oil rig vs semi

oil rig

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

semi

noun
  • A semi-trailer; a tractor-trailer; an eighteen-wheeler; an artic. 

  • A semifinal. 

  • A semi-detached house. 

  • A partial erection of the penis. 

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