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Assessing the corrosion degradation of aboveground tank bottom plates is a critical challenge for the industry. Internal inspections are a useful way of assessing the integrity of assets but might severely impact normal plant operation. In 2006, Chang et al. conducted a study on storage tank accidents and concluded that 74% of reported accidents occurred in petrochemical refineries, and 85% of them had caused fire and explosions.
Aboveground storage tanks require regular inspections of possible corrosion attacks on the tank bottom which typically calls for internal access leading to downtime and increased operational cost. A failure probability-based inspection approach utilizing non-intrusive technologies is the ultimate goal for operating above ground storage tanks in the most cost-beneficial way.This article describes a novel approach that combines ElectroMagnetic Acoustic Transducer (EMAT) technology with Finite Element Method (FEM) modeling for the detection and prediction of corrosion growth and the remaining structural strength of the tank while remaining in service.The tank bottom is scanned from the outside by EMAT technology. The resulting B-scan with identification of the corrosion attack locations is post-processed and mapped on a virtual twin model of the tank. The corrosion conditions at the bottom tank plate are determined based on polarization measurements in a drain water sample taken from the tank and inserted into the model. A 3D FEM model with intelligent high-resolution meshing around the defect areas allows calculating the current distribution and thus estimating the corrosion rate at the defect locations. This model with high-resolution meshing is then also further used for calculating the mechanical stress on the tank bottom while accounting for the wall thickness loss at the defect locations.Periodic inspections, combined with virtual twin model simulations, allow for monitoring the structural health of assets and optimizing a failure probability-based inspection strategy.
In industrial plants such as oil & gas and chemical plants, the plant piping is covered with insulative materials such as mineral wools and metal cladding for thermal insulation. The piping under insulation is subject to more severe corrosive environment than that exposed to the outdoor, due to rainwater entering through the cladding joints and condensation caused by temperature fluctuation. In addition, since the piping is covered with the insulation materials, it is impossible to identify the corrosion from the outside, increasing the risk of leakage accidents due to delays in corrosion mitigations.
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Mixed metal oxide (MMO) is widely used as anode material for the cathodic protection of storage tank bottoms due to its advantages such as low consumption rate long service life and ideal current distribution. However with the elapse of service time the MMO CP systems have been found with such issues as low polarization level and uneven potential distribution threatening the operation safety of storage tanks. In this study numerical modeling and subscale experiment were chosen to clarify the main factors responsible for the aforementioned issues. The results indicated that the inadequacy of polarization and the lack of uniform potential distribution were related to a combination of multiple factors including the anode-to-tank bottom distance the impressed current level and the number of drain points. On the basis of these systematic analyses this paper conclusively proposes certain design principles and maintenance suggestions for the cathodic protection system of tank bottoms.
The Brazilian cost of corrosion was estimated at 3% of the GPD in 2018, that percentage is equivalent to approximately $US 49 billion, according to an ABRACO(1) journal released in 2020.1 It is estimated that from this cost $US 19 billion could have been saved through anticorrosive actions. In another research conducted by the EPRI(2) the results showed that at least 22% of corrosion costs could be avoided through adequate mitigating actions.2