65899223 niigata replacing bottom plates of oil storage tanks

5/22/2018 65899223 Niigata Replacing Bottom Plates of Oil Storage Tanks

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n vv method for repl cingcorroded ottom pl tesof oil stor ge nks

Abstract

Wataru TsudaAkira IsedaKoichi Yamazaki

Nippon Petroleum Refining o LtdNiigata onstruction o LtdNiigata onstruction o 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 hy-draulic jacks a costly and time consuming exercise.

Nippon Petroleum Refining Co. and Niigata Construction Co. havejointly developed a new non jacking method that allows th e tankbottom plates to be replaced by supporting the tank with simple jigs.

Engineering analysis and strain gauge measurements prove thatt he method does not generate unacceptable stress levels in the tankeven from earthquake and wind loads during the work.

The method has been successfully applied to bottom plate replace-ment of more than 260 tanks and it has demonstrated remarkablecost and time savings when compared w ith t he conventional Jackup Method.

Reprinted from a paper to be published in the Journal the Japan Petroleum nstitute



Introduction

Stric t regulat ions have been enforced in recent yearsthroughou t Japan concerning the acceptable bottom platethickness in large storage tanks . These regulat ions wereone of the results of an oil spillage accident at a Japaneserefinery in 1974 and local government agencies require aregular and systematic inspection of all tank bottom plates.The Jack-up Method is conventionally used for bottomplate replacement work, involving jack mount ing attachments to the t ank and localized foundation reinforcementunder the jacks. It is labour intensive, ties us cos tly hydraulic jacking equipment, and it takes a long time toapply.

Work ow 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 s tudy , and is refer red 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 Procedure2.1 Reinforcing the shell

A. reinforcing ring is normally installed around the inneror the outer circumference of the shell plates (Fig. 1). Thisprevents any distort ion of the shell plates from the residualstresses which may have accumulated during construct ionand 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 cut ting work needs to be doneaccurately and carefully because the cut surfaces becomethe new joint faces between the shell and annular plates.

Shell plate

Fig. Installation of the reinforcing ring Fig. 3 Cutting out the shell and annular plates inprogressive stages2.2 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

Mark cr line fo r .rut om u i~ l l LIt t i 11t l ( LIi \1111l 11t

Transient marker line

utt im: liner I r 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 wi th 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 in toposition.

Shell plate

Fig. 4 Support pieces, other jigs and initial welding ofthe annular platesFig. 4. shows the temporary tank support measures which

are then taken , s tarting with the first suppor t pieces andshoes. These are posit ioned 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 pro tect the new annular platesfrom subsequent gas cut ting 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 spec if ied 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 thi s analysis, a tankwith the following specifications was used for both thecalculations and field measurements:

also narrows the unwelded radia l gap between adjacentannular plates and can sometimes cause cracking to theexisting weld bead end. So, before complet ing 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 annularpl ate s and the bottom plates. Fig. 5. shows the o rder ofthese individual welding operations.2.7 Finishing

After the welding work has been completed, all thesupport 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

TotalThe stresses induced in the shell plate during this operation

must be lower than the stresses in the support pieces andmust also be at an acceptable level to comply with legalsafety standards and codes of practice.

2) Tank WeightShellAccessories

Shell plate

Fig. 5 Order of welding operations

Guide plates are ins tal led to maintain the correct shapeof the shell p la tes and to guide th em when the ta nk 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 pla te annular

segmentinser ting and tacking the new bottom plate segmentinto positioninstalling the support pieces, support plates, guideplates and wedges

is continued around the circumference of the tank until allthe bottom plate annular segments have been replaced. Thetank is t hen compe te ly supported on the. new annular

, plates via the support pieces.2.5 Setting-down the tankBefore lowering the tank, the annular p la te s are buttwelded 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 l iquid 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) ou t of the support pieces ina progressive sequence until the cut-back shell plates arecompletely supported on the rep lacement annular plates.When an internal roof-supporting structure exists, its lengthis also adjusted during the setting-down operation.2.6 Welding Afte r th e tank has been lowered on to the new annular

_ plates and jigs have been installed for any adjustment to thecurvature 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)P w = C . q . A l)A = h . D 2)q = P V o 2 h / h o ) ~ 3)

Ps = k.Wps : earthquake loadk : earthquake factorW : tank weight

= 82.7ton= 0.1= 827ton

W MoP =- + 1)su N ZN. )Z = 2)Z

P u support piece load = 3.31 ton/pieceN : number of support pieces = 28 4Mo : overturning moment =2,496ton.mZ : modulus of support piece section = 6,471 mY : tank radius = 45.57m

