Sea State Conditions Research Articles

Genetic algorithms to determine JONSWAP spectra parameters

The genetic algorithm (GA) model presented here provides specific JONSWAP parameters that can be used for wave modelling. This work describes a validated heuristic model based on GA, to select JONSWAP spectra parameters, regardless of water depth restrictions and sea state conditions. The identification of the JONSWAP spectra parameters is difficult, as the alpha and gamma coefficients have scattered distributions that modulate the spectral peak energy. In addition, the selection of alpha and gamma coefficients from in situ free surface records may be difficult and time-consuming, because of the amount of data and nonlinearities involved. The proposed model uses either in situ or numerically modelled wave data and has three main steps: (1) generation and crossover, (2) minimisation of the cost function ΔHs, defined as the minimum difference between the calculated artificial significant wave height and the in situ wave height (instrumented or modelled), and (3) mutation and natural selection. To apply the model, in situ wave data measured by an acoustic Doppler current profiler over 5.5 months was used in this research. The results show a high correlation (r2), of 0.95, between the best fitted curves of modelled spectra and measured data.

Wave Excitation Force Estimation Using an Electrical-Based Extended Kalman Filter for Point Absorber Wave Energy Converters

Accurate real-time knowledge of the wave excitation force affecting a wave energy converter (WEC) – either through measurement or by estimation – is crucial for implementing effective control strategies that ensure optimum power absorption, system reliability, and durability. The estimation of the excitation force using other readily available measurements is deemed a cost-effective solution given the technical difficulties associated with directly measuring the excitation force on the WEC’s floater hull. In this study, an electrical-based extended Kalman filter (E-EKF) estimator for estimating the wave excitation force, floater’s heave displacement, and velocity is proposed. The estimator is derived using a holistic nonlinear wave-to-wire model of a direct-drive heaving WEC. A continuous and differentiable approximation of the well-known Tustin friction model is utilized to incorporate the friction force model into the estimator. The proposed E-EKF estimator requires only the measurement of the three-phase permanent magnet linear generator stator currents using current transducers. A practical approach is provided to overcome the need for measuring the wave surface elevation and velocity. Simulations are conducted to assess the goodness of the proposed E-EKF under various sea-state conditions, modeling mismatches, and electric loading scenarios. For the sake of comparison, the performance of the E-EKF estimator is measured against mechanical-based extended Kalman filter and linearized mechanical Kalman filter estimators. The E-EKF estimator exhibits superior performance in terms of nearly all performance metrics, with an excitation energy percentage error score not exceeding 9 %, while being immune to measurement noise.

Estimation of the Significant Wave Height from Marine Radar Images without External Reference

In the context of the sea state monitoring by means of the X-band marine radar, the estimation of a significant wave height ( H s ) is, currently, one of the most challenging tasks. For its estimation, a calibration is usually required using an external reference, such as in situ sensors, and mainly buoys. In this paper, a method that allows us to avoid the need for an external reference for H s estimation is presented. This strategy is, mainly, based on the correlation between a raw radar image and the corresponding non-calibrated wave elevation image to which varying its amplitude by using a scale factor creates a mathematical model for the radar imaging. The proposed strategy has been validated by considering a simulated waves field, generated at varying sea state conditions. The results show a good estimation of the significant wave height, confirmed by a squared correlation coefficient greater than 0.70 for each considered sea state.

Global strength analysis for a split hopper barge subject to head equivalent design wave

Due to the Classification Societies’ regulations, in the preliminary design stage for new ship constructions it is necessary to prove with calculations that the stress and deformation distributions in the ship structure do not exceed the stress limit values, regardless the sea state such as calm water, sagging or hogging wave conditions. Normally, it is considered that the hogging and sagging conditions are the worst head wave conditions, when the vertical shear forces and bending moments can reach high values and the risk of ship structure collapse can be high. The main purpose of this study is to assess, using the Finite Element Method (FEM), the global strength for a split hopper barge in three sea state conditions: calm water, hogging and sagging head design waves.

