bigger and cheaper lng tanks

8/12/2019 Bigger and Cheaper LNG Tanks

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BIGGER & CHEAPER LNG TANKS?

OVERCOMING THE OBSTACLES CONFRONTING

FREESTANDING 9% NICKEL STEEL TANKS UP TO

AND BEYOND 200,000 m3

DES RESERVOIRS DE GNL PLUS GRANDS

ET MOINS CHERS?

COMMENT SURMONTER LES OBSTACLES AUXQUELS DOIVENT

FAIR FACE LES RESERVOIRS AUTOSUPPORTANTS EN ACIER AU

NICKEL A 9% DUNE CAPACITE DE 200 000 m3ET PLUS

Bob Long

Technical DirectorWhessoe LGA Gas Technology Ltd.

Brinkburn RoadDarlington DL3 6DS, U.K.

ABSTRACT

Much was made at LNG 11 of the high cost of the storage components of LNGprojects to the point that the cost of LNG tanks could have an adverse influence on theeconomic viability of potential projects. This together with the steadily increasing unitcapacity of LNG tanks, driven by financial and effective use of space considerations,make the constraints on tank capacity and cost imposed by the design codes more than aninconvenience.

Both API 620 Appendix Q and BS 7777 : 1993 include limitations on allowabledesign stress and maximum shell plate thickness and impose hydrostatic testingrequirements which limit the effective capacity of LNG tanks built to economic (height todiameter) proportions. This is illustrated by means of examples.

The opportunity exists to rewrite some of these rules within the current deliberationsof the CEN Committee TC 265 as it seeks to produce a new Eurocode for low

temperature storage tanks. This paper presents the case for the use of the higher (BS7777) stresses with the partial hydrotest requirements, which would allow tank capacitiesup to, and beyond 200,000m3 without excessive shell thickness. The conventionalwisdom relating to the benefits of a full hydrostatic test, with particular reference to 9%Nickel steels and Nickel based alloy weld metals, will be challenged by consideringcurrently available material quality and crack initiation and/or arrest fracture controlphilosophies, based on the wealth of material test data already available to the industry.


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The influence of these proposals on tank capacity, cost and construction timescalewill be illustrated by way of examples.

RESUME

Pendant la LNG11, le cot lev des composants entrant dans la constitution dessystmes de stockage de GNL a fait lobjet dune grande attention. En effet, le prix desrsevoirs pourrait avoir des rpercussions ngatives sur la viabilit conomique desprojets potentiels. Ceci ajout laugmentation croissante de la capacit des rservoirs,pour des raisons financires et dutilisation optimale de lespace, rend encore pluspnibles les contraintes lies la capacit et aux cots imposes par les normes deconstruction.

Les normes API 620 Annexe Q et BS 7777 :1993 dfinissent des limites sur lescontraintes admissibles ainsi que lpasseur maximale des parois et imposent des essaishydrostatiques qui limitent la capacit utile des rservoirs de GNL construits avec desproportions conomiques (hauteur par rapport au diamtre). Cette question est illustrepar des exemples.

Lopportunit de modifier certaines de ces rglementations se prsente dans le cadredes dlibrations actuelles du Comit CEN TC 265 charg de produire un nouveauEurocode pour les rservoirs de stockage basses tempratures. Ce texte prsente desarguments en faveur de lemploi de contraintes plus leves (BS 7777) avec des essaishydrostatiques partiels qui permettrait de porter la capacit des rservoirs

200 000 m3 et plus sans augumenter de faon excessive lpaisseur des parois.Lattitude conventionnelle relative aux avantages dessais hydrostatiques complets, avecun rfrence particulire aux aciers au nickel 9% et aux alliages dapport base denickel, est remise en question par ltude de la qualit des matriaux actuellement

disponibles et des philosophies relatives au contrle de la fissuration, cest--diredclenchement et/ou arrt de la fissuration, en se basant sur le grand nombre de donnesdessais disponibles dans lindustrie.

Linfluence de ces propositions sur la capacit des rservoirs, leur cot et les dlais deconstruction est illustre au moyen dexemples.


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BIGGER & CHEAPER LNG TANKS?

INTRODUCTION

A significant amount of attention has been focused in recent years on the need to

reduce the CAPEX in LNG projects from wellhead to end user. The storage componentof both export and import terminals represents a significant proportion of the total costs,particularly in the latter instance. The cost of LNG Storage tanks was discussed at LNG11 in Birmingham and will doubtless be subject to close attention at LNG 12.

