Sea State Conditions Research Articles

A Bayesian hierarchical spatio-temporal model for significant wave height in the North Atlantic

Bad weather and rough seas continue to be a major cause for ship losses and is thus a significant contributor to the risk to maritime transportation. This stresses the importance of taking severe sea state conditions adequately into account in ship design and operation. Hence, there is a need for appropriate stochastic models describing the variability of sea states, taking into account long-term trends related to climate change. Various stochastic models of significant wave height are reported in the literature, but most are based on point measurements without considering spatial variations. As far as the authors are aware, no model of significant wave height to date exploits the flexible framework of Bayesian hierarchical space-time models. This framework allows modelling of complex dependence structures in space and time and incorporation of physical features and prior knowledge, yet at the same time remains intuitive and easily interpreted. This paper presents a Bayesian hierarchical space-time model for significant wave height. The model has been fitted by significant wave height data for an area in the North Atlantic ocean. The different components of the model will be outlined, and the results from applying the model to monthly and daily data will be discussed. Different model alternatives have been tried and long-term trends in the data have been identified for all model alternatives. Overall, these trends are in reasonable agreement and also agree fairly well with previous studies. Furthermore, a discussion of possible extensions to the model, e.g. incorporating regression terms with relevant meteorological data will be presented.

Long-term response analysis of FPSO mooring systems

The design of mooring systems for floating production units usually considers extreme environmental conditions as a primary design parameter. However, in the case of FPSO (Floating, Production, Storage and Offloading) units, the worst response for the mooring system may be associated with other sea state conditions due to the fact that its extreme response may be associated with a resonant period instead of an extreme wave height. The best way to deal with this problem is by performing long-term analysis in order to obtain extreme response estimates. This procedure is computationally very demanding, since many short-term environmental conditions, and their associated stochastic nonlinear time domain numerical simulations of the mooring lines, are required to obtain such estimates. A simplified approach for the long-term analysis is the environmental contour-line design approach. In this paper a Monte Carlo-based integration procedure combined with an interpolation scheme to obtain the parameters of the short-term response distribution is employed to hasten the long-term analysis. Numerical simulations are carried out for an FPSO at three different locations considering a North Sea joint probability distribution for the environmental parameters. The long-term analysis results are compared against those obtained using extreme environmental conditions and environmental contour-line methodology. These results represent the characteristic load effect for the design of mooring systems of floating units using the reliability analysis for mooring line. The results show that the long-term results are usually more critical than those obtained with the other approaches and even different mooring lines can be identified as the critical ones.

Ocean Surface's Scattering Coefficient Retrieval by Delay–Doppler Map Inversion

Global navigation satellite system reflectometry of signals is a promising technique to remotely sense a number of Earth's geophysical parameters, and it has been proposed for ocean monitoring applications such as mesoscale altimetry or sea state monitoring. So far, the following two main approaches have been considered to retrieve a sea state descriptor from measured delay waveforms or delay-Doppler (DD) maps (DDMs): 1) fitting the measurements to a model tuned with the desired parameter or 2) directly linking a property of the measurements with the parameter to be retrieved (e.g., volume of the normalized DDM). However, these approaches provide a single sea state descriptor related to the overall glistening zone where sea state conditions may not be homogeneous. In this letter, the relationship between the physical space and the DD domains is exploited, and a method to retrieve the bistatic scattering coefficient distribution over the observation ocean surface from measured DDMs is proposed.

A new formula for the sea state and structural parameters influencing the stability of homogeneous reshaping berm breakwaters

The berm recession of a reshaping berm breakwater has a very important role for the stability of this kind of structure. Based on a 2D experimental modeling method in a wave flume, the recession of the berm due to sea state and structural parameters has been studied. Irregular waves with a JONSWAP spectrum were used. A total of 215 tests have been performed to cover the impact of sea state conditions such as wave height, wave period, storm duration and water depth at the toe of the structure, and structural parameters such as berm elevation from still water level, berm width and stone diameter on berm recession. In this paper, first a new dimensionless parameter is introduced to evaluate the combined effect of wave height and wave period on berm recession using results of the experimental work. Then, a formula that includes some sea state and structural parameters is derived using the new dimensionless parameter for estimating the berm recession. A comparison is made between the estimated berm recessions by this new formula and formulae given by other researchers to show the preference of using the new dimensionless parameter. The comparison shows that the recession estimated by the new formula has not only a better correlation with the present experimental data, but also has an improved correlation with other experimental results within the range of parameters tested. Outside the range of parameters tested the Lykke Andersen (2006) formula performed best.

