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Periodic Report Summary - EXTREME SEAS (Design for ship safety in extreme seas)

Project context and objectives:

The EXTREME SEAS is a Collaborative project (CP). The project consortium consists of two shipyards (MEYER WERFT and ENVC), two classification societies (Det Norske Veritas and Germanischer Lloyd AG), one model basin (CEHIPAR), one provider of meteorological services (Norwegian Meteorological Institute, met.no), one research institute (Institute of Applied Physics of the Russian Academy of Sciences, IAP RAS) and four universities (Universitá di Torino, Dip. Fisica Generale, Instituto Superior Técnico, University of Duiburg-Essen and Technische Universität Berlin), the last of which operates a model basin. The ES partners are representing six European countries. EXTREME SEAS is coordinated by DNV (the contact person: Elzbieta Bitner-Gregersen, e-mail: Elzbieta.Bitner-Gregersen@dnv.com).

The risk for ships to encountering dangerous sea states has been emphasised by news-media within the last years with increasing frequency. Especially accidents with subsequent pollution of large coastal areas (Erika, Prestige, MSC Napoli), ship damage (Caledonia Star, Bremen, Schiehallion, Explorer, Voyager, Norwegian Dawn) and human casualties (e.g. Norwegian Dawn) have highlighted that improvements are needed to reduce the risk of these types of accidents. The recent hurricanes in the Gulf of Mexico have confirmed that extreme sea states can be dangerous for marine structures. It is likely that the significance of severe sea state conditions for ship traffic will even grow in the future because of the expected increase of frequency and severity of extreme weather events associated with the global warming.

The existing ship accident databases do not include sufficient information allowing identification of an extreme wave event and estimation of frequencies of casualties associated with loss or damage of life / vessels / cargo. The EXTREME SEA project is overcoming these shortcomings by the assessment of the occurrence of extreme and extraordinarily steep and / or large (rogue) waves in advanced numerical and physical simulation models as well as by studying ship behaviour in these waves.

Project results:

EXTREME SEAS is studying physical and statistical properties of extreme waves, and developing advanced numerical and physical simulation models for wave-structure interaction. A further objective is to develop warning criteria for marine structures for extreme sea states and extraordinarily steep and / or large waves, and to implement them in a marine weather forecasting system operated by a meteorological office belonging to the consortium.

The developed methodology and tools will be generally applicable to different ship types. The case studies considered in EXTREME SEAS are devoted to container vessels, to passenger ships, to Liquefied natural gas (LNG) carriers and to product and chemical tankers. Weaknesses of the current design procedures for ship structures will be highlighted.

In the EXTREME SEAS project the work is organised in seven main inter-linked Work packages (WPs). The wave WPs 1-3 are research activities dedicated to non-linear wave modelling and improved understanding of mechanisms generating extreme wave events as well as to development of warning criteria for extreme and rogue waves. The sea surface and response of the ship models in extreme waves is measured in WP5 while the technology development is carried out in the ship analysis WPs 4,6. WP7 is dedicated to applications and assessment of the impact of the EXTREME SEAS developments on current ship design procedures.

In the first 18 months from the project start ES WP1 focused on explanation of mechanisms generating rogue (freak) waves, development of numerical codes for description of extreme and rogue waves, comparison and validation of wave models with experimental data and some field data, as well as on establishing link to the project ship WPs. The evolution of extreme and extraordinarily steep and / or large waves in time and space by non-linear solutions have been studied. Identification of mechanisms responsible for generating of extreme events is crucial for providing a satisfactory definition of a rogue event and for selection characteristic parameters describing it for design. Particular attention in the project has been given to one of the generation mechanism, modulational instability, being responsible for formation of unstable wave packets characterised by steep waves with a narrow band spectrum which can be identified by use of the Benjamin-Feir index (BFI).

Stochastic wave simulations have been performed within the frameworks of the approximate envelope models (the nonlinear Schrödinger equation and the Dysthe equation), and strongly (high-order spectral method) and fully (Euler equations in conformal variables) non-linear codes. Different initial conditions for unidirectional relatively narrow-banded waves have been considered (different wave intensities, spectral widths and shapes). The modules of slow switching on the nonlinear effects and wave breaking regularisation have been introduced to the HOSM code. Using the HOSM and BMNLS codes simulations have been carried out also for directionally spread wave fields. An approach allowing comparison of water surface time series with experimental results from wave tank laboratory has been proposed. Space and time have been interchanged in the approach using a group velocity.

The comparison performed between the unidirectional simulations within the Dysthe envelope model and the full equations of hydrodynamics shows that the waves, which are well-captured by the envelope approach, may be quite steep (up to about ka equal to 0.2, where k is the wavenumber, and a is the wave amplitude) and the wave packets are short (thus, a relatively wide spectrum). The approach was tested versus application to the in-situ data. Further, the analysis of laboratory data has shown that the envelope concept may not give satisfactory results when applied to wave crests. Investigations of the applicability of the envelope concept for description of extreme and rogue waves are still ongoing.

