Settlement Criteria for Steel Oil Storage Tanks
Ali Akhavan-Zanjan A khavan-Zanjanii Research Student, Department Department of Civil engineering, University of Tehran, Tehran, Iran
[email protected]
ABSTRACT This paper discusess the criteria of settlement in steel tanks which are used to storage oil or gasoline. The steel tank is as a representative of many steel tanks constructed in south of I.R.Iran, that has a ratio between the diameter and the height of order 4 with slenderness ratio (radius to thickness) of the order of 1000 (first coarse) to 3750 (last coarse). weakness of the site soil causes settlement to be more than usual so the most economical solution is to find how much can the settlement be.
KEYWORDS:
steel tank, settlement, tilt, shell
INTRODUCTION The settlement of the foundation in large, thin walled shells has been of great concern in the past and there is some codes and articles about it that are so useful. this paper wanted to show that what is the Criteria of allowable settlement of a large and small steel tank. so the paper considered some large and small steel tank that are the representative of many steel tanks constructed in south of I.R.Iran. According to D’Orazio and Duncan, examination of the settlement measured for the tanks shows one fact clearly: Steel tank bottoms can undergo a wide variety of types of distortion as they settle”. However, most analytical studies concentrate on just one type of distortion: a vertical displacement pattern at the base of the shell that follows a harmonic shape. In another paper, the same authors state: “Because their walls have significant stiffness and ability to span local soft spots, the settlement profiles of tank walls tend to be smooth and free of sharp variations. Here is one of the disasters that happen because of a tank failure that have been reported in the literature notably is the report of the failure of a 26.15 m radius shell storing hot-oil in Japan in 1974. The consequences of this failure were manifold: “The contents flooded much of the refinery property and flowed into the adjacent inland sea causing severe damage to the fishing industry. As a result, the 270,000 bbl/day refinery was shut down for about nine months, largely because of public reaction. By the time the refinery was permitted to resume operation. The accident had cost
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the refinery more than $150,000,000. This shows how important and dangerous can the damage of steel tank specially large steel tank be. The paper is organized as follows: section 2 contains the most usual settlement that would happen, section 3 is the case studies and review of literature, section 4 is about the comparison of cited allowable settlement and section 5 wanted to refer recommended settlement.
DEFINITIONS Various forms of settlements could take place so it is crucial to define all required variables at the beginning of this chapter as follows: • • • • •
D= R= H= L=
Diameter of the tank. Radius of the tank. Height of the tank. distance between two points with differential settlement. Δ max = Total maximum settlement: This type of settlement illustrates in Figure (2-1 ).
Figure 2-1. Total Maximum Settlement of Steel Steel Tank
•
Figure 2-2. Average Average Settlement of a Steel Tank
Δ ave = Average settlement: This type of settlement is an average of the settlement of all
points of a tank (Figure 2-2).
Figure 2-3. Tilt of a Steel Tank Figure 2-4. Bottom-Edge Differential Settlement of a ste s steel tank w = Tilt: This component rotates the tank in a tilt plane (Figure 2-3). • • •
δ = Differential settlement settlement between two points. δ bottom =
Edge settlement occurs when the tank shell settles sharply around the periphery, resulting in deformation of the bottom plate near the shell-to-bottom corner junction, (Figure 2-4), or the depth of the depressed area of the bottom plate, (Figure 25).
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δ shell = This component of settlement at the bottom edge leads to the lack of δ
circularity and creates stresses in the shell. shell is defined as differential outline settlement between settlement of one measurement point with respect to the average of settlements settlements of its two adjacent points (Figure 2-2). δ i = U i − (0.5 × U i +1 + 0.5 × U i −1 ) Eq. 2-1. δ i = Differential settlement between one point and average settlement of its adjacent
points U i = Settlement Settlement of each points in Figure (2-6).