Parameterizations and Algorithms for Oceanic Whitecap Coverage

Abstract Shipboard measurements of fractional whitecap coverage W and wind speed at 10-m height, obtained during the 2006 Marine Aerosol Production (MAP) campaign, have been combined with ECMWF wave model and Quick Scatterometer (QuikSCAT) satellite wind speed data for assessment of existing W parameterizations. The wind history trend found in an earlier study of the MAP data could be associated with wave development on whitecapping, as previously postulated. Whitecapping was shown to be mainly wind driven; for high wind speeds (>9 m s−1), a minor reduction in the scatter of in situ W data points could be achieved by including sea state conditions or by using parameters related to wave breaking. The W values were slightly larger for decreasing wind/developed waves than for increasing wind/developing waves, whereas cross-swell conditions (deflection angle between wind and swell waves between ±45° and ±135°) appeared to dampen whitecapping. Tabulated curve fitting results of the different parameterizations show that the errors that could not be attributed to the propagation of the standard error in U10 remained largely unexplained. It is possible that the counteracting effects of wave development and cross swell undermine the performance of the simple parameterizations in this study.

Using a Bayesian Network to Summarize Variability in Numerical Long-Term Simulations of a Meteorological–Marine System: Drift Climatology of Assumed Oil Spills in the North Sea

Climate-related scientific analyses of meteorological–marine systems are often based on numerical long-term simulations at high spatial and temporal detail. Such comprehensive data sets require much resources and specific evaluation tools, which sometimes hampers their use within interdisciplinary projects. In the present study, we propose the use of a Bayesian network to represent simulated transports in the North Sea depending on variable external forcing in terms of conditional probabilities. Eliciting probability tables from multi-decadal numerical simulations ensures that all realistic weather and resulting sea state conditions are covered in agreement with the frequency of their occurrence. The probabilistic representation conveniently allows for conditioning numerical simulations on either external forcing (weather conditions) or observed transports. In the latter case, the Bayesian inversion formula becomes involved to transfer information in a direction opposite to causal dependencies encoded in the underlying mechanistic model. We show that simulated travel time distributions even allow for taking into account a substance’s specific half-life, although this was not an issue in the original passive tracer simulations.

Modelling, control and Pontryagin Maximum Principle for a two-body wave energy device

Frequency-domain analysis is applied to a wave energy device composed of two coaxial axisymmetric bodies. For each frequency optimal damping coefficient values which maximize absorbed power are obtained. Several displacement amplitude restriction scenarios are considered. A stochastic model to describe the device’s behaviour in irregular waves is developed. Optimal mechanical damping and spring coefficients are computed. Considering different sea state conditions, probability density functions are defined for relevant parameters and time-averaged absorbed power values are obtained.A time domain model is also developed for the device. A non-linear power take-off mechanism configuration, consisting in a hydraulic circuit with low-pressure and high-pressure gas accumulators, is devised. Time-averaged absorbed power is maximized in terms of characteristic mechanism parameter. A sub-optimal method of phase control by latching is applied to the device in order to improve its performance. Analytical development of Pontryagin Maximum Principle is used to establish an algorithm for device’s control.

