SAFETY EVALUATION OF MARK III TYPE LNG … Conferences/2007... · HAZID and HAZOP study focused on MARK III cargo containment system have been ... Brief FMEA worksheets are shown

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SAFETY EVALUATION OF MARK III TYPE LNG CARRIERS UNDER BARRIER LEAKAGES

Junhong Bae

Kihun Joh

Hobyung Yoon

Heesung Lee

Munkeun Ha

Samsung Heavy Industries Co., Ltd. Geoje-Si, Gyeongsangnam-Do, Korea

[email protected]

ABSTRACT

Liquefied Natural Gas (LNG) carriers of MARK III type cargo containment system are widely used over the world since they provide more space to carry and lower fuel cost. Recently developed LNG fields around artic areas need more robust LNG vessels enough to be safe from vibrations and unexpected collisions against arctic ices. In this situation there have been concerned about cargo containment system in case of severe damages during voyage.

Safety evaluation will be made for the MARK III type LNG carriers, in case both primary and secondary barriers are damaged from unexpected forces. The endurance of the MARK III system will be checked according to the international gas code standard using a commercial numerical code. In the calculation phase and temperature change of LNG at each part of the cargo system will be inspected with two and three dimensional models. A rather simplified model calculation might not be enough to confirm its validity. So if necessary, the test to confirm the numerical model may be planned under clearly defined procedure and environment for considerable time. Through theoretical and or experimental investigations will show if the MARK III type is safe even under extreme cases.

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1. INTRODUCTION

There have been studies on risk assessments in case of LNG spill over due to unexpected accidents such as collisions or terrorist attacks. And recently needs for HAZID and HAZOP study focused on MARK III cargo containment system have been raised by ship owner sides and class societies. But the quantitative estimation for various risk scenarios on cargo itself and hull structure of MARK III system may not be easy because of complicated multi-phase motion of LNG. For these safety assessments we try to use commercial computational fluid dynamic code for simulation of multi-phase LNG as well as simple fluid dynamic theory with single liquid phase motion of LNG.

2. FMEA STUDY FOR LNGC CARGO TANKS

Failure Mode and Effect Analysis is a vigorous pro-active qualitative engineering method that helps identifying and avoiding defects in the early conceptual phase of products and processes. It is a practicable tool even for non-specialist when it is applied to manufacturing. Because of that, FMEA was carried out to find out the major incidents making defects on primary barrier of LNG Carriers through the whole manufacturing process. The study was especially focused on the failures of manufacture and inspection for MARK III type Cargo Containment System (CCS).

2.1 FMEA study for Manufacture and Inspection of LNGC Cargo Tanks

Brief FMEA worksheets are shown below. First, system failure that stands for fatal incident was listed and classified through the manufacturing and inspection process. Second, cause and effect were described to understand and find out the incidents procedure. Finally, mitigation and compensation methods were presented to show our preparation for removing errors.

