Paper presented at ASME PVP 2002 Conference, Vancouver, Canada, 4 - 8 August 2002
Abstract
The application of risk-based life management practices has generated considerable interest in industry. Plant safety and availability can be demonstrably improved through the application of risk-based methods alongside good plant management practices.
The needs for and the experience of risk-based plant life management however, vary greatly across industry sectors. The principles of risk-based methods are documented and many risk-based techniques are widely available, but the practical interpretation of the principles and the use of the most appropriate techniques are subjects of great debate.
This paper will identify technical and organisational requirements to implement risk-based methods. As part of this paper, a questionnaire survey has been carried out among companies to gain better understanding of the reality of plant life management and the needs of plant operators. This survey indicates that the benefits of risk-based methods for inspection (RBI) and maintenance (RBM) optimisation are recognised by different industrial sectors. There appears to be a lack of established and documented uniform RBI/RBM policy or guidance for application throughout the industry sectors. There is also an indication of insufficient resources and training to implement risk-based methods. Development of risk-based techniques by a competent team and an integrated user-friendly software based on a sound methodology remain as key issues.
A case study is described of the application of RBI to an oil refinery process unit.
Keywords
Risk-Based Inspection (RBI), Risk-Based Maintenance (RBM), Fitness-for-Service (FFS), RISKWISE TM.
Introduction
Background
This paper is prepared in response to the current level of interest in risk-based inspection within several industrial sectors. It has become clear that the needs for, and the experience of, risk-based plant life management vary greatly across industry sectors. Companies already practising risk-based methods need re-assurance about their practices and companies who wish to implement risk-based inspection (RBI) programs require concise guidance. There is also wide interest in various fitness-for-service assessment standards and their relationship to risk-based methods.
Objectives
The objectives of this paper include:
- A review of the published literature and codified generic documents, strengths and weaknesses of RBI and Fitness-for-Service (FFS) methods from technical and sector application viewpoints.
- An investigation of risk-based practices in industry via a user-experience survey, providing feedback on currently available RBI practices.
- An identification of best practice and the provision of guidance on implementation of RBI within various sectors.
- A report on a case study to illustrate the application of risk-based inspection using TWI's RISKWISE TM software.
The nature of risk
Risk is a combination of the probability of occurrence of a hazardous event and the magnitude of the consequences of the event. Risk is defined by three components: the event, probability of the event occurring, and the undesirable consequences. Perception of risk is often strongly influenced by the consequences rather than probability.
The practice of risk-based life management is complex. A balanced approach to life management is to consider all political, economic, commercial, technological and human aspects. In this process, a wide range of pertinent issues should be considered, including:
- The nature and tolerability of risk,
- Principles of management of health and safety at work,
- Process of and methods for risk assessment and management,
- Industry guidelines on risk-based plant safety, availability and inspection,
- Risk related decision making, and
- Fitness-for-service.
Industry is recognising that benefit may be gained from adopting formal risk-based approaches to plant integrity management through improved targeting and scheduling of inspections and maintenance activities. The primary benefits of RBI and RBM are: improved health and safety management; cost savings derived from extending inspection intervals, avoiding unnecessary inspection; increasing plant availability; and optimum repair and replacement scheduling.
Overview of UK Regulations Governing Industrial Risks
In the United Kingdom, the law requires industries to ensure, so far as is reasonably practicable, the health, safety and welfare at work of all their employees and to conduct their operations in such a way to ensure, so far as is reasonable, that the public is not exposed to risks to their health and safety. The Health and Safety Executive (HSE) in the UK is responsible for drawing up the guidelines on tolerability limits of risk at work.
[1] The HSE requires the risks within the tolerable limits to be reduced as low as reasonably practical, commonly known as the ALARP principles. Practices of industrial risk regulation and management have been under continuous development over the last three decades. One of the challenges today in industrial risk management is that society becomes less tolerant of risks. Industrial operations are expected to work to a much lower level of risk than that associated with daily life activities. It is therefore crucially important to understand that risk can only be reduced to ALARP.
Risk assessment and management must answer the following questions:
- What can go wrong?
