A fuel tank arrangement in a marine vessel has a tank container, a first fuel line having a first fuel line port opening into the tank container so as to provide a flow path exclusively for a first fuel, a second fuel line having a second fuel line port opening into the tank container so as to provide a flow path exclusively for a second fuel. The first fuel line port and/or the second fuel line port is provided with a closing means so as to close the flow path from the tank container into the fuel line.