Table 1. FMEA Worksheet for Prevention of Damage on Primary Barrier

Process No./System Failure Cause Effect Mitigation/ Compensation

Panel Capsizing Binding imperfection

Member damage Replacement of broken panel

Binding with band

Clash with forklift Careless driving

Member damage Replacement of broken panel

Regular education for operator

Panel Installation

Jig & tool falling Handle without care

Worker injury Replacement of broken panel

Keep in tool box

Remaining of inspection material

Unskilled workman Inspection error Reconfirmation for

liquid NH3 Test for LNG Cargo Paraphernalia

falling Management without care

Falling items broken Replacement of broken panel

Keep the paraphernalia to the appointed location

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Fault construction by dust

Insufficient removal of dust by cutting works

Seam error Hole re-formation

Wear a gas mask and carry a vacuum cleaner

Jig & clamp falling Unskilled workman

Delay of schedule Replacement of broken panel

Connect sheet with jig to prevent falling-down

Membrane Installation

Fire by spark Leaving acetone behind


Keep acetone in a tool box

Fire by electric leakage

Electric leakage


Shut a main switch/plug out

Carriage falling Leaving broken gear behind


Check a link part of clamp gear Welding

Fire at back side of welding plate

Overheating of welding back-side


Keep welding standard, check electric current

Moving member falling

Binding imperfection

Replacement of broken panel

Use a safety net to prevent the damage of cargo

Member falling when pump tower turnover

Binding imperfection


Remove unfixed members

Fire by inflammables Smoking


No smoking in a cargo tank

Pump Tower Installation

Fire by spark Handle without care


Cover the bottom of working area with incombustible material

Install & removal members falling

Absence of bottom plywood


Install handrail and safety net

Clash with forklift Careless driving


Regular education for operator

Scaffolding Install/ Removal

Scaffolding collapse Arbitrary scaffolding demount

Scaffolding damage Replacement of broken panel

Regular inspection and education

In case of “panel install” main incidents are panel capsizing, forklift accident, and jig and tool falling. Those could damage to the primary barrier already installed. If the incidents are happened, every panel around the incident place will be inspected and replaced if the need arises.

In case of “NH3 test for LNG cargo” main incidents are remaining of inspection material and paraphernalia falling. Those could damage to the primary barrier already installed. To prevent these, inspectors always check the remainder before and after the

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inspection and it is recommended that every worker inspector must keep their paraphernalia to the locker before entering a tank.

In case of “membrane install” main incidents are defect during construction by dust or fire from spark during welding. Fire could make severe damage to the primary barrier already installed. Thus, if the incident is happened, every panel around the incident place will be inspected and replaced if damage is identified. And, dust could make defect to the membrane install, so dust removing work using vacuum cleaner have to be carried out before the installing works.

In case of “welding work” main incidents are fire by electric leakage or fire at back side of welding plate. Those could damage to the primary barrier already installed. If the incidents are happened, every burned panel around the incident place will be inspected and replaced if the need arises.

In case of “pump tower install” main incidents are member falling by turnover of pump tower and fire by inflammables. Those could damage to the already installed primary barrier. If those incidents are happened, every dent panel around the incident place will be inspected and replaced if the need arises.

In case of “scaffolding install/removal” main incidents are install and removal member falling and scaffolding collapse. Those could damage to the already installed primary barrier. If the incidents are happened, every dent panel around the incident place will be inspected and replaced if the need arises.

As mentioned above, the accidents being on manufacturing and inspecting may be happened, but there always be preparation, mitigation, and compensation on the process. So, even if the accidents are happened on the way of working, the defect on the primary barrier of under constructing LNGC can be traced out and will be disappeared immediately.

3. STUDY FOR LEAK PHENOMENA

3.1 Simulation of the leak effect in secondary barrier

A commercial CFD program named CFX was used to simulate effects on secondary barrier. In this calculation, the possible evaporation of the leaked LNG was considered for various leak position, size and flow rate.

3.1.1 Velocity of the leaked LNG. The velocity of LNG at a leaked position is calculated using fluid dynamic theory of Bernoulli and continuity equations [2]. In this calculation the viscosity of LNG was neglected and LNG filling level was assumed to be in full up to 98.5%. Table 2 shows velocity at each leaked position described in Figure 1 with some notations. According to the calculation, the velocity of leaked LNG does not depend on defect size but on its level from ground.

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22

22

11

21

22ghPVghPV

++=++ρρ

(Eq. 1)

2211 VAVA ρρ = (Eq. 2)

h

98.5 %

0 m

A1

A2 V2

V1

Figure 1. Notation of the Calculation

Table 2. Velocity according to defect position and size

V (Velocity at hole area, m/s) Depth (h) 2gh A2(d=0.5cm) A2(d=1cm) A2(d=2cm)

0 m 0.00 0.00 0.00 0.00 4 m 78.48 8.86 8.86 8.86 8 m 156.96 12.53 12.53 12.53 12 m 235.44 15.34 15.34 15.34 16 m 313.92 17.72 17.72 17.72 20 m 392.40 19.81 19.81 19.81 24 m 470.88 21.70 21.70 21.70 27 m 528.92 23.00 23.00 23.00

3.1.2 Nitrogen gas flow rate for the evaporation of the leaked LNG. We consider the evaporation effect because LNGC has N2 generator. It means some amount of nitrogen with ambient temperature can be supplied into the barriers. At first we consider the heat transfer between LNG and nitrogen gas using commercial software named HYSYS [3]. We check the necessary amount of nitrogen to evaporate the leaked LNG for different flow rate of leaked LNG. Table 3 shows the calculated result in accordance with equilibrium temperature and the amount of nitrogen gas. According to Table 3, at least 270 m3/h of nitrogen gas should be needed to evaporate 1 m3/h of LNG with -110 ℃ of equilibrium temperature.

