A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
page 1
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
A Practical Approach to Modelling LNG Train Design for Minimizing Measurement & Allocation Uncertainty
www.daniel.com
ABSTRACT
among JV partners for equity on a multi-train LNG complex. This
Our ability to measure accurately has formed the basis of
situation can arise if the ownership among LNG trains and the
trade in all industries. As a consequence, the performance
commercial terms governing their operation are different. Thus
of measurement systems will affect the profitability of any
a single LNG facility using shared resources where applicable,
business. In the Oil and Gas Industry measurement systems
may have multiple trains (plants) where there can be several
are used for fiscal and custody transfer purposes, for process
owners and several feed gas streams, as is the case at Atlantic
plant operations and for plant efficiency monitoring. It is of
LNG Company of Trinidad & Tobago.
great importance therefore that these systems are engineered,
operated and maintained to industry standards and within
The problem of accurately determining the LNG production of a
equipment specifications.
train arises because of the following:
1. The absence of reliable LNG flow measurement at the
This paper highlights the implications of measurement
outlet of the trains.
uncertainties on LNG product allocations and how, through
2. Shared LNG storage at the facility.
a study of these uncertainties and the implementation of a
3. Recycling of LNG vapours from tank storage into the
Measurement Upgrade Enhancement Project, the final Allocation
production streams.
Uncertainty was more than halved. The study addressed each
of the metering elements associated with the allocation process
The first constraint is dictated by the manufacturers’ products.
and identified which of these had the greatest impact (and/
Reliable dynamic LNG measurement technology is actively
or required attention). The measurement upgrade addressed
being researched by a number of flow measurement vendors
issues affecting key meter performance such as flow profile
with the main challenges being the ability to accurately measure
effects and valve noise with respect to ultrasonic measurement.
LNG leaving the trains and the ability to prove such a meter.
In addition, there are no dynamic LNG flow measurement
A major factor in the success of the project was the use of a
standards.
Measurement Exposure Model (MEM) which determined the
measurement uncertainties associated with key metering points,
The second constraint is from a standpoint of practicality. It is
and the impact they had on the final LNG Product Allocation. By
good to have the option to pump LNG production to several
using the MEM to focus on the effect of these measurement
tanks. With the cost of an LNG storage tank being in the vicinity
uncertainties,
of 250-300 million USD it is prudent to share this facility since
unnecessary
modifications/upgrades
were
avoided and savings made in the overall project.
this would strengthen the project economics. However, this
complicates ownership traceability where LNG production is
INTRODUCTION
1 MEASUREMENT AND ALLOCATION
commingled across several storage tanks.
The following are basic questions that should be asked of any
The third constraint is the recycling of the LNG vapours from
LNG Production facility:
storage, loading and LNG cooldown to production facilities
•
•
•
What does X% uncertainty of LNG measurement
means that shared production from one train/owner/shipper can
mean and how can this be improved?
be allocated to another.
To what extent do the plant measurement systems
affect the LNG product allocation?
As a result of the above issues, the allocation process used to
What is the minimum acceptable financial risk to Joint
determine the LNG production of a train on a multi-train facility
Venture (JV) partners on an LNG complex and how
can be very complex. This product allocation will be governed
can this be improved?
by the commercial terms and will be largely a function of mass,
energy or standard volume balanced equations, for example,
These are some of the issues which have to be addressed
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
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Train Production = Inlet - Fuel Consumption-NGL Production Train Losses
In order to build the MEM, an accurate assessment had to
It is worth noting that static custody transfer measurement, as
be made of the measurement systems associated with the
outlined by The International Group of LNG Importers (GIIGNL),
Allocation System. This was accomplished by completing a
takes place on LNG tankers at an uncertainty of ±0.2-0.3%,
comprehensive Audit of the metering elements feeding into
however what about dynamic LNG train production?
the Allocation System for the LNG Pant and assessing their
respective measurement uncertainties. The current operating
The key question then becomes, what are the uncertainties
Measurement Uncertainties of meters used for Custody and
associated with these measurement points and what impact
Allocation purposes were then used as an input by the Model
does it have on the LNG product allocation?
(the MEM) to establish the final Allocation Uncertainties in order
to quantify the financial “risk” to partners.
