Heave Motion Research Articles

Effects of Mooring Line with Different Materials on the Dynamic Response of Offshore Floating Wind Turbine

The influence of mooring systems with lines of different material on the dynamic response of a floating wind turbine is studied using a 5 MW OC4-DeepCwind semi-submersible wind turbine as a representative prototype in this study. Two types of mooring systems were designed using the MoorDyn module in OpenFAST software (v3.1.0): one uses chains, and the other uses a hybrid mooring line composed of chains and high-strength polyethylene (HMPE) ropes. A wind turbine with two types of mooring systems was simulated using the OpenFAST software. The results show that the floating wind turbine moored with the hybrid lines exhibited a larger heave and pitch motion than that moored using chains alone. At the same time, the surge displacement was smaller than that of the wind turbine using chains alone. In terms of mooring line tension, the mean and amplitude values of the hybrid mooring system at the location examined were smaller than those of the chain mooring system. Thus, using HMPE ropes in the mooring system can significantly reduce line loads. In addition, the HMPE ropes used in the floating wind turbine mooring system did not affect the power generation of the wind turbine. This study provides promising support data and observations for applying high-strength polyethylene (HMPE) ropes in mooring systems for floating wind turbines.

Effect of a rotational damper on a moored and articulated multibody offshore system in waves

Motions and loads of an articulated and moored floating platform consisting of multiple bodies in waves are investigated through numerical analysis. The wave–structure interaction (WSI) problem is solved using a high-fidelity viscous–flow solver that couples nonlinear rigid body motions, multibody interactions with an internal connection, and mooring dynamics. The study focuses on two modular floating structures (MFSs) connected by a flexible joint, with and without a rotational damper, and positioned using four symmetrical mooring lines. Multibody responses and the corresponding loads acting on the mooring lines and hinged joints are analyzed. Our results reveal that the influence of the damper on heave motions is less significant. Notably, the presence of the rotational damper has a noticeable impact on pitch motions between the two hinged MFSs. Introducing a rotational damper on the flexible joint effectively dampens the highly dynamic pitch motions while not imposing additional loads on the flexible joints.

Numerical Study on Aerodynamic Performance of Floating Dual-Rotor Wind Turbines in Heave and Surge Motions

Numerical Study on Aerodynamic Performance of Floating Dual-Rotor Wind Turbines in Heave and Surge Motions

Two-body dynamic simulations of a combined semi-submersible floating offshore wind turbine and torus wave energy converter

Dynamic responses of a combined wind and wave energy system which consist of a 5MW semi-submersible floating offshore wind turbine (FOWT) and a torus-shaped wave energy converter (WEC) are investigated. In the investigated configuration, the torus WEC is constrained to move only in the heave direction with respect to the FOWT. This results in a two-body dynamic system that allows for the extraction of wave energy through the relative heave motion. A Modelica-based multi-body system (MBS) code is used to perform fully coupled dynamic analyses of the combined system. The wind turbine loads are calculated using the state-of-the-art Blade Element Momentum (BEM) method, while the wave power take-off (PTO) system is modelled as a linear spring and a viscous damper. A case study is presented where the mass of the torus is varied to investigate the impact on the two-body dynamics of the combined system. The system performance is assessed in terms of the maximum wave power absorption and the quality of wind power production.

A two-dimensional Reynolds averaged Navier–Stokes investigation on gap resonance and wave elevation around two side-by-side bodies with motions

The coupled dynamics of wave elevation and motions around bodies floating in proximity may be sensitive to radiation and diffraction effects. In this paper, the influence of radiation on wave elevation is examined for two side-by-side boxes subject to forced sway, heave, and roll oscillations in a two-dimensional numerical wave tank. The effects of diffraction on wave elevation and the joint influence of radiation and diffraction are investigated in regular waves assuming free heave motions. Heaving of one or two boxes and single-box sway or roll excites a piston mode of water motions in the proximity gap. Synchronous sway or roll induces sloshing. The close relationship between gap resonance and rapid water exchange in and out of the gap is confirmed. Vortices within the gap drive water exchange and influence the gap wave elevation. Their impact is determined by both spatial and temporal distributions.

