A lot of work have been done on standardization of LNG bunkering this year, including a launch of an ISO guideline and a Recommended Practice. One key topic of those documents is the establishment of safety zones around the operations. So I am happy to let my colleague Maartje Folbert share some more detail about how to work with safety zones. Maartje Folbert, risk analysis expert, DNV GL Rotterdam:
Now that ports in several countries are moving forward to develop LNG bunkering facilities, the operational safety issues related to LNG bunkering have increasingly become a topic of international discussion. The establishment of safety zones around bunkering operations is one of these topics. Until recently no international consensus was reached as to how this should be done. With the development of the draft ISO guidelines for systems and installations for supply of LNG as fuel to ships as well as the draft version of DNV GL’s RP on LNG bunkering, a direction has been given as to how the establishment of safety zones around the bunkering activities should be dealt with.
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.
The application of the methodologies and the hearing process of both the draft ISO guidelines and the draft RP will without a doubt demonstrate some practical issues related to the above described methodologies, but we have certainly moved one step closer to an international consensus on how LNG bunkering can be done in a safe way.