Wave Headings Research Articles

Wave energy extraction for an array of dual-oscillating wave surge converter with different layouts

Array configuration of oscillating wave surge converter (OWSC) devices in nearshore is a preferable option for realizing a cost-balance of extracting wave energy and reducing installation expense due to closer installed place to coastline. The goal of the present work is to assess energy extraction of an array of dual-OWSC system with different layout schemes in comparison with an isolated OWSC, which can be regarded as a guideline for multi-array configuration in realistic wave farm. The coupled three-dimensional (3-D) hydrodynamic model is established based on the potential flow theory with fully nonlinear boundary condition in time domain. A non-dimensional approach is conducted to focusing on the accurate effects of multi-body interaction, wave nonlinearity, wave resonance, mechanical damping, layout scheme and oblique incidence as optimization design. For a front-back array system, wave resonance in dual-module gap enhances significantly the energy extraction of the front OWSC but does not contribute much to that of the back OWSC. Furthermore, wave resonance in wide gap has a positive effect on the capture efficiency in large wave periods. An in-line array system has a beneficial performance in small wave periods, while a staggered array system realizes more energy extraction in resonance region. A strong wave disturbance between flap sides of an in-line and staggered system, leads to the increase of energy extraction for the back OWSC with imposing small incident wave heading. Therefore, the combination of multi-triple-array OWSC with different gap distances will provide a desirable configuration which is independent on oblique wave conditions.

Time domain TEBEM method of ship motion in waves with forward speed by using impulse response function formulation

A numerical method for solving 3D unsteady potential flow problem of ship advancing in waves is proposed. The flow field is divided into inner and outer domain by an artificial matching surface. The inner domain includes ship-wetted surface, matching surface and free surface inside. The free-surface condition is formulated by perturbation about the double-body flow (DB model) or uniform incoming flow assumption (NK model). The outer domain contains matching surface, free surface outside and infinite far-field boundary, where only NK model is applied. The simple Green function and transient free surface Green function are used to form the boundary integral equation (BIE) by the impulse response function method for the inner and outer domain. Taylor Expansion Boundary Element Method (TEBEM) is utilized to solve the BIEs. Matching conditions for the inner-domain and outer-domain flow are enforced by the continuity of velocity potential and normal velocity on the matching surface. In this paper, several ships are selected to predict the hydrodynamic coefficients and displacement at different forward speed. The experiment measurements in physical tank is also used to validate the accuracy of numerical results. Good agreement of 6DOF ship motions in various wave headings can be obtained using TEBEM method.

Study on the characteristics of trimaran seakeeping performance at various speeds

Based on open-source CFD tool OpenFOAM and the hybrid-method solver qaleFoam, the numerical study about the effect of sailing speed on the motion of trimaran in waves of various headings is carried out. The wave heading ranges from head waves to stern waves, and three sailing speed of trimaran are chosen for simulation. The grid independence test is carried out first, and the computed results are compared with experimental results. Then, the numerical method is used to simulate the trimaran’s motion in waves of different parameters. It could be seen that the influence of the sailing speed on the added resistance is more significant than that on motion, especially in head wave and oblique head wave. In beam wave, the effect of sailing speed on the trimaran's motion is not noticeable, but the larger speed could reduce the added resistance. In the stern wave of low encounter frequency, the low sailing speed will reduce the added resistance and pitch.

Experimental Study and Numerical Analysis of Floating Crane Catamaran Mooring Tension in Intact and Damage Conditions Using Time-Domain Approach

