_ Real-time communication of rotary steerable system (RSS) measurements and status to surface is critical for optimizing drilling performance and staying on target. In most North American land drilling rigs that utilize RSS, the measurement-while-drilling (MWD) tool and RSS are separate systems without a direct communication link. Instead, short-hop communication systems are used to bridge the gap between the RSS and the MWD. Short hops typically send data over short distances of 10 to 30 ft and utilize wireless communication techniques. The two most common short-hop technologies are electromagnetic (EM) and magnetic. EM short hops utilize gap subs, which provide an insulating gap in the bottomhole assembly (BHA) across which a voltage can be applied. The transmitter applies a voltage across its gap, and current flows through the drilling fluid and formation. The receiver then measures the corresponding voltage across its gap and decodes the signal. Both the transmitter and the receiver in an EM short hop require a gap sub. Gap subs are typically constructed using an insulating epoxy or ceramic section between two halves of a drilling sub, electrically isolating what is above from what is below. This technology has its shortcomings, because they can fail, resulting in a lost-in-hole event that can cost upward of 1 million. Gap sub reliability has increased over the years, but gap sub failures still occur. Magnetic short hops communicate by transmitting and receiving magnetic fields directly, which does not require gap subs. A magnetic transmitter typically consists of a coil or solenoid driven by a transmitter electronics assembly that creates a time-varying magnetic field. This magnetic field passes through the nonmagnetic drilling collars and is received in the MWD by magnetometers or a receiving coil. Magnetic short hops typically have less range than EM short hops, but their primary advantage is that they do not require gap subs in the BHA. Another advantage of magnetic short hops is that they are not affected by drilling-fluid conductivity or formation resistivity. Operational Challenges One of the primary reasons short-hop systems are needed is that the majority of MWD systems used in North America utilize bottom-mounted positive mud pulsers to transmit mud-pulse telemetry to surface. Bottom-mounted pulsers have been proven and refined for decades. They are also economical to operate and reliable. It is very difficult and impractical to get a wired connection through the bottom-mounted pulser assembly all the way from the RSS to the MWD, thus necessitating a short-hop communication system to get data from the RSS to the MWD when bottom-mounted pulsers are used. MWD systems that utilize top-mounted pulsers can connect directly to the RSS and do not typically require a short-hop communication system. However, top-mounted pulsers are more expensive to maintain, and their reliability is typically lower than that of modern bottom-mounted pulsers. One of the largest challenges with magnetic short-hop systems is that magnetic field strength decreases approximately with the cube of the distance from the source. Generating a magnetic field strong enough to detect even 10 to 15 ft away with the power available downhole is one challenge.
David Erdos (Mon,) studied this question.