Please confirm that in dynamic analysis, for checking of the code stress, the first combined case is:
SUS + D1 and
for checking of the nozzle loads (restrains) the second combined case is:
OPE + D1.
If this is correct, can someone explain why if I add snubbers to dampen down vibrations the nozzle loads increase. Shouldn't dampening of vibrations reduce loads on the nozzles?

Your load case assumptions are correct.

As to whether or not a snubber will reduce your nozzle loads, that depends on:
- where you placed the snubber
- and how the system response changes due to the snubber.

You can't just assume that a snubber will solve your problems.

The snubbers are located at the place of maximum movements due to vibrations. The system response, displacements, when snubbers are addednts are drasticaly reduced, but nozzle loads are larger then when system is free to vibrate. Does this make sense?

Hi,
It is not the answer you are looking for. Based on prior experience, snubbers are not suitable for slug flow lines because of limitation in rated cycles. Please be cautious using snubber in this case.
Regards,

As suggested by others/ it is difficult to visualise your problem without an idea of the problem geometry and the slug load point of applications. Sometimes the Friction components can be influencing the loads. Suggest do a quick No-friction case check.

The Snubber will certainly increase your system rigidity in the Dynamic/ shock load cases.