An experimental study on the hydraulic stability of Icelandic-Type berm breakwaters

One of the key aspects of breakwater design and safety assessment is their stability evaluation. In the present paper, hydraulic stability of a multi-layer berm breakwater (MLBB) has been studied through varying the parameter of eroded area. To this end, a 2-D experimental model has been carried out in the wave flume of Tarbiat Modares University using irregular waves with a JONSWAP spectrum. A total of 150 tests have been performed to cover a wide range of sea state conditions and geometrical parameters such as wave height, wave period, water depth, berm width, berm elevation, stone class I height, front slope and nominal diameter of stone class I. In this study, a new dimensionless parameter is introduced to indicate that the wave breaking type plays a fundamental role in stability of MLBBs. Eventually; a new formula is derived to estimate the damage of MLBBs, based on different sea state conditions and various geometrical parameters. Comparisons are made between the estimated damage parameter of MLBBs by the proposed formula and those estimated by existing formulae. The results denote that the present formula not only performs well for MLBBs experimental data, but also indicates a reliable performance for some of Lykke Andersen's (2006) data in statically homogenous berm breakwaters (HBBs) outside the range of given formula. Furthermore, an innovative approach is presented to examine the eroded area based on the berm recession and the intersection point of reshaped and initial profile from SWL, which performs faultlessly rather than the existing one.

Abundance, distribution and diet of the common dolphin,Delphinus delphis, in the northern Aegean Sea (Greece)

AbstractThe abundance, distribution and diet of the short‐beaked common dolphin were investigated as part of the first detailed study on cetacean populations in the North Aegean Sea. Since 2004, the area has been proposed by national and international entities as a marine protected area for common dolphins and other cetacean species owing to its high biodiversity.Abundance and distribution were investigated between 2005 and 2013 through dedicated scientific marine transect surveys, covering 14,701 km, in sea conditions of Beaufort sea state 3 and below. The line‐transect sampling method was used to estimate relative abundance, using Distance 6.0 software. Encounter rate for this species in the study area was estimated to be 0.24 groups/100 km (1.5 dolphins/100 km), with a mean group size of 6.88 (SE = 1.90).Common dolphin sightings were analysed for correlations with eight environmental variables (distance from the coast, depth, slope, median salinity, gradient of salinity, median temperature, gradient of temperature and mean current) using generalized additive modelling. Common dolphin sightings were significantly correlated to depth, temperature and salinity.Stomach‐content analyses were performed on eight suitable samples from common dolphins stranded in the study area. The trophic level of the species was calculated, indicating that this species is a top predator.This research provides the first estimates for abundance and distribution and the first diet analysis for this Mediterranean Endangered dolphin species in the study area based on a year‐round long‐term study.

#Portichiusi: The Human Costs of Migrant Deterrence in the Mediterranean

Using daily data on forced migration from the IOM and Frontex, I compare trends in flows and mortality across three major migration routes in the Mediterranean, analysing the effects of the introduction of rescue-deterrence policies in Italy. Controlling for exogenous shocks which affect push and pull factors in mobility, along with sea state conditions and route-day fixed effects, I find that the reduction in refugee migration flows in the Central Mediterranean has been modest, at best. At the same time, these policies have generated a permanent increase in daily mortality rates in the Central Mediterranean, having grown by more than 4 deaths per day. Finally, I investigate whether variations in mortality are sufficient to offset migration flows. Increases in mortality rates, however, are only accompanied by a short-term negative displacement effect, as migration attempts are delayed by increases in absolute mortality, rather than being prevented.

Nonlinear control algorithms for efficiency-improved course keeping of large tankers under heavy sea state conditions

In order to solve the problem of more energy-saving and safer course keeping control for large oil tanker under heavy sea state conditions, a hyperbolic tangent function was used as a new nonlinear feedback function to re-examine the nonlinear feedback in this paper. The new control algorithm was theoretically proved by describing function, and the stability analysis of the system was carried out through Nyquist curve. Taking "YUKUN" and "New Triumph" oil tanker as examples, systematic comparative simulations of hyperbolic tangent function and sine function were carried out. It was found that sine function was better than the hyperbolic tangent function for the course keeping control of general ship under normal sea state conditions. However, for large oil tanker under heavy sea state conditions, the hyperbolic tangent function could achieve almost the same control effect as sine function while the maximum rudder angle decreased by 39% and the average rudder angle decreased by 36%. Therefore, from the numerical investigations, the nonlinear algorithm modified by hyperbolic tangent has obvious advantages in the course keeping control of large oil tanker under heavy sea state conditions.

Motion Fidelity Requirements for Helicopter-Ship Operations in Maritime Rotorcraft Flight Simulators

The research presented in this paper is part of a longer-term project to develop overall fidelity requirements for simulated helicopter shipboard operations to inform and support first-of-class flight trials. The paper reports the results of motion cueing assessment and optimization research, conducted in a six-degree-of-freedom motion flight simulator, to develop simulator motion drive laws capable of providing high-fidelity motion cueing for simulated shipboard operations. To do this, a novel objective technique, vestibular motion perception error (VMPE), has been developed. The technique was used to optimize the motion cues for simulated helicopter landings on a naval single-spot destroyer at different wind and sea-state conditions. New simulator motion tuning sets were derived offline and then tested experimentally to compare the objective VMPE predictions with subjective assessments from a test pilot. Results show the influence of different motion cues, airwake conditions, and ship motion states on the pilot’s overall perception of self-motion, control strategy, task performance, and workload. It was found that high-fidelity motion cueing becomes more desirable for the pilot at higher wind conditions and sea states, for which an “Optimized” motion setting was obtained using the new technique. Moreover, the use of an “Optimized” motion setting generated by the VMPE methodology resulted in reduced pilot workload, leading to improved simulated maritime helicopter operational capability. The technique provides a rational methodology for motion tuning, which could be applied in training and engineering simulators.

An efficiency-aware continuous adaptive proportional-integral velocity-feedback control for wave energy converters

The main objective in hydrodynamic control of wave energy converters (WECs) is the maximization of the energy captured from the waves. Latching control, model predictive control and “PI” control are examples of implementable strategies surveyed in the literature. “PI” control is the common name of a form of hydrodynamic control where the control force applied to the captor is a proportional-integral feedback of captor velocity. While suboptimal, it has the merit of being simple, requiring only straightforward computations and can be considered a standard solution for WECs with a four-quadrant power takeoff (PTO) system. Adaptive “PI” control has been already discussed in the literature, usually using a gain-scheduling approach, with optimal gains precomputed off-line for a representative set of sea states and applied as a function of estimated sea state conditions. In most literature, only average on-line estimations of sea states have been proposed, with time windows of several minutes. Such intermittent adaptive control laws are clearly suboptimal in terms of energy recovery, since the control gains are not continuously updated whereas the sea state is continuously time-varying. In this paper we present a continuously adaptive “PI” control strategy, whose gains are adapted on-line on a wave-to-wave basis, based on a real-time estimate of the dominant wave frequency of the wave force. The PTO efficiency is taken into account. The proposed control method is validated and compared through experiment for irregular sea states.

Feature identification using acoustic signature of Ocean Researcher III (ORIII) of Taiwan

Feature identification using acoustic signature of Ocean Researcher III (ORIII) of Taiwan

Rogue waves during Typhoon Trami in the East China Sea

As concluded from physical theory and laboratory experiment, it is widely accepted that nonlinearities of sea state play an important role in the formation of rogue waves; however, the sea states and corresponding nonlinearities of real-world rogue wave events remain poorly understood. Three rogue waves were recorded by a directional buoy located in the East China Sea during Typhoon Trami in August 2013. This study used the WAVEWATCH III model to simulate the sea state conditions pertaining to when and where those rogue waves were observed, based on which a comprehensive and full-scale analysis was performed. From the perspectives of wind and wave fields, wave system tracking, High-Order Spectral method simulation, and some characteristic sea state parameters, we concluded that the rogue waves occurred in sea states dominated by second-order nonlinearities. Moreover, third-order modulational instabilities were suppressed in these events because of the developed or fully developed sea state determined by the typhoon wave system. The method adopted in this study can provide comprehensive and full-scale analysis of rogue waves in the real world. The case studied in this paper is not considered unique, and rules could be found and confirmed in relation to other typhoon sea states through the application of our proposed method.

Wave-induced real-fluid effects in marine energy converters: Review and application to OWC devices

The performance assessment of industrial marine energy converters involves the integrated treatment of their hydrodynamic design and the optimization of their device hulls. Nowadays, such tasks require extensive experimental work and simulation plans, consuming considerable resources and time. In this comprehensive review of integrated approaches to numerical and experimental testing, the advantages and disadvantages of existing tools, from full-scale prototype and wave tank models to Computational Fluid Dynamics (CFD) and potential flow simulations, are all analysed. Likewise, current challenges such as experimental scale effects, numerical viscosity, and turbulence treatment are all studied. The novelty of this research is an integrated approach that employs experimental wave tank tests to validate a numerical wave tank model based on CFD that serves to calibrate a fast potential flow solver with Morison's correction terms. The model allows running, on tight resources, the necessary simulation for the design and optimisation of marine energy converters under multiple sea state conditions. Given the operating regimes of conventional marine energy converters, the results show that the influence of turbulence may be small, due to the unsteady nature of the oscillatory boundary layer flows.

Visualisation of Probabilistic Access Forecasts for Offshore Operations

Access forecasting aims to predict the quality of transfer of maintenance technicians to/from vessels and the constituent offshore structures at the wind farm. This is highly dependent on sea state conditions as well as other environmental factors such as visibility. Typically, scheduling or dispatch decisions are made on the basis of deterministic forecasts of significant wave height, often coupled with service contracts where transfers are expected to be attempted below a threshold significant wave height. However, there is always uncertainty in a weather prediction which can be accounted for by probabilistic forecasts. The aim of this work is to explore visualisation ideas for transforming vessel specific access forecasts into an interpretable and intuitive decision-support tool. Three simple methods are proposed based on a score out of 10, classification of transfer conditions, and a threshold score. Methods for summarising access conditions for 2–5 days ahead are also developed. This new forecasting and visualisation capability has significant implications for marine coordinators and skippers who will be able to make better informed safety-critical decisions, with the potential for reductions in the cost-of-energy offshore.

A Blade Load Feedback Control For Floating Offshore Wind Turbines

The platform-pitch motions of Floating Offshore Wind Turbines are highly influenced by the blade-pitch controller. However, in rough sea state conditions, the platform-pitch motions are dominated by the incident wave dynamics. Then, the properties of the designed controller to reduce the platform-pitch motions can be changed due to the sea state conditions. In this study, the drawbacks of a previously designed Aerodynamic Platform Stabiliser controller performance in rough sea state conditions are analysed. Furthermore, an additional blade load feedback control loop has been designed to improve the performance of the controller in rough sea state conditions. The preliminary results presented here show the potential of the blade load feedback control loop effectiveness to improve the Aerodynamic Platform Stabiliser control loop performance in rough sea state conditions. The time-domain simulations were carried out with the NREL 5-MW wind turbine mounted on the ITI Energy’s barge in the fully coupled non-linear aero-hydro-elastic simulation tool FAST.

Tether analyses of offshore triceratops under ice force due to continuous crushing

Oil exploration and production in ultra-deep ice-infested waters demands an innovative structural form to withstand the ice loads. Offshore triceratops is one such new-generation innovative, compliant platforms, which show good adaptability in extreme sea state conditions even under the combined action of wind, waves, and current. As tethers are the crucial components in compliant platforms like a triceratops, it is of paramount importance to study the effect of dynamic ice loads on tether tension variation. The main aim of this study is to carry out the tether tension analysis under the ice force action in different ice sea states. The ice force time history is developed from the continuous crushing ice spectral model, and numerical studies are carried out using Ansys Aqwa by applying the ice force on buoyant legs. The study reveals that the tether tension variation is less than 10% even under extreme ice sea state. The parametric studies are also carried out to assess the effect of ice velocity, ice crushing strength, and ice thickness on tether tension variation. It is found that the maximum stress developed in the wires of tethers is very less compared to the yield stress of the material, ensuring no tether failure. However, a large number of stress cycles may lead to fatigue failure of tethers. Hence, fatigue analysis is also carried out to assess the fatigue damage and service life of tethers. The results show that fatigue damage increases from normal to extreme ice sea state. The results are also compared with open-water load case. Studies carried out under lock-in ice condition shows the increase in the fatigue damage and reduction in the service life of the tethers.

A Systematic Review of Remotely Operated Vehicle Surveys for Visually Assessing Fish Assemblages

