Edge Settlementof

Storage Tank Bottoms

API Standard 653

Appendix B

Presented by:

Steven L. Braune, PETank Industry Consultants

API-653, First Edition

� The allowable edge settlement limit

� Very conservative

� Based on several published articles

� A recommendation only

R37.0B ====

R

R

B

B

R37.0B ====

Early Experience

� Observed edge settlement typically exceeded API recommendations

� API-653 permitted a more detailed evaluation

� Many oil companies developed methods

Early In-House Practice(Experience-Based)

R

B

B = 0.37(R)

B = 2.0(R)

B = 1.6(R)

Another Approach(Elastic-Plastic Beam Model)

R

B

B = 0.37(R)

B = 0.45(R2)

Tank Settlement Revision Task Group… mid-1990s

� Initial recommendations revised

� Several API-member companies shared in-house practices

� Current recommendations are about 2 to 4 times greater than original limit

Finite Element Models

� Two oil companies had extensive experience using FEA for edge settlement evaluations

� Analysis procedures and evaluation criteria differed… similar in principle

� Formed basis for API Task Group work

Appendix B RevisionsSettlement Curves

� Settlement curves varied by diameter

� Accepts two inches of settlement

� Additional NDE required for edge settlements > 75% of allowable

� Limits based on fillet weld strength

Bew–Maxim

um Allowable

Settlement (inches)

Figure B-10 for Bew

R – Radius of Settled Area (feet)

Figure B-10… Lap Welds Approximately Parallel to Shell

R – Radius of Settled Area (feet)

Figure B-11… Lap Welds Approx. Perpendicular to Shell

Be–Maxim

um Allowable

Settlement (inches)

Figure B-11 for Be

Figure B-10 for Bew

120 ft Diameter

Tank

R = 3 feet

Bew=3.8 in.

Figure B-11 for Be

120 ft Diameter

Tank

R = 3 feet

Be=4.7 in.

Bottom Weld Orientation

� Edge settlement, B limited to…

� Bew= Allowable for weld parallel to shell

� Be= Allowable for weld perpendicularto shell

� Weld angle relative to radial line

αααα−−−−−−−−==== sin)BB(BB ewee

A Rigorous Stress Analysis

� If actual settlements exceed recommended limits…

� Repair the tank

� Perform a rigorous stress analysis

A Recent Result…

� A rigorous stress analysis was performed on a tank with edge settlements of near 90% of limit

� Client wanted to know if tank should be repaired now or could they monitor settlement for a few more years

Finite Element Analysis Results

90% of API limit

150% of API limit

Tank Shell

Current Settlement

Profile

Projected Settlement Profiles

QUESTIONS?

Steven L. Braune, PETank Industry Consultantswww.TankIndustry.com

804.897-7176