Surge Motion Research Articles

Robust adaptive event-triggered path following control for autonomous surface vehicles in shallow waters

This paper utilizes an adaptive event-triggered mechanism to investigate the path following control of autonomous surface vehicles (ASVs) in shallow waters. Given the challenges posed by the shallow water environment to ASV maneuverability and heading stability, the controller design must enhance the robustness and reduce model complexity. Firstly, a time-varying drift angle-based course guidance law was proposed using a position error dynamics model and a reduced-order extended state observer. Secondly, to simplify the controller's design steps, the ASV's surge and sway motion models are decoupled and transformed. Then, an event-triggered mechanism controller was designed to activate the ASV's rudder angle and shaft speed. In addition, a new robust term was introduced to further estimate the residuals of the RBF neural network, the external disturbances and the event-triggered residual term, thus eliminating the requirement for an upper bound on the unknown disturbances and residuals. The proposed control method based on the robust adaptive event-triggered mechanism can guarantee that all estimation and tracking errors are uniformly ultimately bounded. Simulation results demonstrate that the controller can ensure effective path following of vessels in shallow waters.

Experimental hydrodynamic assessment of a cylindrical-type floating solar system exposed to waves

In this paper, an experimental investigation on the wave loads and structural motions of two semi-fixed semi-immersed horizontal cylinders type rafts in the free surface zone is conducted. The physical models are tested at the 1:4.5 scale and exposed to a range of regular and irregular waves in a wave flume at Queen's University Belfast. The physical models and experimental setup are discussed alongside an investigation of the hydrodynamic phenomena, surge forces, and dynamic responses that each structure exhibits in the coastal wave climates. Furthermore, an investigation into the wave attenuation by both models is carried out. The results show that the surge forces have a positive correlation with wave steepness for both models. Hydrodynamic phenomena such as wave runup and overtopping, radiative damping and reflected waves, constructive interference, diffraction and flow separation were identified during the experiments. A negative mean heave displacement is observed during the monochromatic sea states which could result in impact loading and submergence of the superstructure components and photovoltaic panels at full-scale. The results presented in this paper may be used to calibrate and verify numerical models that calculate the global responses and hydrodynamic forces. It may also benefit the design processes of geometrically similar floating solar technologies by providing data on surge loads, motion responses and hydrodynamic observations.

A CFD investigation of the effects of passing ship on a small size ice floe

A RANS (Reynolds Averaged Navier-Stokes) CFD STAR-CCM+ model is used to investigate the hydrodynamic interactions between a ship and a circular ice floe. The overset mesh technique is employed to simulate the advancing movement of the passing ship. The ship has a hull of similar configuration to the KCS MOERI container ship. The ice floe has a diameter of 30% of the ship’s length and a thickness of 3 m. The paper primarily focuses on the motions of the ice floe and the hydrodynamic forces induced by the ship progressing in different speeds and calm water conditions. A parametric study on the influence of the ice floe surge and sway motions and associated hydrodynamic loads is presented. It is concluded that ship-generated waves can significantly affect the motions of the ice floe and the influence on sway is greater than that on surge. The ship speed is also found to have a distinct impact on ice floe motions.

Preliminary stability design method and hybrid experimental validation of a floating platform for 10 MW wind turbine

Platform motions induced by wind and waves import strong nonlinear coupling effects in the aerodynamic characteristics of floating offshore wind turbines (FOWTs), which exacerbate the power instability of the wind turbine and should be considered during the preliminary platform design stage. For this reason, a platform motion-based method was proposed to estimate the power stability and successfully applied to the preliminary platform stability design in this paper. Firstly, a high-fidelity computational fluid dynamics (CFD) model of a 10 MW wind turbine was established using the oversetting grid and superposition motion technology. Then the influence laws of platform surge and pitch motions on the power characteristics of the wind turbine were summarized. Subsequently, a semi-submersible floating platform structure for supporting the 10 MW wind turbine was designed preliminarily. The power stability of the wind turbine and tilting stability of the platform were analyzed based on the hydrodynamic analysis results. Finally, a hybrid scaled model of the 10 MW FOWT was built, incorporating a six-degrees of freedom load actuation system based on multi-fans. The scaled model test was carried out in a wave basin, demonstrating that both the power stability and the tilting stability of the platform meet the proposed stability requirements.

