oil platform vs oil rig

oil platform

noun
  • A large structure used to house workers and machinery needed to drill and then produce oil and natural gas in the ocean. 

oil rig

noun
  • A rig, consisting of platform and machinery, used to drill for oil and other petroleum products like natural gas. 

How often have the words oil platform and oil rig occurred in a corpus of books? (source: Google Ngram Viewer )