Irregular Sea Research Articles

Drift stability of HyStOH semi-submersible supported by airfoil shaped structures

This study presents the results of the German research project HyStOH funded by German Federal Ministry of Economic Affairs and Energy (BMWi). The project consortium of German universities, wind turbine designers, wind farm developers and certification bodies design a novel semi-submersible steel structure with a single point mooring and self-aligning capabilities. The tower, which carries a 6 MW two-bladed downwind turbine, has an airfoil shaped cover, which supports the self-alignment of the full structure towards the main wind direction. The downwind operating wind turbine in combination with the lift force generating tower enables a passive yaw system acting against hydrodynamical impacts from waves and currents. Simulations in time domain applying the fully coupled aero-hydro-servo-elastic code Bladed_4.8 demonstrate the drift sensitivity and the self-aligning capabilities of the HyStOH design. The hydrodynamic coefficients for the simulations have been adjusted by calculations with an 1:1 model of the panel code panMARE, developed by Technical University of Hamburg-Harburg. The motivation of this analysis is to capture the complex coupled motions of an innovative FOWT by numerical simulation tools [1]. The analysis presents dynamic simulations of the HyStOH design operating in turbulent wind and irregular sea state with special focus on the yaw drift behaviour of the FOWT. The sensitivity study of yaw drift is based on numerous variations of wind-wave-current misalignments during normal operation. The results of the simulations demonstrate the weathervane capabilities of the airfoil shaped structures.

Lifetimes of Rogue Wave Events in Direct Numerical Simulations of Deep-Water Irregular Sea Waves

The issue of rogue wave lifetimes is addressed in this study, which helps to detail the general picture of this dangerous oceanic phenomenon. The direct numerical simulations of irregular wave ensembles are performed to obtain the complete accurate data on the rogue wave occurrence and evolution. Purely collinear wave systems, moderately crested, and short-crested sea states have been simulated by means of the high-order spectral method for the potential Euler equations. As rogue waves are transient and poorly reflect the physical effects, we join instant abnormally high waves in close locations and close time moments to new objects, rogue events, which helps to retrieve the abnormal occurrences more stably and more consistently from the physical point of view. The rogue event lifetime probability distributions are calculated based on the simulated wave data. They show the distinctive difference between rough sea states with small directional bandwidth on one part, and small-amplitude sea states and short-crested states on the other part. The former support long-living rogue wave patterns (the corresponding probability distributions have heavy tails), though the latter possess exponential probability distributions of rogue event lifetimes and generally produce much shorter rogue wave events.

CFD design-load analysis of a two-body wave energy converter

Wave energy converters (WECs) must survive in a wide variety of conditions while minimizing structural costs, so as to deliver power at cost-competitive rates. Although engineering design and analysis tools used for other ocean systems, such as offshore structures and ships, can be applied, the unique nature and limited historical experience of WEC design necessitates assessment of the effectiveness of these methods for this specific application. This paper details a study to predict extreme loading in a two-body WEC using a combination of mid-fidelity and high-fidelity numerical modeling tools. Here, the mid-fidelity approach is a time-domain model based on linearized potential flow hydrodynamics and the high-fidelity modeling tool is an unsteady Reynolds-averaged Navier–Stokes model. In both models, the dynamics of the WEC power take-off and mooring system have been included. For the high-fidelity model, two design wave approaches (an equivalent regular wave and a focused wave) are used to estimate the worst case wave forcing within a realistic irregular sea state. These simplified design wave approaches aim to capture the extreme response of the WEC within a feasible amount of computational effort. When compared to the mid-fidelity model results in a long-duration irregular sea, the short-duration design waves simulated in CFD produce upper percentile load responses, hinting at the suitability of these two approaches.

Experimental study on hydrodynamic performance of SWATH vessels in calm water and in head waves

An extensive experimental investigation on four SWATH hull forms has been conducted in calm water and in regular waves at University of Naples Federico II. Calm water tests have been analyzed in the range of Froude number FrT from 0.1 to 0.6. For all four SWATH configurations at the speed, corresponding to FrT 0.32, the behaviour in regular waves has been tested. The results of heave, pitch and vertical accelerations are presented in nondimensional form as RAO. For the “most promising” SWATH #4 configuration, a set of stabilizing fins have been designed and an active stabilization system has been developed. The developed SWATH#5 has been tested in calm water on three displacements in the range of FrT from 0.1 to 0.65. The dynamic wetted surface has been identified and the residual resistance coefficient CR as well as RT/Δ are reported. Seakeeping tests have been performed in regular head sea and in head and following irregular sea at FrT = 0.50. The conditions for the occurrence of dynamic longitudinal instabilities have been identified. The results allows to comment the effect of slenderness of struts and SWATH’s immersed bodies on resistance and seakeeping and concerns the applicability of SWATH concept to small craft.

