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THE BEST REMEDY TO PROLONG LIFETIME OF LNG STORAGE TANKS : A DIAGNOSIS & OVERHAUL FOR LNG STORAGE TANKS
Chusung Son
Senior Manager, Terminal Improvement Team, LNG Terminal Operation Section
Korea Gas Corporation 120, Cheomdan-ro, Dong-gu, Daegu, Korea
In-Kyu Cho General Manager, Terminal Improvement Team,
LNG Terminal Operation Section Korea Gas Corporation
120, Cheomdan-ro, Dong-gu, Daegu, Korea [email protected]
Youngkyun Kim, Senior Research Engineer LNG Technology Center
KOGAS Research Institute Korea Gas Corporation
960 Incheonsinhang-daero, Yeonsu-Gu, Incheon, Korea
Seungchul Ju Assistant Manager
Terminal Construction Team Pyeongtaek LNG Terminal
Korea Gas Corporation 175-88, Namyangman-Ro,
Posung-Up, Pyeongtaek-Si, Korea [email protected]
Jeongyeon Lee Senior Staff, Terminal Improvement Team,
LNG Terminal Operation Section, Korea Gas Corporation
120, Cheomdan-ro, Dong-gu, Daegu, Korea
ABSTRACT Korea Gas Corporation (KOGAS) has been operating 69 storage tanks in four LNG receiving terminals for approximately 30 years. For the first time in the world, diagnosis and overhaul of LNG storage tanks started with KOGAS in 2007. Since there was an abnormal datum at Incheon LNG receiving terminal, the company’s task force team inspected four in-ground membrane tanks. It was a risky challenge because there were no experience, techniques, procedures, and specialized engineers available for taking action on the overhaul of the operating tanks. However, the researchers succeeded the first overhaul without any accident and problem. As a result, the company had accumulated new technologies such as the sealing of steel roof, dispersion prevention cover of internal unloading pipe, vent hole of flash gas and so forth. With the overhaul of four tanks as a beginning, KOGAS has completed four in-ground membrane tanks in Incheon and eight above-ground membrane tanks in Pyeongtaek. In addition, the diagnosis and overhaul of six tanks are on progress by seven special engineers on two task force teams. KOGAS has made three kinds of overhaul manuals regarding in-ground membrane, above-ground membrane and above-ground 9%-Ni tank. The long term operated tanks' auxiliaries renewal is progressing as connected with their overhaul. If the abnormal factors of tanks are detected in periodic diagnosis, their overhaul will be conducted in accordance with the company’s procedures and standards. Its diagnosis and overhaul technology would become the best remedy to prolong the lifespan of LNG storage tanks in the LNG receiving terminals around the world. This paper discusses the countermeasures required to extend the useable life of the long term operated LNG storage tanks and especially offers the overhaul procedure of a membrane tank in which KOGAS has a lot of experience and know-how.
1
1. Introduction Korea Gas Corporation is operating 69 LNG storage tanks in four LNG terminals namely Pyeongtaek, Incheon, Tongyeong, and Samcheok as shown in Figure 1. These tanks have the capacity to store up to 10,660,000kl of LNG as shown in Table 1. Especially, the three largest LNG storage tanks in the world with the capacity of 270,000kl are being constructed in Samcheok LNG terminal.
[ Pyeongtaek Terminal : 23 Tanks ] [ Incheon Terminal : 20 Tanks ]
[ Tongyeong Terminal : 17 Tanks ] [ Samcheok Terminal : 9 Tanks1 ]
Figure 1. LNG Terminals and Operational LNG Storage Tanks in KOGAS
Terminal Tank No. Capacity(kl) Engineering Type Remark
Pyeongtaek 1 ~ 10 100,000 SN-TGZ(FRA) Membrane
Above-ground 11 ~ 14 140,000 KOGAS-Tech(KOR) 9%-Ni
15 ~ 23 200,000
Incheon
1 ~ 10 100,000 TKK(JPN) 9%-Ni Above-ground 11 ~ 12 140,000 MHI(JPN)
Membrane In-ground 13 ~ 14 200,000 IHI(JPN) 15 ~ 18 200,000 KHI(JPN) 19 ~ 20 200,000 JFA(JPN)
Tongyeong
1 ~ 3 140,000 KHI(JPN)
9%-Ni Above-ground
4 ~ 5 140,000 TKK(JPN) 6 ~ 12 140,000
KOGAS-Tech(KOR) 13 ~ 16 200,000
17 140,000 Samcheok 1 ~ 9 200,000
Table 1. Operational Status of LNG Storage Tanks in KOGAS Terminals
1 There are 9 operational tanks and 3 under construction tanks (270,000kl) in Samcheock Terminal.
2
Recently, the operational tanks have had to adapt to changing circumstances as below; § A few tanks have deteriorated after many years (more than 15 years) of operation § Performance of tanks' auxiliaries has degraded after many years of operation § LNG storage tanks’ management criteria have been tightened following consolidation of the
gas industry law and regulation in Korea. § Safety of internal tank has to be checked and guaranteed considering tanks’ deterioration. Therefore, the view of maintenance against an LNG storage tank has to be changed. Also, the diagnosis, repair, and renewal of internal tank, outer tank, and auxiliaries are necessary through the tank’s overhaul to meet the changed circumstances. This paper deals with the process to secure safety and extend the lifespan of the long term operational membrane tank through its diagnosis and overhaul.
