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Engineering Critical Assessment - Fracture Analysis

An engineering critical assessment (ECA) – also known as fracture analysis – is a fitness-for-service procedure that uses fracture mechanics principles to determine the defect tolerance of safety critical items.

When evaluating the integrity of structures such as pipelines and pressure vessels, platforms, rigs and wind turbines, an ECA enables the user to make informed and confident decisions on the most appropriate remedial measures to take.

An ECA is used to decide whether a given flaw is safe from brittle fracture, plastic collapse, fatigue crack growth or creep crack growth under specified loading conditions. It can therefore be used:

  • During design, to assist in the choice of welding procedure and/or inspection techniques.
  • During fabrication, to assess:

    a) the significance of known defects which are unacceptable to a given fabrication code; or 
    b) the maximum critical flaw size, minimum fracture toughness or maximum operating stresses.
  • During operation, to assess flaws found in service and to make decisions as to whether they can safely remain, or whether down-rating, repair or replacement are necessary.

There are standardised methods for ECA and these are now used routinely by the oil and gas, nuclear, aerospace, petrochemical and power industries to determine the safety of their structures. TWI has been deeply involved in the development of methods for conducting ECAs for more than 40 years, and can offer this service to its members on a consultancy basis. Our broad range of expertise and facilities, including advanced NDT, fatigue and fracture testing allows us to gather in-depth data as the basis for our ECAs.

Expertise

Standards

ECA methods have been standardised in codes like 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”. TWI has a proven track record in using these internationally recognised fracture mechanics procedures, but can also devise ad hoc solutions to particular problems, such as using specialist FE modelling or full-scale testing.

TWI experts are actively involved in the research and development of ECA standards including the recently updated BS 7910:2013. This means that our fracture mechanics engineers have an in-depth knowledge of ECA codes and can provide a prompt and accurate response to fracture-related problems.

Software

To support our ECAs TWI has developed CrackWISE®, a decision-support software designed to assist engineers in evaluating the integrity of pipelines, pressure equipment and structures containing flaws, in accordance with BS 7910.

Our 'Structural Integrity Assessment and Practical Application of BS 7910 Procedures for the Assessment of Flaws in Metallic Structures' training course introduces the concept of ECA and provides practical training in fracture and fatigue assessments using CrackWISE® software.

Finite Element Analysis

Modelling techniques are sometimes needed to augment these standardised methods such as Finite Element Analysis (FEA) to determine the crack loading and collapse loads of cracked structures undergoing complex modes of loading.

TWI uses FEA to determine crack tip constraint in test specimens and for cracks in structures to determine whether lower constraint specimens can be used to measure toughness. FEA is also used to make predictions of welding residual stresses and to predict their contributions to crack tip loading.

Aggressive environment

The ECA of pipelines and other structures exposed to aggressive environment is complex due to the lack of understanding of the behaviour of material under such conditions. We have established expertise in understanding materials’ response to external loading while exposed to aggressive environment, and carry out ECA taking into account the adverse effect of environments using our extensive bespoke testing facilities. As the methods for the majority of such tests are not yet standardised, TWI has developed its own testing procedures based upon long-term testing experience in this area.

Benefits

The integrity of pipelines, plant, equipment and structures is vital to ensure a continued, safe and economic operation. Flaws such as cracks, welding defects and corrosion damage can occur during manufacture or service life. For safety critical items like pipelines and pressure vessels, platforms, mooring chains and risers, rigs and wind turbines the failure of a single component due to the presence of a flaw can threaten human life, as well as presenting severe economic and environmental consequences.

Other flaws may be harmless, as they will not lead to failure during the lifetime of the component. Replacement or repair of such 'insignificant' flaws is unnecessary and economically wasteful. ECA enables us to make this assessment of whether a known flaw is 'critical'.

Case studies:

FAQs:

For more information, please email contactus@twi.co.uk.