Dual Drawworks Provide Operational Redundancy and Reduce Cut and Slip Costs

Abstract

Abstract Dual drawworks is a new concept based on the AC gear-driven Active Heave Drawworks. If one of the two independent units fails, the other can still run the block and provide redundancy. Dual drawworks also improves rig efficiency due to higher capacities and savings of cut and slip operations. Introduction When the Active Heave Drawworks (AHD) was introduced to the marked in the late 90s, it represented a trend setting new technology. In addition to the normal hoisting function this single gear driven AC drawworks acts as a dynamic brake and as an active heave compensator. Active means that the heave compensation does not need hook load variations but works equally well if string is attached to bottom or not. The removal of the heavy and complicated crown block compensator is one of the greatest advantages because it lowers the rig's center of mass and increases the available deck load capacity. Because the AHD is a single system, it lacks redundancy. If it fails -by a power shut down or another failure- it simply stops. The dual drawworks overcomes this drawback, provided that the two units are controlled and powered from independent sources. Figure 1 shows a schematic view of the dual drawworks concept. The two units, which are single gear drawworks driven by AC motors, can be located on opposite sides of the rig (as shown here), or stacked on top of each other. The latter option gives a smaller footprint but an uneven load on the derrick. Location on the same side also requires that the direction of drill-line bending must be flipped over. Hoist and trip speed capacity The key data for the dual drawworks, used as an example in this paper, are listed in table 1. The pull force versus hoisting speed is visualized in figure 2 for both dual and single modes. The intermittent duty (solid) curves are calculated with an overload factor of 140%. This overload can typically be applied in periods of 15 minutes before the motors are overheated. The capacity calculations are valid for second drum layer. The pull capacity curves reflect the torque versus speed characteristics of the AC motors, which can be run up to 2300 rpm. The differences between the continuous hold load and the low speed continuous hoist load are due to differences in the drawworks efficiency. Because synchronously operated drawworks sees only half the number of lines, the dual mode has a higher reeving efficiency than the single mode. This is clearly seen in figure 2. Also this figure shows that the speed capacity in the single mode is only 50% of the dual mode.