Reconstruction and prediction of global whipping responses on a large cruise ship based on LSTM neural networks

This study presents a model based on Long Short-Term Memory (LSTM) neural networks for the reconstruction and prediction of global whipping responses using ship motion data. The model is fine-tuned and trained based on a dataset of heave, pitch, surge motion data, vertical acceleration data, and vertical bending moments (VBM) from a large cruise ship model experiment by utilizing 5-fold cross-validation. The established model is tested with a pre-split test dataset and used to carry out reconstruction and multi-step prediction of the VBM amidship in following and head seas. The reconstructed results demonstrate a high degree of fit with the measured VBM amidship under both regular and irregular wave cases, indicating the model's ability to capture whipping responses. The findings of multi-step prediction indicate that the prediction accuracy and the model's ability to capture whipping responses decrease as the horizon increases, and the model is more accurate in predicting whipping responses when they are stronger.

Coupling effect of time-varying axial tension and combined unsteady flow on vortex-induced vibration of flexible risers

The surge and heave motions of the floating platform have a non-negligible effect on vortex-induced vibration (VIV) of marine risers, which generates time-varying axial tension of risers, i.e., parametric excitation, and unsteady flow and complicates the prediction of VIV. A semi-empirical model is improved to simulate the VIV of flexible risers with parametric excitation under combined unsteady flow. Both the flow velocity and the stiffness matrix are updated with time. Fifty-five cases are simulated to investigate the separate and coupling effects of parametric excitation and combined unsteady flow on VIV. The phenomena such as sum and difference frequencies, Mathieu-type resonance, intermittent vibration and frequency fluctuation are observed. The effects of parameters such as excitation amplitude dT, excitation frequency fp, combined ratio r and oscillation period P on VIV are discussed in detail. Some interesting conclusions are drawn: VIV may be significantly suppressed when transition-low-frequency parametric excitation and short-period combined unsteady flow act together. Larger r makes the effect of combined unsteady flow more significant. VIV is mainly dominated by the parametric excitation when fp is twice the natural frequency of the riser corresponding to the dominant mode.

Fully coupled aero-hydrodynamic modelling of floating offshore wind turbines in nonlinear waves using a direct time-domain approach

In standard operating conditions, a floating offshore wind turbine (FOWT) suffers from the combined action of nonlinear waves and wind. It is important to explore the effect of higher-order hydrodynamic loads induced by the nonlinear waves on its motion response. In this work, a fully coupled aero-hydrodynamic analysis method for FOWT is developed. The hydrodynamic part is based on the nonlinear potential flow theory and the perturbation approach, using a higher-order boundary element method in the time domain. Blade element momentum theory is applied to evaluate the aerodynamic load on the turbine rotor. The mooring system is modelled by the catenary theory. The developed method is verified against the published data of the NREL's 5-MW baseline wind turbine supported by the DeepCwind semi-submersible platform. The verifications are conducted stepwise in the following ways, including hydrodynamic wave excitation load, free-decay response, the load-displacement relationship of the mooring line, the steady-state response of the wind turbine, and fully coupled aero-hydrodynamic interaction. It is found that the nonlinear effect mainly influences the surge motion. The coupled configuration has little effect on the linear wave force and platform displacement but notably impacts the nonlinear wave force and displacement. The FOWT motion is found to be critical in changing the wave field, the maximum wave elevation position around the platform, and the nonlinear wave force acting on the floating platform. The results illustrate that the traditional indirect time-domain method using the Quadratic Transfer Function (QTF) based on assumed response cannot appropriately evaluate the nonlinear wave force of a FOWT in coupled motions. The results also reveal that nonlinear wave hydrodynamics is necessary to understand the motion response of a fully coupled FOWT.

