exposed www.flyingfocus.nl


tions corresponding to annual exceedance probabilities of say 2*10-2


or lower, (i.e. a


return period of 50 years or larger), is rather limited, Mr Haver adds.


This means that empirical knowledge is limited to occurrence rates much more frequent than those that could pose a threat to a properly designed structure. According to some design codes an offshore structure is expected to withstand the 10-4


- annual


probability wave event - with at most, some minor local damage. This means that a considerable extrapolation from this empirical experience has to be done in order to predict wave events corresponding to occurring frequencies in the order of 10-4 per annum.


The 10,000-year wave Mr Haver stresses that he prefers to refer to extreme waves and responses by their annual oc- currence probability rather than the return period. His experience is that most people are more concerned when the wave is


defined as occurring during next year with probability 10-4


, rather than the 10,000-


year wave – although – of course, it is the same wave.


The extrapolation referred to above, needs support from the underlying physics. “How- ever, the belief regarding governing physics can only be consistently verified from avail- able measurements and yet, we do not have any measurements from the most extreme sea states!”, he points out. “Can we be sure that governing physical mechanisms are the same for the 10-4


ity weather event as for the 10-1 weather events?,” he asks.


– annual probabil- – 10-2


Many readers will associate Mr Haver with the New Year Wave of January 1, 1995, which was measured at a Statoil platform.


Mr Haver took on the role of “presenting this strange beast of a wave” at conferences. Initially, he did a presentation at a wave conference in Banff. But at this time he did


not say anything about how unlikely it was. A few years later he presented more details about the wave in Melbourne in Florida. Here, Mr Haver comments he gave his opin- ion, based on gut feeling, on how the sea surface could have been one or two minutes immediately prior to the wave, during the wave and a few minutes after the wave. Mr Haver was then asked by Professor Al Osborne for the measured time series including the Draupner Wave because he wanted to see if a mathematical model could explain that sort of behaviour. The rest as they say is history. Since then, the Draupner platform wave has been analysed many times by people all over the world.


Do freak waves exist? Mr Haver also has personal experience of this type of wave event. Prior to that winter some work was done from a temporary working plat- form underneath the cellar deck. The work was not completed before the winter so Mr Haver had to consider whether a temporary deck could be left there over the winter.


report 7


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