Motion Response Research Articles

Initialization Algorithms of a Rigid–Flexible Coupling Fluid–Structure Interaction System by Considering Length-and-Area Constraints

Rigid–flexible coupling fluid–structure interaction systems are expected to be future solutions for reducing energy lost in water. The dynamics of these systems is usually investigated via numerical simulations. However, in existing numerical works there is no accurate algorithm for the initialization of the flexible filament, which ensures both the length and area constraints, leading to inaccurate results or even severe numerical instabilities. We propose two alternative initialization algorithms, respectively, the “Trapezoidal arrangement” and the “Quartic curve arrangement”. The performances of both of these two algorithms are investigated in numerical simulations by using the immersed boundary method. The motion responses and force characteristics of the flexible filament are analyzed carefully, verifying the capability of the proposed algorithms. Specifically, “Quartic curve arrangement” is further recommended due to its good property of convergence.

Numerical analysis of the effect of tunnel hydrofoil - stern flap on the motion stability of a double M-craft in regular waves

Abstract The double M-craft is a new type of high-performance multi-hull vessel that combines the gliding characteristics of a planing boat with the resistance-reducing characteristics of a hovercraft. But it also suffers from motion instability in regular waves. At present, there is scarce study on the effect of appendage on the motion stability of double M-craft in regular waves. By using the software STAR-CCM+ to numerically simulate the pitching and heaving motions of a double M-craft installed with tunnel hydrofoils and stern flaps in regular waves. Based on the overset mesh technology, the volume of fluid (VOF) method is used to capture the water-gas two-phase flow field. The 2-degree of freedom (2-DOF) motion of the rigid body is simulated by dynamic fluid body Interaction (DFBI). After investigating the effects of these combined appendages on the hydrodynamic performance, running attitude, and motion response of a double M-craft in regular waves, the optimal mounting parameters of the combined appendage have been obtained. The results have shown that all nine types of combined appendages can reduce the heave and pitch. The optimal combined appendage installation parameters include a hydrofoil longitudinal mounting position of 1/8L, angle of attack of 3°, stern flap length of 1.5%L, and flap angle of 5°, which can effectively reduce the pitching response by about 8% and reduce the swaying response by about 2.6%, and enhance its longitudinal motion stability.

Effects of mooring failure on the dynamic behavior of the power capture platforms

In recent years, for enhancing the ocean energy capture, it has been prevail to combine multiple wave energy converters (WECs) with the floating platforms. A power capture platform concepts is proposed in this paper based on the point-absorber WEC array and the SPIC semi-submersible platform. The present study conducts the time-domain dynamic analysis on the performance of the power capture platforms with mooring failure. After the validation of the numerical models, ANSYS-AQWA is employed to investigate the platform motion response, the remaining mooring lines' tension response, and the WEC array's power output. The results show that the platform motion and the remaining mooring lines' tension appear significant transient overshoots after mooring failure. The mean motion response of the platform increases because of the reduction of mooring stiffness. Meanwhile, the energy distribution of the platform slow-drift and roll motions at the low-frequency region increases as well. Moreover, the mooring line tension adjacent to the failed mooring line increases significantly, while that of other mooring lines decreases. Notably, mooring failure has slight effects on the WEC array's energy conversion performance, and the total absorbed power among Model 2 is more than that among Model 1. These important findings provide some insights into the design of power capture platforms. Moreover, to ensure the floating system stable and reliable, the effects of mooring failure and the resulting changes should be evaluated in advance.

An analysis of floating wind turbine towing operations using OpenFAST

Abstract The floating wind industry is slowly entering the commercialisation phase and array-scale floating wind farms are imminent. Irrespective of the type of floater used, towing operations represent a crucial marine activity required during installation, major repairs and decommissioning phases. Numerical analysis of towing operations is vital for predicting hydrodynamic motions and towing loads, enabling the identification of metocean limits crucial for achieving optimised safe and efficient execution. This paper presents a unique approach to utilise OpenFAST for the examination of towing operations. Compared to other numerical analysis tools available in the market, OpenFAST stands out as a free, open-source tool specifically designed for wind turbine analysis. The study introduces and evaluates an analytical method using OpenFAST, demonstrating its efficacy in modelling towing dynamics, predicting forces and assessing motion responses of a semi-submersible floating wind turbine in varying wave conditions. The numerical simulations are compared against the results from an experimental campaign. The capability of the method in simulating towing dynamics and the prediction of motion responses are discussed.