2. Historical Trail of LNG Storage Tank Overhaul in KOGAS Terminal The first overhaul of an LNG storage tank was for an in-ground membrane type that started in 2007 at the Incheon terminal. A total of 6 tanks were subjected for inspection and overhaul because more methane was sensed in IBS2. In accordance with KOGAS operation manual, the concentration of methane shall be maintained as 1.25vol%(1/4 of LEL) by N2 sweeping since it can exist in IBS because of membrane’s porosity. However, the concentration of methane in IBS continued to increase gradually and the relevant tanks were within warranty period. KOGAS determined to open and inspect the tanks and then started to overhaul the 6 tanks in consecutive order presidentially by constructors and with the support of KOGAS. The above-ground membrane tanks in Pyeongtaek terminal which are similar to the in-ground membrane tanks in the Incheon terminal but are operated on a long term period (more than 20 years) were checked based on the operational datum in IBS. Then, KOGAS started to proactively inspect and overhaul those in Pyeongtaek terminal in order to guarantee the their safety. After the first overhaul, KOGAS built the criteria of LNG storage tank overhaul as shown in Table 2 and then has progressed tanks’ overhaul as shown in Table 3.
Item 9%-Ni
Membrane Remark Dike No Dike
Inspection Period per tank 1 implementation per 2 years All of LNG Storage Tanks
Steel Roof ○ ○ ○ Sealing Welding Reinforcement
Inner tank ○ ○ ○ Non-destructive Inspection &
Repair (If required)
Vapor Barrier ○ ○ N/A Non-destructive Inspection & Repair (If required)
Auxiliaries ○ ○ ○ Renewal during operation stop
Table 2. Criteria of LNG Storage Tank Diagnosis & Overhaul
2 IBS(Inner Barrier Space) : IBS is isolated from an inner tank and filled with N2.
3
Terminal Tank No. Overhaul Period Status Overhaul
Contents Remark
Pyeongtaek
#1,7 03-2011 ~ 07-2012
Completion Steel Roof Membrane
Etc.
Conducted by KOGAS
#2 08-2013 ~ 09-2014
#3 04-2014 ~ 06-2015
#4 12-2009 ~ 03-2011
#5 03-2013 ~ 03-2014
#6 10-2014 ~ 10-2015
#9 07-2015 ~ 09-2016
Incheon
#14 06-2007 ~ 10-2008
Completion
Defect Repair &
Improvement of Lead Pipe
Conducted by
Constructor (Defect Repair)
#15
#16 08-2007 ~ 12-2008
#17 08-2007 ~ 11-2008
#13 05-2009 ~ 10-2010 Improvement of Lead Pipe
#18 06-2011 ~ 09-2012
#1 03-2016 ~ 05-2017 In Progress Steel Roof
9%-Ni Etc.
Conducted by KOGAS #2 10-2015 ~ 02-2017 Completion
#3 03-2017 ~ 09-2018 In Preparation
Tongyeong
#1 10-2017 ~ 02-2019 In Preparation Steel Roof
9%-Ni Etc.
Conducted by KOGAS #2 10-2016 ~ 02-2018 In Progress
#3 06-2017 ~ 10-2018 In Preparation
Table 3. Status of LNG Storage Tank Diagnosis & Overhaul
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3. Diagnosis & Overhaul of Above-ground MembranTank
The following flow chart as shown in Figure 2 is the basic procedure for the above-ground membrane tank in KOGAS terminal.
Figure 2. Flow Chart of Above-ground Membrane Tank
LNG final pumping and natural vaporization
Tank System Separation & Quality Verification Test
Nitrogen Purge
Aeration & Tank opening
Temporary Equipment Installation
Disassembly to Deck Upper Area
Steel Roof & Membrane Inspection
Roof repair, reinforcement & Inspection
Tank Nozzles Connection &Cool Down
LNG Filling & Pump Test Run
De-commissioning
Nitrogen & FG Purge Commissioning
Tank Scavenging
He Test, NH3 Test, PT, VBT, ATT
LMS leakage inspection after FG purge
Flammable gas LEL of 25% or lower
Flammable gas LEL of 10% or lower and oxygen concentration of 20% or more & Global Test
Isolation of Nozzles.
