July 2022

Maintenance and Reliability

Storage tank settlement and soil-side corrosion assessment with optimized repair strategy

Aboveground storage tanks are used in several industries with different fluid services. These

Alsaiari, M., Alghamdi, F., Saudi Aramco

Aboveground storage tanks are used in several industries with different fluid services. These industrial tanks are normally huge structures built at a bigger scale to store the selected valuable fluids. However, the structures of these tanks are often thin and are subject to failure under any unexpected ground deformation.1

Most aboveground storage tanks are supported on soil, concrete slabs, gravel compaction, ring walls and pile cap foundations.2 Aboveground storage tanks are classified into two main categories, based on their operating pressure range: atmospheric and low pressure (not exceeding 15 psig), according to API-650/620 standards.3,4 Tanks usually are constructed with either metallic or non-metallic materials. However, these tanks are designed and manufactured with a flat bottom and a cylindrical vessel shape.5,6

Tanks typically have different types of roofs,5 but some are operated without roofs and others have either fixed or floating roofs.5,7 Aboveground storage tanks have two main concerns that impact their reliability: corrosion and bottom-supporting configuration. Throughout the oil and gas industry, tank bottom plate corrosion is a typical issue that causes high maintenance cost and equipment outage.8,9 Conversely, storage tank-supporting configuration failure may lead to catastrophic damages that affect the environment and human life, since they contain large volumes of hazardous products.2

Several studies were conducted to analyze and evaluate storage tank settlements and bottom plate support configuration.10,11 Tank settlement classifications, per API-653, are uniform settlement, planar tilt, non-planar settlement, shell settlement, edge settlement, bottom settlement near the shell, and localized bottom settlement remote from the shell.12 API-653 does not consider the foundation stiffness influence, plate-shell section stiffness and plate thickness, which give approximately 20% allowable edge displacement of the maximum amplitude.13

Overview and assessment of a tank failure

An aboveground storage tank used to store distilled water in a processing facility—constructed in 1940s—suffered soil-side corrosion in several locations in the bottom plate critical zone (within 3 in. away from the shell). Edge settlement was also observed, as shown in FIG. 1. Based on these findings, a risk assessment and technical evaluation based on inspection findings were deemed urgent before returning the tank to service.

FIG. 1. Tank edge settlement layout.

Magnetic flux leakage (MFL) floor scanning was conducted on the tank bottom plates. Additionally, ultrasonic thickness testing was carried out within 18 in. from the weld of the bottom to the shell joint where MFL is inaccessible. The tank bottom plates were observed with corrosion from both sides. Additionally, localized pitting corrosion was noticed close to the shell-to-bottom weld joint. However, the most severe metal losses were caused by underside (soil side) corrosion located at the critical zone.

The bottom plate’s original thickness was 6 mm, while detected thickness readings ranged from 1.6 mm–2.7 mm. These values are very low (i.e., below the minimum required thickness) and it was determined that repair was required. The root causes of this corrosion formation are attributed to an ineffective cathodic protection system and lack of drip ring. The tank was constructed with a ring wall foundation type, but that was 70 yr ago when there were no standards that mandated drip ring installation to prevent water ingress between the bottom plates and the foundation.

The settlement was evaluated structurally, particularly on the deflected bottom plate at the tank perimeter. Additionally, an integrity inspection of the bottom-to-shell weld joints was completed and found satisfactory. The tank settlement was classified as edge settlement, according to API-653.12 The tank settlement appearance was uniform, and this type of settlement does not cause any serious stresses in the tank structure.14 Moreover, most standards do not express concern about the uniform tank settlement structurally, except for the associated piping system.1 In this case example, the existing piping system had sufficient piping flexibly with no risk that might require enforcement.

