measures should be taken very soon. The yellow areas indicate a medium risk where action is required by a certain due date. The acceptable risk is presented by the green areas.

The risk increases with the operation time due to time-dependant failure modes. Time-dependant effects are e.g. growth of corrosion flaws or cyclic loads during the operation. After inspection, corrective measures can be considered, e.g.
• Reassessment of the flaws,
• Rechecking and replacement or strengthening of critical parts.
This may lead to a reduction of the risk predicted before the inspection. When the inspection scheduling is based on risk evaluation, the current risk matrix is required. This approach allows the focus on high risk areas for the inspection instead of time-based inspection of the entire system.

Failure Probability

The probability of failure can be estimated by different methods. The approach used most often is the evaluation of historical damage data. This approach can be used to identify the major threats and their contribution to pipeline failure. An evaluation of different data bases shows regional differences between the failure distributions. As the failure modes have time dependent as well as time independent aspects, they have to be considered both. The approach proposed here covers both effects in different ways:
• Index procedure,
• Remaining life time.

The index procedure describes the general condition of the pipeline and the remaining life time considers directly the time dependent effects. The total failure probability will be obtained by the highest failure probability of both assessments.

Index Procedure

 

Figure 3: Structure of Index Procedure

In Figure 3 it is shown the structure for estimating the failure probability when using the index procedure. Each index part (e.g. design, corrosion, operation) will be assigned a value between 0 and 100, where 100 is the best result and 0 the worst case. These values will be scaled with suitable weighting factors. The sum of all weighting factors is equal to one. Thus, the sum of the weighted index part values is also a result between 0 and 100 and can then be used for the assessment. Each index value is either derived by or by qualitative assessments. The design index is used to describe the influence of the design parameters to the condition of the pipeline. Design failure can cause serious damages and a conservative design leads to a reduction of the failure probability. As corrosion is an important failure mode the corrosion index is introduced. Corrosion can lead to a leak as well as to fracture of a pipeline. Third party damage occurs for offshore as well as for onshore pipelines. Regarding e.g. offshore pipelines an anchor impact or interaction with fishing equipment can lead to failure. These failure modes are assessed in the third party index. The operation conditions of the pipeline also influence the failure probability. If e.g. the pipeline is operated close or above the design pressure it will fail more likely than a pipeline operated at 50% of the design pressure. These types of dependencies are covered in the operation index. The combination of all four index values by considering weighting factors leads to the result of the index procedure.

 

Remaining Life Time Approach

The remaining life time approach is an alternative approach for estimating the probability of failure. The remaining life time of the pipeline can be calculated taking into account different issues.

Figure 4: Remaining life time approach

The remaining life time with respect to corrosion can be estimated based on e.g. measured flaw dimensions and growth rate. Also the design life should be considered. If the design life is reached a detailed assessment of the pipeline should be carried out before further operation. Fatigue due to operational loads and especially free spans are time dependent issues for pipeline failures and should therefore be considered.
 

Consequences of Failure Different types of consequences are regarded:
• (Human) safety,
• Environmental impact,
• Assets (economical consequence) and
• Reputation / political consequence.
The first three items are of similar importance for the total consequence. The consequence with respect to reputation or political aspects has to be determined and weighted in agreement with the operator, see Table 1.

Safety (of persons) is the most important issue. Here, the location, specifying the population density close to the pipeline, type of content, pressure and spill volume have to be considered. The environmental impact is mainly influenced by the medium transported, the potential release (spill) volume, and the location (protective areas like the Wadden Sea). The financial aspect is very client specific and therefore not included in this paper.
 

Pipeline Segmentation

As the risk is not constant along a pipeline route, it is beneficial to divide the pipeline in different segments. The idea is that every single pipeline segment has approximately a constant probability of failure and a constant consequence of failure. Figure 5 shows a pipeline divided into different parts, so called “TAGs”. The pipeline shown in Figure 5 starts at an offshore platform with a riser, which normally has a higher wall thickness than the pipeline. The consequence of a failure is also higher in the vicinity of the platform, because personnel could be directly affected. Along the pipeline route the probability and consequence of failure may change due to different wall thickness, environmental conditions, water depth, locations like crossings, etc. For each segment the PoF and CoF should be nearly constant and have to be assessed.

Figure 5:Pipeline divided into several TAGs

Risk Based Inspection and Maintenance

Based on the results of the risk approach suitable inspection / preventive maintenance frequencies have to be determined. They depend on the considered TAG and on the operator requirements; i.e. they have to be agreed and adapted with the operator. Different approaches can be used for the determination of the suitable maintenance frequency. One possibility is the determination of a so-called inspection frequency factor (IFF). Based on the obtained risk the inspection frequency factor can be determined. An example is given in Table 2. Note that the inspection frequency factor is also related to the risk color according to Figure 3. Based on the “normal“ inspection interval, provided by the operator, the new interval can be determined risk based, by considering the corresponding frequency factor from Table 2.

Source:

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