Drilling Fluid Systems

Two pressure regimes in one loop

The mud system is a closed loop with one high-pressure section. Centrifugal charge pumps feed the mud pumps from the active pits. The mud pumps discharge at up to 5,000 or 7,500 psi (34.5 or 51.7 MPa) through the standpipe manifold and rotary hose into the top drive and drill string. Returns reach the surface at close to atmospheric pressure and pass through the flow line to the shakers, degasser, hydrocyclones and centrifuges before they reach the pits again.

The split matters when you assess used equipment. On the high-pressure side, a hydrostatic test chart and traceable pressure ratings decide whether a unit can go into service. On the return side, condition, capacity and hazardous area certification count for more than pressure rating. The mud gas separator is the exception, a low-pressure vessel whose design pressure and vent line still have to be checked like any other pressure vessel.

How the sub-Categories compare

Choosing across the group

Hold the pressure chain together

The lowest-rated item between the pump fluid end and the drill string sets the working pressure of the whole line. That chain includes the pulsation dampener, the discharge relief valve, the discharge piping, the standpipe manifold valves, the rotary hose, the gooseneck and the wash pipe on the top drive. If you fit 7,500 psi pumps to a 5,000 psi standpipe, the relief valves must be set to protect the 5,000 psi items, and the extra rating is wasted.

Size the return side for peak flow

Shakers and degassers must pass the highest circulating rate the pumps can deliver, with mud at its heaviest and the finest screens in use. Adding a third pump or larger liners without checking shaker capacity sends whole mud over the screens and loses it with the cuttings.

Check the mud type

Oil-based and synthetic-based mud attack some elastomers, so pump pistons, valve inserts, hose liners and seals must be rated for the base fluid. These mud types also bring cuttings discharge limits that may call for a cuttings dryer.

Standards and class across the group

• API Spec 7K covers piston mud-pump components, high-pressure mud and cement hoses, and pressure-relieving devices for high-pressure drilling fluid systems.
• API RP 7L sets inspection, maintenance, repair and remanufacture practice for equipment in the 7K scope.
• API Spec 6A is the usual design basis for standpipe manifold gate valves. Its rated working pressures are 2,000, 3,000, 5,000, 10,000, 15,000 and 20,000 psi, so a 7,500 psi mud valve is built to an OEM rating rather than a 6A class.
• API Spec 16C covers choke and kill equipment, including flexible choke and kill lines. A 16C line is not a substitute for a 7K rotary hose.
• API RP 13C defines how shaker screens are tested and labelled and how processing equipment is evaluated.
• DNV-OS-E101 (drilling facilities) and the ABS Guide for the Classification of Drilling Systems apply on units with a DNV DRILL or ABS CDS notation. A replacement pump, manifold or hose on such a unit needs certification acceptable to that society.

The IMO MODU Code and IEC 61892-7 class enclosed spaces holding open parts of the mud system as hazardous areas. A shaker room, which handles mud before the final degassing point, is normally Zone 1. An enclosed pit room downstream of the degasser is normally Zone 2, unless gas can collect there. Every motor, junction box and instrument in these spaces needs Ex certification to the IEC 60079 series or ATEX for its zone, gas group and temperature class.

Integration points that cut across the group

• Power. Most modern offshore pumps run on AC motors fed by variable frequency drives. Older rigs use DC motors on SCR drives. A pump from one system needs new motors or drives to run on the other.
• Supply frequency. Shaker vibrators, agitators and centrifugal pumps built for 50 Hz run faster on a 60 Hz supply, which changes G-force, head and motor load.
• Footprint and weight. Check deck load, skid size and lifting points against the space you have, especially in a pit room built into the hull.

Sourcing used fluid-system equipment

Value sits in a few parts. On a mud pump it is the power end, on a manifold the piping and valve bodies, on a centrifuge the bowl, conveyor and gearbox. Liners, pistons, valves, screens and hose assemblies are consumables, so price a used unit as if you will replace them. Unused surplus from cancelled newbuilds and stacked rigs is common in this group, but elastomers and hoses age on the shelf, so read build dates. Naventr can arrange an inspection on request and answer your questions from the Seller's records before you commit.