Effect of Interaction of Flapping Hydrofoil Motion Parameters on Thrust Pulsation Performance

This study presents a novel index to assess the thrust propulsion performance of flapping-hydrofoil, and identifies the order of priority of motion parameters that affect the thrust pulsation amplitude. The flapping motion is simulated using the Udf (user-defined motion) feature in the commercial software Fluent, and the impacts of three flapping motion parameters on flapping thrust are investigated by manipulating these parameters. The results of the numerical simulation indicate that there is a positive correlation between the heave amplitude and the thrust pulsation amplitude. Additionally, the thrust pulsation amplitude is interactively affected by the pitch amplitude, the phase difference between horizontal and heave motion, and the heave amplitude. The Levenberg-Marquardt Algorithm is employed to fit the function that relates the thrust pulsation amplitude with the three motion parameters. The coefficients obtained from the fitting function indicate the impacts of the interaction effect, as well as the effect of each motion parameter on the pulsation amplitude.

Structural Response Analysis for Multi-Linked Floating Offshore Structure Based on Fluid–Structure Coupled Analysis

Recently, offshore structures for eco-friendly energy, such as wind and solar power, have been developed to address the problem of insufficient land space; in the case of energy generation, they are designed on a considerable scale. Therefore, the scalability of offshore structures is crucial. The Korea Research Institute of Ships & Ocean Engineering (KRISO) developed multi-linked floating offshore structures composed of floating bodies and connection beams for floating photovoltaic systems. Large-scale floating photovoltaic systems are mainly designed in a manner that expands through the connection between modules and demonstrates a difference in structural response with connection conditions. A fluid–structure coupled analysis was performed for the multi-linked floating offshore structures. First, the wave load acting on the multi-linked offshore floating structures was calculated through wave load analysis for various wave load conditions. The response amplitude operators (RAOs) for the motions and structural response of the unit structure were calculated by performing finite element analysis. The effects of connection conditions were analyzed through comparative studies of RAOs and the response’s maximum magnitude and occurrence location. Hence, comparing the cases of a hinge connection affecting heave and pitch motions and a fixed connection, the maximum bending stress of the structure decreased by approximately 2.5 times, while the mooring tension increased by approximately 20%, confirmed to be the largest change in bending stress and mooring tension compared to fixed connection. Therefore, the change in structural response according to connection condition makes it possible to design a higher structural safety of the structural member through the hinge connection in the construction of a large-scale multi-linked floating offshore structure for large-scale photovoltaic systems in which some unit structures are connected. However, considering the tension of the mooring line increases, a safety evaluation of the mooring line must be performed.

Impact of flapping trajectory and foil gap on induced thrust of a flapping foil

Wings and fins with tandem configuration are commonly observed in a flock of birds or schools of fish for enhanced performance while flying and summing. Emerging study on these subject underpinned by flapping foil is challenging. However, our study focuses on the tandem hydrofoil by varying the conventional (vertical heave and pitch motion) flapping motion to two new flapping trajectories: (i) elliptical and (ii) fishtail flapping trajectories. The hydrodynamic efficiency, such as propulsive efficiency and wake structure of the tandem hydrofoil, are examined using computational methods. The effect of Strouhal number (St) and inter-foil spacing are also discussed along with the flapping trajectories. The results show that by implementing the two new flapping trajectories, the induced thrust of the hind foil can be enhanced up to 45% compared to the simple flapping. This study shed new light on improving the biomimetic propulsion device, as it aims to enhance the elicited thrust of the hydrofoil.

Design Longitudinal Control System Using Suitable T-Foil Modeling for the Offshore Wind Power Operation and Maintenance Vessel with Severe Sea States

In order to reduce the offshore wind power operation and maintenance vessel motion induced by severe sea states, a suitable stabilizer with the ship based on linear quadratic regulator strategy is proposed in this paper. First of all, the dynamics of the ship motion model are established to study the longitudinal control system. The six degrees of freedom nonlinear motion model and nonlinear coupled longitudinal motion (heave and pitch) model are described in detail in this paper. Secondly, this work presents matching suitability between the T-foil and the operation and maintenance vessel. Therefore, the most suitable installation position and the optimum strut’s height of T-foil are determined by meshing the ship hull model, setting the water channel, and a series of corresponding computer fluid dynamic simulation. Following that, the linear quadratic regulator controller is studied with active longitudinal control system based on the suitable T-foil. Furthermore, a longitudinal control system is built, including free vessel module and the suitable T-foil stabilizer-based proposed controller module. Finally, the simulation results indicate that the designed T-foil and the longitudinal control system are feasible and effective to ensure the heave and pitch motion reduction based on the proposed controller.