Numerical Modeling of Wear Damage in Seals of a Wave Energy Converter with Hydraulic Power Take-Off under Random Loads

Understanding and managing the safety and reliability issues of ocean energy converters is a key factor in the development of viable technologies. The present article deals with wear in the high-pressure hydraulic power take-off machinery in a heaving-buoy wave energy converter. In this study, an abrasive wear model for such machinery is developed for the steady-state operation by considering the effects of sealed pressure, relative velocity, material properties, time-varying environmental conditions, and lubrication, with a focus on the piston ring. In particular, the dependence of the wear damage on the sea state condition is investigated. The dynamic parameter named instantaneous wear work rate (Load × Velocity) is numerically calculated in the time domain. An estimate of the average wear work rate in each sea state is computed on the basis of a wave scatter table (significant wave heights versus peak period). Long-term wear damage can then be accounted for by summing up the short-term wear damage multiplied with the occurrence probability of each sea state. Finally, the relative contribution of wear damage from different sea states is obtained.

Evaluation of a Roughness Length Model and Sea Surface Properties with Data from the Baltic Sea

AbstractThe exchange of momentum between the oceans and atmosphere is important for many atmospheric and oceanic processes and is mainly governed by the roughness of sea surface. The roughness can be expressed by a roughness length z0. A roughness length model, based on the concept that z0 is determined by stochastic wave breaking, is presented. The model performance is evaluated using measurements from the Östergarnsholm site, in the Baltic Sea, and pertinent information from other recent investigations. The wave field and the roughness length variations are investigated during various sea state conditions dominated by wind-driven waves. It is found that several parameters, describing the characteristics of the wave field, are dependent on the amount of energy that long waves have relative to the energy of short, wind-driven waves of the sea spectrum (called the swell ratio). The impact of swell ratio on z0 can explain the discrepancies found in various results among relevant investigations. The roughness length model can well reproduce the observed roughness length.

Multistatic and MIMO Distributed ISAR for Enhanced Cross-Range Resolution of Rotating Targets

In this paper, we present a new technique to exploit the data acquired simultaneously by multiple radar sensors carried by multiple air platforms to increase the cross-range resolution of inverse synthetic aperture radar (ISAR) images of rotating targets. This distributed ISAR technique is devised for two different cases: 1) multiple-input-multiple-output (MIMO) case with each platform carrying an active radar that transmits and receives RF waveforms and 2) multistatic case with a single platform carrying an active radar (transmitting and receiving) and the remaining platforms equipped with passive sensors (namely, receiving only). The processing chain proposed for the distributed ISAR is shown, together with the results obtained against simulated ISAR data for both the MIMO and the multistatic cases. The performance analysis shows that the proposed technique is able to provide an increase of the cross-range resolution up to the number of platforms in the multistatic case and even higher in the MIMO case, if the platforms are properly located. This is of great benefit in applications where the target rotation angle is insufficient to guarantee the desired resolution. A typical case is the imaging of ship targets with rotation induced by the sea swell structure under low sea state conditions. To make the results appealing for practical application, the performance degradation is also analyzed arising from errors in the knowledge of both the target rotation motion and the acquisition geometry. Experimental data collected by a ground-based radar operating together with a rotating platform are processed by following the presented distributed ISAR technique to validate the proposed approach.

모바일하버 선박의 계류안정화시스템 및 의장장치 개념설계

Mobile Harbor(MH) is a new paradigm for maritime transport system introduced in Korea, the target of which is to carry out ship-to-ship cargo operation rapidly and effectively even under a condition of sea state 3. A MH ship is moored alongside a large container vessel anchored at the defined anchorage and also equipped with gantry cranes for handling containers. The MH study concerned includes rapid container handling system, optimum design for floating structure, hybrid berthing & cargo operation system, design for cargo handling crane, etc. This paper is to deal with a conceptual design of a stabilized mooring system and mooring equipment under a condition of ship-to-ship mooring. In this connection, we suggest a positioning control winch system in order to control heave motions of the MH ship which is to add constant brakepower and stabilized function to an auto-tension winch and mooring equipment used currently in large container ships.

