JONSWAP Wave Spectrum Research Articles

Performance Analysis of a Sea Javelin Wave Energy Converter in Irregular Wave

ABSTRACT Liu, H.; Ao, J.; Chen, H; Liu, M.; Collu, M., and Liu, J., 2018. Performance analysis of a sea javelin wave energy converter in irregular wave. In: Liu, Z.L. and Mi, C. (eds.), Advances in Sustainable Port and Ocean Engineering. Journal of Coastal Research, Special Issue No. 83, pp. 932–940. Coconut Creek (Florida), ISSN 0749-0208. Wave energy, as a kind of eco-friendly energy due to its wide distribution and high energy density, has become a hot research topic at home and abroad and is likely to become a new industry in the future. In this article, we adopt a sea javelin wave energy converter (SJ-WEC) which is a floating vertical axisymmetric buoy. At first, semi-analytical solution of SJ-WEC is studied using eigen-function in frequency domain. Then, wave force and hydrodynamic coefficients of frequency domain can be applied into impulse response function (IRF) method to obtain motion and power of SJ-WEC. In the end, we can select ISSC wave spectrum and JONSWAP wave spectrum to represent the irr...

Fatigue damage mitigation of offshore wind turbines under real wind and wave conditions

Offshore wind turbines (OWTs) subjected to combined wind and wave loadings experience excessive vibrations which will increase fatigue loadings on the structure and reduce the fatigue life. In this paper, a three-dimensional pendulum tuned mass damper (3d-PTMD) is attached to the OWT to mitigate the bi-directional vibrations resulting from wind-wave misalignment so as to increase the fatigue life. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established using the Euler-Lagrangian equation. To predict long-term metocean condition, a statistical analysis for different properties of wind and wave loading such as, wind-wave misalignment, significant wave height and wind velocity is carried out. The aerodynamic wind loading is calculated using the blade element method and the wave loading is computed using the JONSWAP wave spectrum and Morison equation. Dual linear tuned mass dampers (TMDs) deployed in the side-side and fore-aft directions are used for comparison. The NREL monopile 5 MW baseline wind turbine is used to examine the performance of 3d-PTMD in a realistic metocean condition. The fatigue damage is estimated based on the rain-flow cycle counting method and Miner’s rule. Results indicate that the 3d-PTMD can increase the wind turbine tower fatigue life by more than 50% in comparison with the dual TMDs.

Nonlinear Simulation of Focused Wave Group Action on a Truncated Surface-Piercing Structure

In this study, we numerically investigated the nonlinear focused wave group action on a truncated surface-piercing structure, and developed a two-dimensional fully nonlinear numerical tank using the higher-order boundary element method. We determined the amplitude of the wave components of the focused wave group based on the JONSWAP wave spectrum. We discuss the effects of the presence of a surface-piercing structure on the characteristics of the focused wave group and find that the location of the structure does not evidently change the focal location or focal time of the focused wave group. The largest amplitudes of the run-up and horizontal force on the structure occur when the front surface of the structure is at the focal location. The critical draught and breadth of the structure occur when the wave run-up reaches its maximum along the structure.

Feasibility Study of Submerged Floating Tunnels Moored by an Inclined Tendon System

Concepts of submerged floating tunnels (SFTs) for land connection have been continuously suggested and developed by several researchers and institutes. To maintain their predefined positions under various dynamic environmental loading conditions, the submerged floating tunnels should be effectively moored by reasonable mooring systems. With rational mooring systems, the design of SFTs should be confirmed to satisfy the structural safety, fatigue, and operability design criteria related to tunnel motion, internal forces, structural stresses, and the fatigue life of the main structural members. This paper presents a feasibility study of a submerged floating tunnel moored by an inclined tendon system. The basic structural concept was developed based on the concept of conventional cable-stayed bridges to minimize the seabed excavation, penetration, and anchoring work by applying tower-inclined tendon systems instead of conventional tendons with individual seabed anchors. To evaluate the structural performance of the new type of SFT, a hydrodynamic analysis was performed in the time domain using the commercial nonlinear finite element code ABAQUS–AQUA. For the main dynamic environmental loading condition, an irregular wave load was examined. A JONSWAP wave spectrum was used to generate a time-series wave-induced hydrodynamic load considering the specific significant wave height and peak period for predetermined wave conditions. By performing a time-domain hydrodynamic analysis on the submerged floating structure under irregular waves, the motional characteristics, structural stresses, and fatigue damage of the floating tunnel and mooring members were analyzed to evaluate the structural safety and fatigue performance. According to the analytical study, the suggested conceptual model for SFTs shows very good hydrodynamic structural performance. It can be concluded that the concept can be considered as a reasonable structural type of SFT.

