dismantling of aboveground lng storage tanks and …

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DISMANTLING OF ABOVEGROUND LNG STORAGE TANKS AND THEIR AGING RESEARCH

Hiroshi Nishigami Maki Yamashita

Shunsuke Ohnishi Nobuhiro Wadama Osaka Gas Co., Ltd

Hiroto Yamaoka Tatsuo Tsuji, Yu Murakami

IHI Corporation

Takehiro Inoue Naoki Saito

Motohiro Okushima Nippon Steel & Sumitomo Metal Co., Ltd

KEYWORDS: aboveground LNG storage tank, aging research, dismantling method]

ABSTRACT

In 2011, Osaka Gas commenced the dismantlement of two aboveground LNG storage tanks with a storage capacity of 45,000 m3 each at the Senboku 1 terminal. The said LNG storage tanks had been in commercial operation for approximately 40 years, and were the first to be dismantled in Japan. Concurrent with the dismantlement of the said LNG storage tanks, Osaka Gas also began the research on the deterioration due to their aging. Aging evaluation of the said LNG storage tanks will contribute to the LNG industry by not only potentially increasing its lifetime, but by improving its functions and safety as well. In this paper, the method of dismantling and the research results with regard to the aging for the two types of LNG storage tanks, respectively made of 9% Nickel steel and Aluminum alloy are reported. The contents of the research will be of the following criteria: (1) Mechanical properties of 9%Ni steel and Aluminum alloy (2) Thickness of steel pipe piles (3) Deterioration of instrument devices (4) Deterioration of insulation materials. After dismantling of the said LNG storage tanks, Osaka Gas decided to construct a large scale LNG storage tank applying newly developed 7% Ni-TMCP steel for its inner tank. The storage capacity of the new LNG storage tank will be 230,000m3, and is scheduled to be completed by November, 2015. The present state of the construction of the innovative LNG storage tank will be reported in this paper as well.

1. INTRODUCTION

In 1972, IHI Corporation (IHI) built three LNG storage tanks with capacities of 45,000m3 in Osaka Gas Co., Ltd (Osaka Gas) Senboku I Terminal. Since then, the tanks, which were of the single containment type with double steel walls, had been successfully operated without any trouble for 40 years. However, the conventional LNG storage tanks were getting more inefficient in the use of receiving terminal because of its lower dike in comparison with the state-of-the-art full-containment LNG storage tanks. (Figure1)

Figure 1. One of the demolished LNG storage tanks

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To keep up with the increasing demand for LNG and for effective practical use, Osaka Gas started to demolish the two conventional LNG storage tanks in 2011, and now a large LNG storage tank of 230,000m3 has been under construction since September 2012.

Demolition of old LNG storage tanks was executed with paying sufficient attention to the safety in consideration of the influences on the other facilities in operation and neighboring companies. Then, Osaka Gas investigated the demolished LNG inner tank’s material and its thermal insulation material jointly with IHI as the constructor and Nippon Steel & Sumitomo Metal Co., LTD (NSSMC) as the supplier of tank’s steel products. Moreover, we examined the steel pipe piles and the instrumentation devices from Osaka Gas’s own point of view. This paper describes the study on dismantling method of LNG storage tanks, the results of the investigations, and also reports on the new material used for the LNG storage tank under construction.

Investigation items:

1) Base metal mechanical properties (chemical composition / macro-micro structure / tensile strength /

Charpy absorbed energy / retained austenite)

2) Weld metal mechanical properties (chemical composition / macro-micro structure / tensile strength /

Charpy absorbed energy)

3) Fracture toughness properties (CTOD test / duplex ESSO test / wide plate test)

4) Corrosion of foundation piles (steel pipe piles)

5) Deterioration of concrete

6) Deterioration of instrumentation devices

7) Deterioration of thermal insulation material

2. DISMANTLING METHOD OF THE LNG STORAGE TANKS

2.1 Specification of the tanks

Table 2.1 and Figure 2.1 show the specifications of the two demolished tanks.

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Table 2.1 Specification of the tanks

1 Internal liquid LNG

2 Capacity 45,000KL

3 Type single containment type with double steel walls

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Dimensions inner tank outer tank

Diameter 44,600mm 46,400mm

Height 28,820mm 31,850mm

Roof radius 35,700mm 36,600mm

5 Design temp. -162℃ ambient temp.

6 Design pressure 0.12kg/cm2 50mmH2O

7 Main material 9%Ni steel(Al alloy) carbon steel

8 Insulation perlite & perlite concrete

9 Thickness of insulation tank shell & roof; 900mm

tank bottom ;1,100mm

10 Foundation slab reinforced concrete

diameter :44,600mm thickness ;800mm

11 Dike

reinforced concrete

height :4,000mm(3,000mm from the ground)

thickness ; 1,400mm

12 Earthquake history ground surface acceleration;

178gal

13 Commercial operation Feb.1972.

Note; ( ) shows Al alloy tank

Figure 2.1 Configuration of the demolished LNG storage tank

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2.2 Dismantling procedure

The LNG tanks were demolished through the following procedure in Figure 2.2.