At the same time there is a clear tendency towards LNG tanks of greater capacity.The tanks constructed at Canvey Island in the 1960's were of 9000m capacity. Duringthe 1970's and early 1980's the British Gas peak shaving tanks were 50,000m. For awhile 100,000m seemed a popular choice and for Dabhol 160,000m tanks are beingconsidered.

The increase in tank capacity is driven by the desire for lower unit costs ($/m stored)arising from scale effects together with savings associated with the reduction in fixedcosts (i.e., pumping systems, instrumentation, utilities, etc.) and economic use of landarea.

This paper examines the constraints on the size of above ground free standing 9%Nickel Steel primary containment which arise from various design conditions, codes andpossible modifications to these codes. The paper also considers the methods fordemonstrating and achieving tank integrity and safety. We are confident that practicaland economic outer tanks of single, double and full containment types can be provided tomatch the inner tank sizes discussed in this paper.

One obvious means of arriving at a lower unit storage cost is to choose singlecontainment rather than full containment. This however is a complex decision involvinglocal codes and regulations, the outcome of comprehensive and detailed hazard analyses,safety studies and the owners preferred storage philosophy. Frequently the seeminglyapparent economic choice becomes less certain when the observers viewpoint choice isbroadened from the single subject of storage to the operating facility as a whole togetherwith its environs. This aspect is not considered in detail in this paper.

THE UNIT COST/SIZE RELATIONSHIP

It has been presumed in the introduction that increases in tank size brings economies

of scale and a reduction in unit cost.

This was demonstrated by the following exercise:

Tank Types

Single containment (Fig 1)Full containment (Fig 2)


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Figure 1

Figure 2


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Tank Capacity

50, 000 m100, 000 m150, 000 m200, 000 m250, 000 m

Design Basis

Code API 620 Appendix Q (Ref 1)Design stress Maximum allowed by codeNPSH heel 1.0 mUllage for slosh, etc. 1.0 mDesign SG 0.48Design Temp - 165CFoundation Ground bearing slab

(No piles required)

The dimensions used for the various tank capacities are listed in Table 1.

Table 1. Tank Dimensions For Unit Cost/ Size Calculations.

Capacity

(m3)

Di

(m)

Do

(m)

Hi

(m)

Ho

(m)

50,000 52 54.4 25.68 27.97

100,000 65 67.4 32.31 34.61

150,000 75 77.4 36.14 38.44

200,000 85 87.4 37.44 39.74

250,000 90 92.4 41.52 43.80

These examples have been costed on a UK supply and construction basis, each beingfitted with the following:

- 4 off intank pumps complete with pump columns, foot valves and handling

equipment.- Tower stairway and roof access equipment.

- Full set of instrumentation including LTD measurement.

- Full electrical installation including base heating.

- The normal pipework to a termination point at grade.

The unit cost/tank capacity is shown in Figure 3.


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Figure 3

This curve shows that the unit cost does decrease as capacity increases for both singleand full containment tanks. The rate of reduction decreases with increase in capacity.Detailed examination of the estimates behind these curves suggests that for the very bigtanks the tonnage of materials, in particular 9% Nickel steel for the inner tank shell andthe carbon steel outer tank shell begins to increase in a disproportionate way. It is alsointeresting to note that the cost difference between single and full containment is modest

and reduces from around 17% to 12% as tank capacity increases over the size rangecovered. We have not included the costs of secondary impoundment for singlecontainment tanks. When these are included together with costs associated withadditional fire protection and economic use of site area, then it is quite probable thatmany real cases will indicate full containment to be the most economic choice.

STORAGE COSTS AS A PROPORTION OF TOTAL COSTS

The costs of a number of typical LNG import terminals have been considered.Clearly the cost of the storage component varies from one project to another dependingon many considerations but on average varies between 45% and 65% of the total facility

cost. This suggests only that storage costs are a significant cost of the total facility costsand as such warrant a detailed review to ensure that their costs are minimised. It is alsousually the case that the use of large tanks maximises the use of available site area.

PARAMETERS WHICH INFLUENCE TANK SIZE

From the foregoing it is clear that greater tank capacity equates to reduced unitstorage cost.

Unit Cost Ratio vs Tank (Usable) Capacity

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

50 100 150 200 250

Capacity x 1000 CuM

Relativecost/m3stored

Single

Full


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The other parameters which will influence or limit the size of tank which can beconstructed are:

i) Design code or method

ii) Material properties

iii) Soil conditions

iv) Base insulation

v) Seismic design data

vi) Local planning rules (i.e., maximum roof height)

Each of these are discussed in subsequent paragraphs.

