Evaluation of Environmental Criteria for Gulf of Mexico Platform Design
Authors: Fu S.L. , Petrauskas C. , Botelho D.L.R. , Carter E.W. , Basaldua E.A. , Abbott B.J. 
Published: October 2008

This paper documents a study assessing three different environmental design criteria for the Gulf of Mexico based on the expected total cost approach. With this approach, an optimum criteria can be derived by minimizing the expected total cost over the life of the structure. Four platforms in various water depths were designed for:API RP2A criteria,Chevron's current criteria used for new platform design, andChevron's original criteria used for new platform design prior to 1990.

In this study, structural weight, capital cost, platform strength, failure probability, and expected total cost for each platform under each criteria were investigated.

INTRODUCTION

The recent completion of proprietary studies involving model scale tests of jacket/deck and cylinders, and joint industry studies, as well as involvement in API activities, led Chevron to revise its criteria for the Gulf of Mexico as well as its wave force calculation procedures. To consolidate some of the ideas imbedded in the new approach and also to evaluate the impact of this approach on platform cost, Chevron carried out an economic analysis based on the expected total cost [1,2,3J. The expected total cost, which is the sum of the initial cost and the expected failure cost, was applied to four platforms designed to each of the three different criteria:the criteria used by Chevron for the design of new platforms prior to 1990 designated as Chevron original (CHO) [4J;the revised criteria used by Chevron starting In 1990, designated as Chevron interim (CHI) [5J; andthe Chevron interpretation of API RP2A criteria, designated as (API) [6].

The four existing Gulf of Mexico platforms designated as A,B, C, and D, located in water depths of 46, 124,211 and 725 feet, are shown in Figures 1a through 1d.

CONSISTENT RISK PROCESS

The consistent risk process carried out in this study consisted of three major steps: Platform design, risk analysis, and expected total cost estimation.

Platform Design

Environmental Criteria/Wave Force Calculation

For each platform location, the appropriate environmental criteria involved specific values for design parameters such as: wind speed, wave height and period, current profile, marine growth; and hydrodynamics coefficients for wave force calculation. The wave kinematics reduction factor and current shielding factors were adopted for the Chevron interim criteria only. The API wind speed was used for all criteria. For illustration, only relevant parameters for the three design criteria for Platform C are given in Table 1. The current profiles are depicted in Fig. 1e. A flow chart for the wave force calculation procedure is shown in Fig. 2.

Design and Analysis

Each platform was designed, for each of the three criteria, to satisfy the requirements of the in-service and pre-service conditions. In-service analysis included both 1DO-year storm and operating storm loading conditions as well as simplified fatigue analysis. The SACS program suite [7J was used for these analyses.