The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Application of FEA in large storage tanks’ shell course
repairs - A case study assessment to determine
membrane deformation and structural stability
Arash Zamani , Yong Wang
Saunders International Limited
November 2014
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Background
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Storage tanks have been in use for
decades in many industrial plants
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Oil refineries, petrochemical plants,
Power plants, water facilities
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They are used to store liquids for
different purposes
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
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Introduction
Composed of a flat floor and a cylindrical shell
The roof can be either fixed or floating
In some cases there is no roof at all (Open top)
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Inspection and Maintenance
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Tanks usually store liquids, such as petrochemical
products, chemicals
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Any failure in storage tanks can cause a disaster
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They should be periodically inspected, assessed
and repaired if necessary
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API653 and EEMUA 159
Failure of a 8 m diameter, 10 m high steel tank
containing 350 m3 of waste solvent in 2009
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Objectives
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The approaches we use to solve challenges in a real maintenance job
How we use FEA for engineering assessment in repair work
What items are Important for calculation
How we assess and accept the results
How we use a simplified manual calculation for verifications
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Problem Definition
Tank
• A 17.5m dia x 18m high open top tank
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Inspection showed heavy corrosions on
bottom strake
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Replace bottom strake plates
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Challenges
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Complex plant
Accessories such as mixer, vibrators, piping
and etc. were installed on top of the tank on a
platform
Two other tanks were located very close to
this tank
Difficulty in access due to congestion
Removal of the existing plates and
transportation and installation of new ones
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Methodology
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Make an opening in the roof platform
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A lifting rail was designed and installed on
one of the top shell courses
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The rail with a hoist provides the ability of
grabbing, lifting and moving the plates
around the tank
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The tank to be on temporary supports
during the replacement of bottom strake
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The temporary supports transfer the
vertical loads to the ring wall and pad
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
FEA Modelling
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FEA is suitable for complex geometry
Shell buckling due to weight of lifted plate
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Temporary structure strength and buckling
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Minimize the shell deflection at supports
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Quad4 plate elements and Beam elements
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A Linear static and linear buckling solver
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Boundary Condition and Loadings
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Cylindrical coordinate system
A portion of the shell was modelled
Symmetric boundary condition on edges
Pin Type boundary at supports
Fixed radial displacement at shell top
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Weight of top platform estimated to be
450 Ton which applied as uniform load
Weight of plate to be lifted 4.2 Ton
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The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Lifting Rail Design
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3x9 meter in 20 millimetres plates (4.2 Ton)
The load impose bending moments to the shell
Permanent deformation
Failure of rail members should be checked
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AS 1170.0: 1.2x DL + 1.5x Q
A SF of 1.2 considered for dynamic load effects
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Lifting Rail Design
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Changing the design configuration
Selecting the appropriate member size
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Stress in structural members : 243 MPa
Von Misses stress in the shell: 150 MPa
Allowable: 85% of the yield strength
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Max shell out-of-roundness : 6.3 mm
Moving concentrated load effects as a
result of hoist movement was studied
along the rail
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Verification: Manual calculation
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1): Axial stress: T/A
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2): BM: (Pv x L )/4 , Sb= BM/Z
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Shell and Support Check
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Possibility of shell buckling can be reduced by
increasing the number of supports, or adding
stiffeners
The number of supports should be selected
based on the shell buckling limitation
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Optimal solution should be selected
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38 sets of supports made of 150x150x5 square
hollow section and 250x150x5 rectangular
hollow section
These configuration transfers the load to ring
wall and pad
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The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Shell Stiffening
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Combined with supports two rows of 100x75x8
unequal angle temporary stiffeners were installed
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Lowering the chance of shell buckling
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Even distribution of load on supports
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Adding stiffeners reduce shell deflection by 22%
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Shell and support Stress
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Maximum VM stresses in the shell : 90 Mpa
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Maximum fibber stresses in the supports:
161 Mpa
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maximum shell deflection :1.8 mm
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Verification: Manual calculation
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1) load is divided equally between the ring and pad
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2) a portion of shell equal to 16 times the plate
thickness is considered to be effective
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3) Simple beam theory was used
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•
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2
5) ��� = 0.5�ℎ�
6) � = � �ℎ�� , � = �
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The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Summary
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A comprehensive engineering assessment for a typical tank repair work
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Involves step by step design and analysis
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Various aspects to be considered in an engineering evaluation of a
maintenance job
The 8TH Australian Congress on
Applied Mechanics (ACAM 8)
Acknowledgement
Saunders International Limited
Specialist in Design, Construction and
Maintenance of Bulk Storage Facilities