Utilizing geological and geotechnical parameters to constrain optimal siting of Mid-Atlantic Bight offshore wind projects

Madsen, John A.As the offshore wind energy sector expands due to government mandates, a thorough understanding of the geologic setting of potential project sites becomes an essential component in the design process. Geophysical and geotechnical parameters yield vital information on the sediments and/or rocks that are present. The variable distribution of sediments, with concomitant variations in geotechnical properties, has significant implications for the selection (e.g., monopile, suction caisson, gravity base, jacket), design, location, installation, and subsequent scouring in the vicinity of wind turbine foundations. Identifying suitable sites based on sediment types allow for optimized engineering design solutions. Because foundations represent approximately 25% of total offshore wind project expenditures, reducing foundation costs with geologic suitability in mind could significantly decrease required initial investments, thereby expediting project and industry advancement. To illustrate how geological and geotechnical data can be used to inform site selection for foundations, geophysical data were analyzed and interpreted (chirp sub-bottom profiling, side-scan sonar, and multibeam bathymetry) from the Maryland Wind Energy Area (WEA). Side-scan sonar data from the WEA show three distinct acoustic intensities; each is correlated to a general bottom sediment grain size classification (muds, muddy and/or shelly sand, and sand with some gravel). Chirp sub-bottom profiles reveal the continuity and thicknesses of various depositional layers including paleochannel systems. Paleochannels consist of heterogeneous infill; creating undesirable conditions for foundation placement. This “desktop” study provides a suitability model for how the interpretation of geophysical and geotechnical data can be used to provide constraints on, and reduce uncertainties associated with, foundation location and type selection. Results from this study revealed 5 distinct subsurface units. The oldest (Unit 5) originated from Middle Pleistocene during Marine Isotope Stages (MIS) 5 & 6. The youngest (Unit 1) consists of the modern surficial sand sheet sediments which have been eroded and reworked during recent Holocene transgression. Several distinct paleochannel systems incise the study area. Though data beyond the boundaries of the study area are scarce a southeasterly channel direction along with results from previous studies suggest these systems originated from Maryland coastal bays. An integrated marine spatial planning approach identified the southernmost portions of the study area as the most unsuitable for wind energy development. Conversely, the same analysis determined that the central-eastern section of the WEA is most suitable. Correlating these data with parameters governing foundation selection revealed that piled-type foundations (either lattice or monopile) are most appropriate for the study area, although suction bucket caisson foundations cannot be definitely ruled out as a possible design solution.University of Delaware, Department of Geological SciencesM.S