Uncertainties in the Fatigue Lives of Tubular Joints
Authors: Xu T. , Bea R.G. , Ramos R. , Valle O. , Valdes V. 
Published: October 2008

Tubular joints are integral components of offshore structures and as such, many codes and standards address their design. The fatigue damage process of tubular joints in offshore structures is inherently variable and uncertain. In addition, potentially, there are important bias and uncertainties introduced by the analytical models used in design codes and standards. Therefore, in the development of reliability based fatigue design and requalification criteria, it is important to recognize these differences and to develop the best estimates of joint fatigue durability.

This paper summarizes a study to identify the primary sources of bias and uncertainty associated with assessment of fatigue lives of tubular joints in offshore platforms. Based on the extensive screened test data, the important uncertainty sources of tubular joint fatigue durability has been examined, and the uncertainty models are developed to facilitate development of reliability based fatigue design and requalification criteria and fatigue inspection guidelines for tubular joints in offshore platforms.

Introduction

The development of the reliability based offshore platform design and requalification guidelines involves defining the scope (class of structure, load cases, etc.) and code format - including definition of characteristics values; selecting reliability measures; assessing uncertainties in loads and resistance; establishing target reliability levels; and determination of factors of safety, and load and resistance factors.

Tubular joints are one of the main technical problems and high cost areas in the design, construction and maintenance of steel, template-type offshore structures. The uncertainty assessment of the tubular joints is therefore one of the most important components in the development of the reliability based design and requalification criteria for offshore platforms. The nature of uncertainties of the joint capacity and durability and methods for their modeling and analysis is a key in reliability based structural design of offshore platforms.

Similarly, it is a key in development of realistic inspection and maintenance programs for these structures.

In the context of development of criteria for design and requalification of platforms1 and floating facilities2 in the Bay of Campeche and development of inspection and maintenance programs for these facilities3,4, this study was intended to address a paradox that had developed from inspections of platforms in the Bay of Campeche.5 This paradox was that when inspections were made of joints in older platforms that were evaluated using detailed fatigue analysis methods6 to have fatigue lives that were in the range of months to years, no cracking could be detected. However, damage was found at other joints where it was not predicted or expected.5 This raised the question of: what was the confidence with the fatigue analyses?

The objective of this paper is to summarize development of uncertainty models of the fatigue strength of tubular joints. Following a brief review of uncertainty of tubular joint fatigue durability, the sources of uncertainties in tubular joint fatigue durability are logically organized and quantitatively and qualitatively evaluated. Based on this evaluation, the uncertainty models are developed to facilitate development of reliability based fatigue design and requalification criteria, and guidelines for the inspection and maintenance of template-type offshore platforms.