Irregular Wave Validation of a Coupling Methodology for Numerical Modelling of Near and Far Field Effects of Wave Energy Converter Arrays

Between the Wave Energy Converters (WECs) of a farm, hydrodynamic interactions occur and have an impact on the surrounding wave field, both close to the WECs (“near field” effects) and at large distances from their location (“far field” effects). To simulate this “far field” impact in a fast and accurate way, a generic coupling methodology between hydrodynamic models has been developed by the Coastal Engineering Research Group of Ghent University in Belgium. This coupling methodology has been widely used for regular waves. However, it has not been developed yet for realistic irregular sea states. The objective of this paper is to present a validation of the novel coupling methodology for the test case of irregular waves, which is demonstrated here for coupling between the mild slope wave propagation model, MILDwave, and the ‘Boundary Element Method’-based wave–structure interaction solver, NEMOH. MILDwave is used to model WEC farm “far field” effects, while NEMOH is used to model “near field” effects. The results of the MILDwave-NEMOH coupled model are validated against numerical results from NEMOH, and against the WECwakes experimental data for a single WEC, and for WEC arrays of five and nine WECs. Root Mean Square Error (RMSE) between disturbance coefficient (Kd) values in the entire numerical domain ( R M S E K d , D ) are used for evaluating the performed validation. The R M S E K d , D between results from the MILDwave-NEMOH coupled model and NEMOH is lower than 2.0% for the performed test cases, and between the MILDwave-NEMOH coupled model and the WECwakes experimental data R M S E K d , D remains below 10%. Consequently, the efficiency is demonstrated of the coupling methodology validated here which is used to simulate WEC farm impact on the wave field under the action of irregular waves.

Experimental Study of a Hybrid Wave Energy Converter Integrated in a Harbor Breakwater

Sea ports are infrastructures with substantial energy demands and often responsible for air pollution and other environmental problems, which may be minimized by using renewable energy, namely electricity harvested from ocean waves. In this regard, a wide variety of concepts to harvest wave energy are available and some shoreline technologies are already in an advanced development phase. The SE@PORTS project aims to assess the suitability and viability of existing wave energy conversion technologies to be integrated in harbor breakwaters, in order to take advantage of their high exposure to ocean waves. This paper describes the experimental study carried out to assess the performance of a hybrid wave energy converter (WEC) integrated in the rubble-mound structure that was proposed for the extension of the North breakwater of the Port of Leixões, Portugal. The hybrid concept combines the overtopping and the oscillating water column principles and was tested on a geometric scale of 1/50. This paper is focused on the assessment of the effects of the hybrid WEC integration on the case-study breakwater, both in terms of its stability and functionality. The 2D physical model included the reproduction of the seabed bathymetry in front of the breakwater and the generation of a wide range of irregular sea states, including extreme wave conditions. The experimental results shown that the integration of the hybrid WEC in the breakwater does not worsens the stability of its toe berm blocks and reduces the magnitude of the overtopping events. The conclusions obtained are therefore favorable to the integration of this type of devices on harbor breakwaters.

Time domain predictions of inertial loads on a drifting ship in irregular beam waves

A proper estimation of the ship accelerations, particularly in the transversal direction, is fundamental for predicting the risk of cargo shifts and inherent structural damages in severe irregular seas. A specific situation worth to be considered in this context is the so-called dead ship condition, in which the ship, because of a loss of steering capabilities, is drifting in beam waves at zero speed. Particularly in this situation, a proper modelling of ship dynamics in wave is deemed necessary for a reliable assessment of inertial loads.In this paper three time domain procedures with different accuracy are applied and compared. The aim is to investigate the dependency of inertial loads on the non-linear effects of ship geometry and on the couplings among the 6DOF of the ship.Simulations are carried out with, in input, the same irregular sea realizations in the time domain, obtained from a digitalisation of JONSWAP spectra. A technique allowing for a fine matching of the digitalised spectra to the analytical ones is developed and applied.The investigation indicates that couplings among ship motions trigger larger inertial loads and, accordingly, a larger probability of occurrence for cargo shifting and structural damages.

