Ocean Engineering Design Research Articles

Hydrodynamic optimization and performance verification of fairings for round-ended cofferdams using dam-break wave experiments and numerical simulations

Round-ended cofferdams are crucial for large-scale marine projects but are vulnerable to the complex marine environment, including waves, tides, and storms. Enhancing the hydrodynamic performance of these cofferdams is essential for improving safety and efficiency in marine construction. This study introduces a novel fairing design aimed at reducing water flow resistance during the quasi-stable stage of a tsunami. The flow field is analyzed using computational fluid dynamics (CFD) simulations, and an adaptive surrogate model is employed to optimize the parameterized fairing shape. The results demonstrate that the optimized shape significantly suppresses vortex shedding, leading to a 16.59 % reduction in the drag coefficient and a 44.3 % reduction in the lift coefficient under flow conditions. Experimental and numerical simulations of dam-break waves are conducted for two designs, R1 and R0. By comparing their flow field and force characteristics, it is found that R1 effectively separates waves during the impulse stage, reduces wave climb height, and decreases impact loads. In the quasi-stable stage, R1 mitigates the blockage effect, reducing the liquid level difference between the front and back, and thus lowering flow forces. Experimental data further reveals that when the downstream is a dry riverbed, R1′s load reduction is particularly notable, with maximum reductions of 45.62 % in the impulse stage and 28.75 % in the quasi-stable stage. When the riverbed is wet, the maximum load reduction rates are 18.04 % and 8.72 %, respectively. Therefore, R1 not only reduces the resistance of round-end cofferdams under water currents but also under extreme wave forces, providing valuable insights for advancing ocean engineering design.

A multi-orientation system for characterizing microstructure changes and mechanical responses of fine-grained gassy sediments associated with gas hydrates.

Fine-grained marine sediments containing veiny and nodular gas hydrates will evolve into fine-grained gassy sediments after hydrate dissociation due to climate-driven ocean warming. The mechanical properties of the fine-grained gassy sediments are basically acquired by ocean engineering design. However, they have not been fully understood, largely due to the lack of microstructure visualization. In this paper, a new system is developed to jointly conduct x-ray computed tomography scans, oedometer tests, and seismic wave testing on a single specimen with temperature being well controlled, allowing varieties of experimental data to be acquired effectively and automatically. The results show that stress history can hardly affect the undrained stiffness of fine-grained gassy sediments, while the drained stiffness of fine-grained sediments without gas bubbles is stress history dependent. After being unloaded, many microstructure changes remain, and examples include the free gas distribution being more concentrated and the connectivity among gas bubbles becoming much better. The multi-orientation system lays the foundation for further studies on the microstructure changes and mechanical responses of fine-grained gassy sediments associated with gas hydrates.

Enhanced Typhoon-Induced Design Wave Height Estimation for Ocean Engineering Applications Using High Performance Computing

Coastal construction heavily depends on accurately estimating design wave parameters. This paper presents a technique for calculating ocean engineering design wave heights that addresses the challenges of inadequate wind speed and measured wave data in the field using High Performance Computing (HPC). The proposed method combines the Simulating WAves Nearshore (SWAN) wave model driven by wind with two different wind field models: the NCEP/NCAR reanalysis wind field and the Jelesnianski typhoon model wind field. By combining these models, the study reduces the underestimation of wind speed and inaccurate depiction of typhoon-related details. The method's effectiveness is demonstrated by applying it to calculate the design wave elements of the deep-sea breeding platform in Dongluo Island, Fujian. The study results show that the platform can experience a maximum wave height of 9.42 m during a 50-year recurrence period. The research shows that the platform's southwest corner is the most vulnerable location. This new calculation method is a significant improvement in ocean engineering design since it overcomes the limitations that come with the lack of wind speed and measured waves. By utilizing multiple wind field models, the calculated wave heights' accuracy and reliability is considerably enhanced. The application of this method to the breeding platform in Dongluo Island has yielded valuable insights for designing resilient structures in similar oceanic environments. This study demonstrates the important application of computational numerical simulations in the field of ocean engineering, providing high-precision predictive capabilities for structural design and risk assessment.

Research on Threshold Selection Method in Wave Extreme Value Analysis

Climate change poses higher requirements on ocean engineering design, and reasonable estimation of design wave heights plays a crucial role in coastal protection and offshore engineering. Extreme value analysis is widely used in frequency calculations of wave parameters, among which the peak over threshold method based on the generalized Pareto distribution is proven to be an effective method, and the different selection of extreme wave samples in this method has a great influence on the calculation results. In this study, long-term significant wave height series were utilized to investigate the long-range correlation of significant wave heights, and thresholds were determined based on the changes of long-range correlations. This approach assumes that extreme events and non-extreme events are generally caused by different physical processes, where extreme events result from massive disturbances leading to abnormal states, and long-range correlations are not affected or minimally affected by extreme events. Thus, thresholds can be determined based on changes of long-range correlations by removing extreme events. Comparing this method to graphical diagnostic techniques, we demonstrated its rationality in determining extreme wave height thresholds. Moreover, the automatic threshold selection offered by this method helps to mitigate errors associated with subjective judgments in traditional approaches.

