1. Introduction
1.1. Objective of Structural Integrity Management
The main objective of a Structural Integrity management system Structural Integrity to ensure that assets are in proper operating condition from implementation to disposal. Thus, for new assets, the first step is to carry out a proper and independently audited project. After installation, however, various situations may lead the asset to conditions different from those considered in the design. Proper integrity management should be able to identify such situations and assess whether, even under these circumstances, the asset remainsfitforserviceorfit for purpose, or whether immediate action needs to be taken.
1.2. Detection of violations and degradations
The identification of these design violations is usually achieved through periodic and special inspections that are capable of detecting, using techniques appropriate for each type of asset, damage or degradation that could reduce the structural capacity of the asset. In addition, reports of unforeseen situations such as impacts, overloads, operating errors, and excessive vibrations also represent ways to detect design violations that need to be analyzed.
1.3. Fitness for Service Assessment and Analysis
When any of these violations are detected, it is up to the aforementioned integrity management system to assess whether the asset in question remains suitable for operation, or whether immediate action should be taken. Therefore, fitness for service (FFS) analysis is not a specific technique, but rather a type of assessment that can use different techniques, depending on the existing failure in the component or specific situation to be assessed. In other words, this assessment is part of a failure management system, which is an important part of any asset management system.
1.4. Importance of results for decisions and planning
The results of this analysis will provide the necessary technical basis for the residual structural capacity of the asset so that the person responsible for Structural Integrity make the necessary decisions for that specific situation, maintaining the appropriate level of risk for the asset and, at the same time, avoiding unnecessary operational downtime. In addition, such decisions make it possible to change the asset's inspection plan based on an estimate of its useful life, or to reclassify it by indicating new safe operating limits for its actual condition. In some cases, the fitness-for-service analysis can also be used to extend the useful life beyond the limits initially planned in the design.
In this article, we will present some examples of fitness for service analysis for non-standard equipment and structures, summarizing the characterization of the problem, the methodology applied and the proposed actions.
2. Methodology
As mentioned above, FFS analysis does not represent a specific technique, but a type of evaluation that can encompass different techniques, depending on the situation you want to evaluate.
For some types of equipment, there are standards and technical publications that already provide for specific fitness-for-service procedures for known types of defects, such as API 579 for pressure vessels. For some mass-produced equipment, such as aircraft, the manufacturer itself already carries out fitness-for-service analyses for the defects commonly identified in inspections and provides the limits and treatments in the maintenance manuals.
2.2 Assessment of non-standard equipment and structures
However, in the case of non-standard equipment and structures in general, even though there are generic technical publications, there is no definitive standard procedure. In these cases, it is up to the Structural Integrity specialist Structural Integrity the most appropriate techniques and methodologies for assessing the residual structural capacity and/or useful life of the asset in each particular case.
2.3. Criteria for defining techniques and procedures
When defining the appropriate techniques and procedures in a fitness for service analysis of structures and equipment in general, the specialist must take into account various aspects, including:
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- Characterization of the fault (type, appearance, dimensions) or damage mechanism;
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- Structure material and its properties;
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- Type, direction and magnitude of loads;
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- Damage evolution rate (stable, variable according to environmental characteristics, variable according to loading characteristics);
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- Applicable failure modes;
2.4. Practical application and corrective measures
Based on this information, appropriate testing techniques and simulations must be carried out in order to determine the residual capacity of the structure and its suitability for service conditions. In cases where the residual capacity of the structure does not prove adequate for operation, reinforcements or restorations must be carried out in order to recover the necessary structural capacity. Until these actions are taken, in order to avoid an operational stoppage, a reclassification (de-rating) of the asset can be studied by, for example, reducing capacity or speed or limiting overloading in specific regions.
In the case of damage mechanisms that evolve over time and/or asset operation, such as corrosion degradation or the propagation of fatigue cracks, the fitness for service analysis must determine not only the current residual capacity of the structure, but also the expectation that this degradation will continue over time, in order to define the feasible deadlines for intervention in the structure. This definition provides a basis for adjusting inspection and intervention deadlines to reduce the impact on operations while maintaining an adequate level of asset risk. These adjustments depend on the damage mechanism involved, as well as the characteristics of the asset and the operation.
