Sea State Conditions Research Articles

Characterisation of the expected weather conditions in the main European coastal traffic routes

The maritime traffic in the European seas is analysed in order to characterize the different areas in terms of traffic density and average navigational distance from the coast, assessing the relative weight of each coastal area with respect to the total European seaborne trades. The main coastal routes are also detected by highlighting the tracks that present a higher density of geo-located ship reports. The state-of-the-art ERA-interim weather database is used to compute the weighted average characteristics of the sea-state conditions in those routes, providing a meaningful reference for the waves that ships are asked to face in the coastal areas, where the occurrence of accidents is higher and further effort is required to minimize the risk at sea.

Statistical analyses of sea state conditions in South China Sea

The statistical characterization of sea conditions in the South China Sea (SCS) was investigated by analyzing a 30-year (1976–2005) numerically simulated daily wave height and wind speed data. The monthly variation of these parameters shows that wave height and wind speed have minimum values of 0.54 m and 4.15 m s−1, respectively in May and peak values of 2.04 m and 8.12 m s−1, respectively in December. Statistical analysis of the daily wave height and wind speed and the subsequent characterization of the annual, seasonal and monthly mean sea state based on these parameters were also done. Results showed that, in general, the slight sea state prevails in the SCS and has nearly the highest occurrence in all seasons and months. The moderate sea condition prevails in the winter months of December and January while the smooth (wavelets) sea state prevails in May. Furthermore, spatial variation of sea states showed that calm and smooth sea conditions have high occurrences (25%–80%) in the southern SCS. The slight sea condition shows the largest occurrence (25%–55%) over most parts of the SCS. High occurrences (8%–17%) of the rough and very rough seas distribute over some regions in the central SCS. Sea states from high to phenomenal conditions show rare occurrence (<12%) in the northern SCS. The calm (glassy) sea condition shows no occurrence in the SCS.

Analysis of the performance of pontoon-type floating breakwaters

ABSTRACTThe transmission coefficient of floating breakwater (FB) is the most important index for gauging their efficiency. It depends on sea state conditions such as incident wave height, wave period, water depth and structural parameters such as the draught and width of the FB. In this study, dimensionless parameters are developed to predict the value of the transmission coefficient. Since the wave excites the motion responses of the FB, it acts like a wave generator. In this paper, dimensionless parameters that are related to the incident wave height and wave length to predict the sway and heave motion of the FB are proposed. The study is based on a total of 167 data extracted from experiments carried out by various researchers.

First order sea clutter cross section for bistatic shipborne HFSWR

This paper studies the development on the first order sea clutter cross section for bistatic high frequency surface wave radar (HFSWR). Based on the received first order electric field expression, a closed-form of cross sections is derived to account for the case of receiving antenna array being mounted on the shipborne platform. The uniform linear motion and sway motion components are assumed to be responsible for the observed differences in comparison with the bistatic fixed antenna case. Correspondingly, simulations are conducted to study the sea clutter spectral characteristics for these two cases versus different system parameters and sea state conditions. It is shown numerically that the forward motion component will spread the Bragg lines severely and the influence triggered by the sway motion can be explained as the Bessel function modulation of the ordinary sea clutter spectra. The obtained results have important implications in the application of shipborne HFSWR technology to ocean remote sensing and target detection.

Wave Load Analysis of the Corvette Ship in the Sea Water of Indonesia

Wave load prediction of ship should be considered in the design stage of ship’s construction. The excessive of wave load might cause a structural failure of ship during operational in seaway. The bending moment might be experienced on the ship structure that it contributes to stress concentration on the particular part of construction. This paper describes the prediction of bending moment and shear forces of the warship corvette type considered to the sea condition of Indonesia. The 3D diffraction theory was adopted to analyze ship’s motion responses, bending moments, and shear forces numerically. In this numerical simulation, the variations of speed and heading angle of ship were performed with respect to environmental condition of sea state 4, 5, 6 and 7. The simulation results had shown that the maximum shear force and bending moment was occurred on the area of mid-ship.

Nonlinear computational methods for hydroelastic effects of ships in extreme seas

We present computational methods to assess slamming-induced hull whipping on sectional loads of ships in regular and irregular waves. The numerical methods solved the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the nonlinear rigid body motion equations of the elastic ship hull. We numerically investigated three containerships in regular waves, in random irregular long crested waves, and in deterministic wave sequences. Comparisons to experimental measurements agreed favorably. We relied on different wave models, including second order Stokes waves and nonlinear wave fields obtained from the solution of nonlinear Schrodinger equations (NLS). Simulations in random irregular waves provided short-term ship response probability distributions under sea state conditions relevant for design loads.

