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Nonlinear coupling is an important safety consideration for offshore floating structures. Traditional mathematical models like linear motion equations are inadequate for predicting the nonlinear resonance of semi-submersible platforms with bracings. In this study, we proposed a mathematical model that considers the nonlinear effects of heave motion on pitch restoring coefficient, pitch motion on buoyancy, and bracings on buoyancy. Regular wave tests and numerical simulations were performed to investigate the characteristics of coupled motions and to validate the proposed mathematical model. Under head, quartering and beam sea conditions, motion instability of the semi-submersible platform occurred when the incident wave period approached the natural period of heave. In addition, a double-period phenomenon and unique pitch-motion profile were observed. Good agreements between experimental and nonlinear numerical results validated the ability of the proposed mathematical model to accurately simulate nonlinear effects and predict motion instability. Using the validated model, factors affecting the motion instability were analysed and measures to minimize instability were recommended.

Nowadays, extensive research has been conducted to generate electricity from ocean waves, but there are not too many studies on the capture of wave energy in a specific ocean area. As the largest sea area in China's offshore waters, the South China Sea contains a large amount of wave energy and is regarded as a natural wave energy power plant. A comparative study was conducted to investigate the effects of wave energy converter (WEC) parameters on the long-term power capture performance of a bistable point absorber WEC installed in the South China Sea in this paper. Combined with the joint probability distribution of significant wave height Hs and peak wave period Tp in South China Sea, a mathematical model was developed for the evaluation of long-term power performance of WECs. The governing equations of the point absorber WEC were established using the hybrid frequency-time domain method. The dynamic response and captured power can be obtained after numerically solving the equations of motion with the 4th order Runge-Kutta Method. The results show that the bistable mechanism has great advantages in the long-term captured power of WEC. In general, a circular truncated cone-shaped buoy WEC with a larger buoy size and smaller nonlinear parameter tend to result in better long-term power capture performance. The insights given in this study provide a motivation and foundation for developing a complete set of performance optimization methods of point absorber WECs installed in South China Sea.

The hybrid model testing technique has been widely used for evaluating the safety of deepwater floating systems. In this technique, a full-depth mooring system is equivalently substituted with a truncated model by employing truncation design. However, for a deepwater platform with a highly compliant mooring system, the large offset resulting from harsh environments makes it difficult to realize truncation design. The conventional symmetrically truncated mooring system starts to lose its accuracy. To achieve sufficient equivalence and enlarge the offset limit of the truncated model, a pre-offset and asymmetric truncation design is proposed in this paper for a deepwater semi-submersible system. The feasibility of the technique was first validated by numerical calculations. Thereafter, hybrid model testing was performed successfully for a semi-submersible with a highly compliant mooring system operating in harsh environments. Numerical reconstruction and extrapolation were performed, and the numerical results showed good agreement with experimental results. However, some remarkable discrepancies were observed in vertical motions because of nonlinear effects such as wave impact and the influence of bracings.

Offshore platforms operating in harsh ocean environments often suffer from severe wave impacts which threaten the structural integrity and staffs safety. An experimental study was carried out to investigate the wave impact load and its effect on the global response of a semi-submersible. First, two typical wave impact events occurring successively in the wave test run are analyzed, including the characteristics of incident waves, relative wave elevations and the spatial distribution of the wave impact load. Subsequently, the corresponding global response of the semi-submersible under these two wave impacts are investigated in time domain. It reveals that compared with the incident wave, the relative wave elevation has a more straightforward relationship with the wave impact load. The relative wave crest height is associated with the spatial distribution of the wave impact load, while the local wave steepness matters more in the magnitude of the wave impact load. The impulsive effect of the wave impact load on the motion behaviors is not obvious. But severe wave impacts can introduce excessive horizontal accelerations and nonlinear behaviors like ringing in the acceleration response.

Recent accidents caused by wave impact loads have attracted increasing concerns from academia and industry. An experimental study was carried out to investigate the local wave impact loads on a semi-submersible exposed to various sea states, focusing on the spatial distributions and interference among the fore column, aft column, deck bottom and deck side. A versatile tool based on the conditional probability is proposed and used along with correlation analyses to evaluate the interference among different components. The results show that the wave impact loads on the semi-submersible are of high nonlinearity and the aft column is the worst-hit component where wave impacts occur most frequently. The transition region between the fore column and deck side is the most dangerous area for extremely large waves, while the connection corner deserves particular concerns for the aft column and the deck bottom. The horizontal variation of wave impact loads is not significant on the fore column, but is remarkable within the width of the deck side. The interference examination reveals that wave impact loads on different components are correlated at varying levels under different wave impact scenarios. The in-depth analysis improves the understanding of wave impact events on multi-column floating structures.

Resonance is a critical consideration in the design of offshore floating structures. This paper aims at analysing the nonlinear effects of bracings and motion coupling on the resonance features of a semi-submersible platform. An improved mathematical model based on potential theory is proposed to simulate the motion response of a semi-submersible platform under irregular wave conditions, considering both the variations of hydrostatic and hydrodynamic forces induced by the bracings entering and exiting the water and the nonlinear coupling induced by the platform motions. For comparison purposes, numerical simulations are also performed using a mathematical model without considering the aforementioned effects. Validated by results of wave basin tests and numerical simulations, the proposed model performs much better in capturing the characteristic resonance features of pitch motion in low-frequency region. The nonlinear hydrostatic effect of bracings leads to the increase of resonance frequency as the motion amplitude increases, while the hydrodynamic force on the bracings and the nonlinear motion coupling only influence the amplitude of resonance spectral peak. In addition, factors influencing the nonlinear effects such as the vertical position and diameter of bracings and the pitch restoring coefficient are further investigated. It is revealed that the deviation of pitch resonance frequency has evident dependence on the ratio between nonlinear and linear volumetric variations, and an empirical formula estimating the resonance frequency is proposed using the observed dependence. Theoretically, both smaller bracing radius and larger pitch restoring coefficient are beneficial for suppressing the resonance induced by the nonlinear effects. The proposed model can be an effective tool for predicting the motion response, and the understanding of the resonance features is helpful for the design of semi-submersibles.