He joined the new German Navy (the Bundesmarine), which was established in 1956, serving until becoming team leader at the Bundeswehr Command and Staff College.

His first assignment was to the armoured cruiser Deutschland, after which he served on torpedo boats and minesweepers, reaching the rank of Kapitänleutnant by April 1939.

[3] After training the boat's crew for several months, Piening set out from Kiel on his first patrol in command of U-155 in February 1942, with the intention of attacking shipping in American waters.

U-155 sank another ship off the coast of the United States re-crossing the Atlantic to the submarine's new base at Lorient, as part of the 10th U-boat Flotilla.

[1][6] He also became well known for developing the "Piening Route" in 1943, hugging the coast of France and northern Spain, to evade Allied patrol aircraft in the Bay of Biscay.

On 1 October 1965 he became team leader at the Führungsakademie der Bundeswehr, serving until 1969, retiring with the rank of Kapitän zur See.