As previously discussed in the article FFS Analysis – what is it?, fitness for service assessments play a crucial role in an asset management system. These studies aim to assess whether a structure or equipment is fit for operation, even in the presence of damage or degradation.
The aim of this article is to present some case examples of the application of FFS analysis. See below:
Structures affected by fire - Steel
A relatively common accident that can lead to serious consequences is fire in structures. In this type of situation, it is common to see damage to the regions most affected by the high temperatures, as well as the redistribution of stresses to adjacent elements, due to the degradation of stiffness in critical regions. It is therefore necessary to assess the residual structural capacity of the asset, based on the extent and intensity of the damage, and to define its necessaryfitness for service.
Figure 1 shows the situation of a metal belt conveyor structure after a fire has occurred on the belt.
Figure 1: Situation of the structure after the fire.
In this event, the initial characterization of the damage involved classifying the structure into macro-regions in terms of its exposure to temperature, according to the consequences caused to the materials. The elements were assessed through visual inspections, observing aspects such as the presence of paint on the profiles, possible paint degradation, flaking and deformations. Where geometric distortions were identified, replacement of the component was recommended.
In addition, non-destructive tests were carried out to determine the consequences of the high temperature on the elements. One of the tests carried out was the surface hardness measurement, shown in Figure 2, which allows the measured values to be correlated with the material's breaking strength limit. In order to quantify thickness losses due to flaking caused by the fire, thickness measurements were also taken using ultrasound.
Figure 2: Surface hardness measurement (a) and ultrasound thickness measurement (b).
Once the damage caused by the high temperatures had been identified and quantified, the losses in thickness and strength were taken into account in a finite element computer model used to assess the adequacy of the structure, which indicated the need for reinforcements/replacement of some components. To ensure a proper repair procedure for the asset, this model was also used to plan the component replacement procedures. An example of a study to determine the feasibility of removing elements is shown in Figure 3 and Figure 4. The analyses indicated that even with the removal of structural elements, the utilization rates of the rest of the structure were within the permissible range.
After carrying out the necessary repairs to the critical points identified, the structure was released for use.
Figure 3: Evaluated disassembly sequence.
Figure 4: Result of the structural analysis with the degraded structure and elements removed.
Thus, the fitness for service assessment was used to enable the planning and execution of all the other works to revitalize the structure affected by the fire, clarifying the risks involved in the procedures and enabling the necessary preventive measures to be drawn up to guarantee the safety of the people and assets involved in the process.
Structures affected by fire - Concrete
Fire can also lead to deterioration and damage to concrete structures. In this sense, a fitness for service analysis should be carried out, in a similar way to that shown for metal structures, to assess the adequacy of the structure or the need for reinforcements.
When it comes to identifying damage, the effects of fire on concrete can be characterized by a change in color. In addition to this visual alteration, there is a loss of resistance which is directly proportional to the temperature to which this type of structure is subjected. In general, failure is expected when the temperature reaches around 600°C, when the aggregates expand and internal tensions develop that fracture the concrete. The assessment of physical damage and degradation of properties should also be validated by means of non-destructive tests and extraction of cores, for proper consideration in computational structural analysis.
Loss of material due to corrosion
Corrosion degradation is one of the most common damage mechanisms in port environments. The damage evolves over time, generating material loss and reducing the structural capacity of the component.
The example presented here involves the fastening of a ship unloader tie rod, which Structural Inspection significant corrosion. In this case, material loss occurred due to a reduction in thickness in part of the component. As this component is subject to tensile loads during operation, this reduction is directly related to a loss of structural capacity. In this case, however, due to the location of the corrosion, the geometry of the component was slightly altered, generating a redistribution of stresses with stress concentration in the affected area, further accentuating the non-compliance.
As this is a degradation mechanism that evolves over time, the corrective measure must be carried out within a period compatible with the evolution of the manifestation and residual structural capacity. A conservative analysis was carried out, considering the critical situation of reduced thickness in the affected region, which still indicated approval for the planned operation.
This result benefits the operation in two ways. Firstly, by allowing scheduled intervention in the asset, avoiding unforeseen downtime. Secondly, by allowing only the recovery of the component and its protective layer by painting, without the need to completely replace the component to recover the design thickness, which would represent a significantly more costly and time-consuming intervention on the asset.
Figure 5: Corrosion with severe mass loss on bonding plates.
Figure 6: Result of static analysis fitness for service– Defect 14.
