The International Ship and Offshore Structures Congress is a forum for the exchange of information by experts undertaking and applying marine structural research.
The aim of the ISSC is to facilitate the evaluation and dissemination of results from recent investigations; to make recommendations for standard design procedures and criteria; to discuss research in progress and planned; to identify areas requiring future research, and to encourage international collaboration in furthering these aims.
Congresses are held every 3 years, the next one will be in San Diego 2003, to discuss the reports of the technical and specialist committees. These reports are written to present and discuss on-going research and to make recommendations on updating of design procedures and on further research.
The mandate of Committee I.1, "Environment", is: Concern for description of the ocean environment, especially with respect to wave, current and wind statistics, in deep and shallow waters, as a basis for the determination of environmental loads for structural design. Attention shall also be given to statistical description of temperature, ice, earthquake, soil condition, marine growth and other phenomena relevant to design of ship and offshore structures.
At its Lisbon interim meeting in March 2001, the Committee established a tentative list of topics for special focus, i.e. a "short list" of half-a-dozen topics that the Committee believes deserve more in-depth treatment, not restricted to the period 2000-2002, because their subjects reach a state of consensus or because of a special request of the maritime or offshore industry for a presentation of the uptodate State of the Art. In addition, the Committee agreed to try to carry out a project, consisting in establishing reference datasets for estimation of long-term design conditions.
The present document is a brief description of these topics that is made public in order to arouse feedback and comments from all concerned bodies.
These datasets shall contain long-term histories of significant wave height and period, and as many additional parameters as possible: wind, short-term wave time-histories, data from other sources such as hindcast, etc.
Three "locations" will be considered, tentatively:
For several decades, ISSC has been recommending the use of a spectral shape bsed on the Pierson-Moskowitz formula. Its presentation based on engineering parameters and its simplicity are appealing. Yet, it suffers from a number of drawbacks that are becoming more apparent as more detailed and precise descriptions of the sea states are required and can be taken into account in the design process. Especially, the spectral bandwidth cannot be adjusted, there is no provision for directionality nor for multiple peakedness of the spectrum, and the assumption of a fully developed sea can certainly be challenged on many occasions.
Current practice favours the JONSWAP spectrum over the ISSC spectrum. In many cases, the lack of a better validated model leads to stretch the application domain for the JONSWAP spectrum far beyond the range of conditions that it was designed for. Cosine to the 2s power is commonly used to model directional spreading without much more justification.
It does not means that those current practices would be invalid or unsafe, but this shows clearly that the ISSC recommendation for a spectrum is no longer sufficient and that it should be updated. The 2003 Committee could set itself the goal to come out with a new recommendation of a model that would remain simple and of easy use, but could take into account on request directional spreading, multiple peaks, realistic energy contents for all frequency bands of interest and the shape characteristics of spectra in regions where climate is very different from that of the North Sea.
(quality, accuracy, bias, precision, ...)
A ship or offshore structure in an ice-free sea area experiences the severity of the oceanic environment in a way than can be related to the sea state described e.g. in terms of the significant wave height. It would be convenient to have a similar numeral expressing the severity of an ice covered sea area. This could then in analogy be called ice state. However, this hope has been frustrated by the complicated structure of the sea ice cover. In very large scales sea ice cover can be described as a quasi continuous nonlinear viscous fluid. In smaller scales the ice cover evolves as a random process which is difficult to link with the larger scale continuous description.
The traditional way to describe sea ice is by a list of properties: ice type, concentration, thickness, floe size, lead and ridge properties, and stress and strain. One possible line of development towards ice state would be to increase precision by adding more properties into this list and/or increasing the number of categories for these. Some other approaches have also been suggested. The Canadian Ice Regime Shipping Control System defines an ice numeral or index of hazardousness of the ice conditions to a ship. The interpretation of SAR images develop constantly and may lead to new ways to characterise ice cover. It is important to recognise that it may not be feasible to try to define the state of an ice cover with no reference to scale. Instead, an ice state should be defined in reference to one scale and combine this definition with connections between different scales.
The existence of uncommonly large waves is not disputed. The question is rather whether these waves are "normal" extremes the statistical population of high waves, or stem out from different generating mechanisms. In the first case, design procedures and standards would be mostly adequate, but in the second case, dramatic modifications might be necessary to reach the target levels of safety. Two approaches are currently being used to solve this question: 1) Analyse observations of extreme waves for consistency with the "standard" population of waves, and with the expected extreme values from conventional design; 2) Investigate with numerical simulations and test tanks on possible different generation mechanisms, and compare with available observations.
On-going work to put observations in common, to compare and to analyse results could lead to significant progress to the answer of the above interrogation, and towards the improvement of the reliability of design methods through a correct taking into account of "freak" or "rogue" waves probabilities and effects.
Members of the Committee:
Michel Olagnon, chairman of ISSC I.1 "Environment" Committee.