Fitness for Service Evaluation of Hydrotest Exemption According to API 653

Hydrostatic Testing – Why?

• To prevent risk of brittle fracture in service

• To prevent product leaks

• To check tank settlement

• Shows the tank is suitable for continued service

Brittle Fracture

• Dependent on • Material toughness • Temperature • Stress level • Crack like defect

• Examples

• Fawley storage tank, UK in 1952 • Pittsburgh storage tank, USA in 1988

Brittle Fracture

Crack-like defect

Fawley storage tank, UK in 1952

• 42 m diameter

• During hydrostatic test

• Fracture tougness lowered after welding

Pittsburgh storage tank, USA in 1988

• Tank was moved

• Reassembled with no inspection

• 14000 m3 Diesel oil spilled

• 3000 m3 into the river

Major Alteration or Major Repair

• Nozzle in shell larger than NPS 12

• Replace or add shell plate exceeding 12 in.

• Remove or replace annular plate material

• Replacement of more than 12 in. vertical weld

Major Alteration or Major Repair

• Replacement of more than 12 in. of radial weld in annular plate

• Installation of new bottom

• Replacement of shell to bottom weld

• Jacking a tank shell

Hydrostatic Test Exemption Engineer Approval and Owner Authorization

12.3.2.2

Fitness for Service Evaluation 12.3.2.1.b/12.3.2.7

Hydrotest Exemption

Appropriate Following Parts 12.3.2.1.a

Shell Repair 12.3.2.3

Bottom Repair within Critical Zone 12.3.2.4

Shell-to-bottom Weld Repair 12.3.2.5

Minor Shell Jacking 12.3.2.6

Hydrotest Exemption

12.3.2.7 Fitness for Service Evaluation

Fitness for Service

Mechanical Finite Element Modelling

Stress Analysis Engineering Critical Assessment

Inspection Tank Dimensions

Filling Height Design Temperature

Thickness Measurements NDT Weld Acceptance Limit

Materials WPQR/WPS

Weld Specimens Old-to-new Fracture Toughness (Charpy V)

Material Properties Test

NDT Acceptance Limit

• Undercut acc. to ISO 5817 • Quality level D, C or B

• D) h ≤ 0.2t, but max. 1 mm

• C) h ≤ 0.1t, but max. 0.5 mm

• B) h ≤ 0.05t, but max. 0.5 mm

Weld Specimens

• Piece of existing tank material

• Piece of new tank material

• Welded with new WPS/WPQR

Charpy V Specimens

• Charpy V Specimens for estimating Impact energy (and fracture toughness)

• Tensile tests may be needed to verify mechanical properties

Charpy Specimen Locations

• Sets of three Charpy V specimens • Weld metal • HAZ, • +2 mm from HAZ • +5 mm from HAZ

Obtain Charpy Impact strength

Obtain Kmat Fracture Toughness from Charpy Tests

Finite Element Model

• Finite element model is based on • Tank dimensions • Inspection reports • Filling height

• Stress analysis

• Determine membrane and bending stresses

Tank Deformation

• Tank bottom lifts from the ground

• Scale x50

Stress Analysis

• Axial Stress

• High stress at weld toe to shell

• Primarily bending stresses

Examples of FE Models

Engineering Critical Assessment

• Stress level from FE analysis

• Fracture toughness from material test

• Partial safety factors

• Determine critical flaw size

Failure Assessment Diagram

Critical Flaw Size

Fitness for Service Evaluation

• Compare critical flaw size with NDT acceptance limits

If critical flaw size is larger than NDT acceptance limit, hydrostatic test is not required

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