June 2011Hannover

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RISK & RELIABILITY BASED FITNESS FOR SERVICE (FFS) ASSESSMENT FOR SUBSEA PIPELINES

By

Ir. Muhd Ashri Mustapha & Dr. Yong BaI.

RAHSIA

1. Introduction

2. Objective

3. Methodology and Principle

4. QRA & Target Reliability

5. SRA, Retaining Pressure Capacity & FFS

6. Examples

7. Conclusion

Table of Contents

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• The risk and reliability based fitness-for-services (FFS) assessmentaddressed in this paper is a quantitative risk assessment (QRA) basedFFS study on subsea oil or gas pipelines.

• The main purpose of QRA is to determine the target reliabilities fordifferent pipeline segments.

• Structure Reliability Assessment (SRA) method is used to calculate themaximum safe operating pressure, which indicates the pipeline retainingpressure capacity.

• QRA and SRA results will be used to conduct traditional FFS, whichindicates whether the pipeline is fit for service or not by a comparison ofpipeline retaining pressure capacity with given MAOP.

1. Introduction

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• To portray pipeline present risk picture and define the target reliability of every pipeline segment;

• To determine the pressure containment capacity of the pipeline at the time it was last inspected;

• To conduct the corrosion assessment to estimate the internal corrosion rates;

• To determine the remaining years for which the pipeline can be safely operated dated from the last inspection;

• To recommend suitable actions to be taken based on the assessment findings.

2. Objectives

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• This risk and reliability based FFS studyprocess will focus on pipeline corrosiondefects only.

• First of all, QRA is performed to derivethe pipeline target reliability.

• Then, using target reliability andstructure reliability analysis (SRA)method, the pipeline retaining pressurecapacity Psafe will be obtained as thepreparation of FFS.

• Finally, traditional FFS will beconducted to indicate whether thepipeline is fit for service or not by acomparison of pipeline retainingpressure capacity with given MAOP.

3. Methodology and Principle

Operating data

Develop defects to remaining design life

Psafe > MAOP?Yes

Inspection data Design data

QRA

Corrosion rate Target reliability

Pipeline Segmentation

Defect assessment one by one based on SRA

method

No

Psafe >MAOP?

Calculate remaining design life capacity

Remaining life to current MAOP

Yes

No

Calculate de-rated capacity

RAHSIA

• This section intends to perform quantitative risk assessment (QRA) to establish the pipeline structure target reliability taking into account pipeline safety, environmental, and economic consequences.

• The QRA process will bring benefits to the following FFS analysis:

– Pipeline Segmentation - precise pipeline segmentation

– Probability of Failure (Pf)

– Consequences of Failure (Cof)

– Target Reliability - choice of pipeline target reliabilities

4. QRA & Target Reliability

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• The risk evaluator must decide on a strategy for creating these sections in order to obtain an accurate risk picture.

• Each pipeline segment will have its own risk as the production of failure probability and failure consequence.

• A significant condition change must be determined by the evaluator with consideration given to data costs and desired accuracy.

• An example of a short list of prioritized conditions is as follows:– Pipeline specification (wall thickness, diameter, etc.);

– Soil conditions (pH, moisture, etc.);

– Population density;

– Coating condition;

– Age of pipeline;

– Environmental sensitivity (Marine Park, Nature Reserve).

4.1 Pipeline Segmentation

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• Pipeline failure usually takes the form of leakage, which is the initiate event resulting to serious consequences.

• Probability of Failure (Pf) is estimated as failure frequencies of different types of degradation mechanisms operating in the pipeline component.

• The failure frequency is calculated based on different damage causes. The main damage causes identified for subsea pipelines are listed below:

– Internal Corrosion

– External Corrosion

– Erosion

– External Impact

– Free-span

– On-bottom Stability

• The famous UK PARLOC 2001 database is proposed to be used for pipeline

Pf assessment.

4.2 Probability of Failure (Pf)

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• Consequence of failure can be expressed as number of people affected (injured or killed), property damage, amount of a spill, area affected, outage time, mission delay, money lost or any other measure of negative impact for the quantification of risk.

• It is usually divided into three categories of Safety, Economic and Environmental consequence to be analyzed respectively by qualitatively or quantitatively way.

• The consequence analysis is an extensive effort covering a series of steps including:

– Accident scenario analysis of possible event sequences (Event Tree Analysis for instance)– Analysis of accidental loads, related to fire, explosion, impact– Analysis of the response of systems and equipment to accidental loads– Analysis of final consequences to personnel, environment, and assets

• Each of these steps may include extensive studies and modeling.

4.3 Consequences of Failure (CoF)

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• To ensure certain safety levels of pipeline or pipeline segments, target reliability need to be settled and has to be met at pipeline design phase.