Maximum steepness of oceanic waves: Field and laboratory experiments

The breaking of waves is an important mechanism for a number of physical, chemical and biological processes in the ocean. Intuitively, waves break when they become too steep. Unfortunately, a general consensus on the ultimate shape of waves has not been achieved yet due to the complexity of the breaking mechanism which still remains the least understood of all processes affecting waves. To estimate the limiting shape of ocean waves, here we present a statistical analysis of a large sample of individual wave steepness. Data were collected from measurements of the surface elevation in laboratory facilities and the open sea under a variety of sea state conditions. Observations reveal that waves are able to reach steeper profiles than the Stokes' limit for stationary waves. Due to the large number of records this finding is statistically robust.

Autotuning Aspects for Dynamic Positioning Systems

Abstract This paper considers aspects related to the problem of automatic tuning (autotuning) of dynamic positioning (DP) controllers for marine surface vessels. Autotuning involves automatic adjustment of controller parameters for shifting vessel operational conditions (VOCs), which encompass changing vessel dynamics and different operational tasks. A controller with fixed gains cannot behave equally well for all VOCs and autotuning can thus be very valuable in practice, saving time for commissioning and improving operational robustness. In this work, we review some previous work and discuss fundamental concepts within the subject area. We also propose a novel performance index for station-keeping purposes and use this index together with an autotuning algorithm which employs data generated from the index. Finally, some simulation results concerning station keeping of an offshore supply vessel operating in challenging sea-state conditions are included.

Assessment of chemical dispersant effectiveness in a wave tank under regular non-breaking and breaking wave conditions

Current chemical dispersant effectiveness tests for product selection are commonly performed with bench-scale testing apparatus. However, for the assessment of oil dispersant effectiveness under real sea state conditions, test protocols are required to have hydrodynamic conditions closer to the natural environment, including transport and dilution effects. To achieve this goal, Fisheries and Oceans Canada and the US Environmental Protection Agency (EPA) designed and constructed a wave tank system to study chemical dispersant effectiveness under controlled mixing energy conditions (regular non-breaking, spilling breaking, and plunging breaking waves). Quantification of oil dispersant effectiveness was based on observed changes in dispersed oil concentrations and oil-droplet size distribution. The study results quantitatively demonstrated that total dispersed oil concentration and breakup kinetics of oil droplets in the water column were strongly dependent on the presence of chemical dispersants and the influence of breaking waves. These data on the effectiveness of dispersants as a function of sea state will have significant implications in the drafting of future operational guidelines for dispersant use at sea.

Constitution et exploitation d'une base de données d'états de mer le long des côtes françaises par simulation numérique sur 23 ans

ABSTRACT A numerical wave data-base (called ANEMOC) has been built by hindcasting seastate conditions over a period of 23 years, from 1979 to 2002, with the TOMAWAC wave model. The wind fields used as input data come from the reanalysis ERA-40 of the European Center for Medium-Range Weather Forecasts (ECMWF). Some statistical processing of the results have been performed, so as to build synthetic charts of the medium and severe wave conditions over the European continental shelf. Finally two different methods are proposed to estimate extreme wave heights with return periods larger than the simulated period. Both methods for statistical extrapolation are applied to a selection of points and compared.

Automatic Oil Spill Detection by Marine X-Band Radars -New System Based on Processing of Digitized Radar Images

The OSD (Oil Spill Detection) system's main objective is to detect oil spills on the sea surface, and to track the oil spill over time. The system provides help both for those supervising oil fields, and those in charge of clean-up operations. The OSD system has been thoroughly tested during several field trials in the North Sea. During these trials the spill size, sea state and wind conditions varied throughout, thus enabling realistic real time testing. The OSD system may operate in complete darkness or fog, and for 24 hours per day. This greatly enabling recovery and skimming operations when visibility is poor.The Miros AS Company has developed the OSD system in close co-operation with the Norwegian Clean Seas Association for operating companies, thus complying with the NOFO practical and operational requirements. To identify an oil spill, the OSD system uses advanced image-processing algorithms on radar images and extracted by the Miros Wavex System. The OSD system is based on the fact that areas covered with oil will reflect less microwave power. This is due to the damping of capillary waves on the sea surface.

