Jamieson, Fausset and Brown on 2 Timothy 4:6

6. Greek, "For I am already being offered"; literally, as a libation; appropriate to the shedding of his blood. Every sacrifice began with an initiatory libation on the victim's head (compare Note, see on Php 2:17). A motive to stimulate Timothy to faithfulness--the departure and final blessedness of Paul; it is the end that crowns the work [Bengel]. As the time of his departure was indicated to Peter, so to Paul (2Pe 1:14).

my departure--literally, "loosing anchor" (see on Php 1:23). Dissolution.