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THE 28 TH CONFERENCE OF THE FEDERATION OF ENGINEERING ORGANIZATIONS CAFEO 28 HANOI VIETNAM, 30 TH NOV. - 2 ND DEC., 2010 TITLE: Offshore Pipeline RBI Methodology M Hisham Noraswad AUTHOR(S)’ NAME(S): Mohd Hisham Abu Bakar, Pipeline Engineer Noraswad B M Daud, Pipeline Engineer ORGANIZATION & DESIGNATION: PETRONAS Group Technical Solutions ADDRESS: Pipeline Department, Level 7, Menara Dayabumi, 50050 Kuala Lumpur, MALAYSIA TEL: 0196618070 FAX: 0327836061 EMAIL: mhisham@petronas.com.my noraswad@petronas.com.my Requirements: 1. The report is obliged to have above information 2. The report should not be longer than 10 pages (A4 size); if longer then an executive summary of the paper is required. 3. Font: Times New Roman, Single line, 4. Font size: 12 Executive Summary This presentation outlines the basic philosophy underpinning the methods of evaluating the probability and consequence of failure, making assessment of the risk level, and concluding on the appropriate actions that need to be taken to manage the identified risk for offshore pipeline system. Risk-Based Inspection (RBI) is a means to design and optimize an inspection scheme based on the performance of a risk assessment progress using historical database, analytical methods and experience & engineering judgment. The objective of RBI is to aid the development of optimized inspection, monitoring and testing plans for meeting specified system acceptance criteria. In RBI analysis process, ‘high risk’ areas and major failure modes are identified and analyzed. Thus, it will lead us to target inspection and maintenance resources at these areas of the structure / system where they can have the greatest effect in reducing risk, the occurrence likelihood and consequences of unplanned failures, and to reduce the cost of unproductive inspections. Among the reasons for selecting a risk based approach to inspection planning are: to focus inspection effort upon items where the safety or economic risks are identified as being high, to correspondingly reduce the inspection effort applied to identified low risk systems, to ensure that the overall pipeline system risk acceptance criteria, as set by the operator, are not exceeded, to achieve an optimal schedule for the inspection activities, to identify and apply the optimal inspection or monitoring methods according to the identified degradation mechanisms. Introduction Detailed Assessment in Risk Based Inspection (RBI) is a method used to effectively manage risk by focusing inspection on high risk items. It optimizes the inspection efforts by balancing inspection costs with inspection benefits and the risks present. The damage causes that will be investigated include metal loss from corrosion i.e. internal corrosion, external corrosion, erosion (if detected), on-bottom stability, external impact & freespan. The Risk is usually defined as a product of Probability of Failure (POF) times the Consequence of Failure (COF). Risk = POF X COF Pipeline failure may result from various causes and its probability of failure (POF) may be determined from knowledge of degradation rates. In the PCSB RBI methodology, six damage causes are considered. The six damage causes are further categorized into two: time-based and event-based. Time-based damage causes are those whose severity increases with time e.g. internal corrosion, external corrosion and erosion. Event-based damage causes are those whose probability of occurrence is relatively constant over time e.g. dropped object. The modeling of POF and COF is explained in the following sections. Risk Modeling To simplify the calculation of probability of failure (POF), we have considered only the causes which have significant impact on the pipelines. Although complete statistical data is unavailable, pipeline operator operational experience indicates that most pipeline failures (leak or burst) in pipeline operator region are attributed to metal loss corrosion. This is time-based failure mode which includes both internal and external corrosion. The least common damage cause is failure due to erosion. It is noted that other degradation mechanisms, whose degradation rate may not be amenable to mathematical modeling, can also cause pipeline failure. The probability of failure (POF) category for event-based failure mode e.g. freespan, on bottom stability and external impact will be based on the PCSB RBI manual. Consequence of failure (COF) may be divided into safety, environment and economic consequences. However, in the absence of rigorous consequence modeling such as a quantitative risk analysis (QRA), the DNV approach of safety class is adopted. According to DNV-OS-F101, the acceptable probability of failure (POF) for ultimate limit state (a condition in which certain limits are exceeded and failure can occur) is related to safety classes as given below: Table 1: Safety class and target annual failure probability for Ultimate Limit State (ULS) Safety Classes Indicating a target annual failure probability for probability of: High < 10-5 Normal < 10-4 Low < 10-3 The safety classes are dependent on category of fluid transported and location of pipeline as presented in the following table and detailed in DNV-OS-F101. Table 2: Safety class dependent on Product, Manning and Location Class Product Unmanned Installation Manned Installation All Location Classes Location Class 1 Location Class 2 Flammable Product Normal Normal High Water & Non- flammable Product Low Low Normal Location classes are categorized as follows: Table 3: Typical location classes Location Class Definition 1 The area where no frequent human activity is anticipated along the pipeline route (> 500 m from platform or shore) 2 The area where frequent human activity is anticipated along the pipeline route (<500 m from platform or shore) Using this approach a defect on a water injection pipeline located on a riser would have an annual POF limit = 10-4 because its location class = 2 (for riser) and safety class = Normal (for water in location class 2). Similarly, a defect on a hydrocarbon gas pipeline, or a crude pipeline on the onshore portion of the pipeline would have annual POF limit = 10-5 because its location class = 2 (for onshore) and safety class = High (for flammable product in location class 2). Both POF and COF, hence risk, are time-dependent. The POF is a function of many parameters among which are pipeline geometry and defect geometry. Defect geometry is in turn a function of corrosion rate as modeled in the following equation: ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ Δ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = t T CR t d t d initial defect depth growth equation From the above equation, defect depth (d/t) will grow over time (∆T) from an initial defect size due to corrosion rate (i.e. CR > 0 mm/yr). The defect depth growth equation contains is modeled as a statistical distribution with a specified coefficient of variation. The use of statistical distribution for CR result in (d/t) that follows the distribution as depicted in the following diagram. The distribution for (d/t) moves to the right as T increases (i.e. T given in number of years elapsed as time progresses). Pipeline operated under internal pressure has an allowable defect depth (d/t)max and length L above which failure of the pipeline is imminent. The shaded area in the diagram indicates the probability of which the pipeline is expected to fail because the defect depth has exceeded the allowable defect depth for a given defect length. The area under the curve to the right of (d/t)max gives POF. This is the region calculated in RBI software as POF. Initially, say at T=1 when defect size is small, the POF is also small. Over time, as the defect grows, its POF increases. The next inspection date is reached when POF has reached 10-4 or 10-5 as appropriate according to the safety class for the defect. Figure 1: Illustration of predicted future deterioration Risk Calculation Using this methodology risk is not explicitly calculated. For simplicity, consequence of failure (COF) is assumed constant and fixed through the safety class approach and risk is increased by the probability of failure (POF) component. The limit for setting the next inspection date is therefore set by the POF. Inspection is recommended when the POF has reached a specified limit. d/t T=1 T=3 T=5 d/t max Where d/t > d/t max , POF d/t T=1 T=3 T=5 d/t max Where d/t > d/t max , POF