• Theoretically, a Life Cycle Cost-Benefit assessment should be a preferred method for determining the optimum target reliability.

4.4 Target Reliability

Reliability

Cost

Optimum Reliability

Failure Cost

Initial Investment and Maintenance Cost

Total Cost

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• The selection of target reliability is based on consequences of failure, location and contents of pipelines, relevant rules, access to inspection and repair, etc.

• When conducting reliability based FFS analysis, target reliability levels in a given reference time period and reference length of pipeline should be selected.

• The selection is based on consequence of failure, location and contents of pipelines, relevant rules, access to inspection and repair, etc.

4.4 Target Reliability

Limit StatesSafety Classes

Low Normal High

SLS 10-1~10-2 10-2~10-3 10-3~10-4

ULS 10-2~10-3 10-3~10-4 10-4~10-5

FLS 10-2~10-3 10-3~10-4 10-4~10-5

ALS 10-3~10-4 10-4~10-5 10-5~10-6

Target reliabilities vs. Safety classes

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• The capacity of each defect will be assessed based on a structure reliabilityassessment (SRA) method and the target reliability above.

• The target reliability will be used according to the maximum allowable failure rateto deduce the maximum value of pipeline safe operating pressure Psafe, which willindicates the pipeline retaining pressure capacity (service limit state).

• The maximum value of pipeline safe operating pressure Psafe is not allowed to beless than the given MAOP.

• The safety index β (API 2A-LRFD) is the most popular measure of reliability inindustry. The safety index is related to the corresponding failure rate by formula:

Where, Φ(.) is the standard normal distribution function.

5. SRA, FFS & Retaining Pressure Capacity

( ) ( )ββ Φ−=−Φ= 1fP

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• A structure reliability assessment (SRA) method is used to calculate the pipeline failure rate and the reliability R= 1-Pf.

• An SRA model for the pipeline failure rate calculation is presented here for damage from corrosion.

• The main steps of SRA method has been illustrated in the left figure.

5.1 SRA Method

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• The target reliability is a structural safety requirement, which means the pipelinefailure probability Pf is not allowed to be greater than it.

• If assign target reliability to failure rate Pf and deduce the value of pipeline safeoperating pressure Psafe by using the SRA method, this maximum value of Psafe willindicates the pipeline retaining pressure capacity (service limit state).

• If this maximum safe operating pressure Psafe is identified to be less than MAOP,the defect is unacceptable and the pipeline is declared to be unfit for service.

• Using the SRA method described before, the pipeline maximum safe operatingpressure (Psafe) equals to the mean load (Sm) divided by its bias:

5.2 FFS & Retaining Pressure Capacity

Smmsafe BSP /=

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• Flow-chart of Pressure Capacity Assessment can be expressed as follow:

5.2 FFS & Retaining Pressure Capacity

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• Corrosion Rate:

– The corrosion caused by the incidences of CO2 represents the greatest risk to the integrity of carbonsteel equipment in a production environment and is more common than damage related to fatigue,erosion, or stress corrosion cracking.

• De Waard’s models for corrosion rate have been programmed in-house softwaresubsea pipeline integrity management software: PaRIS.

• The purpose of corrosion rate calculation is to predict corrosion defectsdevelopment.

• According to the corrosion rate value (CR) and the retaining pressure capacity(Psafe), the pipeline remaining life can also be obtained.

5.2 FFS & Retaining Pressure Capacity

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• One subsea oil export pipeline is installed at the year 1982, with design life of 20years. The table below is the general data of the pipeline with inspection results ofcorrosion defect at the 2003 incorporated:

6. Examples

Parameter Symbol [Unit] Value

Outer diameter D[mm] 273.05

Wall thickness t[mm] 8.5

Standard deviation �σt[mm] 0.5

Design factor F 0.72

SMYS SMYS[MPa] 358.5

MAOP MAOP[MPa] 9.3

Operating Pressure Pop[MPa] 3

Corrosion rate r[mm/year] 0.17

Standard deviation �r[mm/year] 0. 5

Measured maximum defect depth do/t 0.45

Standard deviation �σdo0.05

Measured maximum defect length σLo [mm] 250

Standard deviation σLo5

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• To determine the pipeline safety level and target reliability accordingly, a completerisk assessment is supposed be performed.

• A sensitivity study at the target reliability has been performed to review thebenefits of using reliability based FFS in comparison to the using of other codeslike ASME B31G and DNV RP F101.

• The results have been illustrated in the tables and figures bellow.

6. Examples

RAHSIA6. Examples

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• The advantage of QRA based determination of target reliability is that the pipelineis segmented more scientifically from a risk perspective and every segment has itsown target reliability.

• This assessment also benefits from making good use of available data and reportsinclude: inspection data, monitoring data, pipeline repair and incident records,corrosion study report and QRA report (if any) etc.

7. Conclusion

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