Storage at Depot LH

Similar to the previous object (5.3.1) the expected frequency of the accident scenarios considered for the LH2 storage at the hydrogen filling station was synthesised from the component failure data associated with each of the identified failure modes, using the FTA approach. The accident scenarios considered for the study objects can be classified into two TOP events, i.e. instantaneous and continuous. Additionally, the continuous release scenario includes hydrogen release both in the liquid and vapour phase. Therefore three fault tree diagrams were developed related to the three Top events: (1) instantaneous release of hydrogen, (2) continuous release of hydrogen in the liquid phase, and (3) continuous release of hydrogen in the vapour phase. A detailed description of the FTAs for the plant is presented in Appendix G2.

Fig. G2.1 shows the first fault tree diagram for the instantaneous release of hydrogen. The fault tree was divided into four branches to be considered as the main contributor to the Top event (e.g. tank rupture). These include: (1) tank overpressure and all PRDs fail closed (blockage); (2) tank excessive under-pressure (low pressure) and the underpressure protection failure; (3) external events (e.g. Earthquake, mechanical impacts, etc.); and (4) spontaneous events (such as low temperature embrittlement, fatigue failure, etc.). Similar to previous system that the two last events (i.e. external and spontaneous events) are presented as undeveloped events. The tank overpressure was mainly caused by tank overfilling or tank loss of vacuum. Human error and instruments failure may great potential leading to overfilling. The expected frequency of the To event is estimated by assigning the failure data (Table G2.1.1) to each basic events of the FTA. The result is shown in Appendix G2.

The second and third fault trees related to the Top events of continuous release of hydrogen for both the liquid and vapour phase are shown in Fig. G2.2 and G2.3, respectively. The main contributors to the top events are piping rupture, connections failure, tank leak, or release through venting circuits. Release during unloading of the tanker truck involving human error, may contribute to the continuous release of hydrogen in the liquid stage. Besides, pipe rupture in various liquid lines may occur if the emergency (remotely operated) shutoff valve fails to close. Similar to the previous plant hydrogen release through the venting system may occur due to pressure relief devices (PRD) failing open prematurely.

The summary of the FTA results for the LH2 storage at depot is presented in Table 5-10. The table shows that the overall expected frequency of the loss of containment of hydrogen from the LH2 tank at the depot is 1.2 x 10-4 /year (once per 10,905 years), with the instantaneous and continuous release are 2.8 x 10-6 /year (once per 357,143 years) and 8.9 x 10-5 /year (once per 11,249 years), respectively. It also shows that only 3.1% of the hydrogen release from the LH2 storage occurs instantaneously.

Table 5-10 Expected frequencies of the LH

[2 storage at depot

Failure modes

5%

50%

Mean

95%

K-95

Percentage

Instantaneous release

2.3E-08

6.4E-07

4.9E-06

1.8E-05

27.5

4.0%

Liquid continuous release

9.1E-07

1.7E-05

8.3E-05

3.2E-04

18.8

67.5%

Vapour continuous release

1.8E-07

4.8E-06

3.5E-05

1.3E-04

26.2

28.5%

Overall

5.0E-06

4.8E-05

1.2E-04

4.6E-04

9.6

100.0%

Table 5-11 Accident outcome frequencies of the LH2 storage at depot

Release Scenarios

Accident Outcomes

Conditional Probability on Release

5%

50%

Mean

95%

Instantaneous

Early explosion

0.0072

3.6E-08

3.4E-07

8.8E-07

3,3E-06

Fireball

0.0287

1.4E-07

1.4E-06

3.5E-06

1,3E-05

Pool fire

0.0005

2.5E-09

2.4E-08

6.3E-08

2,3E-07

Late explosion

0.0000

1.1E-11

1.1E-10

2.8E-10

1,1E-09

Flash fire

0.0000

4.6E-11

4.4E-10

1.1E-09

4,2E-09

Continuous

Jet Fire

0.4801

2.4E-06

2.3E-05

5.9E-05

2,2E-04

Pool fire

0.0768

3.8E-07

3.7E-06

9.4E-06

3,5E-05

Late Explosion

0.0017

8.6E-09

8.3E-08

2.1E-07

7,9E-07

Flash Fire

0.0069

3.5E-08

3.3E-07

8.5E-07

3,2E-06

No effect

0,3957

2.0E-06

1.9E-05

4.9E-05

1.8E-04

Overall

0,9976

2.3E-05

8.8E-05

1.2E-04

3.4E-04

The accident outcome frequencies of the above initial frequencies were calculated by multiplying the expected frequencies with the outcome probabilities resulting from the event tree diagram for LH2 release (Fig. 4.5), and the result is shown in Table 5-11. The table shows that most of the accident outcomes (which account for 59%) are dominated by fire, and only 1% of this may result in explosion. The rest (about 40%) of the accident may have no effect to the population.

