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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

• Storage tanks have been in use for

decades in many industrial plants

• Oil refineries, petrochemical plants,

Power plants, water facilities

• They are used to store liquids for

different purposes



• 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)

Introduction



Inspection and Maintenance

• Tanks usually store liquids, such as petrochemical products, chemicals

• Any failure in storage tanks can cause a disaster

• They should be periodically inspected, assessed and repaired if necessary

• API653 and EEMUA 159



Failure of a 8 m diameter, 10 m high steel tank

containing 350 m3 of waste solvent in 2009

Objectives

• 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



Problem Definition

Tank

• A 17.5m dia x 18m high open top tank

• Inspection showed heavy corrosions on

bottom strake

• Replace bottom strake plates



Challenges

• 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



Methodology

• Make an opening in the roof platform

• A lifting rail was designed and installed on one of the top shell courses

• The rail with a hoist provides the ability of grabbing, lifting and moving the plates around the tank

• The tank to be on temporary supports during the replacement of bottom strake

• The temporary supports transfer the vertical loads to the ring wall and pad



FEA Modelling

• FEA is suitable for complex geometry

• Shell buckling due to weight of lifted plate

• Temporary structure strength and buckling

• Minimize the shell deflection at supports

• Quad4 plate elements and Beam elements

• A Linear static and linear buckling solver



Boundary Condition and Loadings

• 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

• Weight of top platform estimated to be 450 Ton which applied as uniform load

• Weight of plate to be lifted 4.2 Ton



Lifting Rail Design

• 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

• AS 1170.0: 1.2x DL + 1.5x Q

• A SF of 1.2 considered for dynamic load effects





Lifting Rail Design

• Changing the design configuration

• Selecting the appropriate member size

• Stress in structural members : 243 MPa

• Von Misses stress in the shell: 150 MPa

• Allowable: 85% of the yield strength

• Max shell out-of-roundness : 6.3 mm

• Moving concentrated load effects as a

result of hoist movement was studied

along the rail

• 1): Axial stress: T/A



Verification: Manual calculation

• 2): BM: (Pv x L )/4 , Sb= BM/Z

Shell and Support Check

• 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

• Optimal solution should be selected

• 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



Shell Stiffening

• Combined with supports two rows of 100x75x8

unequal angle temporary stiffeners were installed

• Lowering the chance of shell buckling

• Even distribution of load on supports

• Adding stiffeners reduce shell deflection by 22%



Shell and support Stress

• Maximum VM stresses in the shell : 90 Mpa

• Maximum fibber stresses in the supports: 161 Mpa

• maximum shell deflection :1.8 mm



Verification: Manual calculation



• 1) load is divided equally between the ring and pad

• 2) a portion of shell equal to 16 times the plate

thickness is considered to be effective

• 3) Simple beam theory was used

• 4) �� = ��28 , � = ��

• 5) �� = 0.5�ℎ� 1� − 1� �� , � = ��

• 6) � = �ℎ� �� , � = �

• A comprehensive engineering assessment for a typical tank repair work

• Involves step by step design and analysis

• Various aspects to be considered in an engineering evaluation of a

maintenance job

Summary



Acknowledgement

Saunders International Limited

Specialist in Design, Construction and

Maintenance of Bulk Storage Facilities



o] ] }v}(& ]vo P } P vl [ Z oo }µ repairs - A case study...

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