Floating Crane Catamaran equipped with a mooring system to keep stable while operating. During operation, wave load causes tension on the mooring system. In this study, the tension of the mooring system was analyzed using experimental studies and numerical analysis with intact and damaged mooring conditions. Experimental studies were carried out by simulating a physical model in the Ocean Basin Maneuvering Laboratory, BTH-BPPT. While numerical using related software. Mooring tension analysis is carried out using the frequency domain approach which refers to the API RP 2SK rules. The sum of the average tension, significant low frequency tension and maximum wave frequency tension is the maximum tension of the mooring system. The low frequency tension and wave frequency tension is obtained by the low-band-pass filter process. The stochastic value is obtained by the FFT of low frequency and wave frequency tension. The results of maximum tension from experimental and numerical at intact conditions, wave headings 90°, Hs 2.5 m, are 373.7 kN and 441.6 kN and at Hs 6.37 m are 565.6 kN and 1741.5 kN. In the damaged condition, wave heading 90°, Hs 2.5 m, the maximum tension is 863.9 kN and 2113.3 kN.

Numerical investigation of ship motions in cross waves using CFD

A computational fluid dynamics (CFD) based Unsteady Reynolds Averaged Navier–Stokes and Volume of Fluid model solver is adopted to estimate ship motion responses in bi-directional cross waves. The characteristics of cross waves are analyzed theoretically and by CFD verification at first. Then ship nonlinear motion responses and green water on deck induced by cross waves are systematically analyzed by adopting a S175 containership model sailing in different cross wave conditions such as various wave lengths, wave heading angles and forward speeds. Finally, ship safe navigational strategy is explored and corresponding operational guidance is proposed to ensure the safety of ships when encountering cross waves.

Effect of Wave Headings on the Dynamic Response of the Continuous Mating Operation of Floatover Installation

The topside floatover installation is always a great challenge and is sensitive to environmental conditions. In this study, experimental analysis on the mating operation of the floatover installation in different wave headings is presented. The continuous mating operation using the rapid transfer technique was experimentally simulated with the assistance of the jacking system and the ballast system. In the continuous transfer modeling, the topsides loads were transferred onto the jacket by several consecutive steps, including the first rapid jack-down for the 30% loads, continuous 30%–70% load transfer and the second repaid jack-down for the remaining 30% loads. Motions of the barge and the topsides as well as loads on the Deck Support Unite (DSU) and the Leg Mating Unite (LMU) in different wave headings were measured. Experimental results illustrated the complex motion behavior and load characteristics of the continuous transfer operation. Results indicate that the rapid jack-down operations will lead to impact loads and larger lateral DSU loads. The bow quartering seas are much more dangerous as it gives rise to the larger motions and loads. Comparisons with the traditional steady-state modeling indicate that the continuous transfer modeling has greater advantages over the steady-state modeling on predicting the loads.

Vessel Velocity Estimation and Tracking From Doppler Echoes of T/R-R Composite Compact HFSWR

Vessel speed and heading are two important kinematic parameters describing its state of motion. However, due to low spatial resolution of a compact high-frequency surface wave radar (HFSWR), vessel speed and heading cannot always be accurately estimated. Since HFSWR can measure vessel Doppler velocity with relatively high accuracy, it is possible to estimate a vessel's vector velocity based on two Doppler velocities measured along two different directions. In this article, a newly developed T/R-R composite compact HFSWR system is introduced, and a corresponding vessel velocity estimation method which employs a target's radial velocity and elliptical velocity, respectively, measured by the monostatic (T/R) and bistatic (T-R) settings is proposed. First, monostatic and bistatic tracks are independently generated using a multitarget tracking algorithm. Then, the obtained monostatic and bistatic tracks are matched using a track-to-track association method to determine the track pair belonging to each target. Subsequently, the associated track pairs are combined to produce fused tracks for improving positioning accuracy. Finally, vessel vector velocity is estimated based on the radial and elliptical velocities as well as the fused target position. Comparisons of vector velocity estimation results from radar field data with corresponding automatic identification system data demonstrate that the average root-mean-square-errors of the estimated speed and heading are 0.48 km/h and 3.9 $^{\circ }$ , respectively, which meets the practical requirements of a maritime surveillance system. Moreover, the velocity estimation error is analyzed via theoretical derivation and experimental verification. The proposed method shows good potential in further improving the tracking accuracy.

