Failure Envelopes of Composite Bucket Foundation for Offshore Wind Turbines under Combined Loading with considering Different Scour Depths

The composite bucket foundation of offshore wind turbines is subjected to a variety of loads in the marine environment, such as horizontal load H, vertical load $V$ , bending moment M, and torque T. In addition, due to the characteristics of its connection section, the water flow around the foundation will produce scour pits of various degrees, reducing the depth of the bucket foundation, which has a nonnegligible impact on the overall stability of the bucket foundation. In this paper, the failure envelope characteristics of different combinations of loads on bucket foundations, including $V$ -H-T, $V$ -M-T, conventional $V$ -H-M, and noncoplanar $V$ -H-M, are numerically investigated with considering different scour depths. The numerical results indicate that the $V$ -H-T, $V$ -M-T, conventional $V$ -H-M, and noncongruent $V$ -H-M failure envelopes gradually shrink inwards with increasing scour depth, and the stability of the composite bucket foundation decreases; the conventional $V$ -H-M failure envelope shows an asymmetry of convexity to the right, and the noncongruent $V$ -H-M failure envelope shows an asymmetry of outward convexity to the left and right. The corresponding mathematical expressions for the failure envelope are obtained through the normalized fitting process, which can be used to evaluate the stability of the bucket foundation based on the relative relationship between the failure envelope and the actual load conditions, which can provide practical guidance for engineering design.