A Detailed Study On The Failure Probability Of ST130 "A" Platform During The Passage Of Hurricane Andrew
Authors: Botelho D.L.R. , Petrauskas Charles , Mitchell T.J. , Kan D.K.Y. 
Published: October 2008

This paper summarizes some the results of a broader study initiated at Chevron to understand failures and survivals of platforms in large storms. Ultimately, this effort will lead to the development of reliability models for offshore platforms that are calibrated, i.e., that more accurately reflect the failures and survivals experienced in the past, In particular, the results presented here deal with ST130 "A" ultimate strength calculations using static nonlinear pushover analysis tools [1], and its detailed failure probability calculation during Hurricane Andrew, using a failure probability calculation model developed in-house [2]. The theory embedded in this model was extensively used in a recently completed joint industry project on the Bayesian calibration of wave forces and resistance calculation procedures [3, 4].

INTRODUCTION

ST130 "A" Platform was among the 10 or so major platforms [5] completely toppled during the period of August 24-26,1992 when Hurricane Andrew moved through the Gulf of Mexico with sustained winds of 140 miles per hour. In addition, Andrew completely toppled 25 satellite platforms (most of them caissons) and caused significant damage to another 26 major platforms and 140 satellite platforms. Clearly, from a risk management viewpoint, there is a great interest not only in being able to determine the probability of failure of individual platforms, but also in being able to determine the probability of failure of fleet of platforms.

The failure probability calculation procedure applied in the study, relating to a single platform only, conceptually involved three main steps:Develop a nonlinear computer model and perform static push over analyses to determine, for various load directions (a) the platform ultimate strength, and (b) platform force-distribution parameters (the parametric relationship of base shear as a function of wave height and current);Define, for the entire storm, the hourly environmental conditions in terms of direction, significant wave height peak period, and in-line current andUsing the information from steps (1) and (2), determine the platform probability of failure as detailed in [2].

PLATFORM DESCRIPTION

ST130"A" was the frost platform installed in the ST131 field of the South Timbalier area of the Gulf of Mexico in about 140 ft. of water. It was an 8-pile self-contained drilling and production platform, designed for a 25-year return period wave criteria in 1957, and installed in 1958. The 25-year design criteria was then a widely used standard. At the time of its demise ST130 "A was considered a "satellite," supporting only a few wells and minimal equipment. The main features of the platform were8 piles/legs, single-battered (1:15) in the broad side direction;20 well slots, 26" and 30" conductors (only 8 installed);30" O.D. piles, 175ft. penetration;33" O.D. legs, 0.5" thickness with 0.75" joint cansgroutedlegs;main deck elevation T.O.S.@43'-9" above MLW,cellar deck elevation T.O.S.@ 33'-6" above MLW. Figure 1 is a double line drawing based on the original drawings of the structure.

pre-Andrew Survey Results

ST 130"A" was inspected in great detail in 1985and 1990. The general condition of the platform did not change appreciable between these years, with the main findings being:topsides (all deck levels) were heavily corroded, including both levels at the "Plane A" elevation, which exhibited cracks, holes, dents and even some members missing