Motion Compensators

Operating principle

A passive compensator holds the string on hydraulic cylinders backed by compressed air or nitrogen in air pressure vessels (APVs). The gas acts as a soft spring. As the rig heaves up, the cylinders stroke and the hook stays near its position. The pressure in the APVs sets the load the system supports. Friction and gas compression mean passive systems never cancel heave completely.

An active system adds a hydraulic actuator or servo-controlled cylinder driven from a motion reference unit (MRU). It corrects the remaining motion, which matters when landing a BOP, a tree or a completion.

Configurations

• Crown-mounted compensators (CMC) put the cylinders under a floating crown block at the top of the derrick.
• Drill string compensators (DSC) hang inline between the traveling block and the hook.
• Active heave drawworks do the job with drawworks motors, with no separate compensator.
• Active heave assist adds an active cylinder or control layer to a passive CMC or DSC.

A CMC keeps cylinders and hoses away from the moving block, but puts weight at the crown. A DSC is easier to reach for maintenance but adds length under the block and swings with it.

Key specifications

Standards and certification

API 8C covers drill string motion compensators as hoisting equipment, and API RP 8B covers their inspection and repair in service. The APVs and accumulators are pressure vessels, built to a pressure vessel code such as ASME Section VIII or the EU Pressure Equipment Directive, and need periodic inspection under that code or class rules. On a classed rig, DNV or ABS surveys the compensator as part of the drilling plant.

Failure modes and inspection

• Cylinder rod coating damage, scoring and pitting, which destroy seals.
• Seal leaks, seen as falling gas pressure or hydraulic fluid loss.
• Internal corrosion in APVs, checked by internal inspection or ultrasonic thickness readings.
• High-pressure hose ageing. Hoses have OEM replacement intervals regardless of appearance.
• Cracks in the crown cradle, lock bars, chains or yokes, found by MPI.
• Sensor and MRU faults on active systems.

Survey work includes a pressure test of the APV system, a function test through full stroke, a lock and unlock test, and NDT of the load path.

Documentation

• API 8C nameplate data and the compensator's rated loads.
• Pressure vessel certificates, with the last internal inspection and test dates.
• Hose register with installation dates.
• OEM service history and the last cylinder reseal or rod refurbishment.
• Control software version and active heave tuning records.
• Class certificates and the last survey on a classed unit.

Integration and compatibility

A compensator is designed around its derrick, crown and hoisting system. A CMC needs a crown frame built for its weight and cylinder layout. A DSC needs block and hook connections that match it and enough derrick height for its length. Active systems need MRU input and an interface with the drilling control system. Fitting a different compensator type is a rig modification that needs engineering and class approval.

Refurbishment and recertification

A typical overhaul reseals cylinders, regrinds or recoats damaged rods, renews hoses, inspects and recertifies APVs, runs NDT on the load path and ends with a function test. Spare cylinders and APVs are a common unused surplus item. Check that their pressure vessel certificates are still valid or can be renewed by the original code authority.

Common offshore compensator OEMs include NOV and HMH. For riser tension rather than string compensation, see riser tensioners. For the motor-driven alternative, compare drawworks.