By Lars Petter Blikom
Lars Petter Blikom has recenly posted on DNV GL blog for LNG – Energy of the future an article providing guidance on how to calculate safety zones for LNG bunkering as follows:
”The purpose of a safety zone is to reduce the likelihood of igniting spilled natural gas. The philosophy is that, although the event of an LNG release should at all times be avoided, if a dispersing cloud is not ignited there will be no fire risk. The prevention of ignition as such is part of the second layer of defence. This is achieved by not allowing any non-essential personnel and activities within the defined safety zone. So, at the same time the number of people in the vicinity of the activity which could be exposed to fire hazards is reduced to a bare minimum which can also be considered a risk reduction.
Safety zones are not to be confused with security zones. Security zones are part of the first layer of defence: their purpose is to reduce the likelihood of LNG releases caused by external impacts such as ship collisions. The reduction is achieved by monitoring ship traffic and other activities in this zone.
The draft ISO guideline suggests two methods to determine the safety zone for a specific situation: a deterministic approach and a probabilistic approach. Following the deterministic approach it is calculated what is the maximum distance from the bunkering activity at which a cloud of methane could still be flammable. This distance is calculated for a maximum credible scenario, which is to be determined in a workshop meeting. This is a simple approach which usually leads to relatively large safety zones, as no safeguards are included in the analysis.
The probabilistic method follows a risk based approach, by assessing the maximum distance to flammable concentration of each possible release scenario as well as its likelihood. The safety zone is then defined by the distance at which the frequency of the occurrence of a flammable cloud is equal to once every million operations. This method will usually lead to smaller safety zones than the deterministic approach as it does not only look into the effects but also the likelihood of the different scenarios, including the applied safeguards. A drawback is that it requires more complex analysis of the operation. Therefore this methodology will typically be used for locations where the available space is limited and no large safety zones can be implemented.”
Read more posts from Lars Petter Blikom at DNV GL Blog
Lars Petter Blikom is Segment Director for Natural Gas at DNV. Lars Petter follows both energy and maritime industries closely.
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