There are a variety of conditions an in-service component (e.g. vessel, tank, piping) can be found in. The purpose of Fitness-for-Service (FFS) is to evaluate the integrity of an in-service component given a certain degraded condition and rate it for future service considering potential for any additional degradation. A degraded condition does not have to just be based on corrosion. Sometimes a component can experience a large deformation due to unexpected one-time loads resulting in stresses greater than yield. When such a large deformation event occurs it is important to inspect the vessel to make sure no cracking occurred during the deformation event. If it is found that the impacted area is defect-free then the next step is to determine if the component can operate in the deformed shape.
One example of a non-corrosion related degraded condition is a bulged tank. Figure 1 shows an example of a tank that bulged due to an over-pressure event. This tank was modeled using the FEA program Abaqus. The general bulged shape in the model can be seen in Figure 2. An elastic-plastic analysis was performed with the mesh shown in Figure 3. The global model used shell elements to save on computational time without sacrificing accuracy for the tank shell while the sub-model on the critical nozzle was created using 3D brick elements for maximum fidelity. This analysis was able to demonstrate that the tank was qualified for continued operation – even with an additional corrosion allowance.
Significant schedule and cost savings can be realized if analysis is performed to demonstrate the current and future integrity of components that otherwise would be replaced based on differences from their installed condition.
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