Typical Damages of Aboveground Steel Tanks

Aboveground steel tanks serve as critical storage facilities for various substances, including oil and other hazardous materials. However, the operational lifespan of these tanks exposes them to harsh environmental conditions and corrosive products, leading to potential damages that can compromise their structural integrity. This article aims to explore the typical damages encountered in aboveground steel tanks, taking guidance from API 650 and real-world storage base assessments. Additionally, we will discuss additional damage mechanisms related to tank bottoms, roofs, foundations, and attached appurtenances.

I. Deviation of Design Geometric Shape:

One prevalent type of damage observed in aboveground steel tanks is the deviation from the intended geometric shape. These deviations can manifest in several ways:

a. Local Loss of Stability:

Specific areas of the tank's shell or bottom may experience instability, resulting in noticeable bulging or deformation.

b. Total Loss of Stability:

The entire tank can be compromised, leading to instability and potential structural failure.

c. Deviation to Inside or Outside of Shell and/or Bottom:

Deformation of the tank's shell or bottom, either inward or outward, can weaken its structural integrity.

d. Movement and/or Bending of Annular Bottom Plates:

Shifts or bends in the annular bottom plates can adversely affect the overall stability of the tank.

II. Typical Damage of the Tank Bottom:

Settlement of the tank bottom is a common type of damage observed in aboveground steel tanks. It can be categorized as follows:

a. Plane Settlement:

Uniform settlement across the entire tank bottom can occur, potentially leading to stability issues.

b. Irregular Settlement:

The tank may exhibit inclinations, with settlement occurring across the entire bottom or specifically along the perimeter. Damages to anti-rotation devices, supports, and equipment on the floating roof may be observed. In severe cases, supporting roof columns may detach from the bottom. Regular inspections and maintenance are crucial to address settlement issues, ensuring the tank's stability and safety.

III. Typical Damage of the Tank Roof:

The tank roof is prone to various damages, including:

a. Paint Degradation and Intense Corrosion on Roof Plates and Attached Nozzles:

Exposure to the elements can lead to degradation of protective paint coatings and subsequent corrosion on the roof plates and attached nozzles.

b. Product Leakage and Retention on the Floating Roof:

Ruptures in the roof membrane can cause product leakage and retention on the floating roof, compromising its functionality.

c. Opening of Gaps Between the Roof Seal and Tank Shell:

Gaps between the roof seal and tank shell can allow fuel evaporation and rainwater ingress, posing both environmental and structural risks.

d. Presence of Rainwater and Moisture in Pontoons of Floating Roofs:

Moisture accumulation in the pontoons of floating roofs can lead to intense corrosion, impacting the roof's structural integrity.

e. Radial Displacement of the Floating Roof and Deformation of Roof Guide Pole Seal:

Radial displacement of the floating roof and excessive deformation of the seal around the roof guide pole can impair the roof's sealing capabilities.

IV. Corrosion-Related Damage:

Corrosion is a significant challenge for aboveground steel tanks, with two primary types of damage commonly observed:

a. Regular Distribution Corrosion:

Uniform corrosion across the tank affects welds and the steel structure, weakening the tank's integrity.

b. Localized Corrosion:

Concentrated corrosion in specific areas leads to localized weakening of the tank structure.

Preventive measures such as protective coatings, cathodic protection systems, and regular inspections are essential to mitigate corrosion-related damage.

V. Typical Damage on Attached Appurtenances:

Attached appurtenances, such as stairways, ladders, handrails, and support frameworks, are also susceptible to damage. Some common observations include:

a. Paint Degradation and Corrosion:

Protective paint coatings on appurtenances can degrade, leading to corrosion.

b. Poor Quality Welding and Inadequate Weld Placement:

Improper welding, including crossing of vertical and horizontal welds and inadequate spacing from adjacent elements, can compromise the integrity of appurtenances.

c. Missing Connection Bolt Assemblies and Misaligned Elements:

Missing connection bolt assemblies on roof manholes and flanges, along with misaligned elements, can undermine the functionality of attached appurtenances.

d. Deformation of Elements:

Deformation, such as hatches, flanges, or other components, can impact the proper functioning of appurtenances.

e. Compromised Bunding Integrity:

The integrity of the bunding, the tank or tank group's surrounding ditch or embankment, may be compromised, potentially leading to environmental hazards.

Conclusion:

Throughout their operational lifespan, aboveground steel tanks are susceptible to various damages that compromise their structural integrity. Deviations from the design geometric shape, settlement of the tank bottom, corrosion-related issues, and damage to attached appurtenances are among the most observed. Adhering to API 650 guidelines, conducting regular inspections, implementing proper maintenance practices, and employing preventive measures against corrosion are essential to ensure the longevity, safety, and reliability of aboveground steel tanks in storage facilities worldwide. By proactively addressing these concerns, industries can safeguard their assets and maintain optimal operational performance without compromising safety.