Preparation
Cause Investigation
Membrane Repair & Inspection
Repair & Reinforcement
Steel Roof Repair & Reinforcement Welding
Defect Repair Welding including NH3 Test, PT, VBT
Tank Test Global & AE Test
Temporary Equipment Disassembly
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3.1. De-commissioning 3.1.1. Pumping of LNG Inside the Tank
For the LNG of the tank which is the target of overhaul, the pump operation is lowered as much as possible to the lowest liquid level so that the time used for the natural vaporization of the remaining LNG is minimized. According to LP pump operation manual, pumping is performed to LNG liquid level of 32m.
3.1.2. Natural Vaporization The leftover LNG that is impossible to be discharged to LP pump in the LNG storage tank is vaporized in the natural state. In order to check the finished state of the natural vaporization of LNG, the temperature inside the storage tank is measured at the following positions: § Steel roof & tank inside space § Membrane bottom, insulation & slab
When the membrane bottom temperature reaches more than -20℃, it is ended.
3.1.3. Tank Isolation The piping connected with the LNG tank is blocked off and the outside flow of LNG, BOG, and FG is blocked in order to open the tank. If required for the perfect block out, the connected piping is cut and a blank or blind flange is installed.
3.1.4. Quality Verification Test
a. Sampler for Gas Leakage Test to the Concrete Roof
During the natural vaporization and nitrogen purge period, a gas sampler is installed to measure and record the concentration of the methane leftover inside the concrete. These are compared with the measured value after overhaul to check the soundness of the steel roof. The inspection is done 3 times per a day during natural vaporization and nitrogen purge period before the opening of tank.
b. Soundness Test of IBS Inner Ring Pipe
After helium is injected to the IBS inner ring pipe before the opening of tank, it is checked if helium is detected or not at the measurement part of the outer ring left and right near the inner ring pipe to check the soundness of the inner ring pipe. Its response time is measured if it is detected.
3 Net Positive Suction Head(NPSH) can be different per each manufacturer
6
The soundness of IBS inner ring pipe is verified based on the above test result and its judgment is done as below: § If helium is detected at the measurement part of the outer ring left and right near the
inner ring pipe immediately after its injection, it is judged to be damaged at the upper side of inner ring pipe.
§ If helium is detected at the measurement part of the outer ring left and right near the inner ring pipe after a certain amount of time, the damaged position of the inner ring pipe is judged according to the detection response time.
c. Global Test After air purge for opening of tank is finished, the change of oxygen concentration in IBS is measured at the pressurized state(20kPa) of tank for 72 hours. Global test is done before and after overhaul and the soundness of tank is analyzed based on the comparison of their results. The global test is done based on the procedures and guidelines as below:
① The oxygen concentration average value of IBS is below 500ppm after nitrogen purge before the test.
② IBS pressure is maintained at 0.3~0.5kPa and nitrogen supply amount is set at lower
than 40N㎥/hr.
③ The pressure inside the tank is maintained at 20kPa and IBS pressure is done at 0.3~0.5kPa.
④ The oxygen concentration of IBS is measured at several points and they are recorded to make the change trend curve.
⑤ The leakage test using soap water is performed on the welding parts, manholes, flanges, and nozzles during global test period.
⑥ If the oxygen concentration is more than 15,000ppm at an IBS measurement point, it is measured at the next point.
⑦ After the test is finished, nitrogen sweeping is performed to lower the oxygen concentration.
3.1.5. Nitrogen Purge The nitrogen purge is the removal of the remaining gas (e.g. methane, etc.) from the internal tank using the piston effect on the initial warm-up. The nitrogen purge process is as follows:
① Nitrogen Supply : Operation pressure is maintained higher than the other tank operation pressures
② BOG Recovery : Flammable mixed gas until 90VOL%
③ Flaring : Flammable mixed gas lower than 40LEL%
④ Atmosphere Vent : Flammable mixed gas lower than 25LEL%
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The gas concentration inside the tank is measured once every 4 hours on deck upper area and tank inner space. After the nitrogen purge is finished inside the tank, the purge of loading pipe, mixing loading pipe, discharge and relevant pipe are performed using the nitrogen inside the tank.
3.1.6. Aeration After the nitrogen purge is finished, dry air is injected into the tank and makes the internal tank condition more than 20% of oxygen concentration for workers to be able to go into the tank. The air supply condition is as below: § Supply Pressure : 0.7MPa § Flow Rate of Supply : 500 ~ 1,000m3/hr § Tank Pressure : Maintained between 5~12kPa (excluding Global Test pressure)
3.1.7. Tank Opening and Initial Entry
Tank opening and initial entry procedure is as shown in Figure 3.
