Working at height and dropped objects

A drilling mast towers 30 to 50 m (100 to 165 ft) above the rig floor, and people work at many levels on it: the derrickhand on the monkeyboard during trips, crew members on the stabbing board when running casing, mechanics at the crown, and workers on the mud tanks, the pipe racks and the top of the substructure. Everything that goes up the mast can also come down: tools, bolts, pins, ice, a hand radio, a piece of a sheave guard. Falls from height and dropped objects are two sides of the same hazard. This lesson covers both on rigs and service units, building on the general fall protection principles taught in HSE-212.

Where rig crews work at height

The monkeyboard, or racking board, is a platform partway up the mast, at the height of the top of a stand. During a trip out, the derrickhand leans out from the monkeyboard, unlatches the elevators from the top of each stand and guides the stand into the fingers of the racking board; during a trip in, the process runs in reverse. On a triple rig, the monkeyboard is about 27 m (90 ft) above the floor. The derrickhand may work there for hours at a stretch, in wind, rain, snow and darkness, leaning over open space while a heavy block moves within reach. It is one of the most exposed jobs in the industry.

Other elevated work positions include the stabbing board, which a casing crew member uses to guide casing joints into the elevators; the crown platform at the top of the mast, used for inspection and maintenance of the crown block; the top drive maintenance platforms; ladders and stairs to the floor and the mud tanks; the tops of mud tanks, pipe tubs and storage tanks; and the mast itself during rig-up and rig-down. On service rigs, the derrickhand works on a smaller board on a lower mast, and on snubbing units workers stand in a basket above the wellhead.

Fall protection on the mast

Fall protection on a rig follows the same hierarchy as anywhere else: eliminate the need to work at height where possible, use guardrails and enclosed platforms, use travel restraint so a worker cannot reach an edge, and use fall arrest only where the other controls are not practicable. On rigs, elimination has made real progress: pipe-handling systems and automated racking let some rigs trip without a derrickhand on the board at all. Where people still climb, the core equipment is the same as in HSE-212: a full-body harness, a connecting device and a suitable anchor, with a rescue plan.

Climbing the mast is done on a fixed ladder with a ladder climbing assist or a fall arrest system, usually a self-retracting lifeline or a rail or cable sleeve system that moves with the climber. Workers must be connected at all times while climbing. Climbing a mast ladder without fall protection, or with a lanyard hooked over a rung, is never acceptable. On the monkeyboard, the derrickhand is usually connected to a fall arrest line or a positioning system that holds them on the board while leaning out to work; the system must be adjusted so that a slip results in a short fall or no fall, not a long drop onto steel. At the crown and on other platforms, guardrails and anchor points provide protection.

Several hazards are specific to rig work. Fall arrest lines can be caught by the moving travelling block or the top drive. Wet, icy or mud-covered steel increases slip risk. A worker who falls and is caught by a harness must be rescued quickly, because suspension in a harness can cause serious harm within minutes, as HSE-212 explains. Rescue from a monkeyboard 27 m up is a difficult job, so rigs must have a written rescue plan, trained people and the equipment to carry it out, which may include a rescue descent device or a pre-rigged system.

Derrick emergency escape

If a fire, blowout or gas release reaches the rig floor while the derrickhand is on the monkeyboard, climbing down the ladder may mean climbing into the danger. Rigs therefore provide a derrick emergency escape device, often called a Geronimo line: a cable running from the monkeyboard at an angle to a ground anchor some distance from the rig, with a trolley and a controlled descent device that lets the derrickhand ride down the line, clear of the floor. The escape line must be installed and tensioned correctly at every rig-up, the ground anchor must be clear of equipment and traffic, the trolley and brake must be inspected, and the derrickhand must be trained in its use. In winter, ice and snow can affect the trolley, so inspection and testing before each trip matter. Some regulators specify requirements for derrick emergency escape devices, and drilling contractors follow manufacturer instructions and industry guidance for their installation and inspection.

Dropped objects

A dropped object is any object that falls from its previous position, either under gravity alone (a static drop, such as a bolt that vibrates loose) or because something moving strikes it (a dynamic drop, such as a sheave guard knocked off by the travelling block). Dropped objects are one of the leading causes of serious injuries and fatalities in drilling, onshore and offshore. Even small objects become dangerous from height: a 1 kg (2.2 lb) tool falling from the monkeyboard reaches the floor at roughly 23 m/s (51 mph).

Industry's response has been led by the DROPS workgroup, an international, cross-industry initiative that publishes free guidance, reliable securing practices and a widely used DROPS calculator. The calculator classifies the potential consequence of a dropped object by its mass and the height it could fall, from slight injury through medical treatment and lost time to a fatality. The program below shows the physics behind it: impact energy grows with both mass and height. The consequence bands in the program are illustrative teaching values only, not the published DROPS calculator chart, which should be used for real assessments.

import math

g = 9.81
def impact(mass_kg, height_m):
    energy_J = mass_kg * g * height_m
    speed = math.sqrt(2 * g * height_m)
    return energy_J, speed

# Illustrative teaching bands only (NOT the published DROPS chart).
def band(energy_J):
    if energy_J < 40:
        return "minor"
    if energy_J < 100:
        return "medical treatment"
    if energy_J < 300:
        return "lost time / serious"
    return "potentially fatal"

objects = [("Bolt",            0.10, 27), ("Hammer",          1.0, 27),
           ("Hand radio",      0.35, 45), ("Sheave guard pin", 0.5, 45),
           ("Ice chunk",       2.0, 30),  ("Slip insert",      0.8, 6)]
for name, m, h in objects:
    E, v = impact(m, h)
    print(f"{name:17} {m:4.2f} kg from {h:2d} m: {E:6.0f} J at {v:4.1f} m/s -> {band(E)}")

Bolt              0.10 kg from 27 m:     26 J at 23.0 m/s -> minor
Hammer            1.00 kg from 27 m:    265 J at 23.0 m/s -> lost time / serious
Hand radio        0.35 kg from 45 m:    155 J at 29.7 m/s -> lost time / serious
Sheave guard pin  0.50 kg from 45 m:    221 J at 29.7 m/s -> lost time / serious
Ice chunk         2.00 kg from 30 m:    589 J at 24.3 m/s -> potentially fatal
Slip insert       0.80 kg from  6 m:     47 J at 10.8 m/s -> medical treatment

Even a 100 g (0.22 lb) bolt falling from the monkeyboard carries over 25 J, and a hammer from the same height carries over 250 J, enough to cause serious injury even to a worker wearing a hard hat. Hard hats are designed to reduce the effect of small impacts; they are not a substitute for preventing the drop.

Controlling dropped objects

Dropped object prevention combines engineering, inspection and behaviour.

A dropped object survey is a systematic inspection of everything at height, often done by a trained person with a checklist, binoculars and access to the mast. Surveys are especially important after rig moves, because raising and lowering the mast, transport vibration and contact with power lines or other equipment can loosen fixtures. Collisions between the travelling block or top drive and other equipment in the mast, such as racking boards, are a common cause of dynamic drops, and many rigs use anti-collision systems that stop the blocks before contact.

Other falls on the lease

Not every fall is from the mast. Workers fall from the tops of mud tanks while mixing chemicals or checking levels, from pipe racks and tubs while handling pipe, from truck decks while loading and unloading, from ladders and stairs, and on the same level on mud, ice and uneven ground. Mud tanks should have guardrails and grating; pipe racks should be designed so workers do not need to walk on rolling pipe; and loading should use platforms or fall protection where workers must work on deck. The general controls from HSE-212, HSE-213 and HSE-106 apply.