In the process of oil and gas development, semi-submersible platforms operate relocation between wells by retracting and releasing mooring chains. In this paper, a fast well relocating optimization method of semi-submersible platform based on greedy algorithm was proposed in view of the traditional method that cannot obtain relocation strategy quickly. Three sets of relocation operations were selected in this study, an optimization analysis of relocation strategy was carried out for windlasses normal operation and windlasses failure operation. The results show that under normal operation, the optimized strategy has a significant improvement in terms of optimization effect and calculation cost compared with the original strategy and the best strategy. Under windlasses failure operation, it shows that the feasibility of relocation is closely related to the distance and direction of the relocation operation. A comparison of relocation strategy was made between the normal operation and the windlasses failure operation, it was found that the retracted length of the mooring chains and the number of operating steps increased under windlasses failure operation, and the stability keeps changing with different working conditions.

In order to investigate the effect of fillets at the leading and trailing edges of flow holes on the noise characteristics of submerged body, a submerged body with a single flow hole was taken as the research object. Based on the large eddy simulation turbulence model and FW-H (Ffowcs Williams-Hawkings) acoustic model, numerical simulations were carried out using STAR CCM+ software. After verifying that the simulation accuracy meets the requirements, the influence of different fillet positions and radii of the orifice on flow noise was then deeply explored at a flow rate of 10 m/s. The results show that the treatment of fillets at the leading and trailing edges of the flow hole can reduce the flow noise of the flow hole, especially the second-order line-spectrum noise. Regarding fillet position, the noise reduction amplitude by the fillets at the trailing edges is greater than that at the leading edge, and the noise reduction amplitude at the outer rounded corners is greater than that at the inner edge. In the studied operating conditions, when the outer edges of the leading and trailing edges of the orifice are simultaneously rounded with a radius of 10 mm, the minimum second-order line-spectrum noise is 94.03 dB, which is 9.48 dB lower than that in the reference operating condition, and the corresponding frequency is reduced by 4.72 Hz. The far-field radiation noise of the orifice is mainly affected by the pressure pulsation on the back wall and bottom of the orifice, and the influence of the orifice shear layer oscillation is relatively small. The research conclusions provide guidance for the optimization of submerged body flow hole structure and the low-noise design.

Due to the difficulty of attenuation of middle and low frequency band sound waves in the process of propagation, the control of middle and low frequency broadband sound waves has become a challenging topic, so it is necessary to develop new materials and structures with low frequency sound absorption and noise reduction functions. The special properties of acoustic metamaterials provide new ideas for the development of sound absorption and insulation. In order to effectively control the noise in the middle and low frequency bands, a new spatial spiral acoustic metamaterial was designed and optimized by using the finite element software COMSOL Multiphysics, and the sound absorption and sound insulation performance in the 100~2500 Hz frequency band were calculated and analyzed. With the help of 3D printing for completing the preparation of metamaterial, the sound absorption and insulation performance of spiral acoustic metamaterial were compared in an experimental study to verify the accuracy of the calculation method.

Vortex-induced vibration (VIV) of a marine riser is a great threat to its service safety. From the perspective of energy, the hydrodynamic force on the riser undergoing vortex-induced vibration was divided into three components, i.e. vortex-induced force acting as energy input, drag force acting as energy dissipation and added mass force acting a neutral role in energy. The energy competition between the first two components determines the final energy effect of the fluid on the structure. Furthermore, the identification method of hydrodynamic coefficients based on the flexible riser model experiment was derived for the new hydrodynamic force model in detail. Through the towing experiment of the flexible riser, the vortex-induced vibration response, and coefficients distribution characteristics under different flow velocities were identified. The results show that the vortex induced vibration response of the flexible riser under uniform flow has multi-mode participation characteristic, which leads to the "jump" phenomenon of hydrodynamic coefficients. The vortex-induced force coefficients and drag coefficients behave significant correlation with the amplitude of VIV. Based on the measured values between hydrodynamic coefficients and response amplitude, an empirical model for hydrodynamic coefficients under energy competition force model was preliminarily established. The research in this paper provides a valuable reference for the development of fast empirical prediction methods of marine risers in the future.

The precise determination of low-frequency wave loads on ship hulls is an indispensable cornerstone and core challenge in hull structural design. Low-cost ship model testing is often employed in engineering to forecast wave loads. However, test data frequently suffer from deficiencies or abnormalities due to various reasons. Consequently, predicting wave loads from data with defects or anomalies, remains a major engineering challenge. This paper presented an efficient method for accurately determining the wave design loads on ship hulls, specifically tailored to handle deficient or abnormal test data. By integrating 5,400 sets of wave load data calculated using two-dimensional strip theory, a machine learning transfer network was constructed. To address deficient data, we innovatively introduced a fine-tuning network layer, and designed a novel loss function that ignores zero terms, thereby enhancing the network's adaptability. This method achieved rapid wave load forecasting by transferring simulation results to ship model tests, with an accuracy better than 90%. This technique enhances design efficiency, reduces labor costs, and maximizes data utilization, providing a reliable and efficient solution for wave load prediction in hull structural design.