An experimental study of a rectangular floating breakwater with flexible curtains as wave-dissipating components

In this study, the flexible curtains are attached to the bottom of a rectangular floating breakwater as wave-dissipating components. Through a comprehensive experimental investigation, the wave attenuation, motion responses, and mooring forces of the proposed floating breakwater are examined, with a particular focus on the effects of wave height and the hanging length and porosity of the flexible curtain. Meanwhile, comparative analyses are conducted with the stand-alone rectangular floating breakwater and with attaching one rigid slotted barrier. Our experimental results indicate that one underhanging flexible curtain can augment wave attenuation across all tested wave conditions, which is comparable to the rigid slotted barrier. Furthermore, attaching two flexible curtains contributes to a more significant enhancement for harbor or coastal protection, especially against long waves. This can be attributed to the buffer function of flexible curtains, and the increased added mass induced by the water body confined between them, which increases the natural period of floating breakwaters. Furthermore, the attachment of the flexible curtains significantly suppresses the motion responses of the breakwater, which can alleviate the undesirable strong mooring forces. In general, increasing the length or decreasing the porosity of flexible curtains leads to similar trends in the performance of floating breakwaters. The flexible curtains have been proven to be effective wave-dissipating components for rectangular floating breakwaters.

An improved frequency domain model for floating offshore wind turbines

Abstract A cost- and time-efficient design evaluation tool can assist the FOWT industry in evaluating many different concepts. An open source frequency domain model for FOWT applications shows promising results for the integral, coupled response, but clear differences to higher fidelity time-domain studies are observed. In this work, the hydrodynamic modelling is improved by using a potential flow diffraction code, and the implementation of the floating feedback mechanism has been improved. Attempts have been made to improve the mooring system modelling by implementing a lumped mass approach, and to include the excitation of a turbulent wind spectrum. A good match in the system’s motion responses is obtained in the wave frequent range when compared to time-domain results. At lower frequencies, the excitation from the turbulent wind spectrum remains to be improved on. At higher frequencies, the mooring line modelling is to be improved for reliable results. For both effects, a clear outline is set up to improve in future work. The current model is capable of accurately modelling fully coupled wave-frequent motions in combined wind-wave environments in an efficient manner. When the suggested improvements are incorporated in future work, it is expected that a good match in the low- and high-frequency ranges can be obtained as well, allowing to study the full motion- and mooring tension response. Finally the tool will give developers and designers the opportunity to evaluate a large amount of designs in a cost and time efficient manner.

A dynamic simulation-based methodology for systematic assessment of workability on floating wind turbines

Abstract Floating offshore wind technology experiences significant motion responses when exposed to environmental wave and wind loads, possibly interfering with technicians conducting maintenance work. Industrial interest is rising in the assessment of workability, as impairments will decrease the availability of the asset and possibly affect the business case for the wind farm project. Quantification of impairments are formed from three workability indicators: Nordforsk Seakeeping Criteria, ISO 2631-1, and ISO 6897. The present work shows a likely workability decrease, quantified to 2.4% for the Nordforsk Seakeeping Criteria, for the UMaine VolturnUS-S reference platform and the IEA 15MW reference wind turbine. Peak wave period and wave heading direction are found to affect the results and indicate the importance of conducting the study in site-representative conditions. In addition, varying results for different indicators and methodological approaches indicate the need for common rules and standards in the floating wind industry to enable transparency during project development.

