Patent GB-1341426-A (GB)
Title: System for transmitting information
IPC: H04B13/02
Filed: 1971-05-27
Abstract: 1341426 Selective signalling COMPAGNIE FRANCAISE DES PETROLES and SOC NATIONALE D'ETUDE ET DE CONSTRUCTION DE MOTEURS D'AVIATION 27 May 1971 [24 June 1970] 17478/71 Heading G4H [Also in Divisions H1 and H4] In a system for transmitting information between an underwater work station and a central telemetering and remote control station interconnected by a metal pipe-line, the pipeline is electrically conductive along its length and has an electrically insulating covering over its external surface and an electrical connection is provided at each station to the surrounding water, a transmitter and/or receiver being electrically connected to the pipe-line at each station, and a sacrificial anode provides corrosion protection. The circuit through the pipeline is completed through the water. The work station may be a well head in an underwater oil field. Digital information regarding the pressure of gas or crude oil, gas content of crude oil, temperature and state of control valves is sent from the work station to the central station for display and valve control information is sent back, either simultaneously (using modulation of different carrier frequencies) or alternately. Generally, signalling is binary using an on/off, frequency or phase modulated carrier, or D.C. amplitude, e.g. by varying the load line impedance with the two states corresponding to open and short-circuited end respectively. The electrical connection at each station to the pipe-line may be direct, or via a winding on a toroidal magnetic core through which the pipeline passes. A plurality of work stations may be connected to the central station by separate respective pipe-lines, or pipe-lines may be shared between work stations in which case different work stations using the same pipeline respond (perhaps simultaneously) to respective frequencies, or time-division multiplexing is used. In time-division multiplexing, detection of a calling frequency at a work station enables an address decoder, and if the address now received is that of this work station the rest of the message is decoded to control a programmer which routes commands and causes transmission to the central station of sensed information. Power consumption is reduced by supplying power to most of the equipment at the work station only when necessary. Sensing of some critical parameters is continuous, an alarm condition causing transmission of an alarm signal at any time. At the central station, a control desk selects the well to be called and the commands to be sent. Cathodic protection of the pipe-lines is provided by sacrificial anodes connected to the pipe-lines via chokes to prevent loss of the transmitted signal. D.C. delivered by sacrificial anodes may power repeaters for the signal, a repeater being linked to the pipe-line via two windings around respective toroidal magnetic cores, the two windings being in series and the pipe-line threading the two cores, the repeater using a transistorized amplifier connected between the sacrificial anode and a point on the pipe-line between the cores, as well as to the two ends of the series of two windings.