Following the company’s standard approach for predicting the crest height level, (result- ing effect of tide, storm surge and waves), corresponding to an annual exceedance of 10-4
per year, Mr Haver concluded that the
deck could be left there. But just after New Year 1995, several messages reached him from the operational unit of the platform. One message was an old friend from uni- versity, exclaiming, “You were wrong!”. It turned out the platform had experienced a 10,000-year wave!
CresT JIP: Towards an understanding of extreme waves and their impact on offshore structures
Wearing his other hat, Sverre Haver is also Chairman of the CresT JIP, initiated by MARIN. This JIP involves nearly 30 wave and offshore experts from industry and academia and aims to improve our understanding of the impact of large wave crests on offshore structures. Since its start in October 2007, numerous field measurements of severe hurricane and major storm environments - including steep wave events - have been analysed by Shell. Hindcast analyses of some of these environments were provided by Oceanweather. A selection of the most extreme environments was calibrated and measured in MARIN’s Offshore Basin by a large spatial wave probe arrangement. Forristall Ocean Engineering then analysed these measurements, examining abnormal waves, wave crest and height distribution. Hydrodynamic responses, loads and their impact on a TLP during these extreme sea states were determined by model tests. Designed by Aker Solutions, the TLP was of moderate size and based on those operating in the Gulf of Mexico. The model was built by MARIN and tested in the Offshore Basin. Impressive wave-structure interactions related to wave run-up and high deck loads, due to airgap exceedance, were observed. MARIN is now analysing this data to provide Imperial College and Ocean Wave Engineering with input for the development and use of non-linear numerical models for the prediction of non-linear wave phenomena. Based on the findings so far, DNV will develop enhanced rules and regulations which quantify the risk due to extreme wave crests.
Fortunately, there was no damage to the platform. But after that event Statoil reviewed its procedure for extreme crest height predictions for the North Sea. In view of the updated crest heights, the annual exceedance probability of the crest height (18.5 m) of the New Year Wave was in the order of 10-3
.
Many questions still fascinate Mr Haver. “Do ‘real freak’ waves really exist?,” for instance. Mr Haver admits that at present it is hard to answer this question with reasonable confi- dence. “Although I think that so-called real freak waves do exist, I will not be deeply concerned provided the observed occurrence rate of ‘freak waves’ is the true frequency, (i.e. it will be valid also if we experience sea states well beyond those we have frequently experienced). If that is the case they will be too rare to affect the design of offshore structures.”
Can offshore platforms be truly safe? Another question always on the industry’s mind is if there is any such thing as a safe offshore platform that cannot fail. “Within the framework of acceptable costs, I don’t think one can design a struc- ture that cannot fail,” he says. The challenge for those involved in the design process is to fulfil all of the rule requirements in a robust way but at a competitive cost,” stresses Mr Haver.
A number of challenges still need to be addressed, he comments. One concerns the life extension of existing structures approaching their design life. Another more fundamental challenge, he says, is still the reliable prediction of the frequency of occurrence of the most extreme weather events.
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