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TECHNICAL PAPERS

# Station-Keeping Tests of Moored Caisson in Strong Current

[+] Author and Article Information
Subrata K. Chakrabarti

Offshore Structure Analysis, Inc., 13613 Capista Dr., Plainfield, IL

Mark McBride

HR Wallingford, Howbery Park, Oxon OX10 8BA UK

J. Offshore Mech. Arct. Eng 127(4), 315-321 (Jan 27, 2005) (7 pages) doi:10.1115/1.2073087 History: Received June 16, 2004; Revised January 27, 2005

## Abstract

A new suspension bridge is being built over the Tacoma Narrows, Washington. The bridge will be placed on a structure mounted on two large concrete caissons. The caissons are being constructed in a floating position by pouring concrete at site. During this construction period, the floating caissons are moored in place and will be subject to high currents in the Narrows at a range of drafts. In order to investigate the motions of the caisson and the mooring line loads, physical model tests were performed at a scale of $1:100$ at HR Wallingford (HRW). The actual bottom contours of the Narrows near the construction site were duplicated in the model. The catenary mooring lines were highly nonlinear. The current forces and moments on the floating caisson included steady and oscillating components due to flow separation and vortex shedding. There is an existing bridge mounted on two piers in the vicinity of the new caissons, which introduced an appreciable flow interference effect. The tests were conducted in both the ebb and flood flow directions so that the effect of the shadowing of the caisson-pier pair could be studied in the tests. The recorded results of the elastic mooring tests were compared in terms of the maximum measured tensions with a time-domain dynamic motion simulation program, MOTSIM. The results of this comparison are presented in this paper.

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## Figures

Figure 13

Measured maximum line tensions vs. Rayleigh extreme–143 ft 7 kts

Figure 12

Rayleigh fit for the measured maximum line load for the highest loaded line—143 ft 7 kts

Figure 11

Current flow past the caisson and instrumentation cables

Figure 10

Comparison of maximum line loads of HRW and MOTSIM data—143 ft draft 8.2 kt flood minus line AL

Figure 9

Comparison of maximum line loads of HRW and MOTSIM Data—143 ft draft 8.2 kt flood intact

Figure 8

Comparison of maximum line loads of HRW and MOTSIM—143 ft draft 7 kt ebb minus line IU

Figure 7

Comparison of maximum line loads of HRW and MOTSIM—143 ft draft (43.6 m) 7 kt (3.6m∕s) ebb

Figure 6

Maximum line tension for damaged 143 ft draft flood test cases

Figure 5

Maximum line tension for intact 143 ft draft ebb test cases

Figure 4

Motion spectra for 61 ft draft caisson at 4.9 kts ebb current

Figure 3

Example time history of motion and mooring line loads

Figure 2

Load deflection curve for 143 ft draft caisson

Figure 1

(a) Mooring system design layout. (b) Catenary moored caisson in the test facility.

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