Development of Bias in Analytical Predictions Based on Behavior of Platforms During Hurricanes
Authors: Aggarwal R.K. , DoIan D.K. , Cornell C.A. 
Published: October 2008

Hurricane Andrew was a very intense storm that significantly loaded hundreds of platform in the Gulf of Mexico in August 1992. This event provided a unique opportunity to study the true behavior of offshore platforms subjected to large hurricanes and improve procedures used in analytical predictions.

A Joint Industry Project (JfP) was initiated by 13 oil companies and the U.S. Minerals Management Service (MMS), wherein a methodology was developed to use information from observed platform conditions resulting horn Andrew and the hurricane hind cast data with capacity, reliability, and Bayesian updating analyses to determine a measure of differences (biases) in the analytical predictions and field observations. The procedures used for structural integrity analysis were also improved as a result of this study. Phase I of this project completed in timber 19931'4defined a global bias factor. A study of foundation behavior was completed following Phase I and determined bias factors specific to foundation failure modes.67 This paper presents the approach followed in the most recent phase of this project in which bias factors specific to jacket and two foundation failure modes (lateral and axial) were developed. This study utilized an updated storm hind cast, improved analysis models, and a mere detailed calibration prcxdure.5 The three bias factors were developed and were found to differ significantly.

The bias factors developed through this study have provided means to further improve procedures used in the assessment of existing platforms. The proper use of these new analytical methodologies and bias factors will produce more appropriate and cost-effective mitigation measures for safe platform operations. The methodology for establishing bias factors developed and proven in these projects is applicable to other offshore regions and production systems with specific environmental, geotechnical, material and structural features.

Introduction

In August of 1992 the Gulf States were subjected to one of the most intense hurricanes in recorded history. Hurricane Andrew exhibited peak wind speeds of over 155 miles per hour and created wave heights in excess of 70 feet. The track of Hurricane Andrew led through southern Florida, where most of its destruction occurred, progressed through the Gulf of Mexico and eventually made its final landfall near Morgan City, Louisiana. Andrew caused over 50 deaths onshore and extensive property damage. Andrew represents one of the costliest natural disasters in US history.

?Ile hurricane path includes a region of the Gulf that is very densely populated with offshore platforms. As shown in the Fig. 1, the center of the hurricane traversed the Mississippi Camp, South Timbale, Ship Shoal and Eugene Island areas, Along its path through the platform areas, Andrew's waves typically exceeded the 100 par return period criteria used for the design of new structures. The region of platforms most significantly loaded by Andrew, as defined by the U.S. Minerals Management Service (MMS), included approximately 700 platforms located in the Eugene Island, Grand Isle, Mississippi Canyon, Ship Shoal, South Marsh, South Pelto, South Timbalier, and West Delta areas. Many of take platforms were older structures that were not deseed to withstand the forces created by a hurricane of Andrew's magnitude.