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In offshore crane systems, the floating platform motion has a significant impact on the dynamics of the cart motion. Nevertheless, previous studies have ignored the dynamic coupling interaction between the crane and the floating platform induced by changes in hydrostatic, hydrodynamic, and mooring loads affecting the offshore platform-crane system response. To address this problem, this study presents, firstly, a comprehensive model of the crane-platform dynamic coupling under realistic surge-roll-heave motions of the floating platform induced by ocean waves. While the payload motion can be known, the surge-roll-heave motions of the floating platform are considered unknown. Therefore, secondly, we propose an output feedback control approach that combines a state feedback controller and an extended high-gain observer to primarily achieve desired trajectories of the cart motion under unknown payload mass, dynamic friction, dynamic coupling, and external disturbances. The extended high-gain observer uses the measured displacement of the cart to estimate the dynamic states and external disturbances, providing the state feedback controller with the necessary information and increasing the robustness of the control system. The effectiveness of the proposed model-based control approach under unknown dynamic and wave motion disturbances is verified through simulation.

Rayleigh lidar equipped on airborne floating platforms has received increasing attention in recent years due to the demand for exploring the middle atmosphere. However, the inevitable attitude fluctuation of the platform affects the measurement accuracy of the photon profile, which greatly affects temperature retrieval. Here, an extensive theoretical analysis model of geometrical transformations between the actual altitude and detection distance under attitude fluctuations was constructed by taking pitch, roll, and observation angles into consideration. Based on this model and measured attitude angles, the influence of platform fluctuation on lidar measurement was analyzed by calculating the deviations between temperature retrieval results and the NRLMSISE-00 model at different observation angles, which demonstrated that the altitude displacement from the variation of pitch angle is a crucial factor in causing temperature retrieval error, especially at large observation angles. Then, an attitude compensation method was designed to eliminate the impact of fluctuations, incorporating the merits of good robustness. Under the observation angle of 45° and average pitch angle of around 4°, the maximum temperature deviation after attitude compensation was reduced from 21.29 K to 0.366 K, a reduction of around two orders of magnitude, indicating that the method can significantly improve the measurement accuracy of Rayleigh lidar.

Floating offshore wind turbines (FOWTs) still face many challenges in improving platform stability. A fully submersible FOWT platform with inclined side columns is designed to tackle the current technical bottleneck of the FOWT platform, combining the structural characteristics of the semi-submersible and Spar platform. An integrated numerical model of FOWT is established considering the fully coupled effect, and the hydrodynamic performance of the novel FOWT, the semi-submersible FOWT, and the Spar FOWT are compared and analyzed under different wave incidence angles and wave frequencies, as well as the blade and tower dynamic response of the three FOWTs under the coupling effect of wind, wave, and current. The results show that the novel floating platform can significantly optimize the hydrodynamic performance and has a better recovery ability after being subjected to external loads. The novel floating platform can significantly reduce the heave peak and its corresponding wave frequency compared to the semi-submersible platform, reducing the possibility of heave resonance. FOWT operation should ensure positive wave inflow as far as possible to avoid excessive wave forces in the lateral direction. Both blade and tower dynamic response are affected by rotor rotation and tower vibration to varying degrees, while tower dynamic response is mainly affected by platform motion. This study suggests that the application of the novel FOWT concept is feasible and can be an alternative in offshore wind exploitation in deep water.

In this article, the effects of the changes in the mass of the floating wind turbine (as a multi-body system) on its nonlinear vertical vibrations are investigated. The fluctuations of the hydrodynamic added mass of the floating platform and the mass of the vibration absorbers, which added to the structure to mitigate the lateral vibrations, change the mass and consequently the dynamics of the vertical vibrations. In this regard, first, the governing equations of the vertical vibrations of the floating wind turbine are derived. The FAST code is used to validate the proposed model of the dynamics of the vertical vibrations through numerical simulations. Then, derived equations are solved approximately by the perturbation method. According to the approximate solutions, the fluctuations of the added mass of the floating platform and the masses of the vibration absorbers increase the frequency and amplitude of the vertical vibrations, which increases the fatigue loads on the tower of the wind turbine as well as moorings of the floating platform.

The working condition of the floating platform will be affected by wind and waves in the marine environment. Therefore, it is of great importance to carry out real-time prediction research on the mooring load for ensuring the normal operation of the floating platform. Current researches have focused on the real-time prediction of mooring load using the machine learning method, but most of the studies are about the application and generalization analysis of different models. There are few studies on the influence of data distribution characteristics on prediction accuracy. In view of the above problems, this paper investigates the effect of data skewness on the prediction performance for the deep learning model. The long short-term memory (LSTM) neural network is applied to construct the mooring load prediction model. The numerical simulation datasets of the deep water semi-submersible platform are employed in model training and data analysis. The prediction performance of the model is preliminarily verified based on the simulation results. Meanwhile, the distribution characteristics of mooring load data under different sea states are analyzed and a skewness processing method based on the Box-Cox Transformation (BCT) is proposed. The effect of data skewness on prediction accuracy is further investigated. The comparison results indicate that reducing the mooring load data skewness can effectively improve the prediction accuracy of LSTM model.

