Safety and reliability is import factor because of bad working condition on offshore plant. Derrick is the highest part in drilling system and should be controlled in safety manual. Normal derrick structure is built at top side of the plant and effected on sea wave, strong wind and risk environment. So, center of gravity at derrick is important factor for controlling attitude of offshore plant. This study shows improvement of more stable structure through vertical moving derrick system. We worked failure proof analysis through RBD & FMEA at early design stage. And structure analysis and mock-up test are carried out to verify vertical moving function at new concept of derrick system.