Shutting down an underwater fluid production well

A production control system for an underwater well, comprises: first electrically operated means for supplying first hydraulic fluid, for opening a first control valve of the well; second electrically operated means, for supplying second hydraulic fluid at a higher pressure than said first fluid, for opening a further control valve of the well; electronic circuitry (SEM1) for providing electrical power for operating said first and second means; and means for controlling the sequence of closing said control valves as a result of a loss of electrical power from said electronic circuitry. The controlling means comprises: electrical power storage means; detection means responsive to said loss of power from said electronic circuitry; and means coupled with said detection means for using electrical power from said storage means to keep said further control valve open for a period after closure of said first control valve and close it after said period.