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Table 3. Nitrogen Flow Rate According to LNG Flow Rate and Temperature

LNG flow rate 1 m3/h 2 m3/h 3 m3/h 4 m3/h 5 m3/h S. Temp (℃) N2 Flow rate (m3/h)

-155 826 1652 2126 2835 3543 -150 624.8 1250 1874 2499 3124 -140 533 1066 1598 2131 2663 -130 445.4 1038 1336 1782 2227 -120 354.5 825 1062 1416 1769 -110 272.1 634 815.4 1087 1358

Figure 2. Sample Figure for Heat Transfer Calculation (HYSYS)

3.1.3 Temperature Confirmation in the Barriers. The temperature of each barrier can be checked using temperature chart of hull and CFD simulation [4] when LNG contact secondary barrier. In the Figure 3, the temperature of hull part is presented when LNG contact secondary barrier with USCG condition. In the figure 4, the temperature of IS barrier which is a void space between secondary barrier and inner hull side, at 80 mm from inner hull is presented. According to these results when leaked LNG contact secondary barrier, the temperature of IS barrier is about -40 ºC.

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Figure 3. Steel Grade Calculation Figure 4. CFD Result in Insulation Space

3.1.4 Flow Rate Confirmation for Evaporation Using Nitrogen. If secondary barrier has a defect when LNG is leaked from primary barrier the inner hull can be affected or not according to the flow rate and the temperature in insulation space. The consideration of previous chapter, we assume the temperature in insulation space will be about -90 ℃ when leaked LNG moves to insulation space. At that time about 1 m3/h of leaked LNG can be evaporated when 23 m3/h of nitrogen is put in insulation space. In Table 4, we calculate the necessary amount of nitrogen according to the flow rate of leaked LNG at the -90 ℃ of insulation space temperature. In Table 5, we calculate that the damaged area can make 1 m3/h of flow rate according to position. When the damaged area is located at bottom of the tank the size of damaged area will be 4 mm in diameter to make 1 m3/h of flow rate.

3.1.5 CFD Simulation for LNG Leak Phenomena. Based on our calculation, we simulate the LNG leak phenomena with 3-dimensional model using commercial CFD software named CFX [5]. In Figure 5 the boundary condition is shown. We select the most severe condition that means the leak position at membrane is on bottom of the tank and the leak position in secondary barrier is located just below the leak position at membrane. Normally, if the leak position is in vertical wall it can help the LNG evaporation during leaked LNG moves to bottom wall. The temperatures in each barrier are determined from the integrated automatic dada acquisition system called IAS of actual operating ship. The amount of supplied nitrogen is based on the capacity of nitrogen generator that installed on al ship and it is converted to modeled area. The property of glass wool which is installed between panels is presented in Table 6. To get the results of leak simulation we select small dimension of modeling as shown in Figure 5. In table 7 the overall physics of CFD analysis is presented.

-70.3oC

-34oC

-120oC

-30.5oC

-37.4oC

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IBS

IS

IBS Opening

170 mm

4 mm100 mm

170 mm 12.5 mm

Air

(0 ℃)

18 mm

IBS inlet

(Nitrogen,

-120℃)

STEELIS Opening

leakage area

30 mm

4 mm

Porous Region - volume porosity : 0.3

LNG inlet

1 atm + 8 mbar

1 atm + 5 mbar

conjugate heat transfer

IS inlet

(Nitrogen,

-20℃)

Figure 5. Boundary Condition for CFD Analysis

Table 4. Nitrogen Flow Rate According to Leaked LNG (-90℃)

LNG (m3/h) N2 (m3/h) 1 22.68 2 118.05 3 220.77 4 294.09 5 366.47

Table 5. Defect Area for 1 m3/h of LNG Flow Rate

Depth (M) Velocity (m/s) Area (cm2) Diameter (cm) 2.0 6.26 0.44 0.75 6.0 10.85 0.26 0.57 10.0 14.01 0.20 0.50 14.0 16.57 0.17 0.46 18.0 18.79 0.15 0.43 22.0 20.78 0.13 0.41 24.0 21.70 0.13 0.40 27.0 23.00 0.12 0.39

