Human induced climate change has increased the urgency to move away from reliance on fossil fuel sources for energy. While wind and solar have dominated the post carbon landspace, researchers across the globe are exploring ways to harness the power of the oceans and bring marine energy into a sustainable energy portfolio. While marine energy designs encompass a breadth of concepts and topologies, many share a common need for anchoring. To meet this need, a research team from North Carolina State University and the University of North Carolina at Charlotte have been investigating a retrievable anchor concept that would be appropriate for both temporary and long term installations.
The Retrievable Ocean Petal Anchor (ROPA) device is designed to be deployed as a cylinder that is inserted into the ocean floor. Once the device reaches the prescribed depth, the sides of the cylinder (petals) are expanded from their initial configuration to increase the effective area and pullout strength of the anchor. This insertion of the ROPA and deployment of the petals are facilitated by the introduction of high pressure into the soil during installation. This high pressure water works to fluidize the soil, dramatically lowering the shear strength of the soil and thus the resistance to inserting the ROPA and deploying the petals. Once installed, and the flow of high pressure water is stopped, the soil returns to it’s non-fluidized properties.
The performance of the ROPA device has been simulated in PLAXIS geotechnical modeling software. In these simulations, the introduction of high pressure water was seen to fluidize the soil surrounding the nozzle, providing a region of low shear strength soil sufficient for installation of the anchor. The PLAXIS simulations have also shown that ROPA device can maintain it’s pullout strength through multiple loading/unloading cycles and that the vertical pullout strength of the can be increased by increasing the depth of installation.
These early results suggest that the ROPA device provides several advantages over current anchoring technologies. The vertical pullout strength of the ROPA is approximately 7 times greater than an equivalent drag anchor. With drag anchors being optimized for lateral loads, their use would require much longer mooring lines than a ROPA device which could be more vertical (directly under the device). The shorter mooring lines needed for the ROPA device would reduce the likelihood of entanglement or interference with nearby devices (a particular concern if you are deploying an array of devices). Helical or screw type anchors can provide similar vertical pullout strength to the ROPA device, but they require more specialized equipment and personnel than envisioned with the ROPA. With high vertical pull out strength, adjustable pullout strength based on embedment depth, and a relatively straightforward installation and retrieval process, the ROPA anchor could be an enabling mooring technology for marine energy devices, particularly in temporary ocean deployments for testing and certification.