Geological and Geotechnical Considerations for Floating Offshore Wind Infrastructure within the U.S. Atlantic OCS

Abstract

Current offshore wind activities within the Atlantic Outer Continental Shelf are within water depths suitable for fixed-bottom foundations, generally considered as water depths shallower than 60 m. Recent BOEM designated call areas within the Central Atlantic include two areas on the continental slope in water depths greater than 200 m that will require floating offshore wind turbines. The objective of this study is to expand upon a previously completed BOEM-funded desktop study and focus on the geological and geotechnical conditions and the engineering constraints for the deep-water Central Atlantic call areas E and F. This study focused on compiling public domain geophysical and geotechnical data to evaluate the seafloor and shallow subsurface geological and geotechnical conditions relevant to offshore wind within the area of interest. Data available from published scientific literature and government agencies were integrated and evaluated using GIS and seismic interpretation software to identify and map geohazards, seabed and subsurface conditions, and key stratigraphic geotechnical units. Our evaluation of the area of interest indicates generally favorable conditions for floating offshore wind development, with site-specific considerations to be factored into engineering design. Soil provinces with similar geologic characteristics and geotechnical properties include the upper slope (roughly 200 m to 2000 m water depths) and the lower slope (greater than 2000 m water depth). General sediment conditions for both these provinces include Holocene-Pleistocene age silts and clays with intervals of sandy clay or sandy layers related to mass transport deposits (MTDs) from gravity flows. A key difference between the provinces is the potential for older geological units, such as the coastal plain deposits (CPD) within the foundation depth of interest proximal to the upper slope, and a higher frequency of submarine landslide deposits within the lower slope. The potential for slope instability and steep seabed gradients will be a constraint for these developments, as well as the potential for hard grounds, such as submarine landslide blocks or authigenic carbonate/benthic features. Based on these conditions, the suitability of various anchor concepts is discussed.