Increasing flexibility in LNG fuel handling master

Increasing flexibility in LNG fuel handling – the LNGPacTM ISO

The option of having LNG as fuel for a variety of different ship types is already available. Emission reduction requirements and cost competitive gas prices are the main driving forces behind this increasing trend towards LNG. The LNGPac™ ISO, a fuel gas handling system based on removable LNG fuel tank containers, is a new way of making LNG fuel available when a stationary tank solution is not possible.

Text: JONATAN BYGGMÄSTAR & SÖREN KARLSSON Photo: -
Increasing flexibility in LNG fuel handling master
Fig. 1 – LNGPacTM ISO layout arrangement from LNG fuel tank containers to the GVU-EDTM.

Wärtsilä’s fuel gas handling system, LNGPac™, has aroused huge interest, and many customers have already selected the LNGPac™ system for their new-buildings or gas conversions. However, while conventional fuel gas handling systems with stationary tanks (LNGPac™) are, and will remain, the most popular choice, they might not be best suited for all ships.

A fuel gas handling system utilizing removable LNG fuel tank containers is an option worth considering in many different cases. For small and medium sized vessels, which do not require a large LNG capacity, such a solution offers a viable alternative to conventional stationary LNG tanks. If LNG bunkering facilities are not available or bunkering is not possible, using LNG as fuel can still be realised by using LNG fuel tank containers. These containers can be transported by road to the nearest LNG terminal for refilling, and then loaded onboard the ship with no bunkering procedures required at the port.

The LNG container can also be used as a cost effective and standardized LNG fuel tank for stationary applications. The frame standard size dimensions and the modularised skid-based fuel gas handling system make installation fast and cost competitive.

LNGPac™ ISO

The LNGPac™ ISO is a fuel gas handling system based on mobile LNG fuel tank containers. Besides the LNG fuel tank container, the system consists of a docking station and an evaporator skid installed permanently on the ship. The LNGPac™ ISO is intended to be installed on an open and naturally ventilated deck.

Tank containers intended for the transportation of cryogenic liquids, e.g. LNG, are an alternative for use as fuel storage tanks onboard LNG fuelled ships. However, a normal tank container intended for transporting LNG cannot be used since it does not fulfil all the requirements for marine LNG fuel tanks. Modifications relating to remote monitoring and safety systems, IMO type C tank requirements, and leakage & spill protection are a few items that need to be specifically considered for marine fuel tanks.

LNG fuel tank containers

Removable and transportable LNG fuel tank containers are used as fuel tanks in the LNGPac™ ISO fuel gas handling system. The containers, which are designed to fulfil all marine LNG tank requirements, are of standard ISO frame dimensions (20 ft, 40 ft and 45 ft) and can be transported by road, rail and sea, although the maximum gross weight may vary in different countries for land transportation.

The fuel tank is an IMO type C pressure vessel enclosed within an outer tank. Both the inner and the outer tanks are made of stainless steel, which means that the outer enclosure will act as a secondary containment. The LNG fuel tank container is fitted with process equipment, namely the valves and instruments required for operational and safety purposes.

The LNG fuel tank container is also fitted with a pressure build-up evaporator (PBE) for building up and maintaining an operational pressure of approximately 5 bar in the tank. The pressurized tank is used instead of having rotating equipment, such as pumps and/or compressors to feed the gas to the engines. Having a PBE on the container makes the LNG fuel tank containers completely redundant. If, for some reason, a container is out of service another container can be easily taken into operation.

The connection points are located at the end of the LNG fuel tank container for easy and smooth hook-up of the LNG fuel tank container to the onboard fuel gas handling system. These connections consist of the LNG discharge, the vent mast connection, heating media connections, and a connection to the water spraying system built onto the LNG fuel tank container. For fuel tanks located above deck, a water spray system is required to cool the LNG tank in case of fire.

Increasing flexibility in LNG fuel handling 1
Fig. 2 – Terminal tractor loading LNG fuel tank containers onboard a RoRo ship.
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Table 1 – A standard ISO tank container used for transport of cryogenig fluids does not fulfil the requirement of a LNG fuel tank on a ship.

Fastening and securing

The LNG fuel tank containers have to be rigidly fastened and secured to the deck. The fastening and securing system has to be designed for the maximum dynamic and static inclinations, as well as the maximum accelerations, of the vessel. A number of feasible solutions exist. On RoRo and RoPax vessels it is possible to use terminal tractors with trailers for loading and unloading the LNG fuel tank containers. The containers are located on cassettes and secured by twistlocks. On board the ship the cassette is secured to the deck, for example with twistlocks and lashing as a secondary fastening arrangement.