Numerical study on damaged ship rolling and capsizing in irregular beam waves during quasi-steady flooding

To study roll dynamics and capsizing occurrence of damaged ship in irregular beam waves, computational fluid dynamics is used to simulate damaged ship motions at rough sea to high sea states. The velocity-inlet boundary and momentum source are combined for wave generation. A modified formula is adopted to calculate turbulent viscosity to prevent unphysical decay of wave. The lateral drift restrained model is integrated with a combined dynamic mesh strategy to handle the ship's sway, heave and roll motions. Benchmarking study of irregular wave generation and roll response of damaged ship in regular waves is performed. Based on the simulations of motions of intact and damaged ships in irregular waves, the effects of cross flooding and damage orientation on the roll response of ship are discussed. For symmetrical flooding, the roll behaviour of ship is insensitive to the damage orientation. Due to the increase of roll damping, the roll response of damaged ship is smaller than that of intact ship. For asymmetric flooding, the ship is liable to roll toward the damaged side with a large angle. The probability of capsizing of ship caused by waveward damage is high at high sea state.

A preliminary study of a novel wave energy converter of a Scotch Yoke mechanism-based power take-off

This work proposes a novel concept to harvest power from ocean waves using a Scotch Yoke mechanism to transmit the heave motion of the buoy into the rotation of an off-the-shelf rotary generator. The novel transmission mechanism is proposed to tackle some of the main obstacles of ocean waves energy harvesters associated with the reliance on resonance and the high expenses related to the large device dimensions and permanent magnet linear generators. The Scotch Yoke mechanism converts the linear wave motion of a harmonic nature into rotary generator motion, coinciding with the nature of ocean waves. A mathematical model is constructed relating the hydrodynamics of the float to the non-linear resistive forces of the power take-off. Simulations are conducted for two different devices, including a typical large-scale-up wave energy converter device and a relatively small scaled-down wave energy converter device that can be placed close to the shore. The simulation results showcase that the large scale device can have a peak harvesting efficiency of up to 42%, while the small scale device can have a peak harvesting efficiency of up to 34%. This is of great importance as the small device can be used with an off-the-shelf rotary generator, and can be placed close to the shoreline further lowering the cost of manufacturing, installation, and maintenance allowing easier access to the device. Also, to validate the concept, a small scaled-down device has been fabricated, and dry-tested. The tested power take-off is proven to be able to harvest energy from linear harmonic oscillating buoy motions with a peak efficiency of 49% (not including the hydrodynamic efficiency).

Prediction and control strategy based on optimized active disturbance rejection control for AHC system

In this paper, a novel control and prediction algorithm is proposed to solve the problem of decoupling control and response delay in the AHC system of electric winch, which effectively improves the compensation accuracy and response speed of the AHC system. The innovation of the control strategy is mainly as follows: 1) Long short-term memory (LSTM) neural network is used for predicting the ship heave motion. The Bayesian algorithm is used to optimize the hyperparameters to improve the prediction accuracy.2) In the design of the AHC controller, the main speed feedforward is used to directly govern the system's output. The position loop adopts the active disturbance rejection control (ADRC) to improve the anti-disturbance and position-tracking ability of the AHC system.3) For the numerous parameters of ADRC, the chaotic particle Swarm optimization (CPSO) algorithm is used to tune parameters. The simulation results show that the algorithm is effective in improving the robustness and compensation efficiency of the system. Finally, the experiment is carried out on the AHC experimental platform of the full-size winch. The compensation results before and after prediction are compared and analyzed.

Numerical analysis on the seakeeping performances of a full-scale container ship hull using strip theory

The race of taking more cargo on a ship has increased the size of ships as well as other aspects, such as their capacity and structural complexity, which affect the stability of ships. For naval architects and academics, accurately predicting seakeeping performances is difficult. In order to address this, the seakeeping performance of a container ship hall built by Korea Research Institute for Ships and Ocean Engineering (KRISO), i.e., KRISO container ship hall (KCS) is described in this research utilizing a numerical method. Maxsurf software based on strip theory was used to determine the results, where the containership hull was considered and input motions were applied with appropriate boundary conditions. Later, the ship's heading and speed were changed to see the effect of the seakeeping performance of the container ship. The current study is concentrated on systematic comparative research on the investigation of the ship's pitch, heave, and roll movements in irregular waves. It has been found that rolling motion was the highest at 22 kn at the 60° heading angle, potentially affecting ship stability. The significant amplitude analysis indicates that the roll motion is the largest at 60° of wave direction. The pitch motion response and the heave motion are near for each heading angle when the wave frequency exceeds 0.5 and 1 rad/s, respectively. The calculation findings show that seakeeping performance is directly related to ship direction and speed. Furthermore, threatening heading angles during sailing are classified, which could also help in enhancing ship stability.

