Coupled Discrete Element-Finite Difference Method Simulations of OMNI-Max Anchor Installation in Granular Seabed
Authors: Yang Yancheng , Liu Haixiao 
Published: December 2020

Abstract
The ocean resources development is becoming increasingly urgent as the depletion of land reserves combine to enhance demand. In some of the deep-water areas, terrigenous deposits are found near landmasses, which are formed from material eroded from the land surface. They are constituted of clay, silt, sand, and granular soil. More and more gravity installed anchors (GIA) are employed as a part of the offshore foundation systems in these deep-water areas. A kind of newly developed GIA, called OMNI-Max anchor, with a mooring arm located near the anchor tip that is free to rotate about the anchor length, are an effective approach for mooring marine devices to the ocean floor. While providing the reaction force, these anchors can maintain the stability of offshore facilities. The ability of the anchor to achieve these duties relies on its keying and diving behaviors after penetration. Both shallow and deep penetrations, including offshore foundations and anchors penetrations, in granular media are what long interests geotechnical and geophysical fields. The final penetration depth of GIA in the granular seabed is also influenced by quite a few factors, such as impact velocity, particle size distribution (PSD) and anchor surface friction. However, this kind of large-strain problem is not agreeable to typical continuum numerical methods. In the current work, we propose that the coupled discrete element method (DEM) and finite difference method (FDM) is a more proper and efficient tool to investigate the penetration of OMNI-Max anchors in granular soil. The effects of the factors mentioned above are considered in the coupled DEM-FDM simulations. The relative ultimate penetration depths for different penetration conditions are presented and quantified. The response of granular material during penetration is applied to provide insight into system response at the microscale. Energy dissipation in the assembly by both friction and collision at the particle scale is considered. Results show that anchor penetration depth grows with rising impact velocity, while it decreases with an increase of anchor surface friction. When the ratio of fluke width and median diameter of granular size is smaller than 5.6, even under a relatively loose state, the application of OMNI-Max anchor is not recommended because of difficulty in assuring the required penetration depth (about 1.3 anchor lengths, 11.90 m). Although, at the similar impact velocity, the GIA tip embedment in sand is quite shallower than that in clay, alternative GIA designs may realize higher penetrations in the sand, and prove to be viable anchoring solution for granular seabed sediments. Finally, the fabric characters after penetrations are presented to analyze and reveal the state of soil experienced drastic disturbance. The characteristics of these distributions tend to particular states depending on the relative position to the anchor, which have a significant influence on subsequent behaviors of OMNI-Max anchor during the keying process.