- What are the causes?
- What are the consequences?
- How likely is it?
- How safe is safe enough?
- How can risks be reduced?
There are three generic steps in the risk assessment and management process:
- Identification of hazards,
- Assessment of the risk, and
- Reduction of risk.
Review of UK Industry Guidelines on Plant Safety
Nuclear safety assessment principles in the UK are set within the 'Tolerability of risk from nuclear power stations (TOR)' and the 'Safety assessment principles for nuclear plants (SAPs)'. [2] The SAPs require that all potential accidents be identified in a systematic manner. For the analysis of accidents, the SAPs use a three-pronged approach. This requires the deterministic analysis of: (a) both design basis accidents (DBAs); (b) beyond design basis accidents (BDBAs), including severe accidents; and (c) probabilistic safety assessment (PSA). PSA has been used for many years in the UK in licensing and regulation and is now an established feature of safety cases.
A framework of risk-related decision support has been published by the UK offshore oil and gas industry. [3] It recognises the need to reach a risk-related decision with a combination of technical and value-based approaches including:
- Engineering codes and standards,
- Good practice,
- Engineering judgement,
- Risk-based assessments, and
- Value based approaches.
The framework provides a means to determine the relative importance of the various methods of assessing risk by referencing the role of standards, quantitative risk assessment (QRA), societal values, etc. Operators are required to judge which combination of these methods is best used to determine whether the risks are tolerable and ALARP.
In 1991 ASME published a general document on the use of risk-based technology for the development of inspection guidelines. [4] This document presented an overall risk-based inspection process. It described techniques and tools to be used at each stage of the process. It also presented examples throughout the text to illustrate the use of those tools.
In 1992 ASME published a second document, Volume 2 - Part 1 Light water reactor (LWR) for nuclear power plant components, in a series covering the development of guidelines for risk-based inspection. [5] Volume 2 is an application of the general methodology in Volume 1, for the inspection of nuclear power plant components. Nuclear power plants are different from any other industry installations in that multiple levels of safety protection are designed and built into plant systems on the principle of 'defence-in-depth'. This means that combinations of unlikely events must occur to cause the breach of the defence and to result in a significant public harm. Since the potential consequences of a nuclear release are severe, and the chain of interacting events leading to the final event of core damage is complex, rigorous probabilistic risk assessments (PRA) are required. PRA have been performed in nuclear safety assessments for many nuclear power plants in the UK. The use of this extensive PRA information is a key feature of the methodology presented in Volume 2 in the quantitative risk assessments.
In 1994 ASME published a third document, Volume 3, covering fossil fuel-fired electric power generation station applications. [6] It addresses the in-service inspection (ISI) of components in fossil fuel-fired electric power generating stations. It considers application of a risk-based methodology to all fossil power plant components that contribute to plant unavailability, but the primary focus of inspection is on components that maintain a pressure boundary.
The ASME approach [4] comprises a qualitative risk ranking as well as a quantitative assessment applied to individual components or equipment items. The quantitative approach recommends that a full FMECA (Failure Modes Effects Criticality Analysis) should be conducted. The use of operating experience databases and analytical damage models together with their probabilistic application is also recommended. The latter is required for establishing inspection periods, although detailed examples of the analytical process are not given.
API BRD 581 'Base resource document on risk-based inspection' (preliminary draft) was published by the American Petroleum Institute in 2000. [7] It was produce for the oil and gas production, oil refining and petrochemical industry conforming to American regulation and industry practice. It was intended for equipment designed and constructed to the ASME and ANSI codes, as well as the in-service inspection guidelines of API RP 510, RP 653, and RP 570. It was mainly for inspection on pressure maintaining equipment such as pressure vessels and piping, but also for other equipment such as heat exchangers and the pressure-retaining components of pumps.
