Wave Drift Forces Research Articles

Reduction of wave drift force on a truncated cylinder in a wide frequency band using the defect effects on cloaking phenomenon

The reduction of wave drift forces acting on a fixed inner truncated cylinder surrounded by several additional truncated cylinders utilizing the cloaking phenomenon is studied. Two kinds of defects of the outer cylinders including the draft and location defects are studied to investigate their effects on the scattered wave energy of the entire cylinders and the wave drift force of the inner cylinder, respectively. The wave interaction theory combined with higher-order boundary element method is adopted to solve this wave-structure interaction problem. The defect effects are firstly studied at the cloaking wavenumber, in which a symmetry phenomenon of the defect effects on the scattered wave energy is found. Then, the investigation on the defect effects is carried out for a wide range of wavenumbers. A wavenumber offset phenomenon of the minimum wave drift force on the inner cylinder caused by the location defects is observed. Finally, the location defect of the entire outer cylinders is adopted to reduce the wave drift force on the inner cylinder at various wavenumbers.

Near-Trapping on a Four-Column Structure and the Reduction of Wave Drift Forces Using Optimized Method

The near-trapping phenomenon, which can lead to high wave elevations and large wave drift forces, is investigated by a floating four-column structure. To solve this wave-structure interaction problem, a numerical model is established by combining the wave interaction theory with a higher-order boundary element method. Based on this numerical model, behaviors of scattered waves at near-trapping conditions are studied; and the superposition principle of free-surface waves is introduced to understand this near-trapping phenomenon. To avoid the near-trapping phenomenon and protect the structure, a way for rotating the structure to change the wave-approach angle is adopted, and improvements of the wave elevations around the structure and the wave drift forces acting on each column are found. Moreover, a genetic-algorithm-based optimization method is adopted in order to minimize the total wave drift force acting on the whole structure at various wavenumbers by controlling the draft of floating bodies, the wave-approach angle and the separation distance between adjacent floating bodies. With the final optimized parameters, the wave drift forces both on each column and on the whole structure can be significantly reduced. The optimized arrangement obtained from a certain wavenumber can work not only at this target wavenumber but also at a range of wavenumbers.

Numerical study on wave-induced motions and steady wave drift forces for ships in oblique waves

This paper considers numerical predictions of wave-induced motions and steady wave drift forces for ships in oblique waves. The velocity potential around the ship is computed via a time domain Rankine panel method. Based on the computed velocity potential, the six degree of freedom wave-induced motions were computed and then the second order wave loads were evaluated using a near field method. An artificial soft spring system was employed to control horizontal ship motions. The steady longitudinal and transverse wave drift forces and the steady yaw moment were computed for the KVLCC2 tanker at zero forward speed and for the S-175 containership at constant forward speed. Predicted results generally compared favorably to published experimental measurements. A systematic analysis of the effects of the soft spring system on wave-induced motions and steady wave drift forces demonstrated that, although the increased stiffness of the system changed the corresponding motion response of the ships, a spring's influence was negligibly small if its stiffness was weak enough.

Study on steady flow approximation in turning simulation of ship in waves

This study considers the effects of steady flow approximation in the analysis of ship maneuvering in waves. To this end, the ship motion due to waves is analyzed using the three-dimensional time-domain Rankine panel method, and the wave drift forces acting on a ship are calculated by applying the near-field method to directly integrate the pressure on the hull surface. In this seakeeping analysis, the steady flow induced by the operation velocity of the ship is approximated by a uniform flow or double-body flow. The ship maneuvering in waves not only advances forward but also drifts and rotates; hence, it can be regarded as a lifting body. Therefore, the vortex flow generated at the sharp end of the hull is also modeled and reflected in the steady-flow approximation. In order to investigate how the seakeeping performance is determined according to the approximation method, ship motions and drift forces induced by waves are computed for the ship moving forward or obliquely. Lastly, the numerical simulation for the turning test in a regular wave is performed based on the time-domain direct coupling method. Based on the accuracies of computation results compared with existing experimental data, the steady-flow induced coupling effects between seakeeping and maneuvering quantities are discussed.

