oil rig vs oilrig

oil rig

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

oilrig

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 rig and oilrig occurred in a corpus of books? (source: Google Ngram Viewer )