Internal solitary waves (ISWs) often seriously threaten the survivability of the underwater submersible. Most of the existing investigations focus on the interaction between ISWs and fixed or suspended submersibles. However, the investigation on the interaction between ISWs and self-propelled submersibles is still scarce, which is a more realistic case in the marine engineering. In this paper, a three-dimensional numerical model for the interaction between ISW and self-propelled submersible is developed. Based on the extended Korteweg–de Vries (eKdV) theory, the ISW is generated in a two-layer fluid numerical wave tank by solving the Navier–Stokes (N–S) equation. By introducing the hydrodynamic loadings provided by the ISW environment into the standard operation equation of the submersible, the six degrees-of-freedom motions of the submersible can be obtained. The dynamic overset mesh technology is adopted to simulate the motions of the self-propelled submersible in the ISW fluid field. The present numerical model is validated by comparing with the experiment on a submerged cylinder in the ISW environment. Using this numerical model, we compare the interaction effects of the fixed, suspended, or the self-propelled submersible in ISW and discuss the influences of propulsive forces of the self-propelled submersible on ISW–structure interaction effects. The numerical results show that the loadings and movements of the submersible change remarkably in the surge, heave, and pitch direction. Especially, the submersible with high propulsive forces can pierce the wave surface and reach a large pitch angle with the amplitude of 36°, which further results in a 35% motion speed decrease in the initial propulsion direction.

When the underwater submersible encounters an internal solitary wave (ISW), its loadings and motions are significantly disturbed. To investigate the interaction mechanism between the suspended submersible and the ISW, a three-dimensional ISW–submersible-interaction numerical model was established, based on the computational fluid dynamics (CFD) method. The generation and propagation of the ISW was simulated in a two-layer fluid numerical wave tank, according to the eKdV theory. The standard operation equation of the submersible was introduced to simulate the six degree of freedom (6DoF) motions of the submersible combined with the overset dynamic mesh method. The motion simulation method was effectively validated by comparing it with published experimental results on the motion responses of a slender body under the ISW. Based on the constructed numerical model, the dynamic mechanisms between the suspended submersible and the ISW were studied, and the effects of the initial submerged depths and the ISW amplitudes on the dynamic responses of the submersible were revealed. According to the numerical results, the motions of the submersible have been significantly determined by its initial submerged depths. The submersible located above the ISW interface has a significant motion along the propagation direction of the ISW and its motion trajectory resembles a counterclockwise semi ellipse. The motion of the submersible located below the ISW interface follows the trace of the lower layer of fluid, which presents as an unclosed clockwise ellipse. The corresponding motions of the submersible would be increased with the increase in the ISW amplitudes.

Floating structures are subject to complex marine conditions. To ensure their safety, reliability analysis needs to be conducted during the design phase. However, because of the complexity of traditional full long-term analysis, the environmental contour method (ECM) based on the inverse reliability method, which can combine accuracy and efficiency, is extensively used. Due to the unique environment in the South China Sea, the probabilistic characteristics of three-dimensional (3D) environmental parameters of wind, wave and current are investigated. The ECs of the target sea are established via the ECM based on both the inverse first-order reliability method (IFORM) and inverse second-order reliability method (ISORM). It is found that the sea state forecasted by ISORM is more extreme and may lead to a more conservative design than IFORM. Furthermore, the wind–wave–current combination coefficient matrixes developed using the 3D ECs are proposed for the design of FOWTs in the South China Sea. The validity and practicality of the contours and matrixes are tested by using a floating offshore wind turbine (FOWT) as a numerical example. Then, the short-term response of the structure under the combined wind, wave and current conditions is calculated, providing a theoretical reference for the design of FOWTs.

In this paper, an adaptive fuzzy disturbance observer (FDO)-based control is proposed for the swing suppression of the mooring-heavy lift crane (HLC)-cargo system in the presence of a wave disturbance. First, the dynamic model of the HLC system is determined by employing the Lagrangian method, and the wave force is modeled as an exosystem with unknown terms based on the Jonswap spectrum. Then, based on the HLC model and the wave force exosystem, an FDO is established to determine the wave disturbances, and a fuzzy approximator is developed to estimate the unknown terms. A novel disturbance estimation error observer is first developed to facilitate the parameter adaptive updating law. Subsequently, by augmenting the HLC system and disturbance estimation error system, an FDO-based fuzzy antiswing control method is proposed in terms of the linear matrix inequality technique to suppress the swing. The closed-loop system stability is examined by using the Lyapunov method. Finally, the effectiveness of the proposed method is validated by numerical simulation.

Internal solitary waves (ISWs) pose a significant threat to underwater submersibles. Unlike low-speed submersibles in the ISW field, the high-speed submersibles form a large pitching angle, which is more dangerous for submersible maneuverability. However, the mechanisms behind the interaction between ISWs and high-speed submersibles still remain unexplored. In this work, the three-dimensional numerical model for ISW–structure interaction is used to investigate the motion response characteristics of high-speed submersibles in the ISW field. Based on the extended Korteweg–de Vries (eKdV) theory, the ISW is generated in a two-layer numerical tank by enforcing velocity inlet boundaries. The ISW evolution is obtained by solving the Navier–Stokes equations. The motion of the self-propelled submersible is simulated by solving the equation of motion of the submersible. Using this model, numerical investigation on the ISW and high-speed submersible interaction is performed. The effects of submersible depth, self-recovery stiffness, and ISW amplitude on the motion response of the high-speed submersible are analyzed systematically. When the initial position of the submersible is above the ISW trough and the initial depth is close to the trough depth, the submersible may pierce through the ISW surface, which further results in the formation of a large pitching angle, motion stall and “falling deep.” It is difficult for the submersibles with the low recovery stiffness to maintain or control the navigation trajectory. They would undergo the large pitching angle and even impact on the seabed.