Fatigue bending test on grouted connections for monopile offshore wind turbines

Wind and wave loadings induce stochastic bending moments in grouted connections for monopile offshore wind turbines; thus, the fatigue behavior under bending moment is extremely important. This paper presents test results on five cylindrical monopile grouted connection specimens with shear keys, which have an outer diameter of 612 mm, and different lengths of 4365 mm, 4475 mm, and 4585 mm. They were designed and tested under constant range of fatigue cyclic bending moments. The load ranges were established based on the ultimate limit state bending capacity of each specimen. It was found out that, although some local degradation exists, the whole grouted connection in the first four specimen can still survive two million load cycles with no obvious degeneracy in bending capacity. Furthermore, no obvious cracks were found at the end section of the grouted connections. Meanwhile, in the longest specimen, due to its maximum load range, the welded structures cracked after about 1.28 million load cycles indicating the end of testing. In addition, no destruction was found in the grouted connection itself. Moreover, after comparing the specimens with different lengths, the authors determined that the recommended minimum design length for grouted connections with shear keys was conservative in the commonly used design code.