Figure 2-5. Bottom-Center Differential Tank S et setttlement of a Steel
Figure 2-6. Shell Differential Settlement of Steel Tank
REVIEW OF LITERITURE AND CASE STUDIES FOR THE SETTLEMENT OF STEEL TANKS It is important to consider steel tanks suffered from excessive settlement in the past. Therefore case studies in addition to the review of literature, design codes/standards and highly referred papers, are presented to help a realistic judgment to be undertaken about allowable settlement. Klepikov (1989) reviewed a large number of references related to allowable settlement. According to Klepikov (1989), steel storage tank, with capacity less than 10000 m3 and between 20000~20000 m3, could ultimately tolerate 110 mm and 180 mm average settlement ( Δavg) respectively. For small tanks with dimensions of D=9 m and H=8 m in P.L.D area of this project, the capacity is equal to 508.7 m3. For large tanks with dimensions of D=53.6 m and H=18.3 m, the capacity is equal to 41271.5 m3. It should be noted that maximum total settlement ( Δmax) is often larger than average settlement ( Δavg) which is referred by Klepikov (1989). This reference reference has L) to be less than 0.004 for bottom of large tanks also recommended differential settlement ratio ( δ / and less than 0.008 for small ones ( L=D), and allowable outline shell settlement is equal to 0.01 and 0.008 for large and small tanks respectively ( L=6m). And finally tilt ( w/H ) of all tanks should be less than 0.007. The limit for visible tilt is equal to 0.004. (B) USACE (1990) published an engineering manual, EM 1110-1-1904, for geotechnical L) for circular steel tanks on procedures. It suggests that allowable differential settlement, ( δ / flexible base, either with fixed or floating roof, could be consider as equal to 0.008. For large tanks in Mahshahr oil product terminal revamp project L=D/2. The mentioned ratio suggests 213 mm and 36 mm mm as differential settlement between center and edge for large and small tanks respectively. As mentioned by a number of authors (Bowles, 1996), differential settlement is conservatively conservativel y equal to 75% of maximum total settlement, ( Δmax). Therefore the allowable total settlement could be estimated.
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(C) API 653 is the most related document to this report because it only concentrates on oil tanks. Klepikov (1989) and USACE (1990) are general documents for various structures. API 653 (1995) includes an appendix for the evolution of tank bottom settlement, (appendix B). The following points should be noted from API 653 (1995): •
Uniform settlement: This component often can be predicted in advance, with sufficient accuracy from soil tests, and does not induce stresses in the tank structure. However, piping, tank nozzles, and attachments must be designed with adequate consideration to prevent problems caused by such settlement.
•
Planer tilt: This type of settlement could affect tank nozzles which have piping attached to them. The tilt will cause an increase in liquid level.
•
Outline settlement of the shell: Use the following formula to calculate the maximum 2 δ shell = ( L × ε y × 5.5) /( H ). δ = allowable outline settlement: settlement: Where: shell deflection in meter, L= arc length between measurement points in meter. It is equal to 6 m for ε =
•
•
large tanks (Figure 2-6) and equal to 3.53 m for small tanks. y yield strain, (conservatively equal to 0.001), and H=tank height in meters. According to this formula the allowable outline settlement between adjacent measurement measurement points is equal to 10 mm for large tanks with length of 6 m is equal to 0.8 mm and with length of 3.53 m on small tanks. Bottom-Edge differential settlement: The maximum allowable bottom-edge settlement δ = B ew is shown in Figure (2-7). Conservatively bottom − edge could be assumed and it means areas with bottom lap welds approximately parallel to the shell. Bottom-Center differential settlement: Use the following formula to calculate the δ = 0.031 × R maximum allowable bottom plate settlement: bottem −center .
Figure 2-7. Allowable Bottom-edge differential settlement, (From API 653 (1995), Appendix-B).
(D) Case histories data presented by D'Orazio and Doncan (1987) are very valuable because they are real data of settlement for steel tanks which are used to store oil materials. D'Orazio and Duncan (1987) wrote a paper on differential settlement of steel tanks which has been extensively cited by design codes and guidelines. Based upon 31 case histories and finite element simulation, D'Orazio and Duncan (1987) concluded that allowable bottom settlement of steel tanks depends on the shape of the deformation. Steel tanks could deform into 3 profiles, as shown in Figure (2-8). Tanks with settlement shapes of a settle most at the center, and their settlement decrease smoothly
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toward the edge. Tanks with settlement profile of Shapes B have relatively flat interiors, with settlements decreasingly rapidly toward the tank wall. Tanks with settlement profile shapes of C settled most at locations about 2/3 of the distance from the center to the edge of the tank. Shape A is the least severe with respect to distortion, and Shape C is the most severe. Because of the importance of settlement profile shape and resulting bottom distortion, it is of interest to examine what factors control the shape. From this report, a number of useful facts may be noted: •
•
•
Tanks may be stable even though the minimum factor of safety against undrained failure is less than unity. This has occurred in cases where loading was slow, or drainage was rapid, or both. Tanks with F min (based on undrained strength) greater than 1.1 and De / T < 4 , had settlement profile A, ( De is the effective diameter of the tank, De= D+T p, D= Actual Actual diameter of tank, T =Thickness of clay layer and T p= Thickness of any granular layers or compacted clay pad between the base of the tank and the top of the clay layer beneath the tank).
F Most tanks with min (based on undrained strength), greater than 1.1 and De / T > 4 , had settlement profiles shape B.
Figure 2-8. Differential Settlement Profiles of Bottom Plate of Steel Tank •
F Most tanks with min (based on undrained strength), less than had settlement profiles of shape C, whenever the tank was stable or unstable when filled.