Coupling analysis between wave resonance in the moonpool and heave motion of the twin hulls with mooring effect

Fluid resonance in the moonpool formed by two identical rectangular hulls in water waves is investigated by employing the Computational Fluid Dynamics (CFD) package OpenFOAMⓇ. The influence of vertical stiffness on the behavior of moonpool resonance coupling with the heave motion response is presented. Numerical simulations show that the free surface oscillation in the moonpool exhibits a two-peak variation with the incident wave frequency, defined as the first and second peak frequencies. A local Keulegan–Carpenter (KC) number is introduced for describing the influence of fluid viscosity and flow rotation on the fluid resonance and heave motion resonance. At the first peak frequency, the free surface oscillation and heave motion response show an in-phase relationship, where increase in the vertical stiffness can increase the relative motion between them. This finally leads to an increase in the KC number, indicating the increased effect of energy dissipation with increase in the vertical stiffness. At the second peak frequency with an out-of-phase relationship between the free surface oscillation and heave motion response, the variation of the KC number is not sensitive to the vertical stiffness. Correspondingly, the influence of energy dissipation is not strongly dependent on the vertical stiffness.

Comparative analysis on the hydrodynamic characteristics of offshore floating photovoltaic systems based on membrane structures under different mooring configurations

In the context of rapidly increasing global energy demand, solar energy is considered one of the most promising alternatives due to its ubiquity and sustainability. Floating photovoltaics, which do not occupy land and offer higher power generation efficiency, are becoming increasingly popular. As a novel type of floating photovoltaic, membrane structures are drawing more attention due to their lightweight nature, easy installation, and cost-effectiveness. Based on the Ocean Sun's floating photovoltaic membrane prototype as a reference, this study designed and fabricated a 1:40 scale model for laboratory experiments. The research investigated the influence of different mooring configurations and lengths on the hydrodynamic characteristics of membrane structures. The conclusions indicate that structures with more anchor chains exhibit smaller amplitudes in various movements and mooring forces. As the mooring length decreases, the amplitudes of various structural movements decrease, but mooring forces increase. Compared to regular wave conditions, mooring has a more significant impact on structural motion responses under irregular waves. In practical engineering, a comprehensive consideration of structural safety and cost-effectiveness is necessary for design. The selection of appropriate mooring configurations and lengths plays a crucial role in ensuring the safe operation and sustainability of membrane structures.

Design and modeling of an eco-friendly anchored fish aggregating device with artificial reef subjected to wave and current

Anchored fish aggregating devices (AFADs) and artificial reefs play a vital role in enhancing marine biomass and biodiversity for marine ecosystem restoration. Improving the coastal ecosystem remains a significant challenge worldwide. However, AFADs are highly vulnerable to damage under marine conditions, which can lead to significant marine pollution. An AFAD, in which an artificial reef is applied as the anchor, is designed to enhance the efficiency of marine ecosystem restoration. This system consists of a biodegradable raft, cotton rope, and concrete artificial reef. Numerical modeling is conducted to simulate the hydrodynamic performance of the AFAD based on an unsteady Reynolds-averaged Navier–Stokes approach with a realizable k–ε turbulence model. Wave forcing based on the Euler overlay method is applied for wave dissipation, followed by theoretical verification. The effects of wave parameters, relative raft length, rope length, raft diameter, and sinker weight on the motion response amplitude operators and relative rope tension are considered. The results show that the wave parameter has a significant effect on the hydrodynamic performance of the AFAD. With a relative raft diameter of 0.6, relative rope length of 2.6–2.8, and relative sinker weight of 0.8, the motion response amplitude operators and relative rope tension are minimized. The proposed AFAD can provide a stable three-dimensional zone to efficiently enhance the aggregation of marine species.

On Hysteresis In A Variable Pitch Fan Transitioning To Reverse Thrust Mode And Back

Abstract A novel hysteresis phenomenon during the transition to and back from the reverse thrust mode in a Variable Pitch Fan (VPF) is identified and characterised in this work. This is done by using a three-dimensional (3D) fully transient Unsteady Reynolds averaged Navier?Stokes (URANS) with the transitioning fan blade aerofoils simulated by an adaptation of the mesh displacement method. The VPF is modelled to be transitioning in a modern 40000 lbf geared high bypass ratio turbofan engine architecture at "Approach Idle" engine power setting in a typical twin-engine airframe with the flaps, slats, and spoilers set for an aircraft touchdown airspeed of 140 knots. The transition to reverse thrust mode involves flow starvation into the engine, formation of recirculation zones in the bypass duct and the establishment of the reverse stream, all of which occurs in the opposing presence of the free stream flow at aircraft touchdown velocity. The transition back to forward flow mode involves the gradual re-establishment of the free stream which is opposed by the presence of the reverse stream within the engine. It is quantified that in the transition to reverse thrust, the blockage develops with a larger time delay than the disappearance of the blockage during the transition back due to the interplay of the temporal dynamics of fan blade motion and flow field response. The hysteresis phenomena described in this work are critical in properly developing control schedules to adapt for potential bi-stable flow field development during the landing run.

