A Verification of Platform Performance when Designed to the Proposed API RP2A Consequence Based Criteria
Authors: Gebara Joseph , Botelho Dirceu , Kondubhatla Subba , Ullrich Ken 
Published: October 2008

Recently recommended changes to API RP2A 20th edition1 include Consequence Based Criteria (CBC) for the design of fixed offshore structures in the Gulf of Mexico. The new criteria will allow medium (Level 2) and low (Level 3) consequence structures to be designed to lower environmental criteria. This paper compares the performance of platforms designed to the 20th edition1 (100-yr), the 19th edition2 (100-yr) of API RP2A, the CBC Level 2 criteria, the 20th edition (25-yr) and the CBC Level 3 criteria.

The paper provides an example of how the proposed Level 2 and Level 3 criteria should be applied. The comparisons include differences in member capacity, pile penetration, member strength, pile capacity, ultimate capacity of the jackets, reserve strength ratios, and notional failure probabilities.

The reserve strength data generated indicates that the Level 2 and Level 3 designs compare well to structures designed to the 19th edition. The platforms also compare well the designs per the 25-yr. return period criteria using the 20th edition loading recipe.

Introduction

An API Task Group has recently completed the development of Consequence Based Criteria for the design of new platforms3,4. The new criteria allows platforms in water depths less than 400 feet in the Gulf of Mexico to be designed to low or medium consequence levels if they satisfy specific constraints outlined by the API task group. Risks were categorized in three levels, and appropriate criteria (expressed as wave, wind, and current criteria) were developed for each of these three levels. The criteria was developed on the basis of replicating the successful performance, over the years, of platforms designed to the API RP2A 19th edition.

The Consequence Based Criteria is geared towards providing operators an opportunity to select a design criterion for a new platform, which is consistent and appropriate for the intended service of the new structure.

The three levels of design criteria presented are expressed as wave, wind and current. The wave height is presented versus water depth, as illustrated in Figure 1. Wave, wind and current loads are to be calculated using the load recipe in the 20th edition of API RP2A1.

In this paper eight jackets were designed and compared to verify the performance of platforms designed to the Consequence Based Criteria. The designs considered one water depth at two Gulf of Mexico locations.

Platform and Locations

Platform Description

The platform selected for the analyses is the same as per the CBC task group3,4. The platform is located in a water depth of 111-ft. A K-braced framing as shown in Figure 2 was used in all the design cases detailed below. A jacket deck weighing 1930 short tons, six conductors, and one boat landing were included in all design cases. The deck was located above the 100-yr crest and was kept at the same elevation for all cases.

Design Cases Location

Two platform locations were studied, the first is a Eugene Island (EI) location and the second is a South Timbalier (ST) location.