Search

Physical events, such as swell or internal waves, have a strong signature on ocean SAR (synthetic aperture radar) images. The spatial periodicity of these phenomena justifies the use of spectral analysis tools for their detection, localization and pertinent parameter estimation (wavelength, direction of propagation, energy, ...). However, these spectral phenomena are highly non-stationary, especially near the coast where the bathymetric conditions greatly influences the propagation. As the study of these variations are important near the coast, because of human activity, spectral analysis tools robust in non-stationary situations have to be considered. For that purpose, investigations were carried out to evaluate the efficiency of the Wigner-Ville distribution. This representation exhibits very good properties and has been quite extensively studied, even if the numerical implementation still presents some difficulties.

Up to this day, design recommendations or practices for floating structures, such as DNVs RP-C205 and F205 for example, do not account for the existence of low frequency free Infragravity (IG) waves. These are low-frequency water waves which represent an additional excitation for (moored) floating structures. In this study, the influence of free IG waves on moored floating structures response is investigated and compared to second-order low-frequency wave forces, as the standard low frequency excitation, for two selected important free IG events in 2013 and 2014 at the SEM-REV (Site d'Experimentation En Mer - Récupération de l'Energie des Vagues) test site near Le Croisic, Pays-de-la-Loire, France. The wave forces and motion response for two example floater-mooringsystems are calculated and compared for the different frequency ranges separately and combined. Using the latest developments in the wind-wave framework WAVEWATCHIII®, e.g. the parameterized implementation of free IG sources at the reflecting boundaries, directional wave spectra including the low-frequency IG wave band are used. This study has been conducted between November 2014 and October 2015 in the scope of the axis 7 «Sea motions and interaction with marine structures» of the cluster of Excellence LabexMER "A changing ocean".

The design of catenary risers, riser towers, and export lines is key to oil and gas production in the deep ocean. Winds, waves, surface currents and current profiles are essential inputs to the design of these riser systems. With regards to current loading, the variety and abundance of current profiles can make it difficult to determine which profiles are most appropriate for determining extreme and fatigue loading on risers. The choices become increasingly difficult on the frontier of deepwater oil and gas developments as risers are increasingly being required to successfully operate in harsher environments and deeper water. However, longer and more detailed current measurements from in-situ deployments continue to become available. Two or three year current profile records are now common. In order to progress in the specification of current profiles and in the methodologies of using these profiles in the riser design, four oil & gas companies, BP Exploration Operating Company Limited, PETRÓLEO BRASILEIRO S.A. - PETROBRAS, Statoil ASA and TOTAL E&P Recherche Développement SAS, have funded the WACUP project (Worldwide Approximations of Current Profiles). The first objective of the WACUP project is to establish best practices for reducing large current profile databases into a smaller, representative set of profiles that can be used for designing risers. Three techniques have been compared in their ability in reducing measured in-situ databases: Empirical Orthogonal Function (EOF), Self Organizing Maps (SOM) and classical Current Profile Characterisation (CPC). For Vortex Induced Vibration (VIV) fatigue analysis we evaluate the skill of these three techniques in estimating VIV damage to a Steel Catenary Riser (SCR). Our measure of skill is the comparison of the results from the reduced data sets with results from the complete database. The standard techniques are modified, improving the accuracy of the database reduction. We recommend that any of these techniques can be made suitable for concept selection or preliminary engineering, but it may be desirable to run the full set of Gold Standard profiles for full detailed structural design. The second objective is to better understand the means to deal with extreme current profiles, in one part for fatigue damage and in another part for static loads analyses, to compare to traditional methods and to propose, if necessary, better practices. For fatigue, due to the complexity of the relation between current profile shape and damage, extrapolation on Gold Standard damage is the only way to produce quality n-year damage return values. Concerning extreme static response, the first analyses show that CCA profiles, used in exhaustive directions, produce realistic n-year return values. The ability of the available numerical current models to complement the in-situ measurements is also assessed. Models are superior to measurements for providing a large scale regional interpretation of key oceanographic processes. They can also be used to cost-effectively quantify spatial variability in the current regime. However they must be properly validated and calibrated before use in any engineering application. The primary source of data for characterisation of current profiles in riser design remains full water column, site specific, in-situ measurement.

The knowledge of extreme events of environmental variables is an issue of increasing concern to the scientific community. Within the branch of coastal and ocean engineering there are many fields of application where an accurate estimation of long term return period events is needed, i.e. coastal defenses design, coastal flooding management, estimation of changes of the littoral morphology, offshore and onshore renewable energy devices design, etc. But not only the engineers are concerned by extreme events; biological communities in the open sea and estuarine systems are also exposed to extreme events that may affect its natural development (i.e. extreme sea levels in estuarine environments could raze fields of plants not able to deal with salt in just a few hours). But the analysis of environmental variables and their extreme behaviors is not an easy task. In most cases the problem to be solved presents a multivariate nature, which makes it of a special complexity. For instance, in the case of flooding analysis, an estimation of joint probability density of astronomical tide, storm surge and waves is needed; or in the case of floating offshore devices the parameters of interest would be waves, winds and currents. Complex mathematical methods and techniques are needed to be able to estimate the joint probability of occurrence of more than one variable at the same time. In addition, some considerations must be taken regarding the quality of the data in order to be able to guarantee the achievement of reliable estimations. In this document, an analysis of the problem of extreme value analyses, the existing methods, their limitations, and some hints to proceed are exposed. The document is divided as follows: a first part will introduce the main problems or aspects that need to be considered concerning data and exposes the main methods to solve univariate problems and the way to estimate statistical parameters; a second part is focused on the main multivariate methods; then, a fourth part will focus on the principal techniques to estimate the uncertainties; finally in the last part the available softwares useful for EV analysis are exposed. In the appendices is presented the application of these methods in two different case studies, one proposed by IFREMER and based on the design of an offshore structure mooring and the second one proposed by BRGM with a focus on a practical application to flood risk in coastal areas.