At the start of the 20th century, oil and gas was a fast paced burgeoning market.  Just a few years previous (1896), the first submerged oil wells in salt water were drilled off the coast of Santa Barbara (Summerland), California. 

Within the next 10-15 years, drilling from fixed structures occurred in the Gulf of Mexico in the tidal zones of Louisiana and Texas.  It would be over 50 years before the first Mobile Offshore Drilling Unit (MODU) called the Mr. Charlie (1953)and the first purpose built semi-submersible drilling unit the Ocean Driller (1964), were delivered. 

While towing the Ocean Driller to its intended location for Shell Oil Company, the crew noticed that the vessel motions were low when the draft of the semi-submersible was about halfway between the pontoon and the bottom of the deck.  The concept of using a semi-submersible hull to provide a stable drilling platform offshore, and the concomitant need for offshore moorings, was born.

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Taking Stock – A Historical Review of the Mooring Industry (pdf)