Considering the international nature of shipping, the attempts to reduce emissions to sea by ships were formalized by the development of ECAs (Emission Control Areas), which was the main driver for the use of low-sulphur fuels at sea. Natural gas, used for decades, has been recognized as a good fuel alternative, allowing the tightest emission restrictions to be met. In addition, natural gas has been used as a fuel on board ocean-going LNG carriers propelled by boil-off from their cargo tanks almost since the first such vessel was built more than 50 years ago.
The technology for using methane in marine combustion engines is readily available and, considering the need for green shipping, the growth of gas-fuelled sea vessels was expected be exponential, as was the growth in LNG fuel supplied to ships, in particular in SECAs. The expected trend was somehow flattened by concerns being raised about “is the LNG transfer at sea and shore facilities safe enough?” which was translated into a proactive question: “how can we make it safe?”
DNV GL has participated actively in assessing and developing safe transfer procedures and provides integrated solutions for using LNG as fuel. While the standards for shore liquefied-gas-transfer terminals were well developed, transfers at sea were the area where the standards had to be developed. A new “Gas Bunker Vessel” DNV GL class notation was introduced in July 2015 and defined equipment and arrangements for a gas carrier equipped for gas bunker transfer at sea.
The notation did not introduce a new vessel type. A gas tanker of suitable size is a proven transport solution and may be a good and versatile platform for the gas bunker transfer. Newbuildings and existing small-scale LNG carriers initially intended to supply smaller LNG hubs may be easily converted into bunker vessels and effectively used, providing LNG bunker fuel where gas-fuelled ships need it.
The main work on developing these rules focused on evaluating hazards and measures that would constructively help to prevent emergency situations from occurring during the transfer and, if they did occur, to mitigate possible consequences. Dry break-away coupling used to transfer small quantities of LNG was backed up with an emergency release system, allowing the operation to be terminated intentionally when the safety of the vessels is jeopardized. An extended gas-detection system will help to find any leak at the connection at an early stage.
In the case of an accidental leak or spill, the cryogenic effect of the liquefied gas will be handled by hull protection that has been developed. The static charge accumulation and galvanic currents are duly considered through the permanent installation of an insulating flange and, for special cases, a bonding system. The nitrogen or inert gas supply system to the manifold area has been optimized for the inerting required due to the frequent connection and disconnection of the transfer hose.
The notation has optional qualifiers allowing operators to enhance the service capability of the gas bunker supply vessel, thus making operations easier and minimizing the workload for the crew. We believe that the rules which have been developed for the notation will improve the safety of a vessel engaged in gas-bunkering operations and add confidence to the crew on board, operating company and operators of receiving vessels and, what is important – coastal state administration and port authorities.
And, not least important, this will also help to preserve the environment for future generations.