Complex sentence: After 6 ports of call Atlantis arrived on December 6th 2015,3 weeks later on January 1st 2016 Atlantis started operations with its main Client PEMEX, in the Akal field area at 00:01 Hrs.
Split into simpler sentences: After 6 ports of call Atlantis arrived on December 6th, 2015. Three weeks later, on January 1st, 2016, Atlantis started operations with its main Client PEMEX, in the Akal field area at 00:01 Hrs.