Consequence-Based Criteria for the Gulf of Mexico: Development & Calibration of Criteria
Authors: Ward Emmett G. , Lee Griff C. , Hall Rudolph A. , Turner J. Ward , Botelho Dirceu L. , Dyhrkopp Felix G. 
Published: October 2008

An API Task Group has recently developed Consequence-Based Criteria for the design of new platforms in water depths to 400 feet in the Gulf of Mexico. Risks were categorized in three levels, and appropriate design criteria (expressed as wave, wind, and current criteria) were developed for each of these three levels. This paper will describe the development and calibration of these new criteria from the philosophy, premises, and assumptions, which are documented in a companion paper OTC 11885. [1] The criteria for the most severe risk level are taken as the criteria first presented in the 20th Edition of API RP2A. The criteria for the intermediate risk level incorporated the favorable history and experience of existing platforms that were designed to the 9th - 19th Editions of API RP2A. The criteria for the least severe risk level were based on a statement of acceptable risk for such structures.

Introduction

A related paper [1] describes the development of the basis for consequence-based design criteria for Gulf of Mexico platforms. Platforms are categorized by different levels of risk relative to life safety and consequences of failure. This resulted in three levels for platform criteria as summarized in Table 1.

The oceanographic design criteria and the methodology for computing design loads described in Section 2 of the 20th edition of API RP2A-WSD, was taken as the foundation for the development of the consequence-based criteria. The design load methodology represents the culmination of decades of experience and research, and represents the best technology available for describing design loads on platforms given specified oceanographic criteria. For the Gulf of Mexico, specific oceanographic design criteria recommendations result in 100-year return period force levels. This is accomplished by specification of a 100-year wave height and associated wind and currents for Gulf of Mexico hurricanes.

Criteria for Level 1 structures were taken as the 100-year condition (i.e., no change from 20th Edition methodology). The criteria basis for the Level 2 and Level 3 structures are actually just expressions for design load levels. Our task here, then, was to develop oceanographic criteria for use with the 20th Edition methodologies that would result in desired load levels for L2 and L3 platforms.

We pursued two main tasks to develop oceanographic criteria consistent with the prescribed design load levels-load reduction factors and reserve strength ratios (RSR's). Load reduction factors (design load/20th Edition design loads) and idealized platforms seemed to offer a simple, direct path to develop L2 and L3 criteria. However, we finally abandoned this approach when it became apparent that it was too simplistic to account for important variations due to directional versus omnidirectional loading, platform geometries and orientation, and water depth.

We then adopted a more detailed approach based on reserve strength ratios (RSR's) and detailed platform design studies. Simply stated, we computed RSR's for platforms designed to L2 and L3 design bases and used these RSR's as a means to develop oceanographic criteria consistent with the 20th Edition methodology and criteria.