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Safety and Reliability

# Optimization of Steel Monopod-Offshore-Towers Under Probabilistic Constraints

[+] Author and Article Information

Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlandsh.karadeniz@tudelft.nl

Vedat Togan

Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkeytogan@ktu.edu.tr

Ton Vrouwenvelder

Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlandsa.vrouwenvelder@tudelft.nl

J. Offshore Mech. Arct. Eng 132(2), 021605 (Mar 10, 2010) (7 pages) doi:10.1115/1.4000401 History: Received November 15, 2008; Revised June 08, 2009; Published March 10, 2010; Online March 10, 2010

## Abstract

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the International Mathematics and Statistics Library (IMSL) routine that is based on the sequential quadratic programming. The first-order reliability method (FORM) is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented.

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## Figures

Figure 1

A monopod tower without and with segments

Figure 2

A two-level RBO algorithm

Figure 3

Variation in reliability indices of the RBO of the tower with three segments under probabilistic stress constraints

Figure 4

Variation in reliability indices of the RBO of the tower with three segments under probabilistic buckling constraints

Figure 5

A monopod tower with six segments

Figure 6

Variation in reliability indices of the RBO of the tower with six segments under probabilistic stress constraints

Figure 7

Variation in reliability indices of the RBO of the tower with six segments under probabilistic buckling constraints

Figure 8

Variation in reliability indices for segments and for the natural frequency of the RBO of the tower with six segments under probabilistic stress and natural frequency constraints

Figure 9

Variation in reliability indices for segments and for the natural frequency of the RBO of the tower with six segments under probabilistic buckling and natural frequency constraints

Figure 10

Variation in reliability indices for segments and for the natural frequency of the RBO of the tower with six segments under probabilistic buckling and natural frequency constraints (design variables are random)

Figure 11

Variation in mass and total constraint violation values with iteration (design variables are random)

Figure 12

Variation in design with a target reliability index (design variables are random)

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