Experimental and numerical studies on linear and nonlinear seakeeping phenomena of the DTC ship in regular waves

ABSTRACT The paper presents a comprehensive numerical and experimental study on linear and nonlinear seakeeping phenomena, including six degree of freedom (DOF) ship motions, added resistance, drift forces and moments of the DTC containership in regular waves. Obtained numerical results by the 3D panel method NEWDRIFT+ of NTUA-SDL are systematically compared with corresponding experimental data generated within the SHOPERA project (2013–2016) at MARINTEK for various wave headings at low to moderate speeds. The herein presented experimental data are useful not only for validating the employed seakeeping software and others as a benchmark yardstick, but also for the deeper analysis of various open nonlinear seakeeping issues encountered in the prediction of the added resistance and drift forces of modern large containerships in waves.

Investigation into the control strategy for a long spine of Edinburgh Duck modules, using an efficient numerical model

An efficient numerical model of a spine of ten Edinburgh duck modules is developed. The spine joints and duck modules are modelled using a linear approach based on the theory of generalized modes, which mitigates the need for a more computationally expensive time- domain solver. This approach also allows for computation of the shear forces acting on the spine joints, and has the added benefit of enabling the use of complex conjugate control. The resulting hydrodynamic model is verified for a three duck spine against an alternative implementation that uses a nonlinear multibody solver to enforce the joint motions. A conservative weighted motion constraint is imposed on the controlled degrees of freedom of the ten duck spine, in order to ensure results stay within the bounds of the linear theory. Pertinent sections of the theory underpinning the constrained complex conjugate control method are elaborated upon for the case in which not all degrees of freedom are controlled. An implementation of this control method for a solo duck is compared against a result from the literature, in order to confirm the suitability of the choice of duck design in this study. The control force coefficients that maximise the absorbed power, subject to the motion constraint, are computed for the ten duck spine over a range of wave periods and wave heading angles. The resulting dynamics of the spine of ducks are explored, with particular emphasis on aspects related to the power extraction and forces acting within the system.

Multi-body hydrodynamic resonance and shielding effect of vessels parallel and nonparallel side-by-side

An attempt has been made to study the hydrodynamic interaction of side-by-side vessels based on the gap resonant modes which were theoretically derived by Molin et al. The hydrodynamic associated wave elevations, coefficients, and wave forces are analyzed for a real hull shaped FPSO and a ship. The resonant characteristics of parallel and nonparallel configurations under different wave headings are analyzed. The resonant mode due to the gap and its impact on various hydrodynamic results are emphasized in this study. The vessel motions are simulated in the time domain by using Cummins’ approach. Calculated motion RAOs by white noise test and relative motion under irregular waves are validated by measurement in the model test. The resonant behaviors and phase differences of each degree of freedom of vessels are investigated. The shielding effect is evaluated numerically and experimentally by exchanging vessels on lee-side and weather-side. Results indicate the different degree of freedom has a distinct preference to react to the resonant modes. The first mode which is superior under beam sea may become less significant under oblique and head seas. Higher resonant mode shifts to lower frequency in the nonparallel configuration. The shielding effect only suppresses the motion caused by the gap resonance, the natural frequency resonance like roll remains without change.

Single Mode Simulation Calculation of Oscillating Buoy Wave Energy Converter with A Slider

This paper presents an oscillating slider wave energy device which is based on a seabed anchoring and uses eagle beak as the absorber. The self-compiled program uses the boundary element theory based on the simple Green’s function to solve the wave forces and hydrodynamic parameters. And the equation of motion, the oscillation of the float and the capture width ratio are obtained by the modal method. The influences of the shape of the eagle beak, the angle of the slider and the wave heading on the capture ability of the device are investigated. According to the calculation results and the wave resources in the sea area, the optimal shape of the eagle beak and external damping can be selected to maximize the wave energy capture capability.

