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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.

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.

The truncation design of a mooring system is the foundation of hybrid model testing techniques used in experimental research on deepwater floating platforms owing to the limited size of wave basin. Screening multi-variables for multi-objectives leads to difficulties in truncation design aimed at achieving acceptable equivalence, especially for an asymmetric mooring system. This paper proposes an efficient four-level screening method for the truncation design. The selected multi-variables of each mooring line are screened individually to generate feasible regions according to pretensions and initial hang-off angles. Subsequently, the feasible regions are concentrated by the second- and third-level screening: design truncated mooring lines and truncated mooring systems according to static equivalence. Finally, numerical forced oscillation is employed as the fourth-level screening to fulfil dynamic equivalence. Case studies of the truncation design of symmetric and asymmetric mooring systems have verified the proposed method.

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.