5.3.3 Hydrogen Filling Station (LH2)

Similar to the previous object (5.3.2) the expected frequency of the accident scenarios considered for the LH2 storage at the hydrogen filling station was synthesised from the component failure data associated with each of the identified failure modes, using the FTA approach. Three fault tree diagrams were fully reproduced, and are shown in Appendix G3. They include: (1) instantaneous release of the hydrogen due to tank rupture (Fig. G3.1); (2) continuous release of hydrogen in the liquid phase (Fig. G3.2); and (3) continuous release of hydrogen in the vapour phase (Fig. G3.3).

In general, the fault trees developed for the filling station are similar to those for LH2 at the depot. The storage operations, however, involve more human activities than the one of storage at depot, especially, during loading and unloading activities. Therefore, human error in the filling station may highly contribute to the Top events.

Table 5-12 Expected frequencies of the LH2 storage at filling station (/yr)

Failure modes

5%

50%

Mean

95%

K-95

Percentage

Instantaneous release

6,0E-09

2,8E-07

4,3E-06

1,3E-05

46,7

0,6%

Liquid continuous release

1,5E-05

1,9E-04

6,2E-04

2,4E-03

12,7

86,8%

vapour continuous release

8,9E-07

1,7E-07

9,0E-05

3,4E-04

26,2

12,6%

Overall frequency (/yr)

4,7E-05

3,4E-04

7,1E-04

2,5E-03

7,3

100,0%

Table 5-13 Accident outcome frequencies of the LH2 storage at fil

Table 5-13 Accident outcome frequencies of the LH2 storage at fil ing station (/yr)

Release Scenarios

Accident Outcomes

Conditional Probability on Release

5%

50%

Mean

95%

Instantaneous

Early explosion

0,0011

5,1E-08

3,7E-07

7,7E-07

2,7E-06

Fireball

0,0043

2,0E-07

1,5E-06

3,1E-06

1,1E-05

Pool fire

0,0000

5,5E-10

3,9E-09

8,3E-09

2,9E-08

Late explosion

0,0000

2,5E-12

1,8E-11

3,7E-11

1,3E-10

Flash fire

0,0000

9,8E-12

7,1E-11

1,5E-10

5,2E-10

Continuous

Jet Fire

0,4970

2,3E-05

1,7E-04

3,6E-04

1,2E-03

Pool fire

0,0795

3,7E-06

2,7E-05

5,7E-05

2,0E-04

Late Explosion

0,0018

8,4E-08

6,1E-07

1,3E-06

4,5E-06

Flash Fire

0,0072

3,4E-07

2,4E-06

5,1E-06

1,8E-05

No effect

0,4086

1,9E-05

1,4E-04

2,9E-04

1,0E-03

Overall frequency (/yr)

0,9994

1,7E-04

5,5E-04

7,1E-04

1,8E-03

ing station (/yr)

The frequencies were estimated by assigning to each basic event of the fault trees the associated equipment failure rates or probabilities given in Table G3 (Appendix G). The summary result of the top event frequencies is presented in the Appendix G3, and is also shown in Table 5-12. The table shows that the overall expected frequency of the loss of containment of hydrogen from the LH2 tank at the fuelling station is 4.7 x 10-4 /year (once per 2,146 years), with the instantaneous and continuous release are 7.7 x 10-6 /year (once per 129,870 years) and 4.6 x 10-5 /year (once per 2,182 years), respectively.

The accident outcome frequencies of the above frequencies calculated by multiplying the expected frequencies with the outcome probabilities resulted from the event tree diagram for LH2 release (Fig. 4.5). The result (Table 5-13) shows that most of the accident may result in fire outcomes (about 59%), and only about 1% may result in explosion. The rest (about 40%) may have no effect to the population.

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