They buy fuel oil from various suppliers, transport, store and blend them and finally deliver to ocean going ships, containers, tanker fleets, time charter operators and marine fuel traders.
en.wikipedia.org Time charter arrangements specify a daily rate, and port costs and voyage expenses are also generally paid by the charterer.
en.wikipedia.org They use four types of contractual arrangements: the voyage charter, the contract of affreightment, the time charter and the bareboat charter.
en.wikipedia.org Tankers are hired by four types of charter agreements: the voyage charter, the time charter, the bareboat charter, and contract of affreightment.
en.wikipedia.org If one took a longer view, say 15 years, the fluctuations would level out and ships on long-time charter would be profitable.
en.wikipedia.org