A LEVEL 3 FITNESS FOR SERVICE ASSESSMENT OF SHELL
PLATE DEFORMATION IN A STORAGE TANK
Arash Zamani , Yong Wang
Saunders International Limited
November 2016
Introduction
• Storage tanks are one of the most massive
steel structures used in industries. Varying from
<10m to >100m in diameter and up to 30m in
height.
• Widely used in petrochemical and power plants,
oil refineries and water facilities for decades.
• Store liquids for different purposes such as
transportation, storage, process and etc.
Background
• Steel storage tanks are composed of a flat
floor, a cylindrical shell and either fixed or
floating roof.
• Shell thickness reduces as tank height
increases.
• Construction material: carbon steel, stainless
steel and aluminium.
Typical tank configuration
(Cone roof / Floating roof)
Shell Buckling
• As thin wall structures, storage tanks are very
sensitive to buckling due to small thickness to
radius ratio.
• Causes of shell buckling:
Vacuum and wind pressure
External forces
Uneven settlement
Inappropriate welding process
• Shell buckling could result in tank collapse.
Maintenance and Repair
• Tanks shell should be inspected regularly for
possible buckling, damages and deformation.
• Deformed area should be repaired or replaced by
new plates.
• Repair can be done by adding stiffeners to the tank
shell and reshape the damaged area.
• Replace can be done by accessing the damaged
area, supporting the shell, cut and replace the
damaged plates.
• Tanks need to be out of service during repair.
• Profit loss and repair costs are expected.
Can Equipment Operate
Without Repair?
Fitness For Service (FFS)
• Fitness for service assessment (FFS) is an engineering method to
evaluate equipment condition.
• FFS assessments are quantitative engineering evaluations, which
are performed to demonstrate the structural integrity of an inservice component containing a flaw or damage.
• Such an engineering assessment should be able to evaluate the
equipment circumstances and recommend whether remedial work
is needed.
•
Using an appropriate FFS approach can save time and money.
• API 579: provides rules and procedures for FFS assessment.
• API 579: discusses assessment procedure for different damage
scenarios.
• API 579: appoints guidelines to determine whether tank shell
deformation is acceptable and if the tank is safe to continue
operation without any repair for a certain period of time.
API 579 Fitness-For-Service
FFS Levels Of Assessments
• API 579 prescribes three levels of structural integrity
evaluations for equipment.
• Level 1 assessment is the most conservative and
simplified criterion that generally includes the use of
charts and tables.
• Level 2 assessment involves detailed calculations
using relatively simple formulas with some
assumptions.
• Level 3 assessment requires a comprehensive
analysis where advanced computational procedures
such as nonlinear FEA are engaged.
Level 1 Assessment
Fitness for Service
Assessment
(API 579)
Level 2 Assessment
Level 3 Assessment
Problem Definition
• Tank under study is a 45.72 x 16.9 m Cone
roof.
• Tank was built in late 1960s.
• Tank was under periodic repair schedule.
• After replacing a strake plate a deformation
appeared between 6th and 7th strake.
• Pre existing deformation as well as thermal
stresses imposed by welding suspected to
be the reason for shell deformation.
• Shell deformation was assessed to check
whether the tank is Fit For Service.
Deformation Contour
• The first step in the FFS assessment is to determine
the pattern and amount of shell deformation.
• Some methods to measure deformation:
- String and weight
- Surveying and use of mathematical calculation
- Laser reference and manual measurement
- Laser scanning
• A 200mm by 200mm grid was marked on shell.
• The grid covers 2.8m in height and 1.8m in width.
Deformation Contour
• According to API 579, a numerical interpolation
is needed for extra node generation.
• The interpolation is required to obtain a smooth
shell curvature by producing interpolated nodes
between measured points.
• API 579 recommends using piece wise cubic
spline curve fitting method.
• A cubic spline curve fit was applied in both
vertical and circumferential directions.
• In total 1716 nodes were generated and
imported to FEA package.
Site measured pattern
Interpolated pattern
Cubic Spline Curve Fitting
FEA Modelling
• Strand7 package was used for FEA.
• The tank shell was modelled by four nodes plate element.
• Beam element represents curb angle.
• Shell deformed area modelled by importing nodes to FEA.
