RISK BASED INSPECTION (RBI)

Transcription

1 1 RISK BASED INSPECTION (RBI) Roy Johnsen Professor, Institutt for produktutvikling og materialer (IPM), NTNU Tlf:

2 2 CONTENT Introduction - Corrosion Management What is Risk Based Inspection (RBI)? How to perform a RBI analysis? What importance has the probability of detection (POD) of a crack/initiation point?

3 3 WHAT ARE YOU INSPECTING FOR? No need for any NDT method to observe this failure but the problem (wall thinning due to corrosion) should have been found before the crack started to grow!

4 4 WHAT ARE YOU INSPECTING FOR? Internal corrosion CS with CO 2 Inspection needed? NDT leak control? NDT important but which method to use?

5 5 PREFERENTIAL WELD CORROSION Normally not included in the inspection program Base metal Weld metal Heat affected zone

6 6 WHAT ARE YOU INSPECTING FOR? Internal corrosion in CS with CO 2 More typical pitting/partly uniform corrosion

7 7 CORROSION UNDER INSULATION Where to inspect and which NDT method to be used?

8 8 INSTRUMENT TUBING IN AISI 316 Marine environment NDT method Visual Inspection

9 9 STAINLESS STEEL Where and how to inspect? Pitting corrosion in AISI 316 Chloride stress corrosion cracking

10 10 DIFFERENT TYPE OF FAILURES On Statoil operated installations WEAR 5% HYDROGEN EMBRITTLEMENT CREEP 3% INSTALLATION 2% LIQUID METAL EMBRITTLEMENT CORROSION 31% FABRICATION BRITTLE FRACTURE FATIGUE 31%

11 11 THE OPERATORS NEED A SYSTEM TO KEEP THE SAFETY ABOVE A DEFINED RISK LEVEL CORROSION MANAGEMENT

12 12 Planning CORROSION MANAGEMENT What is included? Risk Based Inspection (RBI) Allocation of resources Condition Monitoring Corrosion Erosion/sand Vibration/fatigue Process monitoring Chemical treatment Chemicals added Analysis Inspection Field inspection Evaluation of results

13 13 CORROSION MANAGEMENT (CM) A CONTINOUS PROCESS A: DEVELOP THE CM PLAN 1. Data/information collection 2. Risk screening 3. Detailed risk analysis 4. Develop the integrity plan PLAN B: IMPLEMENT THE PLAN 1. Prepare list of actions 2. Implement list of actions TO DO D: REVIEW THE PLAN 1. Review the integrity plan 2. Propose improvements 3. Implement improvements IMPROVE C: ASSESS THE PLAN 1. Evaluate available data 2. Compare with performance indicators MEASURE

14 14 ELEMENTS IN RBI FOLLOWED BY INSPECTION 1. Preparation and acceptance of the procedure for RBI/Inspection planning 2. Collection and systemization of documentation 3. RBI analysis Probability of Failure (PoF) Consequence of Failure (CoF) Risk level Definition of Inspection interval 4. Inspection planning Specification of number of inspection points and area to be inspected Definition of inspection method When to inspect Preparation of inspection drawings 5. Field inspection Reporting Evaluation of findings 6. Revision/Updating of inspection program based on input from Inspection Process monitoring Corrosion/erosion monitoring

15 15 RBI STANDARDS/SPECIFICATIONS Available documents: DnV RP G 101 Recommended practice for Risk Based Inspection of Topside Static Mechanical Equipment API 580/581 Risk Based Inspection Internal Company specifications

16 16 OBJECTIVE The objective of this Recommended Practice is to describe a method for establishing and maintaining a risk-based inspection (RBI) plan for offshore pressure systems. It provides guidelines and recommendations which can be used to customize methods & working procedures that support the inspection planning process.

17 17 RISK DEFINITION RISK = PoF x CoF Where: PoF = Probability of Failure CoF = Consequence of Failure Risk normally presented as a value or in a matrix format

18 18 RISK SCREENING PROCESS Screening process to be done by a team comprising of personnel having competence in the following areas: inspectors with experience from the installation materials/corrosion engineering safety/consequence analysis plant operation/process knowledge maintenance

19 19 RISK SCREENING PROCESS MAIN QUESTIONS: Is it probably that a leak will occur during the lifetime of the installation? Will a leak cause unwanted consequences? Probability of failure (PoF): Negligible A leak is not assumed under normal operation Consequence of failure (CoF): Acceptable A leak is not ignitable or is low pressure and non-toxic (Safety/personnel) A leak will only result in small non-toxic pollution (Environment) A leak will not result in downtime or loss of production (Economy)

