Warship refueling at sea. This is known as UNREP (underway replenishment) or RAS (replenishment at sea).
First a line-throwing gun is used to shoot a line between the two ships:
Then the line is used to connect a larger fuel hose:
More:
https://en.wikipedia.org/wiki/Underway_replenishment
Previously:
Motion compensating ship personnel transfer gangway - GIF
Boarding a moving cargo vessel - GIF
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