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Sub-Sea TBM Tunneling: A New Approach to Oil and Gas Production

Offshore oil production is an important sector of business worldwide, but recent events have highlighted its inherent risks.  We have witnessed catastrophic accidents on offshore drilling platforms that damaged the delicate marine environment and hurt businesses and families.  Inclement weather and technological unreliability compound the problems that come with accessing oil fields in deep water.

Making offshore oil drilling as safe as possible is a high priority task—and one that could be advanced through TBM tunneling.  Building oil production facilities in controlled environments underground has the potential to take at least some of the variables out of the equation.  The tunnels would provide human access to the well head in deep water, and could be monitored for leaks and other malfunctions.

Schematic of offshore oil tunnel, courtesy of the Norwegian Tunnelling Society (NFF).

Extensive sub-sea tunnels for this purpose are being investigated in Norway, where schematics were drawn up for tunnel access to offshore oil fields in the early 1980s.   Personnel access, transportation of drilling equipment, exploration and drilling for oil, and eventual piping of oil to the shore are all performed in tunnels and caverns underground. This enables access to the drilling sites and intervention by humans, rather than by remotely controlled and operated machines thousands of feet below water. Some concepts, such as the one below, feature a triple cross section—one tunnel would be used for materials transport, one is used for transport of personnel, and the third would be used as the oil supply pipeline.

Schematic of triple cross section of concept subsea tunnel, courtesy of the Norwegian Tunnelling Society (NFF)

More than 40 subsea tunnels have been completed in Norway since their excavation was first proposed: 25 road tunnels, 8 oil pipeline tunnels, and 8 water supply tunnels. All of these tunnels were excavated by conventional drill & blast methods (D&B).  The longest tunnel was 7.9 km (4.9 mi) (Bømlafjord), and the deepest is 287 m (942 ft) below sea level (Eiksund). Most of the tunnels were excavated in hard gneiss rock, with a few in shale and sandstone.

The slow relative excavation rates of drill and blast excavation, however, make this method of excavation on long sub-sea tunnels impracticable.  Many new underground oil fields up for development will require long subsea tunnels of up to 50 km (31 mi) in length. These long tunnels must be excavated in the shortest possible time and as inexpensively as possible to make economical sense. At these tunnel lengths, mechanical full-face tunneling is the most realistic option.  TBM technology for seabed excavation has been used on multiple projects since the 1980s, from the Channel Tunnel to the Boston Harbor Outfall and multiple tunnels worldwide.

New tunnels, as in the schematic below, would be built far below the sea bed, out of unstable regions that might experience sea water infiltration.

Schematic showing the depth of a modern oil and gas tunnel, compared to the depth of the Channel Tunnel.

In addition TBM tunneling would control the amount of over-break compared to drill and blast, by limiting the amount of material removed from the tunnel.

With today’s experience and know-how it is possible to excavate long tunnels under the sea in good time and with reasonable cost.  The technology exists to improve oil and natural gas production from offshore oil fields, all that remains is for the method to gain acceptance in the industry.

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