Step 2 Perlite extraction from the annular space Step 4 Removal of the outer tank roof

Step 6 Removal of the outer tank shell plate Step 7 Removal of the inner tank shell plate

Figure 2.2. Dismantling procedure

<Special notes>

(1) Since the tank structure changes at every step during the tank dismantling, the dismantling steps should be observed by FEM analysis, ABAQUS. Figure2.3 shows some examples of the results by the analysis.

(2) The tanks were dismantled by 150 tons cranes. (3) Perlite was removed by a new extraction machine. (4) Some samples for the study were taken before the tank dismantling.

1. Setting of entering road into the dike

2. Perlite extraction from the annular space

3. Removal of the pipe, pipe frame etc.

4. Removal of the outer tank roof

5. Removal of the inner tank roof

6. Removal of the outer tank shell plate

7. Removal of the inner tank shell plate

8. Removal of the outer tank bottom plate

& bottom insulation

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Figure 2.3. The examples of the strain diagram

3. INSPECTION AND TEST RESULTS

3.1 Visual inspection

3.1.1 Inner tank K11 tank (9% Ni steel) and K31 tank (AL alloy) were found to be in good condition. No damages and buckling were detected due to the Hanshin Awaji Great Earthquake in 1995. There was no influence on the soundness of the tank. There was no damage to the instrumentation devices (level gauge, thermometer) installed in the tanks.

3.1.2 Outer tank (a) Manhole for perlite filling Some corrosion was found at the blind flange of perlite manhole. The doubling plate and the root of the manhole were also corroded.

(b) Outer tank roof plate and shell plate Some corrosion was seen at the welding seam. It is estimated that the repair painting had not been done enough.

(c) Roof walkway Corrosion was seen just right under the protecting seal due to the deterioration of it. In addition, corrosion was found at checker plate of walkway.

(d) Anchor bolt Some anchor bolts especially under the pipe rack were corroded. After removing a weather seal, the surface of several bolts were corroded and decreased its thickness.

(e) Inner tank anchor strap As a result of the investigation, the conspicuous deterioration, damage, deformation and corrosion were not seen. The reason of these phenomena was that the annular space had been filled with nitrogen since the time of the construction.

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3.2.1 Base Metal of Inner Tank Table 3.1 shows sampling parts and investigation items of each tank. Details are given as follows.

Table 3.1. Sampling Parts and Tests (Base Metal)

Tank Sampling Part Chemical

Component Analysis

Tensile Test Charpy

Impact Test CTOD Test

Duplex Esso Test

K31 (A5083)

1st Course ○ ○ - - - 13th Course ○ ○ - - -

Annular Plate ○ - - - - Bottom Plate - ○ - - -

K11 (9%Ni-Steel)

1st Course ○ ○ ○ ○ ○ 13th Course ○ ○ ○ - -

Annular Plate ○ - - - - Bottom Plate - ○ - - -

(a) Chemical Compositions of Base Metal Chemical analyses were carried out on representative samples taken from the 1st and 13th shell courses and annular bottom plate of each tank. Table 3.2 shows the results of K31 tank (made by aluminum alloy), and Table 3.3 shows that of K11 tank (made by 9%Ni steel).

Table 3.2. Chemical Composition (A5083 Base Metal)

Sampling Part

Si Fe Cu Mn Mg Zn Cr Ti B

Shell Plate 1st Course

0.08 0.10 0.01 0.67 4.51 <0.01 0.10 0.01 <0.005

Shell Plate 13th Course

0.08 0.12 0.01 0.65 4.39 <0.01 0.10 0.01 <0.005

Annular Plate 0.11 0.13 0.01 0.67 4.44 <0.01 0.10 0.01 <0.005 Spec

JIS H 4000 ≦0.4 ≦0.4 ≦0.1

0.3 ~1.0

4.0 ~4.9

≦0.25 0.05

~0.25 ≦0.15 -

Table 3.3. Chemical Composition (9%Ni-Steel Base Metal)

Sampling Part C Si Mn P S Ni Shell Plate 1st Course

0.09 0.30 0.59 0.006 0.005 8.76


0.08 0.28 0.59 0.004 0.002 8.80

Annular Plate 0.09 0.28 0.62 0.0054 0.004 8.67 Inspection Certificate

0.09 0.26 0.55 0.009 0.008 8.80

[K31 Tank (made by A5083)]

These results conform to the specs of JIS (Japanese Industrial Standards) H 4000. And it is apparent that the base metal doesn’t across the ages by immersing LNG for a long time. Concentration of Mg, which influences the strength of A5083, was different among each sampling part, but there is no problem for soundness of tank.