Whitecap coverage in coastal environment for steady and unsteady wave field conditions

Breaking waves represent a “key” parameter for many applications involved with a large number of environmental phenomena. In particular, it is well recognized that the whitecap cover induced by breaking waves allows substantial enhancement of heat, momentum, gas and particle transfer at the air–sea interface. A large number of studies were conducted during the last decades on the variation of the whitecap fraction, commonly noted W. The results presented in this paper deal with the evolution of the whitecap coverage in coastal zone. In such areas, the wave field is often unsteady with an important variety of sea state developments. The present analysis is based on an extensive series of data obtained during an experimental campaign which took place on the Mediterranean coast in 2001. The results allow observation of the influence of the sea state conditions of the wave field on the whitecap coverage. In addition, this paper confirms the occurrence of a peak in the variations of the whitecap fraction with the wave age for coastal areas as suggested by Lafon et al. [Lafon, C., Piazzola, J., Forget, P., Le Calvé, O. and Despiau, S., 2004. Analysis of the variations of the whitecap fraction as measured in a coastal zone. Boundary-Layer Meteorol., 111: 339-360.]. A wave age dependent model for the whitecap fraction is then proposed, which takes into account both the wind and the wave influence, and hence, is characteristics of the different sea state conditions.

Antarctic Shallow Drilling Project provides key core samples

The understanding of Antarctica's climate, cryosphere, and biosphere evolution is limited, which is due in part to the paucity of outcrops and cores that record changes during the Tertiary. This problem has been partially rectified using conventional drill ships, such as the JOIDES Resolution, but access to key areas of the continental margin has been restricted because of the inability of these ships to operate in ice‐covered waters. While researchers are restricted in their ability to acquire long cores in ice‐prone areas, nature has provided an alternative method.During past glacial maxima, ice sheets advanced onto the continental shelf, eroding deeply into the stratigraphic section. Several areas of the shelf are characterized by seaward dipping strata a few meters beneath the seafloor. However, these older strata are overlaid by glacial sediments that have proven to be impenetrable by standard piston coring. The Shallow Drilling Project (SHALDRIL) began with the idea of using an icebreaking vessel as the drill platform so that drilling could be conducted in areas that are inaccessible by standard drill ships.Though, even icebreaking vessels with a drill stem hanging beneath them are vulnerable to ever‐present changes in sea state, winds, and ice conditions. Thus, the approach, by necessity, needs to be one of ‘drill and run.’ Two cruises, in the austral falls of 2005 and 2006, have shown the efficacy of this approach.

Survey of stochastic models for wind and sea state time series

The knowledge of sea state and wind conditions is of central importance for many offshore and nearshore operations. In this paper, we make a complete survey of stochastic models for sea state and wind time series. We begin with methods based on Gaussian processes, then non-parametric resampling methods for time series are introduced followed by various parametric models. We also propose an original statistical method, based on Monte Carlo goodness-of-fit tests, for model validation and comparison and this method is illustrated on an example of multivariate sea state time series.

Float Over Installation Method—Comprehensive Comparison Between Numerical and Model Test Results

Installing a large deck onto a platform, such as a spar, using the floatover method is gaining popularity. This is because the operational cost is much lower than other methods of installation, such as modular lifts or a single piece installation by a heavy lift barge. Deck integration can be performed on land, at quay side and will not depend on a heavy lift barge. A new concept for a floatover vessel has been developed for operations in the Gulf of Mexico and West Africa. In this application sea state conditions are essential factors that must be considered in the Gulf of Mexico, especially for transportation. In West Africa, swell conditions will govern floatover deck (FOD) installation. Based on these two different environmental conditions, Technip Offshore, Inc. developed the FOD installation concept using semi-submersible barge type vessels. A significant amount of development work and model testing has been done on this method in recent years on spar floatover. These tests have validated our numerical methods. Another test was conducted to investigate the feasibility of a deck float-over operation onto a compliant tower for a West Africa project. The project consists of a compliant tower supporting a 25,401metricton(28,000s.ton) integrated deck. This paper will describe comparisons between model test data and numerical predictions of the compliant tower floatover operation.