Method of making safe an undersea bottom-to-surface production pipe when production is stopped

An installation for producing fluid, the installation comprises:

• a ship or floating support on the surface having at least two tanks;
• an undersea bottom-to-surface connection production pipe comprising a first pipe portion resting on the sea bottom from a well head to the bottom end of a second pipe portion rising to a ship or floating support on the surface;
• a first auxiliary pipe for transporting gas extending at least from the ship or floating support on the surface to the bottom end of said second pipe portion;
• and
• a plurality of valves comprising:
• a valve (V6) suitable for isolating or putting into communication said first auxiliary pipe for transporting gas and the bottom end of said second production pipe portion;
• a valve (V3) suitable for isolating or putting into communication said first production pipe portion and said second production pipe portion, end to end;
• a valve (V5) suitable for isolating or putting into communication the proximal end of said first production pipe portion and the bottom end either of a fourth auxiliary pipe rising directly to the surface, or else a bottom portion of said first auxiliary pipe connected via an isolating or communicating valve to a top portion of said first auxiliary pipe, said first portion of said first auxiliary pipe being connected to said valve (V6) suitable for isolating or putting into communication said first auxiliary pipe and the bottom end of said second production pipe portion;
• a second auxiliary pipe extending at least from a first or second tank containing an inert replacement fluid or a first separator gel reagent on board the ship or floating support on the surface to a first static mixer, said second auxiliary pipe being suitable for transferring said inert replacement fluid or first separator gel reagent into said first mixer;
• a third auxiliary pipe extending at least from a third tank containing a second separator gel reagent on board the ship or floating support on the surface to a first static mixer, said third auxiliary pipe being suitable for transferring said second separator gel reagent into said first mixer;
• and
• a first separator gel-forming chamber leading at its other end to the proximity of the end of the first pipe portion resting on the sea bottom that is closest to the well head.