Harmonic analysis of measured initial geometric imperfections in large spiral welded carbon steel tubes

• Thick spiral welded carbon steel tubes widely used in many structural applications.
• Detailed laser scans of the surface imperfections of spiral welded tubes performed.
• Data set subject to extensive 2D harmonic analyses and statistical treatment.
• Comparison to tolerance requirements of relevant European standards.
• Algebraic characterisations of girth welds, spiral welds and coil welds proposed.

Geometric imperfections have long been known to play an important role in determining the buckling resistance of metal cylindrical shells and tubes. Though the effect is more important in thin shells rather than thicker tubular members, it may still have a significant impact on the strength of tubulars where buckling occurs in the plastic range.Spiral welded carbon steel tubes with D/t ratios in the approximate range 50–150 are often used as primary load-bearing members together with sheet piling in deep retaining walls. A recent European study on such tubes aimed to devise improved and more economical design guidelines for their use. As a central part of this project, a representative selection of 18 tubes was subject to a laser survey to obtain detailed scans of the initial imperfections found on their outer surfaces, in addition to careful wall thickness measurements. The resulting high quality data set is considered to be the first of its kind.The surface imperfections of the full set of 18 tubes were collectively analysed using a combination of single and double Fourier series to assess the dominant imperfection modes and their amplitudes. It was found that the spiral welding process results in a very unique pattern of surface imperfections which is here characterised algebraically. The systematic peak geometric deviations of the tube surfaces were found to be modest and consistent with the imperfection amplitudes defined by EN 1993-1-6 for this D/t range.