16. Tank Foundation
Transcript of 16. Tank Foundation
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A.P.I. 650 and 653 only give general guide lines to
the type of foundation
Number of different types, evaluated on the basis for:
•Risk
•
Cost
•Constructability
•Schedule
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Grass, moss growing out from under the tank
What to look for around tank to show Fault
Foundation
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Tree’s growing from under the tank
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Tank floor laid on river stone
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Tank shell has edge settlement
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Tank is below ground
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Water lying around ring wall foundation
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Ground outside foundation cracked
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Water lying around tank
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Timber railway ties laid under shell holding moisture
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Concrete foundation has large cracks
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Tank floor annular plate is starting to delaminate
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Tank shell is flattening out at the top of the tank
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Floating roof jams on the way up
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Dike wall out of level
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Tank is not level
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Tank floor annular is hidden
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Insulation holding moisture
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Tar laid on top of annular hiding corrosion
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Bottom shell course bulges out
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Concrete ring wall slopes under tank
Foundation Failure Causing Underside
Corrosion
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Poorly constructed or inadequate foundation material
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Moisture held in the foundations
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Welder sweepings under last floor sheet
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Welding of the tank floor not to any code
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Steel or wood leveling or layout pins
C t Ri W ll d G t
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If the steel tank floor is in contact with concrete,
the moisture in the concrete and the galvanicaction between the dissimilar metals will, over a
period of time, cause the steel to corrode at an
accelerated rate.
Concrete Ring Walls and Grout
Double Bottom Method
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Double Bottom Method
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Developed in the 1980’s as the only viable
secondary containment method, has failed
in a number of cases
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With the old steel floor and a new steel floor
spaced apart with sand or concrete, the oxygenand moisture makes a perfect battery cell and the
new floor becomes the sacrificial anode.
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The new steel floor corrodes out at a rate 4 times
faster than the old floor.
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C.P.must be designed and installed by experienced
contractors or it may result in increasing the rate of
corrosion of the tank floor.
Any change in the amount of moisture in the
foundation material will cause the resistivity of
the material to change and this may result in theC.P. breaking down.
In some cases where oil impregnated fiber board or
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In some cases, where oil impregnated fiber board or
oil sand has been used, nature has done its work
and eaten the oil out of the sand. This can reduce theresistance of the sand and this condition has proven
to accelerate corrosion.
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NDT inspection has limitations and flaws and
cannot inspect 100% of the tank floor. It cannot
inspect the critical zone around the shell to
annular weld, lap joints, under the roof
columns, around tank sumps. Sometimespacked rust shows up as good metal.
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We have identified that many of theproblems with aboveground storage tanks
start at the foundations. Tank’s not properly
founded are bound to fail.
Tank Foundation
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Tank Foundation
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When built on good foundations tanks have
lasted 50 and more years
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EEMUA Publication No 159
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EEMUA Publication No. 159
2.2.2 Problems Caused by Soil Settlement
Even soil settlements are generally not dangerous
because they occur gradually and increaseuniformly. When they become excessive however,
some remedial action may be necessary. The extent
of settlement should therefore be checked, to avoidproblems during operation of the tank.
Pipe connection. Excessive settlement may cause
problems for the pipe connections at the tank shell.
Alternatively the pipe support may settle faster than
the tank on its foundation. Any difference in the level
may seriously overstress the pipe connection.
EEMUA Publication No 159
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EEMUA Publication No. 159
2.2.3 Additional Problems caused by Uneven SoilSettlements
Uneven soil settlement under the shell of the tank placesthe bottom periphery at greater risk than even soil
settlement. The weak flat bottom plates will follow the
uneven pattern of the soil settlement. The tank will try toform a bridge over the area where local increased
settlement occurs. The bottom plates at that location may
not be properly supported by the tank foundations, but
may be suspended from the shell.
This is an unfavourable condition for the shell to bottom
connection and ultimately a rupture at the inner filletweld may occur.
Tank Settlement into the Foundation EEMUA 159
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TankShell
Tank Bottom
Location of Possible
Corrosion
Water and Debris
Collected
Shape afterRepair
Tank Settlement into the Foundation EEMUA 159
Difference in Settlement between Center and
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Tank Shell
Tank Bottom
Difference in Settlement between Center and
Periphery
Settlement at center 30% greater than at Shell
EEMUA 159
Shell settlement
Local uneven Settlement under the Tank
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Tank
Shell
Tank Bottom
Local uneven Settlement under the Tank
Shell
Possible
Fracture
EEMUA
159
Local uneven settlement under the tank should always be
treated seriously
EEMUA Publication No. 159
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a) Edge settlement
- Edge settlement may be caused by a tank pad shoulder
with insufficient width or poor quality construction.
- Edge settlement may be caused when the tank pad
shoulder is damaged or eroded by rain and wind
- Edge settlement may occur when the shell
penetrates into the foundation due to poorcompaction of the tank pad under the tank shell
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EEMUA Publication No. 159
2.2.5 Leakage
If external inspection reveals leakage from the tank
bottom, it is an indication that product is penetratinginto the foundation. This may cause a serious
condition for the safety of the tank and at the same
time cause soil pollution.
