Two U.S. Navy vessels have collided in the Strait of Hormuz, near the Arabian Peninsula. The USS Hartford, a submarine, crashed in the USS New Orleans amphibious ship Friday morning. No serious injuries occurred, but gallons of fuel spilled into the water.

The USS Hartford and the USS New Orleans ran into each other during the early morning hours, at about 1 a.m. local time. Fifteen sailors from the Hartford are reported as “slightly injured.” They have all already returned to duty.

The Navy says the USS Hartford’s nuclear propulsion plant was not damaged. The USS New Orleans, however, suffered a “ruptured fuel tank.” Because of that, 25,000 gallons of diesel fuel marine were spilled.

Military officials say both ships were headed into port when the collision happened. The submarine was underwater at the time. An investigation is now underway.

The U.S. is very lucky there wasn’t a nuclear incident.