Naval mines should not despawn after 15 minutes in Naval EC. The games can take hours to finish, 15 minutes is a flash in the pan for a Naval EC game, and naval mines were designed to last for months if not years. The mines should last for the entirety of the match unless swept or otherwise detonated, at least so long as the player that set them is still in the game, and they are still playing with the vessel that set the mines (you can die and respawn, but can’t change your vessel, so if you start setting minefields in a minelayer, you’re now locked into a “defense” role for your team if you want your minefields to stay).
Unlike other armaments, mines do not get reloaded at caps. The Drache can only carry 40 mines in-game, however, in reality, she could carry 240 naval mines (120 on deck, 120 in her hold forward of her stack which is currently just empty space). Source: The Royal Yugoslav Seaplane Tender & Minelayer Zmaj [Zvonimir Freivogel] [Warship International Vol. 38, No. 1 (2001), pp. 46-55 (10 pages)
The HMS Abdiel is similar, she could carry 158 mines but is also limited to 40 in-game. Allowing mines to stay for an entire Naval EC game until swept or otherwise detonated, giving players the ability to set the depth of their mines, and giving minelayers their full complement of mines would give minelayers and minesweepers a real strategic purpose aside from just being big gunboats. The Drache and HMS Abdiel should have their class changed to minelayers and given their full suite of mines. Other nations should get dedicated minelayers as well. There are numerous coastal vessels in the game already that were dedicated minesweepers in real life, such as the M-17, M-802, and USS Candid. Other vessels have the capability too, as you can see paravanes on their sterns.
This would also give players a reason to work together and use coastal vessels in Naval EC. Mines can also be detonated harmlessly with large HE shells, artillery strikes, depth charges, or aerial bombs with near misses, though all of that would be less efficient and more time-consuming than having a minesweeper clear a path through the minefield. As a downside to prevent excessive mine spam, mines could be made even more vulnerable to detonation when damaged (making it even riskier to carry them), have a lengthy “ready rack replenishment” from their holds and capture point resupplies, and limiting minelayers to 1-3 per team. In the attached pictures, there are explanations and the same suggestions on how to improve minelayers and minesweepers.