Table 6 Properties of Glass Wool Used in LNGC

Volume porosity 0.3 Resistance loss coefficient 1242 m-1

Density 24 kg/m3 Cp 700 J/kg-K

Thermal Conductivity 0.038 W/m-K

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Table 7 Overall Physics of CFD Simulation

Simulation Type Transient Fluid Multi-component & phase (CH4(liquid, vapor), Air, N2) Solid Steel, Glass Wool

Domain Multi Domain (Fluid, Solid, Porous Domain) Turbulence Model k-ε Model (Scalable Wall Function)

Heat Transfer Thermal Energy (Conjugate Heat Transfer) Buoyant Yes

Boundary Condition

Velocity Inlet Symmetry

Opening Pressure Outlet No Slip, Fixed Temperature Wall

Time Scale 10-4 s

3.1.6 Results of CFD Simulation. Based on the boundary condition as mentioned above, we do the simulation for LNG leak phenomena. Before doing the CFD simulation we thought that when leaked LNG evaporate in inter barrier space the pressure should be increase and because of this high pressure the amount of leaked LNG could be affected. After simulation of LNG leak phenomena we could find that the increased pressure affect LNG outflow and during some time LNG cannot drain into inter barrier space. The leaked LNG is evaporated because of heat transfer and inner hull is not affected by LNG. It means that certain amount of leaked LNG could be evaporated. From Figure 6 to Figure 8 the results of CFD simulation are presented. Especially in Figure 7 the phenomena for blockage effect of LNG can be found due to the high pressure during evaporation of leaked LNG. It means the high pressure of evaporated natural gas can reduce the flow rate of LNG which flows from damaged area. From Figure 6 to Figure 8 the calculation time of simulation is quite short because of the analysis time. For the analysis we use total 16 CPUs with 2.4 GHz of CPU speed and 4 GB RAM capacity. Even though we used that amount of CPUs it took 3 weeks to get the results from 0 second to 0.2 second after LNG leaks from membrane. The reason was that we considered the heat transfer with phase changing using large numbers of mesh in the model. It was announced when we want to solve the heat transfer and phase change we had to use very small mesh size and very short time gap between steps. In our analysis we used 1.3 million cells and 10-4 time gap between steps to get the reasonable results.

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Figure 6. Volume fraction of evaporated NG

Figure 7. Volume fraction of leaked LNG

Figure 8. Pressure distribution at the same time with Figure 7

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4. FIRE AND EXPLOSION SAFERY OF LNG IN THE BARRIER SPACE

We check the possibility of fire and explosion with assumption that LNG is leaked because of the defect at each barrier.

As it announced, LNG is a cold, cryogenic, nontoxic, non-explosive substance composed primarily of methane. LNG vapors are flammable only under strict conditions and with confinement. Within the flammable range auto-ignition occur only at 596oC or higher. When cold LNG comes in contact with warmer air, it becomes a visible vapor cloud. As it continues to get warmer, the vapor cloud becomes lighter than air and rises. When LNG vapor mixes with air it is only flammable if it’s within 5% - 15% natural gas in air. If it’s less than five percent natural gas in air, there is not enough natural gas in the air to burn. If it’s more than 15 percent natural gas in air, there is too much gas in the air and not enough oxygen for it to burn. But the IBS and IS spaces which are two independent void spaces separated by secondary barrier between primary membrane and inner hull, are filled with nitrogen gas free from oxygen. Therefore LNG in the space is not burn. LNG in liquid form itself will not explode within storage tanks, since it is stored approximately -160oC and at atmospheric pressure. Without pressure or confinement of heavily obstructed clouds of the vapors, there can be no explosion. An explosion from a release of LNG vapors is in the flammability range, vapors are in a confined space and a source of ignition is present. Since the IBS and IS spaces are free from oxygen LNG in the space is not exploded. Figure 9 shows the flammable range of hydrocarbons, air and oxygen. The concentration of hydrocarbon is outside of flammable range so fire and explosion is not occurred.