In recent years, shipowners have become increasingly concerned about the actual performance in the real marine environment. At present, ship design optimization is mainly conducted based on performance in still water which has a certain difference from real-sea performance. The factors influencing the powering performance of ships in wind and waves were investigated taking series oil tankers, bulk carriers, and container ships as the research subjects. The theory of two-dimensional strip was used to calculate the added resistance under typical sea states BF6 and BF8. The propeller was redesigned using graph method to research the influence of light running margin in the wind and waves. The results show that the principal dimensions have a significant impact on the added resistance of oil tankers and bulk carriers, and have a relatively small impact on container ships. Therefore, it is necessary to consider the ship type separately when optimizing the powering performance in wind and waves. In addition, with the increase of the light running margin, the reserved space for power becomes increasingly sufficient.

The generation of internal wave wakes by submerged objects in density-stratified environments is closely linked to the navigation speed. This study develops a technique of step-layer injection in a wide-scale density-stratified simulation tank and proposes a high-precision multi-array conductivity detection method. Experimental investigations on the excitation of internal wave wakes by an underwater sphere driven by cyclic towing were conducted. Using probability density statistics, root mean square analysis of wave amplitudes, and other analytical methods, this research delved into key issues such as spatiotemporal probability distribution density of Froude number correlated with internal wave, transition zone delineation, and vertical displacement field characteristics, etc. The findings demonstrate that the experimental system and techniques employed can accurately capture the fluctuation information within the stratified flow field and precisely determine the relationship between the characteristics of internal wave wakes and the Froude number. The probability distribution density of the internal-wave-correlated Froude number reveals the transition process of internal wave wakes and clearly identifies the transition zone as approximately 1.6 ⩽ Fr < 3.3. It is discerned that, after the transition, the dominant internal wave correlated Froude numbers within the wake wave system approximately fall within the ranges of [0.3, 0.4] and [2, 2.8] for the inner and outer layers, respectively. Additionally, Lee wave correlated velocity can still be detected in the outer layer region.

As offshore floating structures continue to grow larger, the rain load of large offshore floating structures under extreme conditions has become one of the focal points of concern for designers. Based on the discrete particle model and the rain load calculation formula, this paper completed the rain load calculation for different wind field and raindrop spectrum combination states of the offshore platform, and the study shows that the rain load caused by fluctuating wind is much more discrete than the raindrop spectrum; the rain load of the offshore platform under the action of fluctuating wind follows Gamma distribution; and the rain load variation caused by time-varying rain field follows normal distribution; when the rainfall intensity R is 800 mm/h, the exceedance probability is 95%, the rain load accounts for 4.65%, and the maximum rain load accounts for 8.07%. The research results help to reveal the influencing factors of rain load on offshore platforms and can provide data support for designers to select rain load reasonably.

Fishnet is a complicated flexible structure, and the drag force coefficient is important for the cage netting load calculation and deformation in the numerical simulation. Firstly, the actual netting samples were obtained through the netting patch hanging test in the open sea area, then they were studied in the laboratory to determine the drag coefficient under different marine growth conditions. The equivalent method of the netting in Morrison model was introduced in the numerical simulation to analyze the hydrodynamic load of the fishnet on the 50,000-cubic meter huge aquaculture cage. The results show that the cage netting load under large amounts of marine growth (solidity=0.69) can be 60% higher than that of the netting under small amounts of marine growth (solidity=0.28), and that the design size of the net rope can be reduced by optimizing the layout of the netting in the cage design.

The study of vortex induced vibration under high Reynolds number is of great importance. While flow sensors are generally fixed to structures, it is of great practical importance to analyze and simulate the measurement information under vibration conditions. In this study, the subcritical Reynolds number three-dimensional flow and vortex-induced vibration of a cylinder are simulated and analyzed using the translational moving reference coordinate method together with a self-developed fluid computation program. The results show that the method accurately simulates the vortex-induced vibration phenomenon.It is found that the phase difference between the lift and the displacement produced a jump within the VIV lock-in region, and the vortex broke up during the vibration, showing strong three-dimensional characteristics. Time-averaged physical quantities near the wall and the wake region were obtained by statistical calculation of the grid in the translational reference frame. The simulation method based on the translational reference system is similar to the observation method of sensors attached to the structure in experiment and on-site testing, which can provide references to model tests and field measurements.