tank settlement

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2n International Conference on New Developments in Soil Mechanics and Geotechnical Engineering, 28-30 May 2009, Near East University, Nicosia, North Cyprus

Since various forms of settlements could take place, it is crucial to define all required variables at the beginning of this chapter as depicted in figure1:

(a)

(b)

(c)

(d) (e) (f) Figure1. Possible settlements in steel storage tanks: (a) Total Maximum Settlement of Steel Tank (bowled-shape), (b) Average Settlement of a Steel Tank, (c) Tilt of a Steel Tank, (d) Bottom-Edge Differential Settlement of a Steel Tank, (e) Bottom-Center Differential settlement of a Steel Tank and (f) Shell Differential Settlement of Steel Tank. Δmax = Total maximum settlement: This type of settlement has been illustrated in Figure (1a). Δave = Average settlement: This type of settlement is an average of the settlement of all points of a

tank (Figure 1b) w = Tilt: This component shows the rotation angle of the tank in a tilt plane. δ = Differential settlement between two points. Edge/bottom settlement: 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 1d). Bottom settlement which is the depth of the depressed area of the bottom plate, (Figure 1e).

δ shell = This component of settlement at the bottom edge leads to the lack of circularity and creates stresses in the shell. δ shell is defined as differential outline settlement between settlements of one measurement point with respect to the average of settlements of i ts two adjacent points (Figure 1f).

δ i

= U i − (0.5 ×U i+1 + 0.5 ×U i−1 )

δ i

= Differential settlement between one point and average settlement of its adjacent points

(1)

U i = Settlement of each point in the Figure 1f 3 AVAILABLE SETTLEMENT CRITERIA Table1 presents a comparison of allowable settlements of steel storage tanks from different references. For a 60 m diameter tank with 14.015 m height, calculated maximum settlement based on the codes and available criteria are given in the Table 2. And this data are compared with the analysis of the tank.

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A numerical study on the effect of uneven settlements of oil storage tank Ali Akhavan-Zanjani, Ali Fakher & Seyed Reza Maddah-Sadatieh

Table 1. Comparison of allowable settlements from different references Total Settlement (mm) (Figure1a)

Δ max

Differential Settlement

Tilt

δ bottom

w (Figure 1c)

δshell

) e 1 r e e r u t g n i e F C (

) d 1 e r e u g g i d F E (

) f 1 e e n r i u l t i g u F O (

e c n e r e f e R

e p y t k n a T

API 653 (1995)

LargeSmall

-

-

-

0.031(R)

-

0.0055(L2) /H

Klepikov (1989)

Large

-

-

180

0.004(D)

-

0.01(L)

Small

-

-

110

0.008(D)

-

0.008(L)

USACE (1990) D'Orazio and Duncan (1987)

LargeSmall

-

-

-

0.008(R)

-

LargeSmall

-

-

-

0.025(D)

-

r e t n e C

) b 1 e r u g Δ ave i F (

e g d E

e t a m i t l U

e l b i s i V

-

-

0.004(H)

0.007(H)

-

-

-

-

-

-

Table 2. Allowable settlements from different references for a 60m diameter tank with 14.015m height oil storage steel tank Differential Settlement Total Settlement (mm) Tilt (mm) (mm) Reference: API 653 (1995) Klepikov (1989) USACE (1990) D'Orazio and Duncan (1987)

Δmax

Center*

Edge

930

-

285

Δave

δ bottom

δshell

w

Center

Edge

Outline

Visible

Ultimate

-

930

170

14

-

-

-

180

240

-

60

56

98

285

-

-

240

-

-

-

-

1786

-

-

15

-

-

-

-

*Conservatively assumed as: δ bottom−center = 0.75(Δ max−center ) 4 RECOMMENDED ALLOWABLE SETTLENEMTS API 653 (1995) and D'Orazio and Duncan (1987) recommended values which are more related to large tanks used for oil storage. Hence, the numerical calculations of this project are compared with those valid allowable settlements recommended by the said references. Based on the Table 1 and Table 2, the allowable settlements are conservatively proposed in Table 3 for the selected steel tank