Using information in this paper, criteria have been selected tolerable amount of differential (δ / D) = 0.025 settlement, as follows: profile shape A bottom −center ; profile shape B (δ bottom −center / D ) = 0.015 (δ / D) = 0.005 ; and profile shape C bottom −center . It should be noted that large tanks of tank farm (D=53.6m, H=18.3m) and small tanks of the
T = 2. 5 m
P.L.D area (D=9m, H=8m), confirm the profile A, shown in Figure (2-8), because p depth of clay layer is more than 50m. Therefore tolerable settlement could be considered.
and
Iran Khak Company (IKCE, 2006) performed a geotechnical investigation in the P.L.D area of this project. They recommended total measured settlement should not exceed 1.4(mm week ) in
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the last week of hydrostatic test of the tanks. By this criteria, it is ensured that much settlement will not take place after the end of hydrostatic test of tanks. However this rate might be used to estimate total allowable settlement. It is not necessary to add that settlement rate much faster at the beginning of hydrostatic test. Conservatively, it could be assumed that, settlement rate of each week is equal to 60% of settlement rate of its past 7 days. By this conservative assumption, total settlement in 120 days of hydrostatic test is about 150(mm). It should be noted that, total settlement of tanks happens in years. So the measured settlement in 120 days of hydrostatic test is less than total settlement of the tanks in their service period. Therefore the recommended value of IKCE (2006) could be used only for hydrostatic test. test.
COMPARISON OF CITED ALLOWABLE SETTLEMENTS FOR STEEL TANKS According to references cited in Section (2-2), a comparison of cited allowable settlements for the steel tanks of Mahshahr oil product terminal revamp project is presented in Tables (2-1) to (23). Table 2-1. Comparison of Allowable Settlements from Different References
e c n e r e f e R
e p y t k n a T
API 653 (1995) Klepikov (1989) USACE (1990) D'Orazio and Duncan (1987)
Large Small Large Small Large Small Large Small
Total Settlement (mm) (Figure2 ) 2 2 1) e Δ ave r u t g n r g i e e d e F ( E C
Differential Settlement
δbottom
Δ max
Tilt
w (Figure2-3)
δshell
i g 2 g i g 2 l t e i g 2 ) n r i e e e 6 e e e F r r - u n F r F ( u ( u O ( u -
b i e s l i V
i t t l a e U m
0.0055(L2)/H
-
-
0.01(L) 0.008(L)
0.004(H)
0.007(H)
-
-
-
0.031(R)
-
-
180 110
0.004(D) 0.008(D)
Figure27 -
-
-
-
0.008(R)
-
-
-
-
-
-
-
0.025(D)
-
-
-
-
As it can be seen in Table (6-1), the allowable settlement depends on the dimensions of steel tanks. Therefore different values could be calculated for large tanks in the tank farm area (D=53.6m, H=18.3m) and small tanks in P.L.D area (D=9m, H=8m). Tables (2-2) and (2-3) compare various values for large and small tanks respectively. Table 2-2. Allowable Settlements from Different References for large tanks of the tank farm.
Total Settlement (mm) Reference: API 653 (1995) Klepikov (1989) USACE (1990)
Δ max Center*
Edge
1106
-
285 285
Δ ave
Differential Settlement (mm) δbottom
Tilt (mm)
w
δ shell
Center
Edge
Outline
Ultimate
Visible
-
830
170
10
-
-
-
180
214
-
60
128
73
-
-
214
-
-
-
-
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1786
-
-
1340
-
-
-
-
Table 2-3. Allowable settlements from different references for small tanks of the P.L.D area.
Differential Settlement (mm)
Total Settlement (mm)
Δ max
Reference: API 653 (1995) Klepikov (1989) USACE (1990) D'Orazio and Duncan (1987)
Center*
Edge
186
-
-
Δ ave
δbottom
Tilt (mm)
w
δshell
Center
Edge
Outline
Ultimate
Visible
-
139.5
75
2.4
-
-
-
110
36
-
60
56
32
48
-
-
36
-
-
-
-
300
-
-
225
-
-
-
-
*Conservatively assumed as: δ bottom − center = 0.75( Δ max − center )
PROPOSED ALLOWABLE SETTLENEMTS API 653 (1995) and D'Orazio and Duncan (1987) recommended values which are more related to large tanks used for oil material so they could be mainly used to select allowable in this project. Based upon Tables (2-1) to (2-3) the allowable settlements are conservatively proposed in Table 24 for the steel tanks of Mahshahr oil product terminal revamp project. Although API 653, (1995) is the most used references for the design of steel tanks, it may gives fairly un-conservative values for this project. The writer recommends API values to be used.Therefore about half of the API values for allowable total and differential settlements, as shown in Table 2-4, have been conservatively considered for presented design. Allowable tilt is proposed based upon visible limit of tilt. Table 2-4. Proposed Allowable Settlements for Steel Tanks in Mahshahr Oil Export Port.
