CA2042864A1

CA2042864A1 - Method for detecting defect in lng tank

Info

Publication number: CA2042864A1
Application number: CA 2042864
Authority: CA
Inventors: Koji Ishihara; Akira Hagiwara; Minoru Kawashima; Noriaki Ikeda
Original assignee: NKK Corp
Current assignee: JFE Engineering Corp
Priority date: 1990-06-05
Filing date: 1991-05-17
Publication date: 1991-12-06
Also published as: FR2662800A1; JPH0440334A; AU7736291A

Abstract

ABSTRACT
Disclosed is a method for detecting a defect in an LNG tank, in which: a lattice sampling pipe arrangement is provided in a thermal insulation section of a containment system of a membrane system LNG tank constituted by the thermal insulation section and an inter barrier section with a secondary barrier, which is an object to be detected, disposed as a boundary between the thermal insulation section and the inter barrier section, the lattice sampling pipe arrangement having holes formed at predetermined intervals and disposed at lattice points thereof; sampling pipes of the lattice sampling pipe arrangement are connected to a single gas densimeter individually through respective valves; a tracer gas is led into the inter barrier section so that the density of the tracer gas leaking through the object to be detected is measured by the gas densimeter with respective to each of the sampling pipes; total data is prepared by using a processing mechanism on the basis of the result of measurement of density obtained from each of the sampling pipes;
and a leakage portion in the object to be detected is identified and detected by estimating the gas density at each of the lattice points on the basis of the total data,

Description

BACKGROUND OF THE INVENTION
:, :
The present invention relates to a method for -:
detecting a defect in an LNG tank, and particularly : - .
relates to a method for detecting a defect in an LNG
~, tank in which liquid-tight defect portions of a secondary barrier in an LNG m~mbrane tank build in a membrane system LNG carrier.

As has been known well, the IMO (International Marine Organization gas carrier code requires provision of a perfect secondary barrier in a membrane system LNG carrier. That is, a membrane system LNG carrier (having an LNG membrane tank as a main structure) has a ~,, .
structure for maintaining liquid-ti~htness to keeping LNG for a certain period b~ a secondary barrier called triplex so that an inner hull (carrier body3 should not ~be at a dangerous temperature to make brittle ~fracture if an ultra low temperature cargo (-162C), that i5, LNG leaks because of generation of a crack in a memhrane (called "primary barrier") directly contacting with LNG. Such a tank system is mainly constituted by above-mentioned primary and secondary barriers and a 2~ thermal insulation layer of P-PUF glass-fiber reinforced polyurethane foam). It is also necessary , . . , . ~ .

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- 2 - ;~
1 that satisfactory liquid~tightness of the secondary barrier can be estimated and guaranteed surely if LNG
leaks out of the primary barrier.
As such a conventional LNG tank of a low boil-off type, there is a system called TGZ Mark III Containment System developed by Techniga~ (TGZ), for example, as disclosed in Nippon Kokan K.K. Technical Report, No.104 (1984), p. 63-69.
As a method for confirming effectiveness of such a secondary barrier, there is a vacuum test. Fig. 8 is a model sectional view illustrating a vacuum test equipment provided to perform a vacuum test for an LNG
tank according to the same standard as that of the above-mentioned TGZ Mark III Containment system. Fig. 8A

shows the~configuration of an LNG tank and the vacuum test equipment, and Fig. 8B is a detail view at the A
portion shown in Fig. 8A. In this case, the vacuum test estimates the eflectiveness of a secondary barrier from its air-tightness.
2~ In Figs. 8A and 8B, the reference numeral represents an inner hull of a carrier body, 2 represènts a secondary barrier called triple~ provided inside the inner hull 1, and 3 represents a primary barrier called membrane provided inside thé secondary barrier 2 and constituting an inner wall of an LNG tank and a thermal insulation layer 7 filled with a thermal insulation ' : , ~: ',, ,' , ', ' ~ ,; ' ,. ', ,. '. ; , ,, :

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.1 panel such as glass-fiber reinforced polyurethane foam (R-PUF) is provided between a front plywood 4 to which the primary barrier 3 is attached and a back plywood 6 fixed therewith through mastics 5 on the side of the inner hull 1. Generally the portion of the thermal insulation layer 7 is called a thermal insulation section sectlon (IS), and the space between the primary barrier 3 and the front plywood 4 is an area called an inter barrier section(IBS) 8.

