44
LNG
INDUSTRY
SEPTEMBER
2014
Vessel and tank measurement
issues
At the vessel, volume measurements are taken twice: before
and after loading or unloading. Level sensors measure the height
of the LNG in the vessel’s onboard tanks. Corrections are also
made for the tank shape and dimensions to obtain the volume.
Additionally, adjustments are made for trim and heel to account
for the vessel leaning forward, aft or to the side. Additional
measurements are taken so that volume can be calculated at
standard conditions. These include temperature, pressure and
density of the LNG.
Because the LNG is maintained near its boiling point, there
will be some natural gas in the vapour phase that must be
accounted for. To relieve pressure in the tanks, the vapour is
vented or used as fuel. Older vessels use various metering
technologies to measure the amount of gas that is lost from the
tanks as boil-off. Newer vessels are able to minimise the loss by
condensing the vapour and re-injecting the liquid into the tanks.
Any boil-off gas (BOG) is accounted for as lost inventory.
Energy content determination also requires a chemical
composition analysis, which a chromatograph performs. The
chromatograph results are used to determine density, which is
needed to calculate mass from the volume.
The information from the vessel is often only in the form of a
hardcopy transaction record or ticket. This requires manual entry
into the database. Sometimes, the information is incomplete
when the vessel is loaded or unloaded. For instance, if a
chromatograph is not available, a sample is taken to a laboratory
for analysis and the results are entered later.
The information from the vessel is also extensive. There are
volume calculations over multiple tanks, calculations to derive
the energy content from the volume and there is accounting for
boil-off. The application must be able to verify the calculations,
which includes re-running them if any input information was
entered incorrectly.
The application must also produce an audit trail, which
confirms all input information, identifies its source and ensures
that calculations abide by industry standards. The audit trail
further confirms agreement among all parties involved regarding
lost and unaccounted for inventory.
The parties must also agree on accounting for measurement
errors. For instance, what happens if there is a failure in a
pressure or temperature sensor? In the calculations to derive
volume or energy, a substitute input value is used. It could be the
last known good reading, a similar reading from another tank or
some other agreed-upon quantity. This is another case for which
the application must be able to calculate corrected quantities
and record the substitution in the audit trail.
If this were an unloading process, the next step for the LNG
would be a tank farm. Except for adjustments, such as list and
heel, facility storage tank level measurements and calculations
are similar to those of a vessel.
Into the pipeline
In terms of measurement information, a gas pipeline is a
completely different process. Flow computers sample live
readings from flowmeters, perform corrected flow calculations
and totalise on an hourly basis. Although information on liquids
is almost always batch ticket-based, virtually all gas information
is hourly. That requires the validation of both the batch and
time-based information.
The application must meet a number of standards, which
vary by world region. For gas flowmetering in Europe, ISO
standards rule, while the US uses API standards and American
Gas Association reports. There are numerous details, which vary
among those standards, e.g. reference pressures and
temperatures as well as engineering units. To cover all bases
requires a thorough measurement application.
The prevailing metering technology also differs by region.
While the orifice meter is the traditional standard for natural gas
metering in the US, the turbine meter is standard in Europe.
However, newer technologies, such as ultrasonic metering,
represent common ground. The measurement application must
accommodate all the meter types.
Meanwhile, flowmeters that are able to measure the flow of
cryogenic fluids represent an emerging technology. This class of
meters, including cone, coriolis, orifice and ultrasonic, allows
dynamic measurements to be used in LNG processes such as
vessel loading and unloading.
2
This class of meter will fit into the FLOWCAL application.
The liquids flow computers that are associated with such meters
totalise the corrected flow on an hourly basis, just like a gas flow
computer. Uploading information, auditing, verification and
balancing operations are similar to those on the gas side.
Figure 3.
FLOWCAL screenshot for system-wide mass balance.
Figure 1.
FLOWCAL screenshot where the mass for an orifice
meter has been calculated.
Figure 2.
FLOWCAL screenshot for liquid mass.
