Dear,
To obtain the necessary loads to be able to design the supports, what load cases do I have to consider?
Also, what happens if I also have seismic load?
Thanks a lot.

Your civil / structural engineers should be able to tell you what they require, rather than you try to tell them what they require.

Furthermore, for restraint loads you want to look at the operating cases typically found at the beginning of the load cases editor. If there's a seismic scenario in the static analysis, it'll be something like "W + P1 + T1 + U1" depending on your model.

Based from my experience the following are the common load cases the structural engineer require:

1) WNC = PIPE WEIGHT
2) W-WNC = FLUID WEIGHT
3) WW-WNC = WATER WEIGHT DURING HYDROTEST
4) PURE THERMAL LOAD WITHOUT FRICTION AT ANCHORS & GUIDES (SOMETIMES THERE IS A DOWNWARD THERMAL LOAD AND IF THERE IS A HOLD DOWN GUIDE, CHECK IF THERE IS AN UPWARD THERMAL LOAD).
5) FRICTION LOAD (NORMALLY THEY CALCULATE THIS ON THEIR OWN BUT BETTER CHECK IF THERE IS A SIGNIFICANT DIFFERENCE BETWEEN THEIR CALCULATION AND CAESAR RESULTS)
6) PURE OCCASIONAL LOADS SUCH AS WIND, SEISMIC, PSV THRUST, SURGE OR SLUG.

In this way, they can combine the loads on their own based on the structural code requirement. If I'm not mistaken the pipe empty, pipe full cycle has some sort of a low cycle fatigue effect?.

Any other opinion is highly appreciated. Corrections are welcome.

Cheers!!!

For piping supports, it is wise to consider the max load it may occur. You will check against the support allowable loads and will be just fine.