F 13 Foundations for ambient

temperature storage tanks

v

This Chapter includes a brief review of various consideralions relating to foundations for above
0 ground, vertical cylindrical storage tanks, taken in the main from the tank design Codes.
n
d- This is a specialist subject, and thosd who wishlo pursue it in more depth are advised to seek
more detailed materialfor further studv.

Contents:
13.1 Introduction
13.2 Design loadings
13.3 Foundation profiles
13,4 As-constructed foundation tolerances
13.4.1 API 650 requirements
'13.4.2 BS 2654 requirements
13.4.3 prEN 14015 requirements
13.5 Site investigations
13.6 Soil improvement
13.7 Settlement In service

13.8 Foundation types

13.9 Leak detection and prevention of ground contamination
13.10 A cautionary tale

13.11 References

STORAGE TANKS & EQUIPMENT 249

13 Foundations for ambient temperaturc stomge tanks

toms in floating roof tanks, with the need to remove all of the
13.1 lntroduction tank contents quicklyfor tank decommissioning and for tank in-
This Chapter concentrates its efforts on the foundations for ternal cleaning operations.
conventional storage tanks, i.e. above ground, vertical cylindri-
Fortanks fitted with central drain connections, a slope down to
cal tanks for the storage of liquids at or above ambient
thetank centre sump ofa minimum of 1:120 is considered suit-
temperatures.
able. These tanks usually have a drain line running within the
It is clearly important that storage tanks are provided with suit- tank, from the central drain to a suitable connection as low as is
able foundations and there are numerous considerations which possible on the tank shell. This is considered a better arrange-
must be taken into account where tank foundations are mentthan running the drain line beneath the tank bottom to the
concerned: tank periphery This has beenthe cause ofleakageand ground
contamination problems in the past.
. The initial shape of the foundation is important to the tank
erector. A level foundation, especially in the area immedi- For tanks with one or more peripheral drains and sumps, the
ately beneath the tank shell, will make the tank erector's tank bottom must be coned up to the tank centre, and a slope of
task easier and helo to ensure that the finished shell is 1:120 is considered suitable. In setting out the as-built slope,
made to good shape tolerances. The various design Codes consideration must be given to the anticipated edge-to-centre
provide guidance as to acceptable foundation tolerances. settlement which will occur during hydrostatic testing and
operation.
. The behaviour of the foundation in the short term during
tank erection and hydrostatic testing, and during service for Tanks with a sloping bottom from one side to the other are quite
the life time of the tank is important. Excessive or uneven unusual, for reasons connected with the difficulties associated
settlement during erection or testing would clearly be an with the cutting and erection of the first course of shell plates
embarrassment in terms of cost, time and reputation to all Again a 'l:120 minimum slope taking account ofanticipated set-
concerned. Rectification of foundations which are inconve- tlement would be normal.
niently located beneath tanks is an expensive and time con-
suming business. The tank itself may suffer damage 13.4 As-constructed foundation tolerances
resulting from the settlement which will exacerbate the
proDlems. To assist in ensuring that a tank is constructed with a shell
shape as true as is possible, particularly important for floating
. Poor foundations may threaten the integrity of the tank. roof tanks to prevent roof jamming, it is important that a founda-
There have been numerous examples of storage tanks tion as close to the design profile as possible, especially around
which have su{fered sudden bottom failures as a result of the periphery ls provided. lt is quite usual that the foundation
foundation shortcomings. contractor and the tank contractor are different companies, ei-
. The initial and ongoing costs offoundations must be given ther both employed by the owner, or one as a subcontractor of
careful scrutiny. A"cheap and cheerful" foundation may ap- the other. The point in time when the foundation is handed over
pear less attractive when the costs and service outages as- from oneto the otheris often a sourceofa contractualand tech-
sociated with excessive settlement are made a part of the nical argument, so it is necessary that clear guidelines are pro-
financial equation. vided as to what is required. The various design Codes make
efforts to define what is required.
. The costs associated with ground contamination, particu-
larly by oil-based products are such that leak detection and
13.4.1 API 650 requirements
provisions to prevent ground contamination are now com-
mon, and in certain parts of the world mandatory
API 650 has much to say on this issue in its attempts to provide
clear definitions and it is probably worth repeating these in full.
13.2 Design loadings The Code divides tanks into those with foundations in a hori-
zontal plane (the vast majority) and those with sloping bases.
The loading on the foundations of storage tanks divide into
three separate areas. For the former:

