Complex sentence: The design of the "Petropavlovsk" was approved in January, 1891, was actually an extended version of the "Imperator Nikolai I", but with supplementary barbettes including four with 203 mm guns.
Split into simpler sentences: The design of the "Petropavlovsk" was approved in January of 1891. It was an improved version of the "Imperator Nikolai I", but with supplementary barbettes, including four with 203 mm guns.