Applied Ocean Research最新文献

Monopile is a popular choice in the foundation supporting offshore wind turbines (OWTs), with local scour significantly impacting their lateral responses. Macro-element model, which encapsulates the response between the monopile and the surrounding seabed soils into a force-displacement relation, has been extensively developed to describe offshore foundations. However, such kind of models specifically targeting monopiles subjected to lateral loading in local scour remain underdeveloped. This work proposes a macro-element model with a succinct hyperbolic hardening relation for laterally loaded monopiles in local scour conditions, using the evolutionary polynomial regression (EPR) machine learning technique for easy and optimal design. First, the finite element model is verified and extended to generate force-displacement responses considering the monopile geometries, soil characteristics, and local scour geometries. These results are then utilised to determine the optimal hyperbolic hardening relation of the macro-element model. Next, the EPR technique is employed to determine the relationship between the hyperbolic hardening relation parameters and the influencing factors. Finally, the macro-element model is successfully evaluated by comparing with measurements from centrifuge tests and numerical solutions by finite element analysis, demonstrating its applicability in practical design and the ability to reproduce FEA results with a significant reduction in computational cost.

The study uses linearized water wave theory to examine the role of detuned frequency on Bragg scattering of surface gravity waves over an array of bottom-standing submerged sinusoidal patches. Explicit formulae for reflection and transmission coefficients are derived using the matrix transfer method in the case of an array of patches, with each patch having a finite number of ripples. Bragg resonance occurs in the case of more than two patches beyond a certain cutoff frequency corresponding to supercritical detuning, while a monotonic increasing trend is observed below the cut-off frequency which is referred to as subcritical detuning. The number of sub-harmonic peaks between two consecutive harmonic peaks is one less than the number of patches. As the number of patches grows, so does the number of zero reflections, while the number of sub-harmonic peaks is invariant with the number of ripples within a patch. The corrugated length of the submerged sinusoidal patches and the resonator length determine the highly resonating/wave trapping features of wave reflection within the resonator and the corrugated patches. The time-domain simulation of surface displacement reveals the scattering and splitting of wave pulses over the submerged patches.

This work aims to determine the wave conditions that generate maximum surge response excited predominantly by second-order difference frequency forces. Standard narrow-band wave conditions have random phase components and obtaining the maximum surge response requires long sea-state durations to cover all combinations and correspondingly long computation times using second-order diffraction–radiation models. Multiple 3-hour random sea-states are typically used to evaluate the expected extreme response. The maximum force may be obtained by shifting phases to be equal between component pairs with a frequency difference equal to the structure’s surge natural frequency. However, this work shows that such an approach gives a highly transient force and the lightly damped surge displacement response does not approach a representative maximum value. The larger motion responses may be achieved by sequential wave groups and here we use a genetic algorithm to optimise the phase distribution to give more regular low-frequency excitation in relatively short sea-state durations, less than 1 h. This is demonstrated with a one degree-of-freedom Fourier model. The method is applied to a lightly-moored spar substructure and compared with an experimentally validated standard six degree-of-freedom time domain model (Orcaflex) showing satisfactory agreement.

This study endeavors to realize the concurrent utilization of marine soft clay (MSC) and industrial waste, specifically calcium carbide residue (CCR) and fly ash (FA), through a series of experimental investigations. The optimal ratio between CCR and FA, as well as the efficacy of the composite agent (CF–1), were examined, and an empirical equation associating the unconfined compressive strength (q_u) of stabilized MSC was developed through unconfined compressive strength (UCS) tests. Microscopic analyses, including X–ray diffraction (XRD), scanning electron microscopy (SEM), and energy–dispersive spectroscopy (EDS), were employed to unveil the intrinsic mechanisms underlying CF–1 stabilized MSC. Subsequently, the suitability of CF–1 solidified MSC as a roadbed filler was ascertained through laboratory tests. Results revealed the optimum CCR:FA ratio for CF–1 to be 4:1, demonstrating superior curing effects compared to individual components such as Portland cement (PC), CCR, and FA, with commendable environmental and economic benefits. The developed empirical equation exhibited effectiveness in predicting the q_u of CF–1 solidified MSC under varying curing dates (T) and dosages (W_g) conditions. Characterization through XRD, SEM, and EDS identified the primary products formed within the stabilized MSC matrix with CF–1 as comprising calcium–silicate–hydrate (C–S–H) gel, calcium–aluminate–hydrate (C–A–H) gel, and a minor amount of calcite. As T and W_g increased, the reduction in pores between soil particles enhanced the structural integrity and macro–strength of the cured MSC. The failure pattern of CF–1–solidified MSC elementary samples depended on the CF–1 dosage and curing duration. The solidification mechanism of CF–1 on MSC involved pozzolanic, ion exchange, and carbonation reactions. CF–1 solidified MSC satisfied all the specified requirements for roadbed filler in the relevant code, demonstrating substantial potential for in–situ solidification projects involving MSC.

The pipe-in-pipe (PIP) system, with good structural resistance and favourable thermal insulation capacity, has been extensively applied in oil and natural gas exploitation in deep waters. In the present paper, a simplified equivalent numerical model of the PIP system for deepwater J-lay operation was developed to evaluate the dynamic response of the outer pipe and the inner pipe under the combined hydrodynamic load and pipelay vessel motion by the software OrcaFlex. The comparison of mechanical responses between the present equivalent model and other available PIP models was performed to verify its reasonability. Considering vessel motion, pipe-soil interaction, wave and current, the dynamic behaviour of the outer and inner pipes was evaluated on aspects of the bending moment, effective tension, equivalent stress and strain. After that, the influences of key geometric parameters on the dynamic behaviour of PIP systems were systematically studied, including the diameter-to-thickness ratios of the outer pipe and inner pipe as well as the core thickness. The findings would provide good guidance for the structural design and the installation analysis of PIP systems using the deepwater J-lay operation.

