Don J. DeGroot
The development of floating offshore wind energy is essential for growth of the renewable energy industry, and to decrease the reliance on fossil fuels and emission of greenhouse gasses. Advances in ease of construction and installation of turbines can make the floating offshore wind industry more marketable and feasible on a large scale; this can in part be achieved by innovation in foundation anchoring systems.
This thesis presents an evaluation of helical pile foundations for offshore deep-water application as anchors for floating offshore wind systems with catenary mooring lines in clay and sandy soils. This research outlines the influence of helical geometry, angle of installation and pile group effects on helical pile capacity and required installation torque. Axial capacities were determined using Terzaghi’s general bearing capacity equations, lateral capacities were determined using American Petroleum Institute Recommended Practice and required installation torque was estimated using a Cone Penetration Test based system of equations. Models of semisubmersible and spar buoy floating offshore wind turbine platforms with single-line and multiline catenary mooring systems were used to generate expected anchor loads for design. Given the variance in functionality, both single piles and pile groups of four and nine were considered in 5 the analysis. Load conditions and pile configurations were compared to suction caissons and evaluated for efficiency and ease of construction.
The use of a spar buoy platform and multiline system decreased the required helical pile size compared to a semisubmersible platform and a single-line mooring system. Helical piles designed for sand were smaller in size compared to helical piles in clay due to greater strength of the sand. Individual helical piles designed for pile groups were smaller in overall geometry than single piles, where battered helical pile groups were lighter and vertical helical pile groups were heavier than single piles in terms of net weight. However, smaller helical piles required a fraction of the torque needed to install the larger, single helical piles. For efficiency in terms of capacity per unit weight, single and groups of helical piles are more efficient than suction caissons, using less steel to produce the same capacity. For catenary line systems the helix of a vertically installed helical pile does not contribute to lateral load resistance but it does provide a means for potentially quieter installation compared to driven piles.