In this paper, we present an overview of numerical simulation methods for the flow around typical underwater vehicles at high Reynolds numbers, which highlights the dominant flow structures in different regions of interest. This overview covers the forebody, midbody, stern, wake region, and appendages and summarizes flow phenomena, including laminar-to-turbulent transition, turbulent boundary layers, flow under the influence of curvatures, wake interactions, and all associated complex vortex structures. Furthermore, the current issues and challenges of capturing these flow structures are addressed. This overview provides a deep insight into the use of numerical simulation methods, including the Reynolds-averaged Navier–Stokes (RANS) method, large eddy simulation (LES) method, and the hybrid RANS/LES method, and evaluates their applicability in capturing detailed flow features.

Biofouling on ships and offshore structures has always been a difficult problem to solve, which not only jeopardizes the structural strength but also brings great economic losses. Ultrasonic cavitation is expected to solve this problem due to its characteristics of no damage to structures and no pollution. Starting from the phenomenon and mechanism of ultrasonic cleaning, this paper introduces the application of ultrasonic cavitation in ship, pipeline and oil cleaning as well as ballast water treatment. By reviewing the existing studies, limitations such as insufficient ultrasonic parameter studies, lack of uniform cleanliness standards, and insufficient cavitation studies are summarized to provide traceable research ideas for improving ultrasonic cavitation technology and to guide the expansion and improvement of its applications.

The compressibility of fluids has a profound influence on oscillating bubble dynamics, as characterized by the Mach number. However, current theoretical frameworks for bubbles, whether at the first or second order of the Mach number, are primarily confined to scenarios characterized by weak compressibility. Thus, a critical need to elucidate the precise range of applicability for both first- and second-order bubble theories arises. Herein, we investigate the suitability and constraints of bubble theories with different orders through a comparative analysis involving experimental data and numerical simulations. The focal point of our investigation encompasses theories such as the Rayleigh–Plesset, Keller, Herring, and second-order bubble equations. Furthermore, the impact of parameters inherent in the second-order equations is examined. For spherical oscillating bubble dynamics in a free field, our findings reveal that the first- and second-order bubble theories are applicable when Ma≤0.3 and 0.4, respectively. For a single sonoluminescence bubble, we define an instantaneous Mach number, Ma_i. The second-order theory shows abnormal sensibility when Ma_i is high, which is negligible when Ma_i≤0.4. The results of this study can serve as a valuable reference for studying compressible bubble dynamics.The compressibility of fluids has a profound influence on oscillating bubble dynamics, as characterized by the Mach number. However, current theoretical frameworks for bubbles, whether at the first or second order of the Mach number, are primarily confined to scenarios characterized by weak compressibility. Thus, a critical need to elucidate the precise range of applicability for both first- and second-order bubble theories arises. Herein, we investigate the suitability and constraints of bubble theories with different orders through a comparative analysis involving experimental data and numerical simulations. The focal point of our investigation encompasses theories such as the Rayleigh–Plesset, Keller, Herring, and second-order bubble equations. Furthermore, the impact of parameters inherent in the second-order equations is examined. For spherical oscillating bubble dynamics in a free field, our findings reveal that the first- and second-order bubble theories are applicable when Ma≤0.3 and 0.4, respectively. For a single sonoluminescence bubble, we define an instantaneous Mach number, Ma_i. The second-order theory shows abnormal sensibility when Ma_i is high, which is negligible when Ma_i≤0.4. The results of this study can serve as a valuable reference for studying compressible bubble dynamics.

In this work, we constructed a neural network proxy model (NNPM) to estimate the hydrodynamic resistance in the ship hull structure design process, which is based on the hydrodynamic load data obtained from both the potential flow method (PFM) and the viscous flow method (VFM). Here the PFM dataset is applied for the tuning, pre-training, and the VFM dataset is applied for the fine-training. By adopting the PFM and VFM datasets simultaneously, we aim to construct an NNPM to achieve the high-accuracy prediction on hydrodynamic load on ship hull structures exerted from the viscous flow, while ensuring a moderate data-acquiring workload. The high accuracy prediction on hydrodynamic loads and the relatively low dataset establishment cost of the NNPM developed demonstrated the effectiveness and feasibility of hybrid dataset based NNPM achieving a high precision prediction of hydrodynamic loads on ship hull structures. The successful construction of the high precision hydrodynamic prediction NNPM advances the artificial intelligence-assisted design (AIAD) technology for various marine structures.