Modelling Sea-Surface Wave Motion and Ship Response Using Smoothed Particle Hydrodynamics and Finite Element Analysis

The response of a ship or other vessel to surface sea waves, including extreme waves, may compromise crew and vessel safety and long-term operational capability. Herein, a novel high-fidelity numerical time-dependent simulation approach is presented using Smoothed Particle Hydrodynamics (SPH) for modelling sea waves coupled with Finite Element Analysis for modelling vessel structural response under wave loading conditions. The results are compared with physical scale model wave tank test results. Good agreement was obtained for heave and pitch motions and vertical bending moments for various forward (head) speeds in regular head waves, heave and pitch motions, and vertical bending moments. High computational demands can be met by the increasing availability of computation power. Ongoing research is outlined. The implications for the design of vessels such as ships and for through-life assessment are discussed.

Dynamic Response of a Rotor Supported on Hybrid Bearings in Air, Water, and Liquid Nitrogen: Measurements and Comparisons to Predictions

Abstract Modern liquid rocket engine turbopumps implement hybrid bearings, which combine hydrostatic and hydrodynamic fluid film principles, in compact and lightweight units known for their superior reusability, durability, and reliability. This study aims to provide extensive and reliable test data for the purpose of anchoring and benchmarking predictive tools for these bearings. This paper provides comprehensive dynamic response measurements of a rotor weighing 10.40 kg and approximately 60 mm in diameter at the bearing locations. The test rotor is supported on two hybrid journal bearings with a bearing length to bearing diameter ratio of approximately 0.42 and a bearing radial clearance to rotor radius ratio of around 0.0017. The bearings are tested with air, water, and liquid nitrogen, serving as surrogate test fluids for common propellants in liquid rocket engines. Rotor run-up and speed coast-down tests are conducted to measure shaft motion responses. The results clearly demonstrate that the rotordynamic characteristics of the test rotor supported on the hybrid journal bearings largely rely on properties of the test fluids and their feeding conditions. Notably, when air is pressurized into the test bearings, the shaft motion responses differ markedly from those observed with water and liquid nitrogen, primarily due to significantly lower stiffness and damping coefficients. Furthermore, rotor speed coast-down tests for each test fluid exhibit notable differences in coast-down speed over time characteristics, depending on the test fluid properties. The predicted rotor imbalance responses exhibit excellent correlation with the measurement data. The steady increase of demand and growth of the fluid film bearing technology for reusable rocket engines require accurate bearing design tools, the extensive component testing, and the implementation of the technology. The validated and developed bearing predictive models from previous research demonstrate well the characteristics of bearings under various operating fluids conditions. The findings and database presented in this work contribute not only to comprehending the performance of hybrid bearings but also to understanding and enhancing the dynamic characteristics of rotor systems supported on hybrid bearings.

Numerical analysis of coupling effect between sloshing and motion responses of a KCS in uni- and bi-directional waves

The seakeeping behaviour can be severely affected by the coupling between sloshing and ship motions in waves. The sloshing behaviour of liquid-loaded ships in bi-directional waves differs significantly from that in uni-directional waves. Given that ships operate in short-crested or multi-directional waves during their whole service period, it is significant to gain a deeper understanding of the effect of wave directionality on the coupling between sloshing and ship motions. In this study, a KRISO container ship (KCS) model with type-B liquefied natural gas (LNG) cargo tanks is adopted for coupling analysis. The 2D long-crested regular wave is adopted as an uni-directional wave, and three dimensional (3-D) cross wave is adopted as a bi-directional wave. The computational fluid dynamics (CFD) method is applied for the sloshing and seakeeping simulation of the ship model in different wave fields. The differences in wave elevation, sloshing forces, and coupling effects between ship motions and sloshing are studied in these two wave fields by adjusting control parameters such as ship heading angle and filling ratio. The comparative results indicate that sloshing behaviour and coupled motion responses in cross waves should be paid enough attention during ship design and evaluation.

