Here are some synonyms:
* Unconcerned
* Indifferent
* Apathetic
* Unworried
* Unbothered
Here's an example:
"Despite the impending storm, the captain remained disconcerned, confident in the ship's ability to withstand the rough seas."
Note: "Disconcerned" can also sometimes be used to mean "concerned," but this is less common and often seen as outdated usage.