Weight Indicator

Key Takeaways

• Weight on bit (WOB) is calculated by subtracting the measured hook load from the drill string's off-bottom weight — WOB = (drill string weight × buoyancy factor) - hook load; when the driller lowers the bit to bottom and sees the hook load begin to decrease, the decrease represents weight being transferred to the bit through the drill collars; if off-bottom hook load is 300,000 lbs and the driller sets down weight until the indicator reads 280,000 lbs, WOB is 20,000 lbs; this calculation is performed continuously by the driller during drilling to maintain the target WOB specified in the drilling program, and in modern automated drilling systems the weight indicator feeds directly into the automated WOB control loop.
• Overpull monitoring with the weight indicator is one of the primary stuck pipe detection methods — in normal drilling operations, the hook load gradually increases as more drill pipe is added to the string and decreases slightly as the bit drills ahead; sudden inability to set the expected amount of weight (the bit won't take weight normally) or inability to pick up the string (hook load exceeds the expected off-bottom weight for the current drill string inventory) are both warning signs that pipe movement is restricted; the driller compares the actual hook load to the expected value continuously during connections and tripping, and any discrepancy above a few thousand pounds triggers immediate investigation; early recognition of restricted pipe movement — before the pipe is fully stuck — gives the crew the opportunity to work the pipe free without the jarring operations and spotting fluid treatments required after full sticking.
• The dead-line anchor is the physical connection between the drilling line tension and the weight indicator reading — the drilling line runs from the drawworks drum through the crown block (stationary sheaves at the top of the derrick) and the traveling block (moving sheaves connected to the hook) with the dead end of the line anchored to the derrick base through the dead-line anchor; the tension in the dead end of the line is proportional to the total hook load (by the number of lines strung through the traveling block), and the dead-line anchor's load cell measures this tension; calibration of the dead-line anchor to the actual drill string weight (confirmed by weighing the string in air when first made up) establishes the reference for all subsequent hook load readings during the well.
• The weight indicator must be zeroed and calibrated to reflect current drill string inventory — as tubulars are added to or removed from the drill string during make-up or lay-down operations, the nominal "off-bottom" hook load changes; the driller must know the expected hook load for the current drill string composition to interpret the weight indicator correctly; modern drilling information systems automatically track drill string inventory and calculate the expected hook load using the known weights per foot of each tubular component, the current mud weight for buoyancy correction, and the total depth to provide a continuously updated reference against which the actual weight indicator reading is compared.
• Derrick and traveling equipment load ratings must not be exceeded as read on the weight indicator — every component in the hoisting system (drilling line, crown block, traveling block, hook, swivel, and derrick structure) has a rated working load limit; the weight indicator provides the real-time monitoring that prevents these limits from being exceeded during operations; overpull events where the driller applies upward tension above the drill string's expected weight to free stuck pipe must be managed within the equipment load ratings, which is why drilling programs specify maximum allowable overpull values and drillers are trained to recognize and respect them.