professor, institutt for produktutvikling og materialer...

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Nivå 3 personell NDT Foreningen November 2011 [email protected]

RISK BASED INSPECTION (RBI)

Roy Johnsen Professor, Institutt for produktutvikling og materialer (IPM),

NTNU

[email protected]

Tlf: +47 93245101

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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?

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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!

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WHAT ARE YOU INSPECTING FOR? Internal corrosion CS with CO2

Inspection needed?

NDT – leak control?

NDT – important but which method to use?

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PREFERENTIAL WELD CORROSION Normally not included in the inspection program

Base metal

Weld metal

Heat affected zone

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WHAT ARE YOU INSPECTING FOR? Internal corrosion in CS with CO2

More typical pitting/partly uniform corrosion

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CORROSION UNDER INSULATION

Where to inspect and which NDT method to be used?

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INSTRUMENT TUBING IN AISI 316 Marine environment

NDT method – Visual Inspection

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STAINLESS STEEL Where and how to inspect?

Pitting corrosion in AISI 316

Chloride stress corrosion cracking

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DIFFERENT TYPE OF FAILURES On Statoil operated installations 1997-2007

FATIGUE

31%

CORROSION

31%

WEAR

5%

HYDROGEN EMBRITTLEMENT

FABRICATION

BRITTLE FRACTURE

INSTALLATION 2%

LIQUID METAL EMBRITTLEMENT

CREEP

3%

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

CORROSION MANAGEMENT

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CORROSION MANAGEMENT What is included?

Planning 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

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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

B: IMPLEMENT THE PLAN 1. Prepare list of actions 2. Implement list of actions

C: ASSESS THE PLAN 1. Evaluate available data 2. Compare with performance indicators

D: REVIEW THE PLAN 1. Review the integrity plan 2. Propose improvements 3. Implement improvements

PLAN TO DO

IMPROVE MEASURE

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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

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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

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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.

../../../FAGARTIKLER/Inspeksjon-RBI/DNV-RP-G101 RBI - October 2010.pdf

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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

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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

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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)

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RISC SCREENING MATRIX

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CONSEQUENCE (CoF)

NON-ACCEPTABLEACCEPTABLE

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)

1. 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)

NEGLIGIBLE

< 10-5

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)

.

VERY HIGH

HIGH

MEDIUM

LOW

> 10-5

PR

OB

AB

ILIT

Y (P

oF

)

CONSEQUENCE (CoF)

NON-ACCEPTABLEACCEPTABLE

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)

1. 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)

NEGLIGIBLE

< 10-5

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)

.

VERY HIGH

HIGH

MEDIUM

LOW

> 10-5

PR

OB

AB

ILIT

Y (P

oF

)

EXAMPLE - SCREENING RESULTS

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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

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RISK MATRIX Detailed Analysis

A tyical 3x3 risk matrix

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TYPES OF CORROSION From NACE

Uniform corrosion

CO2 /H2S corrosion

Galvanic corrosion

Pitting/Crevice corrosion

Bacterial corrosion

Erosion corrosion

SCC (Chloride Stress Corrosion Cracking)

SSCC (Sulphide Stress Corrosion Cracking)

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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

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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).

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PROBABILITY OF FAILURE Examples of PoF

For corrosion in carbon steel

For Corrosion Under Insulation

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CONSEQUENCE OF FAILURE CoF

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RISK MATRIX

Risk is transferred to ”Time-to-next-inspection”

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INSPECTION INTERVAL

Example for carbon steel

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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

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WHAT ABOUT OTHER MATERIALS THAN CARBON STEELS?

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PoF Example for noble alloys in seawater

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PoF FROM DNV-RP-G101

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PROCESS MONITORING PROGRAM

Parameters to monitor:

Temperature

Velocity

Water content

Chloride content

Residual chlorine

Oxygen level

CO2/H2S content

Amount of sand particles

Operation window for stainless steel alloy exposed to chloride containing

environment

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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

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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

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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?

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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

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WE DO NOT WANT THIS TO HAPPEN

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