Gas is set to become the second largest source of energy by 2025, with renewables and natural gas accounting for 85% of energy growth and its potential has not been lost on maritime.
It’s clear that the shipping industry has already embraced LNG as a fuel, and according to data from Clarksons (2019), the existing world fleet is made up of 668 LNG-fuelled and LNG-ready ships with an additional 409 in the orderbook. LNG is both a technically proven and a commercially viable solution for shipping today and is a good “transition” fuel, which is able to meet existing and imminent emissions requirements for NOx, SOx, and CO2.
However, the safety and technology considerations of integration of the LNG in a ship requires shipyards, owners and designers involved to understand the challenges and what considerations need to be made to ensure safety at every stage.
Design considerations for the receiving ship
In order to be able to design, build and operate an LNG fuelled ship, one needs to know how to integrate the associated LNG sub-systems, including the fuel containment systems, ventilation and piping arrangements. LR’s consultancy services can help out clients through this process step by step.
- For shipyards we can perform LNG assessment analysis and risk assessment studies.
- For ship designers, there are various aspects that need to be considered to optimise LNG bunkering on the receiving ship.
Boil-Off Gas (BOG) Management: firstly, natural gas is cooled to cryogenic temperatures to create the LNG, so it has a very low temperature compared to traditional fuels and its volume/energy ratio is about 1.6 times. The liquefied gas is stored in insulated tanks, keeping it in a liquid state. However the heat ingress from the tank’s surroundings will increase the temperature inside the tank, causing the liquid to evaporate, generating boil-off gas (BOG). This BOG needs to be managed to prevent fuel tanks becoming over pressurised, which can lead to gas venting. If heat ingress into the LNG fuel tanks is not controlled, venting of harmful gas can occur, causing a health and safety risk to crew and the surrounding area. There are ways of handling BOG, such as managing LNG bunkering operations, burning the BOG or finding other ways of sub-cooling the fuel through a facility onboard the ship. These options need to be considered during the ship design to best suit the ship’s operational profile.
Bunkering Operations Optimisation: An essential part of the receiving ship’s design is the compatibility with the bunkering vessel to reduce the time taken to complete the bunkering operations. For efficient LNG bunkering, both ships should be side by side and should be as close as possible to each other. This reduces the piping arrangement lengths and improves hose handling, ultimately reducing the time it takes to complete the bunkering operation, considering the time required for inerting and cooling the bunkering lines during connecting and disconnecting operations. Additionally, minimising the distance between the bunkering station and the LNG fuel tanks should be considered, as this will also reduce the risk of cryogenic leakage. Incompatibility can be minimised by reviewing the geometric dimension and bunkering arrangements of the two ships, preventing possible movement interference between them, which can delay the bunkering operation.
Main challenges larger ships fuelled by LNG
Maximising Cargo Space: for larger ships, such as Ultra Large Container Ships (ULCS), consideration should be made to optimise fuel capacity and minimise loss of cargo space. A simple way of doing this is to locate the LNG storage fuel tank underneath the accommodation space onboard the ship. The additional safety considerations of this arrangement to the crew can be carefully managed by installing containment systems around the LNG and the surrounding area of the tanks; thus, in the event of a fire or leak, the crew are protected by the containment system.
Containment Systems Considerations: there are different containment systems available - one such example is the membrane technology, which is designed to contain LNG cargo at cryogenic temperatures (i.e. -160°C). It consists of a small layer of membrane supported through insulation by the adjacent hull structure. Alternatively, an independent self-supporting prismatic tank can be used. With both such containment systems it is possible to maximise the ship hull volumetric efficiency and fit practically the entire fuel tank beneath the main deck without any significant reduction of the cargo space. Both systems are provided with a secondary containment system to prevent cryogenic risk to the ship structure. To ensure safety, the fuel tank is separated from the cargo spaces by using a cofferdam.
Refuelling: LNG is transferred from the bunkering vessel to the ships fuel tank, which is located underneath the accommodation. Crew accommodation onboard ULCSs tend to be fitted on the forward section of the ship where the side shell lines are not parallel. The receiving and bunkering ships need to be positioned within the parallel body to improve compatibility. If the fuel tank is located at the front of the ship, and the ULCS bunkering station is located aft, the bunkering pipes will be longer and therefore LNG will have to flow through the pipes along the ship. To prevent hazards, pipes need to be double-layered and ventilated to have sufficient flexibility to accommodate ship movements and possible thermal stress. Flexible hoses can be used to compensate for the ship movements during bunkering operations to prevent the pipes breaking.
For Very Large Crude Carriers (VLCC), the LNG fuel tank can be located above the main deck, so cargo capacity is not affected. Furthermore, to ensure sufficient endurance of the ship, the tank capacity should be in the order of 7,000 cubic meters and will require at least two large tanks positioned on deck. Structural detailed consideration and adequate appraisal of stability and visibility issues are needed as a result of having such large tanks on deck.
Designers can also evaluate the possibility of placing the LNG fuel tank under the deck within the cargo area of the VLCC, using membrane or prismatic containment technologies, with minimum impact on the ship’s cargo capacity.
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