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Wave run-up and air-gap are key issues for the safety of semi-submersible platforms. Real-time wave run-up prediction is helpful to ensure the safety of offshore activities. Based on the long short-term memory (LSTM) network, the extreme short term online prediction method is developed for predicting the wave run-up of semi-submersible platforms using wave and motion sequences. With the help of large sets of data from the model test, the LSTM model is trained and tested. The study shows that when the forecast durations are 6 s and 12 s, the average accuracy of the prediction results are 92.90% and 84.09%, and the relative errors of the maximum wave run-up height are lower than or equal to 19.69% and 30.66%, respectively. In addition, the model has a stable and exact prediction of extreme values of wave run-up height when the forecast duration is within 6 s, which confirms its ability to provide valid technical support for the early warning of wave slamming and overtopping during the operation of offshore platforms.

In order to study the characteristics of vortex-induced motion and galloping of the classic Spar platform, a model test with a mooring system is conducted in basin. The sway motion characteristics of the Spar platform at different current velocities are analyzed. By comparing the results in currents, waves, and wave-current coupled conditions, the coupling effects of current and wave on vortex-induced motion and galloping of the Spar platform are also studied. The results show that galloping is induced by the currents with high reduced velocities. Compared with the vortex-induced motion, the galloping phenomenon has a longer period, larger amplitude, and randomness. The coupling of current and wave would not change the mode of flow-induced motion, but it significantly affects the motion amplitude.

Nonlinear relative wave motions and air gap performance greatly affect the operation and security of semi-submersibles in harsh environments. An appropriate air gap is essential to preventing potential impact loads, and accurate prediction for long-return-period events is of great concern. Given the random and nonlinear nature of relative wave motions, a reliable approach for their prediction is a challenging task that warrants further research. In this study, a modified three-parameter Rayleigh distribution model for nonlinear relative wave crests was derived utilising a quadratic transformation of incident waves. The method of LH-moments was used to estimate the unknown parameters from sample data, and the relationship between the parameters and LH-moments was derived. The sample data used for the probability analysis were obtained from a model test programme for a semi-submersible. The first-order parameter of the model was found to decrease with increasing η, whereas the second-order parameter was found to increase. A larger second-order parameter indicates higher nonlinearity of the relative wave motions for a 100-year-return sea state than for a 1-year-return sea state. In general, using LH-moments to estimate the parameters of the probability distribution results in better prediction of large relative wave crests with low probabilities of exceedance, because the estimation of the three parameters in the nonlinear Rayleigh model using LH-moments released the constraint on the linear term. [ABSTRACT FROM AUTHOR]

A variety of concepts of deep draft semi-submersibles (DDS) have been developed in order to improve the vertical motions. However, the DDS may experience critical vortex-induced motions (VIMs) stemming from the fluctuating forces on the columns. Aiming to investigate the excitation loads and the fluid mechanism of VIM, this paper presents experimental and numerical studies of the three-dimensional (3-D) flow around four circular section cylinders in a square configuration at subcritical Reynolds numbers (Re). The experimental measurements were carried out by using the piezoelectric load cells in a wind tunnel, while the numerical simulations were conducted by employing the Large Eddy Simulation (LES) method. The effects of three spacing ratios (L/D) and four array attack angles (α) ranging from 0° to 45° in 15° increments were investigated. The results show that the vortex structures are the key to understand the behaviours of the forces and the Strouhal number (St). Consequently, the downstream cylinders are normally subjected to smaller mean drags but more serious fluctuating forces than the upstream cylinders, due to the influence of unsteady wake vortices. Furthermore, at the present range ofL/Dratios, the effect of spacing ratio on the excitation forces is not significant. [ABSTRACT FROM AUTHOR]