Numerical Evaluation of Ship Turning Performance in Regular and Irregular Waves

AbstractIn this study, ship's maneuvering performance in waves is evaluated using numerical computation. To this end, three degrees-of-freedom (3DOF) planar motions are considered, and modular-type maneuvering model (maneuvering modeling group model (MMG model)) is applied. As external force of the equation of motion, hull force, propulsion force, rudder force, and wave drift force are adopted. In order to calculate wave drift force, seakeeping program which is based on a higher-order Rankine panel method is used by considering wave frequency, wave heading, ship's forward speed, and ship's lateral speed. This wave drift force is pre-calculated, made into database, and used in time domain simulation. The developed simulation program is validated by comparing the computation results of a turning test in regular waves with experimental data. Using this program, turning performance in irregular waves is evaluated and sensitivities for time signal of wave elevation are investigated. Through this study, it is confirmed that the simplified method based on the MMG model including wave drift force can provide good agreement with experimental data in a practical point of view. It can be observed that the simulation results considering the lateral speed show better agreement with the experimental data than those without consideration. In the case of irregular wave condition, the turning performance can be affected by wave random phases. When the ship encounters the beam sea during the turning operation, the wave elevations at that time play an essential role in the change of ship speed and turning trajectory.

Study on Hydrodynamic Performance Model of Offshore Wind Turbine System Based on Time-domain Coupling Analysis

Wu, H.-T. and Gu, X.-P., 2019. Study on hydrodynamic performance model of offshore wind turbine system based on time-domain coupling analysis. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 337–341. Coconut Creek (Florida), ISSN 0749-0208.The development of wind energy alleviates the issue of energy shortage at present, but the development technology of offshore wind energy is not mature, so the research on the performance of offshore wind turbine system is the key to develop clean wind energy. Based on the theory of time-domain coupling analysis, this paper studies the hydrodynamic performance of offshore wind turbine under environmental load, and probes into the hydrodynamic performance of offshore wind turbine system. It is found that the offshore wind turbine will stop operation when the wind speed is greater than 28m/s, and the calculation results show that the offshore wind turbine in this study meets the requirements of the specification for the stability of large inclination angle. The environmental load determines the initial offset of the offshore wind turbine system, in which the mean wave drift force is much smaller than the offshore wind force. With the increase of wind speed, the wind turbine system can automatically adjust the pitch of the blades. When the wind speed exceeds the limit wind speed, the wind turbine stops working. When operating at rated power, wind turbine occurs under the action of wind force will have an offset, which is large. The offset of wind force and wind turbine decreases with the increase of wind speed. This study provides a theoretical basis for the application of offshore wind energy development system.

Numerical and experimental studies on cloaked arrays of truncated cylinders under different wave directions

The cloaking phenomenon in arrays of truncated surface-piercing cylinders under different wave directions is investigated by numerical and experimental methods. Two arrangements, consisting of an inner cylinder surrounded by either four or eight outer cylinders, are considered to confirm this cloaking phenomenon and to find the effect of the wave direction. The cloaking configurations of the cylinders for a certain wave frequency and a certain wave direction are determined via a real-coded genetic algorithm to minimize the scattered wave energy of the cylinder array. Experiments in a wave basin are designed to measure the free-surface elevation at multiple locations in the vicinity of the inner cylinder and the wave drift force on the inner cylinder at various wavenumbers under different wave directions. Both the wave drift force and the wave elevation show good agreement between the numerical and experimental results. It is found that the reduction in the wave drift force acting on the inner cylinder in a cloaking configuration is due to the improvement of the wave elevation in the vicinity of the inner cylinder. Furthermore, the effect of the wave direction on the cloaking phenomenon is significant for the configuration with four outer cylinders, but it is negligible for eight outer cylinders.

Minimization of drift force on a floating cylinder by optimizing the flexural rigidity of a concentric annular plate

The deployment of suitable configurations of mutually interacting floating bodies for efficiently controlling their hydrodynamic interactions towards the reduction of the wave drift forces and, thus, of the mooring lines’ loads, has, nowadays, gained a great scientific interest. In this paper, the hydrodynamic behaviour of a floating cylinder and a concentric annular flexible plate is analysed in the frequency domain aiming at the minimization of the drift forces acting on the cylinder by optimizing the flexural rigidity of the plate. The diffraction/radiation problem is solved using a higher-order boundary element method. The analysis is implemented assuming that both floating bodies oscillate freely in heave, while for the plate, flexible modes are, additionally, considered for describing its structural deformations. The required modes shapes are determined in vacuum (“dry” mode superposition approach) through analytical expressions. The flexural rigidity of the plate, D, is optimized at a specific wave number using a real-coded genetic algorithm. Initially, results are compared with numerical results of other investigators for the case of two rigid concentric floating cylinders. Next, extended results are presented, focusing on the effect of D, including its optimum value, on various physical quantities describing the behaviour of both the cylinder and the plate. Contrary to the isolated cylinder, the presence of the plate introduces sharp peaks in the variation pattern of the drift force of the cylinder, bounded at specific wave numbers, where resonance of the seiche mode of water motion in the annular cavity or of specific flexible modes of the plate occurs. However, by reducing D to its optimum value, the cylinder’s drift force obtains practically zero values at the target wave number, due to an efficient improvement of the wave field in the annular cavity around the cylinder. Moreover, a great reduction of the drift force compared to the isolated cylinder is achieved in the subsequent high frequency range.

