Life Cycle Reliability &Risk Characteristics Of Minimum Structures
Authors: Bea Robert , Craig Michael , Miller Tom 
Published: October 2008

Minimum structures are used primarily because of their low initial investment and fast-track schedule characteristics. These structures generally cost less and take less time to commission than more traditional structures such as 4-pile (leg) platforms. A wide variety of configurations of these structures have been proposed and installed in offshore locations around the world. This paper details a reliability and risk based approach that has been used to characterize the life-cycle quality (safety, durability, serviceability, compatibility) characteristics of three minimum structures and one traditional 4-pile structure. The three minimum structures included a tripod, a braced caisson, and a guyed caisson. The structures are located in 30 m to 66 m of water in the Gulf of Mexico. The structures were all designed according to current American Petroleum Institute guidelines. Results from the life-cycle (design, construction, operation) analyses of the quality and risk characteristics of the alternative structures are summarized. The occurrences and influences of human and organizational errors throughout thelife-cycle are included The importance of damage and defect tolerance (robustness) in the alternative structures is demonstrated

Introduction

In a project funded by UNOCAL Corporation, a study of minimum structures has been performed to develop and apply a practical process to characterize the life-cycle reliability and risk characteristics of one traditional 4-pile structure (Fig. 1) and three minimum structures.1 The three minimum structures included a tripod, a braced caisson, and a guyed caisson (Figs. 2- 4). The structures are located in 30 m to 66 m of water in the central Gulf of Mexico. The structures were all designed and constructed according to current American Petroleum Institute(API) guidelines.2

A primary objective of this study was to develop and apply an approach that could be used to assess the life-cycle risk costs associated with the quality characteristics (safety, durability, compatibility, serviceability) of each of the structures. The riskcosts were based on the likelihood of compromises in the quality characteristics of the structures and the costs associated with the compromises. The risk costs included those due to inherent or natural causes and those due to human and organizational errors that were detected I corrected and not detected I corrected. The combined life-cycle risk costs were intended to provide insights into how the quality characteristics of the structures might be improved and to help identify which structure might be the 'best' for a particular development.

This study addressed four specific UNOCAL structures sited at four different locations. Because of the unique characteristics of these structure, the specific practices of UNOCAL, the designers, and the constructors, the results from this study are not general. The conclusions drawn from this study mayor may not be applicable to other similar structures. The approach used is general and can be used to perform similar assessments of alternative minimum structures.