Butt Welds in Unstiffened Plates

Butt welds are important connections in most types of welded structures. Examples are plated structures used in ships, floating offshore platforms, frame structures fabricated from tubular sections, tethers connecting tension leg platforms to the sea bottom, shell structures used in semi-submersible platforms for oil and gas production, towers for support structures for wind turbines, foundation piles, pipelines, risers, umbilicals, and so on. Many of these structures are subject to dynamic loading, and a reliable assessment of the hot spot stress at these connections is required for calculation of fatigue life.

A stress concentration factor (SCF) can be defined as a stress magnification at a detail due either to the detail itself or to a fabrication tolerance, with the nominal stress as a reference value. The maximum stress is often referred to as the hot spot stress and is used in conjunction with S-N data for fatigue life calculation. It is also termed geometric stress or structural stress. This hot spot stress is derived as the SCF multiplied by the nominal stress. The effects of misalignment due to eccentricities at plate thickness transitions and fabrication tolerances on the hot spot stress also need to be considered. In scaled plate test specimens, additional stress due to angular misalignment resulting from welding distortion may also be of importance for the resulting hot spot stress, as explained in Section 2.2.1. However, such an effect is normally considered to be of less significance for actual structures than laboratory test specimens due to differences in boundary conditions during fabrication and loading. The angular deviation may be more significant for structures with thinner plates.

SCFs due to misalignment at butt welds in plates were presented by Maddox (1985) and have been included in fatigue design rules for plated structures for many years. A simple butt weld between two plates, as shown in Figure 5.1, is considered as an introduction to the derivation of SCFs for butt welds. It is assumed that the plates are welded together from plates of the same size, with an eccentricity, δ, and without angular misalignment. The plates are subjected to a membrane loading per unit width N = σnominal · t, where σnominal is nominal stress and t = thickness of the plates.