The effect of geometric imperfections on the flexural buckling of tapered spirally welded steel tubes

Abstract

Taking more advantage of wind and other sources of renewable clean energy is essential for a sustainable future. To this end, the cost of wind generated electricity needs to be significantly reduced. One solution is to build taller turbine towers taking advantage of less turbulent and higher velocity wind at higher elevations, which in turn results in producing more power per turbine and reducing the wind energy production costs. However, taller towers require larger base diameters to be able to carry the flexural loads from the turbine. Currently, transportation limitations restrict tower base diameters to 4.3 m in the US, therefore any tower taller than 80 m cannot be optimally designed (in a lowest material weight sense). To overcome this challenge, spiral welding procedure used in the pipeline industry is being adapted to manufacture tapered tower sections on site. However, spiral welding causes unique pattern and magnitude of geometric imperfections, and since the buckling behavior of slender tubes is highly imperfection sensitive, the impact of imperfections associated with the spiral welding procedure on the ultimate flexural strength of the manufactured tubes needs to be thoroughly investigated. Geometric imperfections in slender tubes can arise for a variety of reasons, and for practical purposes can essentially considered to be random. Thus, statistical characterization of the manufacturing-induced imperfections is necessary to better understand the uncertainties in the buckling capacity of slender tubes. The research completed in this dissertation enhances the design of slender spirally welded tubes for use as wind turbine towers. The work includes proposing a simplified approach and a novel measure of imperfection severity that is designed to be insensitive to noise for predicting buckling strength of slender tubes using high-resolution geometric imperfection measurements; characterizing randomness in the weld-induced geometric imperfections of spirally welded tubes and proposing a probabilistic scheme capable of simulating random realistic geometric imperfections for slender tubes; providing a foundation for reliability-based design of spirally welded tubes by presenting a probabilistic view on their buckling capacity considering the randomness in the geometric imperfections; and examining the relationship between spiral welding, induced geometric imperfections, and induced residual stresses.