Abstract: Jacket foundations are considered as the most promising solution for offshore wind turbines (OWTs), due to its large lateral stiffness and adaptability to different marine environments. Nowadays, the study on jacket foundations mainly focuses on the bearing and deformation behavior, with significantly less attention being paid to their fatigue damage. Besides, the actual pile-soil interaction is always ignored when assessing the fatigue damage or fatigue life of the foundations for OWTs. This paper presents the development of a three-dimensional numerical model of tetrapod piled jacket foundations, with the interaction between the corner piles and the soil being implemented into it. The numerical model is validated against centrifuge tests. Then, a time-domain fatigue analysis method for OWT jacket foundations is proposed. The influences of pile-soil interaction, wind-wave load coupling effect, and lateral loading direction on the fatigue damage of the jacket are examined in detail. Main findings include: (1) Neglecting the pile-soil interaction will yield significant underestimation, i.e., approximately 40%, of the fatigue damage of OWT jackets, and this becomes even more discernible at the lower oblique support joints, exhibiting an underestimation up to 90% in certain scenarios. (2) Wind load plays a dominant role in determining fatigue damage of the jacket, and the proportion of fatigue damage caused by winds is much greater than that caused by waves. (3) The lateral wind-wave loading along the diagonal direction develops a greater displacement of the jacket than that along the orthogonal direction. However, the corresponding fatigue damage is relatively small, about 70% of that along the orthogonal direction.

Key words: jacket foundation, fatigue analysis, pile-soil interaction, wind-wave coupling