DESIGN CODE OR METHOD

The two most commonly used codes for the design of LNG tanks are API 620Appendix Q and BS 7777:1993 (Ref 2).

API 620 Appendix Q

This standard was written for single containment tanks and has served the industrywell for many years. It requires a minimum hydrostatic test equivalent to 1.25 times themaximum product load on the tank foundation and bases the allowable stresses for designand test on the weld metal properties allowing maximum values of 400 N/mm Yield and690 N/mm2UTS. This means that the shell thickness is based on the operation condition.The maximum permitted shell thickness for the purposes of this exercise has been

assumed to be 38.1 mm (1 ).

BS 7777:1993

This standard has its origins in the EEMUA 147 document (Ref 3) which was inmany ways a reaction to the incident in 1977 at Umm Said in Qatar where the plant wasdestroyed by the failure of a liquid propane tank in a brittle manner. The subsequenttechnical and legal processes served to throw little light on the real causes of this event.This was however a very influential event which through the EEMUA 147 documentpushed the industry towards secondary containment, full height hydrostatic testing and inthe case of 9% Nickel steels, increased fracture toughness. This has been discussed insome detail by J.B. Denham (Ref 4).

BS 7777:1993 incorporated all of the major requirements outlined in EEMUA 147.

Those most significant to this paper are the increased hydrostatic testing level to the fullproduct height (for all products), the increased minimum thickness requirements for shelland bottom plates and the increased allowable stress for the operating conditions.

CEN TC 265

This is the new Eurocode which will have a section devoted to low temperaturetankage. This document is in the process of being written, although the rate of progressleaves something to be desired. The creation of a Eurocode means that all national codes


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within the EEC are subject to standstill (including BS 7777) and consequently cannotbe amended.

If the industry wishes to promote any new ideas and design methods for lowtemperature tanks, then the Eurocode is perhaps the only suitable vehicle.

Worked Examples

In an effort to quantify the influence of the design codes together with a variety ofpossible amendments to those codes, we have carried out a number of designs of a 9%Nickel steel inner tank with a useable capacity of 200,000 m . A number of differentheight to diameter ratios have been examined. The basis of these calculations is the sameas stated above. The codes used and the modifications made are listed in Table 2. Theresults of this exercise are shown in Figures 4 and 5 where the calculated total inner tankweight (shell, shell stiffening, bottom and annular plates) has been plotted against tankdiameter and D/H ratio.

Table 2. Design basis assumed for designs of 200,000m3

LNG Inner Tank shown in Figs

Design Stresses

Case Hydrotest Sd

(N/mm2)

St

(N/mm2)

Modifications to

Basic Code

BS 7777 Full 260.0 340.0 None

API 620 Partial 229.9 340.0 None

BS 7777 (mod 1) Partial 260.0 340.0 Hydrotest

BS 7777 (mod 2) Partial 260.0 340.0 HydrotestAdopt API min thickness

BS 7777 (mod 3) Partial 275.0 340.0 HydrotestAdopt API min thickness

Increase Sd


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Figure 4 Figure 5

Figure 6

Weight vs Diameter for various design codes

1500

1700

1900

2100

2300

2500

2700

2900

7577

.5 8082

.5 8587

.5 90

Diameter (m)

Weight(Te)

API

BS

BS(mod1)

BS(mod2)

BS(mod3)

Weight vs Ratio D/H for various design Codes

1500

1700

1900

2100

2300

2500

2700

2900

1.6 1.8 2 2.1 2.4 2.5 2.8

D/H

Weight(Te)

API

BS

BS(mod1)

BS(mod2)

BS(mod3)

1st Course Shell Thickness vs Tank Diameter

20

25

30

35

40

45

50

55

75 77.5 80 82.5 85 87.5 90

Diameter (m)

1stcourset(mm) API

BS

BS(mod1)

BS(mod2)

BS(mod3)


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From Figure 4 a number of issues become apparent. The first is that BS 7777unamended produces a very uneconomic design. In fact the bottom course shellthickness plotted in Figure 6 exceed the thicknesses allowed by the code and whichexperience would suggest as sensible. The main difference between the unamended BS7777 designs and the remaining cases considered is due to the requirement full/partialhydrostatic testing which is discussed below.

The ratio of D/H has an influence on the total weight. The larger diameter designsare lighter that their smaller taller alternatives by between 14% and 22%. Whilst this isinteresting it is probable that costs associated with the outer tanks, the insulation and theland usage may consume all or part of these differences. These are also practicalconsiderations which may dictate D/H ratios associated with local soil conditions orseismic design problems.