泡沫覆盖不规则海面的非均匀空-水信道量子密钥分发

泡沫覆盖不规则海面的非均匀空-水信道量子密钥分发

State space modelling approach for rudder roll stabilization

The control objective of the Rudder Roll Stabilization (RRS) system is to deploy the rudder, which is primarily a path controlling device, to reduce the roll motion without interference in heading of ship. To achieve the control of both roll and yaw motions, the only control input is the rudder angle and hence the RRS system is referred as a Single Input, Two Output (SITO) system. Rudder roll stabilization is insignificant at low forward speed of the ship, but can give significant control at higher speed when fast rudder movement is applied. This paper presents a closed loop state space model for accurate simulations on rudder roll stabilization in irregular seas considering the 3-degree of freedom motions, i.e., sway, roll and yaw. The computational model is developed to analyze the effect of the rudder movement on sway, roll and yaw in forward speed conditions in irregular sea conditions. The Sea State conditions are modelled as wave perturbation models using the method of shaping filter established by filtered white noise. The control system has been designed using optimal linear quadratic regulator (LQR) method. The control loop contains both the signal for the autopilot action to trigger the heading angle correction as well as the signal for rudder based roll motion control. The simulations are carried out with rudder roll control system ON and OFF mode to analyze the effect of the rudder on steering and motion stabilization. In both cases the autopilot is in active mode to correct deviations in the course heading. The simulations are analyzed for three different ship speeds in two different Seas State conditions with a low and fast rudder movement to show the efficacy of the model. The performance is evaluated and presented based on the RMS value. Since the rudder based roll motion stabilization may also result in unnecessary motions of sway and yaw, besides the desirable roll reduction, the result presents the sway-roll-yaw responses as applicable under the particular speed and Sea State conditions.

Fuzzy Logic based hybrid type control implementation of a heaving wave energy converter

This study presents the results of the Fuzzy Logic based control application for a heaving wave energy converter operating in realistic sea-state conditions. It has been shown in literature that the power capture performance of a heaving wave energy converter depends on meeting the resonance condition with the incident wave frequency. In regular sea wave conditions, the task of setting the control parameters is a well-known easy task where the wave frequency is known. However, in irregular sea conditions, the wave frequency needed for control settings is not clearly defined. One of the approaches proposed in literature is to use of the dominant wave frequency that can be estimated by a model using the Discrete Fourier Transform approach. The presented model utilises the estimated wave frequency in Fuzzy Logic controller for determination of Power Take-Off control settings. This approach combines widely used fast tuning technique with a new slow tuning method. As a part of the study, the developed realistic simulation model and proposed Fuzzy Logic based controller are used to combine the fast and slow tuning techniques to form a novel hybrid control technique. The simulation results for this new control technique in realistic sea conditions are presented.

Statistical analysis of parametric roll in irregular seas on clusters with large roll amplitudes

The statistical evaluations on the numerical and experimental parametric roll time series in long-crest irregular heading waves are conducted. In the time series of parametric roll under irregular seas, strong “grouping phenomenon” or “cluster structures” can be observed. A pick-out procedure based on parametric roll detection scheme is applied to get the clusters with large roll amplitudes to form the truncated realizations. The statistical properties of the truncated realizations are investigated and compared with those of the original realizations. It is found that the confidence bands of variances of the truncated realizations are significantly larger than those of the original realizations, which indicates that the truncated realizations, i.e. the clusters with large roll amplitudes, are more dependent and narrow-banded than the original realizations. The distribution of the truncated realization is close to normal distribution with the kurtosis slightly larger than the normal distribution, while the distribution of the original realization is leptokurtic with sharp peak and thick tail. The kurtosis of the truncated realizations can be viewed as the lower limit of kurtosis for the possible distribution of the parametric roll. The estimated lower limit of kurtosis with 95% confidence interval can be given approximately as 3.650 ± 0.127.

Investigation on Ship Hydroelastic Vibrational Responses in Waves

The hydroelastic vibrational responses of a large ship sailing in regular and irregular head waves are investigated numerically and experimentally. A 3D time-domain nonlinear hydroelastic mathematical model is established in which the hydrostatic restoring forces and incident wave forces are calculated on the instantaneous wetted hull surface to consider nonlinear effects of the rigid motion and elastic deformation of the hull in harsh waves. Radiation and diffraction wave forces are computed on the mean wetted surface based on the 3D frequency-domain potential flow theory. The slamming loads are calculated by momentum theory and integrated into the hydrodynamic forces. The 1D Timoshenko beam theory is adopted to model the vibrational structural response and is fully coupled with the presented hydrodynamic theory in time-domain to generate the hydroelastic equation of motion. Moreover, self-propelled segmented model tests were conducted in a laboratory wave tank to experimentally investigate the hydroelastic responses of a target ship in regular and irregular head seas. The numerical and experimental results are systemically compared and analyzed, and the established hydroelastic analysis model turns out to be reliable and effective in the prediction of ship hydroelastic responses in waves.

