Lifetime assessment of steel structures
Lifetime assessment is crucial in operation of most steel constructions, and it helps you to plan maintenance and repair, avoid failure and minimise lost turnovers.
Lifetime assessment may be approached in different ways such as assessing the lifetime of a welded joint in relation to fatigue, or estimating the time before an interior welding defect will reach a criticality that entails risk of failure. Lifetime assessment may also depend on expected corrosion rate compared to the actual corrosion rate in the (spectrum of) environments to which the construction is exposed.
Assessment of fatigue in welded joints
We offer different forms of lifetime assessments, and depending on the background for your wanting a lifetime assessment, we may help you in different ways. We have many years’ experience within assessment of the fatigue strength of welded joints and fracture mechanics analysis of welding defects.
To perform fatigue assessment of the strength of the structures’ welded joints, we have to know the stress spectrum of the welding. This may be achieved by different methods; one of them is strain gauge measurement that collects data on stresses and strains in the weld. As to strain gauge measurement, we offer both fitting, data collection and data processing.
Another method is to calculate the stresses based on measured loads, which gives a representative picture of size and number of the loads that the component/structure must withstand during one year.
If you have complicated welded joints, we also offer Finite Element Analysis (FEM) to determine stresses in the welded joint. FEM is a good solution if analytic calculation or strain gauge measurement of the given welded joint is not possible.
Information on the weld details of the structure is necessary to give you a correct picture of the fatigue life of the weld. This means that we must know the quality of the weld, and whether you have performed stress relief annealing or another form of subsequent treatment such as grinding.
We assess the lifetime of welded joints according to current standards relevant for the application. Most often lifetime assessment of welds is monitored with suitable inspection intervals to ensure that a fatigue crack has not initiated and in order to make repairs before failure occurs.
Fracture mechanics analysis of found welding defectsIf you have found defects in your welds by NDT (non-destructive testing), we offer fracture mechanics analysis of the found defects. A defective weld does not necessarily mean that repair is necessary. You can save money by letting us perform fracture mechanics analysis of the found defects, where we can estimate factors such as:
- Does the defect have a critical size?
- How fast will the defect grow to a critical size?
- Should you repair or let the defect stay unrepaired with suitable inspection intervals?
Just as fatigue assessment, fracture mechanics analysis requires knowledge of the stress spectrum in the weld. Further, you need to know the mechanical properties of the material, including the fracture mechanics properties of the material, and consider whether the environment is corrosive.
When you receive a fracture mechanics analysis of welding defects, you will, with suitable safety factors, get an estimate whether the defects are of a critical size, and whether they will grow to critical sizes during their expected lifetime. The safety factors are determined so that you get a suitably low failure probability compared to the consequences of a failure.
We use current standards within fracture mechanics analysis, and you are therefore certain that your component/structure lives up to legislative requirements and norms thus enabling you to continue production during the total lifetime.
Does a risk of high local corrosion rates exist?
The design of your turbine and foundation takes its starting point in the corrosion rates you expect of the materials and the effect of your corrosion protection. When the construction has been in service for some time, you can examine whether these assumptions were correct so that you are able to find the remaining lifetime.
Maybe incidents or changes have occurred that make the corrosion rates considerably higher than you expected.
In a lifetime assessment, we consider the construction as a whole and include its present conditions – e.g. remaining material thickness measured by ultrasound, actual environments, effect of coatings and cathodic protection, consequence of different types of local corrosion attacks etc. Corrosion monitoring is an extremely powerful tool because corrosion rates in the actual environment can be predicted with great certainty.
We can help you in all stages of the process including during design by forming the best basis of subsequent lifetime assessments, remaining lifetime extensions and advice on selection of method.