API BRD 581 [7] comprises qualitative and quantitative approaches. The qualitative approach is based on a series of failure likelihood and failure consequence factors and delivers equipment positioning within a five by five risk matrix. For the quantitative method in API BRD 581 the likelihood evaluation process starts with a generic failure frequency for the type of equipment in question. This value is then modified by factors relating to: the specific equipment (F E) and the safety management regime (F M). F E takes account of items such as damage type, inspection effectiveness, condition, design and fabrication, process control and safety management and F M addresses the potential impact on mechanical integrity of all process safety management issues from API RP 750. The factors F E and F M are obtained from an exhaustive scoring system based on questionnaires or workbooks. The quantitative assessment of failure consequence within API BRD 581 is based on a systematic multi-stage process to determine costs relating to explosive release, toxic consequences, environmental clean-up and business interruption.
The API and ASME approaches are similar in that they both advocate progression from a relatively simple qualitative risk ranking method to a far more complex quantitative method requiring significant effort and expertise to execute. Although the ASME approach considers the time dependence of failure probability, neither approach offers clear and formal methods to translate risks into inspection frequencies.
Fitness-for-Service (FFS) Assessment and its RBI Relation
BS 7910 provides a 'Guide on methods for assessing the acceptability of flaws in metallic structures'.
[8] Emphasis is placed on welded fabrications in ferritic and austenitic steels and aluminium alloys, but the procedures in BS 7910 can be used for analysing flaws in structures made from other metallic materials and in non-welded components or structures. The methods described can be applied at the design, fabrication and operational phases of a structure's life.
API RP 579 [9] is an industry standard for the fitness-for-service (FFS) assessment of pressurised equipment in the oil and gas industry. It is intended to supplement and augment the requirements in API RP 510, RP 570 and RP 653. In principle, pressure equipment subject to RP 579 assessments should be designed and constructed according to ASME and API codes, or other recognised international and internal corporate standards. The assessment procedures are based on allowable stress methods and plastic collapse loads for non crack-like flaws, and fracture mechanics failure assessment diagram methodologies for crack-like flaws.
An important aspect of mechanical integrity management is to inspect and monitor the equipment as damage may occur during its service life. RBI is used to prioritise equipment for targeted inspection based on risk. FFS is usually applied to a single component with respect to a specific failure mode resulted from some operating damage mechanism. RBI may be used to decide the frequency, location and sampling size of an inspection. Once in-service damage is detected, FFS is applied to evaluate whether the equipment is safe to continue operating.
Guidelines on Assessment of Inspection Frequency
In the UK, the Safety Assessment Federation (SAFed) in 1997 produced a set of guidelines on the periodicity of inspections.
[10] The guidelines adopt a basic qualitative risk assessment approach and further state that they should only be adopted after proper consideration of individual circumstances pertaining to each pressure system. The European Confederation of Organisations for Testing, Inspection, Certification and Prevention (CEOC) has developed advisory guidelines aimed at harmonisation within the EU with respect to inspection frequency of boilers and pressure vessels.
[11] An approach is proposed whereby likelihood of failure scores are used to establish position on a risk matrix. The maximum period between inspections is then related to the assessed risk. The UK's Institute of Petroleum (IP) has also issued a code of practice for inspection of pressure vessels in the petroleum industry.
[12] This approach groups equipment into different categories, e.g. process pressure vessels, storage vessels, etc. and uses a grading system to assign inspection intervals. The grade is set by the operator after the first examination. If operational practices are uncertain or if the rate if degradation is expected to be high, then the inspection interval can be reduced by the operator. For cases where the degradation rate is as expected and more predictable the grade achieved allows an extended inspection interval.
In summary, the regulatory environment in the UK is such that where reliable damage rate predictability exists, there is scope for operators to adopt formal methods for determining inspection periods based on risk and remaining life considerations.
Survey of risk-based life management practices
As part of a major joint industry project, TWI carried out a questionnaire-based survey to gain a better understanding of the reality of plant life management and the needs of plant operators. Approximately 90 of the questionnaires that were distributed worldwide, were returned to TWI. A summary of this survey is given below.
- The opinions expressed in this survey were by and large equally representative of all the regions of the world except for Africa and South American-Caribbean.
- The majority of respondents (53 out of 90) were employed by companies that are not Industrial Members of TWI.
