Most of geothermal plant are designed with transportation pipes connecting production or injection wells to the plant a few kilometres away. Transporting very hot geothermal water or steam is a technical challenge in term of thermal expansion management and corrosion issues. These pipes are most of the time above ground like in Iceland or Tuscany. However, surface transport pipes are not feasible in urban or densely cultivated area. Pipe in pipe system is a promising technology for underground transportation pipes but has never been tested on hot (150°C – 200°C) and highly salted geothermal water like the one in Upper Rhine Graben.
As pipe in pipe system are mostly designed with carbon steel service pipe, corrosion would probably be an issue. Service pipe can be designed with corrosion resistant metallurgy instead of carbon steel. However, these alloys can increase drastically the cost of the transportation pipes. Another option would be to have a corrosion resistant coating on inner surface of the pipe.
Thus, from September 2020, different coatings are going to be tested on Soultz-sous-Forêts power plant in the Upper Rhine Graben (France). If coated pipes are cheaper than high corrosion resistant alloy, joining two coated pipes is a challenge. The easier technic is using flanges, but it is not compatible with pipe in pipe technology due to the inner ring space. Two innovative joining solutions are going to be tested. The first one, presented in Figure 1 (a and b), uses internal sleeve protecting the coating on welding area. The second technology is a joining method based on a connection between a cold-expanded “bell-end” and an opposing slightly larger “pin-end”.
Figure 1: View of an internal sleeve (a), view of coating, cross sectional view of an installed sleeve (b) and view of the test pipes tested in the hot condition of the Soultz-sous-Forêts geothermal plant (c)
A second test will also start in September 2020 on the cold geothermal water (40°C – 70°C) at the injection. This test will investigate and compare different technical solutions to transport the cold water: coated carbon steel, fiberglass and austeno-ferritic stainless steel.
Both experiments, involving ESG and ICI, will run for 6 months and the pipes will then be dismantled to determine the efficiency of corrosion resistance and assess the best cost-effective solutions to transport hot and cold geothermal water in Upper Rhine Graben context.