The development of an in-house computer program for determining the motions and loads of advancing ships through sea waves in the frequency domain, is described in this paper. The code is based on the potential flow formulation and originates from a double-body code enhanced with the regular part of the velocity potential computed using the pulsing source Green function. The code is fully developed in C++ language with extensive use of the object-oriented paradigm. The code is capable of estimating the excitation and inertial radiation loads or arbitrary incoming wave frequencies and incidence angles. The hydrodynamic responses such as hydrodynamic coefficients, ship motions, the vertical shear force and the vertical bending moment are estimated. A benchmark container ship and an LNG carrier are selected for testing and validating the computer code. The obtained results are compared with the available experimental data which demonstrate the acceptable compliance for the zero speed whereas there are some discrepancies over the range of frequencies for the advancing ship in different heading angles.

One of the crucial and challenging issues for researchers is presenting an appropriate approach to evaluate the aerodynamic characteristics of air cushion vehicles (ACVs) in terms of system design parameters. One of these issues includes introducing a suitable approach to analyze the effect of geometric shapes on the aerodynamic characteristics of ACVs. The main novelty of this paper lies in presenting an innovative method to study the geometric shape effect on air cushion lift force, which has not been investigated thus far. Moreover, this paper introduces a new approximate mathematical formula for calculating the air cushion lift force in terms of parameters, including the air gap, lateral gaps, air inlet velocity, and scaling factor for the first time. Thus, we calculate the aerodynamic lift force applied to nine different shapes of the air cushions used in the ACVs in the present paper through the ANSYS Fluent software. The geometrical shapes studied in this paper are rectangular, square, equilateral triangle, circular, elliptic shapes, and four other combined shapes, including circle-rectangle, circle-square, hexagonal, and fillet square. Results showed that the cushion with a circular pattern produces the highest lift force among other geometric shapes with the same conditions. The increase in the cushion lift force can be attributed to the fillet with a square shape and its increasing radius compared with the square shape.

The effect of porosity on surface wave scattering by a vertical porous barrier over a rectangular trench is studied here under the assumption of linearized theory of water waves. The fluid region is divided into four subregions depending on the position of the barrier and the trench. Using the Havelock’s expansion of water wave potential in different regions along with suitable matching conditions at the interface of different regions, the problem is formulated in terms of three integral equations. Considering the edge conditions at the submerged end of the barrier and at the edges of the trench, these integral equations are solved using multi-term Galerkin approximation technique taking orthogonal Chebyshev’s polynomials and ultra-spherical Gegenbauer polynomial as its basis function and also simple polynomial as basis function. Using the solutions of the integral equations, the reflection coefficient, transmission coefficient, energy dissipation coefficient and horizontal wave force are determined and depicted graphically. It was observed that the rate of convergence of the Galerkin method in computing the reflection coefficient, considering special functions as basis function is more than the simple polynomial as basis function. The change of porous parameter of the barrier and variation of trench width and height significantly contribute to the change in the scattering coefficients and the hydrodynamic force. The present results are likely to play a crucial role in the analysis of surface wave propagation in oceans involving porous barrier over submarine trench.

At present, the measurement of the near wave field of ships mostly relies on shipborne radar. The commonly used shipborne radar is incoherent and cannot obtain information on wave surface velocity. Therefore, the mathematical model of wave reconstruction is remarkably complex. As a new type of radar, coherent radar can obtain the radial velocity of the wave surface. Most wave surface reconstruction methods that use wave velocity are currently based on velocity potential. The difficulty of these methods lies in determining the initial value of the velocity integral. This paper proposes a wave surface reconstruction method based on an artificial boundary matrix. Numerical simulation data of regular and short-crest waves are used to verify the accuracy of this method. Simultaneously, the reconstruction stability under different wave velocity measurement errors is analyzed. The calculation results show that the proposed method can effectively realize the reconstruction of wave field.

A three-dimensional mathematical hydrodynamic model associated with surface wave radiation by a floating rectangular box-type structure due to heave, sway, and roll motions in finite water depth is investigated based on small amplitude water wave theory and linear structural response. The analytical expressions for the radiation potentials, wave forces, and hydrodynamic coefficients are presented based on matched eigenfunction expansion method (MEFEM). The correctness of the analytical results of wave forces is compared with the construction of a numerical model using the open-source boundary element method code NEMOH. In addition, the present result is compared with the existing published experimental results available in the literature. The effects of the different design parameters on the floating box-type rectangular structure are studied by analyzing the vertical wave force, horizontal wave force, torque, added mass, and damping coefficients due to the heave, sway, and roll motions, and the comparison analysis between the forces is also analyzed in detail. Further, the effect of reflection and transmission coefficients by varying the structural width and drafts are analyzed.