Structural deformation
Structural deformations are damages commonly observed in Structural Integrity inspections. It is a very broad term, which must be treated differently depending on the nature of the deformity, type of component, and the stresses acting on it. Elastic deformations, in general, do not represent structural problems, as long as they do not cause second-order effects that are not accounted for in the structure, nor cause unforeseen interference or impacts. On the other hand, plastic deformations lead to stress redistribution and local changes in material properties, representing a more common cause for concern.
In the example shown here, there is a localized plastic deformation in a ship loader boom, caused by impact, which also generates an angular deformity of the boom as a whole. In this case, the fitness for service assessment must include checks for both, which have different impacts on the asset.
Figure 7: Deformation on the lower table of the equipment's boom.
For local plastic deformation, the assessment seeks to reproduce the observed level of deformity in the computer model, computing the distortions due to the impact plus those resulting from the planned operation. In addition to the maximum admissible limits, which vary with each material, the analysis must also consider whether the damage has a tendency to propagate (which can lead to failure due to cyclic plasticity) or to stabilize after the stresses are redistributed and new loads are applied, which can affect the deadline for intervention, if necessary. The state of residual stress in the component can also affect the possibility of panel instability, which should also be studied.
Analysis of the entire superstructure is recommended in the case of global torsional deformation of the boom, since this implies an imbalance between the loads on the tie rods.
In this example, the level of stress expected in the plastic deformation region, in some specific operating situations, exceeds the recommended limits for the material, indicating the need for repair. Correction using the hot mechanical straightening technique was suggested.
Figure 8: Finite element model of the equipment with implementation of the defect.
Figure 9: Static stress analysis after implementing the defect in the model.
Trinca
One of the main initiators of the fitness for service study is the identification of structural cracks, the main concern with this damage mechanism being possible propagation to component failure. In these cases, based on the characterization of the defect identified (dimensions, location, shape), a fracture mechanics assessment is carried out, taking into account the geometry and loads expected on the component, to determine the intensity of stresses at the crack tip and its tendency to propagate.
If a tendency to spread is identified, the analysis also seeks to determine the speed and direction of this development, as well as the critical length, in order to define the time available for intervention and the best actions to be taken.
In the example below, a crack was identified in the connection of a ship unloader tie rod, which is a critical component for the integrity of the asset. Due to its location, the repair intervention is costly and time-consuming. Based on the characterization of the discontinuity and the properties of the structural material, a fracture mechanics study was carried out to determine the expected time until failure, at different operating rates, allowing the person responsible for the asset to plan the maintenance stoppage more appropriately, based on operational demands. Until the repair was carried out, the growth of the defect was monitored on a daily basis.
Figure 10: Identification of a crack in a ship unloader tie rod.
Figure 11: Calculation of crack propagation for various operating rates.
Railroad
The second example of the fatigue damage mechanism concerns railway components. In this case, the client identified a large number of cracks in railway bogie sleepers, leading to the scrapping of the components.
Most of these sleepers had been in operation for decades, indicating that the cracks were in fact related to an end-of-design-life and it could therefore be expected that other cracks would appear in the other hundreds of these components that remained in operation. Therefore, due to the large number of assets and the inherent limitations of the railroad's downtime, it would be beneficial for the client to identify an admissible timeframe for replacement, based on the characteristics of the discontinuities observed, thus creating a criterion for scrapping these sleepers. With this, replacements can be carried out on a scheduled basis, minimizing the impact and without the need for early replacement of components that still have adequate residual capacity.
In order to determine these scrapping criteria, a finite element computer model of the element was created, based on a 3D scan of the part's geometry. The sleepers were also instrumented with strain gauges to acquire the acting loads during a journey of approximately 1,000 km. The measured loads were treated statistically and applied to the structural model in order to calculate crack propagation using fracture mechanics theory and assess the component'sfitness for service for each crack configuration. The results were then compiled to draw up the criteria and scrapping plan for the crossbar.
Figure 12: Location of the extensometry points on the beam.
Figure 13: Example of crack configuration evaluated to define scrapping criteria.
Conclusion
From these examples, it can be concluded that FFS analysis has a wide range of applications, emphasizing the importance of these assessments in asset management. From metal and concrete structures affected by fires, to components subject to corrosion, deformation and cracks, each case highlights the need to assess the residual operating capacity of assets even in situations of degradation.
FFS analysis makes it possible not only to identify the severity of damage, but also to determine feasible deadlines for interventions, minimizing unexpected downtime and safely extending the useful life of components. The use of finite element computer models, non-destructive testing and detailed inspections are key to drawing up repair and replacement plans, ensuring the continuity of operations without compromising the integrity of the assets.
Thus, this type of study has established itself as an essential tool for strategic planning for maintenance and improvement of resources in industrial and railway systems. Consult our team and learn about our services!
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