Wind-induced drift of objects at sea: the leeway field method

A method for conducting leeway field experiments to establish the drift properties of small objects (0.1-25 m) is described. The objective is to define a standardized and unambiguous procedure for condensing the drift properties down to a set of coefficients that may be incorporated into existing stochastic trajectory forecast models for drifting objects of concern to search and rescue operations and other activities involving vessels lost at sea such as containers with hazardous material. An operational definition of the slip or wind and wave-induced motion of a drifting object relative to the ambient current is proposed. This definition taken together with a strict adherence to 10 m wind speed allows us to refer unambiguously to the leeway of a drifting object. We recommend that all objects if possible be studied using what we term the direct method, where the object's leeway is studied directly using an attached current meter. We divide drifting objects into four categories, depending on their size. For the smaller objects (less than 0.5 m), an indirect method of measuring the object's motion relative to the ambient current must be used. For larger objects, direct measurement of the motion through the near-surface water masses is strongly recommended. Larger objects are categorized according to the ability to attach current meters and wind monitoring systems to them. The leeway field method proposed here is illustrated with results from field work where three objects were studied in their distress configuration; a 1:3.3 sized model of a 40-ft Shipping container, a World War II mine and a 220 l (55-gallon) oil drum.Comment: 33 pages, 12 figures, 3 table

The Leeway of Shipping Containers at Different Immersion Levels

The leeway of 20-foot containers in typical distress conditions is established through field experiments in a Norwegian fjord and in open-ocean conditions off the coast of France with wind speed ranging from calm to 14 m/s. The experimental setup is described in detail and certain recommendations given for experiments on objects of this size. The results are compared with the leeway of a scaled-down container before the full set of measured leeway characteristics are compared with a semi-analytical model of immersed containers. Our results are broadly consistent with the semi-analytical model, but the model is found to be sensitive to choice of drag coefficient and makes no estimate of the cross-wind leeway of containers. We extend the results from the semi-analytical immersion model by extrapolating the observed leeway divergence and estimates of the experimental uncertainty to various realistic immersion levels. The sensitivity of these leeway estimates at different immersion levels are tested using a stochastic trajectory model. Search areas are found to be sensitive to the exact immersion levels, the choice of drag coefficient and somewhat less sensitive to the inclusion of leeway divergence. We further compare the search areas thus found with a range of trajectories estimated using the semi-analytical model with only perturbations to the immersion level. We find that the search areas calculated without estimates of crosswind leeway and its uncertainty will grossly underestimate the rate of expansion of the search areas. We recommend that stochastic trajectory models of container drift should account for these uncertainties by generating search areas for different immersion levels and with the uncertainties in crosswind and downwind leeway reported from our field experiments.Comment: 25 pages, 11 figures and 5 tables; Ocean Dynamics, Special Issue on Advances in Search and Rescue at Sea (2012

Workshop on identification of future emerging technologies in the ocean energy sector

As part of the Commission's internal Low Carbon Energy Observatory (LCEO) project, the Joint Research Centre (JRC) is developing an inventory of Future Emerging Technologies (FET) relevant to energy supply. A key part of the LCEO initiative is the consultation of external experts, addressing both those with in-depth experience in specific fields and those with a broad perspective on relevant science and engineering aspects. In this context, on March 27, 2018 the JRC organised a Workshop on Identification of Future Emerging Technologies for Ocean Enery, on it premises in Ispra. The workshop was organized on the idea of a colloquium between international experts to discuss about future emerging technologies considering different aspects such as their technology readiness level (TRL) , the potential advantages and challenges affecting their development, and evaluating the possible speed of development . A number of different technological solutions were discussed, identified directly by the invited experts on the condition that they respected the following criteria: • To be a technology for energy supply/conversion in the field of ocean energy. • To be a radically new technology/concept, not achievable by incremental research on mainstream technologies (this should match the concept of the Future Emerging Technology in the Horizon 2020 work program http://ec.europa.eu/programmes/horizon2020/en/h2020-section/future-and-emerging-technologies). • To be in an early stage of development: their Technology Readiness Level should not be more than 3. Questionnaires were sent to experts for the identification of ocean energy FETs. The templates can be found in Appendix B. The structure of the workshop was builtupon the inputs received from the experts and on in-house analysis undertaken by the JRC. The aim of this document is to gather, organize and highlight all the knowledge and information, provided by the external and internal experts, which were discussed during the workshop.JRC.C.7-Knowledge for the Energy Unio

