A nevv, method for replacing corroded bottom plates of oil storage fanks
Wataru Tsuda Akira Iseda Koichi Yamazaki
Nippon Petroleum Refining Co. Ltd. Niigata Construction Co. Ltd. Niigata Construction Co. Ltd.
Abstract The bottom plates of oil storage tanks sometimes need to be replaced due to corrosion or rivetted joint leakage. This replacement work usually involves lifting the tank with hydraulic jacks, a costly and time-consuming exercise. Nippon Petroleum Refining Co. and Niigata Construction Co. have jointly developed a new non-jacking method that allows the tank bottom plates to be replaced by supporting the tank with simple jigs. Engineering analysis and strain gauge measurements prove that the method does not generate unacceptable stress levels in the tank, even from earthquake and wind loads, during the work. The method has been successfully applied to bottom plate replacement of more than 260 tanks, and it has demonstrated remarkable cost and time savings when compared with the conventional J ackup Method.
Reprinted from a paper to be published in the Journal of the Japan Petroleum Institute
1. Introduction
Strict regulations have been enforced in recent years throughou t Japan concerning the acceptable bottom plate thickness in large storage tanks. These regulations were one of the results of an oil spillage accident at a Japanese refinery in 1974 and local government agencies require a regular and systematic inspection of all tank bottom plates. The Jack-up Method is conventionally used for bottom plate replacement work, involving jack mounting attachments to the tank and localized foundation reinforcement under the jacks. It is labour intensive, ties us costly hydraulic jacking equipment, and it takes a long time to apply.
,
Work Flow Chart
Planning & design analysis
Preparatory work (covers, enclosures, piping, etc.)
Progressive replacement
A simpler method was introduced by Nippon Petroleum Refinery Co. and Niigata Construction Co. in 1977 after two years of study, and is referred to as the "Support Piece Method". The particular merits of the Support Piece Method are a typical 30% reduction of both repair costs and time when compared with the Jack-up Method. More than 260 storage tanks (of all types and sizes) have been successfully repaired by the Support Piece Method, including large tanks in the 100,000 kilolitre range.
2.3
2. On-site Procedure 2.1
Reinforcing the shell A. reinforcing ring is normally installed around the inner or the outer circumference of the shell plates (Fig. 1). This prevents any distortion of the shell plates from the residual stresses which may have accumulated during construction and service, and retains the correct circular profile during the rectification work.
Fig. 1
Cutting the shell plates An opening cut 30 ft (9m) in length is made around the shell plates (Fig. 3). All shell cutting work needs to be done accurately and carefully because the cut surfaces become the new joint faces between the shell and annular plates. Shell plate
Fig. 3
Installation of the reinforcing ring
Marking off A cutting line is normally marked a nurumum of 1" (25mm) above the base of the shell plates. This dimension is governed by the following: Working space for replacing the annular plates. Welding and inspection of new annular plate butt joints. Removing existing weld metal from the shell and annular plate joint. The extent of corrosion at the base of the shell plates. The position of reinforcement for existing nozzles. The false marker line for the automatic gas/oxygen flame cutting equipment together with a transient line are marked at the same time (Fig. 2). Before marking, any paint, rust and oil which are close to the marking area are thoroughly removed. An automatic gas cutting machine is then installed against one of the marker lines, taking care to ensure the straightness and angle of the cutting plane.
Cutting out the shell and annular plates in progressive stages
•
2.2
2.4
Annular plate replacement and temporary supports The first annular segment of the tank bottom plate complete with the heel of the shell plate. is then cu t ou t and withdrawn through the side plate aperture, taking care not to damage the shell plate or tank foundations. A replacement annular plate, cut precisely to size in the shop, is next maneuvered through the aperture and tacked into position.
Shell plate
•
Shell plate Ma rk c r line for .rut om.u i.. 11~1111l' l"LI t t i 11t-'- l'(! LIi \1111l' 11t
Fig. 4 Transient marker line
Cut t im: line rlr,I r-L-
I--...I_----l---l.
Fig. 2 Marking the cutting line, transient marker line and marker line for the automatic flame cutting equipment
----,
Support pieces, other jigs and initial welding of the annular plates
Fig. 4. shows the temporary tank support measures which are then taken, starting with the first support pieces and shoes. These are positioned at appropriate intervals to suit the weight of the tank. The support pieces are welded to the tank shell plates and supported via shoes on the replacement annular plate. These shoes protect the new annular plates from subsequent gas cutting operations and dist rihu te the loading stress from the support pieces over the new annular plate.
,
Guide plates are installed to maintain the correct shape of the shell plates and to guide them when the tank is later lowered on to the new annular plates. These guide plates also prevent any horizontal movement by wind force when all the bottom plate annular segments have been replaced. As a final safety precaution, wedges are inserted at regular intervals into the gap. This procedure of: -- cutting and removing the old bottom plate annular segment inserting and tacking the new bottom plate segment into position installing the support pieces, support plates, guide plates and wedges is continued around the circumference of the tank until all the bottom plate annular segments have been replaced. The tank is then competely supported on the. new annular plates via the support pieces.
