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AN OVERVIEW OF LIQUEFIED NATURAL GAS (LNG) SHIPS – LNG SHIP MARKET DEVELOPMENT AND PREDICTION, SHIP STRUCTURES, PROPULSION SYSTEMS AND APPLICATIONS

  • 11 Pages
  • Published On: 28-10-2023

Introduction

Liquefied natural gas (LNG) carriers are ship tankers designed for transportation of liquefied natural gas (LNG). Ahn et al. (2017) report that increasing population and improved standards of living across the world are driving growing demand for energy. Noble (2010) reports that gas reserves are unevenly distributed in different regions, and the rising demand for the commodity makes LNG transportation a serious imperative. LNG is a “cleaner” fuel when compared to the closest alternatives such as diesel. With increasing environmental awareness driven by the need to promote environmental sustainability, individuals, groups, organisations and countries are increasingly adopting “clean” energy such as LNG

In most cases, it is not possible (or too expensive) to transport LNG by pipeline. In such instances, LNG ships are worthy replacements given that LNG, when in liquid form under standard conditions of pressure and temperature occupies 1/600 of the volume of natural gas (Hansen & Lysebo, 2004).

This essay provides an in-depth analysis of LNG ships (carriers) and an extensive overview of the LNG ship market development and prediction, ship structures, propulsion systems and applications. The information presented has been sourced from major manufacturers and researchers.

LNG Ships

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As aforementioned, LNG ships or gas carriers are oceangoing tankers used in the transportation of liquefied natural gas. According to the International Gas Union (IGU) report of 2016, the global LNG shipping fleet consists of over 440 vessels with an installed capacity of 60 mmcm. As of January 2017, Ahn et al. (2017) report that there were over 100 builds on order across the world.

LNG Ship Market Development and Prediction

According to METI (2016), traditional LNG tankers were built according to specifications of a given project. As such, LNG was transported to particular LNG terminals using the specially built and customised LNG carriers. However, in recent times, spot trading and high demand projections for LNG have led to the development of LNG ship markets. According to MAN Diesel and Turbo (2013), natural gas is widely regarded as the fuel of the twenty-first century. For this reason, advancement in the design of carriers is pivotal in the growing market. Noble (2010) states that growth in ocean trade and production of LNG has averaged about 6.5 percent annually but has spiralled to 9 percent in the last few years. Going by the huge growth margin, LNG design has become an attractive area of research with the aim of developing new fuel efficient, cheaper and safer LNG carriers. Munko (2012) notes that in recent years, researchers have embarked on studying design optimisations such as larger block coefficients, the effect on membrane tank sloshing loads, propulsion options and alternative container technologies. According to IGU (2016), total LNG trade reached 244.8 MT in 2015 up from 4.7 MT in 2014, representing the biggest growth ever in LNG trade. Correspondingly, there was a massive increase in the number of new LNG carriers to transport the commodity. IGU (2016) reports of a wave of new-build orders that began in late 2012 into 2013. The new-build orders were associated with major LNG projects in the United States and Australia. In 2015, speculative building contributed to the oversupply of LNG vessels (Ahn, et al., 2017). In today’s market, there are over 40 unchartered vessels. IGU (2016) further observes that the LNG market is currently characterised by a huge appetite for larger LNG ships. In 2016, 46 tankers were delivered from shipyards, and there is an oversupply of LNG carriers going by increased number and bigger tonnage (see Appendix B). In fact, Ahn et al. (2016) clearly demonstrate that in 2017, the order book represents over 40 percent of existing fleet and the oversupply will impact LNG shipping market shortly. It is important to note that owners of older vessels create value by converting old LNG ships to FSRUs or use them as floating storage units.

IGU (2016); Ahn et al. (2017) predict a huge increase in average capacity of LNG ships by a margin of 10 percent. With the expansion of Panama Canal, the orders for larger vessels spiralled. In fact, over 87 percent of the total fleet has already registered with the New Panamax Class of large ships. In the next five years, supply is set to reduce given that in the current stage, there is already an oversupply of capacity (See Appendix A). MAN Diesel and Turbo (2013) observes that the demand for lower LNG transportation costs is adequately met by the expanding fleet of LNG carriers and their expanding capacities.

LNG Ship Structures

According to Lloyd's Register (2016), there are two type structures for LNG ships namely self-supporting tanks (free standing) and integral tank construction (see Appendix C). In self-supporting tank structures, cargo containers are independent of shipping hull which creates an allowance for expansion and contraction (Munko, 2012). The main advantage of this structure is that it facilitates easy inspection for leakage and immediate affirmative action. The main disadvantage is that there is a waste of cargo space and capacity due to the gaps left between the cargo tanks and the ships’ hull.

Integral tank construction structure reduces the dead space between cargo tank and ship hulls. For this reason, there is better utilisation of space on board (see Appendix D). The main advantages include saving on space, less dead space to be monitored and purged and similar construction technique for all tanker sizes. The main disadvantage is difficulty in the insulation of the cargo tanks (Noble, 2010).

