Structural Integrity Assessment

Structural integrity assessment of mechanical and pressurised equipment can be undertaken to demonstrate integrity at the design stage, throughout operating life or following abnormal loading conditions.

In addition to experience in the use of assessment methods based on the design-by-rule approach, our engineers have extensive expertise in the use of finite element stress and thermal analysis techniques, fatigue life prediction and defect assessment methods which can be used, for example, to formulate safety cases for economical plant life extension.    While fatigue life assessment on new plant is most often undertaken using rules in ASME VIII-2 Part 5 or PD5500 Annex C, fitness-for-service and defect assessment of existing structures is normally carried out in accordance with established methods in ASME FFS-1 / API 579 & BS 7910, in which the higher 'levels' of assessment can provide increasingly accurate and less conservative results.

Two areas in which the use of integrity assessment methods can be of benefit are the re-rating of existing equipment and in plant life assessment and extension.

The need for re-rating of existing equipment can be driven by any one or a combination of the following:

  • operational requirements, which may require an increase in the operating envelope to achieve improved throughput
  • the need to qualify aged plant against modern design codes, when the code used for the original design work is outdated
  • the need to assess the integrity of plant which has suffered mechanical and/or metallurgical damage

Such work is sometimes required by third parties as part of the safety case submission required to grant a licence for plant operation. FCL’s approach to re-rating, employing a combination of established design code rules and the application of design-by-analysis methods, together with their experience in the presentation of such work to third parties, can provide a cost effective alternative to equipment replacement and/or modification.

The requirement for life assessment can arise in the original mechanical design specification, as a result of changes in operating conditions during normal plant life, or as a result of adverse findings from routine inspection. 

Fatigue life assessments can be carried out on the basis of established design code methods using cyclic stresses calculated using code, classical or design-by-analysis methods, or in line with rules based on experimental work. While the design of new plant for cyclic service is covered as part of FCL’s design services capability, the application of fatigue life and defect assessment methods on existing plant can offer a cost effective alternative to rectification or equipment replacement. 

FCL’s structural integrity assessment service also allows the significance of defects found during routine non-destructive examination or changes in cyclic operating conditions to be assessed. 

The increasing use of fitness-for-service and defect assessment procedures such as ASME FFS-1 / API 579 & BS 7910 is evidence of the economic benefits that can be achieved using this approach, by allowing plant operators to defer repair or replacement of major items.