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a wood-fired steamboat engine. Until the 1940s and 50s, steam engines drove almost
every rig. But, as powerful and portable diesel and gas engines became available,
mechanical rigs began to supplant steam rigs. Oil people called them “mechanical
rigs” or “power rigs,” because the engines drove special machinery, which, in turn,
provided power to the components. Then, in the 1970s and 80s, electric generators,
driven by diesel engines, began to replace the mechanical equipment used to drive
rig components. Today, these “electric rigs” or “diesel- electric rigs” dominate the
drilling scene.
Whether mechanical or electric, virtually every modern drilling rig uses
internal-combustion engines as a prime power source, or prime mover. A rig’s engines
are similar to the one in a car except that rig engines are bigger, more powerful, and
do not use gasoline as a fuel. What’s more, most rigs require more than one engine to
furnish the needed power. Most rig engines today are diesels, because diesel fuel is
safer to transport and store than other fuels such as natural gas, LPG, or gasoline.
Diesel engines do not have spark plugs as do gasoline engines. Instead, heat
generated by compression ignites the gaseous fuel-air mixture inside the engine. Any
time a gas is compressed, its temperature rises. Compress it enough (as in a diesel
engine), and, if the gas is flammable, it gets hot enough to ignite. Thus, diesel engines
are sometimes called "compression-ignition engines." Gasoline engines are often
called "spark ignition engines". A rig, depending on its size and how deep a hole it
must drill, may have from one to four engines. Naturally, the bigger the rig, the
deeper it can drill and the more power it needs. Thus, big rigs have three or four
engines, all of them together developing up to 3,000 or more horsepower (2,100
kilowatts). Of course, once the engines develop all this power, it must be sent or
transmitted to other rig components to make them work. Electric generators transmit
power on most rigs. However, a few older rigs use machinery to transmit the power.

Mechanical Power Transmission

As mentioned before, during the 1950s and 60s, most rigs were – mechanical
that is, the engines drove big chains and sprockets, which, in turn, powered various

parts of the rig. Later, diesel-electric rigs began to dominate the scene because
electrical power transmission had so many advantages over mechanical
transmission. Thus, today, the majority of rigs are diesel-electric. But, mechanical
rigs are still around, so it’s worth a brief look at mechanical power transmission.
Figure 80 is a schematic of a mechanical rig. It shows three 700-horsepower
(490-kilowatt) engines hooked up to a compound. The compound consists of several
heavy-duty sprockets and chains. The engines drive the sprockets around which the
chains are wrapped. The chains drive the various rig components. This chain-and-
sprocket arrangement is known as the compound because it compounds or connects
the power of several engines. With compounded engines, the driller can use one,
two, or all of them at once if required.

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