Parametric study of catenary mooring system on the dynamic response of the semi-submersible platform

In this work, frequency and time responses analysis of the Amirkabir semi-submersible platform were obtained under the combination of wind and wave loads. The effects of buoy size and stretch of mooring line in dynamics response of semi-submersible platform are investigated. The JONSWAP wave spectrum and API wind spectrum are considered as environmental conditions. In this analysis for simulations three dimensional radiation/diffraction theories and Morison's equation of ANSYS/AQWA software are used. The obtained result shows that increasing stretch of mooring line could reduce motions of the platform, while increase the mooring line tension. Also increase in buoy size increases the amplitude of semi-submersible motion and decreases mooring line tension.

Discrete Near-Optimal Wave Disturbances Rejection Control for Offshore Platforms With Delays in Measurement and Actuator

The wave disturbances rejection control problem for linear discrete jacket-type offshore platforms with delays in measurement and actuator is studied. Taken the delays in measurement and actuator into account, the linear model of the jacket-type offshore platform is established in discrete domain, in which the external wave forces are generated from a designed exosystem based on the JONSWAP wave spectrum and linearized wave theory. A delay-free model from the model of the jacket-type offshore platform with delays in measurement and actuator is formulated based on a designed discrete transformation vector, and the system output with delays in measurement is reformed into a corresponding delay-free form. Then, the discrete near-optimal wave disturbances rejection controller (NOWDRC) is derived from the reformed delay-free system and the corresponding system output in respect of an average quadratic performance index, which is made up of a system state feedback item, a wave disturbances feedforward item and a controller memory item. Finally, simulation results demonstrate that the system output under proposed NOWDRC, including the displacement and velocity of the jacket-type offshore platform, can be offset to smaller values than those under a classic feedback optimal vibration controller, and the delays in measurement and actuator can be compensated effectively.

Spatial Distribution of the Baltic Sea Near-Shore Wave Power Potential along the Coast of Klaipėda, Lithuania

Wave power is an abundant source of energy that can be utilized to produce electricity. Therefore, assessments of wave power resources are being carried out worldwide. An overview of the recent assessments is presented in this paper, revealing the global distribution of these resources. Additionally, a study, which aims to assess the spatial distribution of the Baltic Sea near-shore wave power potential along the coast of Klaipėda (Lithuania), is introduced in this paper. The impacts of the wave propagation direction and decreasing depth on wave power resources were examined using the numerical wind-wave model MIKE 21 NSW. The wave height loss of the design waves propagating to shore was modelled, and the wave power fluxes in the studied depths were calculated using the JONSWAP wave spectrum modified for the Baltic Sea. The results revealed that all waves that propagate to the shore in the Baltic Sea near-shore area along the coast of Klaipėda from 30 m depth to 5 m depth lose at least 30% of their power. Still, most common waves in this area are low, and therefore, they start to lose their power while propagating to the shore at relatively low (10–14 m) depths. To turn this into an advantage the wave power converter would have to work efficiently under low power conditions.

