Source of Energy: How to select generator sets for todays oil & gas drill rigs
Written by Steve Besore
Modern
horizontal drilling techniques used in oil and gas exploration require
reliable and fuel-efficient on-site generator sets to supply electric
power for the draw works, drilling, mud pumping and camp loads. Today’s
oil and gas drill rigs have to drill deeper and faster than ever
before. In addition, they have to use unconventional drilling
techniques such as horizontal drilling and fracturing to improve
petroleum extraction from less permeable geologic structures, such as
oil and gas-bearing shale.
Today’s new drilling realities require more power than conventional
wells and have given rise to the development of the AC/DC silicon
controlled rectifier (SCR) drill rig powered by multiple generator sets.
While AC/DC electric rigs with SCR controls dominate petroleum
exploration today, operators are constantly looking for ways to
increase total power availability, reliability and fuel efficiency,
requiring generator sets to deliver high specific power, low fuel
consumption and less maintenance.
AC/DC rigs with SCR
Oil and gas drill rigs tend to be classified by the type of power used to operate the equipment on the rig.
Mechanical rigs use dedicated diesel engines to provide
motive force for the mud pumps, draw works, rotary drill table and
other loads through a system of clutches and transmissions.
Hydraulic rigs have dedicated diesel engines running hydraulic pumps, which, in turn, provide power to the necessary equipment.
DC/DC electric rigs use dedicated diesel-electric direct-current generators to power DC motors that run the equipment.
While mechanical, hydraulic and DC/DC systems are still used for
conventional and shallower wells, they can be costly to operate and
maintain and lack flexibility. In addition, these older systems are
less reliable. Since individual engines are dedicated to single
functions such as driving the mud pump or operating the draw works, a
failure on any one engine can halt drilling altogether.
Today,
the majority of the new oil and gas drill rigs are AC/DC electric rigs
with SCR controls. These rigs use multiple diesel-electric generator
sets running in parallel to produce the two to four megawatts of power
needed at the drill site, including the power needed for camp loads
such as lighting, heating and air conditioning for crew quarters. Power
is generated as alternating current (AC) and then converted to direct
current (DC) by a unit called an SCR (so called for the banks of
silicon-controlled rectifier semiconductors that it contains). The SCR
unit allows precise control of the flow of power to any of the rig’s DC
motor loads while the generators run at a constant speed.
The number of generator sets running at any one time can be varied,
depending on total load, to save fuel. This configuration is also more
reliable because a failure of one of the generator sets does not
necessarily cause a shutdown of drilling operations even though it may
reduce the total amount of power available. An additional advantage of
paralleled generator sets is that individual units can be taken offline
for maintenance without greatly affecting the drilling operation.
Selection criteria
In response to the power needs of modern oil and gas drill rigs, rig
power manufacturers have developed special generating sets that are
designed to stand up to the rigors of the petroleum patch while
delivering maximum power and fuel economy with minimum maintenance.
When selecting generator sets to power a modern drill rig, look for
these key attributes:
1. Base frame stiffness, durability.
A prerequisite for any electric drill generator set is rugged
construction to take the often-severe operating environments and rough
handling that are typical in the field. The stiffness of the generator
set base is a critical factor in its longevity because any distortion
could affect the alignment of the coupling between the engine and
alternator, resulting in severe vibration and damage. Ordinary
structural steel does not have the necessary stiffness to prevent base
frame distortion under severe handling or if placed on uneven ground. A
base frame that uses high-strength, low-alloy steel withstands the
rigorous operating conditions. A three-point mounting system with
rubber vibration isolators provides the best stability of the engine
generator.
2. Ratings and performance characteristics.
Drill rig generator sets are designed for continuous operation, and
therefore are conservatively rated in terms of their kilowatt (kW)
output. A typical drill rig generator set has a nameplate rating of
about 1,100 kW, although there are both larger and smaller units
available. Since these units are likely to be subjected to severe
service, generator sets with a 10 percent overload capability beyond
their nameplate rating will meet most requirements. While engines on
typical commercial 60 Hz generator sets operate at 1,800 rpm, well
servicing companies have found that engines that operate at 1,200 rpm
have a better record of longevity in the field.
3. Overload capacity.
Due to the severity of the operating conditions in the field, generator
sets are often called upon to deliver their maximum output — and then
some. Generator sets should have at least a 10 percent overload
capability beyond their nameplate rating. For further assurance,
compare the ratio of cylinder displacement to rated horsepower of the
generator drive engine. Generally speaking, engines with a larger
displacement to horsepower ratio combined with a longer piston stroke
will have more built-in reserve horsepower and torque than smaller,
shorter stroke engines of the same horsepower rating. They will also
exhibit greater fuel economy and durability.
4. Fuel consumption.
Since drill rig generator sets operate continuously, fuel consumption
accounts for the largest operational cost. Just a few percentage points
of better fuel economy can add a significant number of dollars to the
bottom line at the completion of a well. Diesel engines tend to be most
fuel-efficient in proportion to their output when operated at 100
percent of their rated load. Engines are typically rated in terms of
their brake specific fuel consumption (BSFC), which varies with the
percentage of rated load. The BSFC rating allows specifiers to compare
the fuel economy of generator sets before the units are in the field.
