Complex sentence: It wasn't until her fifth patrol, following a shift in patrol zones to the frozen seas around Novaya Zemlya and of flotilla to the 13th U-flotilla, that "U- 365" experienced success.
Split into simpler sentences: It wasn't until her fifth patrol, following a shift in patrol zones to the frozen seas around Novaya Zemlya and of flotilla to the 13. Unterseebootsflottille, that "U- 365" experienced success.