Although the investigation on the effect of loaded out-of-plane braces on the values of the stress concentration factor (SCF) in offshore tubular joints has been the objective of numerous research works, a number of quite important cases still exist that have not been studied thoroughly due to the diversity of joint types and loading conditions. One of these cases is the multi-planar tubular KK-joint subjected to axial loading. Tubular KK-joints are among the most common joint types in jacket substructure of offshore wind turbines (OWTs). In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified against available experimental data, was used to study the effects of geometrical parameters on the chord-side SCFs in multi-planar tubular KK-joints subjected to axial loading. Parametric FE study was followed by a set of nonlinear regression analyses to develop three new SCF parametric equations for the fatigue analysis and design of axially-loaded multi-planar KK-joints.
The primary structural part of a jacket-type offshore wind turbine (OWT), i.e. the jacket substructure (Fig. 1a), is fabricated from tubular members by welding one end of the branch members, i.e. braces, to the undisturbed surface of the main member, i.e. chord, resulting in what is known as a tubular joint (Fig. 1b). As a result of the formation and propagation of cracks due to wave induced cyclic loads, tubular joints are susceptible to fatigue-induced damage during their service life.
The significant stress concentrations at the vicinity of the welds are considerably detrimental to the fatigue performance of the joints. Hence, it is important to accurately determine the magnitude of stress concentration and to reduce it to a reasonable level. In the design practice, a parameter called the stress concentration factor (SCF) is used to evaluate the magnitude of the stress concentration. The SCF, defined as the ratio of the local surface stress at the brace-to-chord intersection to the nominal stress in the brace, exhibits considerable scatter depending on the joint geometry, loading type, weld size and type, and the considered position for the SCF calculation around the weld profile.