The above-mentioned members are those mainly constituting a membrane tank. In the primary barrier 3, stainless (304L) corrugated membranes formed in a special shape are welded to and supported by an anchor strip 9 built in the front plywood 4, and the membranes attached thus are overlaid and welded to each other.
The standard pitch of corrugations is 340mm, and the corrugations are perpendicular to each other all over the tank wall, so that the corrugations are transformed by the expansion/contraction caused by thermal change and carrier body transformation at the time of carrying LNG so that exceed stress should not be Produced in the membranes. Since the load is transmitted to the carrier body through the thermal insulation layer 7 upon application of the liquid pressure to LNG (cargo) 10, all that the membranes should perform is only to maintain the liquid tightness.

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.1 Ne~t the secondary barrier 2 is formed by contacting glass cloth with both sides of an aluminum sheet, built i~side the thermal insulation layer 7 so that the injury of the primary barrier 3 or the inner hull 1 should not give a direct effect to the secondary barrier 2, and having a liquid-tightness maintaining structure as a backup system so that the carrier body should not be brought not a low temperature even if a crack is produced in the primary barrier 3.
The thermal insulation panel constituting the thermal insulation layer 7 has a sandwich struc~ure in which the panel is bonded to the front plywood 4 and the back plywood 6, the thermal insulation thickness thereof can be changed correspondingly to the required boil-off-ratio(BOR). The edge portion of the back plywood 6 is pressed by a not-shown backing plywood, and the backing polywood is penetrated by a stud bolt welded to the inn~r hull 1 so that the thermal insulation panel attached to the back plywood 6 is :
fixed to the inner hull 1 by fastening the stud bolt with a nut. In addition, slits 11 having the same :;
pitch as the corrugations of the membrane 3 are provided in the front polywood 4 of the thermal insulation layer 7 and the R-PUF between the primary Z5 barrier 3 and the secondary barrier 2, so as to prevent an ex.cess stress from flowing in.
'': ' .

In the vacuum test for detecting a defect in an LNG -tank, air in a thermal insulation section (IS) formed by the thermal insulation layer 7 is exhausted by use of a vacuum pump 12 and a valve 13 so as to establish a certain vacuum level in the thermal insulation section (IS) so that the air-tightness of the secondary barrier ~ ~-2 is estimated on the basis of a pressure increase curve obtained by measuring the pressure increase thereafter by using a mercury manometer 14. Fig. 9 is ~ a diagram illustrating an example of a pressure "' increase curve having been obtained as the result of :, ;
this vacuum test. In Fig. 9, the abscissa indicates the time, and the ordinate indicates the vacuum level. As understood from Fig. 9, when a defect of leakage is .
produced and air-tightness is deteriorated, the time ~to return to the atmospheric pressure is short as shown in the curve ~. On the contrary, when there is less defect as shown in the curves ~ and ~, the time to return to the atmospheric pressure is long. A valve 13a is that used in the case of exhausting air of the area of the inter barrier section (IBS) 8.
If it is proved that there is a defect In the LNG
tank as a result of the vacuum test, the portion of the defect is detected by the use of an infra-red imaging method. Fig. 10 is a ;nodel diagram illustrating a method for detecting defect by use of an infra-red , . . .. , . ., : .. . . . .

h imaging method. First, hot air is fed into an LNG tank 1 15 by a not-shown means so as to raise the temperature of a membrane sheet of the primary barrier 3 uniformly, and, at the same time, a nitrogen gas (cool air~ of about 0C is fed into the thermal insulation sec~ion (IS) 7 through a blower 16. At the time of the presence of enough temperature difference by this manner, the air is exhausted by the valve 13a, and the pressure in the inter barrier section (IBS) 8 is raduces into a certain vacuum level, so that the cool air rushes out of ~he portion of a defect and membrane sheet therein is cooled partially. Since this cooled portion indicates a leakage portion of the secondary barrier Z, the portion is detected by an infra-red camera 17 brought into the LNG tank 15, and a portion or the whole of the membrane sheet is displayed on the screen of an image controller 18, so that by picking up the distribution of the partially cooled portion, it is possible to detect the defect portion.
As has been described above, a conventional method for detecting a defect in an LNG tank has been performed by the use of b~th a vacuum test method and an infra~red imaging method. By such a conventional method for detecting a defect, however, there has been a problem that it is impossible to specify a portion of leakage, while the vacuum test can judge the air-: .. ' , . ~, ' ' , , , ~ , , ,. ' . ~ ' , '' ' ,, , ' , ., , :

