Hello borge ,my grandfather was born in Fredrishald but lived most his life in Onso now a bit of Fredrikstad,he was 31 when he left for Scotland but was a merchant seaman then,you said it might be possible to trace him if knew where he lived will this help.
Yes, records of his maritime career as a Norwegian mariner should be kept at the State archive covering Østfold, which is Oslo. You can ask them to send you copies of the records, which they will only do if they do not have to do research to find him. So you will have to give them enough info about him to let them find him easy, like date of birth and place of residence in Norway. If they can't find the records easy you will have to go there, of get someone else to do research for you. Here is the contact info for the "Statsarkivet i Oslo":
Statsarkivet i Oslo Postboks 4015 Ullevål stadion 0806 Oslo Norway