Fully Coupled Analysis of a 10 MW Floating Wind Turbine Integrated with Multiple Wave Energy Converters for Joint Wind and Wave Utilization

The study focuses on a semi-submersible wind-wave integrated power-generation platform, which consists of an OO-Star semi-submersible platform equipped with a DTU 10 MW wind turbine and a set of wave energy converters. A hydrodynamic model was established using ANSYS-AQWA (2023 R1), and by incorporating upper wind loads and utilizing the open-source program F2A, a fully coupled time-domain model of the integrated power-generation platform was constructed. The primary objective is to explore the interaction mechanisms between the upper wind turbine and the lower wave energy devices under the combined effects of irregular waves and turbulent wind through a series of operational conditions. Additionally, the safety of the mooring system was assessed. The results indicate that, compared to the wave period, the power generation of the lower wave energy devices is more significantly affected by wave height. Overall, the integrated power-generation platform demonstrates optimal performance under the third operational condition. In survival conditions, the introduction of oscillating buoys can improve the motion responses of the platform in terms of sway, roll, pitch, and yaw to a certain extent, but it also increases the surge and heave motion responses and the associated mooring loads. The mooring system can ensure the safety of the integrated power-generation platform under extreme sea conditions.

Multi‐Objective Optimization and Experimental Research of Ship Form Based on Improved Bare‐Bones Multi‐Objective Particle Swarm Optimization Algorithm

ABSTRACTShip form optimization poses a complex and high‐dimensional engineering challenge. Therefore, when conducting multi‐objective optimization research of ship forms, traditional intelligent optimization algorithms are prone to falling into local optima solution and difficult to converge. In order to effectively improve the diversity and convergence performance of the algorithm, this paper improves the bare‐bones multi‐objective particle swarm optimization (BBMOPSO) algorithm by dynamically adjusting the local and global search step sizes, and verifies the algorithm's reliability through standard function testing. Then, a multi‐objective optimization design framework with high efficiency and high integration is constructed. Taking DTMB 5512 as the research case, Free Form Deformation (FFD) method is used for hull deformation, and the proposed algorithm is used for multi‐objective optimization of resistance performance and motion response. Ship model tests were conducted on the DTMB 5512's original hull. And the numerical simulations were compared with the ship model tests. Finally, under the constructed multi‐objective optimization design framework, satisfactory solutions were obtained through the improved algorithm, which confirms the effectiveness and practicality of the improved algorithm. The results show that the algorithm improved in this paper can provide some theoretical basis and technical support for green ship design and low‐carbon shipping.

Study on nonlinear coupled dynamic response of the integrated polar ocean nuclear energy platform

An integrated polar ocean nuclear energy platform is capable of providing adequate electrical and thermal energy essential for polar oceanic development activities. Precisely forecasting the platform's motion response within the marine environment is pivotal for the assurance of its safety. A scenario where the natural frequencies of heave, roll, and pitch of such an integrated polar ocean nuclear energy platform align closely to a 2:1:1 ratio, and when the wave frequency nearly matches the aggregate of the platform's natural heave and pitch frequencies, could induce a phenomenon known as sum-type nonlinear internal resonance. This phenomenon can significantly amplify the swing (roll or pitch) motion of the platform, posing a grave risk to both personnel and equipment onboard. Investigating these nonlinear internal resonance phenomena within the nuclear energy platform is critical for informing both the design and practical deployment of these platforms. Following the validation of the numerical simulation against experimental results, a detailed analysis of sum-type nonlinear internal resonance phenomena in the integrated polar ocean nuclear energy platform under regular wave conditions was performed using a combination of numerical simulations and nonlinear dynamics theory. It was observed that upon exceeding a certain wave height threshold, the platform enters a state of sum-type nonlinear internal resonance, engendering highly complex motion patterns. Notably, irrespective of initial conditions, this resonance triggers the roll motion of the platform, facilitating the external dissipation of energy in a half-sub-harmonic vibration manner. Increasing damping can effectively suppress the occurrence of nonlinear internal resonance phenomena in the platform. In designing the integrated polar ocean nuclear energy platform, it's crucial to avoid having the natural frequency ratios of heave, pitch, and roll align as 2:1:1. If unavoidable, adding side plates to increase damping can mitigate the effects of sum-type nonlinear internal resonance.

