Influence of flange dimensions and geometrical imperfections on stress concentrations at welded flange necks

Abstract

Ring flange connections for tubular towers, like those for wind turbines or chimneys, are subjected to significant fatigue loading. Next to the bolts, the weld connecting the flange to the tower shell also needs to be checked against fatigue failure. The flange causes local bending moments in the shell, which increase the meridional stress, i. e. stress concentrations occur. In this paper, the influence of geometrical imperfections on such stress concentrations is quantified and the influence of flange geometry on resulting stress is investigated. Recommendations are given for flange dimensions and the design procedure.

Imperfections in realistic metal wind turbine support towers: a one‐at‐a‐time (OAAT) study

“…As the stress state from closing the parallel flange gap results in meridional compressive stress at the end of the gap (see Fig. 7 in [5]), the gap was centred at θ b = π/2 + π/6 so as to align its associated peak meridional compressive stress most detrimentally with the direction of bending as illustrated by Fig. 1.…”

“…The parallel flange gap [3][4][5] is a gap relating to the interface flatness tolerance of prEN 1993-1-6 [8] and occurs 'in between flanges before bolts are tightened' [4]. The sinusoidal shape of the gap has been defined by Equation ( 5) based on Equation 4 of Holst and Rotter [10] as…”

“…This gap is first closed by the tightening of bolts (simulated by an initial 'gap closing' computational step) before any additional load is applied. The stresses induced in the shell wall by this gap have been previously examined [3,5] and shown to cause a region of meridional tension at the gap centre, and meridional compression at the ends of the gap after gap closing. Whilst the implications of closing parallel flange gaps has been considered in relation to flange design [4,5], their impact on the buckling capacity of the WTST has not yet been studied.…”

“…In computational studies [1] assessing the strength of WTSTs, the axisymmetric circumferential weld depression [2] is often considered to be a particularly detrimental imperfection: however, it is not the only imperfection present. The sensitivity of a WTST segment to four realistic geometric imperfections which are likely to affect modern WTSTs is considered here; axisymmetric circumferential weld depressions, unintended joint eccentricity at curved plate boundaries caused by imperfections in the original plate lengths, global ovalisation of the tower segment, and the closing of a parallel flange gap [3][4][5] occurring at the base of the tower. Other factors such as residual stresses and plate thickness variations are thought to have a less significant impact on the buckling strength and thus have not been considered in this study.…”

Corrosion fatigue analysis of NREL’s 15-MW offshore wind turbine with time-varying stress concentration factors

“…The analytical approach was compared with a 3D finite element model showing good agreement. In a later paper, Seidel et al [17] investigated the impact of flange dimensions and imperfections in the flange geometry on these SCFs. It was found that connections with large shell thicknesses are increasingly likely to be critical, additionally, it was noted that geometric imperfections have negligible influence on the SCFs.…”