The drilling rig and its systems

A drilling rig is a temporary factory that moves from location to location, builds a hole several kilometers long and then leaves. In a few weeks a crew raises a mast, makes up thousands of meters of pipe, circulates metric tons of drilling fluid, runs and cements casing, and hands the well over to a completion crew. Almost every serious hazard in the oil and gas industry is present on a rig at once: heavy suspended loads, rotating equipment, high-pressure fluid, work at height, flammable and toxic gas, power, noise, weather and a crowded deck where several companies work side by side. This course, the second in the oil and gas sites track, explains those hazards and the controls that keep rig and well site crews alive.

HSE-330 gave the orientation every worker needs before stepping onto a lease: the life cycle, the regulators, gas detection, hydrogen sulfide basics, pressure and stored energy, and emergency response plans. This course assumes that background and goes onto the rig floor, the derrick, the mud tanks, the service rig and the fracturing spread. HSE-332 follows with facilities and pipelines, and HSE-333 covers permits to work and isolation. Where another course already teaches a topic in depth, this course points to it rather than repeating it: HSE-212 for fall protection, HSE-219 for cranes and rigging, HSE-218 for mobile equipment, HSE-241 for noise, HSE-242 for silica and HSE-244 for hydrogen sulfide.

This first lesson walks through the rig itself. You do not need to be able to run a rig to work safely on one, but you do need to know what each system does, where its energy is stored and which parts of it can reach you. A floorhand who understands why the driller is watching the weight indicator, or why the derrickhand clips in before leaving the monkeyboard, makes better decisions than one who simply follows orders.

The five systems of a rotary rig

Every modern rotary rig, from a small single used for shallow wells to a large pad rig drilling long horizontal Montney or Bakken wells, has the same five systems. The hoisting system raises and lowers the drill string and casing. The rotating system turns the bit. The circulating system pumps drilling fluid down the pipe and back up the hole. The power system drives everything. The well control system closes in the well if formation fluids enter it. Understanding the five systems gives you a map of where the energy is.

Hoisting: the heaviest loads on site

The hoisting system is a large block and tackle. Drilling line, a steel wire rope typically 29 to 38 mm (1-1/8 to 1-1/2 in) in diameter on land rigs, runs from a storage reel through a deadline anchor, up to the crown block sheaves at the top of the mast, down through the traveling block sheaves and back several times, and finally onto the drum of the drawworks. Spooling line onto the drum lifts the traveling block; letting it off lowers the block. Because the line is strung through many sheaves, the tension in the fast line is only a fraction of the hook load, but the hook load itself can exceed 200 metric tons on a large rig. Lesson 4 works through the arithmetic.

The hoisting system holds the most gravitational energy on site. A dropped string of pipe, a parted drilling line or a traveling block that runs into the crown can kill people instantly. Rigs therefore carry several layers of protection. A crown saver or crown-o-matic device stops the drawworks if the block travels too high, and modern rigs have electronic block position limits that also protect against running the block into the floor. The driller watches a weight indicator that shows the hook load, and a sudden change in weight warns of stuck pipe, a dropped string or a parted line. The drilling line is slipped and cut on a schedule based on the work it has done, measured in metric ton-kilometers or ton-miles, so that no single section wears out at the sheaves. Inspection of the line, the sheaves, the deadline anchor and the mast itself follows the manufacturer's instructions and industry practice.

Masts and derricks are structures that are raised and lowered at every move. A conventional derrick is built up piece by piece; most land rigs today use a telescoping or folding mast that is raised in one lift by the drawworks or by hydraulic cylinders. Raising and lowering are among the most dangerous operations on a rig because the whole structure is under unusual loads and people may be tempted to stand where they should not. Lesson 9 covers rig moves, raising and lowering in detail.

Rotating: top drives, kellys and the rotary table

Older rigs turn the drill string with a rotary table set in the rig floor. A square or hexagonal pipe called the kelly passes through a kelly bushing in the table, and the table turns the kelly, which turns the pipe. Newer rigs, including nearly all pad rigs drilling horizontal wells, use a top drive: a large electric or hydraulic motor that hangs from the traveling block and screws directly into the top of the drill string. A top drive can rotate and circulate while pipe is moved up or down, which makes drilling faster and safer in many situations, but it also puts a heavy, rotating machine above the heads of the floor crew.

Whatever the drive, the floor crew handles pipe at the well center. Slips are wedges that grip the pipe in the rotary table so that the string hangs from the floor while a connection is made or broken. Tongs are large wrenches used to make up and break out threaded connections to the right torque. On older rigs the crew used manual tongs pulled by a cathead or a hydraulic cylinder; many rigs now use an iron roughneck, a machine that spins and torques connections without workers holding tongs. Automation has greatly reduced the number of hands at well center, but it has not removed people from the floor, and the energy in high-torque connections is still enough to throw a worker or crush a hand. Lesson 4 covers rig floor hazards in detail.

Circulating: the mud system

Drilling fluid, usually called mud, is pumped down the inside of the drill string, out through nozzles in the bit and back up the annulus between the pipe and the hole. It cools and lubricates the bit, carries cuttings to surface, supports the walls of the hole and, most importantly for safety, provides hydrostatic pressure that holds formation fluids in the rock. Mud pumps are large piston pumps that can deliver fluid at pressures of 3,000 to 5,000 psi (20,000 to 35,000 kPa) through the standpipe and a flexible rotary hose to the top drive.

At surface, returning mud passes over shale shakers, vibrating screens that remove cuttings, and then through degassers, desanders, desilters and sometimes centrifuges before returning to the mud tanks. The tanks hold the active volume of mud, and a pit volume totalizer measures it. A gain in pit volume is one of the clearest signs that formation fluid is entering the well, which is why the mud system is part of well control (Lesson 8). The mud system also brings chemical hazards to the rig: oil-based fluids, caustic additives, barite and other powders, hydrogen sulfide in returns from sour zones, and hydrocarbon vapors at the shakers. Lesson 7 covers those exposures.