- The majority of respondents were based in the Oil & Gas Refining, Chemical and Petrochemical industries. The Nuclear Power Generation, Oil & Gas Transportation and Onshore Oil & Gas Production industries were under-represented in the survey.
- The majority of respondents classified themselves as Producers, Operators or Manufacturers and Engineering Service Contractors. Engineering Insurers, Equipment Manufacturers or Suppliers and Safety Regulating Authorities were under-represented in the survey.
- The majority of all respondents' companies (69%) had 'Previously implemented' RBI/RBM, or were 'Currently implementing' RBI/RBM. Several respondents, in all sectors, indicated that they were currently implementing RBI/RBM.Petrochemical, Oil & Gas Refining, Fossil Fuel Power Generation and Chemical companies had the largest proportion of respondents that were considering implementation.
- The only sectors in which respondents had indicated that senior management was 'Unconvinced' that an RBI/RBM program would be beneficial to their company were the Petrochemical, Offshore Oil & Gas Production, Multi-sectoral and Chemical sectors. Approximately 60% of all respondents indicated that senior management were 'Convinced' or 'Very convinced' that an RBI/RBM program would be beneficial to their company.
- Approximately 98% of all respondents (whose company's had previously undertaken a RBI/RBM assessment), indicated that the results of their RBI/RBM program had 'Met expectations' or 'Exceeded the expectations' of their company.
- General Pressure Vessels, Process Piping and Shell and Tube Heat Exchangers are the types of equipment to which RBI/RBM is more often applied. Structures, Safety Relief Valves and Pumps, Turbines and Compressors are the equipment to which RBI/RBM is least often applied.
- Approximately 20% of all respondents indicated that their company had established and documented a uniform RBI/RBM policy or guidance for application throughout their company. In the Fossil Fuel Power Generation sector respondents indicated that their company has not, and is not preparing to, produce a uniform policy or guidance document.
- With regard to how many people were currently responsible for managing the day-to-day introduction of RBI/RBM, 47%, 10%, 28% and 15% of respondents across all sectors indicated that their company had set up a 'Part time -Individual', 'Full time - Individual', 'Part time - Project team', and 'Full time - Project team', respectively.
- Approximately 60% of all respondents whose company is currently using, or intends to use RBI/RBM software, indicated that their software was not linked to any other electronic data management system, or other software system.
- Approximately 24% of all respondents indicated that both the input variables and output results of their RBI/RBM software were linked to another data management or software system.
- Respondents indicated that there is no substantial increase in the accuracy of Semi-quantitative compared to Qualitative (Subjective) RBI/RBM methods. However, respondents believe that Qualitative (Subjective) methods were substantially faster to apply than Semi-quantitative RBI/RBM methods. Quantitative (Probabilistic) methods were considered to be the most accurate of the three methods, but also the slowest of the three RBI/RBM methods to apply, Fig.1.
- Approximately 57% of all respondents indicated that their Safety Regulating Authority (SRA) accepted RBI/RBM as an alternative basis for determining inspection and maintenance intervals. However, there was no one region of the world in which RBI/RBM was entirely accepted as an alternative basis for determining inspection and maintenance intervals.
- The respondents were grouped on the basis of their personal experience in the implementation of RBI/RBM. For those respondents that had previously undertaken RBI/RBM assessments, the two most important reasons for implementing a RBI/RBM program were: (a) Improving the overall safety of critical plant; and (b) Reducing the duration of inspection or maintenance outages.
- Similarly, for those respondents that were currently undertaking RBI/RBM assessments the two most important reasons for implementing a RBI/RBM program were: (a) Improving the overall safety of critical plant; and (b) Extending the interval between inspection or maintenance outages.
- The two overall least significant reasons for implementing a RBI/RBM program were: Classification of plant in terms of potential for environmental damage; and reducing the duration of inspection or maintenance outages.
- For those respondents that had previously undertaken RBI/RBM assessments, the most critical success factors for RBI/RBM implementation programs were: (a) Having the 'right' people in the assessment team; (b) Appointing a suitable assessment team leader; and (c) Reliability of the RBI/RBM analysis methodology.