Wave Drift Forces' Calculation on Two Floating Bodies Based on the Boundary Element Method—Attempt for Improvement of the Constant Panel Method

An improved constant panel method for more accurate evaluation of wave drift forces and moment is proposed. The boundary element method (BEM) of solving boundary integral equations is used to calculate velocity potentials of floating bodies. The equations are discretized by either the higher-order boundary element method or the constant panel method. Though calculating the velocity potential via the constant panel method is simple, the results are unable to accurately evaluate wave drift forces and moment. An improved constant panel method is introduced to address these issues. The improved constant panel method can, without difficulty, employ the near-field method to evaluate wave drift forces and moment, especially for multiple floating bodies. Results of the new evaluation method will be compared with other evaluation method. Additionally, hydrodynamic interaction between multiple floating bodies will be assessed.

Viscous-flow-based investigation of Cloaking phenomenon among multi-floating bodies and experimental verification

The safety of mooring systems on offshore platforms seems more and more significant with the utilization of offshore space and resources, so the reduction of wave drift force is the key issue in this wave-body interaction problem. The wave drift force acting on the inner floating body surrounded by multiple small floating bodies can be reduced obviously with the occurrence of a phenomenon, which is called the Cloaking phenomenon. The Cloaking phenomenon refers to the reduction or complete elimination in amplitude of the scattered waves. In this paper, a real-coded genetic algorithm was used to optimize the parameters of outer floating bodies to minimize the scattered wave energy, and then the wave drift force acting on the inner body can be reduced. Furthermore, associated CFD simulations and experimental research were conducted with the above optimized parameters to investigate and verify the Cloaking phenomenon more systemically. It is shown that the wave drift force acting on the inner floating body in the Cloaking configuration can be reduced obviously both in numerical and experimental research, and the reduction of the wave drift force is closely related to the change of wave field around the structure.

Analysis of the Effect of Supplying Vessel on Wave Drift Force of Semi-Submersible Platform

Wave drift force is the key factor affecting the mooring ability of semi-submersible platform. Aiming at the mooring system composed of replenishment ship and semi-submersible platform, the influence of hydrodynamic interference on semi-submersible platform is analyzed. Based on the three-dimensional potential flow theory and AQWA software, the effects of different wave directions and spacing on the wave drift force of semi-submersible platform are considered. The results show that the hydrodynamic interference of the replenishment ship will affect the wave drift force of the semi-submersible platform, and the influence of the distance between adjacent vessels and the wave direction angle is more sensitive in the middle and high wave frequencies. This paper can provide support for the research of hydrodynamic interference of semi-submersible platform.

A Study on Viscous Wave Drift Force on Semi-submersible and Prediction of The Extreme Value of Mooring Force

A Study on Viscous Wave Drift Force on Semi-submersible and Prediction of The Extreme Value of Mooring Force

Real-Time Simulation of a Rescue Ship Maneuvering in Short-Crested Irregular Waves

Research on the mathematical model and the real-time simulation of the ship motion is investigated. An equation of three degrees of freedom (DOF) nonlinear maneuvering motion is established by means of the unified model in short-crested irregular waves in deep water. The resistance and propulsion dynamics are modeled by the classical methods as well as forces and moments of the rudder and the viscous crossflow. The three-dimensional boundary element method (BEM) using Green function source is adopted to solve the boundary value problem (BVP) in the frequency domain. The added mass and potential damping coefficients are calculated by Cummins method, and the mean wave drift forces by far-field formulas along with numerical verifications. After that, the turning motions of two ships (a rescue ship NHJ111 and the Mariner) in calm water are performed, and the relative error is approximately 7% by comparing calculated results with the experimental data. The maneuvering motion of the Mariner in short-crested irregular waves is then carried out. The turning trajectory, surge speed, and drift forces agree well with the theoretical results in regular waves. Finally, the synchronous interpolation method with two-thread double-timer is conceived. The real-time maneuvering simulation of NHJ111 using the validated model is then realized. The developed method can lay the foundation for fast computations of predicting the rescue ship motions in waves.