Numerical study on motion responses and added resistance for surge-free KCS in head and oblique waves based on the functional decomposition method

A hybrid approach of potential and viscous flows is developed, based on the functional decomposition model SWENSE (Spectral Wave Explicit Navier-Stokes Equations), to investigate the seakeeping performance of the surge-free ship models accurately and efficiently. In the SWENSE method, the total physical field, including velocity, pressure and the signed distance function of the free surface, are decomposed into the incident field and the complementary field. For the incident wave field, the linear wave model is used and the physical field is extended to the flows up to the air zone using the Wheeler stretching method. With respect to the complementary field, the variables are governed by SWENSE. The adapted single-phase level-set model is used to capture the free surface. Based on this hybrid approach, an in-house code HUST-SWENSE is developed. The spring-mass-damper module, 4DOF (Degrees of Freedom) motion equations of the rigid body, three coordinate systems and the dynamic structured grid with the overset technology are applied to deal with the 4DOF ship motions. Firstly, the size of the computational domain for the simulations of the oblique waves is discussed. The numerical results can be convergent for HUST-SWNESE when the radius R/L of the cylindrical computational domain is larger than 2.0. After determining the grid system and the time step based on the verification and validation study, simulations are conducted using the HUST-SWENSE to predict the surge, heave, roll, and pitch motions of the KCS in regular oblique waves. Wave conditions are based on the benchmark cases provided by the Tokyo2015 CFD workshop. The simulated results are then compared with experimental data and other numerical results. The comparison shows that the respective differences were less than 2% for incident regular waves, 10% for heave and pitch motion responses, 5% for roll motions and 20% for added resistances. Therefore, the HUST-SWENSE model can be considered reliable for the prediction of ship motions and added resistance in head and oblique waves. Finally, the influences of different DOFs, wavelengths and wave heights on the motions and added resistance for the surge-free ship model in oblique waves are investigated in detail. It is found that the importance of surge motion in the seakeeping performance prediction of the ship in astern seas, especially for the added resistance. Moreover, the nonlinearity of wave heights is analyzed, and the results indicate that the nonlinearity of the large-amplitude roll is mainly induced by the roll restoring moments, while the nonlinear features of the incident, radiation and diffraction wave systems were responsible for the nonlinearity of added resistance and surge motions.

Hydrodynamic performance investigation of the multi-degree of freedom oscillating-buoy wave energy converter

Wave energy is considered to be a kind of promising renewable energy due to its high energy density and enormous reserves. This study aims at examining the hydrodynamic performance of the multi-degree of freedom oscillating-buoy Wave Energy Converter (WEC). A three-dimensional numerical wave tank was developed using Star-CCM + software to investigate the interaction of waves with WEC. A mathematical model is built based on the structure of the multi-degree of freedom oscillating-buoy WEC. This paper investigated the variation in the device's energy conversion efficiency for different wave periods and spectral analyses of the motion of the floater. The effect of different elastic and damping coefficients on the device's performance was compared. The results demonstrate that the surge motion is the leading cause of the periodic variation in amplitude of floater motion. Heave motion is the main contributor to energy capture. Wave height has an influence on the contribution of the elastic and damping coefficients to the energy conversion efficiency.

Numerical study on dynamic analysis of a nine-module floating aquaculture platform under irregular waves

A numerical study of the dynamics of a nine-module floating aquaculture platform in irregular waves is proposed, to examine the effect of the connector type between net cages and incident wave direction on the hydrodynamic response. A numerical model based on the coupled boundary element method and lumped-mass model was used to simulate the dynamic behavior of the aquaculture platform. Time-domain statistical analysis, and time-frequency analysis based on Hilbert-Huang transform on the motion responses of net cages and the mooring force responses of the platform were then carried out. The results indicate that the mooring system has a decisive influence on the frequency-domain characteristics of surge motion, while the influence of connector can be ignored. The surge motion under flexible hawser connector is less sensitive to wave incident direction, while the wave direction has a greater impact on surge motion under spherical joint connector. The amplitudes of pitch motion under two different connectors decrease significantly under oblique incident waves, compared with perpendicular incident wave condition. The wave incident direction has little effect on the overall mooring force response of the platform. The mooring force responses at different positions show completely different frequency characteristics when the cages are connected by spherical joints. The larger the pitch response of the cage, the smaller the mooring force response of the mooring chain connected to the net cage. There is a positive correlation between the gravity frequency of the mooring force response and the gravity frequency of the pitch response of the net cage.

