Rig Systems

Every drilling rig, land or offshore, is built from the same five systems.

1. Hoisting

The derrick or mast, the drawworks with its drum and brakes, the travelling and crown blocks, and the drilling line. Together they raise and lower the drill string, which weighs hundreds of tonnes, and support it while drilling.

Handling pipe — tripping in and out of the hole — is where a large share of rig time and a large share of injuries occur. Mechanised pipe handling, iron roughnecks and automated systems have substantially reduced both.

2. Rotating

Rotation is provided either by a top drive, a motor suspended in the derrick that turns the string directly, or by the older rotary table and kelly. The top drive is now standard because it allows drilling in long stands, circulation and rotation while tripping, and far better control when the hole is troublesome — a significant safety and efficiency gain.

Below, the drill string is drill pipe, heavyweight pipe, drill collars providing weight on bit, and the bottom hole assembly with stabilisers, measurement tools, motors and the bit.

3. Circulating

Mud pumps push drilling fluid down the drill string, out through the bit nozzles, and back up the annulus carrying cuttings. At surface the fluid passes through shale shakers, desanders, desilters, centrifuges and a degasser, and returns to the pits.

The circulating system is the well's lifeblood: it cleans the hole, cools and lubricates the bit, transmits data by mud pulse, and — most importantly — holds back formation pressure.

4. Power

Diesel engines driving generators, with electric motors on the drawworks, pumps and rotary. Diesel-electric power gives flexible distribution and precise control. Power demand peaks sharply during hoisting, which drives generator sizing.

5. Well control

The blowout preventer stack, with annular preventers that seal around any pipe size and ram preventers including pipe rams, blind rams and shear rams; the choke manifold; the accumulator unit storing hydraulic energy to close the stack; and the kill lines. This system is covered in detail later, and it is the system on which everything else depends.

Rig types

• Land rigs, sized by depth capability and increasingly designed to move quickly between closely spaced pad locations.
• Jack-ups — legs to the seabed, for shallow water.
• Semi-submersibles — floating, moored or dynamically positioned, for deeper water.
• Drillships — dynamically positioned, for deep and ultra-deep water, with high transit speed and large variable deck load.
• Platform rigs and tender-assisted rigs on fixed installations.

Floating rigs introduce heave compensation, a marine riser connecting the well to the rig, and a subsea BOP stack — each of which adds capability and complexity.