For 1,200-rpm generators sets with a rating of about 1,100 kW, BSFC
should be less than 200 grams of fuel per kW-h generated. And, for
maximum fuel economy, always operate the generator sets as near their
nameplate rating as possible.
The
“Overload capacity” graph illustrates that different engine
manufacturers have different ratios of cylinder displacement to brake
horsepower (BHP). This ratio is an important factor in a generator
set’s ability to respond quickly to changes in load and maintain
voltage and frequency. Generator drive engines with the highest
displacement to BHP ratio have more reserve horsepower, the lowest fuel
consumption and the best durability.
The
“Fuel consumption” graph shows the fuel efficiency advantage that a
large displacement to BHP ratio confers for Brand X. Fuel is one of the
major operating costs on a drill rig and there are significant
differences in fuel consumption rates between brands of generator sets.
Since diesel engines are most fuel efficient at full power, it is
important to not oversize the generator sets for the job.
5. Oversized alternator.
While the SCR unit on AC/DC drill rigs allows for efficient and precise
control of power to the various DC motors loads, it causes the current
in the system to lag the voltage, resulting in a low 0.7 power factor
(PF) load on the generator sets. Ordinary generator sets are designed
to operate optimally at about a 0.8 PF. The 0.7 PF load on the
generator sets causes field heating in the alternator, and unless the
alternator is properly oversized, damage can result. For a 1,200-rpm
generator set with about 1,100 kW of capacity, look for its alternator
to be oversized to at least 1,750 kVA to meet the low PF requirements.
Also look for a 50-degree C ambient rating and minimum 80-degree C
temperature-rise capability.
6. Control and monitoring.
Precise control of the generator engine’s speed and operating
parameters yields good regulation of the power output and quick
response to changes in the load. Generator sets with engines that
feature an integrated electronic engine governor and electronic engine
management system provide the most accurate control, protection and
monitoring.
These systems will also monitor alarm conditions and protect the engine
from damage. They are also capable of communicating with external
control systems for remote monitoring and control of the generator
sets. This can be convenient when paralleled generator sets need to be
started or stopped to match the load conditions. Mechanical gauges on
the generator set skid should display variables such as lube oil
pressure, oil temperature, engine coolant temperature, engine speed and
operating hours. Also available are units that feature multi-page color
LCD display panels for various performance and status readouts.
7. Maintenance requirements.
All diesel engines require periodic maintenance to ensure good
performance and reliability. Besides regular inspections, the most
important maintenance procedure involves changing the engine oil
approximately every ten days or 250 hours of operation. Generator sets
that feature an engine-mounted lube oil centrifuge as standard
equipment reduce the downtime required for oil changes. This device can
significantly extend the lube oil change intervals, save money on oil
and filters and increase generator set availability.
8. Single-source supplier.
Generator sets from a single-source supplier that manufactures and
tests the complete drill modules in a factory setting provide assurance
of a higher quality product and faster repairs when they are necessary.
There are many drill rig generator sets that are assembled by
integrators using engines from one manufacturer, alternators from
another and controls from still another. If the generator set breaks
down for any reason, parts or service availability can be a serious
problem if an integrator assembled the unit. Drill rig downtime caused
by delays in getting repair parts can be very costly. n
Steve Besore deals with oil and gas applications at MTU Detroit Diesel. For more information, visit www.mtu-online.com. This was previously published in REM.
Modern
horizontal drilling techniques used in oil and gas exploration require
reliable and fuel-efficient on-site generator sets to supply electric
power for the draw works, drilling, mud pumping and camp loads. Today’s
oil and gas drill rigs have to drill deeper and faster than ever
before. In addition, they have to use unconventional drilling
techniques such as horizontal drilling and fracturing to improve
petroleum extraction from less permeable geologic structures, such as
oil and gas-bearing shale.
Today,
the majority of the new oil and gas drill rigs are AC/DC electric rigs
with SCR controls. These rigs use multiple diesel-electric generator
sets running in parallel to produce the two to four megawatts of power
needed at the drill site, including the power needed for camp loads
such as lighting, heating and air conditioning for crew quarters. Power
is generated as alternating current (AC) and then converted to direct
current (DC) by a unit called an SCR (so called for the banks of
silicon-controlled rectifier semiconductors that it contains). The SCR
unit allows precise control of the flow of power to any of the rig’s DC
motor loads while the generators run at a constant speed.
The
“Overload capacity” graph illustrates that different engine
manufacturers have different ratios of cylinder displacement to brake
horsepower (BHP). This ratio is an important factor in a generator
set’s ability to respond quickly to changes in load and maintain
voltage and frequency. Generator drive engines with the highest
displacement to BHP ratio have more reserve horsepower, the lowest fuel
consumption and the best durability.
The
“Fuel consumption” graph shows the fuel efficiency advantage that a
large displacement to BHP ratio confers for Brand X. Fuel is one of the
major operating costs on a drill rig and there are significant
differences in fuel consumption rates between brands of generator sets.
Since diesel engines are most fuel efficient at full power, it is
important to not oversize the generator sets for the job.