. The central area of the base during operation is subject to . Where a concrete ring wall is provided under the shell, the
uniform loadings from the tank product and non-uniform top of the ringwallshall be level within t 3 mm (%") in any I
loadings arising from the influence of the seismic events on m (30 ft) of the circumference and :! 6 mm (%") in the total
the contained liquid which are described in Chapter '15. Dur- circumference measured from the average elevation
ing tank testing this area of the foundation is subjected to
loadings from the hydrostatic head of the test water' For col-
. Where a concrete ringwall is not provided, the foundation
umn-supported roofs, there are point loads associated with under the shell shall be level within t 3 mm (%") in any 3 m
the column feet which are a combination of the self-weight (10 ft) of the circumference and within :t 12 mm (y""1in lhe
ofthe columns plus the relevant parts ofthe roofloadings total circumference measured from the average elevation
. The areas of the foundation immediately beneath the tank . Where a concrete slab is provided, the first 0.3 m (1 ft)ofthe
shellare the su bject of line loadings arising from a combina- foundation (or width of the annular plate), measured from
tion of self-weight, insulation weight, wind, snow vacuum the outside ofthe tank shell radiallytowards the centre, shall
and seismic loadings. comply with the concrete ringwall requirements. The re-
mainder of the foundation shall be within :! 13 mm (%") of
. Where the tanks are fitted with holding down bolts or straps,
the design shape. lt is not made clear if this latter require-
the foundation must be designed to resist the calculated up-
ment is to be applied to the complete perimeter onlyorto the
lifts arising from the various loadings. The derivation of
whole base slab area. lf it is the latter, then this seems an
these loadings is described in Chapter 4
onerous requirement for the foundation contractor'
For the sloped foundations the elevations around the circum-
13.3 Foundation profiles ference shall be calculated from the high point and the actual
(measured) elevations shall notdeviate from the calculated flg-
It is usual for tanks to be fitted with drains for reasons assocl-
ated with the removalof unwanted impurities such aswaterbot- ures by more than the following:

250 STORAGE TANKS & EQUIPMENT

 13 Foundations for ambient temperature storage tanks

. Where a concrete ring wall is provided I 3 mm (%") in any 9

Diameter of tank Difference
m (30 feet) ofthe circumference and i 6 mm (%") in the total
circumference D

. Where a concrete ringwall is not provided t 3 mm (%") in mm

any 3 m (10 feet) ofthe circumference and !12 mm (%") in D< 10 10

the total circumference
10<D<50 D / 1000
The Code states that the measurements shall be made prior to
the water test rather than prior to building the tank. lf this in- 50<D 50
cludes the foundation tolerances, which it appears to do, then
this is unhelpful in sorting out the possible differences between
Fgure 13.1 Foundaton surface loefances
contractors and providing well-defined hand over criteria. Fram prEN 14415, table 16.2.3

