RSS.The RSS is an evolution in directional-drilling technology that overcomes the drawbacksin steerable motors and in conventional rotary assemblies. To initiate a change in thewellbore trajectory with steerable motors, the drilling rotation is halted in such a position thatthe bend in the motor points in the direction of the new trajectory. This mode, known as thesliding mode, typically creates higher frictional forces on the drillstring. In extreme ERD, thefrictional force builds to the point at which no axial weight is available to overcome the dragof the drillstring against the wellbore, and, thus, further drilling is not possible. To overcomethis limitation in steerable motor assemblies, the RSS was developed in the early 1990s torespondto this need from ERD. The first RSS was used in BP plc¶s Wytch Farm (U.K.)extendedreachwells.RSSs allow continuous rotation of the drillstring while steering the bit. Thus, they havebetter penetration rate, in general, than the conventional steerable motor assemblies. Other benefitsinclude better hole cleaning, lower torque and drag, and better hole quality. RSSs aremuch more complex mechanically and electronically and are, therefore, more expensive to runcompared to conventional steerable motor systems. This economic penalty tends to limit their use to highly demanding extended-reach wells or the very complex profiles associated withdesigner wells. Additionally, the technology is still very new. As a result, the current generationof systems (2002) is climbing a very steep learning curve in regard to run length,performance,and mechanical reliability.There are two steering concepts in the RSS point the bit and push the bit. The point-thebitsystem uses the same principle employed in the bent-housing motor systems. In RSSs, thebent housing is contained inside the collar, so it can be oriented to the desired direction duringdrillstring rotation.Point-the-bit systems claim to allow the use of a long-gauge bit to reduceholespiraling and drill a straighter wellbore.The push-the-bit system uses the principle of applying side force to the bit, pushing it against the borehole wall to achieve the desiredtrajectory.The force can be hydraulic pressureor in the form of mechanical forces.Ingeneral,either a point-the-bit or a push-the-bit RSS allows the operator to expect a maximumbuild rateof approximately 6 to 8°/100 ft for the 8½-in.-hole-size