Abstract
The long-term performance of monopile foundations supporting offshore wind turbines along the United States East Coast remains uncertain. The uncertainty stems from several factors, including limited relevant historical data and questions regarding the reliability of existing soil stiffness estimation methods when applied to monopiles and the marine sediments in which they are installed.
The objective of this research is to investigate, apply, and compare existing methods and tools used to estimate soil stiffness to assess their accuracy and effectiveness for monopile applications. To ensure relevancy to the problem at hand, the geotechnical conditions and offshore wind turbine monopile foundation specifications from the Coastal Virginia Offshore Wind (CVOW) Research Project have been applied. The method of research was divided into two primary categories: analytical methods proposed by other researchers and one-dimensional spring models using the Rocscience Incorporated software RSPile and the PLAXIS Monopile Design Tool (MoDeTo).
The Norwegian Geotechnical Institute (NGI) conducted related research through which they estimated soil stiffness for the CVOW Research Project using PLAXIS3D and INFIDEP. The results of their study have been considered for comparison with the results of the methods used in this research. The analytical methods provided small-strain stiffness values in good agreement with NGI’s result, but there are considerations that must be made to determine their relevancy to the matter in question. Results of the MoDeTo analysis demonstrated better agreement with NGI’s results than those from the application of RSPile.
In concurrent efforts beyond the specific scope of this research, various structural monitoring technologies have been employed to actively document the behavior and performance of the two CVOW Research Project offshore wind turbines. The structural monitoring effort, in conjunction with this research, will facilitate the identification of more accurate estimations of monopile foundation performance. With better estimates of monopile foundation stiffness, the industry can improve monopile designs, mitigate construction and operation risk, and reduce excessive costs associated with over-conservative assumptions used to address uncertainties of soil-monopile interaction.