13,4,2 BS 2654 requirements that the ability of the soil to bear the imposed loadings, the ne-
cessity for soil improvements and the anticipated settlements
BS 2654 does specifically address the handover of the founda- can be evaluated. [,4any storage tanks are constructed at
tion from one contractor to another and suggests that it is nor- coastal locations on poor estuarine soils with poor load bearing
mal for the owner to provide the foundation to the tank contrac- properties. In these situations it is often found necessary to en-
tor. lt states: hance the load bearing properties of the soil, or to modify the
tank proportions to decrease the imposed loadings.
The too of the foundation levels shall be checked at a handover
stage to the tank erector and the differences in level ofthe sur- Some storage tanks are built at sites where the nature of the
face of the tank foundation between any two points 10 m apart sub-soil is well known. In these cases much useful information
around the periphery of the tank shall not be greater than t 6 can be obtained by the study of the performance of similar
mm and the envelope of the peripheral surface levels shall lie structures on these sites.
within 12 mm above to 12 mm below the design levels.
Where this information is not available, a geotechnical site in-
These are locally, and in some cases globally less demanding vestigation must be carried out. The tank design Codes provide
that the API reouirements. some guidance regarding this matter
It does suggest that forfloating rooftanks, for the reasons men- API 650 suggests that the necessary information should be ob-
tioned above, that tighter tolerances may be required. tained from soil borings, load tests, sampling, laboratory testing
and analysis carried out by suitably experienced persons or
13.4.3 prEN 14015 requirements companies, preferably familiar with similar structures in the
same area.
This drafr Standard also addresses the handover ofthe founda- BS 2654 suggests that a site investigation is carried out in ac-
tion tothe tankcontractor. lt requiresthat, before the erection of cordance with BS 5930 (Reference 73. t).
the tank, the erector shall ensure that the location, height,
shape, geometry horizontal plane or slope, surface finish and prEN 14015 suggests that wherever possible, storage tanks
cleanliness of the supporting foundation shall conform to the should be sited in areas where the subsoil conditions are homo-
following: geneous, and have good characteristics in respect of load
bearing and settlement. Prior to the start of the design and con-
. Peripheraltolerances struction of the foundation, a thorough geotechnical investiga-
The purchaser shall specify the datum height of the tion should be conducted to determine the stratigraphy and
foundation and its permissible variation physical properties of the soils underlying the site. lvleasure-
ments should include soil resistivity, conductivity and Iocal wa-
- The difference in level between any two points around
ter table depth and variability. In areas subject to seismic excita-
the foundation shall not be more than 24 mm
tjons, either the local building regulations should be consulted,
- The difference between any two points 5 m apart around orifthese do not provide sufficient data, then a Seismic Hazard
the periphery ofthe tank shall not be greater that 0.1% Assessment (SHA) should be conducted by persons or
of their oerioheral distance companies suitably experienced and skilled in this type ofwork.
The tolerance the erector accepts on the inclination or The Codes are agreed that certain sites should be avoided, or if
slope of the foundation shall be such as to enable the fi- they must be used, perhaps for economic reasons, then must
nal vertical tolerances of the tank to be achieved be subjected to special consideration. API 650 provides the
. most comorehensive list which is as follows:
Foundation surface tolerances
. Sites on hillsides, where part of a tank may be on undis-
- The sag in the as built surface measured with a 3 m long
turbed ground or rock, and part may be on fill or another
template shall not exceed 10 mm
construction where the depth of fill is variable
- The difference between the design level and as bujlt
. Sites on swampy or filled ground, where the layers of muck
level shall not exceed the values given in Figure 13.1
or compressible vegetation are at or below the surface, or
This document also has some sensible advice on the provision where corrosive materials may have been deposited as fill
of detailed information for any holding-down devices which will
require accommodating in the foundation and for the dimen- . Sites underlain by soils, such as layers of plastic clay or or-
sional checking of anchor pocket positions and the anchor ganic clays, that may support heavy loads temporarily, but
installation. settle excessively over long periods of time
. Sites adjacent to water courses or deep excavations, where
1 3.5 Site investigations lateral stability of the ground is questionable
At any site where it is proposed to construct storage tanks, it is . Sites immediately adjacent to heavy structures that distrib-
necessary to have knowledge of the sub-surface conditions so ute some of their load to the sub soil under the tank sites,