2) Buckling load 5)

P_c_ = 3. 3 > 1.5 is maintainedPsu

\ _ I. SOCTll )- 1 .O1. .Ol lll I / i ~CU -Vf_ __ ;I,,

I r

, I t

h -l.ucm

11000

A f1 + : . ~ 2tK= 2)vT2

Pc = - - - - - -= 315ton= 1. 0= 1444m2= 15.846m= 91.135m= 218kg/m 2= 0.115kg. sec 2/m 4= 60m/sec= 15 m= 16.346m

Pw: wind loadC : wind facto rA : projected w in d a re ah : tank heightD : tank diameterq : air pressure : air densityV 0: design wind speedh o : const. heighth : height from ground

3.1.3 Sliding ResistanceRw =W .

Rw : sliding resistance = 413.5 to nW : tank weight = 827ton : coefficient of friction =0.5 9)

When Rw Ps or Pw, th e tank is safe from h o rizon talsliding. If t he o pp os it e case, action must be taken to increase Rw .)3 .1.4 Overturning Resistance

Pc : max. compressive load = 10.8tonA : support piece cross-sectional area=6.4cm 2f : compressive strength = 3 ,400ton/ cm 2n : constant safety factor) = a : Rank ine facto r = 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 a nd s u pp o rtpiece. 6)

2)sC =V

Psu2 . C . 0 2 77 a=r = W . D / 2 I)Mo = R, Pw).H . 2)H = 3)M, : overturning moment resistance = 37,684ton.m

H : height above ground of center of gravity= 7.923rn

When M, Mo, th e tank is safe from overturning. If th e opposite case, action must be taken to increaseMr)

Mo : overturning moment = 2,496ton.m L/ J - . . ,/1

II. i1--I

F : combined longitudinal, bendingan d shear stress = 0.78 toni em 2

C : throat of fillet weld = 0.56cmS : fillet weld size = 0.8cm : weld efficiency = 0.85 7)L : u p per width of

support piece = 5.0cm



0 : length of fillet weld = 12.0cmp : permissible shear

stress =950kgjcm 2(ASTM A570 Gr. 33) 8

When Fa < pthis fillet weld joint is .safe.3.2 Field measurements3.2.1 Measured stressesA three-dimensional finite element analysis program was

used for shell s tress calculation. Ideally elastic deformationand rigid foundations were assumed. In addition, Fig. 6.shows the measured results with' s train gauges duringreconditioning of a 96,000kl floating roof tank. These re-sults are for support pieces 283 and 284, and for theshell plates immediately adjacent to them. The supportpieces were cu t progressively in numeri cal o rd er f rom the

ef i r s t ( I) to the last ( 284). The stress level reached amaximum when the unsupported length was 30 - 40meters.A lthough the calculated stress continued to increase inproportion to this length, in practice minor elastic deformation of the shell l imited the maximum unsupported span to23meters. Beyond this length, the support pieces werebrought into contact with t he annul ar pl ate 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 cu tat 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)orj Sy (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 toPm + Pb + Q was 2 x maximum shearing stress (14.1 kgmrn so that a safety factor of 3.8 existed without creat-ing any distortion or safety hazards.

4. Principal advantages of the new method1) 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 measures

taken - support pieces, guide plates and wedges are many,so t ha t t ank stability is maintained throughout the work.3) Complete replacement of the bottom plates and

reconditioning of certain tank foundations can be done atthe same time

4) The tank dike is no t damaged because all work takesplace inside the dike and additional ground reinforcementis unnecessary.

Unsupported length I l l

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 concerningtechnical standard for controlling dangerous objectsof th e FDB (Japan)

4) 3.1.2 (6), Welded s teel tanks for oil storage,JIS B-8501 (1979)

5) Rankine's formula6) 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-8501 (1979)

9) Appendix IV, Welded steel tanks for oil storage,JIS B-850 1 (1979) P120

28 3 support piCL C-- \k- ShL 1I adjacent to 283 support piCL l- . - 284 support pi

hell adjacent to 284support piece

Fig. 6 Stresses in the shell p la te s and support piecesduring operations

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

Wire strain all Swere 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 no t exceed3Sm



d NIIGATA ONSTRU T ON CO LTDShuwa Shiba Park Bldg. 4 1 2 Chome Shibakoen Minato ku Tokyo Japan

Telephone: 03 433 8231 Telex: 4 53 4 NCCTO J

65899223 niigata replacing bottom plates of oil storage tanks

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