Subidence-induced Sediment Motion: Foundation Design Criteria for Platforms at Main Pass 299

Abstract

Foundation design criteria were developed for fixed base conventional jacket structures in Main Pass 299 in consideration of variations in the surface and subsurface soil movements expected to result from sulphur mine-induced subsidence. During subsidence, some structures will settle 60 to 65 ft and experience horizontal movements of more than 25 ft towards the center of subsidence. Vertical downdrag loads on piles will be caused by differential motions (between pile top and pile bottom) of up to 5 ft in some extraction stages; equal updrag will occur in other stages. The soil motion data necessary to the criteria development were obtained from influence function and finite element methods. The soil motions data for the 500-ft penetration obtained from the influence function method were used as input boundary movement data for finite element analyses of soil motion details in the foundation region from the mudline to about 350-ft penetration. This paper presents details of the concerted use of all of the available information in the development of foundation design criteria appropriate to mine-induced subsidence. Selected results are presented from the 15 different platform sites evaluated. Among the cases presented are examples of various relative importance for each type of soil motion criterion, ranging from minor considerations to dominant design consideration. INTRODUCTION Plans to undertake development of sulphur reserves of about 67 million long tons from deep below the seabed in Main Pass 299 have been initiated. A complex facility of drilling platforms, bridges, power plant, and related structures will be constructed to extract the sulphur. The structures will be fixed base, pile supported jacket platforms. The mining facility as it will be initially constructed is shown in plan on Fig. 1. Drilling platforms (Production 1 and 2) are located at the end of a mile-long bridge. The storage platform is at the other end of the bridge. Two other large structures, the Power Plant and the Quarters platforms are a part of the bridge. Also, there is a Heliport platform (Y-3) about halfway along the bridge. When the sulphur beneath production platforms is depleted, the platforms will be moved to new locations and reconnected to the bridge system. As many as 9 drilling platform locations may be required before complete extraction of the sulphur reserves, which is expected to take about 30 years. Main Pass 299 is located in the Gulf of Mexico about 20 miles east of the delta of the Mississippi River. 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. The sulphur is contained within salt dome cap rock buried beneath some 1400 ft of deltaic sediments and overlying anhydrite. The sulphur-bearing rock is expected to collapse under the weight of overlying strata as the sulphur is extracted. During the economic life of the facility the seafloor is expected to experience regional subsidence, encompassing over 3300 acres of seafloor and up to 65 ft at its deepest.

Reliability-Based Design Criteria for Freeport McMoRan Main Pass Block 299 Sulphur Mine Platforms

“…This ultimate limit state deformation capacity is somewhat greater than the horizontal differential movements predicted as the worst case for the production platform #2 [19]. The plastic horizontal component would be equal to Ap = 0.5 feet.…”

“…Figures 5 and 6 summarize the comparisons of vertical, horizontal movements and respective strains at the Production Platform #2 (-700 feet subsea formation level) for each of these cases [19]. The time aspect of these plots can be related to the mined tonnage of sulfur (about 2 MM tons of sulfur per year).…”

“…Analyses of the near-surface soil movements at Production Platform #2 based on the worst case or lower bound subsidence conditions at -700 feet subsea provided information to evaluate the deformation demands that would be placed on the foundation piles during the lifetime of the facilities [19]. Figures 7-10 summarize the results as a function of the extraction level (2 to 40 MM tons) and penetration below the sea flooT (0 to 500feet).…”

“…Based on the results of the studies of the time rate of subsidence induced vertical deformations [19], due to creep relief of the negative skin fkiction forces, it was assumed that the axial strains induced in the piles due to the subsidence were small compared with the flexural strains. Given the very slow pile loading rates associated with the subsidence related deformations, and given high soil stresses induced by these deformations, creep or stress relaxation in the soil can be expected to partially relieve the strains induced in the piles.…”