Corrosion degradation, for example, which involves loss of material, is generally assessed according to the rate of thickness loss calculated from the characteristics of the environment and the determination of minimum admissible limits that maintain adequate residual structural capacity. However, the way in which thickness loss progresses in the component is also dependent on the characteristics of the component itself and the operation. Cross-linked profile structures generally suffer greater corrosion at the ends, implying a reduction in the final dimensions, loss of thickness and even the shape of the profile. This change in shape can be exacerbated by local conditions of exposure to the environment (areas where water accumulates, for example, or areas with degraded protection). In serious situations, this change in shape can even alter the component's failure mode, which must be duly taken into account in the analysis.
2.5. Influence of Fault Detectability on Deadline Setting
Another aspect of great influence in determining inspection and intervention deadlines is the detectability of the failure. In the case of cracks in components subject to variable loading, which can lead to fatigue, determining these deadlines usually involves fracture mechanics analyses to calculate the speed and direction of crack propagation, starting from a certain initial length.
In these cases, one aspect that influences action planning is the inspection equipment available and the limitations of such instruments for each crack size. In different materials, the crack growth curve has different shapes. Thus, it is important to calculate the theoretical crack size to define the inspection interval and assess whether the instrument's detectability is adequate. Otherwise, an incorrect inspection result can compromise the entire analysis and the integrity management decisions based on it.
3. Conclusion: the crucial role of FFS in asset management
From reading this article, it can be concluded that Fitness For Service analysis plays a crucial role in the asset management system. It should also be noted that the study must be carried out by trained professionals, since various aspects, such as the characterization of the failure, the material of the structure and its respective properties, the type, direction and magnitude of the loads, the rate of damage evolution and the applicable failure modes are some of the aspects that must be observed during the assessment.
4. Fitness for Service Analysis is at Kot Engenharia
If you, like our more than 150 clients, are looking for specialized solutions in structural analysis or failure prevention such as deformation, vibration, and corrosion, consult our team and count on Kot Engenharia.
Since 1993, we have been offering engineering consulting services through technical studies using non-destructive testing, field instrumentation, and computational simulations (FEM, DEM, and CFD) for highly complex diagnostics in concrete and metal structures and industrial equipment.
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FAQ
1. What distinguishes a Fitness-for-Service (FFS) analysis from a conventional technical inspection?
Inspection plays a role in the initial diagnosis: it involves going to the field to identify whether damage exists and measuring its dimensions (such as the length of a crack or the loss of thickness due to corrosion). FFS analysis comes into play immediately afterward as analytical intelligence: it takes this real-world field data and assesses the structural impact of the defect. The goal of FFS is not merely to find the defect, but rather to mathematically calculate whether the equipment is still safe to operate under those conditions and to predict its remaining service life.
2. How is the FFS analysis conducted when the equipment or structure does not have a specific standard (non-standardized)?
For pressure vessels, engineering relies on well-established guidelines, such as the API 579 standard. However, for complex metal structures or custom industrial equipment, there is no definitive standard procedure. In such cases, the integrity specialist must design a customized methodology. This involves mapping in detail the material’s mechanical behavior, the direction and magnitude of actual operating loads, the rate of damage progression over time, and the likely failure modes (such as fatigue, buckling, or yielding).
3. What is de-rating (operational reclassification), and what is its practical use in asset management?
Derating is a temporary risk mitigation strategy. If the FFS analysis indicates that the degraded structure no longer has the residual capacity to support 100% of its original design load, rather than taking the equipment out of service and suddenly shutting down the plant, engineers recalculate lower, safer operating limits. This may involve reducing operating speed, decreasing the volume of material transported, or limiting overloads in specific areas, thereby keeping the asset generating value until the next scheduled maintenance shutdown.
4. How can corrosion-induced degradation alter a structure’s “failure mode”?
Severe corrosion does not merely cause uniform loss of thickness; it deforms the cross-section of metal profiles, particularly at the ends of truss structures and at points where water accumulates. This drastic geometric change can cause a component originally designed to fail due to global bending to suddenly collapse due to local buckling or shear. FFS analysis must account for this geometric change to adjust the simulation criteria to the asset’s new physical reality.
5. What is the risk of ignoring the “detectability limit” of instruments when analyzing crack propagation?
When a structure develops cracks and is subjected to cyclic stresses (fatigue), fracture mechanics is used to predict the rate at which the crack will grow. A critical error occurs if the planned inspection interval is based on a crack size smaller than what the field inspection tool (ultrasound, magnetic particle, etc.) can physically detect. If the instrument lacks the sensitivity to detect the defect in its early stages, the crack will continue to propagate invisibly on the actual graph, potentially causing the structure to collapse before the time estimated by engineers.
