Lorient, France, on 31 December 1941. First patrol Hartenstein's first patrol (24 December 1941 – 10 January 1942) was an uneventful transfer patrol taking U-156 from Kiel to Lorient. U-156, together with and , transited through the Kaiser Wilhelm Canal, passing Brunsbüttel where they joined up with a pathfinder and . The convoy received aerial protection from Messerschmitt Bf 109 fighter aircraft and split up once they reached Cuxhaven. Passing south of the Shetland Islands, bypassing the Hebrides, U-156 slowly approached Rockall, where two weather buoys (, or WFB) were released: "WFB 33" on 7 January 1942 at , and "WFB 32" on 8 January 1942 at 02:44 at in the Porcupine Bank. U-156 received a radio message on the evening of 8 January ordering Hartenstein to proceed at high speed to Lorient, where it arrived on 10 January 1942. Second patrol On his second patrol (19 January 1942 – 17 March 1942) Hartenstein commanded a wolfpack of U-boats (Gruppe Neuland—Group New Land) during the Attack on Aruba in February 1942, attacking an oil refinery. U-156 along with under the command of Günther Müller-Stöckheim and under the command of Jürgen von Rosenstiel departed from Lorient under the protection of a pathfinder in the early morning of 19 January 1942. Hartenstein's orders were to conduct a simultaneous surprise attack of the Gruppe Neuland against the shipping traffic off Aruba and Curaçao. Besides U-156, U-67 and U-502, Gruppe Neuland also included the submarines under the command of Nicolai Clausen and under the command of Albrecht Achilles. On 16 February, after observing the area for a few days, U-156 came around to the refineries. There in front of her target were two Lago Company flat-bottom steamers, and , both British-owned oilers. At 01:31, U-156 surfaced in San Nicolaas Harbour some offshore and attacked the two British tankers at anchor. Hartenstein fired one torpedo from his bow tubes at Pedernales. The torpedo attack was successful and Pedernales was hit amidships. Loaded with crude oil, the steamer immediately burst into flames, killing eight of her 26 crewmen and wounding her captain Herbert McCall. Oranjestad then began to lift anchor and steam away but she was too late and was hit by a second torpedo fired from U-156. She too burst into flames and sunk an hour later in about of water. Fifteen of her 22 crewmen were killed. At 03:13, U-156 attacked the Texaco-owned tanker