Power

Most land rigs are powered by large diesel engines, increasingly with natural gas or dual-fuel engines and sometimes with grid power or battery storage where it is available. The engines drive generators, and an SCR or VFD house converts their output to run the drawworks, top drive, mud pumps and rotary table. Rigs also carry air compressors, hydraulic power units, boilers or steam generators for winter heating, and many electric motors. Power brings electrical hazards (HSE-214), hot exhaust surfaces, fuel handling, noise above 100 dBA near engines and generators (HSE-241) and, for gas-fueled rigs, gas supply piping on the lease. The motorhand looks after the power system and is often the first to notice abnormal engine behavior, which can be an early sign of gas reaching an engine air intake.

Well control equipment

The well control system is the last barrier between formation pressure and the people on the lease. The primary barrier is the hydrostatic pressure of the mud column. If that barrier fails and formation fluid enters the well, called a kick, the crew shuts the well in using the blowout preventer stack. A typical land stack has an annular preventer at the top, which seals around any shape of pipe, and two or more ram preventers below it: pipe rams that close around a specific size of pipe and blind or blind-shear rams that close the open hole or cut the pipe. An accumulator unit, often called a koomey after an early manufacturer, stores hydraulic fluid under nitrogen pressure so that the preventers can close even if rig power fails. A choke manifold lets the crew control the flow from the well while they circulate the kick out, and a mud-gas separator, or gas buster, separates gas from the returns and sends it to a flare.

The BOP stack and accumulator are pressure equipment in their own right. Accumulator bottles hold nitrogen precharged to around 1,000 psi (7,000 kPa) and charged with fluid to 3,000 psi (about 21,000 kPa). Lines from the accumulator to the stack, from the stack to the choke manifold and from the manifold to the flare carry fluid at well pressure during a kick. Function tests and pressure tests of this equipment are frequent and are themselves hazardous operations that call for barricades and clear communication. Lesson 8 explains well control at the level every crew member needs.

Kinds of rigs

Not every rig drills new wells, and the hazards shift with the type of rig.

Pad drilling has become the norm in the Montney, the Duvernay, the Bakken and the Permian. A walking rig can move from one well to the next on the same pad in hours rather than days, which reduces rig moves on the highway but adds a new hazard: drilling a new well a few meters from wells that may already be completed or producing. Simultaneous operations, often called SIMOPS, require careful planning between the drilling contractor, the operator and any production or completion crews on the same pad. HSE-333 covers the permit and isolation side of SIMOPS; this course points out where it appears on the rig.

Pinecrest: the rig arrives

The running example continues at the invented Pinecrest pad of the invented Larchwood Resources, about 40 miles south of Watford City, North Dakota. In HSE-330 the pad held eight producing Bakken wells and a tank battery. In this course Larchwood has contracted a pad rig, Rig 31 of the invented Northline Drilling, to drill four new horizontal wells, numbered 9 to 12, on an extension of the same pad. Each well reaches a true vertical depth of about 9,500 ft and a measured depth of about 20,000 ft. The associated gas still contains about 0.5 percent hydrogen sulfide, so the rig crew holds current H2S training and the site has an H2S contingency plan. After drilling, an invented completions company, Ridgeback Pressure Pumping, will fracture the wells, and an invented service rig company will run production tubing. All people, companies and incidents at Pinecrest are invented for teaching.

The program below lists the major pieces of Rig 31 by system, with invented but realistic ratings, and counts how many involve each of the energy forms from HSE-330's energy wheel. It is a simple way to see that the hoisting and well control systems carry the most severe energies.

# Invented equipment list for Rig 31 at Pinecrest, tagged by energy form.
equipment = [
    ("Mast and crown block",        "hoisting",     ["gravity"]),
    ("Drawworks and drilling line", "hoisting",     ["gravity", "mechanical"]),
    ("Block and top drive",         "hoisting",     ["gravity", "mechanical", "electrical"]),
    ("Iron roughneck",              "rotating",     ["mechanical", "pressure"]),
    ("Slips and manual tongs",      "rotating",     ["mechanical"]),
    ("Mud pumps and standpipe",     "circulating",  ["pressure", "mechanical", "sound"]),
    ("Shale shakers and tanks",     "circulating",  ["chemical", "sound"]),
    ("Engines and generators",      "power",        ["electrical", "temperature", "sound", "chemical"]),
    ("VFD house",                   "power",        ["electrical"]),
    ("BOP stack",                   "well control", ["pressure"]),
    ("Accumulator unit",            "well control", ["pressure"]),
    ("Choke manifold, gas buster",  "well control", ["pressure", "chemical"]),
]
count = {}
for name, system, forms in equipment:
    for f in forms:
        count[f] = count.get(f, 0) + 1
for f, n in sorted(count.items(), key=lambda kv: -kv[1]):
    print(f"{f:12} {n:2d} items  " + "#" * n)
systems = sorted({s for _, s, _ in equipment})
print("Systems:", ", ".join(systems))

mechanical    5 items  #####
pressure      5 items  #####
gravity       3 items  ###
electrical    3 items  ###
sound         3 items  ###
chemical      3 items  ###
temperature   1 items  #
Systems: circulating, hoisting, power, rotating, well control

Counting is not the same as ranking risk, of course. A single parted drilling line can do more harm than a dozen noisy motors. The count simply reminds a new worker that the rig is full of energy of many kinds, and that the floor, where hoisting, rotating, circulating and well control all meet, is the place where those energies concentrate.