The present study examines alternative design configurations on the main tubular welded connections of a prototype deep-water floating tension-leg platform suitable for hosting a wind turbine and three wave energy converters under extreme loading conditions. Several configurations of the brace-to-cylinder connection are examined numerically to improve the structural response of the welded connection. The influence of the longitudinal and transverse stiffeners of the vertical cylinders and the effect of the connection reinforcement with external stiffeners or reinforcing plate are investigated.
The evolution of cost-effective marine energy production systems encounters a multitude of challenges regarding the structural performance of offshore installations, particularly under operational and extreme environmental loading conditions (Moan, 2014). In the context of offshore steel tubular platforms, ensuring the integrity of welded joints between intersecting tubular members is crucial for structural safety. This is primarily due to the emergence of concentrated stresses and strains at critical positions near the welds, commonly known as "hot spots". These localized stresses, also called "geometric" or "hot-spot" stresses, largely arise from the complex geometry of the connection and the localized bending of the primary connection member ("chord"), coupled with cross-sectional distortion. Consequently, severe monotonic loading induces significant inelastic strains at these locations, which may lead to weld fractures. Furthermore, cyclic loading may contribute to the initiation and propagation of fatigue cracks, posing a substantial threat to the welded joint's integrity and, consequently, the overall stability of the platform (Schaumann et al., 2021). Extensive research on the ultimate and fatigue strength of welded tubular joints has been conducted (Wardenier, 1982; Marshall, 1992), which resulted in the provisions of modern design standards and recommendations.
Research program REFOS (2016-2019) has been funded by the European Commission (Mavrakos et al., 2020), aimed at developing a multi-purpose steel floating platform, suitable for the combined exploitation of offshore wind and wave energy. The innovative REFOS platform adopts the tension-leg platform (TLP) design, which is suitable for water depth of about 200 m, and incorporates an array of oscillating water column (OWC) devices for wave energy conversion, as shown in Fig. 1. The concept of a TLP structure with a 5 MW wind turbine was initially introduced by Mazarakos et al. (2015). Subsequently, it underwent further development in REFOS, particularly focusing on enhancing its structural performance, for application in a 10 MW turbine.
