Most offshore drilling rigs now have top drive systems installed in the derrick. A top drive system consists of a power swivel, driven by a 1000 hp dc electric motor. This power swivel is connected to the travelling block and both components run along a vertical guide track which extends from below the crown block to within 3 metres of the rig floor. The electric motor delivers over 25000 ft-lbs torque and can operate at 300 rpm. The power swivel is remotely controlled from the driller’s console, and can be set back if necessary to allow conventional operations to be carried out.

A pipe handling unit, which consists of a 500 ton elevator system and a torque wrench, is suspended below the power swivel. These are used to break out connections. A hydraulically actuated valve below the power swivel is used as a kelly cock.

top drive system

A top drive system replaces the functions of the rotary table and allows the drillstring to be rotated from the top, using the power swivel instead of a kelly and rotary table (Figure 13). The power swivel replaces the conventional rotary system, although a conventional rotary table would generally, also be available as a back up.

The advantages of this system are:
•     It enables complete 90′ stands of pipe to be added to the string rather than the conventional 30′ singles. This saves rig time since 2 out of every 3 connections are eliminated. It also makes coring operations more efficient
•     When tripping out of the hole the power swivel can be easily stabbed into the string to allow circulation and string rotation when pulling out of hole, if necessary (e.g. to prevent stuck pipe)
•     When tripping into the hole the power swivel can be connected to allow any bridges to be drilled out without having to pick up the kelly

The procedures for adding a stand, when using a top drive system is as follows:
1.    Suspend the drillstring from slips, as in the conventional system, and stop
circulation
2.    Break out the connection at the bottom of the power sub
3.    Unlatch the elevators and raise the block to the top of the derrick
4.    Catch the next stand in the elevators, and stab the power sub into the top of the stand
5.    Make up the top and bottom connections of the stand
6.    Pick up the string, pull slips, start pumps and drill ahead

Top drive systems are now very widely used. The disadvantages of a top drive system are:
1. Increase in topside weight on the rig
2. Electric and hydraulic control lines must be run up inside the derrick

When drilling from a semi-submersible under heaving conditions the drillstring may bottom out during connections when the string is hung off in the slips. This could be overcome by drilling with doubles and a drilling sub which could be broken out like a kelly. This method however would reduce the time-saving advantages of the top drive system.

Institute of Petroleum Engineering, Heriot-Watt University
 

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