Tape of Tank
D(m)
H(m)
Total Settlement (mm)
Differential settlement (mm)
Tilt (mm)
Large
53.6
18.3
500
375
73
Small
9
8
100
75
32
Allowable settlement for ordinary buildings, such as residential and office buildings, water towers and shelters, is more restricted but the proposed values are commonly agreed in the literature and text books and Table (2-5) is used in different projects. Table 2-5. Proposed Allowable Settlements by Skempton and MacDonald for Conventional Buildings.
Type of Soil Sand Sand Clay
Type of Foundation Isolated Raft Isolated
Differential Settlement 25 mm 25 mm 40 mm
Total Settlement 40 mm 40~65 mm 65 mm
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8 Raft
40 mm
65~100 mm
Although clayey soil exists in site but considering that it is likely that a layer of compacted gravel and sand to be constructed beneath the foundations so the allowable total settlement presented in Table (6-6) have been considered for buildings. Where a large loaded area is founded on a relatively incompressible stratum (e.g. dense gravel) overlying compressible soil, settlement of the structure will occur due to the consolidation of the latter layer, but it will not take the form of the bowl-shaped depression. The effect of the dense layer, if thick enough, is to form a rigid raft which will largely eliminate differential settlement. Therefore, the allowable settlement of ordinary buildings is suggested in this project as if they are founded on sandy layers. Table 2-6. Proposed Allowable Settlements for Buildings in P.L.D Area.
Type of Foundation Isolated Raft
Total Settlement 40 mm 65 mm
For pump stations and heater, no allowable settlement is recommended by design codes like API 610, but it is noted that the differential settlement values should be very small. According to definitions of NIOEC-SP-00-01 (2006) pumps in this project are heavy machinery because total weight of as pumps is greater than 23kN. It suggested that the weight of the heavy rotary machinery foundation like the pumps in Mahshahr project shall be at least 3 times the weight of machinery. Therefore the pump itself should be placed on a fairy rigid raft foundation to avoid relative movement between pump supports. However tilt and total settlement could be a problem for the connections of pump and pipes so the use of expansion loop or expansion joint or both of them is highly recommended. Design of such expansion loop needs flexible analysis. Therefore the allowable settlement of pumps is a function of pipe-pump connections design, but it assumed as 2 cm for foundation selection and land reclamation design.
REFERENCES 1-API 653, Appendix-B. TENTH EDITION, NOVEMBER 1998, ADDENDUM 1, JANUARY 2000 ADDENDUM 2, NOVEMBER 2001 2-Timothy B.D’orazio, A.M. ASCE and James M. Duncan, F. ASCE (1987) "Differential settlements in steel tanks"journal of Geotechnical Engineering,vol. 113,NO 9,pp 967-83 3- D’Orazio T, Ducan JM, Bell RA. (1989) Distortion of steel tank due to settlement of their walls. Journal of the Geotechnical Engineering Division ASCE 115(6):871–90. 4-T.Y. Wu, G.R. Liu (2000)" Comparison of design methods for a tank-bottom annular plate and concrete ringwall", International Journal of Pressure Vessels and Piping,NO 77,pp. 511-517 5-L.A. Godoy, E.M. Sosa (2003) "Localized support settlements of thin-walled storage tanks"ThinWalled Structures,NO 41,pp 941–955 6- Brown GD, Peterson WG. (1964) Failure of an oil storage tank founded on sensitive marine clay. Canadian Geotechnical Journal;1:205–14.
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7- Green PA, Hight DW (1964) The failure of two oil storage tanks caused by differential settlement, Proc. of the Conf. Settlement of Structures, British Geotechnical Society, Cambridge, UK, 353-60. 8- Clark JS (1969) Survey of oil tank failure. Annales de l’Institute Belge du Petrol; 6:15–24. 9- Myers P. (1997) Aboveground storage tanks. New York: McGraw-Hill 10-D'Orazio, T. B and Duncan J. M. (1982) “CONSAXA: Computer Program for Axisymmetric Finite Element Analysis of Consolidation." Research report No UCB/GT/82-01, Dept. of Civil Engineering. Univ. of California, Berkeley, Calif 11-Kamyab H, Palmer S C (1991) Displacements in oil storage tanks caused by localized differential settlement. J Pressure Vessel Technol, Trans ASME; 113:71–80. 12-Kamyab H, Palmer S C (1989) Analysis of displacements and stresses in oil storage tanks caused by differential settlement. J Mech Eng Sci, Proc IMechE Part C 203:61–70. 13-Teng J G (1996) Buckling of thin shells: recent advances and trends. Appl Mech Rev;49(4):263–74.
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