1. Field of the Invention
This invention relates to downhole drilling, and more particularly to apparatus and methods for transmitting data along a downhole drill string.
2. Description of the Related Art
For half a century, the oil and gas industry has sought to develop downhole telemetry systems that enable high-definition formation evaluation and borehole navigation while drilling in real time. The ability to transmit large amounts of sub-surface data to the surface has the potential to significantly decrease drilling costs by enabling operators to more accurately direct the drill string to hydrocarbon deposits. Such information may also improve safety and reduce the environmental impacts of drilling. This technology may also be desirable to take advantage of numerous advances in the design of tools and techniques for oil and gas exploration, and may be used to provide real-time access to data such as temperature, pressure, inclination, salinity, and the like, while drilling.
In order to transmit data at high speeds along a drill string, various approaches have been attempted or suggested. One approach that is currently showing promise is to incorporate a “network” of data transmission cable and other communication equipment into the drill string. Due to the length of drill strings, which may exceed 20,000 feet, such a network may require placing network “nodes” at selected intervals along the drill string. These nodes may act as repeaters to amplify the data signal and provide points of data collection along the drill string. Communication elements, such as magnetic couplers, may be incorporated into the ends of downhole tools to transmit data across the tool joints. Transmission lines, such as electrical cables, may be incorporated into the downhole tools to transmit data therealong.
Unfortunately, unlike conventional above-ground networks, a downhole network is constrained by the physical limitations of the downhole drill string. In particular, a downhole drill string is a linear structure, making it very difficult to build redundancy (and thereby reliability) into the downhole network. As a result, any break or malfunction in the data transmission path along the drill string may cause communication to be lost between the surface and downhole components. Because the drill string may include many hundreds of downhole components (e.g., sections of drill pipe, drill collar, bottom-hole assembly components, etc.), a single break or malfunction in any downhole component can break the communication path and cause the network to lose much if not all of its functionality.
In view of the foregoing, what are needed are apparatus and methods to provide multiple redundant paths of communication in a downhole network. Such apparatus and methods may be used to significantly improve the reliability of downhole communication networks.