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Since the Rogue Waves 2000 and 2004 workshops, researchers have pursued activity in the proposition of models and descriptions that can improve our understanding of rogue waves. The Brest Rogue Waves 2008 workshop gathered again many scientists and engineers to an opportunity to confront and discuss their views on the subject. It provided an assessment of the State of the Art of research on those matters, and was one more step forward towards efficient mitigation measures., Depuis les colloques Rogue Waves 2000 et 2004, la recherche s'est poursuivie pour la proposition de modèles susceptibles d'améliorer notre compréhension des vagues scélérates. Le colloque Rogue Waves 2008 a de nouveau rassemblé à Brest de nombreux scientifiques et ingénieurs qui ont pu y confronter et discuter leurs positions sur le sujet. Les dernières avancées dans le domaine ont pu y être évaluées, et un nouveau pas a été ainsi fait vers des mesures appropriées face au risque de ces événements.

The purpose of this study is to validate the use of the so-called Iterative Component Addition (ICA) formulas for the assessment of fatigue damage of marine structures subjected to complex (multimodal) wave loading conditions, as prevail in West Africa areas where the sea states are usually described as combinations of three wave systems (e.g. main swell, secondary swell and wind sea). In this context, operators specifications require to consider a very large set of fatigue loading cases, which lead to excessive computation times if the conventional procedure is used for fatigue assessment. The use of the ICA formulas is expected to simplify the evaluation of the damage and to allow a drastic reduction of computation time, keeping a slight level of conservatism. The present work intends to validate those formulas on an industrial application. One is especially interested in the accuracy of the ICA-based approach for fatigue damage assessment, the ease of use of this approach and the computation times it involves. The scope of the study is limited for simplicity purposes to cases where the structural dynamic response is linear. Its actual industrial application deals with the assessment of the fatigue damage induced by the vertical bending moment of an FPSO hull girder subjected to wave loading in a West Africa area. In this report, we define the occurrence probabilities of all the fatigue loading cases to be applied as required by the operator specifications. Then, to validate the use of the ICA formula, the fatigue damage is estimated according to two main approaches. The first one, which serves as a reference solution, uses a conventional method and computes the damages of the sea states from the metocean data base only. The other one uses ICA formulas and compute the damages of all possible sea states defined by the operator specifications. For this last approach, a full application of ICA formulas based on a partition of wave systems response spectra is also carried out and some analytical formulas for the damage of unimodal spectra are set up. Between both approaches, some intermediate computations are made to estimate the individual effects of the approximations introduced. The occurrence probabilities of the fatigue loading cases derived from the metocean specifications exhibit some significant discrepancies from the statistics of the metocean database. Namely, they provide a lower significant height for the sea states with only a main swell and a larger one for sea states with the three wave systems. As to the application of ICA formulas, it is shown to provide accurate results. We can note that the ICA results are close to the reference solution and that most of the conservatism comes from the discretization defined for the statistical analysis. Moreover, the implementation of the ICA-based approach is simple on the actual industrial application. With appropriate change of programming language and optimization of the procedure, dramatic improvements have been obtained in the computation time, with nearly 2 million combinations of sea states processed in less than one minute. Two main difficulties were encountered in this study. The first one concerns the two-slopes S-N curve, as a combined spectrum may have cycles amplitudes that are larger than the stress thres hold of the S-N curve while its individual components do not. That was solved by providing a method to estimate two-slope damage from the two single-slope ones. The second difficulty stems from the discrepancies observed between the statistics on the metocean database and the sea states probabilistic model derived from the metocean specifications, which raises some questions on the construction of fully relevant metocean specifications. Though some progress was made, that second difficulty is not yet satisfactorily solved. However, this study appears as a mighty step in the development of a global framework for fatigue damage assessment under multimodal loading conditions with the ICA formulas.

Since the Rogue Waves 2000 workshop, significant advances have been made in the proposition of models and descriptions that may improve our understanding of rogue waves. Questions arise now on how those results should influence standard practices in the shipbuilding and offshore industries, and whether they allow to improve forecast and warning systems. The Brest Rogue Waves 2004 workshop gathered again many scientists and engineers to an opportunity to confront and discuss their views on the subject., Depuis le colloque Rogue Waves 2000, des avancées significatives ont été réalisées dans la description et la proposition de modèles susceptibles d’améliorer notre compréhension des vagues scélérates. Les questions qui se posent maintenant concernent l’influence que ces résultats doivent avoir sur les normes et pratiques de la construction navale et offshore, et s'ils apportent des possibilités d'amélioration pour les systèmes de prévision et d’alerte. Le colloque Rogue Waves 2004 de Brest a de nouveau rassemblé de nombreux scientifiques et ingénieurs qui ont pu y confronter et discuter leurs positions sur le sujet.

