Time is money! Every hour an LNG carrier can sail away earlier from a FSRU will have a significant positive financial impact. So, it is important that a FSRU can always operate at its maximum flow rate to minimize transfer time. Consequently, the liquefied gas sampling pressure at the cross-over manifold at the FSRU‘s will be at its minimum. In practice it can be lower than 1 barg.
In the May 2022 issue of LNG Industry ASaP’s founder: Hans-Peter Visser answers the question how to measure and sample LNG in a cost-effective and accurate way to make sure every Btu counts.
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In this article about LNG
- Liquefied Natural Gas custody transfer in short
- Keep your head cool with our products & services
- The right tools for the job – May 2022 issue of LNG Industry
- The ultimate solution
- It is the complete package that counts
- LNG test unit – The 3rd generation design from ASaP
- Need help choosing your next LNG solution?
Liquefied Natural Gas (LNG) custody transfer in short
Liquefied Natural Gas (LNG) custody transfer is the process of transferring custody and ownership of LNG from one party to another. Custody transfer occurs when LNG is sold or transported between different entities, such as producers, suppliers, buyers, or shipping companies.
The custody transfer process involves several key steps to ensure accurate measurement and accountability:
- Measurement: Accurate measurement of LNG quantity is crucial during custody transfer.
- Sampling: Along with measurement, representative LNG samples are taken during custody transfer. The sampling of the Liquefied Natural Gas (LNG) for custody transfer is done by means of an analytical vaporizer connected to a sample container and on-line analyzer. The location of the sampling point(s) is at the cross-over manifold. Nearly all analytical sampling systems until now operated at pressures above 2,5 barg.
- Documentation: Detailed documentation is prepared to record the transfer of custody.
- Verification: Independent inspectors or surveyors may be involved to verify the measurements, sampling procedures, and documentation.
- Transfer point: The physical transfer of custody usually takes place at designated transfer points, such as LNG terminals, storage facilities, or loading/discharging ports.
- Confirmation: Confirmations, notifications, and updates regarding the transfer are exchanged between the buyer, seller, shipping company, and any other relevant entities.
- Insurance: Parties involved in the custody transfer often have insurance coverage to mitigate potential risks associated with the LNG cargo.
The custody transfer process ensures that accurate quantities of LNG are exchanged between parties and that ownership is legally transferred. It provides accountability, transparency, and reliability in the LNG industry, enabling smooth operations and financial transactions.
The Catch-22 is: lose money on waiting time of the carrier (increase the pressure by squeezing flow), or lose money on inaccurate LNG analysis?
Keep your head cool with our LNG products & services
The right tools for the job – May 2022 issue of LNG Industry
Hans-Peter Visser, ASaP B.V., the Netherlands, considers how to measure and sample LNG in a cost-effective and accurate way to make sure every Btu counts.
Just Google ‘LNG sampling and analysis’ and there will be a list of companies stating they can sample and measure LNG accurately. The main question is: how do you test it? And once it has been tested with a factory acceptance test (FAT) or a site acceptance test (SAT), how can it be ensured that the measuring results remain as accurate as initially intended?
Given the current global situation and energy prices, this topic has a higher financial impact than ever. Reuters reported on 4 March 2022 that “Asia LNG spot prices hit a record high of more than US$59/million Btu, tracking a surge in European gas prices on concern over tight supply after Russia’s invasion of Ukraine. Price agency S&P Global Commodity Insights’ Japan-Korea-Marker (JKM), which is widely used as a spot benchmark in the region, climbed to US$59.672/ million Btu on Thursday, its data showed.”1
In general, good measurement starts at the beginning, taking a representative sample off from the process stream by means of a sample take-off probe. Due to its cryogenic characteristics, good and reliable sampling of LNG is a whole different ballgame.
It all started at the third (L)NG analytical workshop, ‘Custody Transfer And Quality Assurance’ held September 2007 in Bintulu, Malaysia, jointly organized by Shell Global Solutions and Malaysia LNG.
During the workshop, the feedback from nearly all LNG end users was the lack of a reliable way of sampling and measuring LNG.
During this workshop, the idea arose to create a completely new design for a combined LNG probe/vaporizer, eliminating all shortcomings and elementary errors of the existing equipment.
Meanwhile, many technical articles and lectures have been dedicated to this most important subject. In a nutshell, it comes down to the following:
The natural gas which is processed prior to liquification consists mainly of methane. Depending on the source of the explored natural gas, the heavier hydrocarbons such as ethane, propane, butane, pentane, and even hexane may vary in their composition. Often some nitrogen is also present. During processing and liquification, LNG is produced as a liquid hydrocarbon mixture with a typical boiling point of -162˚C or -260˚F at atmospheric conditions.
During (un)loading and custody transfer of the LNG, the energy content and density of the LNG must be almost constantly sampled to dedicated cylinders and continuously measured as per most common sales and purchase agreements (SPAs). The composition of the LNG is typically measured by an online gas chromatograph (GC), derived from the composition of the physical properties such as calorific value (Btu value), density, etc. of the gasified LNG are determined. In general, an online GC gives a measurement update between 3 and 15 min.
The most crucial and critical part of the measurement is the sampling. Specifically, at the point where the LNG sample is taken from the LNG transfer line and transported to an analytical vaporizer. In the vaporizer, the LNG must be converted into a stable and homogeneous hydrocarbon gas mixture which represents the LNG at the time it passes to the transfer line. The process of sample taking and vaporization of LNG is continuous and instantaneous.