Meshless generalized finite difference method with a domain-decomposition method for solving Helmholtz equation and its application to caisson resonance problems

This paper aims at presenting a meshless Generalized Finite Difference Method (GFDM) for solving the Helmholtz equation. In particular, in order to tackle the problem where a fluid domain is divided into different parts by a thin-wall structure, a Domain-Decomposition Method (DDM) is introduced to prevent interactions between node pairs coming from different parts of the flow domain. Two rigorous benchmarks are performed to validate the convergence and accuracy of the present GFDM-DDM model. Furthermore, caisson resonance problems are also investigated and discussed based on the novel numerical tool. It is shown that the present GFDM-DDM model can provide satisfactory predictions for resonance modes in caisson resonance problems, and hence can be treated as a powerful numerical tool in ocean engineering design due to its accurate and efficient characteristics.

Experimental study on microbubble drag reduction on soil-steel interface

The soil-structure interface behavior is an important concern in ocean engineering design. In some cases, it is necessary to reduce the drag of structure, especially the shear stress on the soil-structure interface. In this study, a series of microbubble drag reduction experiments were first carried out on soil-steel interface. The effects of microbubble volume, moisture content, rotation speed and water types (freshwater and seawater) were investigated. It is shown that the average drag reduction ratio ranged from 15% to 92% under various conditions. With the increase of microbubble volume, the drag reduction effect is improved. Meanwhile, the characteristics of drag reduction were also explored under different moisture content. The drag reduction effect of microbubbles is relatively stable at different speeds.Its is proved that the microbubble drag reduction method can obviously reduce the soil-steel interface drag.

Co-occurrence probability of typhoon surges affecting multiple estuaries in the northern coastal region of Taiwan

Taiwan lies on the northwest Pacific and is usually invaded by typhoons in summers and autumns. When typhoons strike the coast, surges usually occur. In addition, if a typhoon surge occurs during a high-tide period, the water level starts to rise. As a result, largely urbanized and highly populated regions around the estuaries may be devastated by catastrophic damage owing to floodwater elevation rising and unable to drain off floodwater. Therefore, in order to allocate regional resources reasonably and prevent and reduce coastal disasters effectively, it is necessary to estimate the co-occurrence probability of typhoon surges in multiple estuaries. In this study, total 36 typhoon processes are applied to calculate the simultaneous-occurrence probability of typhoon surges in three estuaries in Nan-Kan River, Huang River, and Lan-Yang River in the northern part of Taiwan. We first use certain extreme-value distribution models to fit the simulated peak values of a typhoon surge. These univariate statistical distributions include the Pearson type III, Weibull, generalized extreme value, and lognormal distributions. Through hypothesis and goodness-of-fit tests, the Pearson type III distribution is chosen as the optimal curve to fit the typhoon surges in these three estuaries. Then, according to the dependence of the typhoon surges in these three estuaries, a three-dimensional compound probability distribution model P-GH-TriP3 is constructed, and the joint return periods are calculated. The results indicate the simultaneous-occurrence probability of disasters in these three estuaries is small, which can provide some reasonable proposal for coastal typhoon disaster prevention and reduction and ocean engineering design.

Evolution characteristics and quantization of wave period variation for breaking waves

The wave parameters (the wave height and period) are important environmental factors in the ocean engineering design. General numerical wave models, such as SWAN and WAVEWATCH, are widely employed to simulate the wave conditions. However, significant differences were observed between the measurement and the simulation for both the wave height and period, which asks for wave model improvements. The differences are mainly due to the uncertainty of parameterizing various physical processes, including the wave breaking. The energy transfer and loss during the wave breaking involves an important physical mechanism, and the energy dissipation and the period changes are not well studied. This paper studies the deep and shallow water wave breaking using the wave focusing and the slope platform random wave experiments. The characteristics of the wave periods under different conditions are studied in detail, including the period variation. The results show that the periods change during the wave propagation and breaking processes. The energy transfer caused by the strongly nonlinear interaction between the wave components, as well as the energy loss caused by the wave breaking, are the primary causes. The corresponding relationships are established by fitting the data. For the deep water breaking waves induced by the wave focusing, the spectrally averaged period (SAP) increases, and a positive correlation between the rate of change and the wave steepness is found. In the shallow water, the nonlinear interactions are stronger than in the deep water, the wave periods are significantly reduced, and a negative correlation between the rate of change and a nonlinear parameter is found. The inherent mechanism of the period variation is analyzed based on the energy spectrum distribution variations. The contributions of the nonlinear interactions and the wave breaking to the SAP evolution are discussed.

Statistical description and modelling of extreme ocean wave conditions

A proper description of the ocean wave climate is of great importance for coastal and ocean engineering design and operation. Marine structures must be designed and constructed so they can withstand wave loads imposed by the environment and the statistics of extreme waves must be taken into account in design and operation. This paper outlines some recent applications of extreme value analysis of ocean waves that are believed to be of relevance for coastal and ocean engineering. These include applications of stationary and non-stationary univariate analyses as well as multivariate analysis and spatial extreme value analysis. Finally, environmental contours are presented as a useful tool to characterise extreme ocean environments for structural design and risk assessment.

