ABSTRACT

The paper presents results from the analysis of data collected during six of the worst storms recorded in the last five years at the Total Oil Marine North Alwyn platform in the northern North Sea. During three of these storms significant damage occurred to offshore installations in the North Sea or West of Shetland. This paper concentrates on individual wave characteristics. It considers the joint probability of waveheight and period and, in more detail, the joint probability of waveheight and steepness for which the suitability of various models has been examined. The horizontal and vertical asymmetry of individual storm waves is presented graphically and simple empirical relationships presented for the relationship between crest height and waveheight. The profiles and the characteristics of the three largest waves in each storm are presented and discussed.

INTRODUCTION

Traditionally storms have been characterised by two parameters Hs and Tz and a spectral form such as the Pierson-Moskowitz for deep sea condition or the JONSWAP for areas of limited fetch. Recent damage to FPSOs West of Shetland and in the North Sea has shown this description to be inadequate. In these cases it was the steepness of waves and the heights of the crests that apparently had as much importance as the waveheight. Clearly more detail is needed of individual wave characteristics. Appropriate characteristics are not automatically generated using linear random wave approaches or deterministic design wave approaches. In the former case this is illustrated by joint probability models based on random linear theory failing to predict accurately observed distributions (e.g. Myhaug and Kvalsfold 1992 and Feld and Wolfram 1996). In the latter case the design wave height is usually predicted using extreme wave statistics however the associated period is not usually particularly well specified.

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