[K11 Tank (made by 9%Ni Steel)]

The same as K31 tank, these results conform to the specs of JIS G 3127.

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(b) Tensile Properties Tensile tests were carried out by using specimens: a full thickness specimen in accordance with JIS Z 2201 No.1A. The tests were performed in both the L and C rolling directions at room temperature in accordance with JIS Z 2241.

Table 3.4 shows the results of K31 tank, and Table3.5 shows that of K11 tank.

Table 3.4. Results of Tensile Test (A5083-O Base Metal)

Sampling Part Rolling

Direction 0.2% Proof

Stress [MPa] Tensile

Stress [MPa] Elongation

[%] Shell Plate 1st Course

L 136 297 23 C 139 303 24


L 153 304 20 C 139 302 21

Bottom Plate L 160 299 18 C 139 303 22

Requirement1) - 125 (0.8<t≦40)

275 (0.8<t≦80) - 120 (40<t≦80)

Table 3.5. Results of Tensile Test (9%Ni-Steel Base Metal)

Sampling Part Rolling

Direction 0.2% Proof

Stress [MPa] Tensile

Stress [MPa] Elongation

[%] Shell Plate 1st Course

L 708 761 36 C 706 762 36


L 722 767 29 C 719 767 27

Bottom Plate L 723 776 28 C 714 769 28

Inspection Certification

- 723 779 35

Requirement1) - 590 690 -

[K31 Tank (made by A5083)]

These results on 0.2% proof stress and tensile strength at room temperature satisfy the requirements1), and shows enough elongation, too. It seems that soundness of tank was kept.

[K11 Tank (made by 9%Ni Steel)]

The same as K31 tank, all results satisfy the requirements1). And as compared with inspection certification, it is apparent that the base metal doesn’t deteriorate with age.

(c) Charpy Impact Properties Charpy impact tests for the base metal of the 1st and 13th shell course of K11 tank were carried out using specimens in accordance with JIS Z 2242. The test specimens were taken from the both L and C rolling directions. The central axis of the specimen taken from 1st shell course was in the plate of 1/4 thickness under the inner-side surface, and that taken from 13th shell course was 7mm. The tests were carried out at -196 deg C in accordance with JIS Z 2242.

Figure 3.1 shows the results of Charpy impact tests. These values are lower than that of current 9%Ni steel, but all plates conform to the requirements1), and as compared with inspection certification, it is apparent that the base metal doesn’t deteriorate with age, too.

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Figure 3.1. Results of Charpy Impact Test (9%Ni-Steel Base Metal)

Test Temperature:-196℃

(d) Other For the 1st shell courses of K11 tank, CTOD test and duplex Esso test were carried out and we make sure of the soundness of material used in the construction days. The details are omitted due to space limitation.

3.2.2 Weld Joint of Inner Tank Table 3.6 shows sampling parts and investigation items of each tank. Details are given as follows. Table 3.7 shows the welding procedure of each weld joint. Table 3.8 shows the pictures of macro structure of vertical weld joint of shell plate 1st course.

Table 3.6. Sampling Parts and Tests (Weld Joint)

Tank Sampling Part Weld Joint Chemical

Component Analysis

Tensile Test

Charpy Impact Test

CTOD Test Wide Plate

Test

Bending Fatigue

Test

K31 (A5083)

1st Course Vertical ○ ○ - - - - 1st&2nd Course Horizontal ○ ○ - - - -

13th Course Vertical ○ ○ - - - - Annular Plate Fillet ○ - - - - ○

K11 (9%Ni-Steel)

1st Course Vertical ○ ○ ○ ○ ○ - 1st&2nd Course Horizontal ○ ○ ○ ○ - -

13th Course Vertical ○ ○ ○ - - - Annular Plate Fillet ○ - - - - ○

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Table 3.7. Welding Procedure of Each Weld Joint

Sampling Part K31 (Aluminum) K11 (9%Ni-Steel)

Process Material Groove Process Material Groove

Shell Plate 1st Course Vertical Weld Joint

Large Current

MIG

JIS Z 3232 A5183-WY

(AWS 5.10 ER5183)

SMAW

JIS Z 3224 ENi 6133

(AWS 5.11 ENiCrFe-2)

Shell Plate 1st&2nd Course

Horizontal Weld Joint

Shell Plate 13th Course Vertical Weld Joint

MIG

Shell Plate 1st Course & Annular Plate Fillet Weld Joint

Bottom Plate One Side

Lap Weld Joint

Table 3.8. Macro Structure of Vertical Weld Joint of Shell Plate 1st Course

K31 Aluminum Tank

K11 9%Ni-Steel Tank

(a) Chemical composition of weld metal Chemical analyses were carried out on representative samples taken from the different parts of weld metal. Table 3.9 shows the results of weld metal of K31 tank, and Table 3.10 shows that of K11 tank.

