Monopiles are currently the preferred concept of support structures for offshore wind turbines. However, experiences from operating offshore wind farms indicate that the current design guidelines (e.g. American Petroleum Institute (API)) under-predict the soil stiffness for large-diameter monopiles. Due to the structural dynamic of a wind turbine, it is unconservative to both over-predict and under-predict the soil stiffness. Only an exact prediction is conservative. The objective with this paper is to introduce an approximate method for determining the soil stiffness of sand regarding large-diameter monopiles by modifying the initial stiffness of the API p-y formulation. The modification introduces both a stress level and a strain level correction derived on basis of sound theoretical considerations without introducing new empirical parameters. It has been shown by benchmarking with full-scale measurements from Walney offshore wind farm that the modified approach provides a more accurate determination of the total soil stiffness, although it is still under-predicted.
The p-y curves evolved primarily from research in the oil and gas industry, as the demand for large pile-supported offshore structures increased during the 1970s and 1980s. Research has included testing of full-sized piles in sand under both static and cyclic loading conditions. The p-y curves for piles in sand described by Reese et al. (1974) and O'Neill and Murchison (1983) led to recommendations in the American Petroleum Institute (API) standards for oil and gas installations (2011). In 2004 these recommendations were adopted in the Det Norske Veritas (DNV) standard (2004), which represents the current state of the art for design of monopiles in the offshore wind industry. The p-y curves for piles in sand were developed based on full-scale load tests on long, slender and flexible piles with a diameter of 0.61m (Reese et al., 1974).