Repair methodology

A specialized engineering team evaluated the tank settlement structurally, considering all involved parameters (tank service age, type of service, settlement severity, etc.). It was recommended to continue operating this tank with close monitoring (i.e., measuring the settlement during every operation outage window). Due to the corrosion severity in the tank bottom critical zone, bottom plate replacement was crucial for long-term safe operation. A specific repair procedure was created to replace all corroded bottom plates that were below the minimum required thickness. Replacing the tank bottom plate is very demanding and requires installing an additional supporting system to prevent shell deformation after its cutting. Because of this, external supports were welded to the tank shell to stiffen the tank shell, as shown in FIG. 2.

FIG. 2. An example of a support welded to the tank shell.

The tank bottom plate replacement sequence was implemented with a skip-on-plate replacement technique by replacing one plate and keeping the other. To prevent water ingress underneath the bottom plate, especially at the critical zone, a drip ring around the tank was installed, as seen in FIG. 3.

FIG. 3. A typical example of an installed drip ring around a tank.

Storage tank and asset integrity management systems

The use of asset integrity management systems is widespread in the oil and gas industry to integrate and enhance asset performance according to its desired design functionality. An asset integrity management system is defined as a system or sets of systems that protect life, environment and property by constructing a well-maintained management environment that governs the asset’s main critical elements.15 The main asset’s critical elements comprise process, personnel, practices and methodologies that ensure asset integrity performance to prevent accident. Successful implementation of an asset integrity management system will ensure the safe and reliable operation of processing hazards fluids. One of the main effective integrity management models used in the oil and gas industry (among others) is called the Swiss Cheese Model. This model is divided into barriers with classification categories: prevention, protection and escalation control. These barriers act as a first defense to prevent failures. Storage tanks fall under pressure vessels below the process containment barrier, as shown in FIG. 4.

FIG. 4. The Swiss Cheese Model Risk Escalation.

Storage tank integrity is monitored with integrity performance standards (IPSs), where sets of assurance requirements are assigned to ensure the integrity of the pressure envelope. However, the IPS will be ineffective unless well-designed assurance requirements and verification tasks are adequately allocated. In this storage tank case study, the drip ring installation was not included in the minimum assurance integrity requirements. This allowed the water to accumulate below the tank bottom plates—this failure was not anticipated due to an outdated IPS.


This article describes a systemic assessment and repair methodology for an aboveground liquid storage tank. This tank was in service for approximately 70 yr in well-maintained condition. Recently, the tank experienced combined damages mechanisms: settlement and soil side-plate corrosion. The tank has a uniform edge settlement that poses no risk to the connected piping system due to its sufficient flexibility. On the other side, the tank suffered from severe soil side-plate corrosion that mandated plate replacement to operate the tank safely.

These damages necessitated a well-structured assessment and repair strategy to identify the most affordable repair methodology, considering the tank service age. The tank was evaluated visually along with MFL and ultrasonic examinations. The ultrasonic testing report demonstrated a metal loss of 50%–75% from the original thickness due to inadequate cathodic protection and lack of drip ring where water ingress underneath the tank plate occurred.

Adhering to API STD 653 requirements, a specific repair approach was proposed to rehabilitate this corrosion while maintaining the existing tank condition. The repair procedure focused on the tank bottom critical zone due to its impact on the tank shell stability. As a result of this assessment, a new drip ring and cathodic protection system were installed and tested to verify their effectiveness. A major finding of this study was the benefit of enhancing the asset integrity management system program by updating the integrity performance standards through revamping the minimum assurance requirements and verification tasks that are reviewed quarterly by the process owner and field inspection team to prevent future such failures.  HP


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  4. American Petroleum Institute (API) Standard 620, “Design and construction of large, welded, low-pressure storage tanks,” 10th Ed., 2004.
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  12.  American Petroleum Institute (API) Standard 653, “Tank inspection, repair, alteration and reconstruction,” 5th Ed., 2018.
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  15.  Saudi Aramco, “Asset integrity management system (AIMS) manual,” online: https://www.scribd.com/document/488069802/Asset-Integrity-Management-System-AIMS-Saudi-Aramco-pdf.

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