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Table 3.9. Chemical Composition (K31 Aluminum Tank Weld Metal)

Sampling Part Si Fe Cu Mn Mg Zn Cr Ti B Shell Plate 1st Course

Vertical Weld Joint 0.08 0.10 0.01 0.67 4.51 <0.01 0.10 0.01 <0.005

Shell Plate 1st&2nd Course Horizontal Weld Joint

0.09 0.11 <0.01 0.66 4.35 <0.01 0.10 0.01 <0.005

Shell Plate 13th Course Vertical Weld Joint

0.08 0.12 0.01 0.65 4.39 <0.01 0.10 0.01 <0.005


0.11 0.13 0.01 0.67 4.44 <0.01 0.10 0.01 <0.005

Spec JIS Z 3232

≦0.40 ≦0.40 ≦0.10 0.50 ~1.0

4.3 ~5.2

0.05 ~0.25

≦0.25 ≦0.15 -

Table 3.10. Chemical Composition (K11 9%Ni-Steel Tank Weld Metal)

Sampling Part C Si Mn P S Cu Ni Cr Mo Nb Al Fe Shell Plate 1st Course Vertical Welded Joint

0.06 0.26 1.79 0.010 0.0032 0.05 63.9 13.6 0.69 2.18 0.098 Bal.

Shell Plate 1st&2nd Course Horizontal Welded Joint

0.08 0.26 1.89 0.009 0.0028 0.10 67.0 15.7 0.73 2.17 0.105 Bal.

Shell Plate 13th Course Vertical Welded Joint

0.07 0.22 1.77 0.009 0.0028 0.05 62.5 12.2 0.65 1.90 0.102 Bal.


0.08 0.28 1.90 0.010 0.0023 0.00 66.7 15.0 0.73 2.20 0.091 Bal.

Inspection Certification of Electrode

0.05 0.29 1.92 0.008 0.010 - 68.3 15.5 0.73 2.29

(+ Ta) - 10.8

[K31]

As the base metal, these results conform to the specs of JIS Z 3232 (AWS 5.10 ER 5183). It is apparent that the weld metal doesn’t across the ages.

[K11]

The same as K11 tank, these results conform to the specs of JIS Z 3224 (AWS 5.11 ENiCrFe-2).

(b) Tensile properties Tensile tests were carried out using specimens: a full thickness specimen taken from different parts of the welded joints in accordance with JIS Z 3121 No.1A, but the excess weld metal was not ground. Each tensile test was performed at room temperature in accordance with JIS Z 2241.

Table 3.11 shows the results of K31 tank, and Table3.12 shows that of K11 tank.

Table 3.11. Results of Tensile Test (K31 Aluminum Tank Weld Metal)

Sampling Part Tensile

Stress [MPa] Fracture Location

Shell Plate 1st Course Vertical Welded Joint

297 Fusion Line

~HAZ Shell Plate 1st&2nd Course

Horizontal Weld Joint 303

Fusion Line ~HAZ


298 Base Metal

Requirement1) 275 -

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Table 3.12. Results of Tensile Test (K11 9%Ni-Steel Tank Weld Metal)

Sampling Part Test after Dismantling

Construction Management Test

Tensile Stress [MPa]

Fracture Location

Tensile Stress [MPa]

Shell Plate 1st Course Vertical Welded Joint

756 Weld Metal 761

Shell Plate 1st&2nd Course Horizontal Weld Joint

756 Base Metal 767


781 Base Metal 788

Requirement1) 655 (at 20 deg C) - -

[K31]

These results on tensile strength at room temperature satisfy the requirements1), and fracture locations are not particular. It is apparent that soundness of tank was kept.

[K11]

The same as K31 tank, all results satisfy the requirements1), it is apparent that soundness of tank was kept.

(c) Charpy Impact Properties Charpy impact tests were carried out on weld joints of K11 tank, machining the notch at center of weld metal, fusion line (FL: 50% weld metal + 50% heat affected zone), FL+1mm, FL+3mm, FL+5mm, using specimens in accordance with JIS Z 2202 and 3128. Notch is 2mm depth, V shape. The central axis of the specimen was the same as base metal. The tests were carried out at -196 deg C in accordance with JIS Z 2242.

Figure 3.2 shows the results of Charpy impact tests. All plates conform to the requirements1). Those

results have indicated that the weld joints don’t deteriorate with age.