Experimental comparison of a dual-spar floating wind farm with shared mooring against a single floating wind turbine under wave conditions

Shared mooring can potentially be a cost-reduction factor for future floating wind farms. To assess the effect of shared mooring on the response dynamics of floating wind turbines (FWTs), experiments have been conducted for two spar FWTs with a shared mooring configuration and for a single spar FWT with catenary mooring, both at a scale of 1:47. Various regular and irregular wave conditions were tested and only one wave heading was considered. From the test results, the response amplitude operators and response spectra of platform motions and statistics of mooring tensions are compared for the two configurations. In the extreme wave condition, the shared mooring configuration leads to a 40% increase in the platform surge motion compared with the single spar FWT. Still, the absolute offset is within allowable limits prescribed by power cables. The fairlead tensions of the anchor lines are not significantly increased in the shared mooring configurations, but the shared line shows extreme tension peak events that are seldom present in the anchor lines. This study demonstrates the technological promises and challenges of a shared mooring system from an experimental perspective. Results and videos are made available and can be used to validate numerical models.

A methodology to assess the dynamic response and the structural performance of floating photovoltaic systems

This study presents a comprehensive methodology for evaluating floating photovoltaic (FPV) structures, focusing on the impact of wind and wave conditions from hydrodynamic and structural perspectives. The methodology is applied to a Class 1 pontoon-type structure with rigid and hinged configurations. A total of 558 simulations are conducted, considering various environmental actions, configurations, and mooring line chain sections. The results provide essential data on loads and motion time series for subsequent structural analysis. Wind forces primarily influence surge and sway motions, while wave forces dominate other motion components. Comparing the hinged configuration to the rigid one, a significant reduction in maximum yaw motions of 32% to 76% was observed, depending on the mooring chain section employed. This reduction in yaw motions may positively impact the energy yield. However, an inverse trend was observed for pitch motions. Hence, a comprehensive assessment of wave-induced motions is crucial for selecting the optimal FPV configuration. Furthermore, heavier chain sections effectively limited surge, sway, and yaw motions, with variations of up to 75% observed in the hinged configuration when comparing different chain options. Structural analysis highlights the importance of wave characteristics, mooring system configuration, and system flexibility. The findings emphasize the need to consider environmental conditions, structural aspects, and energy efficiency in optimizing FPV configurations.

Effects of surge and roll motion on a floating tidal turbine using the actuator-line method

This paper employs a dynamic and sliding mesh in the simulation of both uncoupled and coupled surge and roll motions of a tidal stream turbine, utilizing a modified actuator-line method. The modification involves the relocation of blade elements in relation to the grid. Detailed analyses are conducted on the Cp and Cz variations in surge, roll, and coupled motions at various frequencies and amplitudes. It is observed that changing the amplitude and frequency of surge and roll motions both impacts the amplitude of Cp and Cz. Interestingly, the Cp and Cz variations in surge motion are inversely proportional to velocity variations, while they are directly proportional in roll motion. The influence of the surge motion on Cp Cz plays a major role, while the addition of the roll motion increases the mean values of Cp and Cz. Due to the combination of the wake characteristics of both surge and roll, the coupled motion wake exhibits a contraction–expansion oscillation pattern. In a coupled motion with equal periods, the ring and strip tail vortex characteristics of both motions are apparent. A surge period increment diminishes the surge's tail vortex characteristic, whereas an increase in the roll period gradually erodes the roll's tail vortex characteristic. The coefficient variation of the tangential and normal forces (cn, ct) in combined motion mirror that of surge motion, presenting a convex table per surge cycle with depressions at the 1/2T and 1T points. The peak of cn and ct in surge motion are approximately 0.28 and 0.03, respectively, while in roll motion, they are around 0.261 and 0.025. The exploration of cyclic stress impacts on the turbine, and the potential instability on the platform could be valuable directions for future research.

