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Design for Ship Safety in Extreme Seas

Periodic Report Summary 2 - 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, 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:

The risk for ships to encountering dangerous sea states has been emphasized 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.

The overall objective of EXTREME SEAS is to provide technology and methodology that need to be a part of design for ship safety in extreme seas. Further, the objective of the project 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.

To reach these objectives EXTREME SEAS is studying physical and statistical properties of extreme waves, and developing advanced numerical and physical simulation models for wave-structure interaction. The developed methodology and tools will be generally applicable to different ship types. The case studies considered in EXTREME SEAS are devoted, however, to container vessels, passenger ships, LNG carriers and product and chemical tankers. Weaknesses of the current design procedures for ship structures will be highlighted. The project is also accounting for the expected trends in storm intensities and will help shipping industry to adapt to climate change.

In the EXTREME SEAS project the work is organised in seven main inter-linked workpackages. The first three workpackages 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. Experimental tests of extreme and rogue waves and response of the ship models in these waves represent a separate activity in the project while the technology development is carried out in two ship analysis workpackages. One workpackage of the project is dedicated to applications and assessment of the impact of the EXTREME SEAS developments on current ship design procedures.

Project Results:
In the first 32 months from the project start focus was given 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 and field data, as well as on strengthen the link between wave and ship project activities. The results from numerical wave model simulations as well as experimental and field data have been used to perform statistical analysis of extreme and rogue waves and to provide their probability of occurrence. Long-term trends of wave and wind climate in the North Sea, Norwegian Seas and the North Atlantic as well as warning criteria have also been studied. Further, model tests have been carried out, without and with a ship present, in the Spanish basin CEHIPAR and the model tank of Technische Universität Berlin. Development and enhancement of advanced numerical and physical simulation models for wave-structure interaction has also taken place in order to provide an efficient method for the prediction of extreme load and response events from the nonlinear wave field.

Significant results have been achieved in the second period of the project duration covering months 19-32. Consensus regarding mechanisms responsible for generating of rogue events has been reached. They include: linear Fourier superposition, crossing wave systems, wave-current interactions, quasi-resonant interaction (modulational instability) and shallow water effects. It has been shown that the known effect of the modulation instability can occur for long internal waves.

Numerical codes allowing studying the evolution of extreme and rogue waves in time and space by non-linear solutions have been enhanced. This includes the approximate envelope models (the nonlinear Schrodinger equation and the Dysthe equation), and strongly (High-Order Spectral Method, HOSM) and fully (Euler equations in conformal variables) nonlinear codes. Wave kinematics can be provided now and the modules of slow switching on the nonlinear effects and wave breaking regularization have been introduced to the HOSM code.

Characteristics of individual extreme waves, wave groups and statistical characteristics of water surface (including waveforms, kinematics etc.) have been obtained through dynamic numerical simulations and verified by laboratory model tests and in-situ field measuremets. The results have confirmed the earlier project findings; wave directionality affects the modulational instability and consequently the statistical properties of water surface elevation and wave kinematics. The two-dimensional Benjamin-Feir Index, which can be used for identification of rogue waves, has been proposed. Probability of occurrence of rogue waves has been indicated.

Climate changes of wave conditions in the North Atlantic have been studied and a procedure allowing incorporates climate trends in joint met-ocean description use today in design has been suggested.

As a part of development of warning criteria for extreme waves the cruise ship Louise Majesty accident has been analysed. The breather solutions of two coupled Nonlinear Schrödinger (CNLS) equations, where the wave parameters for the solutions are taken from the WAM hindcast, were used to explain the accident (see Figure 1).

Models for non-linear extreme ocean waves have been implemented into sea-keeping software considered by the project. Other technical improvements in order to assess the response in extreme seas have been introduced too and verified by model tests. While in the phase the model tests carried out consisted on generating the target deterministic wave sequences and irregular waves and measuring their kinematic, in the second phase of the tests ship models instrumented to measure motion and structural responses have been tested. The modern container vessel, the passenger ship, the LNG carrier and chemical/product tanker, have been studied. Collaborate agreement regarding the container ship was singed with the EC project TULCS in 2011.

The procedure which enables the response based optimization of a critical wave sequence in a nonlinear numerical wave tank developed in the first period of the project has been modified via combination of a wave/structure interaction analysis tool with an optimization program. A long-term analysis aiming at identifying extreme responses has been carried out.

An importance of application of a holistic approach to development of design procedures for ship safety in extreme seas has been demonstrated and discussed with Carnival and ColorLine in April 2012.

The project results have been published in recognized journals and conference proceedings and presented at the TRA Conference in Athens 26 April 2012. The collaboration workshop with World Meteorological Organization (WMO) aiming at enhancing safety of the future merchant fleet in extreme seas was organized 3-5 October 2011 in Geneva.

Potential Impact:
The EXTREME SEAS project is expected 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. Weaknesses of the current design procedures for ship structures will be highlighted.

Rogue waves are not explicitly included in classification societies’ rules and offshore standards today due to lack of consensus about their definition and probability of occurrence. Such consensus, however, is essential for the evaluation of possible revision of offshore standards and classification society rules. It is expected that EXTREME SEAS will contribute to reaching such consensus. Further, the project will proposed a consistent risk-based approach combining new information about extreme and rogue waves in a design perspective which is lacking today.

The project by accounting for the expected trends in storm intensities and providing procedures allowing inclusion of the climate trends in met-ocean conditions in current design practice will help the shipping industry to adapt to climate change. It is also important to be aware that climate changes, like increase in storm activity (note intensity, duration and fetch) in some regions, may lead to secondary effects such as increased frequency of occurrence of extreme wave events. The frequency of occurrence of combined wave systems like wind sea and swell/swells may increase also in some ocean areas due to the increase of storm intensity and change of storm tracks. This may consequently lead to more frequent extreme wave events. Therefore knowledge about extreme and rogue waves and ship behaviour in these waves will be becoming of increasing importance in the furure.

The project will relate and adapt to safety regulations including Classification Societies' Rules. EXTREME SEAS is expected to contribute to enhancing safety at sea not only in Europe but also world-wide.

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