Complex sentence: The owning and operation of oil tankers was mainstay of Leif Höegh & Co for decades.In the 1960's the company diversified, introducing new concepts for transportation.
Split into simpler sentences: The owning and operation of oil tankers came to form the mainstay of Leif Höegh & Co for decades. From a process of transition in the 1960's the company diversified into new activities also introducing new concepts for transportation.