Structural Design Techniques for Subsidence Used in Analysis of Main Pass 299 Sulphur Mine Platforms
Authors: Sanzgiri S. , Llorente C. 
Published: October 2008

ABSTRACT:
This paper addresses numerical analysis of platforms subjected to gradual and continuous settlement of the sea floor. The methods of numerical interpolation of soil movements and application of these movements to the soil-pile-structural model are described. The projection of a structure on a grid of soil movements, both horizontal and vertical, gives numerical offsets of soil movement applicable on piling. The resulting interaction analysis gives insight into the behavior of piles responding to these movements. The techniques developed permitted analysis of platforms subject to settlement and /or soil movements to be carried out in a fast-track design environment.

INTRODUCTION:
Phase I and II of Freeport's Main Pass 299 Mine comprises 15 discrete platforms, most of which will be subject to varying degrees of platform subsidence and/or lateral soil movements. Additionally, Oil and Gas Platforms within this subsidence zone are also subject to the same loads. The settlement generally occurs in the shape of a subsidence bowl, deepest at the center of extraction. A conventionally piled platform structure located within the influence of this subsidence is subjected to interactive forces between pile and soil movements. This paper describes the analytical methods used in projecting global soil movements on the structure and interacting these soil movements with soil elasto-plastic properties to obtain soil motion-induced forces. Further, these motion-induced forces are integrated with existing gravity, operational, and environmental loads, to achieve soil-pile-structure equilibrium. Soil forces acting on piling include down-drag and up-drag, and principal and out-plane lateral movements. Consequential effects of such uneven movements are also addressed in this paper.

Scope
The scope of this paper is to describe an analytical technique developed to apply in-plane and out-plane soil movements caused by subsidence on the platform structure submerged below the mudline. The soil movements are based on Freeport- McMoran extraction boundary conditions and are obtained by the Geotechnical Consultants [4]. This paper also describes the effects of soil subsidence on pile design for piles restrained by platform structure above the mudline. Once the effects of soil subsidence are projected on platform piling by the technique contained in this paper, the platform analysis and design is conducted, using some unique techniques [3]. Additionally, if the pile subject to such loading is susceptible to non-linear behavior, it is specially studied [5]. This paper also attempts to compare subsidence and mud-slide loads. The entire analysis is conducted within the scope of Reliability-Based Design Criteria [1].

HISTORY
In the analysis phase of all platforms designed in the Main Pass 299 Mine contained within the bowl of expected subsidence, the consensus of opinion of the designers was to first understand subsidence as a concept, then apply subsidence loads on individual piles, and later interact these loads with other piles through the stiffness of the jacket structure. Between January and July, 1989, the concept of subsidence was related numerically to quantums of sulphur extraction. Soil subsidence profiles were developed as 2-D planar finite element grids.