other devices, cannot withstand pressure higher than
atmospheric. Therefore, a fiber optic penetrator is used
to prevent the modules from being flooded with water.
On other devices, such as subsea pumps and wellheads,
which are potentially exposed to the reservoir shut-in
pressures, pressure ratings can climb up to 15,000 psi
when combined with high temperatures.
The penetrator serves an environmentally important
function. An optical failure would mean a loss of sensing
capability, but a mechanical failure would release fluids
from the well into the environment.
Strong Cables Protect Fibers
While fibers have high tensile strength to withstand
longitudinal pulling, they can be easily broken or damaged
if not protected correctly. As a result, fiber optic cables
typically have their own armoring. While aramid yarn
is typically used, more robust designs require metallic
armoring. The high hydrostatic application pressures
can increase attenuation in a fiber. The following three
approaches may be used:
Fiber in Steel Tube (FIST) – This places the fiber in a
solid stainless-steel tube to protect against hydrostatic
pressures, high temperatures, and corrosive environments.
FIST packaging is a loose-tube design, which can
accommodate several fibers loosely held within the tube
and encapsulated in gel. Because the fibers “float” within
the tube, the length of the fiber is slightly longer than the
tube to ensure low strain. FIST technology is the simplest
and lowest cost approach and maintains low strain on
the fiber by decoupling stress on the tube. If the cable
stretches during installation or use, the excess fiber can
accommodate the stretching without straining. Loose tube
designs are also very forgiving of extreme temperature
excursions but are less suited to the most rugged
applications such as extreme depths and cable lengths.
FIST also offers high-density packaging of multiple fiber
in the tube and, of the three options, is the easiest to
STEEL-LIGHT armoring – This option uses strands of
precisely sized plow steel concentrically arranged around
the fiber buffer to protect the fiber from breakage.
ELECTRO-LIGHT armoring – This is similar to STEEL-
LIGHT armoring but uses copper in place of steel. The
copper can also be used for power to allow composite
cables to be designed with a smaller outside diameter.
Continued on page 30
Dream big in small sizes
High flexibility in system design
Robust connection to the PCBs by additional SMT hold-down devices
Suitable for automatic assembly process and reflow soldering processes