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Houston, TX, United States

Papoulia K.D.,University of Waterloo | Panoskaltsis V.P.,Democritus University of Thrace | Kurup N.V.,Houston Offshore Engineering | Korovajchuk I.,Sest, Inc.
Rheologica Acta | Year: 2010

We develop rheological representations, i. e., discrete spectrum models, for the fractional derivative viscoelastic element (fractional dashpot or springpot). Our representations are generalized Maxwell models or series of Kelvin-Voigt units, which, however, maintain the number of parameters of the corresponding fractional order model. Accordingly, the number of parameters of the rheological representation is independent of the number of rheological units. We prove that the representations converge to the corresponding fractional model in the limit as the number of units tends to infinity. The representations extend to compound fractional derivative models such as the fractional Maxwell model, fractional Kelvin-Voigt model, and fractional standard linear solid. Computational experiments show that the rheological representations are accurate approximations of the fractional order models even for a small number of units. © 2010 Springer-Verlag.

Udoh I.E.,Houston Offshore Engineering
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2014

Model testing of deepwater offshore structures often requires the use of statically-equivalent deepwater mooring systems. The need for such equivalent systems arises due to the spatial limitations of wave basins in accommodating the dimensions of the direct-scaled mooring system. With the equivalent mooring system in place and connected to the model floater, the static global restoring forces and global stiffness of the prototype floating structure can be matched (to within some tolerance) by those of the model for specified offsets in the required degrees of freedom. A match in relevant static properties of the system provides the basis for comparisons of dynamic responses of the model and prototype floaters. Although some commercial programs are capable of designing equivalent mooring systems, the physics applied in these programs are protected by intellectual property, and their methodologies are generally inflexible. This paper illustrates a concise approach to the design of statically-equivalent deepwater mooring systems. With this approach, either manual or advanced optimization techniques can be applied as needed based on the complexity of the equivalent system to be designed. A simple iterative scheme is applied in solving the elastic catenary equations for the optimal static configuration of each mooring line. Discussions cover the approach as applied in developing a fit-for-purpose tool called STAMOORSYS, its validation, and its application to the design of an equivalent mooring system for a spar platform in deepwater. The spar model parameters are representative of a structure which could be tested in the Offshore Technology Research Center, College Station, Texas, USA. Results show that the method is capable of producing good design solutions using manual optimization and a genetic algorithm. Copyright © 2014 by ASME.

Gar S.P.,Houston Offshore Engineering | Gar S.P.,Texas A&M University | Head M.H.,Morgan State University | Hurlebaus S.,Texas A&M University
ACI Structural Journal | Year: 2013

Aramid fiber-reinforced polymer (AFRP) tendons, which are inherently corrosion-resistant, can be used to replace steel prestressing strands in bridge girders to enhance bridge sustainability. Despite ongoing experimental research, there is a lack of uniformity and consistency in testing procedures, definitions of material characteristics, and results. Therefore, a robust computational model is needed to perform a refined nonlinear analysis of full-scale AFRP prestressed girders. This paper presents the development of a computational model to numerically evaluate the flexural behavior of an AASHTO I-girder (Type I) prestressed with AFRP tendons in comparison to its conventional prestressing steel counterpart. Numerical results match experimental test data with a high degree of accuracy and reveal that an AASHTO I-girder prestressed with AFRP meets service and strength limit states. Numerical results also show that the deflection equation in ACI 440.4R overestimates the maximum deflection of the AFRP prestressed girder. Copyright © 2013 American Concrete Institute.

Pirayeh Gar S.,Houston Offshore Engineering | Head M.,Morgan State University | Hurlebaus S.,Texas A&M University | Mander J.B.,Texas A&M University
Journal of Bridge Engineering | Year: 2014

Abstract While application of full-depth precast bridge deck panels may be appealing because of an accelerated construction schedule and improved safety conditions, corrosion of steel reinforcement is a major factor affecting the structural durability of precast panels and overall serviceability of a bridge deck. Herein, the concept of using aramid fiber reinforced polymer (AFRP) bars as a substitute for conventional steel reinforcement to overcome corrosion issues is verified. For this purpose, a full-scale bridge deck slab consisting of full-depth precast panels reinforced and prestressed with AFRP bars is constructed and experimentally evaluated in terms of load capacity, deformation, crack pattern, and failure mode. The precast panels are reinforced and prestressed in parallel and perpendicular to the traffic directions, respectively, and supported by reinforced concrete beams. Realistic dimensions, boundary conditions, and structural details are physically modeled to represent an actual bridge deck condition, and different concentrated load configurations including wheel and tandem axle loads are applied on both the slab interior span and overhang. The experimental results show the average failure load of the interior spans and overhangs, respectively equal to 3.9 and 1.4 times the maximum factored wheel load, where the deflection serviceability criteria are met and satisfactory deformability performance is achieved. © 2013 American Society of Civil Engineers.

Lee M.-Y.,Chevron | Zeng J.,Kvaerner | Poll P.,Houston Offshore Engineering
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2014

The use of semi-submersible platforms has become increasingly popular due to its ability to carry large topsides and the possibility for quayside integration. With recent exploration successes in ultra-deepwater fields of the Gulf of Mexico, major oil and engineering companies are keen to look for a safe, reliable and cost-effective dry-tree option to maximize the value of deepwater field developments. Dry-tree semi-submersible (DTS) emerges as such an option to overcome the water depth and size limits imposed by TLP and Spar, respectively, and enables the platform to carry a large well array and payloads in ultra-deep water. This paper presents the offshore industry's multi-year efforts to mature two promising semi-submersible platform concepts that can accommodate long-stroke dry-tree risers and have large drilling and production capabilities. Results of technology development and qualification will be highlighted with details on hull performance and hull/riser interfaces. Key structural, mooring and riser analyses and scaled model test results including the long-stroke riser tensioning system will be presented. Remaining challenges that need to be overcome to advance the DTS concepts from "technology acceptance" to "project readiness" will also be discussed. Copyright © 2014 by ASME.

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