Subsidence-Induced Sediment Motion: Implications for Production Facility Siting-Main Pass 299
Authors: Green L.B. , Hooper J.R. 
Published: October 2008

ABSTRACT
The preliminary phase of the foundation investigation for the Main Pass 299 offshore sulphur mine required definition of soil motion characteristics during subsidence as they would apply to pile-supported structures Based on; the geologic nature of the sulphur ore body the technology of Frasch Process mining historical records; and comparatively simple analytical models it was concluded that the most important design considerations would be; horizontal strain of the seafloor; differential shearing between layered sediments of sand and clay and vertical strains. These would be reflected as lateral loads transmitted from the piles to the platform jacket, bending of piles in the ground, and downdrag and updrag loading on the piles. The Freeport-McMoRan Influence Point Method was used to determine the regional development of the subsidence trough in response to sulphur mining. A finite element-type analytical method was selected to analyze local foundation sediment movements. The influence Point Method is the input to the finite element model.

INTRODUCTION
Drilling platforms, bridges, a power plant, and related structures will be constructed offshore in Main Pass 299 to extract sulphur from ore bearing rock on a salt dome about 1500 ft below the seabed. The structures will be pile supported. It was recognized that a complex interaction would take place between the sulphur extraction, collapse of the cap rock, and movements that would then occur in the sediments that support the piling. Analytical tools and methods would have to be selected to address all of the possible deformational forms that might arise in the foundation strata during a planned 30-yr life of the facility. The initial goal of the foundation investigation was, therefore, to create a geological model of subsidence phenomena and in particular, the problems they would create for pile foundations. The model would be used to select the best foundation design techniques. This paper describes the main considerations in the derivation of the geologic model. Companion papers in this conference.1,2,3 describe the methods used in design analysis and the results of the foundation investigation.

REGIONAL SETTING
Main Pass Block 299 is about 20 miles east of Main Pass, Mississippi River Delta. This area is just east of the modern Belize deltaic lobe of the Mississippi River but is well beyond the zone of frequent mudflows associated with the prograding delta. Seafloor gradients in the block are typically less than 0.l percent.
Soil borings show a surficial layer of very soft clay about 40-ft thick probably of Holocene age. Underlying these sediments, geophysical data (Fig. 1) show an erosion surface probably dating from the late Wisconsin sea level lowstand about 12,000 years ago. Below the erosion surface is an Upper Pleistocene deltaic sequence informally named the Lagniappe Delta 4. Comprise. d .of a series of overriding deltaic sequences that may have originated from the ancient Pearl and/or Mobile Rivers, the Lagniappe Delta is believed to extend to depths of about 60 to 130 ft below the seafloor.