I think Bobby's first question is right on the money -- first determine exactly what is in your pipeline. You have called it "ethylene gas at P=80 bar (Pc = 50 bar according to DIPPR) and T=environment (Tc=282 K or 9 C according to DIPPR)". I am reminded of a day in an early PChem class where we talked about what to call a fluid that is well above the critical pressure but near the critical temperature (note that we decided not to call it a "gas" without some careful clarifications).
At 80 bar, there would certainly be temperatures (maybe above 290 or 300 K) where ethylene might behave more like a gas.
At 80 bar, there would certainly be temperatures (maybe below 280 or 270 K) where ethylene might behave more like a liquid.
Even though there is no clearly identifiable phase boundary, there would be a transition region where ethylene's behavior is ambiguous.
You have not said what your expected environment temperatures are. 250 to 320 K would seem to cover most of the planet, though, so it is not difficult for me to hypothesize that the original engineers anticipated that the pipeline could contain ethylene in a liquid like state and designed it for "thermal expansion" with appropriate safety valves.
As Bobby said -- you need to figure out exactly what is in your pipeline. I would go further and suggest that you pull up your phase diagrams and/or equations of state and/or whatever else you want to use to estimate the PVT properties of ethylene and figure out exactly what is in that pipeline at your expected conditions and what happens if/when the pipeline is blocked off and allowed to warm up.