Hydrocarbon Processing - July 2022 - 75

Maintenance and Reliability
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.
The tank bottom plate replacement
sequence was implemented with a skipon-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.
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.
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.
Takeaway. 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 wellstructured
assessment and repair strategy
to identify the most affordable repair
methodology, considering the tank serFIG.
2. An example of a support welded to the
tank shell.
FIG. 3. A typical example of an installed drip
ring around a tank.
FIG. 4. The Swiss Cheese Model Risk Escalation.
Hydrocarbon Processing | JULY 2022 75
Hydrocarbon Processing - July 2022

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