The influence of the minimum shell and bottom plate thickness requirements of theBS and API codes are more than anticipated contributing a difference in total weight ofbetween 5.0% and 6.5%. Clearly there are differences unseen by this exercise associatedwith the increased erection costs for the thinner shells and the increased complexity of the

shell stiffening. It would however seem that this is a worthwhile and uncontentioussaving which equals or exceeds the possible and more contentious savings associatedwith increasing the allowable stress in the tank shell.

MAXIMUM SHELL THICKNESS

BS 7777 proposes a maximum thickness for types IV and V 9% Nickel Steel of 30mm but leaves the door open for thicknesses in excess of this figure to be agreed betweenthe purchaser and the manufacturer.

Fabrique de Fer de Charleroi of Belgium who supplied the 9% Nickel plate for the

tanks in Greece and Trinidad, have provided plate and welding data which indicates thatthe required properties can be met in the plate and weld metal in thickness up to 38.1 mm

(1). This is considered therefore a sensible and supportable maximum thickness forthe purpose of this exercise.

Hydrotest Test Level

The European Standard EN 1473: 1997 (Ref 5) requires the primary container to betested to 125% of the operating base loading and states quite bluntly that higherhydrostatic test heights with the purpose of stress relieving the steel are pointless forcontainers made from 9% nickel steel. There is a considerable support for this view,

particularly where the new enhanced toughness crack arresting grades of 9% nickel steelare concerned. The development of enhanced grades of 9% nickel steels, the test workand the development of crack arrest criteria for refrigerated gas storage tanks iscomprehensively covered by Denham (Ref 6).

API 620 Appendix Q was not influenced to change its test height criteria as a result ofthe Qatar incident which it viewed as relevant only to LPG tanks and chose to amendonly Appendix R. Thus a considerable practical body of satisfactory experience relatingto the adequacy of the partial hydrotest exists for LNG tanks built to the API rules.


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Inspection

The main problem on 9% Ni tanks is the density difference between the parent plateand the high nickel weld metal which on the radiograph creates a marked difference inphotographic density. Thus in order to achieve the requirements for sensitivity anddensity the parameters of the radiographic technique need careful adjustment betweenfilm type, Kilovoltage and exposure time.

The resultant technique must also provide an overall exposure time which producesan acceptable production rate as radiography is commonly a critical path item on theprogramme. This is a particular problem when 100% radiography is specified for bothvertical and circumferential seams.

Production rate can be exacerbated by the size of the equipment, i.e., physicalhandling problems within the tank. As plate thickness increases so does the size of theequipment with typically 250-320 kV being required for 25-35 mm thicknesses and atthese power rates only metal ceramic X-ray heads/power sources are suitable for siteproduction. However, it still requires proper study to provide the correct handling and

support systems for the equipment in order to maximise production. As thicknessreduces then X-ray power requirement reduces which eases handling problems in termsof weight but increases them in terms of height above grade (dependent on constructionmethod).

It is considered that more effort should be given to justifying the use of Gammaradiography. This technique is acceptable to the major codes but not commonly acceptedby clients or consultants.

The Gamma ray technique has been shown to be capable of achieving density andsensitivity requirements and by using implanted defect plates its ability to detect crack

like defects has been demonstrated.

In the future more detailed work on the use of ultrasonic inspection using time offlight techniques for recording purposes in conjunction with longitudinal shear waveprobes could provide the basis for an alternative inspection system. Real timeradiography (i.e., pulsed X-ray with computer enhanced real time imaging) is capable ofapplication to 9% Ni tank on a mechanised basis and could dramatically improveproduction rates. However, clients, consultants and codes would have to accept thetechnique and potential equipment costs are high.

The commonly specified requirement for 100% radiography for both horizontal andvertical seams is an expensive luxury. If we consider the basic requirements laid down in

API 620 Appendix Q, taking the 200,000 m3 tank as an example, the length of seamradiographed is some 35% of the total. This is a substantial saving in money and maywell take radiography off the critical path. There is a good case to be made for the basicAPI level of radiography based on an ECA approach or for circumferential seams only tocombine radiography with some ultrasonic examination (which does not require separateshift working), a combination which could be used to advantage.


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MATERIAL PROPERTIES FOR LNG INNER TANKS

Materials Requirements

The integrity and safety of the 9% Ni Steel inner tank, defined as the avoidance ofcatastrophic failure depends on three factors.

i) 9% Ni Steel plate and weld metal properties

ii) Fracture resistance properties as controlled by the principal stress, secondarystress and residual stresses and the fracture behaviour under the influence of adefect.

iii) Freedom of the welds (including the heat affected zone) from deleteriousflaws.

