typical damages of above ground steel tanks

8/8/2019 Typical Damages of Above Ground Steel Tanks

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INSPECTION OF ABOVEGROUND STEEL TANKS

The vertical cylindrical steel tanks for oil and oil products storage are dangerous facilities. During the

long period of their exploitation the different process decrease tank security and increase the risk of

accidents. For example such processes are settlement of foundation, corrosion, fatigue of material,deviation of the construction in comparison of project form .

Accidents with steel tanks cause big financial loss (the product stored in the tank is much more

expensive then the facility itself), the pollution of environment, the risk of fire and victims.

The periodical inspection and diagnosis is a must in order to decrease the risk of partial or full

distraction of tanks and to avoid all the problems that are the consequences of destruction. The

inspection and diagnosis include several activities, related to the inspection of tanks assessment of

the fitness of different elements for further exploitation, determination of zones joints, connections

or construction elements in which the repairing works are a must, determination of safe way of

exploitation assessment of residual resource.

According to specialists that made inspection of aboveground steel tanks, it is necessary to be

carried out regular inspections of steel tanks in service. Various ways of tanks inspection with

specific scope are:

a) external inspection it could be done by personnel who exploits the facility and who is especially

trained for this purpose. It includes examination of external surface of tank for discovering of:

- leaks from bottom and / or shell;

- various damages and serious deformations in the shell;

- symptoms of settlement of the foundation;

- damages of heat insulation.

Results from external inspection have to be formed as report and written in diary of the tank.

The period between two external examinations depends on real conditions but must not exceed one

month.

b) partial inspection it must be done by qualified persons inspectors . Exploitation of the tanks

could not be stopped. The inspection includes:

- introducing with technical documentation;

- visual control of construction of tanks and welding joints;



- determination of the kind and the degree of corrosion damages upon the accessible surface of the

shell, joint between bottom and the shell supporting joint and tank roof;

- measuring of the real thickness of elements of roof and shell;

- measuring of deviations of geometrical form of the shell;

- check of status of foundation;

- measuring settlement of the foundation.

Discovered defects and damages must be entered in sketch that becomes part of tank

documentation.

After analysis of collected data about tank's condition must be done design calculations of tank's

structure and assessment of residual resource of the facility. Real condition of the tank must be

evaluated and to be written instructions to further exploitation, repair or rejection.

The period between two partial inspections could be smaller of 5 years or 1/4 of residual corrosion

life of the shell

c) full inspection it must be done by qualified personal and purpose is full assessment of technical

condition of the tank. In this kind of inspection tanks must be out of service. The facility must be

empty. Gas must be cleaned with the control of concentration of carbon vapors and cleaned. The

following kind of works is necessary to be done:

- introducing with technical documentation for exploitation, technology and design of steel tank;

- inspection of the surface outside and inside of all constructive elements;

- measuring of real thickness of all constructive elements of the tank;

- measuring of geometrical characteristic of all constructive elements of tank which measure

exceeds the determined limits

- check of status of foundation;

- determination of kind and value of settlement;

- design calculations of structure of the tank and lose of stability of the shell, including earthquakeresponsibility;

- assessment of residual resource of the facility and determination of next inspection.

The intervals between two full inspections shall be determined by the corrosion rates measured

during previous inspections or anticipated based on experience with tanks in similar service and



assessment of residual thickness of tank's bottom The actual inspection interval shall be set to

ensure that the bottom plate minimum thicknesses at the next inspection are not less than the

values as follow:

- 2 mm when is not present leak detection system;

- 1,3 mm when leak detection system is present.

The maximum period between two full inspections could not be bigger than:

- when corrosion rates is unknown - 10 years;

- when corrosion rates is known - 20 years.

The technical report and conclusion must be prepared according to the results of inspection and it

must be integral part of the documentation of the tank. Referring to this report the repairing work

must be carried out aiming to increase the safety and to make longer the period of theirexploitation.

When the damages are especially serious or irreparable, when the tank has not residual resource,

the exploitation of the tank must be stopped.

TYPICAL DAMAGES OF ABOVEGROUND STEEL TANKS

During theirs exploitation the above ground steel tanks bear unfavorableinfluence of the

environment and stored product. Their status becomes worse and many damages are collected into

a tank and they can cause serious failures of the steel facility.

Referring to this thesis the author visited many oil storage bases, where he saw and evaluated the

current damages of the tanks in the exploitation.

The correctness of one project decisions could be better evaluated after determined period of

exploitation of the facility. Different constructive solutions taken during the years can cause

different damages of the above ground steel tanks.

1. Deviation of design geometric shape:

- locallose of stability;

- total (of whole tank) lose of stability;

- deviation to inside or outside of shell and bottom;

- movement and / or bending of annular bottom plates.



a) plane settlement:

- settlement of the whole bottom;

- settlement of the perimeter of tank.

b) irregular settlement:

- inclined tank;

- by the whole bottom;

- by the perimeter of the tank;

- key on the floating roof with damages of antirotation device, supports and equipment;

- tearing of supporting roof columns from the bottom.

2. Damage of corrosion

- corrosion of welds;

- corrosion of steel structure.

a) the corrosion has regular distribution;

b) local corrosion.

Pictures below show typical damages that could be met in aboveground steel tanks in service

Corrosion of anchors



Settlement of the tank

Sinking in foundation

Lifting of tank and plants under bottom

Cavity under the steel bottom



Corrosion of shell inside the tank

Corrosion of roof plates

Corrosion under heat insulation



Corrosion of bottom plates

Plants on spiral stairway

Local deformations of the shell



Deviation of shell into inside

Surface damages, lap joints



and reinforcement of welds

Lap vertical joint in the shell

Holding of a water on the floating roof

typical damages of above ground steel tanks

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