# Economic Feasibility of Retrofitting an Ageing Ship to Improve the Environmental Footprint
## 2 Retrofitting the Propulsion System
### Limitations
The smallest size, i.e., a 30 m\({}^{3}\), is selected and the tank is located on the poor deck, in front of the superstructure, taking up space of four containers (TEU). The tank will be isolated from the top side by a specially built metal platform located 4 m from thedeck and designed with a load capacity of 250 tons, sufficient to place four 40 ft ISO LNG containers fully loaded and secured by standard foundations and quick-release fittings used on container ships. In addition to the main tank, this permit reaches a capacity of 132 m\({}^{3}\) LNG. The volume of the mobile containers is 33 m\({}^{3}\)[59]. Such an arrangement meets the requirements, and the length and width of the metal platform are estimated to be 12.2 m and 4.92 m, respectively, while the vertical clearance from the main deck is 4.00 m. The loading and unloading of the adjacent rows of containers are not hindered. All equipment and systems for regular operation and processing of LNG are installed inside the TCS (tank connection space). It is a closed, gas-tight enclosure, with independent ventilation from the other spaces, made of stainless steel, which acts as a second barrier that prevents a possible LNG leak from affecting the ship's hull. All LNG storage tanks [59] are highly insulated, but gradual heating is inevitable, leading to BOG (boil-over gas), which must be managed. The BOG can be controlled to a certain point for vessels equipped with Type C tanks by allowing the pressure to build. The IGF Code requires that the tank pressure be maintained below the set point of the pressure relief valves without venting gas into the atmosphere. The IGF code proposes that the minimum holding time for pressure vessels (type C vessels) is 15 days. Another challenge during the retrofitting project is the selection and positioning of the bunker station. LNG bunker operation used to be a critical situation due to the potential risks this operation involved. Apart from the installation of the station, including all necessary pipes, it is essential to consider that a drip may must be installed. Drip trays shall be fitted where leakage may occur, which can cause damage to the ship structure or where limitation of the area which is affected by a spill is necessary. The drip tray shall also be thermally insulated from the ship's structure so that the surrounding hull or deck structures are not exposed to unacceptable cooling in case of leakage of liquid fuel. Finally, drip trays must be fitted with a drain valve for rainwater. All these facts are essential to consider when installing on the deck. A length of 3 m, a width of 1.5 m and a height of 2 m were chosen for the size and dimensions of the bunker station. There are no specific size requirements for these stations and it is sufficient to have room for piping and to provide space for air isolation. The most suitable location is below the deck, where a 1.55 \(\times\) 1.98 m hydraulic hatch will be constructed to open the station during bunkering. This arrangement of the bunker station will prevent obstructing the deck from passing from the accommodation to the container bays. With this setup, the fuel pipelines will be brought directly to the deck above the bunker station and the construction of additional insulation and casings will not be necessary. There is a need to install a module related to the use of diesel, such as the module for initial gas ignition, which is also directly related to the gas combustion operation. The module will be located next to the fuel separators in the ER, where the necessary space is 1500 \(\times\) 800 mm.