Another possible fastening and securing option is to directly secure the LNG fuel tank container with twistlocks and lashing to the deck. This would be a suitable solution for container feeders and other vessels where the containers can be lifted on and off. It is also possible to utilize these containers as stationary LNG tanks, which are not removed frequently for filling. In this case, a bunkering station can be installed to allow the LNG fuel tank container to be bunkered directly on the vessel

Docking station

The docking station is the module whereby the LNG fuel tank containers are connected to the fuel gas handling system onboard the ship. The number of LNG fuel tank container slots in the docking station is defined according to the required LNG capacity for the specific vessel. The engine gas consumption, sea voyage length, and the interval between changing the containers defines the required number of LNG fuel tank containers.

All the necessary connections between the LNG fuel tank container and the fuel gas handling system are located in the docking station for easy and practical connecting operations. Flexible hoses fitted with quick couplings are used to attach the process connections. The quick couplings have a closing valve in both coupling units to prevent any leakage when connecting and disconnecting the hoses.

The instrument readings and control signals for the remote controlled valves on the LNG fuel tank container are connected to a junction box in the docking station. There is a data connection and a hard wired cable connection for increased safety.

Preparation of the gas

The LNG is discharged from the fuel tank containers via the docking station to the evaporator skid. The evaporator skid is the module where the LNG is vaporized and heated to the conditions required by the engine (i.e. 0 – 60 °C). The master gas fuel valve, which is the last safety related stop valve in the gas supply system outside the machinery spaces, is installed after the main gas evaporator on the evaporator skid.

The LNGPac™ ISO is controlled and monitored by a control and safety system. All modules, including the LNG fuel tank containers, are monitored and controlled by a single dedicated PLC-based automation system.

Ship arrangement

The LNGPac™ ISO is intended to be located on an open deck where natural ventilation is ensured at all times. Drip trays must be installed under the skids, and also under the hose connections between the LNG fuel tank containers and the docking station. This is to prevent possible LNG leaks from damaging the deck beneath the skids.

Increasing flexibility in LNG fuel handling 3
Fig. 3 – LNG fuel containers loaded and connected to the onboard fuel gas handling system.
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Table 2 – Typical characteristics of LNG fuel tank containers of different sizes.

A design and feasibility study has been conducted together with Germanischer Lloyd (GL) for the conversion of a RoRo vessel to gas using the LNGPac™ ISO as the fuel gas handling system. Four LNG fuel tank containers, the docking station, and the evaporator skid were located on the naturally ventilated aft deck. The LNG fuel tank containers were located on cassettes, and secured with twistlocks and lashing.  A second set of LNG fuel tank containers would be refilled in advance so as to be ready for switching with the empty LNG fuel tank containers when the ship is in port.

In the area where the fuel gas handling system was located, potential existing ventilation inlets and outlets, as well as the electrical equipment, would have to be modified due to the hazardous zones around the fuel gas handling system. For protection of the surrounding ship structures, and for weather protection of the equipment, the docking station and evaporator skid would be located on drip trays in naturally ventilated shelters. Escape routes were planned from all areas, especially from the docking station where the flexible hose connections to the LNG fuel tank containers were located.

As part of the study, a comprehensive risk analysis was performed of the fuel gas handling system. A risk analysis is required for a gas fuelled ship where operational risks and risks associated with physical arrangements are indentified and eliminated or mitigated. The major hazard for a fuel gas handling system on a ship is LNG leakage, and the subsequent damage to the vessel. Where LNG leakages can occur, two important things have to be incorporated into the design. Firstly, no damage that can harm the integrity of the ship can be allowed to happen. Secondly, there has to be a way to detect and identify the leakage in order to stop and limit the leakage and the consequences. Potential leakage sources were identified for the LNGPac™ ISO modules and corrective actions were taken.

CONCLUSIONS

Wärtsilä is today recognized as a leader in propulsion solutions for gas fuelled vessels. The company's strong and early commitment to this goal has created in depth knowledge of the use of natural gas and LNG. The Wärtsilä LNGPac™ ISO is an addition to Wärtsilä’s portfolio of solutions for the LNG fuelled ship market. It represents further proof of Wärtsilä’s expertise and knowledge of LNG applications, as well as its dedication to make LNG available for all ship operators and owners.

Increasing flexibility in LNG fuel handling 5
Fig. 4 – Environmentally friendly RoRo vessel sailing on gas.

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