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Mud Treatment Systems
Mud treatment systems take gas out of drilling mud and reduce the oil on drilled cuttings before disposal. This Category covers mud gas separators, also called poor boy degassers, vacuum and centrifugal degassers, and cuttings dryers, including vertical basket dryers that recover oil-based and synthetic-based mud from shaker cuttings.
What each unit does
A mud gas separator is a vertical vessel downstream of the choke manifold. During a kick it takes choke returns, drops the mud onto baffles or an impingement plate so free gas breaks out, and vents the gas up a vent line to a safe height. A liquid seal, or mud leg, at the bottom keeps gas from following the mud to the shakers.
A degasser removes the small bubbles still entrained in gas-cut mud. In a vacuum degasser, a vacuum pump lowers the pressure in the tank and the mud spreads in a thin film over internal plates so the gas expands and escapes. A centrifugal degasser uses spin instead. The degasser sits after the shakers and ahead of the hydrocyclones, because gas-cut mud makes their feed pumps lose suction.
A cuttings dryer is a vertical basket centrifuge. It spins cuttings from the shakers against a fine screen basket, recovering mud and lowering the oil on cuttings before disposal or discharge.
Configurations
- Mud gas separators. Open-bottom or closed-bottom vessels with a U-tube liquid seal, sized for the rig's kick tolerance and vent line. Some carry an H2S service rating.
- Degassers. Vacuum units, horizontal or vertical, and centrifugal units mounted on the pit.
- Cuttings handling. Vertical basket dryers are the common offshore unit. Thermal desorption units, which heat cuttings to drive off residual oil, are a separate class of plant.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Separator diameter and height | Set gas handling and retention time | in (mm), ft (m) |
| Liquid seal height | Sets the back pressure the seal holds | ft (m) of mud |
| Vent line bore | Sets friction pressure at peak gas rate | in (mm) |
| Separator design pressure | Limits safe operation | psi (bar) |
| Degasser throughput | Must match circulating rate | gpm (m³/h) |
| Dryer capacity and oil on cuttings | Sets the discharge you can meet | t/h, percent by weight |
Standards and certification
A mud gas separator is a pressure vessel, often built to ASME Section VIII Division 1. G.R. MacDougall's SPE paper 20430, which API 53 refers to for separator sizing, is the common sizing method. It treats blow-through as the failure to design against. If vent line friction pressure at peak gas rate exceeds the hydrostatic pressure of the mud leg, gas reaches the shakers. On a classed unit the separator and vent line normally fall within the DNV or ABS drilling plant scope. Equipment in sour service needs materials qualified to NACE MR0175 / ISO 15156. Degasser and dryer motors need IEC 60079 or ATEX certificates for the zone they sit in, which is usually Zone 1 near the shakers and degasser.
Cuttings discharge limits are set by the region. In the OSPAR area, OSPAR Decision 2000/3 bars discharge of cuttings contaminated with oil-based fluid at more than 1 percent oil by weight on dry cuttings, which most dryers cannot reach without further treatment.
Failure modes and inspection
- Separator shell and internals. Erosion concentrates at the inlet, impingement plate and baffles. Survey wall thickness by ultrasonic testing and inspect internals through the manway.
- Liquid seal. Solids settle in the mud leg and drain. A plugged leg loses its seal height.
- Vacuum degassers. Check the vacuum pump, the float or level control and the discharge eductor nozzle.
- Cuttings dryers. Screen baskets wear and tear. Check flights, bearings and gearbox, and ask for vibration readings at operating speed.
Documentation
For a mud gas separator, ask for the pressure vessel data book with design code and calculations, the nameplate, the last hydrostatic test, a recent wall thickness survey and the original sizing calculation with its design gas rate. For degassers and dryers, ask for motor Ex certificates, the overhaul history and the dryer's oil on cuttings test results.
Integration and compatibility
Check that the separator's inlet matches your choke manifold outlet and its vent line can be routed to the derrick or a safe vent point at the bore it was sized for. Rerun the sizing with your own mud weight, kick volume and vent line length, because a separator sized for another rig may blow through on yours. A dryer needs a cuttings feed from the shakers and a route for recovered mud.
Refurbishment and recertification
A used separator needs internal inspection, thickness survey and a new hydrostatic test before service. Degassers rebuild simply with new pumps, seals and level controls. A cuttings dryer overhaul replaces the basket, bearings and gearbox parts and rebalances the rotor. The separator takes returns from the choke and kill manifolds, and dryers process cuttings from the solids control equipment. SLB's M-I SWACO makes D-GASSER degassers, mud gas separators and VERTI-G dryers.
FAQ
Frequently asked questions
What is the difference between a mud gas separator and a degasser?
A mud gas separator handles large volumes of free gas from the choke manifold during well control, while a degasser removes small entrained bubbles from gas-cut mud in the pits. The separator is a pressure vessel with a vent line and liquid seal. One cannot do the other's job, so most offshore rigs carry both.
How do I check whether a used mud gas separator is the right size?
Rerun its sizing with your own peak gas rate, mud weight and vent line length, using the SPE 20430 method that API 53 refers to. The liquid seal must hold more pressure than vent line friction at peak gas rate, or gas blows through to the shakers. Ask for the original calculation.
What documents should come with a used mud gas separator?
A used mud gas separator should come with its pressure vessel data book with design code and calculations, the nameplate, the last hydrostatic test certificate, a recent ultrasonic thickness survey and the original sizing calculation. Without the sizing basis you cannot judge whether it suits your wells.
What does a cuttings dryer do?
A cuttings dryer spins oily cuttings from the shakers in a fine screen basket to recover drilling fluid and lower the oil on cuttings. It returns usable mud to the system and reduces waste volume. Where the local limit is very low, such as the 1 percent OSPAR limit, dried cuttings still need further treatment or containment.
Can a standard mud gas separator be used in H2S service?
Only if its materials are qualified to NACE MR0175 / ISO 15156 and its vent arrangement suits sour gas. Check the data book for material certificates and the sour service statement. A separator built for sweet service cannot be upgraded by paperwork alone.
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