Chazal forbade to halachically nullify a moveable vessel from its function on Shabbos. For example: Do not place a bowl under a dripping lamp to catch the oil – the oil is muktza and will cause the bowl (a bosis) to be immovable, (ShA3 – MB6,9)
Why is the oil muktza? Because it was set aside to fuel a flame during bain hashmashos. Even though now it is no longer fueling a flame, once it gets a din muktza on bain hashmashos it remains with that status for the entire Shabbos. (ShA3 – MB9)
If the muktza oil falls into a bowl of regular muttar oil, it all becomes ossur. The issur is not bottel – “dovor sh’yash bo mattirin lo bottel”. Muktza isn’t bottel since it will become muttar anyways when Shabbos is over. (MB8)
You are allowed to put a bowl underneath a burning candle if you are concerned that it may fall and catch fire. (MB5)
You are allowed to touch muktza as long as you don’t move it. (Rama3)
Touching a hanging lamp moves it – so it is ossur even when no flame is burning in it. If there is a flame burning in it, it’s much worse! You may cause the oil to sway towards the flame (mav’ir) or away from the flame (m’chabeh). (MB10,11)
Are you allowed to set up water under a ner before Shabbos? If your intention is just to raise the level of the oil it is okay, but if you want the flame to extinguish in the water and the water is in a separate vessel it is ossur – you may come to raise the water towards falling sparks on Shabbos. If the water is directly under the flame in the same vessel some poskim hold it is muttar – you may rely on them if need be. (ShA4 – MB15, 18)
In case of fire hazard, you may instruct a minor to put a bowl of water under a falling wax candle. (MB16)