This paper highlights the implications of measurement
uncertainties on LNG product allocations and how the use
3 MEASUREMENT UNCERTAINTY ASSESSMENT
of a Measurement Exposure Model (MEM) assisted in the
It should be appreciated that no measurement can be absolutely
implementation of a Measurement Upgrade Enhancement
precise,
Project thereby reducing the LNG and NGL product allocation
introduced as a result of the measurement instruments chosen,
uncertainty by an average of 2 %. In executing this measurement
their calibration and installation. Uncertainty then is an estimate
upgrade, the issue of valve noise and ultrasonic measurement
of the limits to which one can expect an error to go, under a given
(inlet feed gas meters) was also addressed since the correct
set of conditions as part of the measurement process. Whilst the
installation and operation of inlet feed gas meters had the
determination of measurement uncertainty is independent of the
greatest impact on the LNG Allocation.
MEM, it forms an essential input into the Model, if an accurate
since
there
are
inevitable
biases/inaccuracies
assessment of an existing Allocation Systems Performance is
2 BUILDING A MEASUREMENT EXPOSURE MODEL
(MEM)
required. All relevant Measurement systems are independently
A Measurement Exposure Model (MEM) is a mathematical tool
using proven Calculation Techniques (API and ISO) complying
that monitors the facility’s measurement points and product
with the relevant Measurement System Guidelines for Design,
allocation equations to determine the individual and/or collective
Operation and Maintenance.
assessed and their Measurement Uncertainty established
impact of each measurement point on the allocation of products.
Figure 1 - Typical MEM Mimic Screen
A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
The calculation process requires such information as:
page 3
•
Laboratory Analysis Uncertainties
•
Calibration Procedures, frequency and tolerance
•
Primary Measurement Element Uncertainties (Orifice,
This information is used to calculate the individual Measurement
Turbine Meter, Coriolis, USM, Etc.)
System Uncertainties and their consequent contribution to the
Figure 2 - Typical Input Screens for Uncertainty Assessment
•
•
Secondary Instrument Uncertainties (DP, Pressure,
Temperature, Densitometer, Flow Computer, etc.)
Correction Algorithms, Process Simulation
Assumptions, etc.
overall LNG Train Metering Allocation Uncertainty (calculated
separately by the MEM).
Typical Custody & Allocation Measurement Systems include:
•
Gas Pipeline Meters (Custody)
Figure 3 - Typical Output Screens of the Uncertainty Assessment
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
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•
Train Inlet Meters (TIM) (Allocation)
identify any “high risk” metering
•
Train Liquid Output Meters (Allocation)
elements in the system and establish the overall Allocation
•
Train Output Meters (Allocation)
Uncertainties.
•
Storage Tank Measurements (Stock/Allocation)
•
Fuel Gas Meters (Allocation
•
Flare Gas Meters (Allocation)
•
Gas Recycle Meters - where fitted (Allocation)
•
Tanker Loading Measurements (Custody)
The results are typically presented as follows:
•
Measurement Uncertainty of each measurement point
expressed as a percentage and in terms of product
•
Measurement Uncertainty associated with the Plant
Using Figure 3 Output Data, the MEM user can enter the
Balance expressed as a percentage and in terms of
estimated measurement uncertainty relating to the metering
product
element Operating Condition, thus enabling the Model to
•
simulate most working conditions and reflect the additional
Allocation expressed as a percentage and in terms of
uncertainty in Allocation for those conditions. This can be
particularly helpful where the operator needs to be cognisant
of the effect that instrument malfunctions or other failures may
Measurement Uncertainty associated with the Plant
product
•
Cost based Risk Analysis to contributing Fields and
Owners for various operating scenario’s
have on the final Allocation uncertainty for the system.
Thus, once the initial uncertainty analysis has been performed on
4 MEASUREMENT EXPOSURE MODEL (MEM)
each metering system to establish the Allocation measurement
The Measurement Uncertainty Assessment forms an essential
point Inputs, then the MEM analysis can be performed to asses
input to the MEM. In cases where the LNG Plant is not yet
the sensitivity of these points on the Plant Balance & Allocation
designed, design estimates of measurement uncertainty can
and the results documented.
be entered into the MEM such that the Design Engineers can
accurately assess their impact on the Plant Allocation. This
Any trouble spots highlighted can be re-assessed in order to
aspect of the MEM is not to be underestimated, as it could save
maximize the Plants performance. In this way, attention need
considerable Capital Expenditure on unnecessary design for
only be focused on those elements that most affect the system
uncritical elements of the Plant Measurement System. In the
and the reduced risk associated with the changes can be
identified by the model.
case of an existing LNG Plant, weaknesses in the
measurement systems
provided can be identified,
thus providing substantial
justification for change,
in terms of misplaced
Allocation Revenue.