Sea state in the Baltic Sea from space-borne high-resolution synthetic aperture radar imagery

ABSTRACTIn this work, remote sensing synthetic aperture radar (SAR) data from X-band TerraSAR-X and TanDEM-X (TS-X and TD-X) satellites have been used to adopt the algorithms for estimating sea state parameters in the specific condition of the Baltic Sea with archipelago islands and where short steep sea state dominates. Since the moving targets can be defocused and shifted in SAR images, sea state consisting of short windsea waves with strong local orbital velocities and wave breaking needs additional effort for accurate estimation of the total significant wave height that consists of swell and windsea parts. The XWAVE_C algorithm, developed for the North Sea, where the long swell waves coming from the Atlantic Ocean are present during storms, was further enhanced for the short steep windsea which dominates under ordinary storm conditions in the Baltics. For the empirical XWAVE_C model function, based on the spectral analysis of subscenes as well as on local wind information, an additional term was incorporated for assessment the minimal windsea significant wave height by applying JONSWAP wave spectra. A term to compensate spectral distortions triggered by windsea waves moving in SAR flight direction has also been introduced. In total, 95 TS-X/TD-X StripMap scenes between 2012 and 2017 were acquired in Eastern Baltic Sea, processed and analysed. The wave height results from SAR images were compared with collocated in situ data from 11 available buoys. The analysed data include both high and low windsea conditions. The comparison of SAR-derived wave heights with measured wave heights shows high agreement with a correlation coefficient r of 0.88. The wind speed, estimated from SAR images, was compared to measurements from 14 collocated in situ stations, yielding a high agreement with an r value of 0.90. This article is focused on the algorithm developments; however, it is also the first study of sea state retrieval in the Baltic Sea using high-resolution satellite-based techniques. The results show the local variability in the wave fields connected to atmospheric features. The observed local wave height can increase by 1–2 m in kilometre-size cells that are accompanied by wind gusts. The developed algorithms are installed in the German Aerospace Center’s (DLR) ground station Neustrelitz and can also be used in near-real-time.

STABILITAS SAMPAN TERBUAT DARI EMBER CAT BEKAS DENGAN BILGE KEEL PADA SUDUT 30 DAN 45 DERAJAT

Ember cat bekas dapat dimanfaatkan sebagai alternatif pengganti kayu yang semakin sulit diperoleh dan pengganti bahan serat fiber yang tidak murah dalam pembuatan sampan. Namun sampan ini memiliki kekurangan, yakni stabilitasnya yang rendah. Bobotnya yang ringan membuat sampan mudah oleng meskipun hanya beroperasi di perairan yang relatif tenang. Oleh karenanya instalasi sirip peredam oleng (bilge keel) diharapkan dapat meningkatkan stablitas sampan ini. Tujuan penelitian ini adalah untuk mendapatkan nilai stabililitas sampan dengan instalasi bilge keel pada sudut yang berbeda dalam rangka mendapatkan stabilitas sampan yang baik. Ukuran sampan yang dijadikan obyek dalam penelitian ini adalah panjang (LOA) 3,15m; lebar (B) 0,64m; dalam (D) 0,30m. Metode simulasi numerik untuk mendapatkan nilai stabilitas serta beberapa parameter seakeeping (pitching, roling dan heaving) digunakan dalam penelitian ini dengan bantuan perangkat lunak yang sesuai. Untuk faktor gelombang, spektrum gelombang JONSWAP digunakan sebagai inputan dalam simulasi numerik. Hasil analisis menunjukkan bahwa terdapat perbedaan stabilitas yang ditunjukkan oleh nilai lengan GZ (righting arm) pada sampan yang dipasangi bilge keel dengan sudut 30 dan 45 derajat sebesar 0,001 m. Bilge keel dengan sudut 30 derajat memberikan nilai stabilitas yang lebih besar dibandingkan dengan sudut 45 derajat. Dari analisis response amplitude operator (RAO) menunjukkan bahwa dengan inputan spektrum gelombang JONSWAP, gerakan rolling, pithcing dan heaving sampan masih berada pada rentang normal