L'absorption Dynamique des Ondes de Gravité en Régime Instationnaire

This study deals with the dynamic absorption of water waves. We focus here our attention on the case of a simple devce, a 2D piston, in order to exploit the well-known expressions of the potential as far as possible in the design of new absorption laws. We consider a semi-infinite two dimensional wave tank closed by a mobile vertical plane. An unsteady wave train generated at infinity impiges on this plate. The problem of the dynamic absorption consists in finding, in real time, the velocity to be given to the plate so that the radiated and the reflected wave trains should cancel each other. We propose here to derive this velocity from the forces measured on the plate. We first derive a frequency dependant transfer function between the optimal velocity of the paddle and the total force for the case of steady time harmonic incident waves. As a consequence of the linear approach, we choose, the time domain velocity leading to the complete absorption of the incident wave train is obtained by convolutiong the inverse Fourier transform of this transfer function with the measured hydrodynamic force. Unfortunately, the impulse response function of the ideal absorber derived that way is not causal; thus, it cannot be used just as is as the control loop of the physical absorbing device. So, we suggest two causal non ideal approximations of the ideal non-causal controller.On considère un bassin de houle bidimensionnel dans lequel on étudie le problème de l'absorption dynamique des ondes de gravité par la translation d'une de ses parois verticales en réponse aux efforts hydrodynamiques qu'elle subit. On se place dans le cadre de la théorie linéaire des écoulements à surface libre des fluides parfaits pesants et on introduit les conditions aux limites classiques sur le contour du domaine fluide. En imposant une condition dite d'absorption optimale aux potentiels réfléchis et de radiation, on obtient une relation fréquentielle d'absorption totale en boucle ouverte caractérisée par une fonction de transfert complexe entre efforts hydrodynamiques et vitesse de l'absorbeur. Le passage au domaine temporel par transformation de Fourier inverse fait apparaître une réponse impulsionnelle non causale rendant le système réalisable. Plusieurs approximations causales du système sont alors étudiées. On détermine dans un premier temps une relation asymptotique basses fréquences du type Sommerfeld-Orlansky pour des grandeurs locales. On détermine ensuite une relation correspondant à un modèle de type feedback/feedforward conduisant au prix d'une hypothèse simplificatrice à une relation causale approchée purement instationnaire. L'efficacité de ce modèle n'étant que légèrement supérieure à celle du mode asymptotique, on montre qu'il est possible de l'améliorer de manière considérable en rajoutant une relation de dépendance vis à vis de la fréquence de l'onde incidente. Toutes les relations de transfert apparaissant dans ces lois de contrôle de l'absorbeur ont été identifiées par une méthode de lissage par exponentielles avant d'être testées numériquement de manière à éviter les intégrales de convolution. On présente ensuite une décomposition de la relation d'absorption visant à obtenir de manière plus générale une boucle de contrôle de type feedback/feedforward ne faisant intervenir que des systèmes à réponse impulsionnelle causale. On présente enfin un modèle expérimental de bassin de houle bidimensionnel muni d'un tel système d'absorption par batteur piston

Caractérisation statistique des conditions d'états de mer multimodales dans le golfe de Gascogne pour le dimensionnement des structures en mer.

