Fitness-for-service assessments
- half a century of consultancy and training at TWI
The SITG department within TWI offers consultancy services, training and advice on the application of fitness-for-service. Ian Partridge reviews how TWI can help you in this crucial field.
In industry, as competitive pressure grows and existing equipment ages, fitness-for-service (FFS) assessments are being used as part of the plant life management process as a means of increasing availability, reliability, efficiency and safety. Today the classic plant life cycle, Fig.1, is being extended and optimised as companies strive to get the maximum value out of their ageing assets. Various asset management techniques are used for this purpose and FFS assessment is one of the most common.
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Fig. 1. Plant life cycle in terms of failure frequency versus age |
FFS assessments can be used at any stage of the life of a structure:
- In the concept and design phase, material property requirements can be set.
- In the construction phase, workmanship specifications can be complemented or replaced, eg if flaws more severe than stipulated in the fabrication code are found, rejection or acceptance can be rationalised using fitness-for-purpose principles. In this way, many unnecessary, costly and potentially detrimental repairs may be avoided.
- During routine inspection, the fitness-for-purpose concept enables decisions on the need to repair defects found, and timing of such repair to be made.
- Towards the end of the design life of a structure, fitness-for-purpose methodologies, together with more accurate inspection techniques and weld quality improvement methods, can be used for re-assessment and re-qualification for continued operation beyond the initial design life, with substantial economic benefits.
FFS assessment is not a new field it is also referred to as fitness-for-purpose assessment, engineering critical assessment as well as remnant life assessment. TWI has a long history in this area with its involvement in the development of several elements of the underlying methodologies, and standards ie BS 7910:1999 and its predecessors PD 6493:1991 and PD 6493:1980. These cover the assessment of welded structures like pressure vessels, pipelines and offshore structures. With the recent introduction of the American Petroleum Institute (API) recommended practice 579 'Fitness-for-service' in 2000, the subject has become a hot topic within the pressure equipment industry.
The benefits of FFS
Over the years FFS assessments have been conducted throughout a wide range of industries. The main drivers for conducting FFS assessments are:
- Improving equipment safety;
- Improving equipment availability;
- Optimising inspection intervals;
- Determining equipment residual life;
- Extending equipment life;
- Integrating lessons learnt from previous equipment failures and;
- Re-rating or operation change;
Any unplanned shutdown of a manufacturing plant is expensive in terms of both loss of production and manpower required to solve a particular mechanical integrity issue. FFS assessments, especially at the lower 'screening assessment' levels, can quickly determine whether equipment is safe for immediate return to service. Higher levels of assessment can be complex and time consuming but still generally require less time than the ordering, supply and commissioning of replacement equipment.
In-service damage mechanisms
During equipment life various service conditions can result in short to long-term damage. The design process normally allows for service-induced damage over a component's life, eg via a corrosion allowance on shell thickness. However, factors such as emergency operational requirements, upsets, operation beyond design life and the presence of previously unanticipated damage mechanisms can cause equipment to become unsafe.
For failure to occur, a detrimental combination of applied stress, damage size and the material's resistance to failure is required, Fig.2.
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Fig.2. Fitness for purpose assessments link stress, defect size and fracture toughness |
Damage to a component can occur in many forms such as:
- Mechanical damage
- Overload
- Overheating
- Corrosion
- Erosion
- Fatigue
- Creep
- Hydrogen
These mechanisms may be grouped into four primary categories, Table 1.
Table 1 Categorisation of in-service damage mechanisms
| Metal Loss | Crack-like flaws | Geometrical defects | Metallurgical flaws |
|---|---|---|---|
| General (uniform) corrosion | Fatigue Cracks | Dents | Toughness reduction |
| Crevice corrosion | SCC | Gauges | Strength reduction |
| Pitting corrosion | Planar fabrication flaws | Out-of-roundness | Corrosion resistance reduction |
Often, a combination of damage mechanisms needs to be considered. For example, a fabrication flaw may initially grow by fatigue to a size where fracture, gross yielding or leakage can occur.
FFS assessment procedures
A FFS assessment is usually defined as a multi-disciplinary engineering analysis of equipment to determine its fitness for continued operation. Worldwide regulatory requirements demand that the assessment must be based on recognised and generally accepted good engineering practice.
Research conducted and knowledge gained during the past 50 years have led to the formulation of international standards and procedures for conducting FFS assessments. The following are a selection of major published standards and procedures in current use:
- BS 7910:1999: 'Guide on methods for assessing the acceptability of flaws in metallic structures', British Standards Institution, 2000.
- ANSI/API RP 579: 'Fitness-For-Service', American Petroleum Institute.
- ASME B31.G / Modified B31.G (RSTRENG): 'Manual for determining the remaining strength of corroded pipelines', American Society of Mechanical Engineers.
- SINTAP: 'Structural integrity procedures for European industry', Structural Integrity Procedure for Europe.
- R6 procedure - R/H/R6: 'Assessment of the integrity of structures containing defects', Nuclear Electric
- R5: Assessment procedure for the high temperature response of structures, Nuclear Electric
The choice of standard is largely dependent on regulatory applicability and industry sector. The major driver for procedural choice is often driven by the type of in-service damage encountered.
BS 7910 covers assessments of crack-like flaws, fatigue, creep, local metal loss and some geometric issues for metallic structures. API 579 covers assessments of metallic pressure equipment susceptible to brittle fracture, crack-like flaws, fatigue, general and local metal loss, geometric issues and fire damage. ASME B31.G covers the assessment of local metal loss for pressure piping. R6 covers assessments of crack-like flaws and fatigue in metallic structures and R5 covers the assessment of high temperature flaws in metallic structures.
There are also many other procedures derived by individuals or single organisations for general use. However, within the pressure equipment community the two most commonly used standards are BS 7910 and API RP 579.
For more information please contact Ian Partridge ian.partridge@twi.co.uk.
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