28
LNG
INDUSTRY
SEPTEMBER
2014
colours – anti-Stokes – is strongly temperature sensitive. The
other – Stokes – is not. By measuring the intensity difference
between the two colours (see Figure 1), it is possible to obtain
a very well calibrated temperature measurement. By doing
this with optical fibre, and by employing Optical Time Domain
Reflectometry (OTDR) technology, one can get the highly
sought after ‘distributed’ temperature sensing capability.
So what are the capabilities of a fibre that measures
temperature along its length? Modern day fibre-based DTS
systems can measure temperature at 1 m intervals and up to
distances of 40+ km. They have no electronics in the field, just
a single optical fibre connected to the electronics box in a
control room. Furthermore, the fibre is immune to almost all
environmental hazards – vibration and electromagnetic fields
cause absolutely no impact on the temperature
measurement. A single fibre cable can measure temperatures
from -196°C to over 300°C. In fact, DTS cables are also
certified to withstand +750°C for over two hours with
measurement up over 1100°C for short periods.
Analysing the system capabilities for an LNG plant, it is
evident that measuring extremely low temperatures is an
advantage. But so is the low risk of ignition. A common
misconception is that fibre-based signals are inherently safe,
but this is not always true. BS EN 60079-28:2007 (IEC 60079):
“Explosive atmospheres. Protection of equipment and
transmission systems using optical radiation” covers just this
topic.
1
All fibre systems should be tested and certified to
international standards such as ATEX. Such ATEX-certified DTS
systems exist even for Zone 0 environments.
Typical uses for DTS systems
on an LNG plant
DTS systems were first used in LNG terminals in relation to
tank annulus leak detection. A fibre in a steel tube would be
installed around the annulus space of a double-walled full
containment tank between concrete and steel before the
perlite fill. Once the tank is operational, the temperature
measurement of the annulus space will indicate any leaks
from the steel wall inner tank. The advantages of this
technology are that the amount of gland space real-estate
required on the roof of the tank is kept to an absolute
minimum and the resulting solution has a very long life
expectancy if the correct fibre is used. The DTS systems
installed in the 1980s still use the same fibre cable, buried in
the perlite.
LNG pipeline integrity is the next application of interest.
Of course it is possible to detect leaks in the pipeline – a
simple fibre attached under the pipe easily detects a -162°C
cold spot even with arctic ambient temperatures. This
solution has been implemented on a great number of sites
and is almost standard now for LNG terminal FEED studies.
But this can be taken further. The same cable can be used to
look at the pipeline insulation integrity. It is not necessary to
wait for -162°C to flag a problem, because a modern DTS
system is capable of detecting small localised temperature
fluctuations and hence report potential insulation failures
before they become a risk to pipeline integrity.
Pipeline cooldown monitoring has been another
interesting area for DTS technology and has been
implemented on LNG pipelines with varying levels of success.
While this is a viable and easy solution for pipelines where
the insulation is to be applied at site, the application of the
fibre on more popular pre-insulated pipes has had some well
documented failures and remains contentious.
The ability to accurately measure LNG temperature brings
with it the possibility to improve LNG processes, hence DTS
instruments have found their uses in coil wound heat
exchangers (CWHEs) for monitoring the temperature through
the entire cooling process. While still mainly based in R&D
work, this application does offer a glimpse of things to come.
Case study
DTS information can also be used to better control LNG tank
pad heaters and hence reduce energy consumption and tank
boil off. Kogas recently implemented such a DTS system
from AP Sensing at its newly inaugurated site in Samcheok,
South Korea.
DTS fibre cable has been threaded through the tank base
pad alongside the heater cables, but unlike previous
installations, the data from the DTS is used to directly control
the heaters on an individual basis. Using the DTS information
to accurately map the temperature of the pad, a feedback
loop can then individually control each heater to increase or
Figure 1.
DTS Raman Stokes and anti-Stokes.
Figure 2.
LNG storage tank in South Korea.
