I don't know what is normal so forgive my ignorance but this raises many questions.
Aren't these reviews a little late, since they came after you received your title? I assume you wanted to have your paper as perfect as possible before you achieved your grade.
If I were you I would connect these matters and perhaps even wonder what was really needed (and what not) to get this PhD.
Why do you need to address these reviews since you have already finished the PhD study?
Naturally you want to show what was good about your paper but it sounds like a re-exam.
And what it is worth if a paper would be rejected? Does it have any consequences?
Can you work forever on your paper, after you've achieved your PhD?
Just curious how this works (at least in Norway).
Well, the PhD dissertation is seen as an independent piece of work, which is evaluated in its own right. The university appoints a committee to evaluate the thesis, and once it is accepted by the committee, the candidate must defend it in public. This means the candidate presents his thesis to an audience, and after the presentation the committee will ask questions about the work. If the defense of the work is also approved, the degree is awarded.
Now, parts of the work presented in a PhD thesis can be submitted to scientific journals (with a bit of reworking to match the format the journal requires). This is a completely different process, independent of the PhD thesis. A scientific journal does not care if a submitted paper is written by a M.Sc. student, a tenured professor or an industrial researcher, as long as it satisfies the quality demands. The scientific journals are independent companies.
When a paper is submitted to a journal, the editorial office will appoint one or more scientific peers (people who are believed to have the knowledge to evaluate the paper), who are asked to evaluate the paper. They have to assess not only the quality of the work, but also how it matches the scope of that particular journal, and whether it does add something of value to the research field. The reviewers are usually given plenty of time to evaluate - often two-three months.
The work I did for my PhD thesis has formed the basis of
four journal papers (four out of five main chapters in the thesis have been worked into journal papers). Two have been accepted and published in
Industrial Engineering & Chemistry Research, one is under review with
Journal of Natural Gas Science and Engineering. The one I am currently reworking was submitted to
Computers & Chemical Engineering, and the reviewer who is negative thinks that the paper does not really fit there. I can't really go in deep detail, but I can say this much:
The paper describes and discusses
challenges related to mathematical modelling and optimisation of a certain type of processes. This reviewer has said that since the paper does not suggest a
solution, it is premature and should be rejected. The other reviewer does not seem to have an issue with this, and the editorial office has also said that we (me and my co-author) need only
partly address it. This illustrates that even for the same journal, reviewers can have different opinions on whether a paper should be accepted or not.
To summarize: Journal papers and the PhD degree are not formally linked at all. For me, the only motivation for continuing this process is personal satisfaction (after all, having your name on papers in a scientific journal carries some prestige) and that I simply don't like having unfinished business. If it turns out that this particular paper was good enough for a PhD thesis but not good enough for
Computers & Chemical Engineering, so be it. The same has happened to many other works.