Numerical Study on Characteristics and Control of Heading Angle of Floating LNG Bunkering Terminal for Improvement of Loading and Off-loading Performance

In this study, heading characteristics and heading control performances were evaluated to achieve the wave shield effect. The wave shield effect originating from heading control reduces the relative motions of moored vessels in a floating liquefied natural gas bunkering terminal (FLBT). Therefore, loading and off-loading performances are improved through reduced relative motion. For the objective of this study and efficiency of the analysis, a simplified model was used that assuming no relative motion of the moored vessels in the FLBT. The simplified model involved modeling the environmental loads and inertia of several floating bodies, including FLBT, into the environmental loads and inertia of a single vessel. The simplified model was validated through comparisons with model tests. With the simplified model, heading characteristics and heading control simulations were performed using low-frequency planar motion equations. The heading characteristics and heading control performances of FLBT were analyzed through the results of simulations under the expected environmental conditions. The capacity of the tunnel thrust for the heading control performance was confirmed to be adequate for improvement of the loading and off-loading performances using the wave shielding effects under the operation conditions. Received 5 February 2020, revised 5 March 2020, accepted 9 April 2020 Corresponding author Seunghoon Oh: +82-51-604-7825, carot541@kriso.re.kr It is noted that this paper is revised edition based on proceedings of the annual fall meeting the KSOE 2019 in Gimhae. c 2020, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 78 Seunghoon Oh et al. as the loading and off-loading operations are generally performed in the environment where one or more floating bodies are moored in the FLBT, evaluation of the heading angle characteristics and control performance of the FLBT with multiple floating bodies is required. In this study, heading angle control was numerically analyzed to evaluate the heading angle characteristics and control performance of the FLBT with multiple moored vessels. Environmental loads on the off-loading vessels, including the FLBT, were calculated through verified model tests and numerical analysis. As the mooring line of the vessel moored at the FLBT is taut, the amplitude of relative motion is small in terms of heading angle control. The relative motion of a vessel moored using the taut mooring can be ignored. Therefore, the heading angle characteristics and control performance were analyzed using a simplified model, in which the vessels moored at the FLBT were considered as a part of the FLBT. The environmental loads (wind, current, and wave) and inertia (mass, moment of inertia, and added mass) acting on the multiple vessels were replaced by the loads and inertia of a single vessel to construct a simplified model. Model tests and comparative studies were conducted to confirm the validity of the simplified model. Low-frequency planar motion simulation of the FLBT was performed using the simplified model that was validated under the environmental conditions of the expected FLBT installation position, and the heading angle characteristics of the FLBT were identified. Finally, the heading angle control performance, which is essential for the ocean wave shielding effect to improve the loading and off-loading performances, was evaluated. Proportional-derivative (PD) control was used for the heading angle control, and the Lagrange multiplier method was used as the thrust allocation algorithm. Through the performance evaluation of the heading angle control, the range of the controllable heading angles was verified, and the result can be used as a guide for the operating procedure. 2. Simplified Model for Loading and Off-loading Scenarios and Evaluation of Heading Angle Control Performance 2.1 Off-loading Scenario The FLBT receives LNG from LNG carriers and supplies the received LNG to the vessels with LNG propulsion systems through the LNG-BS. The FLBT developed in this study is shown in Fig. 1. It was designed to receive LNG from a 170K LNG carrier and unload the LNG to 30K and 5K LNG-BSs simultaneously. According to a study on the operating procedure of the developed FLBT, it can dock up to three vessels simultaneously, and the total operation time when three Fig. 1 Arrangement by the scenario of loading and off-loading operations Fig. 2 Maximum relative motion response in irregular wave conditions (Jung, 2019) Numerical Study on Characteristics and Control of Heading Angle of Floating LNG Bunkering Terminal 79 vessels are docked in the FLBT for off-loading was analyzed to be significantly short compared with the entire off-loading operation time. However, to determine the range of loading and off-loading, the operating conditions when the maximum number of vessels is docked should be considered. Kim et al. (2017), Kim et al. (2018), Jung et al. (2018), and Jung (2019) conducted model tests and numerical analysis of the relative motion of docked vessels in an FLBT during operation to examine the loading and off-loading performances of the FLBT. They confirmed that it is necessary to have the shielding effect from the sea waves and maintain the heading angle to improve the loading and off-loading performances of the 5K LNG-BS. The results of the study conducted by Jung (2019) confirmed that all the vessels docked in the FLBT with the heading angles ranging from 202.5° to 225° were capable of stable loading and off-loading as shown in Fig. 2. In this study, based on studies on the operating procedure of the FLBT and the loading and off-loading performances (Kim et al., 2017; Kim et al., 2018; Jung et al., 2018; Jung, 2019), the heading angle characteristics and heading angle control performance will be evaluated to improve the loading and off-loading performances. 2.2 Environmental Conditions Based on the data survey on the environmental load, the wave deformation and seawater flow were numerically analyzed in the estimated installation area to evaluate the design conditions according to the frequency. As shown in Table 1, it was assumed the operating condition, under which docking, loading, and off-loading are performed, with a one-year return period. The environmental conditions were determined by selecting the expected installation area, as shown in Fig. 3, to evaluate the design conditions of the FLBT. Fig. 3 Proposed sites for FLBT operation Table 1 Operation conditions (1-year return period)