Validation of a distributed simulation of ship replenishment at sea with model tests

A simulation of ship motions during replenishment at sea was validated using dedicated model test data. Motions of the supply ship and receiving ship are influenced by hydrodynamic forces and forces from replenishment gear used to transfer payloads. Simulated entities include rudders and propellers, with associated controllers. The composite simulation was implemented as a distributed simulation using the high-level architecture. An experimental model test program was developed for a tanker and destroyer conducting replenishment in head seas at a speed of 15 knots. The simulation gives generally good agreement with the model tests for heave, roll, and pitch motions, with some noted exceptions. For dynamic positioning of the destroyer relative to the tanker, there is very good agreement for mean values but underprediction of lower frequency sway and yaw motions. The variation of heave and roll motion accuracy is likely due to the hydrodynamic force prediction method using the frequency domain Green function for zero forward speed, thus not accurately modelling the propagation of radiated and diffracted waves between vessels. Prediction of viscous roll forces using semi-empirical methods likely contributed to differences between simulated and model test roll motions. The underprediction of lower frequency sway and yaw motions for the destroyer in waves could be caused by the ship motion predictions not including second-order wave drift forces.

Real-coded genetic algorithm optimization in reduction of wave drift forces on an array of truncated cylinders

To reduce wave drift forces acting on an array of truncated cylinders, the optimization of geometrical dimensions of the structure using a genetic algorithm is presented in this paper. Wave drift forces are always the major challenge for offshore floating structures facing to waves. Recently, a cloaking phenomenon has attracted the attention of researchers, which can reduce the scattered wave energy of offshore structures even to zero. In a diffraction problem of water waves, the term cloaking means that there are no scattered waves existing outside the structures. The configuration that a truncated inner cylinder surrounded by four truncated cylinders is considered as an effective way to realize this phenomenon. To accurately calculate the hydrodynamic characteristics of the truncated cylinder array, the Wave Interaction Theory combining with a higher-order boundary element method is adopted. Based on the above numerical methods, three parameters including the radius and the draft of the outer cylinders and the distance between the inner cylinder and outer cylinders are optimized to minimize the scattered wave energy and the wave drift force for a certain wavenumber. It is found that the effect of the draft of outer cylinders on the scattered wave energy is insignificant. Therefore, the optimization with two parameters is carried out additionally without consideration of the draft of outer cylinders. Moreover, the relationship between the wave drift force and the wave distribution around the inner cylinder is discussed.

Dynamic modeling and simulations of the wave glider

• A 4 degree-of-freedom mathematical model of the wave glider is established using Newton–Euler approach. • A time domain simulation program is developed using Simulink. • Motion simulation of the wave glider is conducted under several typical sea states. • The wave glider's motion behavior is analyzed in detail. Wave glider is a new wave-powered autonomous marine vehicle, which is composed of a submerged glider connected to a surface floater via a tether. Such an advanced vessel is designed to harvest energy in ocean surface waves to generate forward thrust. Equipped with solar panel and battery as well as some dedicated sensors, the wave glider is able to achieve long duration missions via sea-side control. In this paper, a 4-DOF (degree-of-freedom) mathematical model of the wave glider is established using Newton–Euler approach. The second-order wave drift force on the horizontal plane and the first order wave force on the vertical direction are considered. The hydrodynamic parameters were calculated using the potential flow theory and empirical formula. Furthermore, motion simulation of the wave glider with respect to the sensitivity analysis to some key environmental factors and the heading control ability is conducted. The simulation results are presented and discussed in detail, which provides theoretical guidance and reference for wave glider design.