Suppression of the hydroelastic responses of a composite very large floating structure by a submerged elastic ring

With the aid of the method of matched eigenfunction expansions, the hydroelastic dynamics of a composite very large floating structure on a two-layer fluid is analytically investigated in the framework of linear potential flow theory. The composite structure consists of a semi-immersed rigid cylinder fixed with a floating elastic ring and protected by a submerged flexible ring for mitigating the hydroelastic responses. A new dispersion relation for the hydroelastic waves in the two-layer fluid containing a submerged plate is derived to obtain the wave numbers with a given frequency. The coupling effect among the oscillation of rigid cylinder, the deformation of the floating and submerged plates and the motion of wave is considered by dividing the problem into scattering and radiation parts. An inner product is introduced to deduce the unknown expansion coefficients of the velocity potentials. The amplitudes of heave and pitch motions for the rigid cylinder are obtained and shown graphically. The effects of the physical parameters are explored to confirm the mitigative effect of the submerged ring on the response of the composite very large floating structure. A submerged porous ring can also eliminate the amplitudes of waves in the surrounding water region.

A Virtual System and Method for Autonomous Navigation Performance Testing of Unmanned Surface Vehicles

An overall framework of the virtual testing system has been established based on the analysis of the virtual testing requirements for autonomous navigation performance of unmanned surface vehicles (USVs). This system consists of several modules, including the environment module, motion module, sensor module, and 3D visualization module. Firstly, within the robot operating system (ROS) environment, a three-dimensional navigation environment was generated by combining actual wave spectra with Gerstner waves. By designing a power plugin for USV navigation, the system was made to reflects the coupled motion model of USVs in wind, waves and currents, along with predictive results. Regarding the four typical sensor information on USVs, the actual sensors were virtualized, and a simulation approach for virtual sensor information is provided. The three-dimensional visualization of USV’s motion enables the intuitive display and analysis of the virtual testing process. Based on the prediction of coupled motion characteristics in wind, waves and currents, the interaction between USVs and the virtual testing system has been realized. A platform for virtual testing experiments to determine the autonomous navigation performance of USVs was established, and the effectiveness of the platform was verified in terms of perception and environmental interference. In virtual environmental interference validation, the average amplitude deviation of the heave motion of USVs under sea state 3 reaches 0.74 m, and the average amplitude deviation of the pitch motion reaches 0.25 rad, showing the gradually increasing disturbance of the sea state. Finally, virtual testing experiments were conducted on a specific USV to evaluate its autonomous navigation perception performance, trajectory tracking performance, and autonomous obstacle avoidance. The evaluation results indicate that the platform can achieve the functionality of virtual testing for the autonomous navigation performance of USVs from the perspective of cost function, taking the reaction distance, regression distance, and obstacle avoidance time into consideration. A representative example is that the cost function deviation rates of overtaking obstacle avoidance between static and dynamic seas reach 5.11%, 8.98% and 18.43%, respectively. The gradually increasing data shows that the virtual simulating method matches the drifting-off-course tendency of boats in rough seas. This includes acquiring perception information of navigation and simulating the motion and navigation processes for visualization. The platform provides new means for testing and evaluating the autonomous navigation performance of USVs.

Comparison of prediction methods for added power and propeller revolution for a 1,800 TEU container ship using resistance tests in regular and irregular waves

This paper predicts the added power and propeller revolution due to irregular head waves for a 1,800 TEU container ship. Resistance tests were conducted with 11 regular waves and irregular waves at SS = 5. Time series (TS) and autocovariance analysis methods were applied to estimate statistical variables and uncertainties. The paper investigated the statistical convergence times of measured data. The wave elevation, heave and pitch motions obtained from the TS agree well with those from spectral analysis in regular and irregular waves. The added resistance from the TS is larger than that from spectral analysis using a regular wave test by 6.1%. The characteristics of response operators and spectra of the added (or reduced) propulsion performances and overload factors are discussed. The power and propeller revolution were evaluated with a variety of methods: SA(LVM-W), LVM-IR, DPM, and RTIM.

Experimental investigation on the nonlinear dynamic responses of the coupled floating submerged artificial seabed-mooring system in internal solitary waves

A new deepwater artificial seabed (DAS) production system is developed as a competitive offshore field development solution in deep and ultra-deep waters. This paper experimentally investigates the acting mechanism of ISWs on the nonlinear dynamic responses of the coupled floating submerged artificial seabed-mooring (ASM) system in a large-scale density-stratified flume. A compositive method for measuring and analyzing the interactions between the ASM system and the ISW is proposed. On this basis, the spatio-temporal features of the dynamic motion responses, the mooring tension variations and the flow field surrounding the ASM coupling system are characterized. The minute motion responses of the test model are captured by a charge-coupled device camera and subsequently processed using a developed image processing method. The temporal tension variations in the mooring lines are measured utilizing a non-contact optical measurement technique. The fluid-structure interaction characteristics between the flow field and the test model are revealed through the particle image velocimetry technique. In addition, the effects of crucial environmental factors on the nonlinear coupled dynamic responses of the ASM system are systematically explored in the experiments. The results indicate that the nonlinear dynamic responses of the ASM system in ISWs are characterized by the significant surge motion (maximum: 14.7% of the water depth), the coupled heave motion with the surge response, and the considerable mooring tension variation (maximum: 31.8% of the pre-tension), especially with the test model located within the density pycnocline.

