A nevv, method for replacingcorroded bottom plates

of oil storage fanks

Abstract

Wataru TsudaAkira IsedaKoichi Yamazaki

Nippon Petroleum Refining Co. Ltd.

Niigata Construction Co. Ltd.

Niigata Construction Co. Ltd.

The bottom plates of oil storage tanks sometimes need to be replaceddue 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. havejointly developed a new non-jacking method that allows the tankbottom plates to be replaced by supporting the tank with simple jigs.

Engineering analysis and strain gauge measurements prove thatthe 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 remarkablecost and time savings when compared with the conventional Jack-up 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 yearsthroughou t Japan concerning the acceptable bottom platethickness in large storage tanks. These regulations wereone of the results of an oil spillage accident at a Japaneserefinery 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 bottomplate replacement work, involving jack mounting attach­ments to the tank and localized foundation reinforcementunder the jacks. It is labour intensive, ties us costly hy­draulic jacking equipment, and it takes a long time toapply.

Work Flow Chart

Planning & design analysis

Preparatory work(covers, enclosures, piping, etc.)

Progressivereplacement

A simpler method was introduced by Nippon PetroleumRefinery Co. and Niigata Construction Co. in 1977 aftertwo years of study, and is referred to as the "Support PieceMethod". The particular merits of the Support Piece Methodare a typical 30% reduction of both repair costs and timewhen compared with the Jack-up Method. More than 260storage tanks (of all types and sizes) have been successfullyrepaired by the Support Piece Method, including large tanksin the 100,000 kilolitre range.

2. On-site Procedure

2.1 Reinforcing the shellA. reinforcing ring is normally installed around the inner

or the outer circumference of the shell plates (Fig. 1). Thisprevents any distortion of the shell plates from the residualstresses which may have accumulated during constructionand service, and retains the correct circular profile duringthe rectification work.

2.3 Cutting the shell platesAn opening cut 30 ft (9m) in length is made around the

shell plates (Fig. 3). All shell cutting work needs to be doneaccurately and carefully because the cut surfaces becomethe new joint faces between the shell and annular plates.

Shell plate

Fig. 1 Installation of the reinforcing ringFig. 3 Cutting out the shell and annular plates in

progressive stages •2.2 Marking off

A cutting line is normally marked a nurumum of 1"(25mm) above the base of the shell plates. This dimension isgoverned 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 andannular 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 flamecutting equipment together with a transient line are markedat the same time (Fig. 2). Before marking, any paint, rustand oil which are close to the marking area are thoroughlyremoved. An automatic gas cutting machine is then installedagainst one of the marker lines, taking care to ensure thestraightness and angle of the cutting plane.

Shellplate

Ma rk c r line for .rut om.u i..11~1111l' l"LIt t i 11t-'- l'(! LIi \1111l' 11t

Transient marker line

Cut t im: line

lr,r I r-L- I--...I_----l---l. ----,

Fig. 2 Marking the cutting line, transient marker lineand marker line for the automatic flamecutting equipment

2.4 Annular plate replacement and temporary supportsThe first annular segment of the tank bottom plate

complete with the heel of the shell plate. is then cu t ou tand withdrawn through the side plate aperture, taking carenot to damage the shell plate or tank foundations. Areplacement annular plate, cut precisely to size in the shop,is next maneuvered through the aperture and tacked intoposition.

Shell plate

Fig. 4 Support pieces, other jigs and initial welding ofthe annular plates

Fig. 4. shows the temporary tank support measures whichare then taken, starting with the first support pieces andshoes. These are positioned at appropriate intervals to suitthe weight of the tank. The support pieces are welded to thetank shell plates and supported via shoes on the replacementannular plate. These shoes protect the new annular platesfrom subsequent gas cutting operations and dist rihu te theloading stress from the support pieces over the new annularplate.

•

3. Engineering analysis

Outside diameter : 91,135mmHeight . 15,846mm

690.8ton136.2ton

W = 827.0ton

2.8 TestingThe new bottom is tested in accordance with the testing

methods specified in API. Std. 650, 5.3. Additionally, amagnetic particle or liquid penetrant examination is con­ducted.

