1. Frangible roofs
How to determine frangibility in existing storage tanks

2. Frangible roofs Alex Joensen
Specialist, Structural Consultancy, FORCE Technology
M. Sc. Eng. & IWE
EEMUA 159 Tank Integrity Assessor
TLF:
Mail:

3. Agenda What is a frangible roof and why are they important
Is the roof-to-shell joint in my tank frangible
-EEMUA 180 approach
-Risk assessment
-BS 2654 or API 650 criteria
-EN 14015
-FINITE ELEMENT ANALYSIS or API 2000
Roofs with external supporting structure
Non-frangibility? - mitigation
Conclusions and Recommendations

4. Frangible roofs
What is a frangible roof and why are they important

5. Frangible roofs Is the roof-to-shell connection in my tank frangible
Ensure that the probability of failure is greater at roof/shell
connection than in the tank shell or the bottom/shell connection
EEMUA 180
-Frangible Roof Joints for Fixed Roof Storage Tanks:
Guide for Designers and Users
Liquid
Vapour space
Roof-to-shell
connection
Bottom-to-shell
Definer AST
Strukturel påvirkning ved indvendigt overtryk
EEMUA 180 Metodik

6. Is the roof-to-shell connection in my tank frangible
Tank identification
Risk assessment
has been performed with respect to necessity of frangible roof joint
Perform risk assessment No
Yes
Not necessary
Follow requirements and calculation method of BS 2654, Appendix F
Analyse frangibility with help of requirements and calculation method of EN 14015, Appendix K
Perform Finite Element calculations and assess failure pressure of roof/shell joint and shell/bottom joint
Assess capacity and diameter of emergency vents acc. to API 2000 and install vent to roof manhole
Further assessment to frangibility of roof/shell joint is not necessary
Alternative 1
Alternative 2
Necessity
of frangibility required
Diameter >12,5m
Tank complies with recommendations
Diameter <5m
Tank complies with requirements and calculation method
Follow recommendations of EEMUA 180 section 8
Is the roof-to-shell connection in my tank frangible
EEMUA 180 FLOW CART
Perform risk assessment to
determine necessity for frangibility
Diameter > 12,5m?
BS 2654, APPENDIX F
(API 650, APPENDIX F)
Nødvendigheden af en fragil tagkantsamling bør vurderes ved udførelse af en risikovurdering.
Har produktet et lavt damptryk
Et højt flammepunkt
Opbygges der brandfarlig gasblanding i dampzonen
Er diameteren > 12,5m?
-Studier udført af EEMUA i halvfemserne af havarerede tanke har vist at fragiliteten i tagkantsamlingen ikke har været tilstrækkelig, ofte deler følgende karekteristika
-Diameter <12,5m
-Kuppeltag (vinklen mellem tagplade ved tagkantsamling mod vandret er over 1/5 (1/6 I h.t. API 653)

7. Is the roof-to-shell connection in my tank frangible
Liquid
Vapour space
Roof-to-shell
connection
Bottom-to-shell
Is the roof-to-shell connection in my tank frangible
Method of BS 2654 (API 650), APPENDIX F
Area inequality rule
Compression area:
𝐴> 50𝑝 𝑅 2 𝑆 𝑐 tan 𝜃
Effective cross-sectional area:
𝐴< 7,07⋅ 10 −3 𝑇 tan𝜃
𝑆 𝑐 Allowable compressive stress
T total weight of shell, shell stiffening and roof
framework supported by the shell
Θ slope of roof
Compression area vs. effective cross sectional area
Begrænsende tagkantringdesign

8. Is the roof-to-shell connection in my tank frangible
Method of API 650 APPENDIX F has more alternatives for shell-to-roof designs

9. Is the roof-to-shell connection in my tank frangible
Method of EN 14015, APPENDIX K
Design roof failure pressure:
Design bottom failure pressure:
Pressures shall verify the following formula
𝑝 𝑟 = 𝛼 min 𝑒 𝑟𝑖𝑛𝑔 ; 𝑒 𝑟𝑜𝑜𝑓 𝐷 𝑟 𝛼 2 ( tan 𝜃 𝛼 𝑒 𝑐𝑦𝑙 𝐷 𝐹 𝛼 4
𝑝 𝑏 = 𝛽 0 + 𝛽 1 𝑋+ 𝛽 2 𝑋 2 𝜖
𝑝 𝑏 ≥1,675𝛾 𝑝 𝑟

10. Is the roof-to-shell connection in my tank frangible
Tank does not comply with, or is not applicable to, the methods described in BS 2654 (API 650) or EN 14015

11. Is the roof-to-shell connection in my tank frangible
Conduct Finite Element Analysis (FEA)
Compare stress at
roof-to-shell junction and shell-to-bottom junction

12. Roofs with external supporting structure

13. Mitigation Inspection Alteration Grind down to seal weld thickness
Top angle of shell
Roof plate
500mm
Roof supporting members

14. Conclusions and Recommendations
Urgency assessment for necessity of frangible roof-to-shell joint acc. EEMUA 180
Tank Data
Value
Unit
Remarks
Terminal
Tank
Year of construction
Design code
Design pressure
Diameter
Tank shell height
Urgency assessment 1
When one of the following questions can be answered by YES, the urgency is HIGH
YES NO
is the diameter of the tank smaller than 12,5 m?
has the tank been constructed before the year 1980?
is the slope of the roof greater than 1:5?
is the throat thickness of the weld of the roof plates to the top curb section > 3 mm?
has the bottom of the tank not been equipped with an annular section with butt-welded joints?
Urgency assessment 2
Have modifications been performed to tank design since the initial construction, such as:
has the top curb section of the tank been increased to cater for insulation fastening structures?
has the tank been equipped with a heating coil?
has the tank been connected to a vapour recovery unit?
has the operating pressure of the tank been increased?
has a handrail been welded close to the roof-to-shell joint?
is the tank equipped with external rafters?
are the roof plates welded also from the underside?
are the roof plates welded to the roof supporting structure?
Conclusions and Recommendations
Urgency assessment
Urgency =akut eller påtrængende

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