STORAGE TANKS & EOUIPMENT 251

13 Foundations for ambient tempercturc storcge tanks

thereby reducing the sub soils capacity to carry additional ward force exerted on the tank bottom corner by the bottom
loadings without excessive settlement plates. There are rules in the various design Codes to allow
. Sires wheretanks may be exposed to flood waters, possibly
these calculations to be made.
resulting in uplift, displacement or scour Clearly a tank with a coned up to the centre bottom is better
suited to cope with this form ofsettlement as it has to pass from
. Sited in regions of high seismicitythat may be susceptible to
the cone up, through flat to the cone down before serious ten-
liquefaction
sile stresses are imposed on the bottom plates. Some owners
. Sited with thin layers of soft clay soils that are directly be- have theirown rulesfor situations wherethis type ofsettlement
neath the tank bottom and can cause lateral ground stability is anticipated. In addition to the cone up preset, some of these
proprems involve an improved bottom plate joint (perhaps a two pass sin-
gle-sided llllet, a double-sided fillet or butt welding) and a
stiffening of the tank bottom corner
13.6 Soil improvement
Tilt, as long as it is pure tilt, is anotherform ofsettlement which
lf the subsoil is found to be inadequate for the imposed loads most tanks can accommodate without undue problems, with
withoutexcessive or uneven settlement, and the tank cannot be the exception of floating roof tanks where some binding may
relocated to another area where the soil conditions are better. occur.
then the Codes are agreed that one of a number of means of
soil improvement may be used: Differential settlement around the tank periphery is usually
problematic. Floating roof tanks change shape giving rise to
r Removal and replacement of unsatisfactory material by roof jamming at quite small settlements of this type, and fixed
suitable compacted fill rooftanks can be distressed by their attempts to bridge gaps. lt
. lmprovement of the soft or loose material by vibration, dy- is often difficult to separate the components due to tilt and differ-
namic compaction or pre-loading with an overburden of ential settlement from a set of bottom level readings. The
other material method given in API 653 (Reference 13.2) is useful and Figure
13.2 is taken from that document showing howthis is achieved.
. Sub-soil drainage with or without pre-loading
SpeciUc guidance as to what represents acceptable limits for
. Stabilization by chemical grout injection the different forms ofsettlement applied to the different types of
. tanks is not easy to find. The design Codes are not helpful. The
Provision of a reinforced concrete raft with or without sup-
poning piles tank maintenance and repaircodes are more forthcoming (Ref-
erences 13.2 and 13.3).
The design, specification and undertaking of these forms of
The hydrostatic testing ofthe tank is the point atwhich the foun-
foundation improvement should be left to those experienced in
dation design is first called upon to perform its intended duties.
this type of work.
BS 2654 includes some sensible advice regarding tank testing.
The testing of the first tank in a new area is critical and should
13.7 Settlement in service be carried out with caution and comprehensive settlement
The prime function of the tank foundatlon designer is to provide
a foundation at an economic cost, which will protect the tank
from excessive settlements during its construction, hydrostatic
test and service life. A conventional storage tank may be sub-
ject to a settlement which is made up of a combination of the
following:
. Globalsettlement. This isthe uniform downward settlement
of the completed structure
. Differentialsettlements:
Tilting of the tank across its diameter
Edge-to-centre settlement along a radial line to the tank
centre
Differential settlement around the tank periphery
Storage tanks have differing tolerances to these various differ-
entforms ofsettlement. The tolerance is also a function ofthe
tank type and geometry For tanks built on poor but uniform
soils wherethe main settlement is globalwith little accompany-
ing differential settlement, and the connecting pipework hasthe
necessary flexibility, settlements measured in meters have
been recorded without undue detrimental effects. There are
sites where this order of settlement is a part of the life cycle of 10 12 14 16 1a 20 22
the storage tanks. They are designed with permanent shell
jacking brackets, or suitably stiffened for lifting by other means
such as airbags. When these tanks have settled by an agreed
O4-ofrh.o h
rl€tcc{on td pol.r'l U,= od{tdano ed€nFd ot pohl '1i
amount, they are lifted and the foundation is refurbished at the sr= Ur- {& Ur-d r,! U'+r), ror@mde (+) wlton abN..e. cl'€:
original elevation. s11- t4r(2uft+1t2!t2l
- (-) u'en bdw aNs rorodrnpJei
4, =(+) 'os
The ability ofa tank to accommodate edge-to-centre settlement
can be calculated with some degree ofconfidence. This form of
settlement is almost invariably a downward movement of the
centre ofthe bottom relative to the tank shell. lts limiting value is Figure 13.2 Graphical represenlationof tankshell settlement
a function ofthe tensile stresses in the bottom plates and the in-
Frcn API 653, tigure B-3