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tightness of a secondary barrier as a whole. ;
Also in the infra-red imaging method, the work o~ -image-picking up the whole in a large tank so as to find out a defect portion therein needs much labor and time, and the work is therefore not effective. In addition, since material such as a membrane sheet having intensive reflectivity provides many noise images, there has been a problem, that sometimes it is difficult to judge whether an image is caused by a defect or caused by ~ noise.
10 : '' SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the foregoing problems in the prior art.
It is another object of the present invention to provide a simpler method for detecting a defect portlon ~ -~
by the use of means for detecting gas density by a . ~
tracer gas.
, In order to attain the above objects, accordlng to an aspect ~ of the present invention, in the method for a ~
`
detecting a defect in an LNG tank: a lattice sampling pipe arrangement is provided in a thermal insulation section of a containment system of a membrane system LNG tank constituted by the thermal insulation section and an inter barrier section with a secondary barrier, which is an object to be detected, disposed as a ; . .

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. , , . .,~:, . , , . . : , ~: : . , , .. ,~ . . . . : ~ , boundary between the thermal insulation section and the inter barrier section, the lattice sampling pipe arrangement having holes formed a predetermined intervals and disposed at lattice points thereof;
sampling pipes of the lattice sampling pipe arrangement are connected to a single gas densimeter individually through respective valves: at tracer gas is led into the inter barrier section so that the density of the traGer gas leaking through the object to be detected is measured by the gas densimeter with respective to each of the sampling pipes: total data is prepared by using a processing mechanism on the basis of the result of measurement of density obtained from each of the sampling pipes; and a leakage portion in the object to be detected is identified and detected by estimating the gas density of at each of the lattice points on the basis of the total data. :-According to the present invention a tracer gas is flown into one of the two sections which sandwich the -~ -object to be detected, that is, the secondary barrier.

The tracer gas leaking through the object to be detected is taken out through the lattice sampling pipes located in the other section and 10d into the gas densimeter so :~
that defective leakage is detected. In this case, the respective sampling pipes have sampling holes at the lattice points, and each sampling pipe has a plurality ~:

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.1 Of sampling holes so as to form a plurality of lattice points. If valves provided at the respective sampling holes are opened, a gas in the vicinity of the plurality of the sampling holes is sucked through the sampling holes. Therefore, if the tracer gas leaks through the object to be detected and exists in the vicinity of the sampling pipe, the density of this gas can be measured by the gas densimeter. In this case, the gas density ;~ measured through one sampling pipe is an average value of the gas density sampled through a plurality of sampling holes. However, if the gas density is measured in a manner as described above through all the sampling pipes arranged in a lattice, the gas density at each lattice point is measured twice through two 15~ ~ongitudinally and transversally provided different sampling pipes. That is, the gas density at a certain lattice point is the gas density obtained through two sampliDg pipes ~crossing each other at the lattice point, and it is possib1e to estimate that a defect exists in the vicinity of a lattice point when the gas density obtained at the lattice point is large. More specifically, if the lattice sampling pipes and the lattice points formed by the sampling pipes are numbered, it is possible to identify the position and quantity of leakage.

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Figs. lA and lB are model sectional views illustrating a leakage test equipment by a tracer gas used for the me-thod for the deect detection according to the present invention;
Figs. 2A through 2C are views illustrating a lattice sampling pipe arrangement according to the present invention;
Fig. 3 i5 a model diagram for explaining a method for measuring gas density through a sampling pipe according to the present invention;
Fig. 4 is a model diagram illustrating a C02 laser , light absorbing gas densimeter with sulfur he~afluoride ~ -for detecting a tracer has according to the present ~ ~
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invention ;
Fig. 5 is a diagram illustratin~ a measured example ~- ~
lndlcating the gas density distribution by the use of ,- -the lattice sampling pipe arrangement according to the :~
present invention; `~

Fig. 6 is a diagram illustrating the gas density distribution in the test surface estimated from the test result of Fig. 5;
Fig. 7 is a diagram illustrating a three- -dimensional image display of a measured examples of the ;

tracer gas density and distribution through real-time measurement;

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1 Figs. 8A and 8B are model sectional views illustrating a conventional vacuum test equipment for an LNG tank;
Fig. 9 is a diagram illustrating pressure increase curves obtained as a re~1lt of a conventional vacuum test; and Fig. 10 is a model diagram illustrating a conventional test equipment by an infra-red image pickup method.

' DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Figs. lA and lB are model sectional views illustrating a leakage test equipment by a tracer gas used for the method for the defect detection according to the present invention. Fig. lA shows an LNG tank and a leakage test equipment associated therewith, and Fig. lB is a detailed diagram at the portion B shown in Fig. lA. In Figs. lA and lB, parts the same as or equivalent to those in the conventional example of Fig. 6 are referenced correspondingly and the description thereof will be omitted here.
First, in Fig. lA, a tracer gas 20 is led into an inter barrier section (IBS) 8 through a valve 13a. In the portion B of Fig. lA, as shown in detail in Fig.
lB, the space for forming mastics 5 is enlarged, a ., . . ~ . . .
- : .

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.. .. . . . .

1 lattice sampling pipe arrangement constituted by orthogonal sampling pipes 21 and 21a is provided in the area of the space, that is, a part of a thermal insulation layer (IS) 7. As shown in Fig. lB, although the lattice sampling pipes 21 and 21a are arranged in a section between a secondary barrier 2 and an inner hull 1, that is, on the inner hull side, they may be buried in a polyurethane foam (R-PUF) 22. Then, a tracer gas is led into the IBS 8 through the valve - ~

10 13a, and the thermal insulation section of the IS 7 is ~ -connected with a vacuum pump 12 through a valve 13 so that the air therein can be exhausted by the vacuum pump 12. One-side ends of the sampling pipes 21 and 21a are connected with a gas densimeter 23 through - F
respective valves 13b (see Fig. 2).
Figs. 2A through 2C are views illustrating a lattice sampling pipe arrangement. Fig. 2A is a model diagram illustrating the connection of the lattice . .. . . .
sampling pipes with a gas densimeter, Fig. 2B is a partial view showing, in detail, a portion in the vicinity of a lattice point C shown in Fig. 2A, and Fig. ~ -2C is a partial view showing, in detail, in a portion at the other end D of a sampling pipe. As illustrated, the lattice sampling pipes 21 and 21a are grouped for every test range on each surface (side surfaces and bottom surfaces) of an LNG tank shown in Fig. lA. As - , , ,. ,, , ; ~ , " ;, 1 described above, the sampling pipes 21 and 21a are sucked by an exhaust system provided on the gas densimeter 23 through the respective valves 13b so that the gas density measurement is performed (the measurement will be described later). At the portion of the lattice point C, sampling holes 24 which serve as gas entrances are formed in the sampling pipes 21 and 21a so that each hole perpendicularly diametrically penetrates each sampling pipe as shown in Fig. 22B.

In the portion D, a blind seal 25 is formed at the other end of each pipe as shown in Fig. 2C. Then, the sampling pipes 21 and 21a are arranged so that the distance of the lattice is made narrow, that is, the distribution of the lattice points is made dense, at portions where the defect generation rate is high. In addition, as shown in Fig. 2A, the valves 13b are numbered into Vi=1, ~ Vi=n~ Vj=1, ~ j=n~
the sampling pipes 21 and 2la corresponding thereto are numbered into Si=1~ ~ Si=n' S~ Sj=n' respect~ively.
Next, the measurement procedure of defect detection will be described. First, the valve 13 is opened so that the portion of the IS 7 is exhaust to a vacuum by means of the vacuum pump 12, and the valve 13a is opened to lead a tracer gas (sulfur hexafluoride) into the IBS 8. Consequently, a pressure ,, , , :