Leakage close to the periphery of the shell should be
considered as a major problem requiring urgentattention. It should therefore be remedied
immediately as the leak may cause a local wash out of
the tank foundation under the shell
EEMUA Publication No 159
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EEMUA Publication No. 159
2.2.5 Leakage
Wash out will cause loss of support for the bottom plates
and may result in a rupture at the inner fillet weld of thebottom-to-shell connection or the bottom lapwelds. This
would cause a sudden complete discharge of the tank
contents.
As it is impossible to predict the probability of a washout
occurring, corrective action should always be taken as
soon as possible
The possibility of a rupture increases when the tank is
filled to a high level
EEMUA Publication No 159
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EEMUA Publication No. 159
2.2.7 Relevelling of Tanks and Foundations
When settlements have reached their maximum limits,
relevelling of the foundations and the tank will benecessary.
The tank is jacked up to a level 8 feet (2.4 meters)above the foundation to provide sufficient space under
the tank for the repair of the tank foundations
Jacking of tanks has been successfully carried out for
tanks up to 300 foot (90 meters) diameter. The jacking
of large tanks requires a contractor with specialised
experience
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2.3 Corrosion of Tanks
General underside corrosion should be slight where
there is a well prepared and maintained foundation.
The major causes of underside corrosion are:
- Poor drainage around tank
- Mill scale, which when present on the underside of the
tank bottom, causes preferential attack
- Water collecting around the tank and migrating under
the tank
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2.3 Corrosion of Tanks
- Significant tank settlement that lowers the tank below
a good drainage level. This may lead to corrosion
of the annular plate around the tank perimeter.
- High natural water or underground springs
- Coral or beach sand with high chlorides used for thefoundation.
- Acidic coal based cinders or slags being included in the
foundation.
- Sharp or large stones being included in the foundation.
This may give rise to deep pitting by oxygen
concentrationcell corrosion.
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2.3 Corrosion of Tanks- Poor quality of bitumen sand mix. B.S. 2654
- Cathode protection may have been installed after thetank has been commissioned. In this case it
makes it difficult to ensure that sufficient
protective current reaches all areas of the bottom.
- In tanks storing heated products, there are large
differences in the tank to soil potential across the
bottom. This can cause deep random pitting.
Potentials become more anodic from the center of
the tank outwards.
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2.3 Corrosion of Tanks- Hot tanks increase water permeation under bottoms and
corrosion rates increase at higher temperatures.
- Stray D.C. currents may cause extremely high local
corrosion. This occurs when an external source of
direct current such as an electric railway causes
current to flow through the ground to the tankbottom.
- Other isolated cases have included rapid galvanic
corrosion resulting from installation of a newbottom on top of an old bottom and higher than
average corrosion rates caused by elevated
temperature.
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Corrosion Triangle
Steel
Moisture Air
If all 3 elements are present then corrosion can occur
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Corrosion Triangle
Steel
Air
If all 3 elements are present then corrosion can occur
Remove moisture = no corrosion
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Corrosion Triangle
Steel
Moisture
If all 3 elements are present then corrosion can occur
Remove air = No corrosion
EEMUA Publication No. 159
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2.4 Tank Bottoms
2.4.3 Bottom Settlement
2.4.1 Shape of Tank Bottoms due to settlement
2.4.2 Ripples in the tank bottom
a) Edge settlement
- Rain water collected in the depression around the
shell will decrease the quality of the foundation
under the shell and increase the possibility of
edge settlement.
EEMUA Publication No. 159
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b) Bulges or Depressions in Bottom Plates
- Sometimes it is proposed that these voids are filled with
liquid concrete to give support to the bottom plates
at that location. How ever this is incorrect. Slurry
will not fill these voids but will flow to those
locations where the bottom can move easily, so thatultimately the tank bottom will rest on a number
concrete points instead of being supported evenly
over its entire surface.
- When bulges are formed and not filled with foundation
material (void) there is a risk of rippling when the
tank is filled and the product contained will press theplate downward.
Typical Tank Settlement
Curve
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Curve
Settlements cause ovality of
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tank shells
The EEMUA
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The EEMUA
No. 183
Publication
EEMUA Publication No. 183
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2. -Tank Foundations: Design
3. -Tank Bottoms: Design
4. -Main Causes of Tank Bottom Leakage
5. -Inspection Records and Techniques
1. -Introduction and Scope
EEMUA Publication No. 183
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8. -Condition Monitoring and Maintenance
9. -Main Conclusions and Recommendations
7. -Available Systems for Detecting leakage andMinimizing Soil Pollution.
6. -Other measures for preserving the Integrity of Tank
Bottoms.
