Reliability and applications of a new design method for calculating hot-spot stress in CHS double K-joints under arbitrary combined loading

This paper presents further development of a simple and accurate new method for calculating hot-spot stress in circular hollow section (CHS) double K (DKK) joints to enable safe application for practical design. Such joints are widely used in the construction of offshore wind turbine (OWT) jackets and use of this design approach offers the opportunity to reduce the weight of these structures. The new hot-spot stress calculation method decomposes any arbitrary brace load combination (axial load, in-plane bending, out-of-plane bending) into six basic load cases and employs only 11 key stress concentration factors (SCFs) to define stress distributions around the weld toes of all four brace-chord connections in the DKK joint. This paper applies the UK Department of Energy (UK DEn) criteria for acceptance to ensure that the new method is safe. This design method is then applied to design two representative jacket structures in the North Sea (Seagreen and East Anglia wind farms) and the structures are compared to those designed using the current industry practice of calculating hot-spot stress. This comparative study indicates that steel weight savings of 10.43% and 7.07% may be achieved using the new hot spot stress calculation method whilst achieving the same predicted service life.