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.1 difference is produced between the sections on the opposite sides of the secondary barrier 2, so that if there is a defect of leakage in the secondary barrier 2, the tracer gas leak is through thus defect into the sampling pipes 21 and 21a. By sequentially swi~ching the valves 13b which are provided in the respective lattice samplin0 pipes, the leaking tracer gas is led into the respective sampling pipes 21 and 21a, and the gas density thereof is measured by the gas densimeterer 23. In a p~actical measuring procedure, first, only the valve Vi=1 of the valves 13b is opened to perform suction and measure gas density. After the gas density measurement through the valve Vi=1 is finished, the valve Vl=1 is closed and the next valve 5 Vi 2 lS opened to measure the gas density. The valves 13b are repeatedly opened and closed successively one after one from the~valve Vi=1 to valve Yi=n and from te valve Vj 1 to the valve Vj=n, so as to measure the gas density with respect to each of all the sampling pipes 21 and 21a successively from the pipe Si=1 to the pipe Si=n and from the pipe Sja1 to the pipe Sj=n. Further, the measurement by such an operation is repeatedly performed a plurality of times. As shown in Fig. 3, a tracer gas is sucked by a sampling pipe 21 through sampling holes 24 near an area E at which the tracer has leaks through the secondary barrier 2, and the ', ' ' :, ' ' . ,' :,' .~, ' h ~J ~ 2d 1 tracer gas is led to the gas densimeter 23 so that the density thereof is measured in the following manner.
As the tracer gas, a sulfur hexafluoride gas, a helium gas, a halogen group gas, etc. may be used, and the type of the gas densimeter is selected depending on the kind of the gas to be used. Fig. 4 is a model explanation view illustrating a method by using sulfur hexafluoride as an example of the tracer gas, in which this tracer gas is irradiated with laser light so that the gas density is measured on the basis of the degree of absorption of this laser light. In Fig. 4, a sample suction pipe 31 is combined with the sampling pipes 21 and 21a shown in Figs. 1 to 3, and this sample . .
suction pipe 31 is connected to an air entrance of a lS suction cell 32. The opposite ends of the suction cell 32 are formed of a material which is transparent with respect to light having a wave length in an infra-red band. The suction cell 32 is shaped into a closed vessel having a predetermined cell length (for example, 30 cm), and ha~ing an entrance and an exit for air. In this embodiment, the air entrance of the suction cell 32 is connected to the sample suction pipe 31, and the air exit of the same is connected to a suction exhaust system 34. If the sampling pipes 21 and 2la are arranged to pass through a leakage portion, SF6-mixed air is led into the pipes. In order to detect the , . . ~ . ~ . ..

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.1 SF6-mixed air, the suction cell 32 is irradiated with, for example, laser light having a wave length of 10.6 ~m (hereinafter referred to as "P(16)-ray laser light") of a carbon dioxide laser (hereinafter referred to as ''C2 laser"). The reference numeral 35 represents the C2 laser which generates P(16)-ray laser light. The reference numeral 36 represents a helium-neon laser (hereinafter referred to as "He-Ne laser"). ~enerating red light, the He-Ne laser 36 is used as a pilot laser for checking an optical path of the C02 laser. The reference numeral 37 represents a spectrum analyzer which is a measurement instrument for measuring the wave length of the light generated from the CO2 laser 35 and the He-Ne laser 36. The reference numeral 38 represents a photo-detector which detects light near the wave length of 106 ~m, converts the detected light into an electric single, and outputs the thus converted electric signal. The reference numeral 39 represents an amplified which amplifies an input signal supplied from the photo-detector 38, and supplies the amplified signal to a display 4d and a leakage discriminator 41. The display 40 displays the output of the amplifier 39, and the leakage discriminator 41 discriminates leakage on the basis of the change of an output signal of the amplified 39. The reference numeral 42 represents a mirror for reflecting light, 43 represents a half .' ' ' . ' .' ' , , '..

' '~. ' . ' "' ' ' ~ . '' ' , , . ~ '' '' " . ' ' ' ', ' ' . : , .1 mirror for reflecting incident light partially and transmitting it partially, 33 represents a pipe connecting the exit of the suction cell 32 with the suction exhaust system 34.
In the gas densimeter shown in Fig. 4, if a tracer gas SF6 is led into the suction cell 32 through the sample suction pipe 31, the quantity of the transmitted P(16)-ray laser light is reduced by ~he absorption of the SF6 gas, so that the SF6 gas density can be measured on the basis of the quantity of the reduction. As ~or a similar gas densimeter, there are a halôgen leak detector using a halogen-group gas, and a helium leak detector of a mass analysis type each of which may be used as a well-known gas densimeter and is suitabIe for use in the detection method of the present invention, but the description thereof will be omitted here.
Fig. 5 is a diagram illustrating a measured example showing the distribution of gas density by the use of a lattice sampling pipe arrangement. Lattice points formed by the sampling pipes Si=1 to Si n and Sj=1 to Sj=n are numbered into P11 to Pnn The gas density obtained from a certain sampling pipe (for example, Si=1) is an average gas density of the values sampled through sampling holes provided at a plurality of lattice points, so that the gas density at a lattice .

