RSS. The RSS is an evolution in directional-drilling technology that overcomes the drawbacks

in steerable motors and in conventional rotary assemblies. To initiate a change in the

wellbore trajectory with steerable motors, the drilling rotation is halted in such a position that

the bend in the motor points in the direction of the new trajectory. This mode, known as the

sliding mode, typically creates higher frictional forces on the drillstring. In extreme ERD, the

frictional force builds to the point at which no axial weight is available to overcome the drag

of the drillstring against the wellbore, and, thus, further drilling is not possible. To overcome

this limitation in steerable motor assemblies, the RSS was developed in the early 1990s to

respond to this need from ERD. The first RSS was used in BP plc’s Wytch Farm (U.K.)

extendedreach wells.

RSSs allow continuous rotation of the drillstring while steering the bit. Thus, they have

better penetration rate, in general, than the conventional steerable motor assemblies. Other

benefits include better hole cleaning, lower torque and drag, and better hole quality. RSSs are

much more complex mechanically and electronically and are, therefore, more expensive to run

compared 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 with

designer wells. Additionally, the technology is still very new. As a result, the current generation

of 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-thebit

system uses the same principle employed in the bent-housing motor systems. In RSSs, the

bent housing is contained inside the collar, so it can be oriented to the desired direction during

drillstring rotation. Point-the-bit systems claim to allow the use of a long-gauge bit to reduce

hole spiraling 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 desired

trajectory. The force can be hydraulic pressure or in the form of mechanical forces. In general,

either a point-the-bit or a push-the-bit RSS allows the operator to expect a maximum build rate

of approximately 6 to 8°/100 ft for the 8½-in.-hole-sized tool.