Irregular Wave Forces on a Large Vertical Circular Cylinder

Substructures of offshore wind turbines are exposed to irregular sea states that are generally defined using wave spectra. The waves undergo transformations due to non-linear wave-wave interactions and due to interaction with the structures they are incident on. In the present study, simulations of regular and irregular wave interaction with a vertical cylinder are carried out using the open- source Computational Fluid Dynamics (CFD) model REEF3D. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations over the entire domain and provides detailed information regarding the wave hydrodynamics including fluid pressure, velocities and the free surface. The non-linear wave-wave, wave-structure interactions and the turbulence in the flow are accounted for in the solution of the RANS equations. In this way, detailed flow features around the cylinders can be visualised and analysed. The numerical model is verified with the analytical equations for the loads on a cylinder under regular waves. Further, simulations are carried out for irregular waves generated using the JONSWAP wave spectrum and the wave force spectrum is calculated. The wave spectrum for the different wave gauges around the cylinder are compared. The free surface features around the cylinder are visualised and correlated to the wave forces on the cylinder. It is observed that the regular waves with higher steepness show a clear diffraction pattern around the cylinder. For the irregular waves, the diffraction pattern is less developed and irregular.

Design of non-linear controller for ocean wave energy plant

This paper proposes nonlinear control strategies for oscillating water column (OWC) wave power-take-off (PTO) systems. The system consists of Wells turbine, doubly fed induction generator (DFIG) and irregular wave models. This study considers a separately excited DC motor for emulating the Wells turbine. Plant emulation needs advanced control algorithms so that it reflects the practical behavior of actual plant under varying characteristics of ocean waves. Therefore, three non-linear controllers (NLC-1, 2 and 3) for OWC plant emulation and DFIG control have been designed using the Lyapunov stability theorem. The Pierson-Moskowitz and JONSWAP irregular wave models which represent realistic sea wave conditions have been applied for validating the proposed controllers. The OWC plant parameters such as Wells turbine flow coefficient, turbine torque and electrical output power has been compared between uncontrolled and controlled cases. The proposed rotational speed controller NLC-2 provides enhanced mean output power against uncontrolled operation of plant. The enhancement in output power is obtained as 25.14% and 39.24% for JONSWAP and Pierson-Moskowitz wave spectrum.

Beyond significant wave height: A new approach for validating spectral wave models

Wave data are required in many engineering applications. At locations where measured records are not available or are too short for design purposes, estimates of wave properties from numerical wave models are often used to characterise the expected wave climate. Typically, model predictions are validated against observations of the sea-state parameters, such as significant wave height, peak wave period, mean wave period and mean wave direction. However, while agreement between observed and predicted sea-state parameters can be good, in some cases the measured and predicted wave spectra can diverge significantly. In these circumstances, simple sea state parameters alone are not sufficient to describe the range of wave conditions that could arise at a given site. In this paper we present a new, alternative approach for assessing wave model performance by applying new parameterisation to the frequency wave spectrum. Seven parameters (significant wave height, peak frequency, peak energy density, squared Euclidean distance, skewness, kurtosis and mean width deviation) are used to better define the characteristics of unimodal wave spectra. Sensitivity tests are undertaken to analyse the performance and sensitivity of these parameters in identifying differences between observed and predicted wave spectra, using a range of idealised JONSWAP wave spectra. We demonstrate that comparing multiple parameters is a better method to distinguish differences between spectra than the results obtained using individual parameters in isolation. As such, application of two-dimensional validation matrices are proposed to provide a better, qualitative overview of the goodness of fit between observed and predicted wave spectra. The advantages of the new approach are demonstrated through validation of a hindcast spectral wave model at Hastings, southeast England. We believe that we have achieved our purpose here to start a discussion on alternative validation techniques that could be enhanced in the future.