All tanks are checked by an engineering analysis beforestarting the work. As an example of this analysis, a tankwith the following specifications was used for both thecalculations and field measurements:

also narrows the unwelded radial gap between adjacentannular plates and can sometimes cause cracking to theexisting weld bead end. So, before completing the butt weldbetween adjacent annular plates, remaking of the grooveand inspection of the existing weld bead end are essential.The final welding operation is the joint between the annularplates and the bottom plates. Fig. 5. shows the order ofthese individual welding operations.

2.7 FinishingAfter the welding work has been completed, all the

support pieces, guide plates and jigs are finally removed andtheir temporary weld marks are finished flat by grinding.

(1) Tank DutyContent CRUDEType F. R. T.

Capacity 96,000kl

Total

The stresses induced in the shell plate during this operationmust be lower than the stresses in the support pieces andmust also be at an acceptable level to comply wi th legalsafety standards and codes of practice.

(2) Tank WeightShellAccessories

Shell plate

Fig. 5 Order of welding operations

Guide plates are installed to maintain the correct shapeof the shell plates and to guide them when the tank is laterlowered on to the new annular plates. These guide plates alsoprevent any horizontal movement by wind force when allthe bottom plate annular segments have been replaced. As afinal safety precaution, wedges are inserted at regularintervals into the gap.

This procedure of:-- cutting and removing the old bottom plate annular

segmentinserting and tacking the new bottom plate segmentinto positioninstalling the support pieces, support plates, guide

plates and wedgesis continued around the circumference of the tank until allthe bottom plate annular segments have been replaced. Thetank is then competely supported on the. new annular

, plates via the support pieces.

2.5 Setting-down the tankBefore lowering the tank, the annular plates are butt­

welded radially over a length of about 12" (300mm)inwards from the outer circumference and the weld surfaceis ground flat as shown in Fig. 4. This gives the finishedsurface on which the tank shell plates will sit. A magneticparticle or liquid penetrant examination is done on thissurface.

All the wedges are then removed and controlled loweringof the tank on to the new annular plates is achieved in smallstages by cutting 3/8" (10mm) out of the support pieces ina progressive sequence until the cut-back shell plates arecompletely supported on the replacement annular plates.When an internal roof-supporting structure exists, its lengthis also adjusted during the setting-down operation.

2.6 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 betweenthe shell and annular plates is welded. This welding operation

3.13.1.1

CalculationsEarthquake Load (horizontal) 1) 2)

3.1.5 Support Pieces(1) Load

3.1.2 Wind Load (horizontal) 3) 4)

Pw=C.q.A (l)A =h . D ' (2)

q = ~. P. Vo2(h'/ho)~ (3)

Ps = k.W

ps : earthquake loadk : earthquake factorW : tank weight

= 82.7ton= 0.1= 827ton

W MoP =- +-- (1)su N Z·····

N.')'Z = -- (2)

Z

Psu support piece load = 3.31 ton/pieceN : number of support pieces = 284Mo : overturning moment =2,496ton.mZ : modulus of support piece section = 6,471 m

'Y : tank radius = 45.57m

(2) Buckling load 5)

P_c_ =3.3 > 1.5 is maintainedPsu

\ _ I. SOCTll()- /' 1'.Oun

1.'.Ol'lll I /' i I"~

~ "CU"-VfL_§ __ ;I,,',"I

r .

, I ,t

h -l.ucm

1

1000

A· f

1 + : . (~) 2

tK=-- (2)

vT2

Pc = - - - - - -

= 315ton

= 1.0= 1444m2

= 15.846m= 91.135m= 218kg/m2

= 0.115kg. sec2/m4

=60m/sec=15m= 16.346m

Pw: wind load

C : wind factorA : projected wind areah : tank heightD : tank diameterq : air pressurep : air density

V0: design wind speedh o : const. height

h' : height from ground

3.1.3 Sliding Resistance

Rw =W . J1

Rw : sliding resistance = 413.5 tonW : tank weight = 827tonJ1 : coefficient of friction =0.5 9)

When Rw ~ Ps or Pw, the tank is safe from horizontal

sliding. (If the opposite case, action must be taken to in­crease Rw .)