Figure 3. Tank Opening & Initial Flow Chart
3.2. Preparation
3.2.1. Temporary Equipment Installation 1) Crane Installation
Field Control Center § Tank manhole entrance installation § Tank personnel management
Pre-Safety Inspection § Tank isolation verification § Air injection into tank § Safety tools, equipment inspection
Pre-Safety Inspection § Test position(deck upper area, tank inner space) § Object animal : Canary, Hamster 1 pair each § Test time : more than 12 hours
Tank Opening § Manhole cover opening § Gas dome spare nozzle opening
Deck Upper Area & Tank Bottom Inspection(1st Stage)
§ Entry personnel : KOGAS 1 & Constructor 2 persons § Oxygen and gas concentration is measured at
each stair landing on the entry ladder inside tank (Oxygen concentration of more than 20%)
§ Verification of flammable gas, liquid gas existence § Gas concentration measurement on the deck
upper area Internal Inspection
A 300 ton crane is used to transfer a 100materials for overhaul of the tank. It of the existing crane for LP pump overhau
2) Installation of Material Rack The material rack is installed near the manhole and the are temporarily stored there
3) Installation of Equipment and Structure for Transportation into the
A winch for transportation of protection nets are installed.
4) Installation of Fall Prevention Scaffolding on Deck Outside Area After the insulation and plywood on the deck outside area are disassembled, the fall prevention scaffolding is installed on the deck outside area and the membrane ladders
5) Installation of Protective Plywood
The protective plywood is installed to prevent passage damage on the deck and the corrugated iron is installed in the circumference direction with a width of more than 3m from the membrane wall in order to prevent objects from falling when operating the gondola. In addition, the protective plywood with a diameter of more than 10m is installed at the lower part of the material hole in order to prevent from falling. Their installation is as shown in
Figure 4. Deck Protective Plywood & Corrugated Iron Installation Diagram
6) Installation of Scaffolding for Steel Roof The scaffolding is installed to inspect the steel roof as shown in
Tank Entry Ladder
Man Hole
Gas Dome
8
A 300 ton crane is used to transfer a 100 ton crane into its dike materials for overhaul of the tank. It is installed on the concrete road under the structure of the existing crane for LP pump overhaul.
nstallation of Material Rack
The material rack is installed near the manhole and the transportedstored there before they are carried into the tank by men
stallation of Equipment and Structure for Transportation into the
for transportation of membrane protection plywood, gondola, etcprotection nets are installed.
Installation of Fall Prevention Scaffolding on Deck Outside Area &
insulation and plywood on the deck outside area are disassembled, the fall olding is installed on the deck outside area and the membrane ladders
Installation of Protective Plywood and Corrugated Iron
The protective plywood is installed to prevent passage damage on the deck and the corrugated iron is installed in the circumference direction with a width of more than 3m
membrane wall in order to prevent objects from falling when operating the ndola. In addition, the protective plywood with a diameter of more than 10m is
installed at the lower part of the material hole in order to prevent Their installation is as shown in Figure 4.
Deck Protective Plywood & Corrugated Iron Installation Diagram
Installation of Scaffolding for Steel Roof Inspection
The scaffolding is installed to inspect the steel roof as shown in Figure
Tank Wall
Corrugated Iron
Falling Prevention Scaffolding
Protective Plywood
Central Work Passage
Entry Ladder
ton crane into its dike in order to transport is installed on the concrete road under the structure
transported materials to the roof before they are carried into the tank by men.
Tank
protection plywood, gondola, etc and fall
Membrane Ladder
insulation and plywood on the deck outside area are disassembled, the fall olding is installed on the deck outside area and the membrane ladders.
The protective plywood is installed to prevent passage damage on the deck and the corrugated iron is installed in the circumference direction with a width of more than 3m
membrane wall in order to prevent objects from falling when operating the ndola. In addition, the protective plywood with a diameter of more than 10m is
installed at the lower part of the material hole in order to prevent transporting materials
Deck Protective Plywood & Corrugated Iron Installation Diagram
Figure 5.
Corrugated Iron
Falling Prevention Scaffolding
Sectional Drawing
Figure 5
7) Gondola Installation for Membrane Inspection The gondola monorail (Ithe Al-sheets of the selected area are
8) Installation of Electric Equipment The electric equipmentarea and tank inside.
9) Installation of Air Injection
The air supply equipment reused in order to maintain an oxygen concentration of more than 20%.The vacuum breaker pipe is disassembled and 2 exhaust frelease the harmful gas and dust occurring from tank inside work to the outside of the tank.
