Response Based Design Conditions in the North Sea: Application of a New Method

Abstract

SUMMARY The estimation of rare, extreme environments from a relatively short oceanographic database is crucial to the calculation of loads and responses of offshore structures. A novel and consistent approach has been developed that makes use of a number of asymptotic properties of extremes. A storm can be adequately characterized by its most probable extreme wave or the resultant structural response. This allows us to treat storms, rather than sea states, as the essential random, independent events; to account correctly for uncertainty in the largest wave and structural response within a storm; and to deal with the uncertainty in the seventy of randomly arriving storms, including the rare storms that are more severe than those in the database. The method provides the long term load statistics essential to reliability analysis and to the calibration of environmental load factors in design codes. We demonstrate the application of the method to the prediction of extreme waves, loads and "response based" environmental design conditions for a drag dominated structure in the North Sea. INTRODUCTION The estimation of extreme waves. The loads induced by extreme storms are critical in the design of offshore structures for location in severe seas, The load arises References and figures at end of paper from a combination of waves, currents and wind, though waves are generally the dominant factor. The problem of specifying meteorological and oceanographic (met-ocean) design conditions is one of estimating these environmental variables corresponding to some return period, typically 100 years, on the basis of measured or hind cast time series extending over a relatively short period, 5 to 25 years. For the North Sea and some other areas, a widely used approach to obtaining design wave heights has involved fitting cumulative distributions to the significant wave heights of successive three hour sea states[1,2]. U is common to neglect both the correlation between successive sea states and uncertainty in the extreme wave of a sea state. In spite of this, the distributions are used to estimate a 100 year return value. Similar processes are used separately foreach met-ocean variable. The consequent 100 year wind, 100 year wave and 100 year current are conservatively assumed to occur simultaneously and act in the same direction. For a typical jacket structure, this will lead to a "design load" that is much more severe than the "true" 100 year load.