Experimental observations of active blade pitch and generator control influence on floating wind turbine response

In this paper, the influence of wind turbine blade pitch and generator controls on the global response of a floating wind turbine is investigated. Several different active turbine controllers are considered and the resulting floating wind turbine global response is compared with that for a baseline configuration employing a fixed blade pitch and a fixed rotor speed. Results from platform pitch free-decay tests as well as a simultaneous dynamic wind and irregular sea state condition are used to understand the controllers' influence on floating wind turbine dynamic behavior.

Evaluation of the wave energy resources in the Cape Verde Islands

The Cape Verde Islands form an archipelago off the African coast in the Atlantic Ocean. Since it is highly dependent on fossil fuels, Cape Verde decision makers have started to take into account also the potential of renewable energies, especially wind and solar. In particular, wind power has already 26 MW installed. From this perspective, the present work aims to be a first step in the evaluation of a different source of renewable energy, the wave energy. Using reanalysis data from ECMWF, the SWAN model was run for a 10-year period, covering the time interval 2004–2013, using a methodology already implemented in other island environments. Moreover, three years of this high resolution data are compared with the available altimetry data. In this way, a dataset of the sea state conditions around Cape Verde Islands was produced. This dataset is further used for wave climate analyses and wave energy resource assessments. This study indicates that the coastal environment of the Cape Verde Islands, and especially some particular areas, present considerable wave energy resources that should be taken in consideration for extraction in the near future.

Effect of scouring in sand on monopile-supported offshore wind turbines

ABSTRACTThis paper analyzes the influence of scour on the overall response of monopile-supported offshore wind turbines (OWTs) in 20-m water depth. Scouring effects on OWTs have been often studied within the geotechnical domain, considering static loads at the mudline. The present work attempts to address the scour-induced problems in OWTs by making use of an integrated aerodynamic–hydrodynamic load approach in sandy soils. The OWT analysis is simulated for operational and shut-down (parked) condition. Under parked situations, the OWT blades are feathered, and power production is suspended, owing to structural safety concerns. The 50 Monte Carlo responses of stochastic sea-state condition (wind speed with turbulence, significant wave height, and peak spectral period) are generated. Irregular, long-crested waves are generated using the Joint North Sea Wave Project (JONSWAP) spectrum. Then from each simulation, the ensemble response is obtained. Sandy soils of varying densities are considered. Results indicate that OWTs founded on loose sands suffer significant stiffness (and hence natural frequency) reductions, shifting the structure into the resonance regime. Lateral responses also show an escalation with reduction in density of sandy soil.

A study on the influence of ageing of coatings on their mechanical properties and fracture in ballast tanks

ABSTRACTInspections have shown that coatings used in water ballast tanks (WBTs) can fracture prior to their target design life. This has led to local corrosion of the substrate and unwanted maintenance which is time-consuming and costly for the shipowner. The objective of the current study is to show that ageing of marine coatings used in WBTs can reduce the coatings’ fracture strain to a level lower than the strain of the substrate. Laboratory tests were conducted to subject three commercial WBT coatings to different types of ageing. Changes in their mechanical properties were confirmed. Finite element analyses of a 4400 twenty-foot equivalent unit container vessel were carried out for a number of sea state conditions. The strains of the hull structure (i.e. the substrate) were calculated and compared with the fracture strains of the aged coatings. It was shown that, in many locations, the strain of the substrate can exceed the fracture strain of aged coatings. Finally, systematic study with proactive coating condition performance monitoring is discussed.

Tracking Breather Dynamics in Irregular Sea State Conditions.

Breather solutions of the nonlinear Schrödinger equation (NLSE) are known to be considered as backbone models for extreme events in the ocean as well as in Kerr media. These exact deterministic rogue wave (RW) prototypes on a regular background describe a wide range of modulation instability configurations. Alternatively, oceanic or electromagnetic wave fields can be of chaotic nature and it is known that RWs may develop in such conditions as well. We report an experimental study confirming that extreme localizations in an irregular oceanic Joint North Sea Wave Project wave field can be tracked back to originate from exact NLSE breather solutions, such as the Peregrine breather. Numerical NLSE as well as modified NLSE simulations are both in good agreement with laboratory experiments and highlight the significance of universal weakly nonlinear evolution equations in the emergence as well as prediction of extreme events in nonlinear dispersive media.

