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June 2011

Hannover

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RISK & RELIABILITY BASED FITNESS FOR SERVICE (FFS) ASSESSMENT

FOR SUBSEA PIPELINES

By

Ir. Muhd Ashri Mustapha & Dr. Yong BaI.

6th Pipeline Technology Conference 2011


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1. Introduction

2. Objective

3. Methodology and Principle

4. QRA & Target Reliabil ity

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) assessment

addressed in this paper is a quantitative risk assessment (QRA) based

FFS study on subsea oil or gas pipelines.

The main purpose of QRA is to determine the target reliabilities for

different pipeline segments.

Structure Reliability Assessment (SRA) method is used to calculate the

maximum safe operating pressure, which indicates the pipeline retaining

pressure capacity.

QRA and SRA results will be used to conduct traditional FFS, which

indicates whether the pipeline is fit for service or not by a comparison of

pipeline 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 waslast 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 study

process will focus on pipeline corrosion

defects only.

First of all, QRA is performed to derive

the pipeline target reliability.

Then, using target reliability andstructure reliability analysis (SRA)

method, the pipeline retaining pressure

capacity Psafe will be obtained as the

preparation of FFS.

Finally, traditional FFS will be

conducted to indicate whether the

pipeline is fit for service or not by a

comparison of pipeline retaining

pressure 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


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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 probabilityand 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 ofdegradation 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 methodfor 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 givenreference 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 reliability

assessment (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 will

indicates 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 (API 2A-LRFD) is the most popular measure of reliability in

industry. The safety index is related to the corresponding failure rate by formula:

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

5. SRA, FFS & Retaining Pressure

Capacity

( ) ( ) == 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 fordamage 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 pipeline

failure probability Pfis not allowed to be greater than it.

If assign target reliability to failure rate Pf and deduce the value of pipeline safeoperating pressurePsafe by using the SRA method, this maximum value ofPsafe will

indicates 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 operating

pressure (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:



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Corrosion Rate:

The corrosion caused by the incidences of CO2 represents the greatest risk to the integrity of carbon

steel equipment in a production environment and is more common than damage related to fatigue,erosion, or stress corrosion cracking.

De Waards models for corrosion rate have been programmed in-house software

subsea pipeline integrity management software: PaRIS.

The purpose of corrosion rate calculation is to predict corrosion defects

development.

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



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One subsea oil export pipeline is installed at the year 1982, with design life of 20

years. The table below is the general data of the pipeline with inspection results of

corrosion 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 t[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 do 0.05

Measured maximum defect length Lo[mm] 250

Standard deviation Lo 5


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To determine the pipeline safety level and target reliability accordingly, a complete

risk 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 codes

like ASME B31G and DNV RP F101.

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

6. Examples


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The advantage of QRA based determination of target reliability is that the pipeline

is segmented more scientifically from a risk perspective and every segment has its

own target reliability.

This assessment also benefits from making good use of available data and reports

include: inspection data, monitoring data, pipeline repair and incident records,

corrosion study report and QRA report (if any) etc.

7. Conclusion


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