The OGP Workshop on the Metocean and Engineering Aspects of Floating Systems which was held at St. Albans in April 2001 identified better discrimination of the low frequency end of the wave spectrum as one of the most pressing problems for the design and analysis of floating systems. The responses of many floating systems have proven to be very sensitive not only to the amplitude and frequency of the swell, but also to the details of the shape of the swell peak in the spectrum. There has been relatively little serious study of the low frequency end of the wave spectrum compared to the effort that has gone into the study of the high frequency tail of the spectrum. Much of the present interest in swell comes from the active deep water developments taking place off West Africa. The West African environment is generally mild, but swell from storms in the Southern Ocean is persistent and can reach fairly high amplitudes with very low periods. Enough data is now available so that a serious study of the characteristics of swell off West Africa can be made. In response to this need and opportunity, Shell Global Solutions, in association with Ifremer and Oceanweather, formed a Joint Industry Project to analyze and compare the available data on swell off West Africa. This report describes the results of that West Africa Swell Project (WASP).

There are now many evidences that some waves do significantly exceed the common height and steepness expectations that can be derived from existing models. A better understanding of why, how, and when these huge waves occur should thus be a research priority. The Brest ROGUE WAVES 2000 workshop gathered many of the scientists and engineers actively working on the subject to an opportunity to confront and discuss their most recent advances concerning the definition, statistics, modeling and prediction of those abnormal waves., C'est aujourd'hui une certitude que certaines vagues outrepassent en hauteur et en cambrure les prédictions fondées sur les modèles courants. L'amélioration de la compréhension des raisons, des mécanismes, et des circonstances de leur apparition se doit donc d'être une priorité de recherche. Le colloque Rogue Waves 2000 a rassemblé à Brest nombre des sçientifiques et ingénieurs actifs sur le sujet, qui y ont trouvé l'occasion de confronter et discuter leurs avancées les plus récentes en termes de définition, de statistiques, de modélisation et de prédiction de ces vagues anormales.

To design offshore structures, to prepare operations at sea and unusual transportations on marine routes, to assess safety levels with respect to marine climate, one needs practical data describing the wave, wind and current conditions that may occur. This essential information is unfortunately very seldom readily available exactly for the geographical area of interest. Where to look for it, how to evaluate and choose amongst what one may find, how to get aware of its adequation and drawbacks for the projected use, whom to call for help, as many questions to which this report is expected to bring some answers. Engineers in charge of coastal or offshore projects, logistics managers implied in sea transportations, underwriters, planners for shore equipments should find here some grounds on which to base their decisions. Data collectors should get better ideas of the interest and uses of their work for operational industry purposes. Also, inquiring minds may measure the contribution of novel sensor technologies, such as satellites, and computer simulations to the understanding of seas and oceans., La conception de structures marines, la préparation d'opérations à la mer et de transports maritimes exceptionnels, l'évaluation des risques liés au climat marin ne peuvent se faire sans disposer de données qui décrivent les conditions de houle, de vent et de courant susceptibles d'intervenir. Il est malheureusement bien rare d'avoir juste sous la main, pour la zone géographique concernée, ces informations indispensables. Où les chercher, comment les juger et choisir parmi celles qu'on trouve, comment connaître leurs qualités et leurs défauts pour l'utilisation projetée, par qui se faire aider, autant de questions auxquelles ce document s'efforce d'apporter des éléments de réponse. Les ingénieurs qui ont la charge de projets côtiers ou hauturiers, les logisticiens impliqués dans des transports par voie maritime, les assureurs, les responsables de l'aménagement des zones littorales, y trouveront des moyens de conforter leurs décisions. Les collecteurs de ces données pourront mieux saisir l'intérêt de leur tâche pour les applications industrielles. Enfin, les esprits curieux mesureront l'apport des nouvelles technologies de capteurs, en particulier les satellites, et des simulations sur ordinateur, à la connaissance des mers et des océans.