252 STORAGE TANKS & EQUIPMENT

 13 Foundations for ambient tempeftture storage tanks

measurement provisions. The testing of subsequent tanks in 13.8 Foundation types

the same area may be adjusted, dependent on the results of
this first test. The Codes are in agreement that a number of different types of
For tanks where the ground conditions are good and settle- tank foundation are acceptable. These are:
ments are anticipated to be modest, it is acceptable to half fill
the tank as quickly as is practicable before stopping and taking Earth foundations without a ringwall. A typical example is
settlement measurements. lt should then be filled to three quar- shown in Figure 13.3. The capping with sand bitumen is
ters full and then to the full height with pauses for settlements at something which both the British and the European Stan-
each Doint. The full water load should be maintained for 48 dards are keen, if not insistent on. API 650 makes no such
hours, and if no significant settlementtakes place, the tank can specific requirement. The plastic tubes are for early indica-
be emptied. tion of bottom leakage and to help to prevent foundation
washout problems. (See Section 13.10).
For tanks built on weak ground, a much more cautious test
method is proposed with slowfilling rates and frequent pauses, Earth foundations with a concrete ringwall. Atypical exam-
some prolonged, for settlement rates to slow or stop. Clearly in ple is shown in Figure 13.4. The ringwall is of reinforced
these situations, sufficient time must be allowed in the con- concrete and details are given in the Standard forthe design
struction programme for the extended test period. of this ringwall. Cautionary words are included in all of the
Standards regarding the possible problems of differential
settlement between the ringwall and the material within the
ringwall (usually compacted fill) and its effects on the local
suooort of the tank bottom.

Earth foundations with a crushed stone or gravel ringwall.

See Figure 13.5 for a typical example. lt is important that
the exposed shoulder is Drotected from erosion. lt should be
remembered that heavy rain falling on a storage tank can
result in a vigorous waterfall around the periphery of the
€nK.

A concrete slab foundation. Figure 13.6 shows a typical ex-

ample. This pafiicular example indicates a thin slab with a
thickened peripheral region. On occasions, the slab diame-
ter is increased to provide additional support to the tank.

A concrete slab foundation with supporting piles. Where

piles are not or cannot have their integrity proven by field
Figure 13.3 Typicallank ioundaiion wiihout a ingwall
From BS 2654, figure 35 testing, it is suggested thai the slab is designed to accom-
modate the failure of an individual oile.

-75 mm (3")mn or @npaccd. creansa.d

Remove a.y lnsuilabe maI€,a aod

rcplace wilh su able l l i lhan

f I
thooushry Mpacl till

Notesi
1. S4 8.42.3 br GquircrunE io. relnfoferent. is not posible. eler lo Acl 316 hr addiisat d@toDment
2. Thb top.r lhe c..crr€ nngell shall be srMlh a.d t€v6r.lhe
d*€ 6tr€ngh lharl be al bas120 MP€ (3000 tbtin.2) arEr 3 Flngwalls lial ex@ed 300 mfr (12 in) in widlh shall haE
2a days. Fatnbmnt rdier nL€t be siaggeEd end shal b3 Bba6 disr.ibuied on boh la@s
hpped io d@rop turl stre.gm h rh€ bo.d. r ,rE!!@.i.e ot ts!6 4. S€e 8.4.2.2 lor be p6nion ol ltE lank shell on |he nn!ral1.

