The present invention relates to the construction and the conversion of vessels. The invention relates specifically to a method for converting a single-walled vessel into a double-walled vessel, comprising of: ¢ creating a recess in the single-walled vessel for placing of a tank compartment, ¢ providing plate material, ¢ connecting the plate material to form a tank compartment outside the vessel, ¢ placing the tank compartment in the recess, characterized in that at least two frame elements, which are placed substantially parallel and spaced apart and to which the plate material is connected, are used to form the tank compartment.