Complex sentence: The intention was for the bombs to detonate in open sea once the ship had cleared the Oslofjord but, because departure was delayed, the bombs went off before the "Donau" reached Drøbak, where the captain managed to beach her.
Split into simpler sentences: The intention was for the bombs to detonate in open sea once the ship had cleared the Oslofjord. But because departure was delayed, the bombs went off before the "Donau" reached Drøbak, where the captain managed to beach her.