Belief that natural gas is the only thing in a dormant natural gas pipeline and that you're working with the same freezing points and mechanical stresses at 70m depth as you are at sea level are excellent strategies to make your dormant natural gas pipeline explode.
Hydrates are well known problems in undersea pipelines, and the problem gets worse if you have a pipeline sitting dormant.
https://oilfieldbasics.com/2022/03/09/natural-gas-hydrates/
So tell me, what gas or liquid freezes above 32 Deg F?
An explosion due to overpressure caused by chemicals mixing, I can buy that.
Contaminants solidifying and causing a blockage? Sure.
Freezing? No.