Figure 3.2. Results of Charpy Impact Test (K11 9%Ni-Steel Tank Welded Joint)

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(d) Other For the vertical weld joint of 1st shell courses of K11 tank, CTOD test and wide plate test were carried out and we make sure of the soundness of welded joints. The details are omitted due to space limitation.

3.3 Corrosion survey on corrosion of foundation piles

3.3.1 Purpose and method Corrosion countermeasure for foundation piles is important for maintenance of LNG tank substructure. Osaka Gas adopts a corrosion allowance by adding 2mm margin to the specification of foundation piles in construction design as a corrosion countermeasure. Meanwhile, heads of piles are bonded in advance during construction so that a cathodic protection can be applied in preparation for significant corrosion progression in service. Moreover, the installation method of electrodes using a horizontal boring technique for cathodic protection has been developed, and the corrosion condition has been monitored in order to judge the necessity of cathodic protection.

The corrosion condition has been judged according to the thickness measurement which is executed by remote field testing and actual pile investigation by excavating the upper portion of piles. Based on the evaluation in this way indicated that there was no necessity of corrosion countermeasure for the next 50 years at least. Because actual measurement data is still not enough, we conducted a survey on actual piles that pulled out on this occasion of tank demolition in order to make up for scant data of actual corrosion condition.

Figure 3.3 shows the section view of the LNG tank. There were 496 foundation piles in total that driven with steel pipe piles; length 25m, outer diameter 406.4mm, thickness of upper pile 12.7mm and lower pile 9.5mm.

As shown in Figure 3.4, the survey was conducted with 31 piles among all foundation ones of LNG storage tank. The thickness of pile was measured down to 6m below pile head, where the lateral bearing force is susceptible due to corrosion. In addition, 5 piles among them were measured over the entire length of pile.

The measurement of thickness was carried out after removing extraneous matters such as corrosion products and residual material. Then the averages of pile thickness were calculated from measured thickness and weight for each 1m.

Also note that the cathodic protection of impressed current method had been applied to the removed tank from one year after start-up.

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Figure 3.3. Specification of tank foundation

Figure 3.4. Framing plan of piles and the measured piles in corrosion survey

3.3.2 Result Observing the appearance of piles, corrosion is found on the outside surface evenly, where local corrosion of particular part is not found, nor rust inside of the steel pipe. Therefore, the corrosion content is estimated from the outer wall of a pile by reducing the measured value from the initial thickness of a pile. Figure3.5 shows the corrosion content according to the depth. The corrosion content is about 0.6mm at a maximum, so it is kept within 2.0mm margin. Also, no specific difference is found in the corrosion conditions between each depth.

The averaged corrosion rate is obtained as 0.014mm/year at a maximum. It also stays within the confines of normal corrosion rate that reported various research literatures. The evaluation criterion is estimated at up to 0.073mm/year before this corrosion survey; the total corrosion rating was calculated from adding 0.042mm/year as macro-cell corrosion rate of steel in concrete to 0.031mm/year as maximum natural corrosion rate. However, the corrosion rate obtained from this survey resulted in far below the conventional value we estimated.

From pile head to 6m below

Whole length

(Measurement range)

Steel pipe pile

Length: 25.0m

Outer diameter: 406.4mm

Thickness: (Upper part) 12.7mm

(Lower part) 9.5mm

Number: 496

Diameter of base slab: 48.0m

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The effective range of cathodic protection related to the arrangement of foundation piles could not be ascertained because there was no specific difference found between the outer and the center piles due to the too small corrosion content.

3.4 Deterioration survey for concrete

3.4.1 Purpose and scope Diagnostic evaluation for concrete structure used for 40 years was undertaken based on chloride ion content, carbonation depth and compressive strength of concrete used for the base slab and dike of LNG storage tank.

Measure points are selected at the marked point in Figure 3.6; 5 points on underside of base slab, 4 points on side surface of base slab, each 4points on outer and inner side of dike.

Figure 3.6. Measure points for concrete deterioration survey

Depth(m)

Corrosion allowance

Corrosion content (mm)

Figure 3.5. Corrosion content

▲ Underside of base slab ▲ Side surface of base slab ▲ Outer and inner side of dike

*Only from K031LNG tank on east side.

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3.4.2 Result

(a) Chloride ion content Chloride ion content as measured at each 20cm depth for 7 measuring points at t underside of base slab and for 5 measuring points at each side surface of foundation slab and dike. Figure 3.7 shows the chloride ion content. The covering depth of base slab and dike is 85~105mm, and the chloride ion content at the reinforcing steels was lower than 2.5kg/m3 that specified as criteria of corrosion durability. Also, it shows that almost no chloride ion content made inroad into the underside of base slab.