Estimating a non-Gaussian probability density of the rolling motion in irregular beam seas

A methodology for predicting the probability density function of roll motion for irregular beam seas was developed in previous research. That work introduced a non-Gaussian probability density function (PDF), which shows a good agreement with Monte Carlo simulation (MCS) results in comparison with linear theory results. However, if the nonlinear damping coefficient in a system is large, the PDF delivers estimations that do not match the MCS results. The procedure reported herein improves the prediction accuracy using a non-Gaussian PDF that takes a nonlinear damping term into account.

Wake effect assessment of a flap type wave energy converter farm under realistic environmental conditions by using a numerical coupling methodology

Ocean Energy Europe has estimated that 100 GW of ocean energy capacity (wave and tidal) could be deployed in Europe by 2050. Along with the European targets it is expected that large farms of Wave Energy Converters (WECs) will be installed in the sea and, as part of the consenting process for their installation, it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the assessment of WEC farms impact on the surrounding wave field (wake effect) through the use of a numerical coupling methodology. The methodology consists of a Boundary Element Method (BEM) solver to obtain the wave perturbation generated by the WEC farm for the near-field accounting for the wave-body interactions within the farm whilst a Wave Propagation Model (WPM) based on the mild-slope equations determines the wave transformation in the far-field. The near-field solution obtained from the BEM solver is described as an internal boundary condition in the WPM and then it is propagated throughout the WPM numerical domain. The internal boundary is described by imposing the solution of the surface elevation and velocity potential at the free-surface at each instant of time along a line surrounding the WEC farm.As a case study the methodology was applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique was done first for a single flap and then for a farm of 5 flaps. Once validated, a realistic scenario was assessed by quantifying the impact of irregular sea states composed of long crested waves on a large WEC farm composed of 18 flaps and located on a real bathymetry. The irregular waves were obtained by superposing the regular wave field solutions for all wave frequencies represented in the considered sea state based on the linear water wave theory. Within the limits of this theory these simulations demonstrate the versatility of the methodology to accurately represent the impact of a WEC farm on the surrounding wave climate. The influence of the peak period and the spacing between flaps on the WEC farm wake effect was assessed as well.

Cretaceous paleomargin tilted blocks geometry in northern Tunisia: stratigraphic consideration and fault kinematic analysis

New stratigraphic data, lithostratigraphic correlations, and fault kinematic analysis are used to discuss the basin geometry and sedimentation patterns of the northeastern Tunisia during Cretaceous times. Significant facies and thickness variations are deduced along the northeastern Atlas of Tunisia. The NW-SE 80-km-long regional correlation suggests a high sedimentation rate associated with irregular sea floor. The fault kinematic analysis highlights N-S to NE-SW tectonic extension during Early Cretaceous. During Aptian–Albian times, an extensional regime is recognized with NE-SW tectonic extension. The Cenomanian–Turonian fault populations highlight a WNW-ESE to NW-SE extension, and Campanian–Maastrichtian faults illustrate an NW-SE extension. The normal faulting is associated to repetitive local depocenters with a high rate of sedimentation as well as abundant syntectonic conglomeratic horizons, slump folds, and halokinetic structures. The sequence correlation shows repetitive local depocenters characterizing the basin during Early Cretaceous times. All the above arguments are in favor of basin configuration with tilted blocks geometry. This geometry is shaped by major synsedimentary intra-basin listric normal faults, themselves related to the extensional setting of the southern Tethyan paleomargin, which persisted into the Campanian–Maastrichtian times. The results support a predominant relationship between tilted blocks geometry and sedimentation rather than E-W “Tunisian trough” as it was previously accepted.

Adaptive Control of a Wave Energy Converter

Energy maximizing controllers (EMCs), for wave energy converters (WECs), based on linear models, are attractive in terms of simplicity and computation. However, such (Cummins equation) models are normally built around the still water level as an equilibrium point and assume small movement, leading to poor model validity for realistic WEC motions, especially for the large amplitude motions obtained by a well-controlled WEC. The method proposed here is to use an adaptive algorithm to estimate the control model online, whereby system identification techniques are employed to identify a linear model that is most representative of the actual controlled WEC behavior. Using exponential forgetting, the linear model can be regularly adapted to remain representative in changing operational conditions. To that end, this paper presents a novel adaptive controller based on a receding horizon pseudospectral formulation. A case study is presented, providing an preliminary evaluation of the adaptive receding horizon pseudospectral control (ARHPC), using simulations based on both linear hydrodynamic modeling and computational fluid dynamics (CFD). The parameter adaptation is shown to behave as expected in both different simulation environments. The ARHPC is observed to perform well in irregular sea states, absorbing more power than its nonadaptive counterpart. The optimal trajectory calculated by the adaptive model is seen to have a smaller motion and power take-off (PTO) forces, compared to those calculated by the nonadaptive linear control model, due to the increased amount of hydrodynamic resistance estimated by the adaptive model, as identified from the nonlinear viscous CFD simulation.