Dependency of the Sentinel-1 azimuth wavelength cut-off on significant wave height and wind speed

ABSTRACTSea wind and sea state estimation by synthetic aperture radar (SAR) measurements is a topic of relevance both on the scientific and user side. The new European Space Agency (ESA) Sentinel-1 constellation is meant to support marine studies and ensure high-quality data. In this paper, we investigate the azimuth cut-off () sea wind speed and significant wave height retrieval approach by taking benefit of two sets of multi-look SAR images with incidence angles varying from 20° to 45°. The images have been co-located with sea surface wind measurements acquired by the scatterometer onboard the Chinese satellite HY-2A (HSCAT) and with the European Centre for Medium Range Weather Forecast (ECMWF) operational model output. This study is meant to analyse both the empirical dependency of SAR on significant wave height () and wind speed (U). Several fitting geophysical model functions (-GMFs) are proposed and discussed. The results show that is strongly correlated with the significant wave height in all sea state conditions, while the correlation with the wind speed is only high for fully developed sea states. The azimuth cut-off based significant wave height retrievals are compared with independent National Data Buoy Centre (NDBC) network observations, showing a root mean square difference of about 0.5 m.

Tailoring the Employment of Offshore Wind Turbine Support Structure Load Mitigation Controllers

The currently available control concepts to mitigate aerodynamic and hydrodynamic induced support structure loads reduce either fore-aft or side-to-side damage under certain operational conditions. The load reduction is achieved together with an increase in loads in other components of the turbine e.g. pitch actuators or drive train, increasing the risk of unscheduled maintenance. The main objective of this paper is to demonstrate a methodology for reduction of support structure damage equivalent loads (DEL) in fore-aft and side-to-side directions using already available control concepts. A multi-objective optimization problem is formulated to minimize the DELs, while limiting the collateral effects of the control algorithms for load reduction. The optimization gives trigger values of sea state condition for the activation or deactivation of certain control concepts. As a result, by accepting the consumption of a small fraction of the load reserve in the design load envelope of other turbine components, a considerable reduction of the support structure loads is facilitated.

The effects of roll motion of the floating platform on hydrodynamics performance of horizontal-axis tidal current turbine

Under the condition of actual sea state, hydrodynamic characteristics of floating horizontal-axis turbine are related to wave characteristics and floating carrier motion responses. We recently published the hydrodynamic performance and axial damping coefficient of the horizontal-axis tidal current turbine influenced by surge motion in constant inflow using CFD simulation (Zhang et al. Renew Energy 74:796–802, 2015). Encouraged by this result, this paper uses sliding mesh to analyze the hydrodynamic characteristics in uniform stream when the turbine is forced to roll and studies influences of different roll frequency, roll amplitude, and tip speed ratio on turbine’s performance. Roll-damping coefficient and added mass coefficient can be derived by torque almanacs curve of rolling turbine by the least square method. Results show that the turbine axial load, roll moment, and energy utilization ratio will fluctuate in roll motion; the more roll frequency and roll amplitude, and the more load and moment wave amplitude of momentary value. The crest value occurs in the balance position of rolling, while the amplitude of oscillation depends on the angular speed of rolling and rotating speed of the turbine. The frequency and amplitude of the roll have little impact on damping coefficient, but rotational speed of the turbine has positive impact on this coefficient. Results of this study can provide data to study motion response of floating carrier for floating tidal current turbine system and check the structural design and control of the electric output.

Integrity Assessment of Offshore Subsea Wells: Evaluation of Wellhead Finite Element Model Against Monitoring Data Using Different Soil Models

Understanding well conductor–soil interaction mechanism plays an important role in well integrity assessment. Evaluation of field data obtained from monitored offshore wells can provide valuable insights into the matter. This paper presents a comparison of the numerical results obtained from a full three-dimensional (3D) finite element (FE) analyses model of the blowout preventer (BOP), wellhead (WH), conductor, and surface casing versus the field measured data obtained during drilling operations. Sensitivity studies were also performed for several parameters that were considered important in the local well response analysis under observed sea state conditions. The conductor–soil analyses were simulated using the Winkler spring p–y curves obtained by two different approaches: American Petroleum Institute (API) RP2GEO and a recently developed model by Zakeri et al. The results of bending moments in conductor and surface casing have indicated good agreement between FE model results and the field measurements.