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of Mahshahr Oil Product Terminal Revamp Project. Although, API 653 (1995) is the most common reference for the design of steel tanks and it may give fairly un-conservative values for this project, it has been used as the reference code (standard). Therefore, about half of the API values (to consider more safety factor) for allowable total and differential settlements, as shown in Table 3, have been conservatively considered for the presented design. Allowable tilt is proposed based upon visible limit of tilt. Table 3. Proposed allowable settlements for the steel tank in Mahshahr Oil Product Terminal Type of Total Differential Tilt D(m) H(m) Tank Settlement (mm) settlement (mm) (mm) Large

60

14.15

500

375

56

Small

9

8

100

75

32

5 PROBLEM DEFINITION 5.1 UTILIZED SOFTWARE Three-dimensional steel storage tank subjected to uneven settlements has been analyzed by using ABAQUS, a commercially available finite element program, and the settlement was coded in FOTRAN. ABAQUS is a general purpose finite-element analysis program developed by Hibbitt, Karlsson & Sorenson, Inc. (HKS, 1998 and 2000). It includes three core products: ABAQUS/Standard, ABAQUS/Explicit, and ABAQUS/CAE. ABAQUS/Standard was used to perform three-dimensional simulation of uneven settlement on the steel storage tank behavior. 5.2 GEOMETRY OF TANK AND MATERIAL DEFINITION Having considered the ratio of thickness to radius of the tank, quadrilateral shell elements with four nodes (S4) were used to model the cylinder and most of bottom and top shell, while for some parts of roof and bottom especially for the center of them triangular elements (S3R) were used. Top angle was modeled by beam elements (B31OS) and soil (table 4) was modeled by spring elements (CONN3D2) that could only tolerate pressure and could not stand tension, k = 0.8 kg m , k= stiffness 3

of the spring and μ = 0.296 , μ = the coefficient of friction between steel tank and the soil, the area settlement below steel storage tank due to consolidation settlement is plotted in Figure 3 .The kinematic behavior of steel was used in this investigation. Top angle steel is ST_37; annular plate (annular plate is the part of bottom plate that is placed below the shell and usually has more thickness or is made of stiffener steel), in this project it is made of stiffener steel is A516M (A516) and material of the other parts of the tank is A 283M (A283). The diameter to height ratio of the tank is of order 4, and the slenderness ratio (radius to thickness) of the tank is order of 1000 (for the first course) to 3750 (for the last course). LAYER

DEPTH (m)

Table 4. The STRATUMS OF ZONE DESCRIPTION

Cu (kPa)

1

0~1.5

Very Stiff

135

2

1.5~12.5

Soft

25

3

12.5~14

Very Stiff

120

4

14~17.5

Stiff

60

5

17.5~18.5

Very Stiff

160

6

18.5~21

Very Stiff

110

7

21~27

Hard

200

8

27~34

Very Stiff

135

9

34~37

Hard

250

10

37~200

Very Hard

350

241


Steel ST_37 A 283M(A283) A 516M(A516)

Table 5. The carbon steel is used in the steel tank F y ( MPa) allowable stress ( MPa ) ε y 240 205 220

144 137 147

0.0011 0.00097 0.001

E s ( MPa) E p ( MPa) 2.1×105 2.1×105 2.1×105

1.05×103 1.05×103 1.05×103

F y = yield stress ε y = yield strain

E s

= elastic modules of steel

E p

= plastic modules of steel

The roof of the desired tank consists of a thin plate and a grid of girder and rafter laying on vertical columns for simplicity, the presented analysis is for the tank without roof In this way the benefit of roof structure will be conservatively neglected. Bottom plate has 10mm thickness.

(a) (b) Figure 2 (a) The shell of the steel storage tank, (b) the symmetric mesh of bottom and shell of the steel tank. 5.3 PROCESS OF LOADING Three successive steps for applying load was considered, in the first step the load of the weight of steel tank was considered, in the second step the pressure of water was applied and in the last step the settlement effect was coded. 6 THE EFFECT OF UNEVEN SETTLEMENT ON THE STEEL STORAGE TANK WITH RINGWALL FOUNDATION 6.1 THE EFFECT OF BOWL-SHAPE SETTLEMENT ON STEEL TANK Maximum acceptable settlement due to maximum-in plane stress was 368mm and the maximum stress occurs at the base of shell. The other noticeable criteria in this project is: the maximum strain must be less than .006 in the shell that is 6.08e-4 in 368mm settlement, the horizontal displacement of the tank must be less than .0016 of the diameter of the tank which is equal to 96mm. The calculated horizontal displacement due to the bowel-shaped settlement is 20mm, hence the settlement criteria of maximum-in plane stress is satisfied.