Study on the Prediction of Motion Response of Offshore Platforms Based on ResCNN-LSTM

In the random sea environment, offshore platforms are influenced by factors such as wind, waves, and currents, as well as their interactions, leading to complex motion phenomena that affect the safety of offshore platform operations. Consequently, accurately predicting the motion response of offshore platforms has long been a key focus in the fields of naval architecture and ocean engineering. This paper utilizes STAR-CCM+ to simulate time-history data of offshore platform motion responses under both regular and irregular waves. Furthermore, a predictive model combining residual convolutional neural networks and long short-term memory neural networks using neural network technology is also studied. This model utilizes an autoregressive approach to predict the motion responses of offshore platforms, with its predictive accuracy validated through comprehensive evaluations. Under regular wave conditions, the coefficient of determination (R2) for the platform’s heave and pitch responses consistently exceeds 0.99. Meanwhile, under irregular wave conditions, the R2 values remain generally above 0.4. Additionally, the model exhibits commendable performance in terms of Mean Squared Error (MSE) and Mean Absolute Percentage Error (MAPE) metrics. The aim of this study is to present a novel approach to predicting offshore platform motion responses, while providing a more scientific basis for decision-making in offshore platform operations.

Nonlinear harmonic resonant behaviors and bifurcation in a Two Degree-of-Freedom Duffing oscillator coupled system of Tension Leg Platform type Floating Offshore Wind Turbine

The surge-heave coupled motion model of a Tension Leg Platform type Floating Offshore Wind Turbine (TLP FOWT) is first established. By taking the tendons stretching and platform set-down motion into consideration, the nonlinear model is a Duffing oscillator coupled system. We analyze the nonlinear feature in the surge-heave coupled motions by the semi-analytical algorithms in nonlinear dynamics. The harmonic balance method is applied to obtain the analytical solutions of the nonlinear system under wind and wave excitations. The analytical solution verifies the excellent accuracy. To further achieve the heave motion response, we organize the bifurcation analysis and discuss the effects of the dynamic parameters on the heave responses. The results reveal the multi components of heave motion, which consist of constant, primary harmonic resonate and higher order harmonic resonate. In addition, the change of dynamic parameters has various effects on the response. The increment of wave load leads to the expansion of heave response, and an instantaneous expansion appear. Both linear surge and heave damping only affect the amplitudes of heave motion. The nonlinear surge stiffness and heave stiffness can barely affect the primary harmonic resonance amplitudes, but they present different effects on the higher order harmonic resonance component.

Investigation of motion response suppression characteristics in floating platforms equipped with novel spiral side plates

AbstractTo enhance the stability of floating wind turbine platforms, this study combines the structural characteristics of helical side plates and proposes the installation of serrated helical side plates on Spar wind turbine platforms. Based on potential flow theory, the present study employs blade element momentum theory and radiation‐diffraction theory. Upon establishing a dynamic data link library, wind‐wave coupling was implemented, and the dynamic response characteristics of platforms with different helical side plates were compared. The results indicate that in the frequency domain analysis, the platform with serrated helical side plates demonstrates reduced sensitivity to waves, with a particularly notable increase in added mass in the heave direction, suggesting excellent hydrodynamic performance. In the time domain analysis, improvements in the platform's surge and heave stability performances were observed, measuring 10.99% and 10.64%, respectively, in extreme conditions. Owing to the unique features of the serrated structure, the tension in the mooring chains on the wave‐facing side is reduced, thereby effectively lowering the fatigue load on the mooring chains.