Cementing Units

What a cementing unit does

The unit mixes dry cement from the rig's bulk system with mix water into slurry at a set density, then pumps it down the casing behind a plug. A recirculating mixer blends cement and water through a jet and recirculates slurry through a densitometer so the operator can hold density. Batch mixers prepare a full volume in advance for critical jobs. Displacement tanks measure the fluid that pumps the plug to the float collar.

Between cement jobs, the unit does high-pressure work for the rig: casing and BOP pressure tests, leak-off and formation integrity tests, and pumping kill fluid in a well control event.

Configurations

• Twin pump units with two triplex plunger pumps, which give redundancy during a job.
• Electric drive from the rig's AC supply through variable frequency drives, or diesel engine drive with transmissions.
• Recirculating mixers, with or without batch mixing tanks.
• Local or remote control, with automatic density control and job data recording.

Key specifications

Standards and certification

API Spec 10A covers the well cements themselves. Class G and Class H are the common oilwell cements offshore. The unit's high-pressure manifold and treating iron must be rated and tested to the unit's working pressure, and API RP 7HU1 covers safe use of 2 in hammer unions. Relief valves protect each pump discharge. The skid needs a DNV 2.7-1 or 2.7-3 certificate offshore, and the engines and electrics need ratings for their hazardous area zone.

Failure modes and inspection

• Fluid end blocks crack from pressure cycles, usually at the intersecting bores. Inspect with dye penetrant or magnetic particle testing at each valve and seat change.
• Valves, seats, plungers and packing wear. Their change records show how hard the pumps have worked.
• Power end bearings and crossheads wear. Oil analysis and hours tell you how far.
• Treating iron erodes and cracks. Ultrasonic wall checks and hydrostatic tests catch it.
• Densitometers and flow meters drift and need calibration.

Years of casing tests and kill pumping can put more fluid end cycles on a unit than its cement job count suggests.

Documentation

• Fluid end and power end service history, including valve and seat changes and any block replacements.
• Hydrostatic test charts and wall thickness readings for the manifold and treating iron.
• Relief valve test certificates.
• Engine or motor hours, drive service records and oil analysis.
• Densitometer and flow meter calibration, and the control software version.
• DNV skid certificate and Ex certificates.

Integration with your rig

Match an electric unit's power demand, voltage and frequency to the rig's supply and spare switchboard capacity. A diesel unit in a hazardous area needs intake shutdown valves and spark arresting exhausts. Check bulk cement and mix water feed lines, the line to the rig floor cement head, and links to the rig's data system. Some units use a nuclear densitometer with a licensed sealed source, which is usually removed before sale. Budget for your own licence or a non-nuclear densitometer.

Refurbishment and when to buy what

A unit from a stacked rig can move to another rig with the same power supply and deck layout. Converting between electric and diesel drive means a new power package, new controls and often a bigger footprint, so it rarely pays. A refurbished unit with new or inspected fluid end blocks, recertified iron and calibrated density control lowers startup risk. SLB, Halliburton and Baker Hughes build offshore cementing units. For the float equipment and centralizers run with the casing, see Casing Equipment.