RISK & RELIABILITY BASED FITNESS FOR SERVICE (FFS) ASSESSMENT FOR SUBSEA PIPELINES By Ir. Muhd Ashri Mustapha & Dr. Yong BaI. June 2011 Hannover All rights reserved. No part of this document may be reproduced, stored in a retrieval system or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the permission of the copyright owner. 1
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
1. Introduction 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 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.
3. Methodology and Principle Operating data Inspection data QRA Design data This risk and reliability based FFS study process will focus on pipeline corrosion defects only. Corrosion rate Pipeline Segmentation Target reliability First of all, QRA is performed to derive the pipeline target reliability. Yes Develop defects to remaining design life Defect assessment one by one based on SRA method P safe > MAOP? No Then, using target reliability and structure reliability analysis (SRA) method, the pipeline retaining pressure capacity P safe will be obtained as the preparation of FFS. P safe >MAOP? No Calculate remaining design life capacity Yes Remaining life to current MAOP Calculate de-rated capacity 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.
4. QRA & Target Reliability 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 (P f ) Consequences of Failure (Cof) Target Reliability - choice of pipeline target reliabilities
4.1 Pipeline Segmentation 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).
4.2 Probability of Failure (P f ) Pipeline failure usually takes the form of leakage, which is the initiate event resulting to serious consequences. Probability of Failure (P f ) 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.
4.3 Consequences of Failure (CoF) 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.
4.4 Target Reliability 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. Cost Total Cost Failure Cost Initial Investment and Maintenance Cost Optimum Reliability Reliability
4.4 Target Reliability 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. 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 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 rate to deduce the maximum value of pipeline safe operating pressure P safe,whichwill indicates the pipeline retaining pressure capacity (service limit state). The maximum value of pipeline safe operating pressure P safe is not allowed to be less than the given MAOP. 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: P f ( β ) = Φ( β ) = Φ 1 Where, Φ(.) is the standard normal distribution function.
5.1 SRA Method A structure reliability assessment (SRA) method is used to calculate the pipeline failure rate and the reliability R= 1-P f. 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.
5.2 FFS & Retaining Pressure Capacity The target reliability is a structural safety requirement, which means the pipeline failure probability P f is not allowed to be greater than it. If assign target reliability to failure rate P f and deduce the value of pipeline safe operating pressure P safe by using the SRA method, this maximum value of P safe will indicates the pipeline retaining pressure capacity (service limit state). If this maximum safe operating pressure P safe 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 (P safe ) equals to the mean load (S m ) divided by its bias: P = S / B safe m Sm
5.2 FFS & Retaining Pressure Capacity Flow-chart of Pressure Capacity Assessment can be expressed as follow:
5.2 FFS & Retaining Pressure Capacity Corrosion Rate: The corrosion caused by the incidences of CO 2 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. The purpose of corrosion rate calculation is to predict corrosion defects development. According to the corrosion rate value (CR) and the retaining pressure capacity (P safe ), the pipeline remaining life can also be obtained.
6. Examples 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 t [mm] 0.5 Design factor F 0.72 SMYS SMYS[MPa] 358.5 MAOP MAOP[MPa] 9.3 Operating Pressure P op [MPa] 3 Corrosion rate r[mm/year] 0.17 Standard deviation r[mm/year] 0. 5 Measured maximum defect depth d o /t 0.45 Standard deviation σ do 0.05 Measured maximum defect length σ Lo [mm] 250 Standard deviation σ Lo 5
6. Examples 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 the benefits 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
7. Conclusion 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.