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Figure 3 obtained differential settlement in the tank area (consolidation settlement). Δmax =

1.96 meter that means the uniform settlement of tank is about 1.04 meter.

Figure 4 Tank bowl-shape settlement. This contour shows that maximum stress happens at the base of shell and the stress reduces in the height of shell.

Figure 5 Stress of tank versus bowl-shape settlement The curves show that the slope of base of shell suddenly reduces after 700mm settlement, because the stress is more than the yield stress of the steel. The numerical results dictate that the bowel-shape settlement has more effect on the base of shell than the other part of steel tank. Concluding, the acceptance criterion of this type of settlement is stress in the base of shell. Maximum shell displacement occurs in the shell about 273mm above the base of shell and is 21.7mm.

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6.2 THE EFFECT OF TILT SETTLEMENT ON THE STEEL TANK For this kind of settlement, 500mm linear settlement was applied by adding a FORTRAN subroutine. No part of the tank yields. The maximum displacement on the top of shell is 3.3mm and the maximum stress equal to 140.03MPa occurs in the shell about 1832mm above the base of shell which is acceptable. In this case the satisfaction of criteria of settlement of pipe, other maintainer and purring of water must be considered. 6.3 THE EFFECT OF EDGE DIFFRENTIAL SETTLEMENT ON THE STEEL TANK

(a)

(b)

Figure 6 (a) the plan and the elevation of the tank before edge settlement (b) edge differential settlement of steel tank. The tank tolerates only up to maximum 42.5mm of this kind of settlement. The formula of the applied settlement is , that b=settlement, and l = the length of settled area that was supposed to be 6 meter, and maximum a is 0.0425.

Figure 7 Maximum in plane stress of shell versus edge differential settlement of the steel tank. The maximum in plane stress occurred in depression part of the tank, e.g. in the height of 2.75m and the effect of this settlement increases in the higher height of the tank. This has been demonstrated in the next figure and the slope of the curve will increase with the height.

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Figure 8 In plane displacement of shell versus edge differential settlement of steel tank. 7 CONCLUSIONS According to the presented study: For bowl-shape settlement, allowable limits presented by klepikov • (1989) and USACE(1990) are very conservative but API(1995) presents reasonable values. Limits propose by D’orazio and Duncan (1987) is much more than others. For tilt settlement, Klepikov(1989) recommended 56mm and 98mm • for visible and ultimate tilts but API(1995) mentioned that in this type of settlements the effect of settlement on the tank nozzles must be consider, the analysis of the tank shows that API(1995) recommendation is acceptable. For bowl-shape settlement the stress in the base of the shell is the • criterion of the settlement while, for the tilt settlement, the allowable settlement of pipe must be considered. For the shell differential settlement, the maximum stress in the height of 2.75m, the horizontal displacement of the top of the shell, and the height of 11.8m must be considered. REFERENCES API 653, Appendix-B.TENTH EDITION, NOVEMBER (1998), ADDENDUM 1, JANUARY (2000) ADDENDUM 2, NOVEMBER (2001) D’orazio Timothy, B. and Duncan James, M.,(1987).Differential settlements in steel tanks, journal of Geotechnical engineering, vol. 113, NO 9, pp 967-983 Duncan James, M., ASCE, F. and D’orazio Timothy, B.D., (1985). Stability of oil storage tanks, journal of Geotechnical engineering, vol. 110, NO 9, September, 1984, paper NO.19125 Godoy, L.A. and Sosa E.M. (2003), Localized support settlements of thin-walled storage tanks, Thin-Walled Structures, NO 41, pp 941–955 Green, P.A., and Hight, D.W. (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. Hibbitt, H. D., Karlsson, B. I., and Sorensen (1997), ABAQUS/ User’s Manual, Version 5.7, Hibbitt, Karlsson and Sorensen, Inc.

Kamyab, H and 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. Kamyab, H. and Palmer, S.C.,(1991) .Displacements in oil storage tanks caused by localized differential settlement. J Pressure Vessel Technol. Trans ASME; 113:71–80. Klepikov, S.N.(1989).performance criteria-allowable deformation of building and damage. general report/discussion 28 Teng JG. Buckling of thin shells: recent advances and trends. Appl Mech Rev 1996;49(4):263– 74.

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