2.5
2.6 _
2.7
Finishing
After the welding work has been completed, all the support pieces, guide plates and jigs are finally removed and their temporary weld marks are finished flat by grinding.
2.8
Testing
The new bottom is tested in accordance with the testing methods specified in API. Std. 650, 5.3. Additionally, a magnetic particle or liquid penetrant examination is conducted.
Setting-down the tank
Before lowering the tank, the annular plates are buttwelded radially over a length of about 12" (300mm) inwards from the outer circumference and the weld surface is ground flat as shown in Fig. 4. This gives the finished surface on which the tank shell plates will sit. A magnetic particle or liquid penetrant examination is done on this surface. All the wedges are then removed and controlled lowering of the tank on to the new annular plates is achieved in small stages by cutting 3/8" (10mm) out of the support pieces in a progressive sequence until the cut-back shell plates are completely supported on the replacement annular plates. When an internal roof-supporting structure exists, its length is also adjusted during the setting-down operation.
&
also narrows the unwelded radial gap between adjacent annular plates and can sometimes cause cracking to the existing weld bead end. So, before completing the butt weld between adjacent annular plates, remaking of the groove and inspection of the existing weld bead end are essential. The final welding operation is the joint between the annular plates and the bottom plates. Fig. 5. shows the order of these individual welding operations.
3. Engineering analysis All tanks are checked by an engineering analysis before starting the work. As an example of this analysis, a tank with the following specifications was used for both the calculations and field measurements: (1) Tank Duty Content CRUDE Type F. R. T. Capacity 96,000kl
Outside diameter : 91,135 mm Height . 15,846mm
Welding
After the tank has been lowered on to the new annular plates and jigs have been installed for any adjustment to the curvature of the bottom shell plates, the T-joint between the shell and annular plates is welded. This welding operation Shell plate
(2) Tank Weight Shell Accessories Total
Fig. 5
Order of welding operations
690.8ton 136.2ton W = 827.0ton
The stresses induced in the shell plate during this operation must be lower than the stresses in the support pieces and must also be at an acceptable level to comply wi th legal safety standards and codes of practice.
3.1.5
3.1 Calculations Earthquake Load (horizontal) 1) 2) 3.1.1
Support Pieces
(1)
Load
P s = k.W
Psu
3.1.2
Wind Load (horizontal) 3)
P w: C : A : h : D : q :
wind load wind factor projected wind area tank height tank diameter air pressure p : air density V 0: design wind speed h o : const. height h' : height from ground
3.1.3
Z = --
= 0.1
P su support piece load = 3.31 ton/piece N : number of support pieces = 284 M o : overturning moment = 2,496ton.m Z : modulus of support piece section = 6,471 m 'Y : tank radius = 45.57m
4)
(l) (2) (3)
(2)
= 315ton
= 1.0 = 1444m 2 = 15.846m
Pc =
-
A· f
-
-
-
1+ :
.
-
-
(~)
= 15m
P _c_ Psu
= 16.346m
Fa
L_§
1000
__
I
(2)
= 3.3 >
h
I
r .
,t
-l.ucm
1.5 is maintained
Fillet weld joint strength between shell and support piece. 6)
•
P
= - - - -su-2 - - 2 . C . £0
•
77
s V2'
C =--
M, : overturning moment resistance = 37,684ton.m
= 2,496ton.m L
/'/1 /J-..,
When M, ~ M o, the tank is safe from overturning. (If the opposite case, action must be taken to increase
I"~
Pc : max. compressive load = 10.8ton A : support piece cross-sectional area = 6.4cm 2 2 f : compressive strength = 3 ,400ton/cm n : constant (safety factor) = ~ a : Rankine factor = 1/7,500 £ : support piece length = 20.0cm K : first moment of area = 0.46cm t : support piece thickness = 1.6cm
Overturning Resistance
: height above ground of center of gravity = 7.923rn
1'.Oun
i
vT2
= 60m/sec
Mr = W . D / 2 ... (I) Mo = R, (Pw).H . (2) H = ~h ..... (3)
SOCTll
/'
2
K=--
(3)
Mr·)
I.
I /'
~ "CU"-Vf;I,,',"
1
t
When R w ~ P s or P w, the tank is safe from horizontal sliding. (If the opposite case, action must be taken to increase R w .)
H
_ ()-
,
2
= W . J1
M o : overturning moment
\
= 91.135m = 218kg/m = 0.115kg. sec 2 /m 4
R w : sliding resistance = 413.5 ton W : tank weight = 827ton J1 : coefficient of friction = 0.5 9)
3.1.4
Buckling load 5)
1.'.Ol'lll
Sliding Resistance
Rw
(2)
Z
= 827ton
Pw=C.q.A A =h . D ' q = ~. P. Vo2(h'/ho)~
M
N.')'