On the Wave Bottom Shear Stress in Shallow Depths: The Role of Wave Period and Bed Roughness

Lagoons and coastal semi-enclosed basins morphologically evolve depending on local waves, currents, and tidal conditions. In very shallow water depths, typical of tidal flats and mudflats, the bed shear stress due to the wind waves is a key factor governing sediment resuspension. A current line of research focuses on the distribution of wave shear stress with depth, this being a very important aspect related to the dynamic equilibrium of transitional areas. In this work a relevant contribution to this study is provided, by means of the comparison between experimental growth curves which predict the finite depth wave characteristics and the numerical results obtained by means a spectral model. In particular, the dominant role of the bottom friction dissipation is underlined, especially in the presence of irregular and heterogeneous sea beds. The effects of this energy loss on the wave field is investigated, highlighting that both the variability of the wave period and the relative bottom roughness can change the bed shear stress trend substantially.

On a submerged wave energy converter with snap-through power take-off

This paper investigates the performance of a bistable snap-through power take-off (PTO) operating inside a submerged wave energy converter (WEC). The equation of motion of the surging WEC is derived in the time domain using the Euler–Lagrange equations. The dynamic response of the WEC in regular waves is studied first. It is found that the wave amplitude has a significant impact on the energy conversion efficiency with the proposed energy extraction mechanism. With larger waves impacting on the WEC, the conversion efficiency of the present nonlinear PTO increases significantly. Three response regimes, i.e. local oscillation, aperiodic snap-through, and periodic snap-through, of the nonlinear PTO system are observed with various wave amplitudes. This nonlinear feature is quite different from the linear PTO mechanism that is independent of the wave amplitude. Further, the dynamic response of the nonlinear WEC subjected to irregular wave sea conditions is investigated. Parametric studies have been carried out to determine the optimum operating conditions of the bistable device in order to maximize the wave energy extraction. The utilization of the snap-through PTO can enhance the efficiency of the WEC over its linear counterpart in irregular waves.

PeWEC: Experimental validation of wave to PTO numerical model

In recent years, several efforts have been performed in order to make Wave Energy Converters (WECs) sufficiently profitable and reliable, for their integration in the renewable energy mix. Looking at the European scenario, the development of technologies has been carried out by the countries bordering on the Atlantic Ocean, where a greater energy potential is available. However, a higher potential means also a stronger wave climate during storms, that may compromise the plants survivability. Looking at the Mediterranean scenario, the lower energy potential is balanced by milder extreme condition that should facilitate devices reliability in such conditions. The research activities described in the present work aims to develop a pendulum converter (PeWEC: Pendulum Wave Energy Converter) specifically designed for the Mediterranean Sea climate. In this work, the mathematical model describing the PeWEC dynamics is given, together with experimental tank testing performed on a 1:12 scale prototype. Experimental data recorded during the testing campaign are used to validate the numerical model previously described, both in regular and in irregular sea states. The numerical model proved to be in good agreement with the experimental data.

Numerical and experimental analysis of hydroelastic responses of a high-speed trimaran in oblique irregular waves

Investigation of hydroelastic responses of high-speed vessels in irregular sea state is of major interest in naval applications. A three dimensional nonlinear time-domain hydroelastic method in oblique irregular waves is developed, in which the nonlinear hydrostatic restoring force caused by instantaneous wetted surface and slamming force are considered. In order to solve the two technical problems caused by irregular sea state, the time-domain retardation function and Proportional, Integral and Derivative (PID) autopilot model are applied respectively. Besides, segmented model tests of a high-speed trimaran in oblique waves are performed. An oblique wave testing system for trimarans is designed and assembled. The measured results of main hull and cross-decks are analyzed, and the differences in distribution of load responses between trimarans and monohull ships are discussed. Finally, from the comparisons, it is confirmed that the present concept for dealing with nonlinear hydroelastic responses of ships in oblique irregular waves is reliable and accurate.