(b) Carbonation depth Figure 3.8 shows the measurement result of carbonation depth. Almost no carbonation progress is found at base slab. Even the carbonation found at side surface of base slab and dike wall are 25mm at a maximum, it shows that the carbonation depth does not reach to the reinforcing steels.

Carbonation depth (mm)

Underside of base slab

Side surface of base slab

Dike

Figure 3.8. Carbonation depth

Minimum covering depth

Chloride ion content (kg/m3)

Depth from concrete surface (cm)

― Underside of base slab ― Side surface of base slab ― Dike

Figure 3.7. Chloride ion content

Criteria of corrosion durability

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(c) Compressive strength Figure 3.9 shows the measurement result of compressive strength. The survey of compressive strength shows that all the specimens satisfy the design compressive strength of 24.0N/mm2.

3.5 Deterioration of Instrumentation Devices

The following are the report of deterioration investigation of instrumentation devices. The devices below are selected as objectives for investigation among the instrumentation devices mounted on the tank.

Thermometer for Roll-over detection

Guide wire for Float Type Level Meter

These devices have not renewed since the construction of the tanks, and they have been in service without any failure. However, major renovation work including hot-up and opening of LNG tank would be necessary if these devices had failure and needed to be repaired. Also, operation of the entire LNG terminal might be interrupted. Therefore, it is important to investigate the deterioration of these devices for better approach to future construction and maintenance of LNG tanks.

3.5.1 Thermometer for Roll-over (a) Outline of Thermometer for Roll-over Here is the detail of Thermometer for Roll-over.

This thermometer has a very important role in monitoring LNG stratification that is considered as a warning symptom of roll-over phenomenon.

As shown in Figure 3.10, 15 thermometers are arranged at intervals of 2m from the top of LNG tank. According to the indicated values of the thermometers, supervised computer system (hereafter SCS) judges the occurrence of LNG stratification. When SCS detects stratification, it sends an alert signal to the center control room (hereafter CCR) and requires the operator to take measures, such as sending out, transfer or circulation.

Compressive strength (N/mm2)

Underside of base slab

Side surface of base slab

Dike

Figure 3.9. Compressive strength

Design compressive strength

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Figure 3.11. Insulation resistance test

Figure 3.10. Arranged thermometers

(b) Objective for Investigation

30 thermometers for roll-over taken from the two removed LNG tanks are used for investigation.

The specifications of the thermometer are described below:

Model : Metal-sheathed resistance thermometer sensor

Sheath material : SUS316

Sheath outer diameter : φ4.8mm

Sheath length : 13m~41m (at intervals of 2m)

Classification : 0.5

Detection element : Platinum resistor

(Standard resistance value : Pt100Ω at 0℃ (JPt100Ω))

Conductor : 3-wire type

(c) Investigation Contents / Method

It has already reported that there are some thermometers indicating the signs of insulation degradation before the LNG tanks removal. If the insulation degradation occurred, the ability of measuring temperature deteriorated. Furthermore, if the insulation completely damaged, temperature measurement became impossible subsequently. Prior to the investigation, we implemented insulation resistance test (applied voltage: 100CV) for all thermometers just before removing them to identify the thermometers having the insulation degradation. Then we cut off the sheath off from the head of the thermometer having insulation degradation and implemented insulation resistance test (Figure 3.11) again to investigate the region where insulation degradation occurred.

(d) Result of Investigation Table 3.13 shows the result of the insulation resistance test. There were 5 thermometers that had lower insulation resistance value than 20MΩ at the time of measurement before removing. However these thermometers had more than 20MΩ of insulation resistance in the second test after cutting the sheath for 1m off from the head. Therefore, it can be said that insulation degradation of the thermometers occurred within 1m from the head.

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Table 3.13. The result of insulation resistance test and region of insulation degradation

Thermometer No. Insulation resistance

[MΩ]

Region of insulation

degradation [m]

(distance from the head)

No.1 LNG tank

1 1 1

3 6 1

5 2 1

12 11 1

No.3LNG tank 1 0.1 1

(e) Conclusion The insulation degradation was found on 5 thermometers among all 30 objective thermometers. However, they did not have severe degradation to the extent of influencing the measuring ability of thermometers. The degradation occurred within 1m from the thermometer’s head according to the investigation of region. It is assumed that the cause of insulation degradation was the influent water from the terminal area covered with epoxide resin. The epoxide resin used for insulating the terminal area had deteriorated.

The insulation degradation was solved by cutting off the deteriorated region in upper 1m. We conclude that the thermometer should be installed with extra length to deal with insulation degradation.

3.5.2 Guide wire for Float type level meter (a) Outline of Float type level meter Here is the detail of Float type level meter.

