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Structural Response of a Multi-Hinged Articulated Offshore Tower Under Seismic Excitation

[+] Author Affiliations
Syed Danish Hasan, Nazrul Islam, Khalid Moin

Jamia Millia Islamia, New Delhi, India

Paper No. OMAE2009-79315, pp. 281-290; 10 pages
doi:10.1115/OMAE2009-79315
From:
  • ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 1: Offshore Technology
  • Honolulu, Hawaii, USA, May 31–June 5, 2009
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-4341-3 | eISBN: 978-0-7918-3844-0
  • Copyright © 2009 by ASME

abstract

Articulated towers are the compliant offshore structures that are designed with high degree of compliancy in horizontal direction and to remain relatively stiff in vertical direction. The nonlinear effects due to large displacements, large rotations and high environmental forces are of prime importance in the analysis. This paper investigates the structural response of a 580 m high multi-hinged articulated tower under different seismic sea environment in a water depth of 545 m. The articulated tower is represented as an upright flexible pendulum supported on the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air; a concentrated mass above still water level (SWL). For computation of seismic loads, the tower is idealized as a “stick” model of finite elements with masses lumped at the nodes. The earthquake response is carried out by time history analysis using real sets of Californian earthquakes. Disturbed water particle kinematics due to seismic shaking of sea bed is taken into consideration. Nonlinear dynamic equation of motion is formulated using Lagrangian approach. The approach is based on energy principle that relates the kinetic energy, potential energy and work of the system in terms of rotational degree-of-freedom. The solution to the equation of motion is obtained by Newmark-β scheme in the time domain that counters the nonlinearities associated with the system in an iterative fashion. It is observed that with the increase in water depth, additional hinges are required to compensate the increased bending moment due to additional earthquake loads. Analysis results are compared and presented in the form of time-histories and PSDFs of various responses along with combined responses due to horizontal and vertical component of ground motion using direct sum and SRSS method.

Copyright © 2009 by ASME

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