The term hydroduct is valid to refer to a pipeline conducting hydrogenand it is written in one word, without space or hyphen.
Examples such as the following can be found in the media: «Germany will join the agreement for the underwater hydroduct between Spain, France and Portugal», «The project is completed with the construction of a ten-kilometre-long hydroduct» or «I questioned the feasibility of converting it into a hydro-duct”.
The proper meaning of Water- is water’so a hydroduct It is, in principle, a conduction of this liquid, meaning in which it is used on occasions. Nevertheless, from hydrogen It is beginning to be applied to what is related to this last substanceespecially as a source of energy. It is a process similar to what they have already experienced photo- to allude to photography (photomontage), auto- in relation to automobiles (car wash) o bio- for what has a biographical character (bioseries).
Being an element that works as a prefix, it is appropriate to write it in a single word, so spellings like hydro-duct e hydro pipeline.
Although minority, variant is also documented hydrogen pipelinewhich lacks the conciseness of hydroductalthough it is more transparent. In both voices, the second element is -ductowhich is also found in aqueduct, pipeline y pipeline.
Therefore, the previous examples are valid, although in the last one it would have been appropriate to write “I was questioning the feasibility of converting it into a hydroduct.”