Comparative analysis of bent and basic winglets on performance improvement of horizontal axis wind turbines

The winglet has been widely deployed in the optimization of the blade design as it reduces the tip loss of the blade and increases the swept area of the rotor. However, previous studies have not adequately investigated the effect of platform motion on winglet performance in wind turbines. The objective of this study is to propose a novel bent winglet structure for wind turbines to improve performance under both stationary and surge conditions. To achieve this, the NREL Phase VI horizontal axis wind turbine (HAWT) is treated as a baseline. The numerical method employed is validated by comparing the simulated power and pressure coefficients of the HAWT with experimental data from the literature. The performance of the conventional winglet with the proposed novel winglet is compared in detail, taking into account the cant, twist, expansion direction, length, and winglet number of the conventional winglet. The results show that the novel bent winglet exhibits superior performance, with a 14.5% improvement in performance compared to the conventional winglet under surge motion. This study provides a feasible scenario for the optimization of onshore and offshore wind turbine designs.

Validation of maneuvering simulations for a KCS at different forward speeds using the 4-DOF MMG method

Ships with a high center of gravity and at relatively fast speed, such as container vessels, may experience significant heel during turning at a rudder angle of 35°. Accurate prediction of the maneuvering motion of such ships requires consideration of roll motion in addition to surge, sway, and yaw motions. This study applies the 4-Degrees of Freedom (4-DOF) Maneuvering Modeling Group (MMG) method, developed by one of the authors, as a practical simulation method that includes the roll-coupling effect, to predict the maneuvering of a Kriso Container Ship (KCS). To validate the accuracy of the maneuvering simulations, the results are compared with those obtained from free-running model tests for turning and zig-zag maneuvers. Furthermore, to capture the effect of speed on maneuvering, the free-running tests are conducted by varying the approach speed of the ship model. The test results reveal that the advance in turning and the overshoot angle in zig-zag maneuvers increase with speed, while the tactical diameter decreases. The simulation results obtained using the 4-DOF MMG method are consistent with the free-running test results with practical accuracy. Therefore, the 4-DOF MMG method proves useful in predicting the maneuvering of ships, such as container ships, at fast speeds.

Hydrodynamic characteristics of a 15 MW semi-submersible floating offshore wind turbine in freak waves

Dynamics of the floating offshore wind turbines (FOWTs) in harsh ocean environments gains increasing attention due to the high maintenance costs in case of structural damage. The freak wave occurring more frequently than believed poses a threat to FOWTs, but mechanism and influencing factors of the wave-structure interaction problem have not been fully explained. In this paper, dynamics of the VolturnUS-S semi-submersible platform designed for the IEA 15 MW wind turbine under freak waves is studied using a coupled computational fluid dynamics and finite element mooring line model. First, the hydrodynamic model is validated using the DeepCwind semi-submersible platform. Then, the validated model is employed for the analysis of the VolturnUS-S semi-submersible platform under freak waves described as a focused group of waves. Hydrodynamic behaviors of the semi-submersible FOWT are described in detail, and parametric analysis of the problem including the effects of focusing crest amplitude, focusing location, incident angle, gravity center location is conducted. Important results include the dramatic surge motion of the platform and fairlead tension of the upstream mooring line, and alleviation of these responses by varying the wave angle to 30° and moving the gravity center to the centroid of the platform geometry.

Effects of the current-wave interaction on a cylinder platform

ABSTRACT This article investigates the motion responses of floating production storage and offloading systems (FPSO) under wave-current interactions. Experiments were conducted on a cylindrical FPSO in a wave-current combined wave basin. Two methods were employed to compare the effects of currents: applying an equivalent force to a specific location in the platform and generating real currents in the basin. The primary objective of this study is to explore the sensitivity characteristics of the platform's dynamic response in the presence of wave-current coupling. The findings reveal significant phase shifts in the low-frequency motions of the sway and pitch responses due to the interaction between waves and currents. Moreover, the six degree-of-freedom motions of the platform experience amplified overall energies in both low-frequency and wave-frequency motions. A notable observation is that wave-current interaction leads to a considerable increase in surge motion amplitude, even when the reduced velocities are smaller than the lock-in region.

