Determination Of The Period Range Associated To The Design Wave

Abstract

ABSTRACT Description of the paper This paper presents two new methods of determination of the range of periods associated to the design wave. The first is based on the analysis of the extreme wave for each storm recorded, the second on an adaptation of our individual wave height-wave period joint distribution previously published. Data used in this work came from U.K.O.O.A. waverider recordings in the northern North Sea for values Of H1/3 greater than 6 m. A total of 340 hours of sea surface recordings collected from 21 major storms at point Stevenson over a three years period were analysed by the zero upcrossing method. Applications The methods developed are adapted to determine the range of periods to be associated to a given design wave height, once given sea state data over a reasonable period of time. Results, observations, conclusions In view of the nature of severe storms, a strong statistical relationship is observed between H1/3 and Tz.u.c at the storm peaks. For a given H 1/3, variations in Tz.u.c from one storm to another and variations in periods of individual waves for a given height in a given sea state both contribute significantly to the width of the period range for the design wave. Continuous zero upcrossing analysis over each storm gives the highest waves encountered from which can be determine a 14 to 17 second range for the period of a 30 m design wave. When an appropriate choice of the spectral width parameter, evaluated by comparison with data, is used in the theoretical individual wave height-wave period joint distribution a probability distribution for the period of the design wave is obtained. This second method, which depends only on mean sea state parameters (H 1/3, Tz.u.c) leads to results quite similar to those of the first method. INTRODUCTION The period to be associated to the design wave (fifty or one hundred year wave height) can be fixed only within a certain period band, because within a given sea state there is a continuous distribution of periods for a given height and because different meteorological conditions will lead to sea states having similar extreme wave heights but different associated periods. At the present time, in the North Sea, for a design wave close to 30 m in height, a range of periods between 12 and 18 seconds is advocated. Within the climatology working group of A.R.A.E. (Association de Recherche Action des Elements) we have sought for methods in order to determine this range of periods objectively and have applied them to data concerning the most severe storms at point Stevenson (61°20' N, 0°0' E), North East of the Shetlands over a three winter period (1973-1976). We are indebted to the U.K.O. O.A. (United Kingdom Offshore Operators Association) which kindly placed this data at our disposal for this study.