# Numerical Simulation of the Force Acting on the Riser by Two Internal Solitary Waves
###### Abstract  
An internal wave is a typical dynamic process. As an internal wave, an internal solitary wave usually occurs between two layers of fluids with different densities. Compared with general internal waves, internal solitary waves have large amplitudes, fast propagation speeds, short-wave periods, and often have tremendous energy. The propagation causes strong convergence and divergence of seawater and generates a sudden strong current. Due to its various characteristics, the propagation of internal solitary waves can cause serious harm to offshore engineering structures. Therefore, studying the effects of internal solitary waves on risers is vital in preventing environmental pollution caused by riser damage. Although the research on internal solitary waves has achieved very fruitful results, the research on structures is mostly focused on a single condition, and the occurrence of internal solitary wave, as a complex ocean phenomenon, is often accompanied by many situations. Therefore, this paper constructs a numerical simulation of the interaction between two columns of internal solitary waves and risers. This study explores the force and flow field changes of the riser under the condition of multiple internal solitary waves using the Star-CCM+ software in the simulation. The improved _K-epsilon_ turbulence model was adopted to close the three-dimensional incompressible Navier-Stokes equation, and the solitary wave solution of the eKdV equation was used as the initial and boundary conditions. The interaction between single and double internal solitary waves and a riser was calculated, compared, and analyzed using numerical analysis. The experiment results indicate that the conditions of two internal solitary waves differ from those of a single internal solitary wave. After colliding at the riser, the waves gradually merge into a single wave, and the flow field reaches its minimum velocity. Under the two-wave condition, the horizontal force on the riser as a whole is less than the single-wave condition. As the amplitude difference between the two internal solitary waves gradually decreases, the horizontal opposing force received by the riser first increases and then decreases, while the horizontal positive force gradually decreases. internal solitary wave; vertical riser; numerical simulation; load characteristics; environmental 11.2022022022022022022022022022020220202202020220202020202202020220202020202020202022020220220202202022020220202202022022020220220202202202022020220202202022020220220220220202202022 internal waves and heat, nutrient flux, and environment, some scholars use field survey and numerical simulation methods to analyze the motion of internal waves in different watersheds, capture key flow characteristics such as shock and flow, and focus on the internal wave breaking and watershed temperature change [5; 6]. To research the difference between static and non-static effects on internal waves, some scholars use the nonlinear and high-precision numerical model with non-static pressure term to simulate the generation and propagation of short-period internal waves in Gibraltar [7]. The results show that there are obvious differences between static and non-static results, and the rupture of the waveform is more obvious. To better discuss and study the interaction among the atmosphere, internal waves, and surface waves, an ocean-atmosphere coupled prediction system is used to simulate the semidiurnal internal waves generated by the Andaman Sea archipelago. The model shows that the semidiurnal internal waves interact with the circulation and surface waves, which changes the flow and stratification in the ocean, and increases the temperature of the thermocline [8]. Some scholars use the CTD (conductivity, temperature, depth) measurement system and Doppler velocimeter to measure the vertical displacement of water quality points near some typical areas and observe the propagation of solitary waves to shallow water areas in a short period through satellite images [9; 10].