Abstract: Well installation electrical transmission system for use in a well installation having a downhole metallic structure (2) and a platform (3) provided above the level of a borehole (B) for the transmission of at least one of electrical power and electrical communication signals between an out of hole station (4) and a downhole station (5). In some instances, the out of hole station is arranged to modulate current applied to the downhole metallic structure by an impressed current cathodic protection system. In some instances the out of hole station is connected electrically between a portion of the platform or of the metallic structure galvanically connected to the downhole metallic structure, and a downhole connection location on a second set of downhole metallic structure in a second borehole.

Abstract: A subsurface safety valve control system including a downhole station 5 for provision in downhole metallic structure (2) of a well installation which comprises a subsurface safety valve V having an open state allowing flow through the valve and a closed state obstructing flow through the valve. The downhole station (5) is arranged for picking up electrical signals from the downhole metallic structure (2) to obtain valve control signals and for outputting control signals for controlling the subsurface safety valve V in dependence on the signals picked up from the downhole metallic structure. The downhole station (5) comprises a hardware electronics channel (5a) for handling electrical signals picked up from the downhole metallic structure and the hardware electronics channel (5a) is arranged for detecting the presence and/or absence of hold valve open signals.

Abstract: A monitoring well installation comprising metallic casing (2) and a sealing material plug (6) provided downhole in the borehole for sealing the borehole against the egress of fluid from a zone below the plug. There is an uncased length of borehole at least 10 m in length in which the plug seals against the formation. There is provided a downhole sensing tool (3) located below the plug (6) and a communication arrangement (31, 5, 4) for use in transmitting data from the sensing tool towards the surface, which comprises an across plug communication apparatus (5) comprising a pair of spaced contacts (51) contacting the casing and/or the formation and a transmitter located below the plug (6) for transmitting signals carrying data across the plug towards the surface.