LNG ships can also be classified according to their cargo containment designs. There are various types of cargo containment systems as follows: Kvaerner-Moss spherical tank (see Appendix E), membrane system, Mark III, No 96, Cs1, Gaz Transport and Technigaz (GTT) and IHI Prismatic (American Bureau of Shipping, 2014).

The Kvaerner-Moss spherical tank design employs spherical tanks made from 9 percent heavily insulated steel or aluminium alloy. As such, the design entails an independent tank with a secondary barrier. Often, plastic foam is used to cover the outer surfaces of cargo tank walls. The main disadvantage of this design is that it has a low utilisation of a limited volume and cannot support flat deck on offshore amenities (International Gas Union, 2016).

The membrane containment system design uses primary membrane made from a nickel-steel alloy called Invar, a reinforced polyurethane insulation foam and a triplex secondary membrane. Commercially manufactured by GTT and known as Combined System 1 (CS), they can be extensively prefabricated through rationalised assembly processes. A similar development is being undertaken by Korea Gas Corp, but its model is much simpler with a single layer of insulation (Lloyd's Register, 2016).

The IHI self-supporting Type B Prismatic Tank is a free tank with a partial secondary barrier as in the conventional frame and stiffened plate system. Sloshing is not a major problem in this design type due to the presence of hardened plate and frame systems. The tanks are insulated on their outer surfaces, and they rest on a system of wooden block supports (Noble, 2010).

Propulsion Systems and Application

LNG propulsion systems can be divided into three categories as follows:

Category 1: HFO/GAS fuel flexibility

Examples include diesel-electric propulsion systems, slow-speed dual fuel diesel engines and steam turbine systems (Hansen & Lysebo, 2004).

Category 2: Pure HFO burning system

The leading example is slow-speed dual-fuel engines with the capacity for Natural Boil-off Gas (NBOG) reliquefaction installed on board (Deeb, 2013).

Category 3: Pure GAS burning system

For instance, combined electric and gas turbine cycle

Steam propulsion systems, gas turbine propulsion systems and combined cycles of gas turbine and steam turbine (COGAS) are used for LNG carrier propulsion. Gas turbine propulsion system uses a diesel tank for ballast voyage but has a low efficiency ranging from 30 – 35 percent (American Bureau of Shipping, 2014). Steam propulsion system uses hot steam and has a lower efficiency ranging from 25 – 30 percent (MAN Diesel and Turbo, 2013). COGAS uses effluent gas obtained from gas turbines for production of steam to generate extra power and efficiency can be as high as 60 percent (Deeb, 2013). According to Sato (2014), COGAS is increasingly gaining market in cruise ships and ocean liners due to the advantages associated with high efficiency of the COGAS propulsion system. Additionally, Yeo et al. (2006) note that COGAS propulsion system has higher power density than other systems, low noise, low emissions and low maintenance costs. See Appendix F for distribution of propulsion systems

Modern LNG ship propulsion has become an area of interest for researchers, and newer developments and breakthroughs are occasionally reported.

In summary, the future for LNG ships is promising given that huge onshore and offshore gas discoveries are occasionally reported. Apart from Asia and the Americas, significant discoveries have also been made in Africa, and the continent has become a major exporter of the commodity. Although the existing LNG carrier capacity exceeds the demand, the situation is likely to change as the world adopts cleaner energy resources. Again, going by the enormous increase in LNG trade from 2014 – 2015, LNG shipping will become a growing area of interest with the aim of optimising designs, structures and propulsion systems.

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References

Ahn, J., You, H., Ryu, J. & Chang, D., 2017. Strategy for selecting an optimal propulsion system of a liquefied hydrogen tanker. International Journal of Hydrogen Energy, Volume 30, pp. 415-424.

American Bureau of Shipping, 2014. Propulsion systems for LNG carriers. New York: American Bureau of Shipping.

Deeb, H., 2013. Structural design of mega LNG carrier. Szczecin: EM Ship Advanced Design. Hansen, J. & Lysebo, R., 2004. Electric propulsion for LNG carriers. LNG Journal, 11(5), pp. 11-12.

International Gas Union, 2016. 2016 world LNG report. New York: IGU.

Lloyd's Register, 2016. Procedure for membrane tank LNG ships. s.l.:s.n.

MAN Diesel and Turbo, 2013. Propulsion trends in LNG carriers-two stroke engines. Copenhagen SV: MAN Diesel & Turbo.

METI, 2016. Strategy for LNG market development. Tokyo: Ministry of Economy, Trade and Industry.

Munko, B., 2012. Economic design of small scale LNG tankers and terminals. Amsterdam: TGE Gas Engineering.

Noble, G., 2010. The next generation of large LNG carriers for long distance and harsh environments. Houston: ConocoPhillips.

Sato, K., 2014. Design of the evolutionary LNG carrier "Sayaendo. Yokohama: Mitsubishi Heavy Industries (MHI).

Yeo, D., Ahn, B., Kim, J. & Kim, I., 2006. Propulsion alternatives for modern LNG carriers. Seoul: Samsung Heavy Industries.

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