6th Pipeline Technology Conference 2011
RISK & RELIABILITY BASED FITNESS FOR SERVICE (FFS) ASSESSMENT
FOR SUBSEA PIPELINES
By
Ir. Muhd Ashri Mustapha & Dr. Yong BaI.
June 2011
Hannover
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Table of Contents
1. Introduction
2. Objective
3. Methodology and Principle
4. QRA & Target Reliability
5. SRA, Retaining Pressure Capacity & FFS
6. Examples
7. Conclusion
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1. Introduction
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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.
2. Objectives
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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;
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To recommend suitable actions to be taken based on the assessment findings.
3. Methodology and Principle
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Operating data
Inspection data
Design data
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This risk and reliability based FFS study
process will focus on pipeline corrosion
defects only.
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First of all, QRA is performed to derive
the pipeline target reliability.
•
Then, using target reliability and
structure 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.
QRA
Pipeline
Segmentation
Corrosion rate
Target reliability
Defect assessment one
by one based on SRA
method
Yes
No
Psafe > MAOP?
Develop defects to
remaining design life
Psafe >MAOP?
Yes
No
Calculate remaining
design life capacity
Remaining life to
current MAOP
Calculate de-rated
capacity
4. QRA & Target Reliability
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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.
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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
4.1 Pipeline Segmentation
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The risk evaluator must decide on a strategy for creating these sections in order to
obtain an accurate risk picture.
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Each pipeline segment will have its own risk as the production of failure probability
and failure consequence.
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A significant condition change must be determined by the evaluator with
consideration given to data costs and desired accuracy.
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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).
4.2 Probability of Failure (Pf)
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Pipeline failure usually takes the form of leakage, which is the initiate event
resulting to serious consequences.
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Probability of Failure (Pf) is estimated as failure frequencies of different types of
degradation mechanisms operating in the pipeline component.
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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
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The famous UK PARLOC 2001 database is proposed to be used for pipeline
Pf assessment.
4.3 Consequences of Failure (CoF)
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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.
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It is usually divided into three categories of Safety, Economic and Environmental
consequence to be analyzed respectively by qualitatively or quantitatively way.
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The consequence analysis is an extensive effort covering a series of steps
including:
–
–
–
–
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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.
4.4 Target Reliability
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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.
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Theoretically, a Life Cycle Cost-Benefit assessment should be a preferred method
for determining the optimum target reliability.
Cost
Total Cost
Failure Cost
Initial Investment and
Maintenance Cost
Optimum Reliability
Reliability
4.4 Target Reliability
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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.
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When conducting reliability based FFS analysis, target reliability levels in a given
reference time period and reference length of pipeline should be selected.
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The selection is based on consequence of failure, location and contents of
pipelines, relevant rules, access to inspection and repair, etc.
Limit States
Safety Classes
Target reliabilities
vs. Safety
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
5. SRA, FFS & Retaining Pressure
Capacity
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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.
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The target reliability will be used according to the maximum allowable failure rate
to deduce the maximum value of pipeline safe operating pressure Psafe, which will
indicates the pipeline retaining pressure capacity (service limit state).
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The maximum value of pipeline safe operating pressure Psafe is not allowed to be
less than the given MAOP.
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The safety index β (API 2A-LRFD) is the most popular measure of reliability in
industry. The safety index is related to the corresponding failure rate by formula:
Pf = Φ(− β ) = 1 − Φ(β )
Where, Φ(.) is the standard normal distribution function.
5.1 SRA Method
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A structure reliability assessment (SRA)
method is used to calculate the pipeline
failure rate and the reliability R= 1-Pf.
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An SRA model for the pipeline failure
rate calculation is presented here for
damage from corrosion.
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The main steps of SRA method has
been illustrated in the left figure.
5.2 FFS & Retaining Pressure Capacity
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The target reliability is a structural safety requirement, which means the pipeline
failure probability Pf is not allowed to be greater than it.
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If assign target reliability to failure rate Pf and deduce the value of pipeline safe
operating pressure Psafe by using the SRA method, this maximum value of Psafe will
indicates the pipeline retaining pressure capacity (service limit state).
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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.
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Using the SRA method described before, the pipeline maximum safe operating
pressure (Psafe) equals to the mean load (Sm) divided by its bias:
Psafe = S m / BSm
5.2 FFS & Retaining Pressure Capacity
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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 carbon
steel 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 software
subsea pipeline integrity management software: PaRIS.
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The purpose of corrosion rate calculation is to predict corrosion defects
development.
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According to the corrosion rate value (CR) and the retaining pressure capacity
(Psafe), the pipeline remaining life can also be obtained.
6. Examples
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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:
Parameter
Symbol [Unit]
Value
Outer diameter
D[mm]
273.05
Wall thickness
t[mm]
8.5
Standard deviation
σ [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
σ
0.05
Measured maximum defect length
Standard deviation
t
do
σ [mm]
σ
Lo
Lo
250
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6. Examples
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To determine the pipeline safety level and target reliability accordingly, a complete
risk assessment is supposed be performed.
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A sensitivity study at the target reliability has been performed to review the
benefits of using reliability based FFS in comparison to the using of other codes
like ASME B31G and DNV RP F101.
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The results have been illustrated in the tables and figures bellow.
6. Examples
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7. Conclusion
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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.
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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.
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