Figure 3.12 shows the float type level meter removed from the tank on this occasion. It is used to measure the liquid level according to the length of the tape that goes up and down depending on the float on the liquid surface. The float moves along the guide wires. Therefore, friction arises between the float and the guide wires every time when the float moves. In this investigation, the deterioration condition of guide wires were inspected for taking into account the risk of broken guide wires.

Figure 3.12. Float type level meter

(b) Objective for investigation There were 2 floating type level meter mounted on each removed LNG tank. One of the guide wires on each LNG tank was selected as objectives. Specification of the guide wire is described below.

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Applied standard : JIS G 3550

Material : SUS304

Product name : Independent strand core rope

Constructive number : 7×19

Length : approx. 37.5m

Diameter of wire : 10mm

Diameter of strand : 0.67mm

(c) Investigation Contents / Method We supposed that the worn volumes were different depending on the positions of a guide wire in consideration of normal liquid level; too high and too low level were less frequent. Therefore, the samples for investigation were picked up for each 1000mm from three positions, upper, middle and lower part. Table 3.14 shows the details of samples. Then we made a comparison of deterioration degree of these guide wires.

Table 3.14. Details of samples

Distance from the bottom of

tank [m]

Total times of

round-trip [time]*

Sample 1 (upper) 33 0

Sample 2 (middle) 17 1,000

Sample 3 (lower) 6 200

* Total times of round-trip after the installation are estimated from liquid level change for a year in 2010.

1) Observation of appearance / gauging of diameter We inspected the worn volume of sampled wires and their strands by visual and a digital microscope. Also, we gauged the diameters of each sample to compare the values to the standard criterion of JIS and among the samples obtained from each position.

2) Tensile test We implemented tensile test of sample wires if these wires satisfy the breaking force specified in JIS G 3550; for tensile test, both edges of wires were bonded by white metal.

We picked up 1 strand of each 6 strand excluding the core as objectives for tensile test of wire, and implemented tensile test if these wires satisfied the breaking force specified in JIS G 4314 under the condition; length of specimen between grips 100mm, tension rate 50mm/min.

(d) Result of inspection 1) Observation of appearance / gauging of diameter

Figure 3.13 shows the pictures of wire’s appearance and their diameters. There was no thinner wire than 10mm (the criterion specified in JIS G 3550) and no difference was found among each position. Red rust was found on some wires however it could be wiped off easily (it is supposed that this red rust adhered at the removing work).

Figure 3.14 shows the pictures of strand’s appearance and their diameters observed by a digital microscope. There were small wears found at the parts marked with red arrows. However these wears were not serious and the wires had enough diameters over the 0.67mm of regulation size. It can be said that there was no wear on the wires.

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Figure 3.13. Wire’s diameters Figure 3.14. Strand’s diameters

2) Tensile test Table 3.15 shows the result of tensile test for wires. All wires at each position satisfy the level of standard breaking force 61.8kN for 10mm wire. Also, no difference was found among each position. Therefore, it can be said that there was no deterioration of wires.

Table 3.16 shows the result of tensile test for strands. Some strands of No.3 LNG tank exceeded over the maximum level of standard breaking force 1850~2100MPa for 0.67mm wire. However, no strands exceeded the minimum level and no difference was found among each position. Therefore, it can be said that there was no deterioration of strands.

Table 3.15. The result of tensile test for wires

Standard breaking

force of wire [kN]

Breaking force of wire [kN]

No.1 LNG tank No.3 LNG tank

Upper

61.8

64.0 68.2

Middle 64.8 71.4

Lower 66.8 72.2

Table 3.16. The result of tensile test for strands

Breaking force of strand [MPa] Standard breaking

force of strand [MPa] No.1 LNG tank No.3 LNG tank

Upper Middle Lower Upper Middle Lower

1 2037 2036 2009 2102 2179 2169

1850~2100

2 2030 1996 2044 2089 2185 2094

3 2100 1962 2052 2089 2127 2149

4 1996 2000 2016 2169 2146 2163

5 2038 1978 2036 2137 2194 2135

6 2056 1989 1990 2143 2165 2114

Average 2043 1994 2025 2121 2166 2137

(e) Conclusion We investigated the deterioration condition of guide wires for Float type level meter that had been installed in two LNG tanks. The guide wires have no specific wear and satisfy the standard specifications for both diameter and breaking force. We conclude that the guide wires for Float type level meter have enough properties that allow continuous use for 40 years.

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3.6 Thermal insulation material

The following thermal insulation materials (i.e. perlite, Mesalite (Mitsui Expanded Shell Light-Weight Aggregate) concrete, perlite concrete block and cylindrical perlite concrete) were investigated (Figure.3.15).

Figure 3.15. Structural drawing of thermal insulation

3.6.1 Perlite Perlite was sampled from the top, the middle, and the bottom of annular space, and also from inside of the cylindrical concrete.

