|Lathif Prasetyo Wibisana||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Muhammad Arif Budiyanto||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
liquefied natural gas (LNG) carriers are the main merchant fleets for the
transport of natural gas for energy. The large LNG carrier has a more efficient
freight cost (approximately 7.8 USD/MMBTU) than a small-scale LNG carrier
(SSLNGC; approx. 12,8 USD/MMBTU). Another method must be introduced to make
SSLNGCs more cost-efficient. As such, this work conducted an experiment to
improve the design of SSLNGCs via design and cost optimization by combining a
value engineering approach and multi-objective optimization to decrease hull
resistance and lower construction material costs by adjusting the ship dimension
ratio. By improving the conceptual design and using semi-integrated numerical
simulations, the final result showed improvements in SSLNGCs by decreasing the
hull shell area by 1.57% to reduce the construction material needed and total
ship resistance by 8.3%.
Hull resistance; Hull surface area; Multi-objective optimization; Small-scale LNG carrier; Value engineering
In the natural gas logistics industry, liquefied natural gas (LNG) carriers have played a major role in the trading, distribution, and shipping logistics processes in terms of transportation, loading, and unloading activities. The LNG supply chain can be defined as a natural gas network that begins with the natural gas from gas fields that move to the liquefaction plant to be changed into liquefied gas that is then stored in LNG storage tanks. LNG is distributed to gas users or end users. In the LNG supply chain, the final logistics costs of providing LNG are highly dependent on the length of the logistics chain, with parameters that include net selling price, LNG liquefaction location and final destination, size and route of LNG vessel size and location of unloading terminal, boil-off gas (BOG) utilization, and the availability of gas need. The lower transportation cost per tonnage of cargo compared to other modes of transportation is an advantage for LNG carriers. In line with this, the LNG industry has continued to grow from large-scale refineries and industries to medium- and small-scale refineries and industries (Rensvik, 2013). This growth has been followed by the need for medium- and small-scale LNG carriers (SSLNGC). However, the constraint in operating small-scale LNG vessels is that the transportation costs are significantly higher (approximately 1.5 USD/MMBTU in shipping costs) than those for large-scale LNG carriers (approximately 1.1 USD/MMBTU in shipping costs). Figure 1 shows the industrial benchmark of the freight costs of LNG distribution; the smaller plant has a higher freight cost (Pratiwi et al., 2021).
Small-scale LNG vessels are ships with a small capacity that carry liquefied natural gas to supply gas needs to archipelagic countries that are difficult to access or do not support the installation of gas pipelines. The volume of this LNG carrier is in the range of 2,500–20,000 cbm, with a voyage of 1000 nautical miles. Based on economic principles, distribution with a larger volume would be more profitable. However, there are some special conditions in which the use of small-capacity vessels would be more efficient due to several factors. One factor is the distribution of LNG to archipelagic countries. Considering that Indonesia is limited by the sea and surrounded by volcanoes and tectonic plates, this means it is not conducive to establishing pipelines because Indonesia has high-intensity earthquakes. Compared to the small-scale LNG vessels, the medium- and large-scale LNG vessels see costs of 1.3 and 1.1 USD/MMBTU. As such, larger ships would carry more cargo and be more efficient. However, as we know, the demand for LNG is not only large scale, but also small and middle scale. Inefficiency could make the transport of small-scale LNG unfeasible because shipping costs are high and unfavorable. This is especially true in Indonesia. Most LNG demand arises on a small scale for gas power generation in eastern/western Indonesia, which are separated and have small-scale gas storage capabilities for power generation. Based on these main issues, this research aims to optimize small-scale LNG shipping to improve efficiency and competitiveness in the shipping market.
Figure 1 Comparison of the average freight rate of LNG carriers (Engblom, 2016)
The experiment showed that the size ratio significantly affected ship resistance. This could also be proved by a common ship resistance empirical calculation. With a combination of shape optimization and value engineering, the methodology can optimize SSLNGC design to the optimal level from the conceptual design to the detail process. This optimization study can be applied commercially because this methodology only uses commercial software without advanced research tools. As a result of the improved design, the propulsion efficiency improved as total resistance was decreased by 8.3% and hull surface area was reduced by 1.57%. An economic study of the optimized design (estimated reduction in investment costs and propulsion costs) was conducted in this experiment.
Combining value engineering and optimization can be practically done, and the combination of the two processes has an impact on the efficiency of ship production costs and total ship resistance. Both the combination and each optimization process contributed to increased efficiency. By modifying the ship's L/B ratio, the total ship resistance was reduced. In line with this, the optimal hull shell area was also obtained.
The research process is still in its early stages. In future research, we will continue to integrate the results of total resistance using CFD software to get a more precise estimate of ship resistance and to validate empirical calculations. In addition, to sharpen the results of the optimization of the shape of the hull ratio in the future, more comprehensive methods such as MOGA/NSGA II can be used as optimization algorithms, and the optimization will use CAESES® for numerical computations. This combination can not only be used for construction and transportation projects, but for marine and offshore projects, as has been done in this study. In the future, this research will be developed not only from the ship's hull components but also holistically from the entire system and machinery in the ship (cargo tanks, fuel selection, boil-of-gas treatment plant, etc.).
project was initiated by the Mechanical Engineering Department University of
Indonesia and supported by funding for Basic Research (PDUPT) from the Ministry
of Research and Technology/National Research and Innovation Agency Republic of
Indonesia for Fiscal Year 2021. Number: NKB-035/UN2.RST/HKP.05.00/2021.
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