• Boundary condition.
• API 579 analysis approaches:
Elastic Analysis, Elastic Plastic Analysis
• Elastic Plastic FEA analysis was used in this assessment.
• Nonlinearity in geometry & nonlinearity in material.
Portion of shell with deformation
Boundary conditions
Stress – Strain Relationship
• API 579 suggests below models for stress-strain
curve:
MPC Model.
Ramberg Osgood Model.
• AS3678 Gr 250 plate:
Module of elasticity: 200GPa
Yield stress: 250MPa
Tensile strength: 410MPa
MPC Model
Coefficients: API 579 Annex F
Load Cases
• Tank understudy is subject to dead load, hydrostatic
pressure, internal pressure and wind force.
• API 579 Global loading cases: (for API 650 storage tanks)
• API 579 Local loading cases: (for API 650 storage tanks)
FFS Assessment
Elastic Method
Protection Against Plastic
collapse
• API 579 level 3 FFS assessment requires below failure
scenarios to be evaluated for the defective tank.
Elastic Plastic Method
• Protection Against Plastic Collapse (PAPC).
• Protection Against Local Failure (PALF).
• Protection Against Collapse From Buckling (PACB).
Limit Load Method
Elastic Method
Level 3 FFS
Assessment
Protection Against
Local Failure
Elastic Plastic Method
Protection Against Collapse
From Buckling
Protection Against Plastic
Collapse (PAPC)
• PAPC criterion studies the overall instability of the
structure subject to Global loading cases.
• PAPC defines plastic collapse load as the load that
causes overall structural instability.
• This load can be determined as the point that any
further increment in the load will result in divergence in
the analysis and inability to achieve equilibrium.
• In practise, PAPC will be satisfied if FEA converges
under all relevant Global loading combinations.
Loading Case: Operation
2.25(P+Pso+D)
PAPC: FEA
• Tank shell model has been analysed subject to
Global load combination cases.
• Using nonlinear elastic plastic solver.
Loading Case: Test
2.1375(Pst+D)+2.475Wt
FEA convergence under all Global loading case:
PAPC Satisfaction
Protection Against Local
Failure: (PALF)
• In addition to PAPC, tank integrity should be
investigated for another criterion called protection
against local failure (PALF).
• According to its definition, PALF discusses the
possibility of local failure due to the emergence of an
imperfection in the structure.
• This criterion needs to be checked against Local
loading cases.
• Using the elastic-plastic method, PALF will be satisfied if
Equivalent Plastic Strain in a location of the component
under investigation, plus any Forming Strain due to
initial forming is less than allowable strain, called
Limiting Triaxial Strain.
PALF: FEA
• Loading case: (operation)
1.53(P+Pso+D)
• Using nonlinear elastic
plastic solver.
• Principal stresses
σ1=307.48MPa
σ2= 107.39MPa
• Equivalent plastic strain
εpeq= 0.00141
Maximum equivalent plastic strain
Maximum principal stress σ2
Maximum principal stress σ1
PALF: Code Check
Protection Against Collapse
From Buckling (PACB)
• Apart from PAPC and PALF, another API 579 criterion called “protection against collapse from
buckling‟ should also be considered in FFS assessment.
• This requirement checks the structural instability of a component with a compressive stress
field under applied load cases.
• For tank understudy, this part of FFS assessment will become important if tank is subject to
high wind forces or vacuum.
• According to API 579, for those analysis where PAPC is performed and imperfections are
explicitly modeled in the FEA, the buckling factors are considered in the relevant loading
combinations.
• Convergence in PAPC analysis will then show that the tank is protected against buckling
collapse as well.
• Other loading cases producing compressive stress should be checked.
Conclusions
• A tank with a local shell deformation has been studied for level 3 FFS assessment according
to API 579.
• PAPC, PALF and PACB satisfied.
• Resultantly, the tank under consideration met all FFS requirements and is able to undergo its
service again. Tank is ‘‘Fit For Service’’!
• FFS helps to predict whether a tank with such an imperfection is able to operate safely until
the next repair interval by using similar assessment and predicting the shell thickness at the
end of interval.
• Loading combinations must be carefully considered in each individual assessment in
accordance with the actual condition of the equipment.
Thanks For Your Attention
Questions?