Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

Fitness for Service

TWI has a long history in the area of fitness-for-service (FFS) and flaw assessment, having been at the forefront of the development and experimental validation of modern FFS methods.

Fitness for Service (FFS) is a standard and best practice used in the oil and gas industry to determine the fitness of in-service equipment for continued use.

Our experts are highly experienced in the development and application of FFS techniques, particularly for the avoidance of brittle and ductile fracture and also general and local metal loss. TWI can help solve problems that may arise throughout the lifecycle of a component – from design to decommissioning – minimising risk and cost.

TWI staff are actively involved in the ongoing research and development of industry standards such as British Standard BS 7910 ‘Guide on methods for assessing the acceptability of flaws in metallic structures,’ API Fitness-for-Service: API 579-1/ASME FFS-1 and the UK nuclear industry’s fracture assessment code R6 ‘Assessment of the integrity of structures containing defects.’


When are FFS assessments used?

Fitness-for-service assessments evaluate the structural integrity of components and their suitability for continuous service. Procedures such as BS 7910:2013, API 579/ASME FFS-1, ASME B31G, DNV-OS-F101 and FITNET enable the integrity of critical pressure components and welded structures to be assessed against different failure modes, using a validated engineering approach.

An FFS assessment provides a quantitative measure of the structural integrity of a component containing flaws. Standards used to carry out these assessments provide guidelines which can be used to make run-repair-replace decisions, assisting plant management in identifying appropriate mitigation actions to ensure that the component can be operated safely.

The method can be used to support design, fabrication, operation, change of service and life extension programmes, and is employed widely in a range of industries, including  power generation,  oil and gas, chemical processing and aerospace. Assessments of this type are widely used as part of the plant lifetime management process, to confidently increase availability, reliability, efficiency and safety.

Any unplanned shutdown of a manufacturing or process plant is expensive in terms of both loss of production and the manpower required to solve a particular mechanical integrity issue. Fitness-for-service assessments, especially at the lower ‘screening assessment’ levels, can quickly determine if 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.

TWI resources

Through established multidisciplinary teams, with specialist knowledge in areas of  fracture mechanics,  fatigue,  finite element analysis, materials, inspection and welding, TWI is able to provide industry-leading services to our international network of Industrial Member companies. We have carried out FFS assessments on pressure vessels, slug catchers, storage tanks, pipelines, piping system and high-temperature equipment in accordance with industry-accepted standards.

TWI's wide-ranging expertise is regularly used to develop non-standard testing and assessment methodologies, conforming to the API 579/ASME FFS-1 Level 3 classification. This capability allows us to take on and solve problems where non-standard geometry, unusual loading, or exotic materials mean that standard FFS approaches are unsuitable.

TWI's expert staff are also available to conduct FFS training, either through our in-house training school or as a bespoke programme, tailored to the needs of individual clients.


  • Support design, fabrication and operation
  • Plant life management
  • Improve equipment availability
  • Optimise maintenance and inspection scheduling
  • Improve equipment safety
  • Determine equipment residual life
  • Integrate lessons learnt from previous equipment failures
  • Life extension programmes
  • Re-rating or operation change


For more information, please email

For more information please email: