The coastal construction has created enormous social and economic benefits, but the construction project has caused sediment transport in the surrounding waters, causing erosion and siltation changes which may even cause the original dock abandoned (Tsoukala et al., 2015; Plomaritis and Collins., 2013; Song et al., 2017; Zhang et al., 2005). Meanwhile, it exerts negative effects on marine ecological environment, arousing wide public concern of scholars (Sravanthi N, 2015; Yao et al., 2018; Gu et al., 2012; Tian and Xu, 2015).The suspended sediment produced during the construction process will form water masses with high suspended matter content within a certain range, weakening or even blocking the light transmission capacity of the water body, affecting the photosynthesis of phytoplankton. The reduction in the number of phytoplankton will cause a corresponding reduction of zooplankton. In addition, suspended sediment will attach to the surface of aquatic animals, interfere with their normal physiological functions, and more seriously enter the digestive system, causing death (Huang et al., 2019). Zhang (2015) used the ECOMSED model to simulate the terminal project of Shandong LNG project. The research obtained the maximum spreading range of suspended sediment produced by excavation of base trenches, stone dumping and dredging works during the spring and neap tides, and analyzed the impact of the project on marine life. Yan (2019) established a two-dimensional model by using MIKE21 FM, simulating the envelope area of the suspended sediment caused by the 10 kv submarine cable laying at Lvhua Island-Huaniao Island. The results show that the construction period has a greater impact on the marine ecological environment, and the service period has basically no impact on the marine ecology. In order to serve human life and protect the environment, numerical simulation has been widely used in engineering construction (Vu, Nguyen and Nguyen.,2020; Agrawal et al.,2019).

Floating LNG storage and regasification unit (FSRU), which integrates LNG receiving, storage, transfer, regasification and export and other functions, it can used as LNG receiving terminal while it is moored at the dock and can also be used for the transportation of LNG. As the emergence of FSRU, it has become an important option for the receiving of LNG, and adopted by more and more countries and regions. According to statistics, up to now, there are 30 projects in operation and another 8 projects area being under construction.

In the loading and unloading operation period, ship to ship operation mode between FSRU and LNG is usually used, as shown in Figure 1. While this mode is used, the hydrodynamic interference and coupled motion between them are more complex. Especially when the two ships are very close to each other, the disturbance between the two ships is aggravated, which brings great safety risks to the loading and unloading operation. Therefore, it is necessary to consider the interaction of the hydrodynamic forces between the two ships, and know well about the hydrodynamic characteristics of the two ships in different conditions and the wave surface rise between the two ships.

The challenges to design Floating Liquefied Natural Gas facilities have attracted enough attention from industry and academia. The FLNG is used for production, liquidation, storage and unloading offshore gas. Therefore, the FLNG system needs lager volume tanks and has no restrictions of filling condition. Liquid tends to slosh in a partially tank during shipping. Sloshing-induced slamming in a tank at low filling depth resulting in the structural local damage during FLNG shipping is one of the main loads in the design of super-large storage tanks (Gavory and De Seze, 2009). Thus, it is important to determine the slamming load and investigate the physical evolution of a wave impact during the sloshing process in a partially field tank. But during liquid impact, the physical phenomena with gas-liquid, gas-solid and gas-liquid-solid are complicated for researchers to predict the evolution of free surface and slamming load with theoretical and numerical methods (Abramson et al., 1974; Lee and Choi, 1999; Faltinsen and Timokha, 2009). For example, Delorme et al. (2008) numerically found that the maximum pressure calculated by their numerical method is greater than the experimentally measured values due to the presence of air. Therefore, from physical mechanics point of view, it is necessary to use experimental methods to study the liquid impact with entrapped air in the tank.

When the rescue ship assistant accident ship, such as transporting the wounded, delivering the supplies or carrying out some other operation, two vessels would have to come to a relatively close proximity. The relative location of two ships is similar to the FPSO–shuttle tanker system (Koo, Kim,2005), however both of the ships are unconstrained. In the situation that the weather is terrible, the progress is actually dangerous for the ships would move violently. In order to avoid the accident, it is necessary to predict the motion of ships.

The multiply body question has been searched for a long time. Hong and Kim (2005) proposed a higher-order boundary element method (HOBEM) and certificate it by experiment. The result shows that the simulation based on HOBEN has a good agreement with the experiment result in ship motion and wave drift force except for the wave drift force in the narrow gap between two ships which has a strong interaction due to Helmholtz resonance. Chen (2005) proposed the damping lid method. He added a dissipative term in the free surface boundary condition inside the gap to get a better accuracy in wave drift of the narrow gap of two ships. After that, studiers start to solve the problem in time domain. Nam and Kim (2016) simulated the berthing progress of the FPSO and shuttle tank in time domain. By using re-mesh algorithm method, they make the calculation more efficient. Pessoa and Fonseca (2016) studied the second order low frequency relative motions between the vessels and related mooring line tensions. Yue，Kang et al (2020) discuss the situation that one FSRU connected with the LNGC by cables. Moreover, the uncertainty analysis is discussed by experiment two model ship in towing tank (Qiu, Meng, Peng and Li,2019).

Liquefied Natural Gas (LNG) is an attractive source of clean energy, and it features easy transportation and relatively low carbon dioxide emission. The offloading of LNG from a floating LNG (FLNG) facility to a LNG carrier in a side-by-side configuration in the open sea is widely applied in offshore industry (Zhao et al., 2018a). Due to the presence of a narrow gap between the FLNG and LNG carrier, resonance of the partially-entrapped water column may occur under certain wave frequencies leading to large fluid motions in the gap. The consequence of large fluid motion may influence the relative motions of vessels, and induce large drift forces which in turn may pose various hazards that affect the cargo transfer operations (Kristiansen, 2009). The gap response is of particular interest because of strong resonant phenomenon where viscous dissipation effects may exert a significant influence.

The study of natural frequencies of standing wave patterns in the gap spurs the interests of researchers. Due to the narrow nature of the gap in between two vessels, the energy is trapped resulting in large response at resonance. Compared to the width of gap, the beam of the LNG carrier and FLNG can be assumed to be infinite. Under this assumption, Molin (2001) analytically studied resonant frequencies and natural modes for a side-by-side configuration in two dimensions and a rectangular moonpool in three dimensions, and explicit expressions were presented. To incorporate the end effect of two vessels, Newman and Sclavounos (1988) suggested that homogeneous Dirichlet conditions can be simply imposed at the ends (velocity potential equals to zero). Based on such assumption, Molin et al. (2002) derived an analytical formula to estimate the natural frequencies of gap resonance. Sun et al. (2010) verified Molin's formula numerically, and reported the sensitive hydrodynamic effects due to gap resonance. Zhao et al. (2017) experimentally studied the resonant fluid response in the gap driven by a transient focussed wave group, and observed that the duration of the liquid motion in the gap is much longer indicating the energy is partially trapped in the gap.