Variable Frequency Drives
Variable Frequency Drives
Optimize Performance and
Protection of Offshore Oil
Electric Submersible
Pumps
Application Notes
Offshore Oil Operations
As global demand for reclaimable petroleum reserves increases, so does the
demand for technological advances in artificial lift systems, especially in the field
of offshore oil operations. Such operations have the reputation of being extremely
expensive with no guarantee that results will yield reserves that will justify such
costs. Artificial lift systems are an essential part of offshore drilling, especially in
maturing oil fields where the reserves lack sufficient pressure to easily bring the
crude oil to the surface. Electric submersible pumps (ESP’s) offer the highest yield
of most deep-well artificial lift systems, but suffer the highest frequency of
expense and repair. This profile outlines how a careful selection of technologies,
specifically the use of electric submersible pumps with variable frequency drives,
can optimize the performance in well extraction on offshore oil production
platforms. This profile also compares ESP systems to other contemporary artificial
lift techniques, discusses electromechanical, solid-state reduced voltage starting
options in comparison with the benefits of continuous duty motor control using
variable frequency drives. In addition, this profile outlines design considerations,
with special regard to powering systems with diesel generators, that are critical
for determining a solution in this given application.
Electric Submersible Pump Technology
Electric submersible pump technology in oil field operations is still in its infancy,
even now in the year 2001. ESP’s had been around for years in applications
pumping water or other fluids of relatively low viscosity, such as fresh water
wells. Up until the early 1990’s, ESP’s saw their first applications in offshore oil
field operations exclusively used in fire fighting or well injection operations.
However, adapting such motors for use in such extreme conditions as downhole
operations had occurred slowly and almost exclusively by North American
Companies, whose main markets were fresh water well applications. In 1994, the
first practical application of downhole ESP’s became moderately successful at an
offshore rig in the Campos basin off the coast of Brazil. The application required
the design of a production string consisting of a submersible pump driven by an
electric motor, separated by a sealing section from the actual pump – to allow for
expansion of gas and fluid and to allow for the installation of a gas/water
separator. With the pump section positioned down the well just below the surface
of the deposit, the separated crude oil is pumped to the surface via conventional
sucker rods. The wellhead is capped with a valve for venting the gas. The gas is
either burned off (flared) to avoid re-infusion of the gas into the reserve or retained
for use in artificial lift or well injection processes. Perforations in the well casing
allow the water to be forced out into the annulus of the well casing and raw crude
mixture to enter.
Variable Frequency Drives
The production string required a pump driven by a two-pole, three-phase AC
induction motor redesigned to fit down a well shaft measuring 4 ½ to 6 ½ inches
in inner diameter and capable of handling thick fluids mixed with abrasive materials
like sand. Horsepower ratings of 750 horsepower were achievable with the
inclusion of additional rotors. However, 50-250 horsepower ESP&