Development of a ship weather routing system

A ship route optimization system aimed at facing the challenges of a demanding maritime trade is presented. The paper describes the complete on-board ship weather routing system highlighting the last updates, which are aimed to give it an operational perspective. Ship responses are modelled for any sea-state condition, taking into account the wave directional spreading. The functionalities of a robust multi-objective evolutionary algorithm are exploited to approximate the most favourable set of solutions, successively ranked according to the ship's requirements. The code modularity allows a great flexibility in the treatment of goals and objectives as well as in the input selection.

Variability of breaking wave characteristics and impact loads on offshore wind turbines supported by monopiles

Most existing and planned offshore wind turbines (OWTs) are located in shallow water where the possibility of breaking waves impacting the structure may influence design. Breaking waves and their associated impact loads are challenging to model because the breaking process is a strongly non-linear phenomenon with significant statistical scattering. Given the challenges and uncertainty in modeling breaking waves, there is a need for comparing existing models with simultaneous environmental and structural measurements taken from utility-scale OWTs exposed to breaking waves. Overall, such measurements are lacking; however, one exception is the Offshore Wind Turbines at Exposed Sites project, which recorded sea state conditions and associated structural loads for a 2.0 MW OWT supported by a monopile and located at the Blyth wind farm off the coast of England. Measurements were recorded over a 17 month campaign between 2001 and 2003, a period that included a storm that exposed the instrumented OWT to dozens of breaking waves. This paper uses the measurements from this campaign to categorize and identify breaking waves and quantify the variability of their impact loads. For this particular site and turbine, the distribution of measured mudline moments due to breaking waves has a mean of 8.7 MN-m, a coefficient of variation of 26% and a maximum of 14.9 MN-m. The accuracy of several breaking wave limits and impact force models is compared with the measurements, and the impact force models are shown to represent the measurements with varying accuracy and to be sensitive to modeling assumptions. Copyright © 2015 John Wiley & Sons, Ltd.

The new Euskalmet coastal–maritime warning system

Abstract. This work presents the main characteristics of the Basque Meteorology Agency (Euskalmet) maritime–coastal risk warning system, with special emphasis on the latest updates, including a clear differentiation on specific warning messages addressing sea conditions for navigation purposes in the first 2 nautical miles, and expected coastal impacts. Some details of the warning bulletin for maritime and coastal risk situations are also presented, together with other communication products and strategies used in coastal and maritime severe episodes at the Basque coast. Today, three different aspects are included in the coastal–maritime risk warning system in Basque Country, related to the main potential severe events that affecting coastal activities. – "Galerna" risk relates to a sudden wind reversal that can severely affect coastal navigation and recreational activities. – "Navigation" risk relates to severe sea state conditions for 0–2 miles, affecting different navigation activities. – "Coastal impact" risk relates to adverse wave characteristics and tidal surges that induce flooding events and different impacts in littoral areas.

Correlation of Wavefield‐Separated Ocean‐Generated Microseisms with North Atlantic Source Regions

Abstract Ocean‐generated microseisms have the potential to be used as a proxy for ocean wave parameters. However, they are often comprised of contributions from multiple, coincidently active source regions. Using a seismic array, it should be possible to separate microseism data consisting of contributions from one or more source areas on the seafloor based on the wavenumber of the signals. Here we investigate the use of frequency–wavenumber filtering on data between 0.1 and 0.25 Hz, corresponding to double‐frequency microseisms, recorded at the Eskdalemuir seismic array in Scotland. As the array is intended for shorter seismic wavelengths than those that occur at microseism frequencies, synthetic and scaled field experiments are performed to establish that the array geometry is suitable. Application is then made to microseism data from the array. Through a comparison with WAVEWATCH3 numerical ocean wave data, we demonstrate that the frequency–wavenumber ( f ‐ k ) filter enables the separation of microseisms into its different geographical source components. This separated microseism data give proxy access to local sea state conditions that are masked in the total microseism wavefield.

Sea state conditions for marine structures' analysis and model tests

The study reviews, based on the state-of-the-art findings, some uncertainties associated with wave data and models currently used in design and operation procedures of ship and offshore structures. Although the same basic principles prevail for hydrodynamic loads on ships and offshore structures, actual problems and methods for assessing these loads in the design and operation stage are not the same. Different wave data and models are used for specifying design and operational criteria for these two types of platforms and different uncertainties are related to them. Wave data and models used to define sea state characteristics are discussed and particular attention is given to the associated sources of uncertainties. Some weaknesses of wave input used in design and operational procedures for marine structures are pointed out. Focus is also given on uncertainties related to model tests as tank testing is an important supporting tool for design and operation. Impact of some selected uncertainties on wave description and wave loads is demonstrated by examples.