Main Pass 299 Sulphur Mine Bridges: Structural Design
Authors: Carr T.N. 
Published: October 2008

The Main Pass Mine sulphur platforms are interconnected by thirteen bridges totaling over a mile in length. The bridges' unprecedented size, their operational requirements, and their response to platform support movement caused by sea floor subsidence all presented design challenges. These unusual design requirements yielded some solutions and design details not typically used for offshore bridges.

INTRODUCTION

Freeport McMoRan Resource Partners' (FMRP) initial development of the Main Pass 299 Sulphur Mine consists of 15 platforms and 13 bridges located in water depths ranging from 208' to 214'(Figure 1). The site is about 15 miles east of the Mississippi River Delta. A salt dome underlies most of the block, which is surrounded by oil and gas platforms. Within the caprock, a layer of limestone contains the sulphur deposit approximately 1,500' below the sea floor. The deposit is about 150' thick at its deepest point.

To extract the deposit, superheated water will be injected into the sulphur-bearing formation, melting the sulphur in place. Recovered with wells at the two Production Platforms, the molten sulphur will be pumped to the Loading and Storage Platform, then loaded into barges for transportation to the onshore terminal in Venice, Louisiana.

It is predicted that the sulphur extraction process will cause seabed subsidence in the shape of a bowl (Figure 2). Vertical subsidence as great as 65' and equally large lateral motions of the soil surface are projected. The two Production Platforms are located near the center of the bowl and will experience the greatest vertical movement. Platforms on the sides of the bowl will not experience so much vertical subsidence, but will be subjected to greater horizontal movements and tilting effects. The large Loading & Storage, Power Plant, and Living & Quarters platforms are located at the fringe of the subsidence bowl over a mile away to avoid or minimize the effects of soil movement on those structures.

Bridges link all Main Pass Block 299 Mine platforms except the Facilities Platform (Figure 3). The bridge system serves two basic functions: it provides personnel and equipment access between platforms, and supports piping and electrical wiring between mine structures. With spans ranging from 236' to 600', the 13 bridges have an average span of 500' (Table 1).

A system of bridges was selected to connect the platforms instead of submarine pipelines after economic studies and operational considerations were evaluated. A bridge span of approximately 500' was identified as optimal for the most cost-effective system of bridges and bridge support towers.

The bridges provide pedestrian and vehicular access directly to the upper decks of the platforms by means of a 14'-wide timbered accessway. A secondary, grated, 3'-wide walkway links platform lower decks and provides access to bridge piping.

All 13 bridges are steel tubular space frames with a rectangular cross section, 25' wide by 28' deep with 42" or 30" diameter chords at the corners (Figure 4). The four upper chords are supported at bridge support platform upper decks and rest on multidirectional structural bearings.