3.2.2. Disassembly of Insulation & Plywood at The box type of insulation installed at the aisle for reuse as shown in For the movement of the gondola and nonoutside area of the deck is disassembled as shown in
Insulation disassembly
Figure
Stair at 4 sections
9
Sectional Drawing of Scaffolding Installation
5. Installation of Scaffolding Under the Roof Plate
Gondola Installation for Membrane Inspection
(I-Beam 300mmX150mm) is installed at the lower area of sheets of the selected area are disassembled for the installa
Installation of Electric Equipment
equipment such as lighting, cable, and so forth is installed to the
Installation of Air Injection and Exhaust Fan
air supply equipment which is installed on the air purge before tank opening reused in order to maintain an oxygen concentration of more than 20%.The vacuum breaker pipe is disassembled and 2 exhaust fans are installed at the tank roofrelease the harmful gas and dust occurring from tank inside work to the outside of the tank.
Disassembly of Insulation & Plywood at Upper Deck Area
The box type of insulation installed at the upper deck area is removed and stored at the work aisle for reuse as shown in Figure 6. For the movement of the gondola and non-destructive test, the plywood installed at the outside area of the deck is disassembled as shown in Figure 6.
disassembly Plywood disassembly
Figure 6. Disassembly of Insulation and Plywood
at 4 sections
Scaffolding board installation section
(1~5 stage)
Scaffolding board installation section
(6~13 stage) Scaffolding board
Roof Plate
Suspended Deck
. Installation of Scaffolding Under the Roof Plate
Beam 300mmX150mm) is installed at the lower area of the deck and for the installation of the gondola.
is installed to the upper deck
before tank opening is reused in order to maintain an oxygen concentration of more than 20%.
ans are installed at the tank roof to release the harmful gas and dust occurring from tank inside work to the outside of the tank.
deck area is removed and stored at the work
destructive test, the plywood installed at the
Plywood disassembly
Plywood
10
3.3. Inspection
3.3.1. Applicable Non-destructive Test The non-destructive test for steel roof is applicable as shown in Table 4.
Application Object Inspection Method Note
Steel Roof
Outside area
Q Section Welding Area Visual/Helium
Including Gusset Plate
R Section Welding Area Visual/Helium
S Section Welding Area Visual/Helium/NH3
T Section Welding Area Visual/Helium/NH3
U Section Welding Area Visual/Helium/NH3
Base Material Area Visual Test
Nozzle Area Visual/Helium/ATT Including
discharge and unloading, etc.
Defect Repair Area
Visual/Helium/ PT/VBT
Including He inlet
Main Body
Main & Stiffener Beam Visual/PT/ATT/VBT
Lug & Bracing Area Visual/ PT/VBT
Basic Material Area Visual/ PT/VBT
Lap Joint PT/He/ATT/VBT including inlet of He and ATT
Hole
Table 4. Application Object & Inspection Method for Steel Roof Non-destructive Test
(S)
Stainless Steel
Stainless Steel (U)
(T)
(Q)
(R)
Carbon Steel
11
The non-destructive test for membrane is applicable as shown in Table 5.
Application Object Inspection Method Note
Membrane
Wall
Welding Area Visual/NH3
Basic Material Area Visual
Defect Repair Area Visual/NH3/PT/VBT Including RLP4
Bottom
Welding Area Visual/NH3
Basic Material Area Visual
Defect Repair Area Visual/NH3/PT/VBT Including RLP
Table 5. Application Object & Inspection Method for Membrane Non-destructive Test
3.3.2. Visual Test (VT) The state of base material and welding parts, the corrosion of structures, and the existence of cracked parts, etc. are performed for the first time.
3.3.3. He Test Helium is injected in the space between the plates of steel roof outside area and the concrete roof and then defects of base material and welding area are inspected. The helium test at steel roof outside area is divided into zone A and B as shown in Figure 7.
Figure 7. Helium Injection Inlet and Detection Section Drawing
If more than 1.0X10-4mbarℓ·/sec which is the helium standard concentration is detected at its measurement point, the helium test is performed at zone A.
4 RLP : Reference Leakage Point
(T)
(Q)
(R)
Stainless Steel(Dissimilar Metal Part)
Helium Injection Entrane (Common Inlet)
(S)
Helium Injection Inlet
Stainless Steel(Membrane)
Zone A (1st )
Zone B (2nd ) (U)
Carbon Steel
If more than 5% is detected at IBS outer ring, the helium test is performed at zone B.The helium test at nozzle parts in the steel roof andas zone C. If more than 1.0X10Two holes for helium injection are made on the areas where the inspector is not able to perform Vacuum Box Test (VBT) due to the interference of stiffener beam. One is used for helium injection and the other is done for The measured helium concentration value shall not exceedone at all of inspected areas.
3.3.4. NH3 Test The gas mixed with nitrogen and ammonia is injected into IBS and ammonia response paint is applied to the membrane welding areas to check if they are defected by the change in color. The response paint is acid and the yellow color is changed to celadon when it reacts with ammonia which is alkaline gas.Reference Leakage Points (RLP) are installed to check the ammonia concentration in IBS, supply the mixed gas, and rinse IBS. The installation of RLP is shown in
Figure 8
When there is no change in color to the parts where the response paint was applied, it is accepted.