At present, the measurement of the near wave field of ships mostly relies on shipborne radar. The commonly used shipborne radar is incoherent and cannot obtain information on wave surface velocity. Therefore, the mathematical model of wave reconstruction is remarkably complex. As a new type of radar, coherent radar can obtain the radial velocity of the wave surface. Most wave surface reconstruction methods that use wave velocity are currently based on velocity potential. The difficulty of these methods lies in determining the initial value of the velocity integral. This paper proposes a wave surface reconstruction method based on an artificial boundary matrix. Numerical simulation data of regular and short-crest waves are used to verify the accuracy of this method. Simultaneously, the reconstruction stability under different wave velocity measurement errors is analyzed. The calculation results show that the proposed method can effectively realize the reconstruction of wave field.

Significant aerodynamic engine instability can occur during the operation of marine gas turbines as airflow enters the compressor through a 90° turning and causes inlet distortion. This study adopts the method of simulating board equivalence to provide the target distortion flow field for ship compressors. The characteristics of the flow field behind the simulated board are obtained through experiments and numerical simulations, through which the relationship between the height of the simulated board and the total pressure distortion is elucidated. Subsequently, the study summarizes the prediction formula to achieve a distortion prediction of 0.8%–7.8%. In addition, this work analyzes the effects of drilling methods and diameters on flow nonuniformity by drilling holes into the simulation board. The results indicate that drilling holes on the board can weaken the nonuniformity of the flow field within a certain range and change the distribution pattern of total pressure in the cross-section. Furthermore, the total pressure distortion no longer changes significantly when the number of holes is too large. The proposed double simulation board structure is capable of obtaining the following two types of distorted flow fields: symmetrical dual lowpressure zones and low-pressure zones with high distortion intensity at the compressor inlet. The distortion equivalent simulation method proposed in this work can obtain various types of distortion spectra, thereby meeting the distortion parameter requirements for the antidistortion testing of marine engines.

At present, the cranes used at sea have several shortcomings in terms of flexibility, efficiency, and safety. Therefore, a floating multi-robot coordinated lifting system is proposed to fulfill the offshore lifting requirements. First, the structure of the lifting system is established according to the lifting task, the kinematic model of the system is developed by using the D–H coordinate transformation, and the dynamic model is developed based on rigid-body dynamics and hydrodynamics. Then, the static and dynamic workspace of the lifting system are analyzed, and the solving steps of the workspace are given by using the Monte–Carlo method. The effect of the load mass and the maximum allowable tension of the cable on the workspace is examined by simulation. Results show that the lifting system has limited carrying capacity and a data reference for selecting the structural parameters by analyzing the factors affecting the workspace. Findings provide a basis for further research on the optimal design of structural parameters and the determination of safe configurations of the lifting system.

In this technologically advancing world, the demand for more energy, oil and gas production is rapidly escalating. To accomplish this, people have inclined more towards completely floating offshore structures, deployed in deep waters. A semi-submersible is selected in the present study, due to its better response characteristics and stability under harsh environmental conditions. The semi-submersible is position restrain with spread mooring lines incorporated with submerged buoy at different locations has been studied. A detailed numerical analysis is carried out using Ansys Aqwa for dynamic response analysis of semi-submersible under the combination of wind, wave, and current forces for 0°, 45°, and 90° directions. It was observed that damping ratios and natural periods had been affected based on the position and number of submerged buoys in the mooring system. Also, reduction in mooring force after incorporating buoy in the mooring lines was observed. Subsequently, a Matlab code based on the S-N curve approach was generated and employed to investigate the fatigue damage of mooring lines under dynamic variation of mooring forces. When pegged with submerged buoys, fatigue life of mooring lines is increased under intact and postulated damaged mooring conditions. Moreover, coupling of motion responses of semi-submersible is observed, and unbounded response is not seen in any degrees-offreedom, even during damaged condition of mooring lines.

Offshore triceratops is one of the successful manifestations of the form-dominant design approaches deployable in ultra-deepwater oil and gas exploration. The deck’s geometric shape and partial isolation from the legs counteract lateral loads. Legs are position-restrained to the sea bed by taut-moored tendons, while ball joints partially isolate the deck from the buoyant legs. However, compliance in the horizontal plane imposes large displacements, intuiting the necessity to examine tendon failure. Numerical analysis of triceratops under wave and wind combined action is carried out under the postulated conditions of a tendon failure. 10-yr, 100-yr, and 1 000-yr post-Katrina hurricane conditions are assumed as loading to the platform. Results confirm a marginal increase in the natural periods of stiff degrees of freedom even under postulated failure conditions, ensuring good adaptability to ultra-deep water. Under postulated failure, the tension of adjacent tendons varies significantly, causing a shift to the mean position of the platform. Fatigue life is significantly reduced under the postulated failure of tendons, making the platform free-floating without affecting its stability. Results also show that the pitch response of the deck is a clear manifestation of the postulated failure, which is otherwise absent due to the presence of ball joints. The attempted study deliberates on the fatigue life of tendons, assessing the platform’s suitability to ultra-deep waters and identifying the vulnerable legs for the chosen load combinations.