The proposals made in this paper are dependent on these three criteria and it can beshown that these conditions can be satisfied.

9% Ni Steel Plates

The most widely accepted specification for 9% Ni steel plates is ASTM A553(quenched and tempered). For this material API 620 Appendix Q requires a minimumCharpy v-notch impact value for full size specimens tested at 196C of 20J (transverse).

In reality, the toughness achieved can be summarised as follows:-

10 mm thick: 95-237J (Whessoe LGA data)

22.3 mm thick: 151-225J (Whessoe LGA data)

38 mm thick: 165 205 J (>1.5 lat exp) (Fabrique de Fer data)

Hence, the quality of steel available exceeds that which is considered to be acceptablefor API 620 Appendix Q.

Weld Metal Properties

The most widely used weld metal is ASME Section 2c SFA SU EN CrMo-6. Thiselectrode provides a weld metal with undermatching strength, however the typical resultsare:

YS ) 420 N/mm2 )UTS ) 690 N/mm2 ) Whessoe LGA data

CV (-196) ) 70J )

When the CTOD values are considered for both the 9% Ni Steel HAZ and the weldmetal, the results exceed 0.3 mm at 165C at thicknesses up to 38 mm.

Thus it can be seen that more than adequate properties can be attained for the weldedstructures.


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Freedom from Defects

Extensive welding experience has shown that the achievement of the workmanshipdefect acceptance levels required by API 620 Appendix Q (i.e., ASME VIII standards)can be achieved without excessive repairs or resort to extreme welding procedures.

When this is coupled with the allowable defect sizes incorporated into the PD 6493

based ECA, it can be concluded that the risk of unacceptable flaws remaining the welds isacceptably low.

Fracture Behaviour

The results of wide plate tests (Ref 7) show that failure in the presence of a largedefect occurs at a factor of 2-3 times the design stress. The fracture behaviourdemonstrated is that of crack growth initiation. It is not possible to conclude whether thecrack will propagate or be arrested subsequently.

There is a view that the fracture control should incorporate crack arrest in the event

that crack growth initiation should occur. This may be an additional feature, but withinthe existing design principles, crack arrest is not required.

SOIL CONDITIONS

Poor soil conditions may limit the proportions or the size of tank which can beconstructed due to limited ground bearing strengths. This may also be a complication tobe considered together with seismic design. This is outside the scope of this discussion.

INSULATION

The majority of LNG tanks are founded on cellular glass manufactured by PittsburghCorning of the USA. For many years this material was manufactured with a minimumaverage strength of 0.74 N/mm (107 psi) and a minimum single value of 0.54 N/mm (78psi).

EEMUA 147 and BS 7777 give permitted factors of safety for cellular glass undercompression arising from operating and hydrostatic test loadings. In the case of BS 7777which required a hydrostatic test to the full product level, the use of this material and thegiven factors of safety will restrict the maximum product to 22.0 m. This is a severerestriction which in the case of the 200,000 m useable capacity example would requirean inner tank of some 110 m in diameter, clearly an uneconomic prospect.

For API 620 Appendix Q which requires a partial hydrostatic test to a level providinga minimum foundation overload of 25%, the use of this material is less restrictiveallowing a maximum product height of 36.7 m which in the case of 200,000 m examplewould require an inner tank diameter of 85 m, not an unreasonable choice.

In recent years Pittsburgh Corning have produced a range of stronger grades ofFoamglas now known as HLB 800 through to HLB 1600. This latter material has aminimum average compressive strength of 1.6 N/mm and a minimum single value of 1.1N/mm. This would permit a tank with full hydrostatic test a maximum product full


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height of 44.85 m. This should allow a tank unrestricted geometry at a modest costpremium.

In addition to the stronger grades of Foamglas there are a number of other materialswhich could be used for LNG tanks base insulation. One which Whessoe LGA havebeen actively considering is Divinycell, a PVC expanded foam made by DIAB ofSweden. This comes in a wide range of strength grades and should not impose any

restrictions in tank geometry.

SEISMIC DESIGN

In recent years a number of LNG terminals have been built in areas of the worldwhich are subject to significant seismic events. Above a certain level the seismic designcriteria begin to dominate the tank design. In Turkey at the Marmara Ereglisi LNGTerminal the tank geometry was restricted (Ref 8) whilst at Revithoussa Island LNGTerminal in Greece the extreme seismic parameters led to the use of seismic isolation ofthe complete full containment LNG tanks (Ref 9).