La caractérisation de la réponse de structures placées en mer et soumises à l'action des vagues passe le plus souvent par des approches fréquentielles nécessitant de fait une bonne description du contenu spectral du chargement. Le contenu fréquentiel de l'état de mer constituant l'excitation est le plus souvent modélisé par une fonction analytique, spectre de densité d'énergie de type JONSWAP ou Pierson-Moskowitz par exemple, fonction dont la forme unimodale ne permet pas de représenter correctement la distribution spectrale de l'énergie au sein d'un état de mer complexe constitué de la superposition de plusieurs systèmes de vagues, houle et mer du vent. Une telle approximation de la distribution de l'énergie peut conduire à des biais importants dans l'évaluation de la réponse d'une structure. S'appuyant sur le jeu de données de partitionnement issu de la base de données HOMERE, une cartographie des statistiques d'occurrence des états de mer complexes est réalisée. Ces statistiques montrent la prévalence des états de mer multimodaux sur la zone considérée et confirment l'importance et la nécessité de prendre en compte une description plus fine de la distribution spectro-angulaire de l'énergie au sein des états de mer. Ces cartographies font également apparaître une variabilité tant saisonnière que spatiale de ce caractère multimodal des états de mer qu'il sera également important de prendre en compte dans les études de climatologies pour le dimensionnement des structures en mer

The impact of the MARINET initiative on the development of Marine Renewable Energy

Marine Renewable Energy Conversion systems comprise wave energy and tidal stream converters as well as offshore-wind turbines for electrical generation. These technologies are currently at different stages of development but are mostly at the pre-commercial stage and require research to be undertaken at a series of scales along the path to commercialization. However each of these technologies also needs specific research infrastructures in order to conduct this research. The aim of the MARINET initiative is to coordinate research and development at all scales (small models through to prototype scales, from laboratories through to open sea tests) and to allow access for researchers and developers to infrastructures which are not available universally in Europe, including test facilities for components such as power take-off systems, grid integration, moorings and environmental monitoring so as to ensure a focusing of activities in this area. The initiative offers researchers and developers access to 45 research facilities as well as to the associated network of expertise at all scales in Offshore Marine Renewable Energy technology research and development. The aim of this paper is to present this MARINET initiative that was started in 2011, bringing together a network of 29 partners spread across twelve countries. Details of the MARINET Transnational Access (TA) program are presented, for which over 260 applications were received throughout the 5 official calls for proposals. In particular, statistics on applications and completed projects are presented which provide an overview of the global development progress of the different offshore renewable energy conversion technologies at a European level. It also provides a good overview of the current research activity, as well as evidence of the requirement for specialised research facilities, in this burgeoning field

A realizable force feedback-feedforward control loop for a piston wave absorber

We consider the problem of the absorption of 2D water waves by the horizontal motions of a vertical plane in response to the hydrodynamical force it experiences. The solution of this problem is straightforward in the frequency domain. Unfortunately, the transposition of the frequency domain solution to the time domain by inverse Fourier transforming leads to a non-causal impulse response function. Thus it cannot be used just as it is, neither as a control loop for physical absorbing devices nor as a non radiating boundary condition (NRBC) in numerical modeling. In this paper, we propose a method to derive causal approximations of this ideal controller. Two time-domain absorbing relations are proposed, differing in whether or not one knows a dominant frequency of the incident wave train to be absorbed. Their performances are compared with the absorption efficiency of the low frequency asymptotic Sommerfeld relation which simply reads, for such a piston device: U(t)=F(t)

Container Drift Assessment - Iroise Sea Experiment. Sar-Drift Project

The SAR-DRIFT project aims at developing a forecast system for localisation of drifting objects, as well as decision aid tools for Maritime Search and Rescue. Its basic principle is to predict the position of drifting objects, when combining environmental measurements and forecast through sophisticated stochastic modelling. Among the different tasks of the project, one is to assess the hydrodynamic behaviour of various objects drifting at sea when submitted to the joint action of wind, waves and currents. Thousands of cargo containers are reported to fall overboard each year. A significant part of these containers will stay afloat from several hours to several weeks and represent a major hazard to navigation. In order to evaluate the drift coefficients of such cargo containers, trials were conducted at sea during which a 20 feet container was let adrift for 24 hours while measuring environmental parameters. Experimental set-up and operational process are described in this report. Main results on evaluation of the behaviour of the drifting container are also presented