Subsea Manifold Risk Based Inspection
A manifold is an arrangement of piping and/or valves designed to combine, distribute, control, and often monitor fluid flow. Subsea manifolds are installed on the seabed within an array of wells to gather production fluids or to inject water or gas into wells.
In the subsea manifold RBI, the following degradation mechanisms are analyzed:
- Internal corrosion (IC);
- External corrosion (EC);
- Internal erosion (IE);
- External impact (EI).
CoF IdentificationThe CoF is divided into three categories with a qualitative ranking system:
- Safety consequences: considers personnel injury or PLL;
- Economic consequences: considered the repair costs and business loss due to interruptions in production;
- Environmental consequences: considers the impact of various types of production release to the environment and the cost of cleanup.
During the initial assessment, the risk limit is settled as high. A risk level that exceeds “high” risk is regarded as not acceptable.
The objective of the initial assessment is to identify the risk level for each degradation mechanism using a qualitative method.
The safety principles adopted are also risk principles, in which the target annual PoF is dependent on the consequences of failure. The safety classes for different manifolds are different, depending on the piping content (product) and the location. The safety classes include:
- Safety class–high;
- Safety class–normal;
- Safety class–low.
The PoF value for a detailed assessment is calculated at the quantitative level, using a probability statistic methodology. The best distribution that reflects actual conditions will be adopted as the basis of the theory. In the specific CoF calculation, the following items are essential:
- The distribution of component capacity;
- The distribution of component subjected loading.
Acceptable criteria for the detailed assessment:
- For safety class–high: target annual PoF is 10E-5;
- For safety class–normal: target annual PoF is 10E-4;
- For safety class–low: target annual PoF is 10E-3.
The objective of the detailed assessment is to reassess the degradation mechanisms that were classified as not acceptable during the initial assessment using a quantitative method. According to the frame flowchart described for an RBI, if the assessment result in the detailed assessment is acceptable (the annual PoF is below the criteria target), then a further inspection will be carried out; if the results are not acceptable (the annual PoF exceeds the criteria target), then an immediate inspection will be performed according to IRP and MRP.
Example for a Manifold RBI
The example that follows describes the specific work flow for a manifold RBI assessment. In this example, we focus only on internal corrosion. Internal corrosion on the manifold piping system was detected by pigging via the MFL (Magnetic Flux Leakage) inspection method, assuming that CO2 is the only reason for the internal defect in the carbon steel material. The defect depth and length are provided by a inspection report. As the defects grow, the capacity will decrease. The RBI methodology for a manifold will ensure that the system safety level is acceptable at any time. The analysis follows.
First the code B31G is used to calculate the piping capacity subjected to internal pressure only under a corrosion defect: Because the value of Pcorr is greater than two times the MAOP, the inspection finding in the inspection year is insignificant.
The zone type is zone 2 and the inspection method for the pigging is relative. In the detailed assessment, the defect failure pressure has to be recalculated using a more accurate method by taking the probability into consideration. To predict the defect growth tendency, the corrosion rate of the defect also needs to be assessed. Second, we check whether the failure probability in the current year 2009 is not acceptable as identified in the initial assessment. In the detailed assessment, the probability of failure will be obtained via the quantitative method in which a normal distribution loading and capacity distribution are used.
In RBI assessment, the inspection plan should be established based on both initial assessment and detailed assessment. However, the detailed assessment result is the major factor. So the RBI competence group should make an inspection schedule and perform the inspection in 2014 for the internal corrosion degradation mechanism.
 M. Humphreys, Subsea Reliability Study into Subsea Isolation System, HSE, London, United Kingdom, 1997.
 Det NorskeVeritas, OREDA Offshore Reliability Data Handbook, fourth ed., Det Norske Veritas Industri Norge as DNV Technica, Norway, 2002.
 Mott MacDonald Ltd, PARLOC 2001, The Update of the Loss of Containment Data for Offshore Pipelines, fifth ed., HSE, London, United Kingdom, 2003.
 Norwegian Technology Standards Institution, CO2 Corrosion Rate Calculation Model, NORSOK Standard No. M-506, (2005).
 M.H. Stein, A.A. Chitale, G. Asher, H. Vaziri, Y. Sun, J.R. Collbert, Integrated Sand and Erosion Alarming on NaKika, Deepwater Gulf of Mexico, SPE 95516, 2005, SPE Annual Technical Conference and Exhibition, Dallas, Texas, 2005.
 O.H. Bjornoy, C. Jahre-Nilsen, O. Eriksen, K. Mork, RBI Planning for Pipelines Description of Approach, OMAE2001/PIPE-4008, OMAE 2001, Rio de Janeiro, Brazil, 2001.
 American Society of Mechanical Engineers, Manual for Determining the Remaining Strength of Corroded Pipelines, ASME B31G-1991, New York, 1991.
 Det Norske Veritas, Corroded Pipelines, DNV-RP-F101, 2004.