Downhole telemetry system for wired tubing
A communications system employing discrete multi-tone (“DMT”) modulated signals on a communications channel formed by inductively-coupled pipe. In one embodiment, the communications system includes a string of wired pipe joints that couple a transmitter to a receiver. Each pipe joint in the string possesses threaded ends configured for mechanically connecting with other pipe joints, a electrical or inductive coupler at each threaded end, and an insulated conductor that electrically connects the couplers. The transmitter and receiver communicate via DMT modulated signals transported by the wired pipe string. The system may further include multiple repeaters in the string. Each repeater may be configured to receive a DMT-modulated signal, to recover information from the DMT-modulated signal, and to retransmit the information in the form of a DMT-modulated signal.
1. A communications method for use in a downhole communications system having repeaters, the method comprising:
listening for a first trigger signal at a first repeater in the downhole communications system;
determining if the first trigger signal identifies a link associated with the first repeater;
activating the first repeater if the first trigger signal identifies the link associated with the first repeater;
forwarding by way of the first repeater a second trigger signal to a second repeater;
establishing Discrete Multi-Tone(DMT)-signal links between the first repeater and the second repeater; and
sending a start command from a transmitter at the surface and thereby initiating communications along a communications path that includes the DMT-signal links.
2. The method of claim 1 , wherein said establishing DMT-signal links includes:
if the trigger signal does not identify the link associated with the first repeater, attempting to establish a DMT-signal link with a subsequent repeater, thereby bypassing said first repeater.
3. The method of claim 1 , further comprising:
allocating non-overlapping frequency bands to adjacent repeaters.
4. The method of claim 3 , further comprising:
preferentially allocating the frequency band having the smallest attenuation to any DMT-signal links that bypass a repeater.
5. A downhole communications system that comprises:
a string of wired pipe joints including multiple repeaters in the downhole communications system;
each of the multiple repeaters listen for a trigger signal, and each of the multiple repeaters enter into a forwarding mode if the trigger signal identifies a link associated with the repeater;
each of the multiple repeaters forward a subsequent trigger signal to a subsequent repeater, and sequentially establish Discrete Multi-Tone(DMT)-signal links along the string;
a transmitter coupled to one end of the string and to generate a DMT-modulated signal; and
a receiver coupled to an opposite end of the string and to receive and demodulate a DMT-modulated signal.
6. The system of claim 5 , wherein said establishing DMT-signal links includes:
if the trigger signal does not identify the link associated with each of the said multiple repeaters, attempting to establish a DMT-signal link with a subsequent repeater, thereby bypassing the said repeater.
7. The system of claim 5 , wherein as part of establishing DMT-signals along the string, the repeaters allocate non-overlapping frequency bands to adjacent repeaters.
8. The system of claim 7 , wherein as part of allocating frequency bands, the receivers preferentially allocate the frequency band having the smallest attenuation to any DMT-signal links that bypass a repeater.