The growing prospect for large farms of floating offshore wind turbines requires a better understanding of wake effects for floating turbines. In this work, large eddy simulations with an actuator line model are used to study the wake of the NREL 5 MW reference turbine mounted on the OC3-UMaine spar and OC4-DeepCwind semi-submersible platforms. The simulations are carried out in the Simulator fOr Wind Farm Applications (SOWFA) coupled with OpenFAST for the platform and turbine motion. The wake location, deficit, and turbulence levels are compared for the two floating platforms and equivalent fixed-turbine cases. The effects of neutral versus stable atmospheric conditions are also compared. Most notably, floating-turbine wakes are deflected upwards compared to fixed-turbine wakes, because of mean platform pitch. The spar wake deflects upwards more than the semi-submersible, while the stable atmosphere increases this vertical deflection compared to the neutral. The time-varying rotor motions do not significantly affect the mid-to-far wake, though the stable atmosphere shows larger fixed-floating differences in horizontal wake fluctuations.

In this paper, the nonlinear bending of the slender flexible cable connected to floating offshore energy platforms is considered. The aim is to find accurate values for the bending stress in the catenaries that lead to fatigue and short lifetime. A new approach called Extended Stiffened Catenary Theory (ESCT) is described and outlined, which accurately predicts the bending stresses such that they can be validated by high-fidelity FEM software, e.g., ABAQUS 2024. It is found that some widely used software, such as Orcaflex 11.4, underestimates these bending stresses. Although the Orcaflex uses built-in FEM software to analyse the stresses, there are substantial differences between the results. Since the stresses are underestimated, it can lead to a wrongly estimated higher fatigue lifetime. Therefore, a critical review of stress analysis in Orcaflex is carried out to find the origins of such underestimation. It is shown that the explicit integration of equations of motion in Orcaflex is the reason for such underestimation, even in static analysis. The ABAQUS can predict accurately because of implicit (standard) integration. It is concluded that using this ESCT allows us to estimate a more realistic and reliable stress, thereby leading to a realistic lifetime for catenary umbilicals and cables for floating platforms.

This paper represents an experimental and theoretical study of dynamic characteristics of coupling resonance between a deep-water riser (DR) and a floating platform. The super long DR used for transporting offshore oil & gas is a high dimensional nonlinear pipeline, and the complex characteristics of coupling response is clarified when the heave amplitude or frequency of the floating platform changes. By using the method of reverse combination of test data (RCTD), a fluid-structure coupling model of DR acted by internal fluid, wave flow and floating platform is constructed. Truncated equivalent method and frequency search method are both adopted to deal with the 1300 m DR during the experimental study on mechanism of coupling resonance. Results of the scaled model tests show that higher order resonance is easy to occur when the DR transports high-density fluid, and amplitude jump phenomenon appears in the resonance region at the DR top, which is closely related to the internal fluid transported. According to the transformation of similarity relationship, the results of scaled model test and numerical calculation are consistent, and equivalent truncation method is convenient to study the large-scale nonlinear DR structures.

Based on the 10 MW OO-Star semi-submersible floating platform, this study proposes internal and external heave plates to enhance its stability and explores their influence on the platform’s hydrodynamic characteristics. The platform’s structural behavior is analyzed in both frequency and time domains using numerical simulation methods. The study investigates the effects of the porosity and number of holes (with an equal porosity) of the inner heave plate and the opening angle (with the equal area) of the external heave plate on the platform’s hydrodynamic characteristics, ultimately obtaining the optimal arrangement for the inner and external heave plates. Results indicate that the best scheme involves a 10% porosity with 16 holes, which reduces the heave amplitude by 5.7% compared to the original structure. Additionally, reducing the opening angle of the external heave plate increases the added mass and natural period in the heave and pitch directions of the platform. At an opening angle of 140°, the added mass in the heave direction can increase by 25.2% compared to the original structure. Overall, the internal and external heave plates effectively reduce the heave and pitch amplitude of the platform under severe sea conditions.

The energy innovation scenario sees hybrid wind-wave platforms as a promising technology for reducing the variability of the power output and for the minimization of the cost of offshore marine renewable installations. This article presents a model that describes the installation of a 5 MW wind turbine on a floating platform designed by Fincantieri and equipped with gyroscopic stabilization. The use of gyros allows for the delivery of platform stabilization by damping the wave and wind induced motion on the floater and at the same time producing extra power. Shetland Island was chosen as the reference site because of its particularly harsh weather. Final results show that the total production of power in moderate and medium climate conditions is considerable thanks to the installation of the gyro, together with a significant stabilization of the platform in terms of pitching angle and nacelle acceleration.