Ship Collision Analysis for the Kawasaki Island Steel Platforms
Authors: Litton R.W. , Puskar F.J. 
Published: October 2008

Construction is currently underway on a bridge-tunnel across Tokyo Bay, Japan. The tunnel segment contains a concrete structure "island" of approx/mate/y 175 m (377 ft) diameter, located in 28 m (92 ft) water depth, for ventilation and access to the tunnel during construction. Due to the large number of ocean going vessels using Tokyo Bay, it was necessary to "protect" the island from ship collision by using a series of 14tubular steel structures which surround the inner concrete structure. The structures have an approximate square plan dimensionof35m(115 ft) and are interconnected by a 1.5m (5 t?)thick composite steel girder and concrete cap. The steel structures are similar in design to offshore platforms used for petroleum operations.

This paper describes the structural analysis used to determine the capability of the platforms to absorb damage from a collision and prevent the vessel from damaging the inner concrete structure. Both simplified 2dimensionai as well as complex 3-dimensional computer models were used.

Energy absorbing non linear elements were used to mimic damage and failure of the platform's tubular members as the vessel impacts and then progressively moves into the platform. Stresses induced in the concrete structure were modeled using a finite element shell program. Energy absorbed by the vessel hull was included in the analysis. Several collision conditions were evaluated including direct head-on collision, sideways "drifting" collision and "glancing" (eccentric) collisions. Vessels considered ranged in size from 5,000 to 130,000 gross tons moving at speeds up to 4.8 kt (5 m/see, 16.4 fps).

INTRODUCTION

The Trans-Tokyo Bay Highway (TTB) will extend approximately 15 km (9.3 mi) across the center of Tokyo Bay from Kawasaki to Kisarazu, Japan. Figure 1 shows a cross section of the TTB which consists of a 5 km (3.1 mi) long bridge on the Kisarazu side, a 10 km (6.2 mi) tunnel section on the Kawasaki side and two man-made islands called the Kisarazu and Kawasaki Man-Made Islands. The Kawasaki Man-Made Island, located in the middle of the tunnel in 28 m (92 ft) water depth, provides a workbase during tunneling and will serve as a ventilation and maintenance facility after completion. The Kisarazu Man-Made Island provides a transition between the bridge and tunnel sections.

The high volume of ship traffic in Tokyo Bay made it Necessary to protect the TTB from ship collision, particularly for Kawasaki Island due to its exposed location in the center of Tokyo Bay. Figure 2 shows a cross section of the Kawasaki Island, which consists of 115 m (377 ft) diameter Inner concrete structure for access to the tunnels. The inner concrete structure is surrounded by a series of 14 tubular steel structures similar in design to steel jacket platforms used for offshore petroleum production. These "jackets" provide a passive defense system that prevents large vessels from directly impacting the inner concrete structure. A 5.2 m (17 ft) thick soil-cement interface separates the jackets from the inner concrete structure.

Figure 3 shows the configuration of a typical Kawasaki Island jacket.