Kelly Down

Key Takeaways

• The kelly is now uncommon in modern drilling operations because top drive systems (which replaced the kelly-rotary table system for most offshore and many onshore drilling operations beginning in the 1980s and 1990s) have eliminated the kelly-down limitation by providing a powered rotary unit that hangs from the traveling block and can drill a complete stand (approximately 90 feet or 27 meters for a triple stand) without requiring a connection, reducing connection frequency from every 30 to 40 feet to every 90 feet and reducing the associated trip time from rig floor operations; however, the kelly remains in service on many onshore rigs (particularly smaller rigs used for shallow or medium-depth wells in North America, Latin America, the Middle East, and developing-country markets where the capital cost of a top drive is not justified by the well program), and the kelly-down condition remains a relevant operational concept for engineers and drillers working on these rigs; the kelly has the advantage of mechanical simplicity and low maintenance cost compared to a top drive (which has electric motors, gearboxes, swivel seals, and hydraulic controls that require regular maintenance), making it preferred in low-well-cost environments where rig utilization rates are low and reliability of a complex power tool is a concern.
• Connection time and frequency during kelly drilling are the primary metrics of drilling efficiency affected by the kelly-down cycle: a 40-foot kelly drills 30 to 35 feet per connection (the usable stroke is less than the full kelly length because the top sub and kelly cock occupy some of the available length), requiring a connection every 30 to 35 feet; at a penetration rate of 60 feet per hour and a connection time of 10 minutes, connections consume 60/35 x 10/60 = approximately 17 percent of rig time (10 minutes of connection per 35 feet, which takes 35 minutes at 60 ft/hr); at a higher penetration rate of 180 feet per hour (achievable in soft formations), the same connection frequency becomes 60/35 x 10/60 = approximately 50 percent of rig time dedicated to connections rather than drilling; this makes connection time a major driver of well cost in fast-drilling sections, and explains why top drive adoption was economically compelling in high-ROP environments -- the ability to drill three joints (90 feet) between connections rather than one joint (30 to 35 feet) cuts connection frequency to one-third and reduces the connection-related non-productive time proportionally; in addition to the direct time saving, top drive connections are made with the string already hung in the slips and the top drive disconnected, enabling single-operator simultaneous management of the connection while other rig operations continue, whereas kelly connections require sequential, manual operations by the driller and floor hands.
• The kelly drive system consists of the kelly itself, the kelly drive bushing (which engages the kelly's polygonal profile and transmits torque from the rotary table while allowing axial movement), the kelly saver sub (a short sub threaded between the bottom of the kelly and the first drillpipe joint that absorbs the repeated makeup and breakout cycles that would otherwise wear the kelly box thread), and the kelly cock (a full-bore ball valve or gate valve located above or below the kelly that can be closed in the event of a well control emergency, preventing flow from the drillstring through the kelly to the rig floor while the kelly is in use): the kelly cock is typically positioned at both the upper and lower ends of the kelly (upper kelly cock for closing when the kelly is on the slips, lower kelly cock for closing when the kelly is engaged in the drive bushing), providing positive pressure shutoff capability at any point in the connection cycle; the kelly saver sub extends the service life of the kelly by bearing the connection makeup loads, since the kelly box thread (the female thread at the bottom of the kelly) is much more expensive to repair or replace than the kelly saver sub; kelly wear is tracked by periodic measurement of the kelly outside diameter at the drive bushing engagement zone (the section that passes through the drive bushing), with replacement indicated when the wear causes excessive play between kelly and bushing that allows vibration and chatter that can damage both the kelly and the drive bushing.
• Kelly-to-top-drive conversion on existing rigs is one of the most common rig upgrade paths for operators seeking to improve drilling efficiency without purchasing a new rig: the conversion involves removing the kelly, drive bushing, and rotary table power transmission equipment and installing a top drive unit (a powered rotating swivel, typically 1,000 to 1,500 horsepower for modern onshore rigs) that hangs from the traveling block and connects directly to the drillstring through a top drive sub; the top drive can be rented (reducing capital commitment for short-term projects) or purchased (reducing operating cost for long-term programs); the conversion also typically requires upgrading the mast or derrick structure (since the top drive adds weight to the traveling block system), upgrading the drawworks braking capacity (since the top drive adds rotational inertia to the block load), and installing a torque track (a guide rail running the full height of the derrick that prevents the top drive from rotating as the drillstring torque reaction is absorbed) or a torque bushing system; the economic payback for a top drive conversion depends on the penetration rate in the formations being drilled, the connection time savings per trip, and the rig utilization rate, with payback periods typically ranging from 3 months (high-ROP, high-utilization programs) to 3 years (slow-drilling programs with long kelly connections).
• Safety implications of the kelly-down condition are particularly significant in well control scenarios where the driller must shut in the well quickly while making a connection: during a connection (when the kelly is broken out from the drillstring and the top drive or kelly is lifted above the rig floor), the drillstring is open at the top (the connection above the first drillpipe joint is exposed to the rig floor atmosphere) and a kick (formation fluid influx) that occurs during the connection cannot be safely controlled until the driller picks up the kelly, makes up the connection, and activates the annular preventer or pipe rams; this sequence takes 3 to 10 minutes in routine connection operations, during which an undetected kick can grow substantially; to minimize this risk, drillers are trained to monitor pit levels and gas indicators continuously during connections, to fill the hole with drilling fluid before breaking out the kelly (to prevent the hole from surging as the drillpipe weight is removed from the column), and to make connections as quickly as possible; top drive systems reduce this risk by allowing the connection to be made in the closed position (the top drive saver sub can be kept stabbed in the drillstring with the top drive valve closed until the connection is made), but the fundamental risk of open-hole connection is present in any drilling system that requires periodic breaking of the drillstring for pipe addition.