Classification Society DNV GL has published the draft of a Recommended Practice (RP) for maritime LNG bunkering which after a six-week consultation period will be published as a practical guide to help authorities, LNG bunker suppliers and ship operators undertake LNG bunkering safely and efficiently. This article overviews their initiative.
LNG bunkering: Rendering courtesy of DNV GL
Currently, 83 LNG-fuelled ships are in operation or on order worldwide, ranging from passenger ferries, Coast Guard ships and cargo vessels to tankers and platform supply vessels. Estimates put the global LNG-fuelled fleet at 3,200 by the year 2025. With the EU poised to invest in helping equip 139 seaports and inland ports with LNG bunker stations by 2025, the time seemed ripe to DNV GL for it to set out RP’s for the design and operation of LNG bunkering on a global scale. But roughly, what dangers are inherent in LNG bunkering?
LNG release by accident
At atmospheric pressure, LNG will boil at -162ᴼ C and represents a cryogenic hazard causing embrittlement of carbon steel structures and potential frost burns to exposed personnel. Evaporated natural gas will be cold and heavier than air, and will thereby be spread by gravity. The natural gas is non-toxic, but can threaten personnel through asphyxiation due to depletion of oxygen.
Illustration of two-phase release of LNG: Courtesy of DNV LR draft RP document
When the natural gas is mixed with air after rainout evaporation as shown above, it will gradually become flammable although only within a narrow range of concentrations in the air. Less air does not contain enough oxygen to sustain a flame, while more air dilutes the gas too much for it to ignite. In the event of a spill, LNG vapours will disperse with the prevailing wind, but ignition will occur if the gas cloud comes in contact with a spark or flame in a suitable air concentration (typically within the range of 5% and 15% for pure methane).
Consequently, protection from cryogenic exposure, as well as from fire exposure, is needed. Since hazardous concentration levels of LNG resulting in asphyxiation are much higher than the combustible range, this additional hazard is usually not considered.
Safety concept: ’Layers of Defence’
The consultation document enlarges upon an earlier ISO guideline concept of concentration on so-called ‘layers of defence’ (LOD) at both the equipment and procedural levels of the bunkering operation, briefly as follows:
Requirements for operations, systems and components aiming at prevention of accidental releases that could develop into hazardous situations.
Requirements to contain and control hazardous situations in the case that a release occurs and thereby prevent/minimize the harmful effects.
Such matters as:
• Rescuing casualties
• Safeguarding /evacuating others
• Minimising damage to property and the environment
• Preventing escalation / bringing the incident under control
Illustrating the global reach of this DNV GL initiative, the draft RP consultation is being presented in the following countries: seminar in Melbourne , Australia hosted by Australian Shipowners Association; in Perth, Western Australia hosted by DNV; in Tokyo, Japan, hosted by Norwegian Embassy; in Seoul, South Korea, hosted by DNV (alongside the IBC LNG bunkering conference), and in Singapore hosted by DNV.
Source: DNV GL (Request registration at www.bit.ly/1av7XiJ to view the comprehensive Draft RP paper)