Coupled hydrodynamic and geotechnical analysis of jacket offshore wind turbine

This paper compares the response of a jacket-supported offshore wind turbine (OWT) under wave loading, when (a) soil–structure interaction (SSI) is ignored and (b) SSI is considered. The jacket is placed in a water depth of 70m and soil conditions off the west coast of India are used in the study. The rotor of the OWT is considered to be parked, under a survival average wind speed of 50m/s, significant waver height Hs=16m and peak spectral period Tp=18s. The significance of includng SSI in OWT studies is investigated by means of pushover analyses and irregular-wave dynamic analyses. Modal studies are performed to determine the variation in the free-vibration response of the two cases. It is observed that ignoring SSI tends to over-estimate the ultimate strength characteristics of the OWT by 3–60% in various modes or increase the tower top displacement above serviceable limit. For dynamics analysis, the wave elevation is generated using wave superposition method. The JONSWAP wave spectrum is discretized using constant area method which introduces additional uncertainty. The analysis shows that approximately 200 frequencies are necessary using constant area method to capture the tail region appropriately. Also the statistical uncertainty in the generation of wave elevation for dynamic analyses is quantified by means of sample convergence studies. The results show that approximately 20–40 samples are required in order to obtain reasonable statistics.

Near-shore wave induced setup along Kalpakkam coast during an extreme cyclone event in the Bay of Bengal

An enhanced sea level during an extreme weather event can result from cumulative effects of reduced sea-level pressure, storm surge, tidal effects and wave induced setup. A numerical experiment was conducted to evaluate sensitivity of model resolution and bottom slope on wave induced setup for an arbitrary coast. Further, an assessment of wave setup at coastal Kalpakkam was investigated, in response to passage of a Category-4 cyclone ‘NARGIS’, in the central Bay of Bengal. The sensitivity experiments cover an arbitrary region of 5km×5km, with varying horizontal resolutions and beach slopes, and subjected to uniform wind forcing and JONSWAP wave spectra prescribed at the open boundary. The computed wave setup using SWAN model reveals that model resolution is not sensitive for gentle bottom slopes, unlike the case with steep bottom slopes. Findings from sensitivity experiments were then applied to evaluate wave setup at three different locations along Kalpakkam coast, for the Cyclone NARGIS event. The study shows that wave setup was highest for a steep slope (1:20), reducing to almost 50% for gentle slopes (1:80). It is inferred that wave setup is an important parameter that needs to be incorporated in an operational storm surge forecasting system.

울진해역의 Freak wave 특성과 스펙트럼 근사에 대한 연구

This paper investigates the statistical properties of waves in the sea area of Uljin, which is located in the East Sea area of Korea. The wave data were measured using AWAC (Acoustic Wave and Current Meter), which was installed at a 16-m water depth from November 2010 to March 2011. The wave data acquisition rate, Hmax, monthly mean Hs, Tz, Tp, and wave direction are summarized. The distributions of Hs and Tz were analyzed using the Hs-Tz scatter diagrams. The measurement wave data were analyzed to investigate freak wave characteristics. By comparing the wave spectrum using the measurement wave data with the wave spectrum obtained by varying the JONSWAP wave spectrum, it was possible to approximate the wave spectrum shape at the Uljin Sea area.

Tuned liquid column dampers in offshore wind turbines for structural control

With offshore wind turbines becoming larger, being moved out further at sea and subjected to ever greater wind and wave forces, it is necessary to analyse the dynamics and minimise the responses of these structures. In this paper, the structural responses of offshore wind turbines are simulated with an attached damper (Tuned Liquid Column Damper (TLCD)) for controlling the vibrations induced within the structure. This requires a realistic simulation of the forces that these tall, flexible and slender structures are subjected to, and consequently the implementation of a damper to control the resulting undesirable vibrations that are induced within the structure. Since sea waves are caused by wind blowing for a sufficiently long time, the state of the sea is related to wind parameters and there exists the possibility of correlating wind and wave loading conditions on structures. The Kaimal spectrum for wind loading is combined with the JONSWAP wave spectrum to formulate correlated wind and wave loadings. The offshore turbine tower is modelled as a Multi-Degree-of-Freedom (MDOF) structure. Cases for flat sea conditions, with which parallels to onshore wind turbines may be drawn, are first simulated. Simulations are presented for the MDOF structure subjected to both ‘moderate’ and ‘strong’ wind and wave loadings. Cases of the blades lumped at the nacelle along with rotating blades are investigated. The reduction in bending moments and structural displacement response with TLCDs for each case are examined. A fatigue analysis is carried out and the implementation of TLCDs is seen to enhance the fatigue life of the structure. An analysis, taking into account the extended fatigue life and reduced bending moments on the structure-TLCD system, is presented.