Anthropogenic activities and greater demands for marine natural resources has led to increases in the spatial extent and duration of pressures on marine ecosystems. Remotely operated vehicles (ROVs) offer a robust survey tool for quantifying these pressures and tracking the success of management intervention while at a range of depths, including those inaccessible to most SCUBA diver-based survey methods (∼>30 m). As the strengths, limitations, and biases of ROVs for visually monitoring fish assemblages remain unclear, this review aims to evaluate ROVs as a survey technique and to suggest optimal sampling strategies for use in typical ROV-based studies. Using the search engines Scopus™ and Google Scholar™, 119 publications were identified that used ROVs for visual surveys of fish assemblages. While the sampling strategies and sampling metrics used to annotate the imagery in these publications varied considerably, the total abundance of fish recorded over strip transects of varying dimensions was the most common sampling design. The choice of ROV system appears to be a strong indicator of both the types of surveys available to studies and the success of ROV deployments. For instance, larger, more powerful working-class systems can complete longer and more complex designs (e.g., swath, cloverleaf, and polygonal transects) at greater depths, whereas observation-class systems are less expensive and easier to deploy, but are more susceptible to delays or cancellations of deployments. In more severe sea state conditions, radial transects, or strip transects that employ live-boating or a weight to anchor the tether to the seafloor, can be used to improve the performance of observation-class systems. As these systems often employ shorter tethers, radial transects can also be used to maximize sampling area at greater depths and on large vessels that may rotate substantially while anchored. For highly mobile species, and in survey designs where individuals are likely to be recounted (e.g., transects along oil and gas pipelines), relative abundance (MaxN) may be a more robust sampling metric. By identifying subtle, yet important, differences in the application of ROVs as a tool for visually surveying deep-water marine ecosystems, we identified key areas for improvement for best practice for future studies.

A Sensitivity Analysis of the Standard Deviation of the Copolarized Phase Difference for Sea Oil Slick Observation

In this paper, a time series of 33 TerraSAR-X copolarized synthetic aperture radar (SAR) imagery collected in Stripmap mode over the Gulf of Mexico in a wide range of incidence angles and sea-state condition is exploited, together with a theoretical framework based on the X-Bragg scattering model, to analyze the effects of noise, angle of incidence, (AOI) and wind speed on the standard deviation of the copolarized phase difference ( $\sigma _{\phi _{c}}$ ) evaluated over sea surface with and without oil slicks. This large data set represents an unprecedented opportunity to analyze, for the first time, the influence of both SAR acquisition and surface parameters on the broadening of the copolarized phase difference probability density function (pdf), $p_{\phi _{c}}(\phi _{c})$ . Experimental results show that the X-Bragg scattering model, here adopted to predict the sea surface $p_{\phi _{c}}(\phi _{c})$ , gives an understanding of the increasing trend of $\sigma _{\phi _{c}}$ with respect to AOI. It is shown that the noise significantly contributes to broaden $p_{\phi _{c}}(\phi _{c})$ over both slick-free and slick-covered sea surface, while the effects of low-to-moderate wind regimes are negligible. In addition, $\sigma _{\phi _{c}}$ exhibits a larger sensitivity to the scene variability, if compared to single-polarization intensity channels, over both slick-free and oil-covered sea surface. This sensitivity is more pronounced at lower AOIs due to the higher noise equivalent sigma zero (NESZ) that affects larger AOIs.

Floating microplastics and aggregate formation in the Western Mediterranean Sea

Pollution by large-sized plastics and microplastic debris is widespread in all Earth environments, also threatening marine ecosystems worldwide. In this study we determine the load of microplastics in the Western Mediterranean Sea and evaluate their aggregation potential into marine aggregates. We report average microplastic abundances of 0.10 ± 0.09 items m−2 (max: 0.50 items m−2). Abundances and chemical composition of microplastics are subject to temporal changes as a function of human activities in the areas of influence and shifting meteorological and sea-state conditions. We find that microplastics were on average smaller in mass than other studies conducted in the Mediterranean Sea indicating longer exposure times in the environment. The microplastic aggregation potential was determined by inspecting formed biogenic aggregates either during sample collection or in the laboratory. Smaller and more angular microplastics dominated in marine aggregates, representing an average 40% in abundance and 25% in mass of microplastics.

Experimental Study of Wave Attenuation in Trapezoidal Floating Breakwaters

A comprehensive experimental study was carried out on the regular wave attenuation with a trapezoidal pontoontype floating breakwater (FB) in deep water. The functionalities of two simple FB geometries consist of a rectangle and a trapezoid with the slope of 60° were investigated under the wave attack. A two-dimensional wave flume was used in the experiment; the incident, transmitted waves, mooring line forces and motion responses of the floating breakwaters were measured. Also the influence of the sea state conditions (incident wave height and wave period) and structural parameters (draught of the structure) were investigated using the trapezoidal FB. Our experimental results indicated that the trapezoidal FB significantly reduced the wave transmission and mooring line force when compared with rectangular FBs. A new formula was developed in order to predict the value of the transmission coefficient in trapezoidal FBs with the slope of 60°. Experimental data showed to be consistent with the results of the formula.