Parametric study of the effects of clump weights on the performance of a novel wind-wave hybrid system

This paper presents a novel catenary mooring system incorporating multiple clump weights (CWs) designed for a SPIC-Wavestar wind-wave energy hybrid system. To enhance the performance of the hybrid system, a parametric analysis was performed on mooring CWs. The motions of the SPIC platform, along with the power absorbed by the Wavestar wave energy converters (WECs), specifically in response to the operational regular wave, were simulated. The hydrodynamic analysis program AQWA was used for the analysis. The results indicated that the surge and pitch motions of the platform comprised low-frequency and wave-frequency components, whereas the heave motion only comprised a wave-frequency component. However, the low-frequency responses of the surge and pitch motions were substantially smaller than their wave-frequency counterparts. In addition, the properties of the CW exerted notable influence on both platform motion and the overall power absorption by the WEC array. The variation trends with respect to the CW weight, location, and number were complex. Overall, the results of this study provide valuable insights into the mooring design of ocean energy hybrid systems.

Fast time-domain model for the preliminary design of a wave power farm

This study presents a novel, fast time-domain model developed for an array of interacting point-absorber wave energy converters. The model is validated using experimental wave tank data. The point-absorbers, based on Uppsala University’s design, are arranged in a symmetric grid and interact with scattered and radiated waves while constrained to the heave motion. The model employs linear potential flow theory to solve the hydrodynamic coefficients in the frequency domain and employs Cummins’ formulation to solve the equations of motion in the time domain. Modeling an array of wave energy converters in the time domain yields a system of integro-differential equations, featuring convolution terms in the excitation and radiation forces. This implies that past waves radiated by the body continue to impact future dynamics. Irregular long-crested waves, generated from the Bretschneider spectrum, serve as the incident waves for the study. The model’s accuracy in capturing the dynamics and power absorption of the farm is demonstrated through validation against experimental data from a 1:10 scaled prototype of a six-point-absorber array. Despite inherent differences between the experimental and numerical set-ups, the model accurately represents the farm’s behavior. Furthermore, an efficiency test reveals that the numerical scheme approximates the performance of wave power farms comprising 6, 12, 24, 48, and 96 interacting devices within a maximum computational time of 20 s. Overall, this research presents a novel and accurate time-domain model for analyzing an array of point-absorber wave energy converters. The model’s ability to capture the dynamics and power absorption, along with its efficiency in simulating larger wave power farms, make it a valuable tool for the preliminary design stage.

Numerical validations and investigation of a semi-submersible floating offshore wind turbine platform interacting with ocean waves using an SPH framework

In this work, we propose numerical validations of the DeepCwind semi-submersible floating platform configuration for a single horizontal axis wind turbine using data from two experimental testing investigations. A Smoothed Particle Hydrodynamics solver is employed to estimate fluid induced loads, whereas the mooring connections are handled via an external library. The first validation setup is based on the DeepCwind offshore wind semi-submersible concept moored with a system of taut-lines and tested for free-decay surge and heave motion (OC6-Phase Ia). The damping evaluation yields a fair estimation of the heave damping behavior, whereas much more dissipation is experienced for the surge. The second validation features a full hydrodynamic characterization of the frequency-related load patterns induced by three different sea-state representations (mono-, bi-chromatic, and irregular waves) (OC6-Phase Ib). The model accurately matches the hydrodynamic load estimation for the whole spectrum of investigated wave components, perfectly capturing the non-linear behavior shown by the considered wave patterns. This work concludes with a systematic study on the motion response, mooring tension, pressure and vorticity, suggesting that: the wave steepness criterion alone cannot identify the most restrictive load case; waves with spectral characteristics close to the heave resonance period lead to higher tensions in the mooring systems, whereas the maximum fluid-induced loads on the hull are decoupled from displacement peaks, showing an average reduction of 30% with respect to the maxima; very steep waves maximize the likelihood of wave overtopping and slamming loads, resulting in locally induced overpressure on the free-board of up to 100% higher than expected for similar wave heights with milder profiles. The input data for these last tests is released for the sake of reproduction.