Prediction of the mean heading of a turret moored FPSO in bi-modal and bi-directional sea states

Ship-shaped platforms like Floating Production Storage and Offloading (FPSO) vessels are deployed in many offshore locations where the sea states are dominated by wind driven local seas combined with long period swells that are incident from different directions. Many of these vessels are held in station by a mooring system that is connected to the platform by a rotating turret. The heading stability of the vessel and its ability to self-align with respect to oncoming environmental forces is studied in this paper. Experiments at 1:120 scale conducted in a wave basin facility for two storm conditions offshore Brazil and West of Africa show that the heading angle is self-limiting to a range of ±20° with respect to head sea direction. Time domain simulations conducted using industry-standard software by modeling the wind-sea and swell with suitable spectrum models approaching from different directions show almost double the heading range seen in experiments. If similar numerical analyses were part of a design process, it is conceivable that this could result in incorrect predictions of the weathervaning of the platform, and hence the latter’s global motions in response to design storm conditions.Previous studies by the authors have confirmed that the delicate balance between various mean wave drift forces and moments is the main contributor to the heading stability of a single-point moored ship shaped vessel. It is shown here that in a bi-directional seastate, the interaction between the two wave components can significantly alter the drift loads on the platform. A simple cross-wave correction is added to the time domain simulations to account for the interaction. Implementing this correction brings the numerical results more in line with the experimental findings. The cross-wave correction term is quantified and recommendations are made for better simulations and experiments of such platforms.

Ocean wave characteristics prediction and its load estimation on marine structures: A transfer learning approach

Statistical and stochastic approaches, commonly used to estimate wave induced loads in the marine and coastal structural analysis and design of offshore vessels and platform, require accurate sea state or wave characteristics, which are usually estimated using computationally intensive numerical wave models. While advances in machine learning helps to overcome the computational challenges of these approaches, they still need measurement of historic data at the region of interest to help prospective estimation of wave loads. This calls for deployment of sensors and allied resources for continuous monitoring and collection of streaming data. Transfer learning, on the other hand, provides a framework to derive representations from an existing region to estimate the characteristics in the region of interest. Deep Belief Networks are excellent candidates for latent representation of the relationship between the different sea state characteristics and the induced loads. In this paper, we first train a DBN based wave characteristic prediction model to predict the sea state characteristics (namely, significant wave height Hs, dominant or peak wave period Tp and average or zero crossing period Tz) using data from 12 stations at 3 geologically different regions (the Gulf of Mexico, the Korean Region and the UK region), using data between the period Jan 1, 2011 and Dec 31, 2014. We validate the model using data in these regions between Jan 1, 2015 and Aug 30, 2015. We then compute the effect of the wave characteristics, namely, the wave-drift force and moment on 12 different marine structures with both regular and irregular waves. Finally, we transfer the DBN representations to predict the sea state characteristics in Irish region for the same period. Evaluations against ground truth and comparisons with state-of-the-art methods show the superior transfer learning performance of the DBN and accurate computations of the wave drift force and moments.

파랑 표류력을 고려한 선박의 파랑 중 선회성능 해석

This paper performs a numerical computation of ship maneuvering performance in waves. For this purpose, modular-type model (MMG (Mathematical Modeling Group) model) is adopted for maneuvering simulation and wave drift force is included in the equation of maneuvering motion. In order to compute wave drift force, two different seakeeping programs are used: AdFLOW based on Wave Green function method and SWAN based on Rankine panel method. When wave drift force is calculated using SWAN program, not only ship forward speed but also ship lateral speed are considered. By doing this, effects of lateral speed on wave drift force and maneuvering performance in waves are confirmed. The developed method is validated by comparing turning test results in regular waves with existing experimental data. Sensitivities of wave drift force on maneuvering performance are, also, checked.

Yaw motion analysis of a FPSO turret mooring system under wave drift forces

In order to evaluate the dynamic behavior of a FPSO-turret system, experimental tests in engineering wave basin have been performed. During the realization of experimental test large slow yaw rotations were detected in the dynamic behavior of the vessel during regular and irregular wave conditions tests. In order to study this phenomenon, an analytical method was developed in order to predict the maximum yaw angle reached under the slow yaw varying oscillations of a single point moored FPSO-turret system dependent of the regular mean wave drift forces and the turret position. The results for the proposed analytical method were verified by comparing it with experimental and simulated results. Then, the proposed method allows to identify the critical regular wave periods than can provoke large yaw rotations as an alternative for the stability tests by bifurcation analysis. On the other hand, the performed yaw motion simulations take into account the following aspects: the turret position, and the force and momentum associated with the mooring system variations (in function of the FPSO center of gravity positions and regular mean wave drift forces). Analyzing the time domain simulations it is possible to identify that large yaw motion is reached faster when the turret is near the CG and so, as the turret moves away, the yaw motions become slower.