Frequently Asked Questions
How failure occurs
Fortunately, catastrophic structural failures are rare. However, when structures such as pressure vessels, storage tanks or pipelines fail, the ramifications can be extensive, in terms of human injury, together with financial and environmental damage. There are failure modes that occur instantly after installation, such as buckling, overload and fast fracture. Other failure modes only occur after a period of time in service, such as fatigue, corrosion, creep, stress corrosion cracking and hydrogen embrittlement. In order to prevent failure there are a number of approaches that can be adopted.
Most failures, and almost all installation failures, can be prevented by ensuring that the design, build, maintenance and inspection of the pipeline or pressure vessel are carried out to a recognised Code or Standard. This may include requirements for post-weld heat-treatment, proof testing and welder and weld procedure qualification. The general approach is to ensure adequate material toughness; design to prevent high stress; to stress relieve thick sections; to fabricate and inspect using qualified welders and procedures; to minimise the incidence of defects, and then proof test.
Proof testing is a traditional method for demonstrating that pressure vessels and pipelines do not contain flaws that can initiate catastrophic failure in service. The vessel is pressurised with water to above the maximum service stress, and if the component survives then the service conditions will be safe. However, failures of large components during proof testing can be very costly, and this method will not take account of any crack growth that occurs during the life of the component.
Failures during service can be due to unforeseen or unaccounted-for cyclic stresses or environmental conditions, causing fatigue or corrosion related problems. Often these issues will be addressed at the design stage. However, cracking that has occurred during service can sometimes be analysed using an Engineering Critical Assessment (ECA).
This is an analysis, based on fracture mechanics principles, of whether or not a given flaw is safe from brittle fracture, fatigue, creep or plastic collapse under specified loading conditions. An ECA can be used during design of a pipeline or plant, to assist in the choice of welding procedure and/or inspection techniques. It can also be used during fabrication, to assess the significance of known defects which are unacceptable to a given fabrication Code, or a failure to meet the toughness requirements of a fabrication Code.
ECAs can also be used during operation, to assess flaws found in service and to make decisions as to whether they can safely remain, or whether down-rating/repair are necessary. The ECA concept (also termed 'fitness-for-purpose analysis') is widely accepted by a range of engineering industries. If the standard ECA cannot demonstrate that a structure is safe then there are other options. It could be assessed using more advanced techniques such as probabilistic analysis, crack arrest or leak before break. Alternatively the structure can be repaired, replaced or downrated, or the operating temperature and/or environment changed.
Fatigue improvement techniques
Fatigue improvement techniques can be used during service to extend a component's life. Small flaws that survive the proof test may grow under cyclic loading in service to an extent that they can cause failure of the component during its lifetime. Fatigue crack growth can be prevented or controlled by the use of standard fatigue design methods, or an ECA can be used for flaws found in service. Removing tiny non-metallic intrusions from the weld toes by methods such as toe grinding, and putting the weld into local compression by peening will also improve the fatigue life.
How can welded joints be assessed?
Engineering critical assessment (ECA).
Are there any methods of improving the fatigue strengths of welded joints?
The following item is only available to TWI Industrial Members:
Catastrophic failures of steel structures in industry: case histories.