THE PREDICTION OF EXTREME VALUE WIND SPEEDS AND WAVE HEIGHTS FROM SATELLITE DATA

The prediction of extreme value (e.g. 1 in 100 year) estimates of wind speed and wave height is an essential element of coastal and ocean engineering design. Despite decades of research on the statistics of extreme values, the consistent limitation faced by practitioners is the requirement for a long (20 plus years) dataset at the location of interest. Long term insitu buoy deployments have started to provide useful records in some geographic locations. Long term numerical model hindcasts have also proved useful. However, buoy deployments are seldom at the location of interest and the accuracy of numerical model hindcasts more than 20 years in the past is questionable. This paper will investigate the use of long-term satellite data sets of wind speed and wave height to provide global estimates of extreme values.

Estimate of the extreme wave height in the South China Sea using GPD method

The South China Sea (SCS) experiences severe typhoon impact every year. That also cause extreme wind waves. The estimate of extreme wave height is of great importance for human activities in the SCS. Since the generation of typhoon is still unpredictable, it is very difficult to estimate the storm waves. In this paper, we did a statistics analysis of the tropical cyclones that can affect the SCS. Based on a hindcast wave simulation, we use the generalized Pareto distribution (GPD) method to estimate the extreme wave height, and discuss the determination of threshold parameters. It is found that GPD method is a more favourable method for the estimation of extreme wave height of storm waves, which is of great significance for ocean engineering design.

Uncertainty quantification in estimation of extreme environments

We estimate uncertainties in ocean engineering design values due to imperfect knowledge of the ocean environment from physical models and observations, using Bayesian uncertainty analysis. Statistical emulators provide computationally efficient approximations to physical wind–wave environment (i.e. “hindcast”) simulators and characterise simulator uncertainty. Discrepancy models describe differences between hindcast simulator outputs and the true wave environment, where the only measurements available are subject to measurement error. System models (consisting of emulator–discrepancy model combinations) are used to estimate storm peak significant wave height (henceforth HS), spectral peak period and storm length jointly in the Danish sector of the North Sea. Using non-stationary extreme value analysis of system output HS, we estimate its 100-year maximum distribution from two different system models, the first based on 37 years of wind–wave simulation, the second on 1200 years; estimates of distributions of 100-year maxima are found to be in good general agreement, but the influence of different sources of uncertainty is nevertheless clear. We also estimate the distribution of 100-year maximum HS using non-stationary extreme value analysis of storm peak wind speed, propagating simulated extreme winds through a system model for HS; we find estimates to be in reasonable agreement with those based on extreme value analysis of HS itself.

Laboratory air‐entraining breaking waves: Imaging visible foam signatures to estimate energy dissipation

AbstractOceanic air‐entraining breaking waves fundamentally influence weather and climate through bubble‐mediated ocean‐atmosphere exchanges, and influence marine engineering design by impacting statistics of wave heights, crest heights, and wave loading. However, estimating individual breaking wave energy dissipation in the field remains a fundamental problem. Using laboratory experiments, we introduce a new method to estimate energy dissipation by individual breaking waves using above‐water images of evolving foam. The data show the volume of the breaking wave two‐phase flow integrated in time during active breaking scales linearly with wave energy dissipated. To determine the volume time‐integral, above‐water images of surface foam provide the breaking wave timescale and horizontal extent of the submerged bubble plume, and the foam decay time provides an estimate of the bubble plume penetration depth. We anticipate that this novel remote sensing method will improve predictions of air‐sea exchanges, validate models of wave energy dissipation, and inform ocean engineering design.

An improved numerical method for calculations in wave kinematics

The prediction of wave kinematics is a necessary basis for ocean engineering design. In this article, an improved method to calculate the wave kinematics especially in the crest region is presented. This method is based on the local approximation method. According to one measured wave surface, the proposed solution focuses on a Fourier series expansion in a somewhat larger segment of the wave surface record. The horizontal and vertical kinematics under the wave crest can be directly predicted. Good agreement between the numerical results and the theoretical Stokes waves verifies the validity of the numerical model. The effectiveness of the numerical method is also proved by the calculation of the wave kinematics of the laboratory focused wave groups.

On the Two-dimensional Diffraction Problem for Lewis-form Sections

To obtain the wave induced pressure distribution and wave-exciting force and moment upon two dimensional bodies, it is necessary to solve the two-dimensional diffraction problem.Tamura has calculated the wave-exciting forces for two-dimensional bodies of Lewis-form section, using the theory of Grim (based on diffraction potential). Hewever, the full solution of the two-dimensional diffraction problem has not been treated so far.In this paper, the author dealt with the diffraction problem for Lewis-form section, following the theory of Ursell and Tasai. The amplitudes and phase differences of the pressure field, rolling moment, sway and heave forces were calculated making use of this method for two Lewis-forms, excited by a given set of incident waves. Results were compared with the data from tests performed at the University of Tokyo's new Seakeeping Basin.The transmitted and reflected wave height were also calculated for various Lewis-forms, in view of their importance in Ocean Engineering Design.