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Among the great books about the oil industry, a neglected classic is The Offshore Imperative: Shell Oil’s Search for Petroleum in Postwar America by Tyler Priest (2007). In 1998, Priest was hired by Shell Oil to write a comprehensive history of the company. However, budget cuts caused the book project to be scrapped. Priest decided to use the research and interviews that he had collected to write his own book about the story of Shell’s offshore exploration, and a fascinating story it is.

Because the international majority shareholders of Royal Dutch Shell did not want the American entity to compete with its other companies, Shell Oil had to confine its operations to U.S. territory. Most of the easy oil had already been discovered, and the best hopes seemed offshore.

The company was already experienced at so-called “marsh drilling” in wetland areas of Louisiana, such as Weeks Island. So, Shell developed “floating derricks,” mounted on barges. Once on location, the barges’ ballast tanks were flooded, and the structures came to rest on the bottom.

Heading offshore. After WWII, the quantities of high-quality crude discovered at Weeks Island led the company to consider the open water, where they had acquired cheap acreage (averaging $8.80/acre) in Main Pass and South Pass near the Mississippi River delta. Explorationists had been excited for some time about large salt domes in that area.

The Main Pass 69 wells were drilled in 10-ft waters by “submergible” drilling barges like those used in the marshes, with old WWII transport vessels anchored nearby as floating breakwaters (and doubling as storage for mud and equipment). The second well drilled on the block was a significant oil discovery. But in an ominous turn of events, the well blew out and burned for 21 days before cratering, swallowing the rig and storage vessels. The lesson learned was that only the very best equipment, training and safety procedures would do in an offshore environment.

Up to that time, “offshore” meant less than 10 ft of water, the limit of the barge-and-breakwater technology. The alternative, a jacket platform with floating tender, was very expensive, and engineers began seeking to design a barge that would be stable in deeper water and could be moved easily.

Oilfield engineer John T. Hayward designed the Breton Rig 20, a platform supported by columns on top of a barge, which could be sunk and refloated in up to 20 ft of water. Not to be outdone, in 1953 the newly formed ODECO constructed the Mr. Charlie, a column-stabilized submersible vessel designed to work in 30–40 ft of water. Shell had been looking for a way to drill in deeper areas of South Pass 27, and signed a five-year contract. (Mr. Charlie worked in the Gulf for 30 years before being retired as a floating museum off Morgan City, La.)

In 1957, ODECO built the world’s largest submersible, the Margaret, the size of two football fields and supported by 10 huge cylindrical columns. The monster, with a catamaran hull, was capable of working in depths to 65 ft. A stupendous engineering achievement to be sure, but it had become apparent that to access even greater depths, the answer would have to be self-elevating jack ups.

In 1955, the Mr. Gus, a hybrid submersible/jackup, drilled a well 63 mi offshore Texas, a record at the time. Unfortunately, that well turned out to be the company’s first million-dollar dry hole. Bad luck was not over for Mr. Gus, which tipped over in 1957 while moving off location. Fixed platforms were more stable and reliable, but the cost of salvaging one in case of a dry hole meant that they were good for development of proven fields, but not exploration.

Deeper still. Water depths around 60 ft were always considered problematic in oil exploration. Jack ups designed for greater depths were prone to capsize, and submersibles were unstable, or had to be designed impossibly large. New technology was imperative. Shell’s Technical Services Division (TSD) began a well-funded (and very secret) program in 1954 to study deepwater drilling, even investigating ideas for a floating vessel capable of working at depths greater than 300 ft. They had their reasons. Geologists were convinced that areas farther south of the original blocks could be highly prolific, and the BLM offered an additional 1.17 million acres off Louisiana and Texas, some of it in quite deep water, for its February 1960 lease sale. Oil companies scooped them up.

Semi-submerged. Shell engineer Bruce Collipp, who wrote his master’s thesis on offshore rig design, sold TSD on a design for a structure with three large columns and a submerged hull. Since most of the mass would be underwater, stability would be greatly improved. This “Trident” design was still just a concept in 1960, when Shell acquired its deepwater acreage. To evaluate its new leases, the company needed a working floating drill platform to drill some test wells before the next lease sale in March 1962.

With no time to build one from scratch, Collipp’s team leased and converted an existing submersible, the Blue Water 1. The hull pontoons were partially flooded so that they remained in stable water below the surface. The vessel was outfitted with an eight-anchor mooring system, utilizing wireline rather than chain, and an automatic ballasting system kept the vessel stable and the rotary table positioned level over the wellhead. Effectively, a rig designed to operate in 60 ft of water could now handle a depth of 600 ft.

Shell sought to have the rig certified, but there was no classification that fit, because there was nothing like it. It was a Coast Guard captain who improvised the colorful description that stuck: “Super Manned Barge–Semi-submersible.”