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Instrumentation & Control
Tank and Volume Monitoring
Tank and volume monitoring covers pit level sensors, pit volume totalizers (PVT), trip tank systems and level transmitters on mud, brine and base oil tanks. A PVT adds the active pit levels into one volume and alarms on gains and losses, the main surface sign of a kick. Float, ultrasonic and radar sensors each suit different tanks, so match a used sensor to its tank and totalizer.
Operating principle
Each active pit has a level sensor. The totalizer converts levels to volume with the tank's strapping table and adds them into total active volume. An unexplained gain can mean formation fluid entering the well, and a fall can mean losses. The driller sets gain and loss alarm limits on total active volume, and many systems also alarm on changes in flow out.
A trip tank is a small, tall tank used while tripping. Its level sensor shows whether the hole takes the right volume of mud as pipe comes out, and gives back the right volume as pipe goes in. Because trip tank volumes are small, its sensor needs fine resolution.
Sensor types
| Type | How it measures | Strengths and limits |
|---|---|---|
| Float | Float position on a guide or arm | Simple, can stick in heavy or gelled mud |
| Ultrasonic | Echo time of a sound pulse | Low cost, upset by foam, vapour and condensation, has a blind zone |
| Radar | Echo of a microwave signal | Copes with foam and vapour, higher cost |
| Differential pressure | Hydrostatic head | Depends on mud density, so needs density input |
Higher-frequency radar, such as 80 GHz, gives a narrower beam from a small antenna, which suits tall, narrow trip tanks with internal obstructions. Ultrasonic sensors need temperature compensation, because the speed of sound changes with air temperature.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Range and blind zone | Must cover the tank from full to empty | Metres or feet |
| Beam angle | Avoids false echoes from agitators and walls | Degrees |
| Output | Matches the totalizer | 4 to 20 mA, HART, Modbus |
| Ex marking | Pit rooms are usually classified | Zone 1 or Zone 2 |
| Tanks and pumps supported | Sets the size of the system | Number of pits and pump stroke inputs |
Standards and certification
Pit rooms and the shaker house are usually Zone 1 or Zone 2, so sensors need ATEX or IECEx certification for the zone. Radar sensors also need radio approval for the country or region where they operate. NORSOK D-001 sets drilling instrumentation requirements, including trip tank monitoring, where an operator applies it. If a level signal feeds an automatic action, IEC 61511 applies to that function.
Failure modes and inspection
- Mud build-up on the sensor face or float, which gives false or frozen readings.
- Condensation on ultrasonic transducers.
- Agitators, guns and baffles producing false radar or ultrasonic echoes.
- Cable and gland damage in wet pit rooms.
- Strapping tables that no longer match a modified tank.
Test each sensor against a measured level at several points, review echo curves on radar and ultrasonic units, and check alarms trip at their set points.
Documentation
Ask for the sensor model and Ex certificates, calibration records, the tank strapping tables, the totalizer configuration and software version, the licence, and wiring diagrams. A complete used PVT is worth more with its tank geometry and configuration, because rebuilding them takes time on site.
Integration and compatibility
A replacement sensor must give the output and range the totalizer expects. Switching from floats to radar usually means reconfiguring the totalizer. On a floating rig, heave and roll move pit levels, so the system needs averaging or several sensors per pit. Pump stroke counters feed flow-in, so check that inputs match your pumps.
Refurbishment and when used makes sense
Used level sensors are good spares when they match your installed type and output. Unused surplus radar suits a rig moving off floats, as long as the totalizer accepts the signal. A full overhaul replaces gaskets, cleans the sensor faces, recalibrates against measured levels and tests every alarm.
Related equipment
The tanks are under mud mixing and storage, and flow-out sensors under pressure and flow monitoring. Naventr can arrange an inspection of a PVT before you decide.
FAQ
Frequently asked questions
How does a pit volume totalizer detect a kick?
It alarms when total active pit volume rises or falls past a set limit. The PVT adds the level readings from all active pits. An unexplained gain can mean formation fluid entering the well, which the crew confirms with a flow check.
Radar or ultrasonic for mud pit level sensors?
Radar copes better with the foam, vapour and condensation over a mud pit, and narrow-beam radar suits trip tanks. Ultrasonic sensors cost less and work well in clean conditions, but they have a blind zone near the sensor face.
What should come with a used PVT system?
The sensors, display or computer, cabling, software licence, and the tank configuration with strapping tables. Without the tank data, the totalizer has to be set up again on site, which takes time and measured fill tests.
Why does a trip tank need a different level sensor?
A trip tank holds small volumes in a tall, narrow shape, so it needs finer resolution and a narrower beam than an active pit. Narrow-beam radar or a dedicated float system avoids echoes from the tank walls.
How do floating rigs handle pit level readings in heave?
They average readings over time and often use more than one sensor per pit. Vessel motion moves the mud surface, so a single snapshot reading would trigger false alarms. Check that a used system offers motion filtering if you work on a floater.
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