The distances to the 10-6 risk level for the different simulation scenarios from category 5 are given in Figure 17.
The risk that is observed for the maximum parameter set seems to be independent of the nautical traffic area. For the minimum parameter set there is a significant difference between the three nautical areas. In general it can be concluded that the risk drivers of category 5 do show a huge similarity to risk driver in category 1 and 2. For most of the scenarios the 10
-6/year risk level is mainly caused by the collision scenario where a 250 mm hole is formed in hull of the large bunker vessel.
Figure 17: LNG bunkering toolkit for category 5
5.2.6 Discussion
From the previous sections it could be concluded that for the maximum parameter set the distance to the 10-6 risk level do not significantly changes over the nautical scenarios. The bunkering of LNG with small bunker vessel, category 1, is an exception where the intense nautical traffic area differs from the low and very low nautical traffic areas. Despite the small difference in distance between the low and very low nautical traffic areas, the 10-6 risk level of the low nautical risk area is dominated by the collision scenario where a 250 mm hole is formed in hull of the small bunker vessel.
For the maximum parameter set of category 1 and 2 it is found that the difference in distance between the 10-6 and 10-5 risk level is small. A detailed analysis of the risk drives reveals that the 10-5 risk level is dominated by the hose rupture scenario where the ESD system works probably. This result clarifies why the distances of the low and very low nautical traffic areas do not significantly differ despite that they do not share the same risk driver.
The maximum parameter set is based on the free field method which means that flammable clouds ignite when the lower flammable limit is reached. Table 4 shows the effect distances (Lower Flammable Limit (LFL)) of the different LOC scenarios from category 1. The largest effect distances are found for the collision scenario where a 250 mm hole is formed in hull of the small bunker vessel.
Table 4: Effect distances LOC scenarios category 1 (likelihood is not taken into account)
Scenario Effect distance [m]
F 1.5m/s D 5m/s
Full bore rupture ESD works 416 179
Full bore rupture due to collision 226 136
25 mm hole ESD works 93 67
250 mm hole in bunker vessel 595 205
The minimal parameter set is based on specific ignition sources that are present in the surrounding of the bunkering activity. In most of the cases the cloud is ignited before it reaches is LFL distance. Figure 18 shows the risk distribution for 5 bunker activities per day in category 1. The figure shows that for the minimal parameters the 10-6/year risk level for intense and low nautical traffic areas is dominated by the collision scenario where a 250 mm hole is formed in hull of the small bunker vessel. In the very low nautical traffic area the 10
-6/year risk level is dominated by the scenario where the hose is ruptures and the ESD system works properly. The same trends can be seen for bunker activities in category 2.
Figure 18: Risk distribution of 5 bunker activities per day in cat 1 for three different risk levels; left, intense nautical traffic; middle low nautical traffic, right; very low nautical traffic
For the bunker categories 3 and 4 is observed that the distance to the 10-6 risk level do not significantly changes over the nautical scenarios. For the maximum parameter set of category 3 and 4 it is found that the distance to the 10-6 risk level is mainly caused by the hose rupture scenarios. An increase of the number of bunker activities shifts the risk driver to scenarios that are less likely to occur. For instance; for the lower bound activities the 10-6/year risk level is mainly caused by the hose rupture scenario where the ESD system works probably, while for the higher bound activities the 10-6/year risk level is mainly caused by the hose rupture scenario where the ESD system fails to work.
The maximum parameter set is based on the free field method which means that flammable clouds ignite when the lower flammable limit is reached. Table 5 shows the effect distances (Lower Flammable Limit (LFL)) of the different LOC scenarios from category 3. The largest effect distances are found in case of rupture of the bunker hose.
Table 5: Effect distances LOC scenarios category 3 (likelihood not taken into account)
Scenario Effect distance [m]
F 1.5m/s D 5m/s
Full bore rupture ESD works 92 63
Full bore rupture due to collision 59 49
25 mm hole ESD works 87 61
The figure shows that for the minimal parameters and 1 bunker activity per week the 10-6/year risk level is dominated by leakage through a 25 mm hole. For the lower risk levels, (10-8/year) the dominant scenarios are shifted to the hose rupture scenarios. The figure also shows that for an increase in bunker activities (10 bunker activities per day) the dominant scenarios that
contribute to the 10-6/year risk level are shifted to the hose rupture scenarios. The same trends can be seen for bunker activities in category 4.
Figure 19: Risk distribution in cat 3 for two different bounds; left, lower bound (1 bunkeractivity a week); right upper bound (10 bunkeractivities a day)
RISK MITIGATION AND FURTHER RESEARCH
This chapter gives an overview of risk mitigation measures that can improve the safety performance of the LNG bunker operations. Two set of measures are given:
General mitigation measures that will ensure safe bunker operations,
More study specific mitigation measures and topics for further research will be given to ensure safe LNG bunkering in the future.