Main Pass 299 Sulphur Mine: Offshore Design in a Soil Subsidence Bowl

Abstract

ABSTRACT The structural configuration and design of the Main Pass Block 299 sulphur mine structures is described. These structures are subjected to significant settlement and severe soil movements caused by the extraction of the sulphur ore. The mine structures consist of 14 platforms connected by bridges and one "stand alone" platform. All sulphur mine platforms are steel jacket structures similar to typical oil and gas structures in the Gulf of Mexico. However, two basic factors related to the sulphur mine operations make these structures different from oil and gas structures: gradual subsidence of the seafloor and a design service life of 40 years. The predicted soil movement induced stresses are so severe at several of the platform sites that 100 ksi yield strength steel has been specified for use in the piles and nonlinear analysis of the piles has been performed. In addition to incorporating the predicted soil-induced movements of the piles in the design of the structures, platform decks and appurtenances such as boat landings and barge bumpers are designed to be elevated to compensate for the platforms' downward movement. Bridges are designed to accommodate significant horizontal movements and differential support settlements created by the motion of interconnected platforms. INTRODUCTION The Main Pass Block 299 sulphur mine initial development consists of 15 platforms and 13 bridges located in water depths ranging from 208 ft. to214 ft. about 15 miles east of the Mississippi River Delta. Figure 1 is an artist's rendition of the mine development. A salt dome underlies most of the block, which is surrounded by oil and gas platforms. Within the caprock lies a layer of limestone which contains the sulphur ore. The sulphur lies approximately 1500 ft. below the seafloor and the deposit is about 150 ft. thick at its deepest point. Nine of the 15 platforms will serve as bridge supports (three of which will also support helidecks). The other platforms include a Loading & Storage, Power Plant, Living Quarters & Warehouse, and two Production platforms. The stand-alone platform is the Facilities platform. Plans call for the present Production platforms to be relocated, as well as new ones installed in other areas of the mine, once the sulphur is exhausted in the initial mining regions. Several of the bridge support platforms and bridges are also designed to be relocated. The Frasch mining process will be utilized to extract the sulphur. Water produced at the Power Plant platform and from brine wells located on Bridge Support 2 will be pumped to the Power Plant Module to be chemically treated and heated. Water will then be piped to the two Production platforms where it will be injected into the sulphur formation. Molten sulphur will be recovered at the Production platforms and pumped to the Storage platform. The sulphur is to be stored in two 90 ft. diameter by 40 ft. high tanks. The sulphur will finally be loaded into specially built double-hulled barges and taken to an onshore terminal near Port Sulphur, La.