Design Analysis of Moored Floating Caisson System

[+] Author and Article Information
Partha Chakrabarti

 Zentech Incorporated, 8582 Katy Freeway, Houston, Texas

Subrata K. Chakrabarti

 Offshore Structure Analysis, Inc., 13613 Capista Dr., Plainfield, Illinois 60544

Adinarayana Mukkamala, Nagaraj Anavekar, Shen Qiang, M. Sri Krishna

 Zentech Incorporated, 8582 Katy Freeway, Houston, Texas 77024

J. Offshore Mech. Arct. Eng 127(2), 75-82 (Oct 06, 2004) (8 pages) doi:10.1115/1.1905640 History: Received July 25, 2004; Revised October 06, 2004

Tacoma Narrows Constructors (TNC) are building a new suspension bridge in Tacoma, close to Seattle, Washington State, USA. The new bridge is being built just south of the existing bridge mounted on two caissons. The caissons are constructed on location after the shallow draft caissons are towed to site. During the construction sequence, the mooring system for each caisson consists of two sets of 16 mooring lines. The lower 16 lines are hooked-up after the shallow draft caisson is towed from the harbor and positioned at the site. The fairlead locations for these lines are kept constant throughout the construction process. The fairlead locations for the upper 16 lines (except three lines on the East Caisson) vary based on the caisson draft. The caissons are subject to a high current from the ebb and flood tide flow in the Narrows. The new caissons are in close proximity to the existing piers and the bottom topography at the site is varying. Therefore, considerable turbulence and vortex shedding is expected in the prevailing current, which will cause current-induced dynamic forces on the caissons. This paper describes the design and analysis of this multiline mooring system for Tacoma Narrows Bridge caissons, based on the construction sequence in the floating condition. The analysis involved optimizing the anchor locations and the line pretensions, determining the dynamic motions of the caissons, the maximum line loads, and the corresponding safety factors. The paper also describes the hydrodynamic analysis for added mass, and damping, the methodology used for the nonlinear moored caisson analysis (MOTSIM ), and the validation of the design tool with other similar models (e.g., STRUCAD*3D). The results of the analysis and the design of the system are discussed.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Caisson geometry

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Figure 2

Sign convention and coordinate system

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Figure 3

Mooring configuration-initial

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Figure 4

Mooring system stiffness

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Figure 5

Comparison of line tensions-dynamic

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Figure 6

Surge damping test in still water

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Figure 7

Data fit to surge damping test in still water

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Figure 8

Surge motion spectra for different damping factors

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Figure 9

Maximum line tensions for different damping factors

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Figure 10

Plot of input surge force, 61ft(18.6m) draft, 7.3Knot(3.8m∕s) flood

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Figure 11

Recommended pretension for different drafts-East Pier

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Figure 12

Dynamic line tensions of the East Caisson-intact-flood

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Figure 13

Dynamic line tensions of the East Caisson-intact-Ebb

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Figure 14

Overall summary of East Pier mooring analysis results

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Figure 15

Overall summary of West Pier mooring analysis results

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Figure 16

Maximum input force, displacement, and tension response



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