Figure 13.4 Example of tank fou ndation with concrete fingwall

From API 650, Appendix B, tigure B-1

STORAGE TANKS & EQUIPMENT 253

13 Foundations for ambient temperature storage tanks

Nore: Any un.!(able rol6n6r 3lr!r be €mftd 6.d .€pr8d€d wm .llrade lir; dre ill rhalr lh€n be
Figurc 13.7 Crushed stone ringwall with under-tank leak detection at the tank
penmeler
Fron API 650, Appendix B, tigure I-2

Figure 13.5 Example offoundation with crushed stone ringwall

Fram APl650, Appendix B, figure B-2

Figure 13.8 Earthen foundaiion with undeFtank leak detection ai the lank per-

From API 650 Appendix I,ligure I-3

4 MeBbrane 7 Bund surfac€ 13.9 Leak detection and prevention of

5 Foundatlon rai I Chat (when rcqlied) ground contamination
3 50 mm sand/bitumen 6 Aurlllarys€al 9 Holdlng doM bolt API 650 has much more to say on this subjectthan do the Brit
ish or the European Codes, which only give a passing mention
to it.
Figure 13.6 Typical concrete slab foundation
From p,EN 14015, figure 14.1-3 ADDendix I of API 650 is devoted to under{ank leak detection
and subgrade protection. lt includes the note stating: "APl sup-
ports a general position of installation of a Release Prevention
The 50 mm thick sand bitumen capping suggested by the Brit-
Barrier (RPB) under new tanks during initial construction. An
ish and European Codes is not universally popular. lt is in-
RPB includes steel bottoms, synthetic materials, clay liners and
tended to provide a measure of corrosion protection to the un-
other barriers or combinations of barriers placed in the bottom
derside of the tank bottom plates. lts effectiveness has been
of, or under an above ground storage tank, which have the fol-
challenged, in much the same way as the usefulness of painting
lowing functions: (a) preventing the escape of contaminated
the underside of bottom plates has. The argument centres
material and (b) containing or channelling released materialfor
around the possible effects of protecting only a part of the bot-
leak detection." Quite a clear statement of intent.
tom plating. That is to say, only a part of the bottom plating is in
contact with the sand bitumen in a similar fashion that only a A number of double steel bottom designs are included in this
part of the bottom plating is protected by paint due to damage category and these are described in Chapter 3, Section 3.4.
by welding operations, making the corrosion situation worse
The Appendix gives detailed requirements for a number of dif-
than protecting none of this surface. There are strongly held
ferent systems. Leak detection for tanks with crushed stone
and conflicting views on this issue. Where cathodic protection
ringwalls and earthen foundations are illustrated in Figures
of the tank bottom plating is to be installed, BS 2654 suggests
13.7 and 13.8.
that the sand bitumen layer is omitted.
Two different systems for tanks supported by concrete slabs
The 300 mm minimum elevation of the finished foundation are shown in Figures 13.9 and 13.10.
above the local grade requirement is to help with drainage of
Provisions required around a draw-off sump are shown in Fig-
water away from the tank, to prevent floating in the event of lo-
ure 13.11, and for a tank with a coned down to the centre bot-
calflooding and to keep the tank bottom above the local water tom see Figure 13.12.
table in the event of settlement for underside corrosion
preventron reasons. This section of the Code also deals with tanks where the bottom
is supported by grillages. The use of a grillage allows the tank
For tanks which require holding-down anchors, the foundation bottom to be visually inspected for leakage, something which is
will normally be of the concrete ringwall or the slab type. To re- considered necessary for a small number of products. Typical
sist the uplifr forces, the dead weight of the ringwallor appropri- grillage arrangements using parallel and radial supports are
ate portion ofthe slab can be used. Tee-shaped ringwalls which shown in Figure 13.13. Grillage support is restricted to tanks
mobilise part of the local sub grade and ground anchors are with shell plate thicknesses up to 13 mm and maximumtemper-
also a possibility. atures of 90 'C. By agreement, the shell thickness limit can be

