Design, Analysis and Verification of Moored Floating Caisson System

Tacoma Narrows Constructors (TNC) is building a new suspension bridge in Tacoma, close to Seattle, Washington State, USA. The new bridge will be built just south of the existing bridge mounted on two caissons, referred to as East Caisson (Tacoma side) and West Caisson (Gig Harbor side). Each pier is about 80’ wide and 130’ long in plan. The mooring system for each caisson consists of two sets of mooring lines: lower and upper. Each set consists of 16 mooring lines. The lower 16 lines consist of anchors that form a radius of about 300 feet. The fairlead locations for these lower 16 lines are kept constant throughout the construction process. These 16 lines are hooked-up after the caisson is towed from the harbor and positioned at the site. For the upper 16 lines (except three lines on East Pier), the anchor locations form a radius of 600’. The fairlead locations for these upper 16 lines vary based on the draft. Due to the proximity of the proposed caissons to the existing piers and the varying bottom topography, considerable turbulence and vortex shedding is expected which will cause current induced dynamic forces on the caissons. This paper describes the design and analysis of this multi-line 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, maximum line loads, and corresponding safety factors. The paper includes 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 design are discussed.