The Design of Fatigue-Resistant Structural Members Excited by Wind
Authors: Rudge Daniel , Fei Chen Yang , Nicholls Stephen , Vandiver J.K. 
Published: October 2008

Wind-induced vibration of long slender members of offshore structures may lead to fatigue damage. A review of a wide variety of design codes revealed gaps in the essential knowledge base and led to a series of wind tunnel experiments in which a flexible cylinder with pinned ends was excited by vortex shedding. The experiment results and a recommended design procedure are presented. A lack of information regarding the probability of occurrence of the necessary wind conditions is identified as the probable explanation of the disparity between predicted and observed failures in the field.

1 Introduction

In recent years there have been a number of well publicized incidence of vortex-induced vibration (VIV) in flare booms [I] [2]. In addition to these cases, significant dynamic response has also been seen in sections of elevated overland pipeline, and in jacket members, during construction and when under tow to the installation site.

The initial purpose of this research was to review the currently available procedures, assess strengths and weaknesses, and recommend a "best" one for possible adopt ion by the American Petroleum Institute for possible inclusion in API-RP2A.

Several design codes, assessing the vortex-induced vibration response of structural members, can be found in the literature [3] [4] [5]. These design codes have been reviewed recently [6] [5]. DnV(1991) [3], like most other empirical methods, stated that the response was primarily dependent on the stability parameter, Ks. This method was simple and explicit, but seems to result in excessively high response predictions and hence stress levels [7]. ESDU 85038 [4] proposed a random vibration model, taking many empirical parameters into account. This method is too complicated, and requires knowledge of several currently uncertain parameters, such as dynamic lift coefficient and spanwise correlation [6]. The Brown & Root simplified design procedure [5] was a compromise between DnV(1991) and ESDU 85038. It adopted K. in its formulation and yet allowed for a variation of the lift coefficient in different Reynolds number regimes. However, the relation between lift coefficient and Reynolds number, as adopted from ESDU 85038, needs to be further verified. The unsatisfactory performance of the existing design codes, indicates that there exist important gaps in our knowledge of aerodynamic coefficients and lock-in mechanisms, particularly with respect to the single span, dynamic, flexible cylinders in air in which our interest lies.

The uncertainties in the literature regarding essential parameters led us to conduct a series of simple wind tunnel tests on a 1.903 inch outer diameter carbon fiber tube with pinned-pinned supports. The results are discussed first in this paper, revealing the following conclusions:response at Ka values greater than 20 is capable of producing fatigue.resonant oscillation with lock-in characteristics is observable at RMS response amplitudes of as little as 1% of a diameter.large response amplitudes, comparable to that predicted by many K. based empirical methods, can be achieved at lock-in, under ideal circumstances in the wind tunnel.