The assessment of a fast computational method in predicting the unsteady loads of vertical axis wind turbines undergoing floating motion

The low center of gravity and low center of thrust of Vertical Axis Wind Turbines (VAWT) are interesting characteristics when considering Floating-Offshore Wind Turbine (FOWT) application. The motion due to the floating platform adds extra complexity to the unsteady aerodynamics of VAWT. Hence, both numerical and experimental studies become very challenging. This paper focuses on the assessment of TubinesFoam, a fast tool consisting of the actuator line method (ALM) embedded in OpenFoam. Then, instead the typical bladed-solved Navier–Stokes equations, the ALM inserts the blade forces into the field as body forces into the momentum equation. A modified version of the turbinesFoam library that includes surge motion for multiple rotors has been evaluated by comparison with an experimental set of data from a twin-rotor performing surge in a wind tunnel. This paper includes the main equations describing the kinematics implemented in the numerical code, as well as a convergence analysis of mesh size, time step and surge cycles. The numerically predicted thrust forces agreed with the experimental results for both investigated tip speed ratios for the case with no surge motion. The numerical and experiment results including surge motion indicated a minimal influence of the surge motion for the cases evaluated here. The last verification using CFD computations from the literature showed that the turbinesFoam tool properly captures the main features of the surge motion. This verified numerical method is a promising tool to understand the effects of geometrical parameters on the performance and the wake development of VAWTs farms in floating-offshore environments.

Coupled dynamic and power generation characteristics of a hybrid system consisting of a semi-submersible wind turbine and an array of heaving wave energy converters

A wind-wave hybrid system consisting of a semi-submersible floating wind turbine and an array of heaving wave energy converters (HWECs) provides a solution for co-located marine renewable energy extraction. The knowledge of the coupled dynamic and power features of the hybrid system is important to the design and operation but is poorly understood. This study proposes a comprehensive numerical model that can tackle the full couplings by bridging the aerodynamic module of OpenFAST and the hydrodynamic simulator WAFDUT through an in-house code based on multi-body dynamics. The coupled dynamic and power generation characteristics of the hybrid system are investigated, with an emphasis on the influence of the HWECs on the wind-induced motion, mooring tension, and wind power generation of the turbine. Results show that the HWECs do not produce negative effects on the wind-induced surge, heave, and pitch motions. The HWECs can slightly reduce the mooring tension and slightly improve wind power generation. The array of HWECs can generate over 500 kW wave power in the waves with periods of 6 s–10 s, which can be a qualified supplement to wind power. The findings offer useful knowledge about the key properties of the hybrid system for practical use.

Conceptual design, parameter optimization and performance investigation of a 10MW semi-submersible floating wind turbine in shallow water

There is lacking of floating wind turbine supporting large wind turbine, i.e. 10 MW, aiming at shallow water applications. In this paper, a 10 MW semi-submersible floating wind turbine named OUCwind is proposed for shallow water, and a scale optimization procedure is proposed during the preliminary design stage. It offers an engineering-practical method to determine the main parameters of a new-proposed floater, and critical floater characteristics, such as floater stability, natural period, platform mass etc. are chosen as important design criteria. The hydrodynamic performance with different scale parameters are investigated and it is found that enlarging distance between columns is a cost-effective way to get better stability and hydrodynamic performance. The structural performance of the new-designed OUCwind floater are compared with other two well-known braceless 10 MW platforms, 10 MW-CSC, and OO-star. From the frequency-domain analysis results, the surge linear transfer function, mean drift force and quadratic transfer function near the surge natural frequency of OUCwind is relatively small compared with other two platforms. In the time domain, the new-designed OUCwind also performs better in surge motion and mooring line tension response compared with 10 MW-CSC and OO-star, especially under extreme condition. Nevertheless, for the heave and pitch motions, the wave-frequency response of the OUCwind platform is relatively large under extreme load conditions.