Three common types of greenwater events – plunging dam breaker (PDB), hammer fist (HF) and plunging wave (PW) – are experimentally modeled in a laboratory wave flume on a rectangular structure, with a focus on investigating their fluid kinematics. To ensure high repeatability for type PW and type HF, a specific wave focusing method was employed, while PDB-type events were generated using a regular wave train. Utilizing a combination of PIV (particle image velocimetry) and BIV (bubble image velocimetry) techniques, ensemble-averaged flow fields were obtained from 20 repeated tests for each event type. The flow patterns at high speed, along with corresponding velocity fields, facilitated a comprehensive examination of flow behaviors, particularly for HF-type events which have received limited study. The maximum dominant speed for type PW was measured at up to 2.76$C$ during the run-up phase, where $C$ denotes the celerity of the incoming wave. For type HF and type PDB, the maximum dominant speeds occurred during the greenwater phase, with magnitudes of 1.37$C$ and 0.79$C$, respectively. The velocity deviation during the greenwater phase is <0.62C for all type events. The greenwater front velocity was measured at 1.36$C$ for type PW and approximately 0.8$C$ for both type HF and type PDB. Moreover, an attempt to evaluating the potential greenwater loads of high spatial resolution is demonstrated by the measured velocity fields for all event types. In this study, the simplest dam break solution is found to effectively capture the horizontal greenwater velocity distribution for all event types. Additionally, other mathematical expressions for the horizontal greenwater velocity have been derived based on flow self-similarity.

The closed sump is a vital inlet structure for low-head tidal pumping stations in coastal regions. The flow instability caused by roof-attached vortices (RAVs) and floor-attached vortices (FAVs) within the sump significantly affects the reliability of the unit operation. Mitigating and eradicating these detrimental vortices is deemed imperative in the realm of engineering applications. Reducing the sources of vortices, improving operating modes, and adding anti-vortex devices (AVDs) are the general ways to suppress the occurrence of vortices. However, few reports exist on the closed pump sumps' joint vortex elimination methods for RAVs and FAVs. Based on a deep understanding of the dynamic evolution behavior of the RAVs and FAVs, a hybrid RANS-LES numerical prediction method is adopted to comprehensively compare the suppression effects of different AVDs on the vortex structure. An effective "elliptical line anti-vortex cone combined with underwater cover plate" joint anti-vortex device (JAVD) is proposed and verified through model experiments. The research results provide analytical ideas for improving the flow field of the pump sump and optimizing hydraulic design.

Wave motion simulators have various applications in the development of marine industrial products. The main factor limiting its performance to meet the needs for extreme sea states simulation is the lack of large stroke, heavy duty, and high response actuators. Therefore, a novel actuator mechanism is proposed in this paper to realize the dynamic output of large stroke, heavy duty and high response. In this paper, a (2P(nR)+PPR)P actuator mechanism composed of 2P(nR)P and PPRP mechanisms is proposed, with the input-output relationship analyzed. Then, this actuator mechanism is applied to a 6-PUS platform. The Newton-Euler method is employed to model and simulate the dynamics of the platform to verify the input-output relationships. Finally, a 6-PUS platform based on (2P(nR)+PPR)P mechanism was designed, built and tested under extreme operating conditions. The results show that the 6-PUS platform with this actuator mechanism can achieve a large stroke of ±45° within 7 s cycle time and a high response motion of ±30° within 3 s under a heavy duty of 10t, which demonstrates that it has the performance of large stroke, heavy duty and high response. This actuator mechanism and its platform are of significant value in wave motion simulators for extreme sea states.

The real-time hybrid model (RTHM) test is adept at addressing the scale contradiction, the lack of fidelity in wind modelling in hydrodynamic testing facilities and spatial constraints inherent in conventional monopile-type offshore wind turbine (OWT) model testing methods, thus emerging as an effective avenue for conducting physical model tests of Monopile-type OWTs. This method entails the reproduction of aerodynamic loads or platform motions using loading device or vibration tables. Time delays in the physical attributes of the loading device and signal transmission processes within the system can result in error accumulation, with the potential to impact overall system stability. Moreover, time delay compensation algorithms for hybrid model test systems with force control loading can easily generate excessive noise, leading to system divergence. As a result, time delay has emerged as a technical challenge in the RTHM test. To address this issue, this paper has developed second-order and third-order polynomial extrapolation algorithms, alongside an adaptive compensation algorithm. The adaptive compensation algorithm employs the least squares method to identify parameters of the loading system, enabling it to address variations in the time delay of the experimental system caused by the nonlinearity of the loading system and changes in the physical properties of the model. The feasibility and effects of time delay compensation for various algorithms are validated through numerical simulation. Results indicate that the adaptive compensation algorithm surpasses second and third-order polynomial extrapolation compensation algorithms in terms of accuracy and compensation effectiveness. To validate the applicability of the adaptive compensation algorithm, a RTHM test was conducted. Across rotor thrust force (RotThrust) and tower top displacement, there was an average reduction of approximately 5 % and 9 % in the maximum and minimum synchronization errors, respectively. This highlights the efficacy of the delay compensation algorithm in practical applications, notably diminishing time delay errors within the experimental system. The adaptive compensation algorithm continuously adjusts and updates parameters, enhancing the adaptability of the compensation process to time-varying systems.