20 20 RISC SCREENING MATRIX

21 21 PROBABILITY (PoF) EXAMPLE - SCREENING RESULTS VERY HIGH HIGH MEDIUM LOW 1. Drill process (11) 2. Wellhead drill (12) 3. Wellhead production (13) 4. Separation/stabilization (20) 5. Separation/stabilization injection (20) 6. Crude handling (21) 7. Gas treatment (24) > 10-5 NEGLIGIBLE < Cooling/refrigerator (40) 2. Seawater (50) 3. Fresh water (53) 4. Open drain (56) 5. Hydraulic power (65) 6. Sewage treatment (66) 7. Lube Oil system (66) 8. Fire water (71). 1. Gas pipeline compression (27) 2. Chemical injection (42) 3. Flare/vent (43) 4. Closed drain (57) 5. Diesel system (62) 6. Compressed air (63) 7. Inert purge (64) ACCEPTABLE NON-ACCEPTABLE CONSEQUENCE (CoF)

22 22 DETAILED RBI Systems selected during the screening process Risk matrix dimension has to be selected 3 x 3? (PoF x CoF) 5 x 5? System to be divided into Corrosion loops (CL) Part of system with similar corrosion conditions Contains pipes, vessels, heater/coolers, Design data for each CL has to be evaluated Type(s) of corrosion for each CL has to be defined Corrosion rate to be defined for carbon steel for each CL

23 23 RISK MATRIX Detailed Analysis A tyical 3x3 risk matrix

24 24 TYPES OF CORROSION From NACE Uniform corrosion CO 2 /H 2 S corrosion Galvanic corrosion Pitting/Crevice corrosion Bacterial corrosion Erosion corrosion SCC (Chloride Stress Corrosion Cracking) SSCC (Sulphide Stress Corrosion Cracking)

25 25 CORROSION ALLOWANCE Used for carbon steel Additional wall thickness to include expected corrosion during the design life Design life: 20 years Corr.rate: 0.15 mm/y Corr.allow.: 20x0.15 mm = 3 mm

26 26 Probility of Failure - PoF Carbon steel The corrosion allowance is used as basis it is assumed that the integrity of the pipe is used when the corrosion allowance is consumed. Stainless steels Actual operation conditions (environment, alloy, process parameters define the PoF value since stainless steel pipes do not have any corrosion allowance).

27 27 PROBABILITY OF FAILURE Examples of PoF For corrosion in carbon steel For Corrosion Under Insulation

28 28 CONSEQUENCE OF FAILURE CoF

29 29 RISK MATRIX Risk is transferred to Time-to-next-inspection

30 30 INSPECTION INTERVAL Example for carbon steel

31 31 DETAILED INSPECTION PLAN For Carbon Steel components: Time to next inspection / inspection interval Which inspection method to be used Where to inspect Area to be inspected Dependant on expected type of failure

32 32 WHAT ABOUT OTHER MATERIALS THAN CARBON STEELS?

33 33 PoF Example for noble alloys in seawater

34 34 PoF FROM DNV-RP-G101

35 35 PROCESS MONITORING PROGRAM Parameters to monitor: Temperature Velocity Water content Chloride content Residual chlorine Oxygen level CO 2 /H 2 S content Amount of sand particles Operation window for stainless steel alloy exposed to chloride containing environment

36 36 INSPECTION PROGRAM Non CS materials Inspection program can be based on RBI Which inspection method to be used and where to inspect? Normally the RBI analysis is used to define the need for condition and process monitoring

37 37 RBI Assessment of fatigue Difficult to base an inspection plan for fatigue on a RBI analysis Field inspection has to be done to find places exposed to fatigue (often vibrations) Mitigatigation or monitoring plans have to be established Small bore pipe exposed to fatigue

38 38 WHAT IMPORTANCE HAS THE POD A FAILURE? The main goals with the RBI methodology is to perform the inspection planning in a systematic and well defined way optimise the inspection work to the high risk areas select the best inspection method, the most probable area for the failure to start and when to inspect The challenges with the inspection plan are do we inspect in the position/area where the worst attack is? do we measure the real condition/attack size (e.g. UT)? is the crack size below the POD level of the actual method? do we inspect with the right frequence?

39 39 WHAT IMPORTANCE HAS THE POD OF A CRACK/INITIATION POINT? The results from inspection is used to update: the status of the Corrosion Management system the RBI analysis the inspection plan Accurate information from inspection is really important for safety, economy and environmental aspects a non detected crack/failure can develop into a catastrophic failure if not detected

40 40 WE DO NOT WANT THIS TO HAPPEN