- For those respondents that were currently undertaking RBI/RBM assessments, the most critical success factors were: (a) Having the 'right' people in the assessment team; (b) Reliability of the RBI/RBM analysis methodology; and (c) Appointing a suitable assessment team leader.
- The overall least important success factor was considered to be the 'Speed of undertaking RBI/RBM assessments'.
- For those respondents that currently use and intend to use RBI/RBM software, the most important attributes of the assessment software were: (a) Overall user-friendliness of the software system; and (b) Based on well-known or published model or methodology.
- For those respondents who do not have and do not intend to use RBI/RBM software, the most important attributes were: (a) Based on well-known or published model or methodology; and (b) Tracking system for actions.
- The overall least important attributes of RBI/RBM software program were the software vendor's operation and maintenance costs and the incorporation of a help feature based on 'expert rules' or 'knowledge base' within the software.
Effective RBI implementation
The process of RBI should form part of an integrated strategy for managing the integrity of all assets and systems throughout the plant or facility. RBI is a logical and systematic process of planning and evaluation. The major steps within the process are as follows: [13]
- Establish requirements and clear statement of objectives
- Define systems, system boundaries and equipment to be addressed
- Specify the RBI management team and responsibilities
- Assemble plant database
- Evaluate failure scenarios, damage mechanisms and uncertainties
- Perform risk audit based on event probability or likelihood and event consequence analyses
- Review risk management measures and develop risk-focused inspection plan
- Implement inspection plan and any associated operational or maintenance measures
- Assessment of inspection findings in terms of remaining life and fitness-for-service, and
- Update and feedback to plant database, risk audit and inspection plan on a continuous basis
For control purposes and to ensure best practice the plant manager should exercise a 'performance audit' on each of the above steps in the RBI process.
The steps are intended to assist industry to evaluate the processes being used for integrity management and inspection planning of pressure systems and other systems containing hazardous materials.
Some of the main requirements and critical issues for the effective implementation of RBI which should be recognised by suppliers of RBI programmes and software products, are summarised below. [14]
User-friendly Software
It is essential that the RBI tool is fully understood and accepted by the user and additionally is easily used without undue complexity. For increased efficiency, the software could be linked with inspection or corrosion data management systems where these are installed such that plant data can be efficiently accessed.
Incorporation of all Damage Mechanisms
The RBI process should take account of all deterioration mechanisms and failure modes to a level of state-of-the-art understanding. Failure databases derived from plant experience together with available material models and associated databases are important input. It should be recognised however, that comprehensive 'expert systems' or prescriptive modules based on knowledge of nominal service conditions, to predict all failure scenarios do not currently exist. Expert judgement based on a combination of operational process experience, design and material degradation issues is thus a key element in identifying both existing and potential damage mechanisms and failure modes.
Audit Team Approach
The risk audit, the evaluation of risk management options and inspection planning stages requires a multi-disciplinary input covering a range of competences. Therefore, it is best performed by a team of experienced plant engineers. The team should include plant personnel with technical expertise covering the following areas:
- Risk analysis
- Process hazards and business consequences
- Local safety management
- Plant design and materials degradation
- Operations, inspection and maintenance functions, and
- Inspection and NDE techniques
The complexity of the plant or facility should determine the size of the team. Safety implications should be addressed by individuals in the team who can demonstrate professional competence. The team leader should preferably be remote from pressures associated with plant production. Consensus is required on all major decisions and a complete record must be kept of all judgements and decisions made. Auditability is essential through all stages from plant data capture to inspection planning rationale.
Further benefits of an interactive audit team approach are the on-the-job training and transference of assessment know-how implicit within the approach.
Risk Management Measures
RBI tools, which only output a relative risk ranking, no matter how comprehensive, do not necessarily provide the solution that operators are looking for. A more holistic approach is increasingly being required whereby the operator is systematically directed through a series of risk management options. These options should not be restricted to inspection or maintenance actions but should also lead the operator to other measures such as, design or engineering modifications, operational changes, etc. The impact of each risk management action should be retained by the software so that cost-risk optimisation can subsequently be undertaken.
