Top-Drive Drilling System Evaluation

Abstract

Summary. Since drilling began, the industry has continuously strived to improve rotating and handling of drillpipe. With these improvements, on-bottom rotating time has been reduced to 25 to 30% of total well time, which has proved acceptable for the U.S. gulf coast. Attention is now being focused on nonrotating time, and with the introduction of the top-drive system, this time can be reduced primarily through pipe handling. Three wells were drilled from a platform in Mustang Island, offshore Texas, and the top-drive system was used and tracked for time savings vs. a typical kelly/rotary system. It has been reported that as much as 20 to 25% time savings can be obtained with the top drive. Definite time savings can be expected with the top-drive drilling system, but the magnitude reported was not experienced on the three wells discussed here. It was established that roughly 6 % of drilling cost or 4 % of total well cost could be saved with the top drive, which represents 11% of drilling time or 8% of total well time. Introduction This evaluation of the top-drive drilling system is based on use of a rig with an existing system that bears an additional day rate for the system. With the introduction of the top-drive system in April 1982, many pipe-handling operations have been improved or eliminated. The top-drive drilling system replaced the kelly/rotary-table system traditionally used (Fig. 1). The drillpipe is rotated in stands by a DC electric or hydraulic motor, and the motor with pipe handler travels up and down the derrick on guide rails. The motor is hoisted by the traveling block and is attached below the rig swivel. Traditionally, during drilling with a kelly/rotary system, the kelly had to be set back for trips and picked up when the trip to bottom was completed or when tight formation was encountered. During drilling, connections were made in the mousehole and the single was then added to the drillstring. If an interval of hole was to be opened or underreamed, that length of the interval of drillpipe would have to be laid down for the operation. Pipe must also be laid down for safety washing and reaming after trips, drilling out of casing, sidetracks, coring, and at the end of the well. Connections are reduced to one-third of the standard kelly/rotary connections when full stands are rotated with the top-drive system. When the topdrive drilling system is used on a multiwell platform, pipe does not have to be laid down at the end of the well. During drilling, the pipe is made up and broken out with the torque wrench or pipe handier (Fig. 1). To trip pipe, the top drive is backed out of the drillstring at the torque wrench, and the pipe is then hoisted by the elevators, as is done with a conventional kelly/rotary system. Because hoisting in and out of the hole is done with the top-drive system, circulation and/or rotation at any time during the trip is made possible by throwing the slips, slacking off, and screwing back into the drillstring. Application of Equipment Because of the complexity of drilling operations and the application of the top-drive system, the operations have been divided into two categories:tangible (those operations that can be measured and assigned a time reduction) andintangible (those operations that cannot be measured or assigned a definite time reduction). The tangible operations that were evaluated and compared with a kelly/ rotary system are connection time, trip time, washing and reaming, setting back and picking up the kelly, underreaming and hole opening, and laying down drillpipe. Tangible Time Savings Connection Time. Connection time was limited to slip-to-slip time because of variance in drilling environments and the amount of time spent leaving and going to bottom after the connection. Several wells have been reviewed for kelly connection time and an average ot3 minutes was recorded. This time was compared with the measured average connection time of 2.5 minutes for the top drive. Table I illustrates the time reduction in connections for each of the three wells vs. a kelly/rotary system. Trip Time. The major difference between the top-drive system and the kelly/rotary system is the elimination of the kelly and kelly handling. To trip pipe. the top drive is backed out of the drillstring at the torque wrench of hydraulic tong jaw (Fig. 1) and then hoisted to pull the drillstring. An additional feature of the top-drive drilling system is the link or bale tilt mechanism, which tilts the elevators outward (Fig. 2) toward the derrickman or mousehole. depending on application. The advantage of the link tilt is that the derrickman does not have to extend out over the monkey board to latch and unlatch stands from the elevators. Stands are rarely dropped across the derrick because the elevators are missed. The track or rails of the top-drive system permit tripping in adverse weather by preventing the travel block and elevators from swinging. For the trip factor (hours per 1,000 ft [304.8 m] for round trips below 1,000 ft [304.8 m]), the top-drive system was found to save an average of 25 minutes over the kelly/rotary system's time. This time saving was rounded to 30 minutes for ease of computations. Note that trip factors are varied and affected by the difference in rig crews. equipment, and trip speeds predetermined or specified by operators. These variances were not taken into consideration when a time-savings comparison was established. Table 2 illustrates the time reduction obtained in trip time only for all round trips below 1,000 ft [304.8 m] with the top-drive drilling system. Washing and Reaming. When this operation is performed with a conventional kelly/rotary system. two to three singles must be laid out and then picked up again to wash and ream down with several minutes of circulation between connections. The top-drive system is made up on the last stand during a trip with circulation and/or rotation instantly available to wash and ream down. If tight hole is encountered anywhere in a trip, the slips are thrown and the top drive is made up on the drillstring, allowing circulation and/or rotation to wipe out the tight interval. The top drive also allows backreaming in full stands when tight hole is encountered on a trip out of the hole, which is a time-consuming operation when done with singles with the kelly/rotary system. The top-drive system (vs. the kelly/rotary system) saved an average of 30 minutes from each washing and reaming operation by elimination of laying out drillpipe when tight spots were encountered and kelly-handling time and circulation required between singles. Table 3 illustrates the time reduction experienced for the three wells drilled. SPEDE P. 43^