Each turn, minefields decay and shrink a little.

Mine_units_after_decay =
   Trunc(Mine_units * (100 - MineDecayRate) / 100)
                     ...if minefield contains normal mines
   Trunc(Mine_units * (100 - WebMineDecayRate) / 100)
                     ...if minefield contains web mines
This is a testing version.
It may be incomplete, and have more bugs (or features) than the public live version at planetscentral.com.