The inclusion of seismic design criteria in this paper will confuse the basic issuesunder discussion and have not been pursued further.

LOCAL PLANNING RULES

Local planning rules commonly give a maximum shell or dome roof height whichwill obviously influence the shape and/or size of tanks which can be constructed on aparticular site. This is clearly outside the scope of this discussion.

CONSIDERATIONS OF INTEGRITY AND SAFETY OF LARGE LNG

INNER TANKS

The design and construction of very large tanks will inevitably cause some concern.Although we are proposing the use of various design rules similar to or identical withAPI 620 Appendix Q which has been the main guideline for the industry for many years,the tanks we are proposing are big and will pioneer a number of new areas of territory interms of size alone.

We have considered two proposals aimed at providing reassurance:

Increase Hydro Test Height

An increase in the hydrostatic test fill height up to the point where the thickness of thelowest course is the same for both operating and test design criteria can be achievedwithout adding cost for the outer tank shell. The only costs relate to the strongerfoundation to withstand the additional loadings and a modest sum for the additional testwater.

This allows the API maximum overstress to increase from 25% to 48% and to pushthe point where the overstress reduces to zero further up the tank shall as indicated inTable 3.


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For the BS 7777 mod 1 design the effect is less marked as can be seen in Table 4where the overstress increases from 25% to 31%.

Table 3Effect of increasing hydrostatic test height such that

course 1 operating and test thicknesses are equal.

(Based on 200,000m3, 80m dia., API 620 APPENDIX Q Design)

Course Width

(m)

Thickness

(mm)

API Test Fill

Ht

(overstress %)

Increased fill

Ht

(overstress %)

1 3.501 33.54 25.0 47.9

2 3.501 30.67 17.2 42.2

3 3.501 27.80 7.8 35.4

4 3.501 24.93 0 27.0

5 3.501 22.06 0 16.4

6 3.501 19.19 0 2.7

7 3.501 16.33 0 0

8 3.501 13.46 0 0

9 3.501 10.59 0 0

10 3.501 9.60 0 0

11 3.501 9.60 0 0

12 3.501 9.60 0 0

Adjust Operating Stress for Courses Which Do Not Experience a Significant Test

Overstress

There is a perception that parts of the tank receiving a small hydrostatic testoverstress are more at risk than those parts which get a higher overstress. A reaction tothis is to reduce the operating stress in certain parts of the tank shell. The details of thisadjustment are described in Table 4.

The penalties in terms of weight and consequently cost are modest.


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Table 4Hydrostatic Test Height increased such that course 1 operating and

test thicknesses are equal.

Effect of reducing operating stress by 10% for parts of the shellreceiving an overstress of


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ASSESSMENT OF TANK INTEGRITY

The proposals described above to provide reassurance of adequate or indeedincreased integrity require some form of confirmation that the desired effect is beingachieved.

To this end an Engineering Critical Assessment was made for one of the 200,000 m3

designs (plus a 50,000 m3 design for comparison purposes) considered in this paper.

The assessment was made by applying PD 6493 1991 (Ref 10) to the designs, todetermine whether brittle fracture would be likely. PD 6493 was selected because it isrecognised world-wide. There are other assessment methods and it would be expectedthat these would provide a similar analysis.

Two postulated flaws were considered;

1) Through thickness flaw - 30mm long

2) Surface flaw - 5mm deep x 100mm long

These flaw sizes were chosen as they would be easily visible on a radiograph anddetected by surface flaw examination techniques, i.e. dye penetrant inspection.

The assessments carried out can only be used as a comparative measure.

The material properties used were based on the weld metal.

At room temperature

Yield Stress = 400 N/mm2

UTS = 690 N/mm2

At -165 C

Yield Stress = 550 N/mm2

UTS = 1000 N/mm2

CTOD value = 0.3mm

The primary stresses used in the assessments were based on the circumferentialstresses in the tank shell, i.e., those applied to the vertical seams.

The residual stress in the weld before hydrostatic testing was assumed to be equal tothe yield strength of the weld metal. When the tank was cooled down the increase inresidual stress, due to the differential contraction between the weld and parent materials,was taken as 50 N/mm2(Ref 11).

The Level 1 approach within PD 6493 was adopted. This is the simplest assessmentlevel and incorporates in-built safety factors for fracture and plastic collapse, averagingabout 2.