EEMUA 1832: Tank Foundations
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2.6 Sand Pad Foundation with Crushed Stone “Piles”
Stone piles will increase the risk of ground water
pollution from leaking tanks by providing good
leakage paths deep into the soil. Such designs are
not therefore recommended for oil/chemical
storage tanks
2.7 Provision of Oiled Sand or Permeable Bitumen Sand
layer
The provision of an oiled sand or permeable bitumen-
sand layer is considered a very important
requirement for inhibiting underside corrosion. (see
Appendix 1, Fig. 2)
3 Tank Bottoms, Designs EEMUA 183
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, g
3.2 Tank Bottoms with Annular Plates
3.4 Double Bottoms
3.3 Tank Bottoms without Annular Plates
3.4 Double Bottoms Fig.10 EEMUA 183
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3.4 Double Bottoms EEMUA 183
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Fig. 10 The “Letter Box” Method
When the new primary bottom is installed, support for
the bottom has to be re-established both outside andinside the tank shell such that the old bottom and
intermediate shell are effectively redundant.
The letter box method of installing a new bottom in a
single bottom tank has been in use for more than 35
years.
3.4 Double BottomsEEMUA 183
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Although there is no requirement to provide support
for the new bottom outside the shell, (API 650) paragraph
1.4.1 requires proper support of the primary bottomand evaluation of the design to verify that the primary
bottom and shell are not over-stressed under the
specified loading conditions including tank settlement.
Such evaluation is not required where the primary
bottom is uniformly supported on both sides of the shell
and not structurally attached to the secondary bottom
or primary bottom support.
3.4 Double Bottoms EEMUA 183
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-The installation of double bottoms should be carefully
evaluated, taking into account the relatively high
cost of the design and of the rectification work
required when tanks undergo progressive
settlement.
-Also the lower or secondary bottom cannot be inspected
even when the tank is out of service and
consequently its integrity is always questionable.
EEMUA 183
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3.4 Double Tank Bottoms
• Complicated to Install
• Relocation of Nozzles & Manholes• Sensitive to tank Settlements
• Releveling virtually impossible
• Removal of contaminated material from doublebottom (following leakage) potentially hazardous
New Tank Foundation Construction with Liner
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7.2 Flexible Membrane Liners in TankEEMUA 183
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Foundation
• No alterations to tank structure
• Installation in combination with (scheduled)inspection/lifting/relevelling
• Future inspection/lifting/relevelling not hampered
• Removal of contaminated soil (following leakage) can beexecuted in open air
2 Foundations of Storage TanksEEMUA 183
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2. Foundations of Storage Tanks
• 5 types
- type A: Traditional Sand Pad
- type B: Sand Pad with Crushed Rock Annular Section
- type C: Concrete Ring Beam Foundation- type D: Concrete raft
- type E: Underpiled Concrete raft
Type A: Traditional
Sand PadEEMUA 183
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Top View Sand PadFoundation
EEMUA 183
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Type B: Sand Pad with CrushedEEMUA
183
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Rock Annular Section
Type C: Concrete Ring
Beam Foundation EEMUA 183
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Type D and E: Concrete EEMUA 183
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Raft Foundation
Soil Improvement on un-equal level
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Soil Improvement on equal level
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Schematic Geometry
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Execution of Soil Improvements
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• To what depth: same depth every where?
• Extend of excavation: D + 2S + 2T + 4d (+Dd)
• At un-equal soil excavation depths the excavation does
not have a circular shape
• Excavation shall not exceed ground water level (No soil
improvements possible)
• High quality of used materials is required
Effect of Shoulder WidthEEMUA 183
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S<1.0 m S>1.5 m
Foundation Conclusions: EEMUA 183
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• Width (S) of Tank Pad Shoulder:
- 1.0 m (3FT) for tanks < 15 m (45ft) diameter
- 1.5 m (5FT) for tanks > 15 m (45ft) diameter
• High Pad down-grades the Stability
- Preferable Height above Grade: 600 mm (24inches)
- Crushed Rock Annular Section increases
Stability• Location of Leak Detection Membrane is Crucial
9. Main Conclusions and Recommendations
EEMUA 183
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-Codes and standards covering the design and construction
of storage tanks are based on over 100 years of
experience. There is no need to question the integrityof the tank structure as a as a primary container,
provided it is properly designed and built to
recognised standards.
-Effective prevention of ground and ground water pollution
from storage tanks depends on the design and
construction of a good foundation, a properly
construction tank bottom and regular, effective
inspection.
9. Main Conclusions and Recommendations
EEMUA 183
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-Tank foundations should have a minimum height above
grade as recommended in 4.2 (24 inches 600mm)
-Tank bottom annular plates should be 3/8 in (8 mm)
minimum thickness and butt welded with backingstrips (see 3.2)
-A course crushed rock tank pad shoulder (annular ring)
under the tank shell is preferred to a sand pad. Fig.
3 (see 2.3)
9. Main Conclusions and Recommendations
EEMUA 183
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-A concrete raft structure with or without piling, though
more costly to build than other foundation types,
may be justified on the basis of long-termmaintenance costs (see 2.5)
-For storage of hot products, tank bottoms should havecone up configuration. However for small tanks a
cone down floor on a concrete raft might be
acceptable (see 4.2)
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Compacted Ground
Tank
Ground levelFoundation
Tank Foundation Settlement
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Using the tank to preload the new foundation
API APPENDIX B
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“RECOMMENDATIONS FOR DESIGN AND
CONSTRUCTION OF FOUNDATIONS FOR
ABOVEGROUND OIL STORAGE TANKS”
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Questions?