Typically the MEM attributes
measurement uncertainties
to each of the measurement
points, then calculates
the flow weighted impact
these uncertainties will
have on Plant Balance
and Allocation. These
calculations can be based
on estimated (or measured)
flow rates through the
various meters. By using
this technique, the MEM can
Figure 4 - Typical Input & Output Screens of MEM
A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
page 5
As can be seen from the Mimic illustrated in Section 2.0, the
Assumptions: Gas Price $ 5.00 per MMBtu,
whole Allocation Process depended on the Train Inlet Meters
Discount Factor of 10%,
(TIM) to enable back allocation of all the Atlantic LNG Plant
Capital Outlay = Loss production + Cost of project= $25.2MM
OUTPUT products to the Gas Delivery Points (GDP), it follows
Project Life cycle 20 years.
that their measurement accuracy is of prime importance. The
MEM demonstrated the cost of the measurement uncertainty
Of interest to note, is that once other projects are undertaken
to the Operators and their Partners by reference to a live Mimic
which increases the 576 TBtu for the facility then this project
responding to reported Allocation data.
financing can be further enhanced, therefore giving economic
benefits to perpetuity. Therefore it is imperative that on an LNG
5 MEASUREMENT PROJECTS JUSTIFICATION
complex inlet measurement uncertainty be optimized.
The objectives of Capital expenditure plant projects can be
generally categorized into two main areas:
6 TRAIN INLET MEASUREMENT
As shown from the MEM an improvement in inlet measurement
•
•
Maintaining and Sustaining Production (Reliability
meant a significant reduction in LNG allocation product
Projects): Major upgrades and/or overhauls to plant
uncertainty. The challenge for the project team would be
equipment aimed at improving the cost of operation
to improve the Train inlet measurement to 1% or better thus
and for mitigating against equipment obsolescence.
improving the allocation uncertainty by 2%. The project team
Throughput and Yield Projects (Optimization
chose to improve on the existing single path ultrasonic meters
Projects): Equipment enhancements and upgrades
by replacing them with multi path ultrasonic meters. Since the
geared towards increasing the production capability of
inlet measurement to the LNG train significantly impacted on the
the asset.
apportioned LNG, it had to be as close as possible to custody
metering systems.
With measurement projects both of the objectives can be
achieved.
Ultrasonic meters (USM) were selected as the preferred flow
measurement technology for the following reasons:
As shown in the MEM, the single biggest measurement
point is the inlet measurement to each of the LNG trains. In
1. Endorsed by various regulatory bodies, standards and
general, although measurement projects may not generally
codes for custody measurement, e.g. AGA Report 9, UK
increase production it is the perception of the production that
Department of Trade & Industry’s Petroleum Measurement
is being addressed. This perception translates into financial
Guidelines and the Norwegian Petroleum Directorate.
exposure risk for all parties. As can be seen from the MEM an
2. High accuracy can be achieved in the field when properly
improvement in the train inlet measurement uncertainty from 5%
installed and maintained.
to 1% translates to an improvement in Allocation Uncertainty of
3. High turndown ratios available for individual meters.
2% or in other words a financial exposure benefit of 2% to the
4. Minimum pressure drop.
shareholders.
5. Strong diagnostic built in capability.
6. Low maintenance as compared to other technologies e.g.
Therefore, using traditional techniques for project justifications
Orifice or Turbine meters.
such as NPV, Pay Pack and IRR measurement projects may be
justified on this basis.
However, the USM can be susceptible to noise generated by
control valves. This noise, typically referred to as white noise
As in the case of the Train Inlet Measurement Upgrade project
affects the performance of the meter. Also, the level of noise
assuming that a 4% reduction in measurement uncertainty can
generated by a control valve is a function of the flow-rate,
be practically achieved then this project should be evaluated
differential pressure drop, pressure and valve trim characteristics.
in relation to other initiatives competing for financial resources.
For example, assuming that three days of downtime on a facility
are required for the measurement upgrade and the total cost of
the project is 1.2 million USD, for an LNG complex with annual
production target of 576 TBtu this gives an NPV (financial risk)
of 444 million USD and a payback in less than 1 year with a
profitability index of 17.
