Subsidence-Induced Sediment Motion: Analysis by Finite-Element Methods for Sites in Main Pass 299
Authors: Templeton J.S. , Little R.L. , Sraders G.A. 
Published: October 2008

Analysis methods were developed for the determination of details in the subsurface deformations and motions of seafloor sediments resulting from mine-induced subsidence. A general purpose, nonlinear finite element program with soil-specific constitutive modelling capabilities was employed. This approach allowed the analyses to model the stratigraphic soil property details as were determined with borings, sampling and laboratory testing. Both effective stress and total stress analysis approaches were invoked, and both twodimensional and three-dimensional idealized problems were solved. The finite element models constructed included up to 18 layers of soil stratigraphy down to a depth of 500 ft below the mudline, covered horizontal extents of up to 4000 ft and employed up to 20,000 degrees of freedom. The nonlinear solutions obtained included full accounting for effects of large strain and essential geometry change as well as nonlinear material behavior.

BACKGROUND

Main Pass 299 is located in the Gulf of Mexico about 20 miles east of the delta of the Mississippi River (Figure 1). Water depths in the block range from 202 ft to 222 ft, with a seafloor slope of about 7 ft per mile to the southeast. In this block, a sulphur reserve is contained within salt dome cap rock and is buried beneath some 1400 ft of sediments. The sulphur-bearing rock is 200 ft thick in one area of the dome and overlies anhydrite. Freeport-McMoRan plans to develop sulphur reserves in the block of about 67 million long tons, requiring 30 years or more to extract. The sulphur will be mined using the Frasch process. The method requires that hot water be circulated to the sulphur in sufficient quantity and adequate temperatures to cause it to melt. The molten sulphur is produced as a liquid. A complex facility of drilling platforms, bridges, power plant, and related structures will be constructed in Main Pass 299 to produce the sulphur. The platforms will be pile supported, fured base, conventional jacket structures. Experience has shown that Frasch process sulphur mining usually causes subsidence. For a project the size of Main Pass 299, the effect will be bowl-shaped and regional in scale, encompassing over 3300 acres of seafloor. During the economic life of the facility the seafloor is expected to experience subsidence up to 65 ft and comparatively large differential movements of the foundation strata are anticipated. The purpose of the work discussed in this paper was to provide methods for determination of subsidencerelated foundation design criteria for the production structures. Subsidence-related phenomena have been studied over the years in both the geologic and engineering context. Most of the accepted methods for prediction of surface movements were derived empirically. The models are usually based on relatively extensive field observations and they have proven to be sufficiently reliable when used in the region for which they were developed. Among the methods in use are a group which relate a mined increment to its surface effect using analytical relationships termed "influence functions". The key to success of these influence function methods has been the inclusion of enough arbitrary constants and parameters in the function to permit calibration to measured subsidence caused by actual mining operations.