Mast

Key Takeaways

• The hook load rating of a mast is its most important structural specification and determines the maximum suspended weight the rig can safely handle during drilling and completion operations — the hook load is the sum of the drill string or casing string weight in air, minus the buoyancy of the drilling fluid acting on the submerged string, plus dynamic loads from acceleration during tripping; modern land drilling masts are rated from 500,000 pounds for light-duty workover masts to 2,000,000 pounds (2 million lb) or more for deep-well drilling masts designed to handle heavy casing strings in wells exceeding 20,000 feet; the hook load rating must be matched to the heaviest casing string in the well design plus an adequate safety margin (typically 10-15%) for dynamic loading; mast ratings are verified through static load testing performed by the manufacturer and periodic recertification by third-party inspection, with any mast that has experienced an overload event, collision, or structural damage requiring engineering recertification before being returned to service.
• Mast erection using the rig's hydraulic system is a defining feature that distinguishes it from the conventional derrick, and the erection procedure is a high-risk operation that must follow a strict sequence to prevent structural overload and collapse — a typical mast erection sequence begins with the rig positioned on the pad with the substructure leveled and pinned, the mast in the horizontal position resting on the top deck of the substructure, hydraulic rams extended slowly to raise the mast from horizontal to vertical while checking structural alignment at each stage, the mast pinned or latched at the top of the erection travel, and the drillfloor and crown equipment connected before the rig is declared operational; mast erection is performed in daylight under calm wind conditions (most manufacturers specify a maximum erection wind speed of 20-30 mph) because crosswind loading on the mast during erection creates bending moments that are not accounted for in the erection load case; fatalities have occurred when masts were erected improperly, without appropriate blocking to prevent hydraulic failure from dropping the mast, or in excessive wind conditions.
• The fingerboard capacity of the mast determines how many stands of drill pipe can be racked back (set back vertically in the derrick) rather than laid down to the pipe rack during a trip — racking back stands saves time because stands do not need to be broken back into individual joints when re-running the string; a mast fingerboard that can handle 450,000 pounds of setback load can hold approximately 600 stands of 5-inch drill pipe (a 30-foot stand weighing approximately 750 pounds), enough for a moderately deep well; for wells where the drill string weight in stands exceeds the fingerboard capacity, the excess string must be laid down rather than racked back, adding significantly to trip time; fingerboard capacity and hook load are both listed in rig specifications and both must meet the well design requirements for the rig to be considered fit for purpose on a given well program.
• Pad drilling programs in shale plays have driven the development of "walking" and "skidding" rigs whose masts are designed to move from well to well on the pad surface without being laid down between wells — a walking rig uses hydraulic legs to lift the entire rig off the pad and "walk" it laterally in increments of a few feet at a time, repositioning from one well center to the next in two to six hours without dismantling any substructure components; a skidding rig slides laterally along rails installed on the pad surface, achieving similar relocation speed; both systems rely on the mast's ability to maintain its erected configuration during the pad move, placing structural requirements on the mast's rigidity and the substructure connection design that fixed-location masts do not share; the economic value of rig walking capability in a 20-well pad program is substantial — if each conventional rig move takes 3 days and a walking rig move takes 6 hours, the walking rig completes 19 moves in about 5 days versus 57 days for a conventional rig, enabling the operator to complete the pad program months earlier and begin production sooner.
• Crown safety devices and mast load monitoring systems are the primary engineering controls that prevent two of the most serious mast-related accidents in drilling operations — crown collision (where the traveling block is unintentionally run into the crown block at high speed, subjecting the mast to a dynamic load that can shear the crown or collapse the mast) and setback overload (where excess pipe weight is racked back in the fingerboard beyond its structural capacity, causing progressive structural failure); crown savers use proximity switches or encoder-based position monitoring to automatically slow and stop the drawworks before the traveling block can reach the crown; setback weight indicators use load cells on the fingerboard beams to continuously display the racked pipe weight and alarm when it approaches the rated capacity; both systems have been industry-standard safety requirements on new drilling rigs for decades, and IADC well control and rig inspection protocols verify their calibration and function before any rig begins a new well program.