What is the Appropriate Wind Gust Averaging Period for Extreme Force Calculations?
Authors: Tromans Peter S. , Forristall George Z. 
Published: October 2008

This paper addresses the question of what wind gust speed should be used in determining the extreme environmental load on a fixed structure. The question is particularly significant for jack-ups where wind generates a much greater fraction of thetotal load than on a conventional steel template. The problem is discussed in the context of structural design practices. We develop a simple probabilistic model of environmental load in terms of a normally distributed wind and normally distributed ocean surface elevation. This allows us to apply an inverse first order reliability method (FORM) and offers a veryefficient means of calculating the appropriate gust factor. The method is successfully tested against wind and wave data. Recommendations and a working method are provided. In many practical cases, the sustained wind speed is adequate for ack-up assessment.

The approach used has, potentially, much wider application.

Introduction

Wind can induce large drag forces on offshore structures. Most often the wind load does not dominate over the wave load in the extreme design condition. In the case of fixedjackets, the wind generates a small fraction, order ten per cent, of the total extreme load and the draft ISO code 1 indicates thatthe sustained wind speed should be used to calculate the extreme global load for design of the sub-structure. A jack-up rig has a relatively large fraction of its frontal area above water-line and wind will induce a larger fraction of the total extreme load. No doubt with this in mind, the SNAME practice2 for site assessment recommends the use of the one minute wind speed rather than the sustained wind speed for assessing extreme global loads. Presumably, there must be intermediate cases that require winds between the sustained value and the one minute gust. At present, no advice is given for these situations in jack-up assessment or other codes. The objective of the present paper is to provide a straightforwardmeans of calculating an appropriate gust factor and give some general recommendations concerning wind speeds for extreme load assessment. Our studies are directed towards drag dominated spaceframe offshore structures such as jackets and jack-ups; the results are of particular consequence for the latter.

Response model and inverse FORM analysis

The approach we use is essentially to apply the first order reliability method (FORM) to identify the environmental combination that generates the extreme response to a sea state. We express a structural response, such as base shear oroverturning moment about mud line, in terms of ocean surface elevation and wind speed. We then treat the surface elevation and unsteady wind as normally distributed within a stationary sea state. The assumption of a normally distributed surface elevation has been tested in the calculation of quasi-static responses of space frames and found to be most satisfactory3 Turbulent velocity fluctuations approach normality when the gradient of intensity is not too great.4 We shall test the validity of the normal wind distribution in the present calculation later.