The numerically simulated waves in WP1 have provided input to WP2 for the statistical analysis of extreme and rogue waves' properties and their probability of occurrence. WP2 has utilised also the laboratory data from the experiments in which WP 1,2 partners were participating, model test data generated in the ES project, as well as existing field data available to the ES partners. Numerically simulated wave records by the NLS equation (2D, 3D), the Dysthe model (2D, 3D), HOSM (2D, 3D) and the conformal method (2D) have been analysed. The project has studied laboratory data from the MARINTEK basin, the DHI basin, the TUB tank, the Hannover wave tank and the CEHIPAR basin, as well as field data from the North Sea. The comparison with model test data generated in the project is delayed due to late signing of the contract. Comparison with field data is still ongoing. A dataset from 20 storms is chosen for this analysis wave measurements are seen to be sensor dependent. The analysis of the numerical, laboratory and field data has shown that wave directionality affects the modulational instability and consequently the statistical properties of water surface elevation. Substantial deviations from theoretical distributions of these parameters (the Normal, Tayfun Forristall and Rayleigh distribution), commonly used in design, are observed when waves are long-crested. However the spreading of the wave energy over a number of directional components can notably reduce the effect of modulational instability. Interesting changes in the wave parameters, particularly in the kurtosis, have been observed during the evolution of a wave train, both in laboratory as well as in numerical simulations. The investigations included both unimodal and bimodal spectra.

The project WP3 has focused on long-term trends of wave and wind climate in the North Sea, Norwegian Seas and the North Atlantic as well as on warning criteria. Hindcast time series of met-ocean parameters from the different hindcast databases used by different ES partners have been compared. The databases are: Norwegian database NORA10, the Portuguese databases REMO and HIPOCAS as well as ERA-40, ERAinterim and NOAA data sets. One location, important for ship design, has been selected in the North Atlantic for comparison of different methodology for extreme analysis used by different ES partners. Extreme significant wave heights have been calculated using the Generalised extreme value distribution, generalised Pareto distribution, peak over threshold model and the Weibull distribution. Maps (in GIS) showing geographical distribution of extreme significant wave height and wind speed have been developed. Further, joint models of met-ocean parameters have been established for this location. Work on the long-term statistics of sea state characteristics responsible for generation of extreme events is still ongoing.

Weather conditions which occurred during the accident of the cruise ship MS Louis Majesty, owned and operated by Louise Cruise Lines have been analysed. This analysis will support the development of satisfactory warning criteria for extreme and rogue waves. An operational wave forecasting of extreme waves cannot be performed in a deterministic way but needs to be expressed in a probabilistic manner. So far the discussion regarding the warning criteria is limited to wave parameters only.

An efficient method for the prediction of extreme response events from the nonlinear wave field is under development in WP4. This WP is collaborating closely with the wave packages and with WP6. When the extreme response event is identified, a fully nonlinear sea-keeping code will be used to get more accurate response calculations. Four ship types are considered by the ES project: a modern container vessel, a passenger ship, an LNG carrier and a product or chemical tanker. Improvements of the codes will be verified using existing benchmark cases and model tests results generated in WP5. Model test matrixes for the planned laboratory tests have been developed. There are plans to collaborate with the EC project TULCS regarding model tests of the container ship. Frames of ship analyses planning to be carried have been developed and initial calculations have been have carried out. Ship types and sizes have been decided in close collaboration with ship yards being the partners of the ES project.

A starting date of WP6 is the month 15 from the project start. Investigations regarding critical conditions in offshore vessel operation have been carried out. For the identification of critical wave scenarios a response based wave generation tool has been developed which enables the response based optimisation of a critical wave sequence in a nonlinear numerical wave tank. WP7 starts the month 25 from the project start. Overview presentations pointing out importance of implementations of EXTREME SEAS results were given at the project progress meetings. An importance of application of a holistic approach to development of design procedures for ship safety has been pointed out. Uncertainties involved need to be included in such an approach.

Three project progress meetings took placed; the kick-of Høvik meeting 3-4 September 2009, the Brussels meeting 11-12 May 2010 and the Torino meeting 4-5 November 2010. The project results developed so far are documented in 29 papers published in recognised journals and conferences proceedings.

The project website has been opened (http://www.mar.ist.utl.pt/extremeseas/). A high level 'interrelation' workshop involving World meteorological organisation (WMO), International Association of the Classification Societies (IACS) and other stakeholders, as considered relevant, is scheduled for 3-6 October 2011 at WMO in Geneva where the project will be presented and implementation of the of the project results will be discussed. Several ES results were presented at the session 'Extreme sea waves' organised by the ES partner IAP RAS at the EGU Conference 8 April 2011, in Vienna, Austria. The EXTREME SEAS session is scheduled within the 'Safety and reliability' symposium organised by the ES partner IST during the OMAE 2011 conference taken place 19-24 June 2011, in Rotterdam, the Netherlands. A brochure for marketing the project has been prepared. The draft technological implementation plan and exploitation plan has been developed.

Potential impact:

The strategic objective of EXTREME SEAS is to enable European shipping industry to improve the design of ship structures that are exposed to rough wave climate, by providing technology and methodology that need to be a part of design for ship safety in extreme seas. The project will relate and adapt to safety regulations including classification societies' rules. It will also account for the expected trends in storm intensities, helping shipping industry to adapt to climate change.

List of websites: http://www.mar.ist.utl.pt/extremeseas/

Related information

Reported by

DET NORSKE VERITAS AS
HOVIK
Norway

Subjects

Security
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