3.3.5. Liquid Penetrant Test (PT) After the steel roof and membrane are repaired, PT welding areas have defects on their surface or not. When there them, it is accepted.
3.3.6. Vacuum Box Test (VBT) It is a test to find the defected parts by applying bubble solution on the surface of
Injection Nozzle
Bottom of Membrane
12
If more than 5% is detected at IBS outer ring, the helium test is performed at zone B.e helium test at nozzle parts in the steel roof and welding parts in gas dome are classified
more than 1.0X10-4mbarℓ·/sec is detected, the helium test is performed.holes for helium injection are made on the areas where the inspector is not able to
perform Vacuum Box Test (VBT) due to the interference of stiffener beam. One is used for helium injection and the other is done for its concentration measurement.
sured helium concentration value shall not exceed the initial helium concentration one at all of inspected areas.
nitrogen and ammonia is injected into IBS and ammonia response paint is applied to the membrane welding areas to check if they are defected by the change in
The response paint is acid and the yellow color is changed to celadon when it reacts with mmonia which is alkaline gas.
Reference Leakage Points (RLP) are installed to check the ammonia concentration in IBS, and rinse IBS. The installation of RLP is shown in
8. RLP Drawing at Bottom & Wall of Membrane
no change in color to the parts where the response paint was applied, it is
(PT)
steel roof and membrane are repaired, PT is used to check if the base metal and welding areas have defects on their surface or not. When there are
It is a test to find the defected parts by applying bubble solution on the surface of
Φ16 Drilling
Φ16 Drilling
Injection Nozzle
Bottom of Membrane
RLP Patch Welding Parts
If more than 5% is detected at IBS outer ring, the helium test is performed at zone B. welding parts in gas dome are classified
is detected, the helium test is performed. holes for helium injection are made on the areas where the inspector is not able to
perform Vacuum Box Test (VBT) due to the interference of stiffener beam. One is used for its concentration measurement.
initial helium concentration
nitrogen and ammonia is injected into IBS and ammonia response paint is applied to the membrane welding areas to check if they are defected by the change in
The response paint is acid and the yellow color is changed to celadon when it reacts with
Reference Leakage Points (RLP) are installed to check the ammonia concentration in IBS, and rinse IBS. The installation of RLP is shown in Figure 8.
. RLP Drawing at Bottom & Wall of Membrane
no change in color to the parts where the response paint was applied, it is
used to check if the base metal and are no defects found on
It is a test to find the defected parts by applying bubble solution on the surface of the
Membrane
Insulation
RLP
Membrane Plane
inspected specimen. The inspector attaches it to the vacuum box, form the negative and find the forming bubbles through the leakage part. § 1st test pressure : -200mbar§ 2nd test pressure : -550mbar§ Vacuum time : 30 sec
When there is no leakage
3.3.7. Air Tightness Test (ATT) In order to maintain air tightness against the steel roof reinforced parts, inside of the reinforcement welding a pressurized state to find the leakage position.The pressure of ATT is maintained from 20kPa to 25kPa at the reinforcement welding areas including the existing welding parts. The test pressure shWhen there is no leakage
3.4. Repair and Reinforcement
3.4.1. Steel Roof Reinforcement The disparate space between the steel roof main beam including steel roof plate is reinforced as welded with the same material to maintainthe tank. The steel roof reinforcement per part is
Main Beam and Steel Reinforcement Welding
3.4.2. Repair Welding of Steel Roof Defects The defects of the steel roof such as He, NH3, PT, VBT, ATT are repaired in order to secure the airtightness of the tank.The method of repair according to defects is as shown in Table
Main Beam
13
. The inspector attaches it to the vacuum box, form the negative and find the forming bubbles through the leakage part. The procedure of VBT is
200mbar 550mbar
ime : 30 seconds
no leakage indicated by the bubble solution, it is accepted.
n order to maintain air tightness against the steel roof reinforced parts, inside of the reinforcement welding area to apply bubble test solution to the welding areas in
to find the leakage position. The pressure of ATT is maintained from 20kPa to 25kPa at the reinforcement welding areas including the existing welding parts. The test pressure shall be maintained for 10
indicated by the bubble solution, it is accepted.
Reinforcement
einforcement
space between the steel roof main beam including stiffenersteel roof plate is reinforced as welded with the same material to maintain
tank. The steel roof reinforcement per part is shown on Figure 9.
teel Roof Plate Reinforcement Welding
Stiffener Beam and between Welding
Figure 9. Steel Roof Reinforcement
epair Welding of Steel Roof Defects
steel roof which are founded by visual inspection and nonsuch as He, NH3, PT, VBT, ATT are repaired in order to secure the airtightness of the tank.The method of repair according to defects is as shown in Table 6.
