Reliability design of monopile foundations in spatially variable soil considering random loads

Due to the spatial variability of soil properties and the uncertainty of marine environmental loads, reliability-based design of monopiles that are extensively utilised in the offshore wind industry is essential. However, there is limited research on the effects of the spatial variability of soil on the capacity and reliability design of monopiles, potentially resulting in over-conservative or non-conservative design approaches. In this study, the effects of the spatial variability of undrained soil on the random individual capacities and probabilistic failure envelopes of monopiles are first investigated. Then, the relationship between the appropriate design (characteristic) quantile of soil shear strength and normalised scale of fluctuation and coefficient of variation of soil strength is established accounting for both soil spatial variability and load uncertainty. Finally, a design framework for monopiles using appropriate design quantile of soil strength is proposed and verified via an example. The results show that the monopile designed based on the proposed framework can meet the two reliability design requirements set in DNV (2013) while achieving a 32% reduction in diameter compared with conventional DNV-based design. The study could be helpful to the design of monopile foundations in practical engineering.