Formal Link to Inspection Frequency
Furthermore, a RBI tool, which only outputs a relative risk ranking, whether qualitative or quantitative, also leaves the user with the problem of selecting an appropriate and safe inspection period.
Health and safety regulations do not specifically prescribe inspection intervals. In the self-regulating environment in the UK, a competent person is expected to use judgement and experience in deciding inspection and maintenance intervals. The UK's Institute of Petroleum [12] and SAFed [10] have issued guidelines on maximum service intervals between inspections.
In spite of the above, to be effective the RBI tool should offer formal guidance on the inspection frequency based on the risk audit undertaken. Where risk management measures are incorporated, the software should accordingly be able to guide the user to the target inspection interval, based on a sound methodology.
In general, in order to determine a safe in-service inspection interval of equipment by means of a formal route, it is necessary to know the remaining life of the equipment item under the damage mechanism, or mechanisms prevailing. In cases where the damage mechanism is time dependent such that damage accumulates continuously, e.g. corrosion, creep and fatigue, then the remaining life can be assessed as long as an appropriate predictive model together with relevant materials databases, future operating conditions and current damage status are known.
Some damage mechanisms such as stress corrosion cracking often arise in an unpredictable manner. In these cases experience in similar equipment and similar service is important in assigning appropriate inspection periods.
Nevertheless, since general and localised corrosion is often the most prolific damage mechanism in industrial installations, in many cases a formal approach based on remaining life estimation is potentially very useful. Calculations should implicitly make conservative assumptions, and issues such as inspection accuracy and process stability also need to be taken into account.
In summary, a formal remaining life evaluation approach to establishing inspection intervals is potentially viable in many instances. The quantitative means to this is entirely possible and has been broadly outlined by means of reliability rule-based methods in the ASME RBI Guidelines. [4] The requirement however is to incorporate the principles of such a formal approach in a user-oriented semi-quantitative RBI route.
Case study using TWI's RISKWISE TM RBI software
RISKWISE TM is a semi-quantitative RBI assessment software tool from TWI which determines a relative risk rating, for individual damage mechanisms within equipment, by categorising the two elements of failure, generally in accordance with the requirements of API RP 580. The study comprised a pilot application of RISKWISE TM on selected equipment items in a Naphtha Hydrodesulphurisation Unit (NHD) within a Japanese oil refinery. The unit was commissioned in the early 1980s. The current inspection period (IP) for pressure equipment was generally 48 months. Piping however, was on a 24-month inspection cycle and storage tanks were inspected every 84 months.
Objectives and Scope
The objectives were to:
- Demonstrate the key steps in the RBI process,
- Suggest ways of optimising inspection plans, and
- Identify ways of reducing the risk of failure.
Table 1 Scope of study
| Item type | Total | Tank farm | Oxygen stripper | Reactor and heater | Hot/cold separator |
|---|
| Accumulator |
1 |
|
|
|
1 |
| Column |
1 |
|
1 |
|
|
| Drum |
1 |
|
|
|
1 |
| Reactor |
1 |
|
|
1 |
|
| Heater |
1 |
|
|
1 |
|
| Piping |
10 |
|
2 |
5 |
3 |
| S&T exchanger |
18 |
|
|
16 |
2 |
| Tanks |
2 |
2 |
|
|
|
| Grand Total |
35 |
2 |
3 |
23 |
7 |
Results
The results are summarised in Fig.2-4. Figure 2 shows the results of the risk audit on a typical risk matrix with likelihood categories 1-5 and consequence categories A-E.
From Fig.4 it can be seen that the computed RLIs vary from zero to 480 months. In view of the currently adopted inspection periods (IP) given above and subject to process controlled shutdowns and statutory requirements, there was clearly scope for optimisation.
Optimised inspection and maintenance plans
Details of selected equipment items, their damage mechanisms (DM), the risk audit results, revised inspection plans, and the risk mitigation recommendations are summarised below.
VAPOUR CONDENSER (BUNDLE): Carbon steel; 38°C; DMs: oxygen pitting, water side; RLI = 0 mths (IP = 48 mths), Risk Class: 5A. Resulting Focus/Defocus proposal: Replace bundle at earliest opportunity (or install air cooler); increased RLI = 48 mths (new Risk Class: 2A).
