24
LNG
INDUSTRY
SEPTEMBER
2014
China National Offshore Oil Corp. (CNOOC) has been
selected to complete the task.
LNG tanks compose an inner and outer tank. The outer
part is formed by steel-reinforced concrete, while the inner
tank is made of metal. Leakages, which can occur on the
inner tank, are one of the greatest threats to the safe
operation of LNG tanks. It is therefore crucial to detect
possible leakages at an early stage. Installing fibre optic
cables as sensors around the foundations as well as at the
walls of the tanks will enable the operator to know the
precise temperature profile of the whole tank and indicate
the potential leakage based on the change of temperature.
This is known as the Joule-Thomson effect.
The tanks’ parameters are as follows:
Diameter of the inner tank: 45 m.
Height of the inner tank: 23 m.
Volume of the inner tank: 30 000 m³.
Temperature of the LNG inside the tank: -168°C.
Traditional method of leak
detection
A traditional method of leak detection is to install the
resistance temperature detector (RTD) at the foundation
of the LNG tank. A sensor is used to measure temperature
by correlating the resistance of the RTD element with
temperature. However, electrical signals are needed to
operate the RTD and are potentially hazardous in some
cases. Furthermore, the RTD is a point-type detector, which
can fail to monitor some blind areas.
In other words, it is difficult for the RTD to detect
leakage at an early stage, when the leaking amount is
relatively small.
However, a new detection method has been introduced
into the industry, namely DTS systems, which deploy fibre
optic cables and can achieve successful detection.
Introduction to DTS systems
DTS systems are optoelectronic devices, which measure
temperatures by means of optical fibres functioning as
linear sensors. Temperatures are recorded along the
optical sensor cable, thus not at points, but as a continuous
profile. A high accuracy of temperature determination is
achieved over great distances.
Optical fibres are made from doped quartz glass.
Quartz glass is a form of silicon dioxide (SiO
2
) with an
amorphous solid structure. Thermal effects induce lattice
oscillations within the solid. When light falls onto these
thermally excited molecular oscillations, an interaction
occurs between the light particles (photons) and the
electrons of the molecule. Light scattering – also known as
Raman scattering – occurs in the optical fibre. Unlike
incident light, this scattered light undergoes a spectral shift
by an amount equivalent to the resonance frequency of the
lattice oscillation.
The light scattered back from the fibre optic therefore
contains three different spectral shares:
The Rayleigh scattering with the wavelength of the
laser source used.
The Stokes line components with the higher
wavelength in which photons are generated.
The anti-Stokes line components with a lower
wavelength than the Rayleigh scattering, in which
photons are destroyed.
The intensity of the so-called anti-Stokes band is
temperature-dependent, while the Stokes band is
practically independent of temperature. The local
temperature of the optical fibre is derived from the ratio of
the anti-Stokes and Stokes light intensities.
DTS systems have the following key advantages:
The fibre acts as a passive sensor. There is no need
for any other electric device or signal to be installed
onsite.
The system can provide a continuous temperature
profile along the fibre, a component which is
Figure 2.
Layout of the fibre optic cable.
Figure 3.
Fibre optic cable during installation.
Figure 1.
Tianjin LNG terminal, the largest in China.