Often the correct measures are not taken when transporting the LNG to the vaporizer. Due to the large temperature difference between the LNG and the ambient temperature, the LNG tends to start boiling uncontrollably before it reaches the vaporizer. This is also referred to as pre-vaporization.
Unfortunately, it is still often the case that, for commercial reasons, cheaper vaporizers are used that are not designed for LNG. This therefore results in non-homogeneous and non-uniform vaporization, also known as pre- and partial evaporation.
The results of pre- and partial vaporization measured by a cyclic measurement such as a GC results in measurement trends which are all over the place or erratic reading. Examples of more than 50% outliers are unfortunately not uncommon.
Figure 1 shows a typical LNG production plant. An LNG loading line and the LNG recirculation line are both equipped with the same brand and model vaporizer, measuring the same LNG by a common GC (stream switching by the GC).
Next, the gross heating value (Btu content) is given of both vaporizers, represented by the dark blue and purple data points (Figure 2). The erratic character is obvious to see.
What makes matters worse is what is seen in the trend lines of both vaporizers, represented in Figure 2 by the yellow and green lines. These trendlines are completely opposite to each other. So, the real question is which result is correct or true? Unfortunately, nobody knows.
What is known, however, is the amount of US dollars related to such measurement uncertainty. Assume an cargo of 170.000 m3 against an Asia LNG spot price of US$59.67 and an uncertainty of 0.52% on the energy content. This results in a measurement error equal to US$1.2 million for each cargo (Table 1).
The ultimate solution
The only way to sample and convert LNG into a homogeneous natural gas mixture is by avoiding pre- and partial vaporization, create flash vaporization, and good mixing. This must be done under tight and controlled circumstances where electrical power and the heat exchange surface area are important factors.
However, the most important step is getting a cryogenic liquid out of the process transfer line and transporting it to the vaporizer. This is the step where there can be challenges. Typically, thermal or vacuum jacketed insulation is used, but that is often not enough. The amount sampled is extremely small compared to the mass of the wetted parts which transport the LNG. Heat ingress is inevitable and pre- and partial vaporization will occur, resulting in incorrect analytical results and therefore losing US dollars on the LNG cargo transferred.
The solution is active sub-cooling. Active sub-cooling is using the characteristics of LNG and its thermodynamic properties. Active subcooling compensates more than sufficiently for the amount of heat ingress and therefore guarantees that no pre- and partial vaporization will occur. This results in outstanding analytical results repeatedly and under all circumstances.
Figure 3 shows the measurements at the LNG transfer at one of the major global LNG producers. The uncertainty of the Btu content measurement is unmatched so far.
This measurement has an error which amounts to US$6.673 per cargo. This is negligible compared to a measurement error of up to US$1.2 million per cargo (Table 2).
It is the complete package that counts
The probe/vaporizer is a part of the whole analytical installation, perhaps the most important, but a chain is only as strong as its weakest link. An LNG sampling and measurement system that complies with ISO 8943: 2007 and G.I.I.G.N.L. 6th edition typically consists of the following main parts:
- Sample Probe.
- LNG vaporizer.
- (Heated) sample lines.
- LNG sampler
The probe till the final data used for the bill of loading (BOL). Typically, the focus was on the hardware performance but now, with dedicated software, the whole measurement chain can be monitored overall and in detail. Monitoring can be done locally or remotely anywhere in the world.
By extending the online measurements for LNG flow and density as well as the BOG flow and composition, the whole energy equation can be done online and instantaneously. This will eliminate the risk of human error and saves a considerable amount of time. But most importantly it makes every Btu count, hence every US dollar count as well. The equation for the BOL can be found in detail in the G.I.I.G.N.L 6th edition as shown in Figure 4.
For such a system, the typical architecture is shown in Figure 5.
Since size matters, these parts will typically be built on one or more so-called skids. Another advantage of skids is that the battery limits are clearly defined and the skids can be pre-tested and transported as one.
In conclusion, keeping a tight handle on finances is more important than ever in the energy industry. It must be ensured that every Btu and every dollar is accounted for. For transfers, this can only be achieved by using the right equipment and techniques in the analysis of the transferred LNG that comply with ISO 8943: 2007 and G.I.I.G.N.L. 6th edition. As demonstrated, LNG is extremely difficult to analyze and it is of key importance to choose the right equipment and the right partner to supply it.
- Marwa Rashad, Reuters, ‘Asia LNG spot prices hit record high, S&P data shows’, (March 2022).
LNG test unit – The 3rd generation design from ASaP
ASaP is actively participating in (re)writing LNG and other International Standards. Therefore the need for testing is well embedded in the ASaP organization.
For the Dutch metrology institute, VSL, ASaP designed, built and tested the first and only commercially available test unit according to EN 12838: 2000. For internal tests ASaP has designed and built 2 generations of test units.
With new market developments emerging, like optical in situ measurements based on Raman and Infrared spectroscopy, ASaP designed a new LNG test unit. ASaP’s 3rd generation test unit is able to test two identical or different devices simultaneously to determine both their uncertainty as well as the differences between two devices and/or analytical methods under identical, controlled and simulated process conditions..
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