Real-time gas analyzer to improve the operation of liquefied natural gas ships


There are different technologies available on the market to measure the quality of liquefied natural gas for evaporation gas applications. Traditionally, gas chromatographs have been used to measure the composition of the gas and its calorific value. These systems require calibration/carrier gases and require manual operation while underway.

Shipowners have expressed interest in alternative technologies. Technologies that reduce both the cost of analysis and manual handling on board ships. Obviously, by maintaining or improving the accuracy of gas quality measurements.

Tunable gas analyzer technology

The gas analyzer measures the composition of the gas using infrared absorption spectroscopy. Each gas component has a unique infrared “fingerprint”. A small stream of gas from the evaporation gas fuel tube is drawn by a sample probe. It is then introduced into the gas analyzer.

By illuminating the gas sample with infrared light at various wavelengths, the analyzer shows us its “fingerprint”. It determines the presence as well as the concentration of the individual gas components. The measurement principle is illustrated in Figure 1. A key component in the analyzer is a (patented) micro-electro-mechanical (MEMS) filter. This filter is widely tunable and capable of scanning the wavelength of infrared light continuously over a wide bandwidth. Wide scanning allows all gas components of interest to be identified and quantified and minimizes cross-interference.

In addition, the fast response of the MEMS filter makes it possible to measure dynamic changes in the mix. Even when sampling relatively small volumes of gas.

Figure 1. Measurement principle of Tunable’s natural gas analyzer

Better data insights for a better understanding of fuel consumption

Some liquefied natural gas carriers have installed gas analyzers to measure the quality of the evaporation gas that is consumed as fuel during navigation. Usually, the shipowner and the charterer agree on fixed rates of evaporation and fuel consumption as part of their charter/transport contract.

However, actual gas consumption and value are affected by the actual gas mix supplied during the trip and may differ from those expected. By having access to online gas analysis of the actual fuel consumed during navigation, the shipowner and charterer obtain accurate data that could benefit and alter their commercial agreements.

Full-scale field test at FSRU Höegh Galleon

Höegh LNG and Tunable started a technological cooperation in 2018. The objective was to test, on board the FSRU (*) Höegh Galleon, a gas analyzer technology that requires less manual support, using the Tunable optical gas analyzer to measure the quality of the gas. evaporation gas. A standard gas chromatograph and Tunable gas analyzer were installed in parallel to verify and compare the performance of the two technologies.

For both technologies, the evaporation gas was extracted using an identical sampling probe. It was introduced into the system of their respective gas analyzers. Figure 2 shows an image of the Tunable gas analyzer installed on the galleon FSRU Höegh.

Figure 2. Gas analyzer installed in FSRU Höegh Galleon

 

(*) FSRU = Floating Storage Regasification Unit = Floating Storage and Regasification Unit

Experience from large-scale field trials

Both analyzers have been in operation since September 2019. Since then they have continuously measured the gas quality and the heating value of the evaporating gas. During this period, the Tunable analyzer has been in continuous operation providing calorific value C1 – C5 + N2 + as can be seen in figure 3.

Figure 3 C1-C5 + N2 data output from FSRU Höegh Galleon tunable analyzer June-September 2021

To analyze the data in more detail, we have shown in figure 4 a month of operation with data from both instruments. In this summary we see that both instruments provide a similar data reading. During the return trip before the next load, the liquefied natural gas tank is sprayed with methane to maintain the temperature of the tank. This operation is identified by the Tunable parser as you can see on the red line.

Figure 4 . FSRU Höegh Galleon methane reading from April to May 2020

 

The goal of the full-scale test, in addition to demonstrating consistent long-term readings, has been to gain operational experience of the system after use offshore. From its commissioning in September 2019 to today, the Tunable analyzer has been in continuous operation with no operational issues.

Analysis of results

During this period, the system has functioned without the need for manual support. Also, since the system does not need any calibration/carrying gas bottles to operate. Additional logistical problems for the ship’s crew have thus been avoided. Remote condition and service monitoring have been successfully tested via datalink during operations, eliminating the need for on-board service personnel.

Installation experience is that the system is small and weighs less than 30 kg for the complete analyzer system. It can also be easily hung on the wall near the location of the sample probe.

Another benefit confirmed by the test is that the gas analyzer system has provided continuous gas flow measurements. In this way it provides rapid data responses to operational changes in gas quality. Being a continuous measurement, the system is less sensitive to the distance between the sampling probe and the analyzer. This allows the data to be used to improve the performance of the engines.

Real-time analysis to improve the performance of liquefied natural gas vessels

Many shipowners are looking for alternative fuels to reduce their GHG (greenhouse gas) footprint. For this reason, we see a substantial increase in ships powered by liquefied natural gas. Variations in gas quality, particularly when natural and forced evaporation gas are combined, make it difficult to operate engines at optimum load. By gaining a better understanding of the quality of the gas entering the engine, it is possible to improve its efficiency.

By providing real-time data, the Tunable Gas Analyzer allows engines to be operated at a higher load when using fuel with variations in gas quality. The direct implication is that ship operators can have a higher load level on one engine before the next one is started without risk of pitting.

This translates into direct savings in fuel consumption. This is so as the motor will run with higher efficiency and it also reduces the running hours of the motors. In addition to direct fuel savings and reduced engine maintenance costs, the shipowner will benefit from a reduction in the ship’s total gas emissions.

Conclusion

The field test demonstrated that the Tunable gas analyzer successfully measured gas mixtures in accordance with the specifications required for LNG carriers. The benefits to shipbuilders are that they get on-site readings without delay and without calibration gas consumption.

Parsers require less support. This implies significantly lower operating and maintenance costs; compared to alternative technologies.

The trial has also shown that the technology is well suited for future exploration of multi-gas data flow analysis for dynamic engine optimization.

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