Hydrodynamic Simulation of the Semi-Submersible Wind Float by Investigating Mooring Systems in Irregular Waves

The present study aims to implement the software ANSYS AQWA to discuss the hydrodynamic analysis of the DeepCwind semi-submersible floating platform in waves based on the potential flow theory by considering the second-order wave exciting force. In this study, the linearized potential-flow hydrodynamic radiation and diffraction problems in the frequency domain were firstly solved by adopting the three-dimensional panel method. Subsequently, the hydrodynamic coefficients and wave loading data were transformed to time domain forms by the Cummins time domain equation as a system loading input. Furthermore, the quadratic transfer function (QTF) matrices with different frequencies and directions deduced based on the near field integration over the mean wetted hull surface were adopted for the calculation of slow-drift forces. In order to represent the damping in a real system for modeling potential flow without Morison’s elements, an additional quadratic damping matrix was added to capture the viscous drag. Eventually, both of the dynamic mooring model based on the lump-mass (LM) approach and the quasi-static mooring model based on the multi-segmented, quasi-static (MSQS) approach are introduced to discuss the mooring effect on the platform hydrodynamics. The effect of wave heading angles on the platform motion is considered as an influential parameter as well.

Regression analysis of experimental data for added resistance in waves of arbitrary heading and development of a semi-empirical formula

In this paper, we first conduct an extensive review of published experimental studies on the added resistance of a ship in waves to establish a common database. The data was enhanced by unpublished data from joint industry projects. Using this newly established database consisting of about 3,000 data points for 130 ships of different type and size, we develop an empirical formula for approximating the added resistance in regular waves of arbitrary heading. For the added resistance due to diffraction effect, we updated Faltinsen et al.'s (1980) asymptotic approach and introduced several semi-empirical parameters to approximate the added resistance in waves of various headings. The limited ship draft effect is examined analytically, while the hull form effects are taken into account empirically by regression of available data. For the radiation effect, the maximum value of added resistance and the corresponding wavelength for the various wave headings are determined by regression analysis of the experimental data. The developed formula is verified by application to typical ship types and sizes, both in the design and ballast conditions, at both design and low speeds. An error analysis for the various tested scenarios shows that the performance of the developed formula is satisfactory.

Parametric study of seakeeping of a trimaran in regular oblique waves

ABSTRACT This paper presents the impact of different geometrical parameters on the seakeeping of a trimaran in regular oblique waves. Experimental tests for eight different arrangements of outriggers were conducted for a range of wave frequencies where a constant wave heading, and wave height was maintained. It was found that the position of outriggers and the hull shape have significant impacts on dynamic performance of the model. Roll motions reduce with an increase in longitudinal spacing of main and demi hulls transoms, separation of outrigger centre lines and buoyancy fraction of outriggers, while heave motion and resistance are increased, and pitch motions are slightly influenced. It was concluded that positioning of outriggers can reduce the roll motions peak amplitude by 80%. This is due to the trimaran geometry and the effect of reflected waves between the hulls. In configurations with minimum roll motion, the added resistance peak is maximised when compared with other configurations.

Numerical and Experimental Investigation of Hydrodynamic Interactions of Two VLFS Modules Deployed in Tandem

This paper numerically and experimentally investigates the hydrodynamic interaction between two semi-submersible type VLFS modules in the frequency domain. Model tests were conducted to investigate the relationship between interactions and wave headings. Numerical studies were performed by solving the radiation-diffraction problem and were validated against the experimental results. Motion Response Amplitude Operators (RAOs) were obtained from numerical and experimental studies. The dependency of the hydrodynamic interaction effect on wave headings is clarified. The influence of different wave periods on the motion responses of two-module VLFS and wave elevations in the gap is studied. The results indicate that the hydrodynamic interactions of the two modules are directly related to the wave headings and the periods of the incident wave. The shielding effect plays an important role in short wave, and the influence decreases with the increase of the incident wavelength. The numerical results based on the diffraction-radiation code can give a relatively good estimation to the responses in short wave while for long wave, it would over-predict the response.