통계해석법에 의한 폰툰식 VLFS의 피로강도해석

The fatigue strength analysis of VLFS is carried out by using a 3-dimensional plate finite element model with a zooming technology which performs the modeling of wide portions of the structure by a coarse mesh but the concerned parts by a very fine mesh of t by t level. And a stepwise substructure modeling technique for global loading conditions is applied which uses the motion response of the global structure from 2-D plate hydroelastic analysis as the enforcing nodal displacements of the concern 3-D structural zooming model. Seven incident wave angles and whole ranges of frequency domains of wave spectrum are considered. In order to consider the effect of breakwater, the modified JONSWAP wave spectrum is used. Applying the wave data of installation region, the longterm spectrum analysis is done based on stochastic process and the fatigue life of the structure is estimated. Finally some design considerations from the view point of fatigue strength analysis of VLFS are discussed.

The SWAN model used to study wave evolution in a flume

The SWAN numerical model is used to model the evolution of JONSWAP wave spectra and hence the significant wave height of waves in a tank. Comparison with experiment has shown that modelling triad interactions in the numerical model leads to too low predictions of spectra and significant wave height and should therefore be excluded. The modelling of the breaking constant was also investigated, by looking at the use of a constant breaking constant, Nelson formula, and Goda formula (added into SWAN for this study). Using a constant value of 0.78 within SWAN gave the best comparison between theory and experiment.

Feedback-feedforward control of offshore platforms under random waves

This study investigates the control of jacket-type offshore platforms. The deck displacement of jacket-type offshore platforms can be controlled using both passive and active control mechanisms. Among the passive control mechanisms, a tuned mass damper concept is studied in this paper. Active control mechanisms considered here include the active mass damper, the active tendon mechanism and the propeller thruster. An optimal frequency domain approach to active control of wave-excited platforms is used in which the H2 norm of the transfer function from the external disturbance to the regulated output is minimized. In this study, the hydrodynamic drag force is evaluated using the JONSWAP wave spectrum. Unlike conventional linearization approaches, the influence of non-linearity in the drag force is retained in this scheme by expressing the non-linear force components in terms of higher-order convolutions of the water-particle velocities. To demonstrate the effectiveness of this scheme, the platform performance with and without control devices under different sea states is evaluated. It is demonstrated that the control devices are useful in reducing the displacement response of jacket-type offshore platforms, especially when the wave forces are concentrated at frequencies close to the natural frequencies of the platform. This becomes especially significant in deep waters because the natural frequencies of jacket-type platforms fall closer to the dominant wave frequencies in deep waters. Adding control devices to deep water platforms will ensure a reduction both in the global response of the platform and in localized effects, such as the fatigue of welded joints. Copyright © 2001 John Wiley & Sons, Ltd.

Use of springs to simulate the wind induced moments on turret moored vessels

This paper presents the results of an experimental study investigating the behaviour of a system of springs used to simulate wind-induced loading effects on turret moored vessels. These vessel types are extensively used offshore for hydrocarbon storage and production with model testing commonly performed at the design stage to gauge the influence of the environment on the vessel motion responses. The performance of the spring system is evaluated using a model vessel in a square complex wave basin over a range of long crested sea states represented by JONSWAP wave spectra and for two turret positions along the vessel longitudinal centre line. The measurements are compared with results from tests using conventional wind fans. It is shown that the spring design may be used as a mechanism during preliminary model testing to simulate the influence of wind on the yawing.