The reduction in residual stress associated with hydrostatic testing is defined in PD6493 as follows;


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When a proof test has been performed for subsequent fracture analysis at lowertemperatures, the residual stress level after the proof test can be assumed to be the lowerof

Yor 14. '

nf

Y

wheren is the net section stress under the proof load conditions

Y is the appropriate material yield strength at the proof test temperature and

f is the flow strength (assumed to be the average of the yield and tensilestrengths) at the proof test temperature.

The results of the assessment are presented in Figures 7 and 8, the Failure AssessmentDiagrams (FADs) for each course of the tank. The axes of the graphs are labelledFracture and Stress.

The bold lines marked on the FADs define the boundaries within which failure willnot occur. If the horizontal boundary is exceeded then failure will be by fracture and ifthe vertical boundary is exceeded then failure will be by plastic collapse.

From the FADs it can be seen that even for a tank which does not receive ahydrostatic test the postulated flaws, which are readily detectable, would not cause failureof the tank by crack growth initiation. This would give a measure of confidence whichwould be increased when a hydrostatic test is applied. Also from the FADs, the benefitsof the effects of hydrostatic testing on the residual stress can be seen. Obviously themaximum benefit is derived on the bottom course with a decreasing effect moving up thetank. At Course 5 any benefit from a 1.25 hydrotest is negligible, similarly at Course 7

the benefit of a 1.48 hydrotest is also negligible for an API 620 Appendix Q design.

In assessing an API design for a 50,000m3 (1.25 hydrotest) tank the results for thelower courses are no different than for the 200,000m3tank for a through thickness flaw.This is because of the comparatively large radii involved with tanks as opposed topressure vessels. Therefore any change in radius of the tank has a minimal effect on theflaw. In this respect there is no significant increase in risk from failure by fracture for an80m dia. tank than for a 52m dia. tank. However for a surface flaw when the thickness ofthe shell is more important, the 50000m

3 tank is apparently more at risk from failure.

There are many API 620 tanks of around 50,000 m3 in service which should providecomfort for the larger size of tanks currently being considered.


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Figure 7a Surface Flaws

COURSE 1

0

0.7071

0 0.8

STRESS

FRA

CTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

COURSE 2

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

COURSE 3

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO1.25 HYDRO (50000)

BS7777 1.31 HYDRO


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5.620

Figure 7b Surface Flaws

COURSE 4

0

0.7071

0 0.8

STRESS

FRAC

TURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

BS7777 -10%

COURSE 5

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

BS7777 -10%

COURSE 6

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

BS7777 -10%

API -10%


21/28

5.621

Figure 7c Surface Flaws

Figure 7c Surface Flaws

COURSE 7

0

0.7071

0 0.8

STRESS

FRA

CTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

BS7777 -10%

API -10%

COURSE 8

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31 HYDRO

BS7777 -10%

API -10%

COURSE 9

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31 HYDRO

API -10%


22/28

5.622

Figure 7d Surface Flaws

COURSE 10

0

0.7071

0 0.8

STRESS

FRACT

URE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31 HYDRO

COURSE 11

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31 HYDRO

COURSE 12

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31 HYDRO


23/28

5.623

Figure 8a Through Thickness Flaws

COURSE 1

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO1.25 HYDRO (50000)

BS7777 1.31HYDRO

COURSE 2

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

COURSE 3

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO


24/28

5.624

Figure 8b Through Thickness Flaws

COURSE 4

0

0.7071

0 0.8

STRESS

FRAC

TURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

BS7777 -10%

COURSE 5

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

BS7777 -10%

COURSE 6

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

BS7777 -10%API -10%


25/28

5.625

Figure 8c Through Thickness Flaws

COURSE 7

0

0.7071

0 0.8

STRESS

FRA

CTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

BS7777 -10%

API -10%

COURSE 8

0

0.7071

0 0.8

STRESS

FRACTURE

NO HYDRO

1.25 HYDRO

1.48 HYDRO

1.25 HYDRO (50000)

BS7777 1.31HYDRO

BS7777 -10%

API -10%

COURSE 9

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31HYDRO

API -10%


26/28

5.626

Figure 8d Through Thickness Flaws

COURSE 10

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31HYDRO

COURSE 11

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31HYDRO

COURSE 12

0

0.7071

0 0.8

STRESS

FRACTURE NO HYDRO

1.25 HYDRO

1.48 HYDRO

BS7777 1.31 HYDRO


27/28

5.627

The results for the BS 7777 (modified) tank show a minimal increase in risk fromfracture for the lower courses, this is due to the higher allowable stresses in the shellplates. The upper courses show a decrease due to the greater minimum thickness that BS7777 imposes, 12mm compared to 9.6mm with API 620.

