Title: Numerical Modeling of Torpedo Anchors

Sponsor: Minerals Management Service

Description: Torpedo-shaped anchors serve as foundations for risers and floating structures in the deep-water marine environment. Such cone-tipped, cylindrical steel pipes, filled with concrete and scrap metal, penetrate the seabed by the kinetic energy they acquire during free fall through water. Estimation of the embedment depth is a crucial part of the design process in that the pull-out capacity is strongly dependent on the strength of the surrounding soil. In this project, we have developed a procedure based on a computational fluid dynamics (CFD) model for the prediction of the embedment depth of torpedo anchors. By means of a representation of the soil as a viscous fluid, the CFD model leads not only to the resisting forces on the anchor but the distributions of pressure and shear in the soil as well (see the Figure below depicting water trapped in the soil during torpedo-anchor installation). These distributions are then imported in another computational tool for finite-element (FE) analysis of coupled deformation and fluid flow in porous media for further simulations of reconsolidation of the soil next to the anchor and, ultimately, short-term and long-term capacity estimation.

Image:

Torpedo Pile

"Volume-fraction-of-fluid" during torpedo-anchor penetration into the seabed (soil shown as "blue" fluid, water as "red").

Publications:

Raie, M.S. "A Computational Procedure for Simulation of Torpedo-Anchor Installation, Set-Up and Pull-Out," Ph.D. Dissertation, The University of Texas at Austin, 2009.

Raie, M.S., and Tassoulas, J.L. "Installation of Torpedo Anchors: Numerical Modeling," Journal of Geotechnical and Geoenvironmental Engineering, American Society of Civil Engineers, Vol. 135, No. 12, pp. 1805-1813, December 2009.

Title: Seabed Scour and Buried-Pipeline Deformation Due to Ice Ridges

Sponsor: Bureau of Safety and Environmental Enforcement

Description: Scour of the Arctic seabed has long been identified as a potentially catastrophic environmental hazard for marine pipelines. A moving ice ridge can scour the soil and destroy a pipeline in its path. In contrast to "upheaval buckling," another phenomenon of grave concern, in which, the relevant (thermal) load is longitudinal, the effects of ice-ridge scouring are associated with lateral loads on pipelines. Also, while upheaval buckling can be prevented by means of heavy (rock or other material) cover placed on top of the pipelines, protection from scouring typically relies on sufficient pipeline-burial depth, or adequate trenching and trench-backfilling requirements. There is general agreement that the ice-ridge scour depth along with the pipeline burial depth (both measured from the top of the soil) can be used in evaluating the outcome of ridge-soil-pipeline interaction. If the scour and burial depths are about the same, there is little doubt that pipeline integrity will be compromised. At the other extreme, if the burial depth is sufficiently greater than the scour depth, the pipeline is not expected to undergo any significant deformation. Survival of the pipeline can then be assumed. The minimum "sufficient" burial depth has not been firmly established but values as low as three times the scour depth have appeared in the literature. In the intermediate range (burial depth equal to a few scour depths), it appears reasonable to expect that the pipeline can be designed so that survival can be ensured, perhaps, with some permanent deformation. Such an outcome may be preferable to the costlier alternative of specifying a greater depth of burial. That these three ranges are meaningful for design purposes has been demonstrated through laboratory and field observations of substantial "sub-scour" soil deformation (at a few scour depths below the top of the soil). However, it is important to keep in mind that sub-scour soil deformation can only serve as an indicator of the loads likely to be experienced by the pipeline. Only through a study of the complete ridge-soil-pipeline system can the actual levels of stress and strain in the pipeline be ascertained. A complete system study is undertaken in the present study.

Image:

Seabed Scour

Seabed scour by Computational Fluid Dynamics simulation: a snapshot of the deformed configuration of soil scoured by a ridge "keel" (in the form of a wedge) moving (right-to-left) at a speed of 0.5 m/s with attack angle of 30 degrees in rather soft soil with undrained shear strength of 2500 Pa (the scour depth is set and kept at 1 m and the width of the "keel" at 2 m; symmetry of the system is assumed; one half of the ridge-soil model is shown).

Publications:

Fadaifard, H., and Tassoulas, J.L. (2012a). "A Computational Framework for Fluid-Structure Interaction," article in preparation.

Fadaifard, H., and Tassoulas, J.L. (2012b). "Analysis of a Fully-Coupled Ridge-Soil-Pipeline System," article in preparation.

Tassoulas, J.L. (2009). "Seabed Scour and Buried-Pipeline Deformation due to Ice Ridges," Phase I Report to the Bureau of Ocean Energy Management, Regulation and Enforcement, http://www.boemre.gov/tarprojects/601.htm, August 6.

Title: Moving Loads on a Layered Half-Space

Sponsor: Ministry of Land, Transport and Maritime of Korean government through the Core Research Institute at Seoul National University for Core Engineering Technology Development of Super Long Span Bridge R&D Center.

Description: The dynamic behavior of a layered half-space subjected to moving loads is of considerable engineering concern. It is relevant in pavement design and particularly significant in the performance of lines for high-speed trains. Accordingly, the response of layered media to moving loads has been explored in many studies. In the present project, using a method based on semidiscretization, responses of a layered half-space to moving line loads are examined. Since the method relies on a discretization of the medium in the direction of layering while analytical solutions are used in the horizontal direction, it is not only effective as a simulation tool but computationally efficient as well. The half-space underlying the layered stratum has often been replaced with a layer on rigid bedrock at sufficient depth. In the present project, a different approach is taken, on the basis of "continued-fraction absorbing boundary conditions (CFABCs)" which are versatile in modeling wave propagation in various unbounded domains. Using this enhancement, dynamic responses of a layered half-space subjected to a series of constant and time-harmonic line loads moving at a constant speed are studied.

Image:

Moving Loads

Effects (at the interface between the layer and the underlying half-space) of a single time-harmonic load on the surface of a layered half-space (see Lee et al. 2012).

Publications:

Lee, J.H.. Kim, J.K. and Tassoulas, J.L. (2012). "Dynamic Analysis of a Layered Half-space Subjected to Moving Line Loads," to appear in Soil Dynamics and Earthquake Engineering.

Title: Bridge Elastomeric Bearings

Sponsor: Vietnam's Ministry of Education and Training.

Description: Elastomeric bearings are widely used in bridge supports to accommodate thermal and other movements. In this study, an earlier investigation of two-dimensional bearing performance to three dimensions is extended to the more realistic three-dimensional case. Large-deformation rubber hyperelasticity is adopted for the description of material behavior and a theoretical model is assembled with the steel-reinforced bearing subjected to compression in the direction through the thickness followed by shear in various lateral directions, including bridge longitudinal and transverse directions. Computations are carried out using a general-purpose, finite-element analysis computer program and the effects of shear direction on bearing behavior are examined.

Images:

Elastomeric Bearing 1

Elastomeric bearing deformed due to vertical load.

Elastomeric Bearing 2

Elastomeric bearing subjected to vertical and longitudinal loads.

Elastomeric Bearing 3

Elastomeric bearing subjected to vertical and transverse loads.

Elastomeric Bearing 4

Slight increase in maximum and minimum principal stresses and more substantial decrease in maximum shear stress as the direction of applied displacement changes from longitudinal to transverse.

Publications:

H.H. Nguyen and Tassoulas, J.L (2010). "Directional Effects of Shear Combined with Compression on Bridge Elastomeric Bearings," Journal of Bridge Engineering, American Society of Civil Engineers, Vol. 15, No. 1, pp. 73-80, January-February.

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