Abstract
The growing demand of renewable energy will inspire the continued development of offshore wind farms in the future. Floating platforms are a viable alternative in water depths exceeding about 50 meters. These platforms must be anchored to the seafloor. There are a number of current research projects investigating the viability of using helical anchors (or helical piles) for floating platforms because of their high capacity to weight ratio. The objective of this study is to find ways to further improve this efficiency by reducing the forces needed to install the anchor while maximizing its resistance to pullout. To reach that goal, a series of small-scale 1g physical model tests on 1/5-scale helical piles was conducted in loose and dense sand. Modifications were made to the helical anchor that included roughening the surface of the helix plate, adding Teflon to the helix plate, as well as performing a so-called “jetting” operation to prevent plugging of the anchor shaft during installation. Anchors were installed to an embedment of about 9 times the helix diameter with constant crowd force, and then pulled out at a constant rate. The results showed that the surface modifications influenced the installation and pullout forces. Generally rough surfaces resulted in higher installation torque and increased pullout load, while smooth surfaces resulted in lower installation torque and lower pullout capacities. The jetting operation significantly reduced installation torque but also compromised the pullout capacity of the pile.
Highlights
Perko (2009) suggests that to minimize disturbance, the anchor should be installed at a penetration rate that is at least 80% of the blade pitch for each revolutions of the anchor
The experimental setup did not allow for the anchors to be installed at a constant displacement and for this reason installation was performed at constant crowd force
For all tests in loose and dense sand, the penetration rate was recorded. These experiments were limited to constant crowd force and could not use a constant penetration rate
Summary
The objective of this study is to find ways to further improve this efficiency by reducing the forces needed to install the anchor while maximizing its resistance to pullout. The objective of this study is to further enhance pile efficiency, by finding ways to decrease installation forces while maximizing pullout capacity. The objective of the study was to increase anchor efficiency by changing the shape of the helical pile itself. The primary objective of this thesis was to find ways to improve helical pile efficiency
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