254 STORAGE TANKS & EQUIPMENT

 13 Foundations lor ambent rcmpetdlurc s@tage ..a -:

y'.*)

zJ
\:r)
Figure 13.9 Reinforced concrete slab with leak deiection al1he oerimeter
Fron APl650 Appendix l, figure l-6

Flgure 13.13 Tanks supported by gr llage members

From AP|650 Appendix l, figure l-11

Pil€s (l Equitsdi Acl3so extended. This section of the Code provides guidance for bot-
lom plate thickness and grillage spacing.

Figure 13.10 Reinforced concrete slab with radlat grooves for teak detect on
Another useful document for those interested in this subject is
From APl650 Appendix l, figure 1-7 EEN,4UA Publicaiion No. 183, (Reference 13.4). This provides a
wealth ofsensible information on tank foundations, tank bottom
design, corrosion prevention, inspection techniques, Ieak de-
tection and sub-grade protection from pollution. lt includes a list
of references and an interesting figure, which gives a simple
correlation between tank age and probability of bottom leak-
age, shown in Flgure 13.'14, based on a statistical analysis of
data from various oil companies.

13.10 A cautionary tale

The subject ofihis tale is a large floating rooftank on a major re-
finery site. The tank was constructed in the 1960s. The tank
D6h pipo wfih opt@l €t6€F.
Dleh.rg6 to l.6k dstecdon was constructed on a base similar to that shown in Figure 13.3
except that the plastic drain pipes were not fitted, which was
common practice in those days. The tank survived its hydro-
Frqure 13.l1 Typicaloraw ofl sumo arrangemenL static test and was put into service. After a brief period in ser-
From APl650 Appendix l, figure l-B vice and at a point when the tank was close to being full of prod-
uct (crude oil), a part of the periphery of the foundation pad
suddenly washed out and the tank discharged its contents into
bond.d lo &mp (Altenstiw the bunded area.

20t
b 15t

:"E 101
gJ
(!

o
10 20 30 40
Tank Bottom Age (years)
Figure 13.12 Centre sudrp for downward-stoped boltom Figurc 13.14 Probabilities ofieakage from tank botloms ptotted agatnst age
Fron API 650 Appendix l, figute l-9 Frcm EEMUA Publicalion No. 183, figure 1

STORAGE TANKS & EQUIPMENT 255

 13 Foundations for ambient temperature stonge lanks

When the tiank was examined, it was found that a substantial focus attention on the design oftankfoundations and helped to
failure had occurred in the welded seams ofthe lap-weldedtank form the guidance that is found in the various Codes today.
bottom plating. The sequence of events was deduced to be as
follows:
. A small leak in the tank bottom plating occuned. This could
13.11 References
I
have been an original defect or had appearedduring the hy- 13j BS 5930:1999 - Code of practice for site investigations,
drostatic test or in oDeration BSI London
I

I
. The lackofdrain pioes meantthatthis leak went undiscov- 13.2 API 653:Second edition December 1995 plus Addanda
ered 1,2 and 3. Tank lnspection, Repair Alteration and Re-
I

I
. The pressure built up behind the tank pad shoulder until it consfrucrbn, API Washington
suddenly washed out locally 13.3 EEMUA 159 (1994) Userb guide to the maintenance
I
. The loss of support for the tank bottom in that area caused and inspection of above ground, veftical, cylinddcal,
the tank bottom plating to fail, and the tank contents were steel storage tanks, EEMUA London
discharged into the bund. 13.4 EEMUA 1 83 (1999) Guide fot the prevention of boftom
I

This was an expensive incident, especially when the cosb of leakage from veftical, cylinddcal, steel storage tanks,
Drevention would have been so modest. It did however serve to EEMUA London

I
I

I
t

I
I

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F.
E

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I

i
i
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256 STORAGE TANKS & EQUIPMENT