The principal benefit of hydrotest is the reduction in residual stress which occurs.The reduction is calculated by PD 6493 as shown previously. The loss of this benefit can

be compensated for by reducing the operating stress such that the sum of the primary andresidual stresses is the same as it would be for a full hydrotest. This would result in somecourses being increased in thickness. It is necessary to decide at what level of overstressthe benefits of residual stress reduction are not significant. For the purpose of thisassessment the level was chosen as 10% overstress. Any course which did not receive a10% overstress during hydrotest, except the minimum thickness courses, was thickenedsuch that the operating stress was reduced by 10%. This figure was chosen nominally inorder to demonstrate the effects of this proposal.

From the results shown in the FADs for the surface flaw, it can be seen that thisincrease in thickness has a small beneficial effect for all courses (more so with the higher

courses) with respect to the fracture failure of the tank. From the FADs for the throughthickness flaws the benefit is less beneficial than for the surface defect.

The only stresses considered in the assessments were the primary membrane andsecondary residual stresses. Other secondary stresses which exist, such as verticalbending in the bottom course have not been considered. A more detailed analysis wouldbe required on an individual basis to determine actual fracture assessments and confirmthe trends indicated in this paper. A hydrostatic test will always be carried out on storagetanks whether it is a 1.25 overload or greater in order to test the foundations. Thereforethe annular to shell weld will always receive a measure of reduction in residual stress as ithas done in the past.

The value used for the CTOD has been validated within the industry. Wide plate testshave been carried out with failures occurring at values two to three times greater than theCTOD values would indicate. (Ref 7)

As mentioned, the circumferential seams are under little or no primary stress andtherefore could survive defects much larger than indicated in this paper for the verticalseams. The circumferential seams would not benefit from a hydrotest with respect to areduction in residual weld stresses and their integrity has been proven over the years. Thetrend towards 100% radiography of these welds in larger tanks, with respect to guardingagainst fracture, seems to be unnecessary

CONCLUSIONS

Tanks of 200,000 m3 capacity in accordance with the API 620 Appendix Q rulesappear to be a safe and economic proposition.

Modifications to the BS 7777 rules lead to further savings. These modified rulecould be taken up by CEN TC 265 if the committee so choose.


28/28

The Engineering Critical Assessment using the rules of PD 6493 suggest that theselarge tanks are safe structures. The two comfort factors suggested (increase hydrotestheight and decrease applied stress) give small increases in integrity when judgedaccording to the PD 6493 criteria.

There is a good case to be made for using the API 620 level of radiography inassociation with some ultrasonic inspection for circumferential seams to take inspection

activities off the inner tank critical path.

REFERENCES CITED

1. API 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks,Ninth Edition February 1996Addendum 1 December 1996The American Petroleum Institute, Washington, DC, USA.

2. BS 7777: 1993, Flat bottomed, vertical, cylindrical storage tanks for lowtemperature service, BSI London, UK.

3. EEMUA 147, Recommendations for the design and construction of refrigeratedliquefied gas storage tanks, Publication No 147, The Engineering Equipment andMaterials Users Association, London, UK.

4. Are storage tank standards holding back LNG import projects? J B Denham FEng,FIMechE, FweldI, LNG Journal, November/December 1996.

5. BSEN 1473: 1997, Installation and equipment for liquefied natural gas Design ofonshore installations. BSI, London, UK.

6. Development of Crack Arrest Criteria for Liquefied Gas Storage Tanks, J B DenhamFEng, FIMechE, FweldI, TWI/HSE/UMIST Seminar, September 1995, TheWelding Institute, Abingdon, UK.

7. Review of Data on 9% Ni steels and weldments, with reference to cryogenic storagetanks, A M Wood and A A Willoughby, The Welding Institute, Abingdon,Cambridge, UK.

8. Seismic design of the LNG tanks at the Marmara Ereglisi, Turkey, LNG Terminal,LV Scorsone and J Hoptay, Pittsburgh Des Moines Inc., Proceedings of Gastech1993.

9. The seismic analysis and design of large cryogenic storage tanks with reference totwo ongoing projects, G N Trott and R O Long, Whessoe LGA Gas TechnologyLtd.

10. PD 6493: 1991, Guidance on methods for assessing the acceptability of flaws infusion welded structures, BSI, London, UK.

11. The use of 9% nickel steel for LNG applicatio, W P Carter and J D Harrison, TheWelding Institute Conference on Welding Low Temperature Containment Plant,November 1973.

bigger and cheaper lng tanks

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