= 82.7ton
p s : earthquake load k : earthquake factor W : tank weight
W
o =-N + -Z - · · · · · (1)
I I
.'i'-'
1-I
F C S 71 L
(2)
: combined longitudinal, bending and shear stress = 0.78 toniem 2 : throat of fillet weld = 0.56cm : fillet weld size = 0.8cm : weld efficiency = 0.85 7) : upper width of support piece = 5.0cm
£0 : length of fillet weld = 12.0cm Tp
: permissible shear
= 950kgjcm 2
stress
(ASTM A570 Gr. 33) 8) When Fa
< T p' this fillet
weld joint is .safe.
3.2 Field measurements 3.2.1 Measured stresses A three-dimensional finite element analysis program was used for shell stress calculation. Ideally elastic deformation and rigid foundations were assumed. In addition, Fig. 6. shows the measured results with' strain gauges during reconditioning of a 96,000kl floating roof tank. These results are for support pieces # 283 and # 284, and for the shell plates immediately adjacent to them. The support pieces were cut progressively in numerical order from the
efirst (# I) to the last (#284). The stress level reached a maximum when the unsupported length was 30 - 40meters. Although the calculated stress continued to increase in proportion to this length, in practice minor elastic deformation of the shell limited the maximum unsupported span to 23meters. Beyond this length, the support pieces were brought into contact with the annular plate and the stress level was contained within competely acceptable limits. The maximum shell stresses were 19 .3 (compressive), 13.1 (tensile) and 7.1 (shear) kg/rum? adjacent to support piece positions #283 and #284 before the support piece was cut at a circumferential distance of about 45meters round from these positions. One support piece (#284) was loaded to beyond its elastic limit without buckling occurring.
~
#283 support piCL'C
--\k-
ShL'1I adjacent to #283 support piCL'l'
-.- #284 support piccc
Where
Pm is the general primary membrane stress Pb is the primary bending stress Q is the secondary stress and Sm is the lesser of 1/3S u (tensile strength)or 2j3S y (yield strength) Suand Sy for this shell material (ASTM A 633GrC) are 53 & 36kgjn1m 2 , respectively. Thus 3 . Sm is 53kgjmm 2 • The maximum measured stress intensity corresponding to Pm + Pb + Q was 2 x maximum shearing stress (14.1 kg! mrn") so that a safety factor of 3.8 existed without creating any distortion or safety hazards.
4. Principal advantages of the new method 1) Simple and repetitive work procedures with minimal specialized equipment and low manhours. Consequently, a typical 30% reduction in both costs and ou t-of-service time is achievable when compared with the Jack-up Method. 2) Gravity does all the tank moving, giving good inherent safety to the method. Moreover, the tank support measures taken - support pieces, guide plates and wedges are many, so that tank stability is maintained throughout the work. 3) Complete replacement of the bottom plates and reconditioning of certain tank foundations can be done at the same time 4) The tank dike is not damaged because all work takes place inside the dike and additional ground reinforcement is unnecessary.
References
- . - Shell adjacent to #284 support piece
'-----,~
-------...,...--------------'
Wire strain !!all!!CS were installed
Unsupported length
Fig. 6
(Ill)
Stresses in the shell plates and support pieces during operations
3.2.2 Evaluation of imposed stress intensity From the measured results, the imposed stress intensity was evaluated. ASME Boiler and Pressure Vessel Code Section VIII Division 2 stipulates that (Pm + Pb + Q) shall not exceed 3S m·
1) Sub-sect. 19 of Sec. 4, Notification concerning technical standard for controlling dangerous objects of the FDB (Japan) 2) 3.1.2 (5), Welded steel tanks for oil storage, JIS B-850 1 (1979) 3) Sub-sect. 20 of Sec. 4, Notification concerning technical standard for controlling dangerous objects of the FDB (Japan) 4) 3.1.2 (6), Welded steel tanks for oil storage, JIS B-850 1 (1979) 5) Rankine's formula 6) K. Enomoto, Yosetsu-Kogaku, Keirin Tosho, 1971, p219 7) 3.5.2 (1), Welded steel tanks for oil storage, JIS B-8501 (1979) 8) 3.9.2 (4.1), Welded steel tanks for oil storage, lIS B-850 1 (1979) 9) Appendix IV, Welded steel tanks for oil storage, JIS B-850 1 (1979) P 120
d;P NIIGATA CONSTRUCTION CO., LTD. Shuwa Shiba Park Bldg., 4-1, 2-Chome, Shibakoen, Minato-ku, Tokyo, Japan Telephone: 03-433-8231 Telex: 2425324 NCCTO J