3.1.4 Overturning Resistance

Pc : max. compressive load = 10.8tonA : support piece cross-sectional area

=6.4cm2

f : compressive strength = 3 ,400ton/cm 2

n : constant (safety factor) = ~

a : Rankine factor = 1/7,500£ : support piece length = 20.0cmK : first moment of area = 0.46cmt : support piece thickness = 1.6cm

(3) Fillet weld joint strength between shell and supportpiece. 6)

•(2)

sC =--

V2'

Psu

2 . C . £0 2• 77

Fa =--------Mr = W . D/ 2 ... (I)Mo = R, (Pw).H . (2)H =~h ..... (3)

M, : overturning moment resistance = 37,684ton.m

H : height above ground of center of gravity

= 7.923rn

When M, ~ Mo, the tank is safe from overturning.(If the opposite case, action must be taken to increase

Mr·)

Mo : overturning moment =2,496ton.mL

/J-..,/'/1II

.'i'-'1--I

F : combined longitudinal, bendingand shear stress = 0.78 toniem 2

C : throat of fillet weld = 0.56cmS : fillet weld size = 0.8cm71 : weld efficiency = 0.85 7)

L : upper width ofsupport piece = 5.0cm

£0 : length of fillet weld = 12.0cmTp : permissible shear

stress =950kgjcm 2

(ASTM A570 Gr. 33) 8)

When Fa < Tp' this fillet weld joint is .safe.

3.2 Field measurements3.2.1 Measured stresses

A three-dimensional finite element analysis program wasused for shell stress calculation. Ideally elastic deformationand rigid foundations were assumed. In addition, Fig. 6.shows the measured results with' strain gauges duringreconditioning of a 96,000kl floating roof tank. These re-sults are for support pieces # 283 and # 284, and for the

shell plates immediately adjacent to them. The supportpieces were cut progressively in numerical order from the

efirst (# I) to the last (#284). The stress level reached amaximum when the unsupported length was 30 - 40meters.Although the calculated stress continued to increase inproportion to this length, in practice minor elastic deforma­tion of the shell limited the maximum unsupported span to23meters. Beyond this length, the support pieces werebrought into contact with the annular plate and the stresslevel was contained within competely acceptable limits. Themaximum shell stresses were 19 .3 (compressive), 13.1(tensile) and 7.1 (shear) kg/rum? adjacent to support piecepositions #283 and #284 before the support piece was cutat a circumferential distance of about 45meters round fromthese positions. One support piece (#284) was loaded tobeyond its elastic limit without buckling occurring.

Where Pm is the general primary membrane stressPb is the primary bending stressQ is the secondary stressand Sm is the lesser of 1/3Su (tensile strength)or

2j3Sy (yield strength)Suand Sy for this shell material (ASTM A 633GrC) are

53 & 36kgjn1m2, 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 creat-ing 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, atypical 30% reduction in both costs and ou t-of-service timeis achievable when compared with the Jack-up Method.

2) Gravity does all the tank moving, giving good inherentsafety to the method. Moreover, the tank support measurestaken - support pieces, guide plates and wedges are many,so that tank stability is maintained throughout the work.

3) Complete replacement of the bottom plates andreconditioning of certain tank foundations can be done at

the same time4) The tank dike is not damaged because all work takes

place inside the dike and additional ground reinforcementis unnecessary.

Unsupported length (Ill)

References

1) Sub-sect. 19 of Sec. 4, Notification concerningtechnical standard for controlling dangerous objectsof 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,

p2197) 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) P120

~ #283 support piCL'C

--\k- ShL'1I adjacent to #283 support piCL'l'

-.- #284 support piccc

-.- Shell adjacent to #284 support piece

Fig. 6 Stresses in the shell plates and support piecesduring operations

'-----,~ -------...,...--------------'

Wire strain !!all!!CSwere installed

3.2.2 Evaluation of imposed stress intensityFrom the measured results, the imposed stress intensity

was evaluated. ASME Boiler and Pressure Vessel Code SectionVIII Division 2 stipulates that (Pm +Pb + Q) shall not exceed

3S m·

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