A LEVEL 3 FITNESS FOR SERVICE ASSESSMENT OF SHELL PLATE DEFORMATION IN A STORAGE TANK

Arash Zamani , Yong Wang

Saunders International Limited

November 2016

• Storage tanks are one of the most massivesteel structures used in industries. Varying from<10m to >100m in diameter and up to 30m inheight.

• Widely used in petrochemical and power plants,oil refineries and water facilities for decades.

• Store liquids for different purposes such astransportation, storage, process and etc.

Introduction

Typical tank configuration

(Cone roof / Floating roof)

• 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.

Background

• 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.

Shell Buckling

• 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.

Maintenance and Repair

• Tanks need to be out of service during repair.

• Profit loss and repair costs are expected.

• 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 in-service 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.

Can Equipment Operate Without Repair? Fitness For Service (FFS)

API 579 Fitness-For-Service

• 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.

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.

Fitness for Service

Assessment

(API 579)

Level 1 Assessment

Level 3 Assessment

Level 2 Assessment

• 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.

Problem Definition

• 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.

Deformation Contour

Site measured pattern Interpolated pattern

Cubic Spline Curve Fitting

• 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.

FEA Modelling

Portion of shell with deformation Boundary conditions

• 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

Stress – Strain Relationship

MPC Model

Coefficients: API 579 Annex F

• 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)

Load Cases

• API 579 level 3 FFS assessment requires below failure scenarios to be evaluated for the defective tank.

• Protection Against Plastic Collapse (PAPC).

• Protection Against Local Failure (PALF).

• Protection Against Collapse From Buckling (PACB).

FFS Assessment

Level 3 FFS

Assessment

Protection Against Plastic

collapse

Protection Against Collapse

From Buckling

Protection Against

Local Failure

Elastic Method

Limit Load Method

Elastic Plastic Method

Elastic Method

Elastic Plastic Method

• 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.

Protection Against Plastic Collapse (PAPC)

PAPC: FEA

• Tank shell model has been analysed subject to Global load combination cases.

• Using nonlinear elastic plastic solver.

Loading Case: Operation

2.25(P+Pso+D)

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 Localloading 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 σ1Maximum principal stress σ2

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?