Graduate Thesis Or Dissertation
 

Numerical Simulations and Microscale Analyses of Offshore Anchor-Granular Material Systems

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  • Anchoring systems are used to hold floating offshore devices (e.g., energy devices, oil platforms) to keep them on their stations. Anchors are embedded into the seabed soils. In some cases, the interface shear between anchor and seabed soils together with anchor self-weight provide the holding capacity of the anchoring systems. However, in other cases, the embedment behavior (e.g., embedment depth) and the geometry of anchor play significant roles on the holding capacity. Anchors embedded into soils are soil-structure interaction problems, which have been widely studied experimentally, analytically, and numerically. Results from experimental tests, analytical and numerical analyses have been reported in the literature looking for the macroscale response. However, few studies focus on looking for the microscale insight of the anchor-soil responses. The discrete element method (DEM) and digital image processing (DIP) can be used for microscale analysis which has been barely applied on the soil-structure practices. In the dissertation, DEM and DIP are used to investigate the macro- and microscale soil-anchor interactions in four chapters. Chapter 3 describes DIP and its application to characterize granular assemblies quantitatively and morphologically. The investigation ranges from obtaining the center of mass of particles, sphericity, local void ratio distribution, moment of inertia, and particle orientation. The void space and particle characteristics have a significant influence on the macroscale response of a granular assembly. Accurately quantification of void space and identification of particle characteristics will give better understandings of granular responses. This chapter presents quantitative approaches of local void space quantification and particle characteristics identification in the aid of three-dimensional digital image analysis. As an in-depth analysis, the authors deepen Oda’s two-dimensional image segmentation approach to three-dimensional by using Delaunay triangulation. Morphological processing (e.g. image erosion, dilation and opening) of three-dimensional binary images is used for both image segmentation and particle characteristic identification. Algorithms of void space quantification, particle characteristics identification and particle seeking are all built as well. The segmentation approach has been verified by applying it to regular packings: simple cubic and tetrahedral. Particle characteristic identification and particle seeking algorithms were verified by application to particles/assemblies with known information (e.g. geometry). Results show that the algorithms are robust and can accurately quantify/identify void space/particle characteristics of the granular assembly. Chapter 4 presents a granular-continuum interface shear model using DEM and investigates the microscale behaviors of the interface. Granular-continuum interfaces are widely present in geotechnical structures, including deep foundations, retaining structures, and anchoring applications. Interface mechanical properties are a function of the properties of the contacting soil and the characteristics of the opposing interface. Therefore, a robust understanding of granular-continuum interface behavior is essential to geotechnical practice. This chapter provides a summary of recent research on the effects of interface roughness, soil density, particle shape, and friction coefficient on interface behavior and strength and results from three-dimensional DEM simulations of granular-continuum interface shear. The trends in DEM results are compared to the previously published physical experiments; and the microscale responses of the interface simulations has been investigated. The results show that there are good agreements between DEM simulation and experimental tests under similar interface roughnesses. DEM simulations give a similar bilinear strength-displacement trend as that previously reported from physical experiments. The interface failure mechanism is contact reorientation for rougher interfaces and contact sliding for smoother interfaces, as shown through microscale investigations. Mobilization of rougher interfaces tends to change to force distribution of the surrounding granular soils. In Chapter 5, the response of plate anchors under quasi-static and cyclic loading conditions is considered. Plate anchors are embedded into the ocean floor to provide holding capacity for offshore structures. Anchor holding capacity is a function of both the anchor and soil properties. Although plate anchors have been widely studied experimentally and numerically, there is still no universally agreed-upon design approach, indicating that the problem of physics remains elusive. In this work, DEM simulations are used to investigate the behavior of plate anchors during pullout in an effort to elucidate some of the microscale physical processes that influence overall system behavior. Macroscale assembly response is compared to published experimental results and empirical solutions. The influence of embedment ratio, anchor roughness, soil density, and anchor size on holding capacity are investigated and system-scale results are shown to reasonably agree with previously published work. Thus, observations of the simulated contact force network and particle velocity during uplift are used to provide insight into anchor failure mechanisms. Finally, the model is used to briefly explore the response of a cyclically-loaded plate anchor embedded in a granular assembly. Results from DEM simulations of torpedo anchor penetration and the associated soil response are presented in Chapter 6. Torpedo anchors are a viable approach for mooring marine hydrokinetic (MHK) energy devices to the seafloor. These anchors can serve to maintain station and to provide the reaction force for an MHK device. The ability of the anchor to perform these duties is a strong function of its penetration depth during installation. This is a large-strain problem not amenable to typical continuum numerical approaches. In the current work, we propose that the discrete element method (DEM) is a more appropriate tool to investigate the shallow penetration of torpedo anchors in sands. The effects of anchor mass, impact velocity, and soil interparticle friction are considered in the DEM simulations. The relative maximum penetration depths for different penetration conditions are quantified and presented. Granular material responses at the microscale during penetration are used to provide insight into system response. Energy dissipation in the assembly by both friction and collision at the particle scale are considered. Results show that anchor penetration increases approximately linearly with an increase in impact velocity or anchor weight. Penetration decreases with an increase in interparticle friction (i.e., soil strength). Observations of microscale behaviors and energy calculations are used to provide insight into overall system response.
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