Overturning Analysis of Maleo Jack-Up Mat Foundation on Soft Clay
Authors: Murff James , Young Alan G. 
Published: October 2008

An analysis of the overturning stability of the Maleo Platform has been conducted using the results of a comprehensive, post installation site investigation and interpretation. The jack-up is idealized as a rigid structure supported by a large "A" shape mat foundation. The mat is considered to be an assemblage of independent strip footings. The structure and foundation are idealized as a rigid-plastic system and the upper bound method of plasticity is used to determine the overturning capacity under wave, current and wind loading. A series of 1-g model tests was carried out to validate the analysis method. Based on this analysis, the Maleo Platform is shown to have a safety factor against overturning failure for the 100-year storm loading significantly greater than the requisite value of 1.5.

Introduction

The foundation performance of mat supported jack up rigs has been very successful since the first rig was built by Bethlehem Steel in 1964 (Hirst et al. 1976). This type of rig has traditionally been used to drill exploration wells, but in recent years the rig has been used as a permanent production platform. The requirements of foundation performance are usually different for these two operations in terms of the design environmental loading.

This paper describes the results of a detailed foundation analysis of the overturning stability of the Maleo platform that was to be used as a production platform. The subject structure is a Bethlehem 250 mat supported jack-up platform which has been placed on soft, normally consolidated clay. The base of the mat penetrated approximately 6.5 ft under a sequence of preloads with a maximum preload of 12,950 kips resulting in an average peak bearing pressure of 599 psf. The details of the structure's geometry, preload history, and design load conditions used herein are described in detail elsewhere (Ooley and Stewart, 2008; Whitley, 1970) and are summarized in later sections.

The soil conditions used in our analyses reflect data collected during the initial site investigation (PT Kalindo Raya Semesta, 2003) as well as additional data obtained from a comprehensive site investigation around the Maleo platform mat foundation after its placement (Fugro Alluvial Offshore Ltd., 2007). The site investigation and resulting interpreted strength profile based on this work are also detailed elsewhere (Fugro Alluvial Offshore Ltd., 2007; Audibert, et al., 2008) and summarized in a subsequent section.

The general approach taken here is to idealize the structure and foundation as a rigid-plastic system and to apply the upper bound method of plasticity (Murff, 2000) to determine the overturning capacity under wave, current and wind loading. We have focused herein on assessing the factor of safety on the overturning resistance in the transverse direction, as that has been shown to be the critical failure mode from our studies. Our definition of overturning capacity for this study is that load at which a plastic failure mechanism is initiated. The basic methodology for carrying out this assessment is detailed in a later section.

The data described below provide the necessary input to assess the transverse overturning stability of the structure foundation. To achieve the design criterion for overturning stability, the foundation must have a factor of safety of 1.5 or higher.

Foundation Geometry and Loading Conditions

The jack-up is idealized as a rigid structure supported by a large "A" shape mat foundation (Hirst et al., 1976). The foundation mat as shown in Figure 1 has outside dimensions of 210 ft by 170 ft providing a total mat area of 21,616 sq ft. The mat height is 10 ft, and there is a 2-ft deep perimeter skirt (Hirst et al., 1976; Whitley, 1970). The shape of the mat has been designed to keep the centroid of the plan area of the mat as close as possible to the centroid of the columns (Whitley, 1970).