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.1 point can be estimated from the gas density values obtained from two crossing sampling pipes. For example, the gas density at the point P11 is estimated by multiplying the gas density Di=1 obtained from the sampling pipe Si=1 by the gas density Dj=1 obtained from the samplin0 pipe Sj=1 Estimating the gas density all over the area from the data of Fig. 5 in the same manner, for example, a gas density distribution diagram can be obtained as shown in Fig. 6. In Fig. 6, the abscissa indicates the position of a lattice point in ~he i direction, and the ordinate indicates the position of the lattice point in the J direction. In such a manner, a leakage defect portion of the secondary barrier 2 (a left upper corner portion of the test surface) can be identified and detected.
As for a method of display the display 40, besides the display example illustrated in Figs. 5 and 6, there :
is a display method by higher data processing. Fig. 7 ~
is a diagram illustrating a measured example in which the density and distribution of a tracer gas is made into a three-dimensional image by a computer. The i-j :
surface indicates a test surface, and the degree of , leakage is displayed in the Z direction, so that a three-dimensional display is realized. In this 2S detection method, a measured value oE gas density supplied into a not-shown computer in real time. The ' ! , ': ' , .' ' ' , ', '., ' . ' ' ' ' , . : ' ' ,: ' , ,,, .'.

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.1 surface subjected to test is divided into unit test sections each including a lattice point. The degree of leakage is estimated by the gas density values obtained from two crossing sampling pipes. For example, the degree of leakage in the unit test section of i=1 and j=1 in Fig. 7 is estimated by multiplying the gas density Di=1 obtained from the sampling pipe of i=1 by the gas density Dj=1 obtained from the sampling pipe of j=1. This is repeated for every unit test section sequentially, and the degree of leakage of the whole the test surface is displa~ed. From this result, it is possible to detect the leakage portion, that is, the i-j coordinates position having the highest pole in the Z direction in each test surface.
As has been described above, according to the present invention, sampling pipes a:re arranged in the form of a lattice, and sampling holes are provided at lattice polnts of the lattice arransement of the sampling pipes to take out a tracer gas, so that it is ~ -20~ possible to make the number of sampling pipes smaller than that in a method for sampling gas through separated sampling pipes each of which has a hole at a single position. For example, in order to measure gas density at 100 points, while the method using a separated pipe arrangement needs 100 sampling pipes, the method using a lattice pipe arrangement according to the present . , . , : ~

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.1 invention needs 20 sampling pipes, so that it is possible to reduce the test cost.
As has been described above, according to the present invention, lattice sampling pipes having sampling holes at lat~ice points in a one-side section of a secondary barrier of an LNG tank upon which a defect of leakage is tested are arranged to detect tracer gas leaking from the other section by a gas densimeter connected with the sampling pipes, so that it is possible to judge a leakage portion of the secondary barrier properly. Consequently, the portion of the secondary barrier to be mended is made clear so . .
as to minimize the cut area of a primary barrier (membrane), so that a great contribution to reducing mending processes can be obtained.

Further, since sampling pipes are provided in a lattice arrangement, it is possiblé to make the number of sampling pipes smaller than that in a method of detection by separated pipes, so that there is an effect to reduce the test cost.

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Claims

What is claimed is:
A method for deteting a defect in an LNG tank, comprising the steps of:
providing a lattice sampling pipe arrangement in a thermal insulation section of a containment system of a membrane system LNG tank constituted by said thermal insulation section and an inter barrier section with a secondary barrier, which is an object to be detected, disposed as a boundary between said thermal insulation section and said inter barrier section, said lattice sampling pipe arrangement having holes formed at predetermined intervals and disposed at lattice points thereof;
connecting sampling pipes of said lattice sampling pipe arrangement to a single gas densimeter individually through respective valves;
leading a tracer gas into said inter barrier section so that density of the tracer gas leaking through said object to be detected is measured by said gas densimeter with respetive to each of said sampling pipes;
preparing total data by using a processing mechanism on the basis of the result of measurement of density obtained from each of said sampling pipes; and identifying and detecting a leakage portion in said object to be detected by estimated gas density at each of said lattice points on the basis of said total data.