CHARGE HEATER TUBES: 9Cr 1Mo steel; 357°C; DMs: sulphidation, creep, vanadate attack; RLI = 480 mths (IP = 48 mths), Risk Class: 1E. Resulting Focus/Defocus proposal: Relax to visual inspection only at next planned shutdown; unchanged RLI = 48mths (unchanged Risk Class: 1E).
FEED TANK: Carbon steel; 25°C; DMs: general and pitting corrosion; RLI = 84 mths (IP = 84 mths); Risk Class: 2B. Resulting Focus/Defocus proposal: Internally coat tank floor with epoxy resin at next opportunity; increased RLI = 168 mths (new Risk Class: 1B)
REACTOR: P11 & 12Cr steel; 380°C; DMs: sulphidation, creep cracking, hydrogen attack, H+ embrittlement; RLI = 90 mths; Risk Class: 2E. Resulting Focus/Defocus proposal: Increase inspection interval for creep embrittlement cracking (only) to 96 mths; unchanged RLI = 90 mths (unchanged Risk Class: 2E)
HOT SEPARATOR: Carbon steel, 101°C; DMs: general/pitting corrosion, HIC, stress corrosion cracking; RLI = 90 mths (IP = 48mths); Risk Class: 2E. Resulting Focus/Defocus proposal: Defer next internal inspection by 48 mths; external UT (only) at normal inspection interval (48 mths); unchanged RLI = 90 mths (unchanged Risk Class: 2E).
FEED LINE: Carbon steel, 24°C; DMs: general/pitting corrosion; RLI = 240 mths (IP = 24 mths); Risk Class: 1D. Resulting Focus/Defocus proposal: Increase external UT interval to 48 mths; unchanged RLI = 240 mths (unchanged Risk Class: 1D)
COMBINED FEED EXCHANGER (BUNDLE): Carbon steel, 170°C; DMs: general/pitting corrosion; RLI = 48 mths (IP = 48 mths); Risk Class: 2E. Resulting Focus/Defocus proposal: Replace bundle in, e.g. Type 321 SS; increased RLI = 180 mths (unchanged Risk Class: 2E)
Summary of Benefits
The major findings and benefits of the study were as follows:
- There was scope for reduction in shutdown inspection for approximately 70% of equipment;
- Incremental run-length extension was feasible after selected equipment modifications.
The study highlighted areas where imminent risk mitigation was needed (e.g. replacement of vapour condenser bundles), on the basis of RISKWISE TM's unique Remaining Life Indicator (RLI).
Concluding remarks
- Risk-based life management is a complex process. A balanced approach to life management is to consider all political, economic, commercial, technological and human aspects.
- The benefits of risk-based methods for inspection and repair or replacement optimisation are recognised by different industry sectors. However, industry survey indicates a lack of established and documented uniform RBI policy or guidance for application throughout the industry sectors.
- Software tools for RBI tend to be qualitative or quantitative. API and ASME guidelines however are based on relatively simple qualitative risk ranking followed by exhausting quantitative routes, which often involve significant resource commitment.
- Expert judgement is essential in identifying all potential damage mechanisms. In addition to the risk audit itself; RBI assessment software should lead the audit team to consider all risk mitigation measures, as well as optimal inspection activities.
- Effective implementation of a semi-quantitative RBI tool has been presented in this paper illustrating the formal consideration of a time dimension of risk. The need to retain application efficiency and user friendliness is an import issue in RBI software tools. The concept of the Remaining Life Indicator (RLI) incorporated in RISKWISE TM has shown that the single semi-quantitative approach could be sufficient for plant-wide inspection planning as well as establishing risk management measures.
- The incorporation of fully quantitative tools in risk-based methods in most cases should be considered only as a refinement in terms of inspection planning although such approaches have their place where the remaining life is critical.
Acknowledgments
This paper is published with the kind permission of the TWI joint industry project's Group of Sponsors, who are all Members of TWI.
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