Sensors that you should have on the rig are as follows
Anchor tension (Heave sensor) : Use for towing and rig move operation.
Bit depth: Use for tracking bit depth
Bit rotating hours: Use for tracking rotating hours. The rotating hours will be used for drill pipe management, preventive maintenance program, etc.
Block height: Use for knowing where the block is in derrick.
Block speed: Use for knowing how fast of block while pulling up or lowering down.
Bulk sensor: Use for knowing how much bulk on the rig.
Choke pressure: Use for knowing how much casing pressure in case of well control situation
Fire Alarm: In case of emergency, fire alram will notify regarding fire on the rig.
Flow In: Flow in from mud pumps.
Flow out sensor (paddle): Flow out at flowline. It will indicate how the well bore is doting.
Gas detector and gas alarm: For detecting gas such as CO2, H2S, etc.
Gas sensor (while drilling): For detecting formation gas coming from wellbore. It’s normally installed at flowline after bell nipple.
Global Positioning System (GPS): It’s used for dectecting where the rig is and it’s very useful for remote area.
Hole depth sensor: For tracking how depth of the well we drill.
Hook load: Know how much drilling line tension.
Leg depth (Jack up rig): Know how depth of the legs are.
Pit volume sensor: Know pit volume on the rig.
Pump stroke: Know how fast we pump.
Rig air pressure: Know air pressure used to supply rig equipment.
Rig water pressure: Know rig water pressure to supply water to all area of the rig.
Stand Pipe Pressure: Know how much pressure at stand pipe.
Torque sensor: How much torque we apply to make up drill pipe or torque while drilling and reaming.
Total stroke count: Track how many strokes the rig has pumped from certain period.
Trip tank sensor: Track how much mud in trip tank. This is very important while we flow check the well, pull out of hole, trip in hole, etc. Due to ability to detect small changes in trip tank, trip tank can tell us how the well dictate to us.
Voltage for jacking system: For leg jacking operation.
Weather condition: Know how current weather condition on the rig and adjacent area.
Weight on Bit: Know how much weight applied to the bit.
Wind direction sensor: Know where the wind direction is and how fast wind blows.
Since I’ve not been working on drill ship or semi rig, please fell free to add your know ledge. This will help share knowledge to other people.