Plate (Bending, SS400)
Welding
Main Beam
Roof Plate Existing Welding
Stiffener Beam
. The inspector attaches it to the vacuum box, form the negative pressure The procedure of VBT is stated below:
bubble solution, it is accepted.
n order to maintain air tightness against the steel roof reinforced parts, air is supplied to the area to apply bubble test solution to the welding areas in
The pressure of ATT is maintained from 20kPa to 25kPa at the reinforcement welding areas all be maintained for 10 minutes.
bubble solution, it is accepted.
stiffener beam and the steel roof plate is reinforced as welded with the same material to maintain the airtightness of
Stiffener Beam and Steel Roof Plate In-
between Welding
which are founded by visual inspection and non-destructive test such as He, NH3, PT, VBT, ATT are repaired in order to secure the airtightness of the tank.
Existing Welding Reinforced Welding
14
Defect Method of Repair Inspection
Notch § Removal of defect by grinding § When the depth of defect is more than 2mm,
overlay welding is applicable PT / VBT
Undercut § Overlay welding after the welding area under cut area is grinded more than 0.5mm PT/VBT
Blow-hole § Removal of defect by grinding § Overlay welding is applicable PT/VBT
Welding Defect § Overlay or patch welding after grinding of defect part PT/VBT
Rust on Welding Area & Base
Material
§ Power brush on the surface of welding area and base material
§ Sealing with rust grip Visual Inspection
Lamination § Removal of defect by grinding § When the depth of defect is more than 2mm,
overlay welding is applicable PT/VBT
Crack § Overlay welding after grinding of defect part § If the defect part is penetrated into the base
material, it is cut and then overlay welding is applicable PT/VBT
Overlap § When the overlap part is more than 1mm, it is grinded until it is removed PT/VBT
Lack of Fusion § Overlay or patch welding after grinding of defect part PT/VBT
Contamination § Removal of polluted one on the welding area with power brushing PT/VBT
Arc Strike
§ After the removal of defect with grinding, PT is performed
§ Overlay welding or rust grip is performed if acceptable through PT
PT
Defects through He Test
§ Removal of defect by grinding § Overlay welding is applicable * Defect of corner part is applied by reinforcement
welding with the angle
PT/VBT/NH3 Test
Defects on Lug Welding Part
§ Removal of welding part § Overlay welding is applicable PT/ NH3 Test
Defects on Lap Joint
§ Removal of defect § Overlay welding is applicable PT/ NH3 Test
Table 6. Repair of Steel Roof Defects
3.4.3. Repair Welding of Membrane Defects
The repair welding is performed on the membrane defects founded through the visual test and non-destructive test such as PT, VBT, and NH3. The repair welding is performed as shown in Tables 7, 8, and 9.
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Division Object
1st ▪ Defect parts by visual/helium/ammonia test
2nd ▪ Insulation sampling areas and defect parts of RLP, 2nd ammonia test
3rd ▪ Defect occurred area after the 2nd ammonia test completion ▪ Defect parts of RLP and 2nd ammonia test
Table 7. Order of Membrane Repair Welding
Damage Type Judgment Standard Repair Method
Surface Dent Round type (under 150µm) No Repair
Spot Hole type (over 150µm) Flat Patch Welding
Surface Scratch When performed Power Brush, Sand Paper
Table 8. Repair Method of Normal Damage Area on Wall & Bottom
Welding Area Position Quantity Non-Destructive Test
RLP
Wall 8 EA
PT/ VBT/NH3 Test
Bottom 13 EA
Wall / Bottom Repair Parts occurred
Leak Point
Wall Repair Parts occurred
Bottom
Table 9. Non-destructive Test of Repair Welding
3.4.4. Metal Silicon and Rust Grip for Steel Roof Airtightness 1) Metal Silicon Application
The metal silicon on the irregular welding surface of the steel roof repair welding and original welding parts is strengthened to increase the airtightness of the welding ones.
2) Rust Grip Application
The rust grip is to prevent the steel roof reinforcement welding parts and original steel plate welding ones from corrosion.
3.5. Test After Repair and Reinforcement
3.5.1. Acoustic Emission Test (AE Test) After the tank’s repair and reinforcement is completed, a sensor for acoustic emission test
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which is installed on the entry nozzle is installed on the steel roof. The obtained signal is used as proof to check the soundness and airtightness of the steel roof. The objects of the acoustic emission test are subject to all the base materials and welding parts of the steel roof. Tank nozzles such as the entry manhole, vacuum breaker, gas dome, safety relief valves are isolated before AE Test. AE test is progressed as shown in Table 10.
