The following picture shows Nimitz-class aircraft carrier USS John C. Stennis (CVN 74) as it steams aside the Military Sealift Command fleet replenishment oiler USNS Pecos (T-AO 197) during a replenishment at sea.
Three SH-60 helicopters appear to be flying around the ships carrying supplies delivered on the aircraft carrier’s flight deck by means of a vertical replenishment (VERTREP) operation.
Although they are nuclear powered (hence they do not need “energy supplies” for some 20 years), aircraft carriers have to take on fuel and supplies (including food, mail, etc.) during the RAS (Replenishment At Sea) to “feed” the embarked planes and personnel.
Image credit: U.S. Navy