Methane

Ethane

Propane

N-Butane

i-Butane

N-Pentane

Air (Vol%)Hyd

roca

rbon

(Vol%

)

Air (Vol%) ([ ]) O2 Vol%

Methane

Ethane

Propane

N-Butane

i-Butane

N-Pentane

Air (Vol%)Hyd

roca

rbon

(Vol%

)

Air (Vol%) ([ ]) O2 Vol% Figure 9. Flammable range of Hydrocarbons

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Table 8 shows the ignition temperature and LFL (Lower Flammable Limit), UFL (Upper Flammable Limit) ranges of hydrocarbons. If the hydrocarbons are exposed to an environment higher than ignition temperature, auto-ignition will be occurred.

IS and IBS spaces are safe from fire and explosion when the barriers are in the following conditions;

1) LNG is cryogenic substance, and IBS and IS space is maintained far from the auto-ignition temperature of LNG vapor.

2) IBS and IS space is filled with inert gas (usually nitrogen is used), therefore reaction between LNG vapor and Oxygen is not occurred. For combustion, oxygen concentration must be between 12 to 21 %.

3) For the purpose of the pressure control in the IS and IBS space, the relief valve is opened at the set point pressure.

4) Venting gas must be diluted with inert gas until the LNG vapor concentration below the LFL.

Table 8. Ignition temperature and Concentration of LNG

Methane Ethane Propane i-Butane n-Butane n-PentaneIgnition

Temperature 595 515 470 462 365 285

LFL 5 2.9 2.1 1.8 1.8 1.4 UFL 15 13 9.5 8.4 8.4 8.3

Vapor Density 0.55 1.04 1.52 2.01 2.01 2.49

5. CONCLUSIONS

FMEA studies on MARK III cargo containment system have been done during its construction and inspection. We Samsung HI have systematic process and inspection technologies to be free from any possible defects in the primary membrane. We have not been reported of any incidents on primary membrane after delivery of LNGC that are operating until now.

We have considered the possibility of fire and explosion during leakage of LNG from primary membrane into insulation area via inter barrier space. There is no possibility of fire or explosion as long as the spaces are filled with nitrogen gas since the flammable limit of LNG is between 5% and 15% in volume concentration in air basis.

We have used simple fluid dynamic model to estimate correlation between leakage in the secondary barrier and its effect on the cargo containment system and inner hull structure of a vessel with assumption of leak size in primary membrane enough to pass LNG flow into secondary barrier leaked area. As it is expected the risk level increases while defects exists lower level. It turned out from the calculation that we need some kinds of mitigating measures to avoid severe cooling of inner hull area up to brittle point.

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Some amount of leaked LNG can be evaporated and it moves to out of cargo containment system because there are lots of heat sources like nitrogen with ambient temperature.

We have also used commercial computational fluid dynamic code CFX to simulate more real situation during leakage of LNG into the insulation space. Multi-phase effect of liquid and vapor motions has been considered to observe pressure and temperature in time near through the leakage path. It was shown that vaporizing of LNG during leakage was so big enough to block or partially sustain the leakage from the primary membrane. So we need to define the effective size of leakage for accurate assessment of safety level if vaporizing effect of LNG is included in calculation.

In spite of more realistic computational fluid dynamic model simulation it is currently too time consumable to simulate the whole cargo containment system. So we are under study to find out much faster methods to cover the whole system with multi-phase effect calculation of liquid and gas of LNG. And we are preparing leak scenario test using a pilot plant mock-up at Samsung HI site to confirm the simulation model and match its boundary conditions.

REFERENCES CITED

[1] Anzai, H., Kobayashi, M., Endo, T., Advanced Risk Assessment of LNG Storage Tanks Based on Risk-Based Maintenance Planning, 14th International Conference & Exhibition on Liquefied Natural Gas, Poster PO-22.

[2] Currie, I.G., Fundamental Mechanics of Fluids, Second Edition, McGraw-Hill, Inc.

[3] HYSYS 2004.1, Process Simulation Code, Aspen Tech.

[4] ICEPAK V4.0, Thermal & Fluid Dynamics Analysis CFD code. ANSYS Inc.

[5] CFX V5.7, Thermal & Fluid Dynamics Analysis CFD code. ANSYS Inc.

Documents

SAFETY EVALUATION OF MARK III TYPE LNG … Conferences/2007... · HAZID and HAZOP study focused on MARK III cargo containment system have been ... Brief FMEA worksheets are shown