Division Detail
Preparation
Steel Roof Sensor Attachment and Sensor Cable Monitor Connection 0% (Atmospheric Pressure). The background noise is measured before pressurization
AE Data Collection
50% (10kPa) AE Signal Collection 100% (20kPa) AE Signal Collection (1st) 100% (20kPa) AE Signal Collection(2nd) 0% (Atmospheric Pressure). The background noise is measured after decompressing
Table 10. AE Test Procedure
When the signal related to the air leakage is not detected, it is accepted.
3.5.2. Global Test Global test is the final inspection method to obtain the soundness of the membrane. The tank inner pressure is pressurized from atmospheric pressure to 20kPa and then the increase of oxygen concentration in IBS is recorded and analyzed to verify the soundness of the membrane. (Refer to 3.1.4 1: Global Test for detail procedure) When the oxygen concentration average, which is the measured value difference between the reference and actual test, is lower than 1,500ppm, it is accepted.
3.6. Temporary Equipment Disassembly The kinds of temporary equipment which are disassembled after the repair and inspection are the following: § Scaffolding boards for steel roof inspection & Scaffolding outside steel roof § Gondola for membrane inspection § Protective plywood & corrugated sheets for repair & inspection § Safety equipment for repair & inspection § Structure & winch for entry of material into the tank § Exhaust fan & temporary material storage on the tank upper area § Crane for material transportation & Electric equipment in the tank
When the gondola and protective plywood are disassembled, the plywood and insulation on the deck are reinstalled.
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3.7. Commissioning
3.7.1. NITROGEN PURGE After the repair, inspection, and disassembly are completed, the nitrogen purge is performed in order to decrease the oxygen concentration and the dew point below the acceptance criteria in the tank as shown in Table 11.
Area Oxygen Concentration Dew-Point Tank Inside Space Lower than 2% Lower than -25 ℃
Dome Space Lower than 2% Lower than -18 ℃ IBS Lower than 2% Lower than -12 ℃
Pump Column Lower than 2% Lower than -25 ℃ Other Pipe Lower than 1% Lower than -30 ℃
Table 11. Acceptance Criteria of Nitrogen Purge
3.7.2. FG Purge
After the nitrogen purge is finished, the left nitrogen in the tank is released and FG is injected to be filled with methane into the tank. The acceptance criteria of FG purge is as below. § Inner Tank : 40 Vol% of Methane Concentration § IBS : No Detection § Tank Roof : Average Methane Concentration is the same before & after overhaul
3.7.3. Tank Nozzle Connection, Cool Down, LNG Filling & Pump Test Run
After the purge & dry-out and FG purge is finished, the tank is normalized as shown in Table 12. No. Procedure Details
1 Preliminary Inspection and
Operation Test When opening and closing the valve, the hindering is checked and the automatic valve interlock test is performed
2 Pipe Normalization
§ Check of unloading line valve close § Installation of evaporation pipe spacer § Disassembly of cool-down pipe blank § Disassembly of inert nozzle and installation of
blind flange § Normalization of other pipes
3 Internal Tank Cool Down § Cooling of internal tank with spray nozzle
4 LNG Filling § Filling Level : 5m or more § Temp. at Bottom : -155℃
5 Unloading and Discharge Pipe § Performed after the normalization of the other pipe
6 LP Pump Test Run § Checking of the current, voltage, discharge pressure, flow rate, vibration, and tank level, etc.
7 Commercial Operation § Normal Operation Resumption
Table 12. Normalization Procedure of Tank
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4. Conclusion During the long term operation of LNG storage tanks, various kinds of deterioration could occur to the internal tank & steel roof without maintenance. The new developed maintenance technique has extended the useable life of LNG storage facility. The diagnosis and overhaul technique of LNG storage tank made the lifecycle longer than generally defined. Such an innovative technology can continue to have the tank’s lifespan further extended by approximately 10% of the new tank construction cost. In order to secure the highest safety level for an LNG storage tank, this tank overhaul technique is able to avoid possible hazardous factors such as gas leakage, BOG increase and so forth. The renewal of the tank offers precious opportunities to improve the LNG terminal’s safety and save construction expense. The changing technology trends in the LNG market environment should be targeted to the diagnosis and overhaul of LNG storage tanks.
n Reference
1) BS EN 14620-1:2006 Design and manufacture of site, built, vertical, cylindrical, flat-
bottomed steel tanks for the storage of refrigerated, liquefied gases with operating
temperatures between 0℃ Part 1:General
2) BS EN 14620-2:2006 Design and manufacture of site, built, vertical, cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied gases with operating
temperatures between 0℃ Part 2:Metallic components
3) BS EN 14620-2:2006 Design and manufacture of site, built, vertical, cylindrical, flat-
bottomed steel tanks for the storage of refrigerated, liquefied gases with operating
temperatures between 0℃ Part 4:Insulation components
4) BS EN 14620-2:2006 Design and manufacture of site, built, vertical, cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied gases with operating
temperatures between 0℃ Part 5:Testing, drying, purging and cool-down