Modeling Behavior of OMNI-MAX™ Anchor for Offshore Floating Platforms, Using the Finite Element Method

Tabatabaei Malazi, Mohammad Hassan | 2018

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51974 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Raie, Mohammad
  7. Abstract:
  8. This dissertation documents the study of Omni-max anchor, and the development of a computational procedure and its application to the analysis of OMNI-MAX anchor behavior under mooring loads. An OMNI-MAX anchor is an offshore anchor designed and developed in recent years, to overcome the uncertainty in the capacity of ordinary offshore anchors.In the process of this research to understand the capacity and mechanisms in which the anchor carries applied loads, the contribution of different parts of the anchor to the total capacity of the anchor is estimated using classical soil mechanics. Then, due to the failure of the used approach to model the dynamic behavior of the anchor and to better simulate its behavior, a computational procedure is developed and applied to model the behavior of the anchor in the saturated clay at the seabed. This procedure consists of a finite element approach with large deformations for porous media and contact models.In the course of developing this procedure, to model the soil, the governing equation of porous media is introduced using mixture theory and by assuming the media as a two-phase domain. Equations of conservation of linear momentum and mass of the domain are written, functionalized and then discretized over the domains and the time. Then to be able to solve the system using the Newton method, discrete equations are transformed into incremental forms. Also for the constitutive model of the soil, the bounding surface plasticity model is used. To model, the anchor, governing equation of a continuum at motion is introduced using, Cauchy motion equation. These equations are functionalized, discretized and then transformed into the incremental form. To model the interface of soil and anchor, types of contacts between interfaces that might occur in the model were distinguished. The relative condition of two surfaces was assessed using a Search algorithm, a Territory Algorithm, and a Distance Algorithm. Then for each type of contact, the proper physical constraint is applied to each surface, using Lagrange multipliers.Finally, to test and verify the correctness of the software's result, various problems aimed to test different parts of the software are modeled. After finding satisfying results and making sure that the result of each segment of the software is correct, a scaled-up model of a small-scale experiment of OMNI-MAX anchor is modeled. Based on this model, it is found that the results of the software correlate with that of the experiment with a coefficient of 0.999 and the maximum error of 15 %. Based on verified results, it was observed that the difference in the dimensions of flukes, and the moment caused by the eccentricity of the anchor's arm, results in rotations and movement of the anchor. These rotations are the main reason for the anchor's loss or gain of embedment depth
  9. Keywords:
  10. Omni-Max Anchor ; Finite Element Method ; Contact Mechanics ; Porous Media ; Soil-Anchor Interaction ; Large Deformation

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