(a) Thermal conductivity Thermal conductivities were measured under the temperature condition of 3 steps(-20~30℃, 5~35℃, 20~50℃). We calculated these values by using the regression equation and obtained the thermal conductivities at the zero Celsius degree. Thermal conductivities of each sample of perlite satisfy the specifications at the time of the construction.

Table 3.17. Thermal conductivity of perlite

Sample Location Thermal conductivity(T.C.)

（W/mK)(at 0℃） Ratio*1

Top of annular space 0.0433 0.98

Middle of annular space 0.0429 0.97

Bottom of annular space 0.0391 0.88

In cylindrical perlite concrete 0.0399 0.90

specification 0.0442 1.00

note;*1:The ratio of the measured values to the specification value

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3.6.2 Mesalite concrete (a) Compressive strength The test was carried out in accordance with JIS A1108 (this standard follows ISO 1920-4). The test results are shown in Table 3.18. Compressive strength of the samples satisfies the specification value at the time of the construction. The specific gravity was almost the same as the value at the time of the construction.

Table 3.18. Compressive strength of Mesalite concrete

specification (N/cm2) Test result (N/cm2) Ratio*1

1470 1955 1.33

note;*1: The ratio of the measured values to the specification value

(b) Thermal conductivity The test was carried out in accordance with ISO 8301(Heat Flow meter apparatus Method). Thermal conductivity was calculated at zero Celsius degree by the equation regression as same as perlite. The test result value shown in Table 3.19 was less for approximately 67% than the construction specification and was a good value.

Table 3.19. Thermal conductivity of Mesalite concrete

Specification (W/mK) Test result (W/mK) Ratio*1

0.93 0.309 0.33


The properties of Mesalite concrete have not been changed since the days of construction.

3.6.3 Perlite concrete (a) Thermal conductivity The test was carried out in accordance with ISO 8301(Heat Flow meter apparatus Method).The test result value shown in Table3.20 was less for approximately 14% than construction specification and was a good value.

Table 3.20. Thermal conductivity of perlite concrete


0.1163 0.10028 0.86


3.6.4 Cylindrical perlite concrete (a) Thermal conductivity The test was carried out in accordance with ISO 8301(Heat Flow meter apparatus Method). The test result value shown in Table 3.21 was less for approximately 7% than construction specification and was a good value.

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Table 3.21. Thermal conductivity of Cylindrical perlite concrete


0.1163 0.10782 0.93


4. PRESENT SITUATION OF THE TANKS UNDER CONSTRUCTION

The inner tank material for above ground LNG storage tanks has mostly been made of 9% Ni steel plate over the 50 years as it has excellent mechanical properties under -160deg.C. During this period, the LNG storage tanks made of 9%Ni steel plate have safety operated. It is known that 9%Ni steel has excellent cryogenic fracture toughness due to the retained austenite and refinement microstructure obtained by nickel content and heat treatment process.

NSSMC, Toyo Kanetsu K.K (TKK) and Osaka Gas jointly developed 7%Ni-TMCP steel having the comparable performance to 9%Ni steel. Newly developed 7%Ni-TMCP is achieved 2% reduction of nickel content from the conventional 9%Ni steel by adopting Thermo Mechanical Control Process (TMCP). Various basic performance tests and fracture toughness tests in development process showed that the base metal and welded joints satisfy the regulatory and technical requirements for LNG storage tanks. In 2010, Japanese Ministry of Economy, Trade and Industry (METI) approved the use of 7%Ni-TMCP steel for newly-built tank of Osaka Gas.

The development of 7%Ni-TMCP steel realizes the reduction of rare metal nickel to be used. Therefore, not only the cost of inner tank’s material but also the cost of nickel in rising market can be saved.

From September 2012, Osaka Gas started the construction of 230,000m3 full-containment tank in Senboku I Terminal. Installation of steel pipe piles had been done at the time of October 2012 and the base slab is now under construction (Figure4). The construction of the tank will be completed by November 2015.

Figure 4. The picture of present situation at construction area

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5. CONCLUSION

Osaka Gas demolished the two LNG storage tanks that had actually been used for about 40 years. On this occasion, we assured the secure dismantling method and verified that the mechanical properties (including chemical composition, tensile strength and Charpy absorbed energy) of 9%Ni steel and Al alloy used as inner tank’s material satisfied sufficient levels. Thermal insulation material, steel pipe piles and instrumentation devices have no significant deterioration. These investigations proved the high integrity of the LNG storage tanks.

The results of these investigations obtained from the demolished LNG storage tanks that had been used for about 40 years provide invaluable actual data and it contributes to the progress of LNG storage tank market in the future.

REFERENCE

1) Recommended Practice for LNG Aboveground Storage; Japan Gas Association 2012

dismantling of aboveground lng storage tanks and …

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