7 USM NOISE REDUCTION STRATEGIES
Most control valve manufacturers achieve their low noise levels
by pushing their audible (20Hz-20kHz) emissions out of the
audible range and into a range (20-200kHz) which typically
affect USM detection signals. This was illustrated during our
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
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flow calibration of the meters where a control valve similar to
The newly purchased 24” 4 path USM were calibrated at the
one used at the inlet of the facility was installed to determine
following points as recommended by AGA Report 9. qmin,
the extent of noise generated, and to see how the meter tuning
0.10qmax, 0.25qmex, 0.40qmax, 0.70qmax, and qmax, together
(noise reduction techniques) would impact the performance.
with two additional points. The meter’s performance was
To combat noise, most
USM manufacturers will
have a combination or
all of the following noise
reduction technologies
embedded in the meter’s
electronics:
•
Signal Stacking
•
Digital Filtering
•
Correlation
Techniques
•
Statistical
Methods
Additionally installing blind
Figure 5 - Initial Calibration Results with USM and Flow Standard
tees on the piping system, locating the meters far from noise
checked with the control valve at 5 times the normal pressure
generating equipment and installation of silencers downstream
drop in operation. The following is the sequence of the testing
will mitigate the impact of noise on the meter.
performed and results from Colorado Engineering Experiment
8 FLOW TESTING RESULTS
As a requirement by AGA Report 9, USM being used for
Station Inc (CEESI):
Calibration of the meter using the Flow standard as a referenced
custody transfer should
be calibrated at a Flow
Test Facility. Although
the Train Inlet Meters
(TIM) were not used for
custody transfer they
have significant impact
to the allocation of LNG
on a multi-train complex,
thus the meters were
calibrated by placing
a laboratory Flow
Standard Meter Bank
in line with the USM
being calibrated. The
Flow Standard Meter
Bank being used to
accurately measure flow,
had been calibrated
using standards that are
traceable to NIST.
Figure 6 – USM Flow Testing at CEESI
A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
page 7
in Fig. 5 below shows the results (out of the box meter) in relation
to the flow standard. A meter factor per test point is applied as a
piece-wise (multi-point) linearization.
The above picture (Fig. 7) shows the valve and meter being
Figure 7 - USM and Control Valve at Flow Test Facility (CEESI)
prepared for noise verification checks by comparing the
performance of the meters with the Flow Standard Meter while
throttling the control valve at various differential pressures. Fig.
8 shows the USM signal detection waveform with no noise
present. Fig. 9 shows the USM signal waveform distorted by
noise generated by the control valve. Fig. 10 and Fig. 11 show
the application of Stacking and Digital Filtering (Noise reduction
Figure 8 - Typical detection used by the USM for flow calculations
(No Noise)
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
techniques) respectively which were required to achieve a good
reading on the USM.
Figure 9 - Noise as seen by the USM (Noise distorts the USM detection waveform)
Figure 10 - USM Noise seen with the Application of Stacking
With Stacking Applied only, the USM was still unable to register
a good reading.
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A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
page 9
Figure 11 - USM with Digital Filtering and Signal Stacking
The USM was able to register good readings
(Calibration Results)
Figure 12 – USM Calibration with Flow Standard and Control valve
The illustration above (Fig.12) shows the improvement of the
next step was to execute the installation and commissioning of the
USM Calibration against the Flow Standard Meter Bank when
meters. The meters were shipped and stored until an opportune
the control valve was throttled at various differential pressures.
time for installation. Having the control valve available at CEESI
9 PERFORMANCE OF USM VS CUSTODY ORIFICE
MEASUREMENT SYSTEM
After the flow calibration of the ultrasonic meters at CEESI the
proved invaluable, since the noise characteristics generated
by this particular valve were analyzed and this significantly
minimized the commissioning time. Furthermore this addressed
the concern as to whether the valve and meter in close proximity
DANIEL MEASUREMENT AND CONTROL WHITE PAPER
could fulfil plant flow regulation and product allocation. It must
be noted however, that in some of the installations, further
noise tuning (custom data filtering) was required. This was
readily observed on installations where the pressure drop was
significantly greater than that tested at the facility.
page 10
3. Vapour recovery measurement will aid in the traceability of
LNG production per train.
11 MAINTENANCE OF MEASUREMENT AND ALLOCATION UNCERTAINTY
Figure 13 - The performance of the ultrasonic meters and the custody metering system on the facility
The diagram shows that percentage deviation between the
With the inlet measurement being so critical to the allocation
USM and the orifice measurement system is of 0.16%-0.6%
of LNG and NGL on a multi train complex, this point of
with the USM reading marginally higher than the orifice
measurement must be effectively maintained. Our experience
measurement.
to date has shown that the USM probes must be kept clean for
10 OTHER LNG TRAIN MEASUREMENT ISSUES
good meter performance.