Effect of Surface Waves on Submarine Pipeline during Lying

Submarine pipelines are essentially used for the transmission of gas and oil across oceans between countries or for transport between shore and offshore installations. The pipeline applications were studied to be installed in deep water, which exposed to different loads such as currents and waves in various directions, barge movements, seafloor interaction, etc. This paper developed a dynamic analysis of the J-lay suspended submarine pipeline during laying taking into account the effect of water depth, the direction of the wave heading, and sea state without vessel movement. The finite element program ANSYS R17.2 is used for modeling and analysis of the pipelines. The random sea state is modeled using the JONSWAP spectrum. It was found that the effect of the direction of wave heading on the bending moment from dynamic analysis of pipeline is obvious in a depth of (2 m) below water surface, and then gradually decreases until it disappears in depth of (100 m). Whereas the effect of wave height is obvious in a depth of (2 m) and then gradually decreases until it disappears in depth of (120 m).

Modelling the motion of a dropped cylinder under 3D second-order regular waves and identification of the governing parameters

ABSTRACT Nonlinear regular wave effects on a dropped cylinder are studied and compared with those of linear waves. A 3D second-order wave model is employed to simulate the falling process of a dropped cylinder under nonlinear regular waves. The calculated results for nonlinear regular wave simulations, nonlinear regular wave effects on the trajectory, landing point and falling time of the dropped cylinder into different water depths, waves at zero and nonzero wave heading angles with different frequencies and amplitudes are compared with the corresponding results under linear regular wave simulations. It is found that wave heading, wave frequency, water depth, wave amplitude, and the selection of regular wave model (linear or nonlinear) are factors that may significantly influence the motion of the dropped cylinder.

Numerical Study on the Coupled Responses of the Steel Lazy Wave Riser (SLWR) Based on the Basis of Design and Moored FPSO

A coupled analysis was performed between the riser to develop oil and gas in ultra-deepwater and the moored floating body. In general, the safety of the riser is conservatively evaluated by considering the maximum offset excluding the coupled analysis with the floating body. In this study, the safety of the riser was analyzed by considering the coupled motion analysis of the moored floating body. The riser is considered steel lazy wave riser (SLWR) applied in the deep sea, and the floating body is determined to FPSO. The methodology was presented on coupled and uncoupled analysis. The coupled effects were analyzed according to the incident wave headings in intact and damaged conditions of mooring lines. The tension of mooring lines, the motion of the floating body, and riser responses were analyzed according to the loading conditions.

On natural frequencies of three-dimensional moonpool of vessels in the fixed and free-floating conditions

The hydrodynamic interactions between the vessel motion and resonant wave response inside the three-dimensional moonpool are studied in the present paper. In particular, the shift of natural frequencies of a three-dimensional moonpool in the fixed and free-floating conditions is computed and examined systematically. Barges with different moonpool dimensions are analyzed to examine how the moonpool configurations affect the shift of resonant frequencies. Detailed analyses are performed for cases with and without recess. Numerical computations are performed for regular waves with wave headings θ=0∘, 45∘ and 90∘. Free-surface elevations inside the moonpool with respect to the incident wave frequencies are computed for both fixed and free-floating conditions. It is observed that natural frequencies of the moonpool in the free-floating condition are higher than those in the fixed condition. For cases without recess, the shift of resonant frequency is more prominent in piston mode resonance, in particular for shallow or long moonpools. In contrast, for cases with a recess, the shift of resonant frequency in the first sloshing mode is relatively larger than that in piston mode. In addition, the effects of motion RAOs on the free-surface elevation inside the moonpool in the free-floating condition are also examined. It is found that the moonpool resonance can induce a local hump in heave and pitch responses. Moreover, through deriving a modified frozen mode approximation model, we reveal that, comparing to the fixed condition, the vessel motion in the free-floating condition yields a reduction in the added mass due to the fluid underneath the moonpool so that the piston mode resonant frequency increases.

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.