Drilling Instrumentation

Operating principle

Hookload is usually measured as tension in the deadline. A hydraulic load cell at the deadline anchor, or a clamp-on sensor on the line itself, turns that tension into fluid pressure. A weight indicator gauge reads the pressure on a dial scaled for the number of lines strung. Hookload is roughly the deadline tension times the number of lines, corrected for sheave friction. Weight on bit is the difference between off-bottom and on-bottom hookload.

Electronic systems add a pressure transducer to the hydraulic line, or use a strain-gauge load pin, and send the signal to the driller's display and recorder. Block position comes from an encoder on the drawworks drum. Top drive torque usually comes from motor current on AC drives or hydraulic pressure on hydraulic units. On a mechanically driven rotary table, a hydraulic transducer under an idler on the drive chain gives torque. Tong torque comes from a hydraulic load cell in the tong snub line.

Configurations

• Hydraulic weight indicator with a mechanical dial, still the backup on many rigs.
• Hydraulic load cell with an electronic transducer feeding a digital display.
• Load pins or strain-gauge sensors integrated into the anchor or crown.
• Tension or compression load cells, which suit different anchor designs.

Key specifications

Standards and certification

Deadline anchors are hoisting equipment under API Spec 8C, so the anchor's rated load sets the upper limit for the load cell installed on it. Sensors and transducers on the drill floor need ATEX or IECEx certification for their zone, and intrinsically safe sensors need a matching barrier. Calibration should use a reference traceable to national standards. NORSOK D-001 adds drilling instrumentation requirements where operators apply it.

Failure modes and inspection

• Diaphragm damage or fatigue in the load cell, which shows as hysteresis or a reading that will not return to zero.
• Fluid loss or air in the hydraulic line, which makes the indicator read low or sluggish.
• Damaged hoses and fittings, and fluid of the wrong type.
• Worn gauge movements, bent pointers and failed dampers on mechanical indicators.
• Encoder couplings that slip, giving a block height that drifts through a trip.

Inspect the diaphragm and housing, pressure-test the hydraulic circuit, bleed and refill to the OEM procedure, and check the gauge against a calibrated reference across its range.

Documentation

Ask for the calibration certificate with its date and reference, the load cell serial number and capacity, the anchor model it came from, the OEM fill and bleed procedure, and the Ex certificates for any electronic sensor.

Integration and compatibility

A load cell made for one anchor model may not mount on another. The indicator scale must match your string-up, or the electronic system needs the right scaling factor. Electronic sensors built for one data system often use proprietary connectors or protocols. Torque from VFD current depends on the drive's motor model, so a used torque display needs the scaling from your drive supplier.

Refurbishment and when used makes sense

A used hydraulic weight indicator in good order makes a sound backup, because the design has changed little in decades. An instrument shop overhaul replaces the diaphragm, seals and hoses, refills the system and calibrates it, and should come with a test report. Unused surplus load cells avoid diaphragm wear. Plan to calibrate the full system after installation, because the reading depends on your anchor, drilling line and lines strung.

Related equipment

Pump and standpipe pressure sensors are under pressure and flow monitoring, and records of these readings under parameter recorders. Naventr can arrange an inspection of a weight system on request.