Owing to the particularity of a polyester fiber material, the polyester mooring undergoes large axial tensile deformation over long-term use. Large axial tensile deformation significantly impacts the dynamic response of the mooring system. In addition, the degrees of large axial tension caused by different elastic moduli are also different, and the force on the mooring line is also different. Therefore, it is of great significance to study the influence of elastic modulus on the dynamic results of the mooring systems under large axial tension. Conventional numerical software fails to consider the axial tension deformation of the mooring. Based on the theory of slender rods, this paper derives the formula for large axial tension using the method of overall coordinates and overall slope coordinates and provides the calculation programs. Considering a polyester mooring system as an example, the calculation program and numerical software are used to calculate and compare the static and dynamic analyses to verify the reliability of the calculation program. To make the force change of the mooring obvious, the elastic moduli of three different orders of magnitude are compared and analyzed, and the dynamic response results after large axial tension are compared. This study concludes that the change in the elastic modulus of the polyester mooring changes the result of the vertex tension by generating an axial tension. The smaller the elastic modulus, the larger the forced oscillation motion amplitude of the top point of the mooring line, the more obvious the axial tension phenomenon, and the smaller the force on the top of the polyester mooring.

The environmental load chart is an important technical support required for the jack-up drilling platform to facilitate its adaptation to different operating waters and ensure the safety of operation. This chart is a crucial part of the platform operation manual. The chart data are closely related to external factors such as water depth, wind, wave, and current conditions of the working water, as well as to the structural characteristics of the platform itself and the number of variable loads. This study examines the platform state under extreme wind, wave, and current conditions during preloading. In addition, this study focuses on the difference between the ultimate reaction force of the pile leg during preloading and the reaction force of the pile leg without considering any environmental load before preloading. Furthermore, the relationship between the difference and the new reaction force of the pile leg caused by the combination of different environmental conditions is established to facilitate the construction of a new form of environmental load chart. The newly formed chart is flexible and simple; thus, it can be used to evaluate the environmental adaptability of the platform in the target well location and provides the preloading target demand or variable load limit according to the given environmental constraints. Moreover, the platform can perform personalized preloading operations, thereby improving its capability to cope with complex geological conditions, such as reducing punch-through risks. This condition reduces the load on jacking system devices and increases its service life.

The complexities of the marine environment and the unique characteristics of underwater channels pose challenges in obtaining reliable signals underwater, necessitating the filtration of underwater acoustic noise. Herein, an underwater acoustic signal denoising method based on ensemble empirical mode decomposition (EEMD), correlation coefficient (CC), permutation entropy (PE), and wavelet threshold denoising (WTD) is proposed. Furthermore, simulation experiments are conducted using simulated and real underwater acoustic data. The experimental results reveal that the proposed denoising method outperforms other previous methods in terms of signal-to-noise ratio, root mean square error, and CC. The proposed method eliminates noise and retains valuable information in the signal.

The demand for high-data-rate underwater acoustic communications (UACs) in marine development is increasing; however, severe multipaths make demodulation a challenge. The decision feedback equalizer (DFE) is one of the most popular equalizers in UAC; however, it is not the optimal algorithm. Although maximum likelihood sequence estimation (MLSE) is the optimal algorithm, its complexity increases exponentially with the number of channel taps, making it challenging to apply to UAC. Therefore, this paper proposes a complexity-reduced MLSE to improve the bit error rate (BER) performance in multipath channels. In the proposed algorithm, the original channel is first shortened using a channel-shortening method, and several dominant channel taps are selected for MLSE. Subsequently, sphere decoding (SD) is performed in the following MLSE. Iterations are applied to eliminate inter-symbol interference caused by weak channel taps. The simulation and sea experiment demonstrate the superiority of the proposed algorithm. The simulation results show that channel shortening combined with SD can drastically reduce computational complexity, and iterative SD performs better than DFE based on recursive least squares (RLS-DFE), DFE based on improved proportionate normalized least mean squares (IPNLMS-DFE), and channel estimation-based DFE (CE-DFE). Moreover, the sea experimental results at Zhairuoshan Island in Zhoushan show that the proposed receiver scheme has improved BER performance over RLS-DFE, IPNLMS-DFE, and CE-DFE. Compared with the RLS-DFE, the BER, after five iterations, is reduced from 0.007 6 to 0.003 7 in the 8–12 kHz band and from 0.151 6 to 0.114 5 in the 13–17 kHz band at a distance of 2 000 m. Thus, the proposed algorithm makes it possible to apply MLSE in UAC in practical scenarios.