Although, the inlet measurement had the single largest impact
The piping system should be designed such that there is no
on the LNG train there are other areas where measurement
accumulation of liquid upstream of the USM. Additionally, if the
improvements can be made to the facility. However, these
USM are in series with an orifice measurement system (custody),
must be done at the design stage thereby improving on the
the similarity of the analytical data in the USM and the custody is
overall allocation process. These areas include:
of great importance to the flow registration alignment. Operating
the orifice custody metering skids at higher differentials per
1. Fuel Measurement. Installing a good single point of
measurement for fuel enhances the quality of measurement
stream also helps with the alignment of the readings registered
by both systems.
that can be installed and simplifies the number of allocation
points. For example on a facility with 3 LNG trains, having 1
The USM should also be installed with double block and bleed
meter per train for the fuel can replace 10 or more metering
isolation valves so that the meter probes can be easily accessed
points per train. This improvement reduces the cost of
for maintenance. Effective isolation also facilitates the easy
operation and maintenance.
removal of the meters for re-calibration exercises, typically
2. Marine Flare Measurement. There are a number of
every six years.
practical challenges with installing good flare measurement
during plant operation (availability issues). Having marine
Maintenance of secondary instrumentation, temperature and
flare measurement implies that losses can be applied to
pressure transmitters must also be done at regular intervals.
individual shippers accordingly if agreed commercially.
A Practical Approach to Modelling LNG Train Design for
Minimizing Measurement & Allocation Uncertainty
page 11
12 CONCLUSION
REFERENCES:
Maintenance of measurement uncertainty greatly affects the
Allen Fagerlund et al, ‘Identification and Prediction of Piping
allocation of products on a multi train LNG facility. It is therefore
System Noise,’ Noise Conference Oct 2005.
important that all plant measurement systems be optimized for
minimum uncertainty. Building of a MEM can help technical
Bill Johansen & Joel Clancy, CEESI ‘Flow Calibrating Ultrasonic
personnel on the facility quickly diagnose mis-measurement
Gas Meters’ International School of Hydrocarbon Measurement
issues and plan for future capital expenditure. Design engineers
May 2003
would also benefit from using the MEM at the conceptual stage
of a project to plan out the specifications for the measurement
Charles Derr, Daniel Measurement and Control ‘Energy
system.
Measurement using Ultrasonic Meters & Gas Chromatography’
International School of Hydrocarbon Measurement May 2003
Train inlet measurement, which is of great concern to all parties,
must be properly designed, operated and maintained to ensure
Gerrit Vermeiren and Sven Lataire, SGS ‘How Accurate is
equity in the allocation. This paper highlighted the issues on
the Shipboard Custody Transfer Measurement system?’ LNG
inlet USM measurement and control valves. Although USM
Journal July/August 2005
have gained custody appeal, their use must be planned and
coordinated with the flow/pressure regulation valve into the
James E Gallagher, Savant Measurement Corporation, ‘Orifice
facility, since USM are susceptible to noise induced by valves.
Flowmeters and the Estimated Uncertainties in Natural Gas
Service,’ International School of Hydrocarbon Measurement
May 2003
Ronald Roberts
Kevin Warner and Klaus Zanker, Daniel Industries, Inc ‘Noise
Instrumentation & Controls Engineer II
Reduction in Ultrasonic Gas Flow Measurement’ 4th International
Atlantic LNG Company of Trinidad & Tobago
Symposium on Fluid Measurement June 1999.
Point Fortin Trinidad & Tobago
[email protected]
Lars Farestvedt, FMC ‘Multipath Ultrasonic Flow Meters
for Gas Measurement’ International School of Hydrocarbon
Justin Walter
Measurement May 2003
Senior Measurement Consultant
Metco Services Limited
Ronald H. Dieck, ‘Measurement Uncertainty Methods and
Aberdeen, Scotland
Applications’ ISA 4th Edition 2007
[email protected]
STANDARDS:
AGA Report No 9 Measurement of Gas by Multipath Ultrasonic
Meters (1998)
ISO/TR 5168 Measurement of fluid flow: Evaluation of
uncertainties
ISO/TR 7066-1 Assessment of uncertainty in calibration and use
of flow measurement devices
ISO 13686 Natural Gas – Quality designation
ISO 14111 Natural Gas – Guidelines to traceability in analysis
ISO 14532 Natural Gas – Terminology
Emerson Process Management
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