A Probabilistic Approach for the Reliability Assessment of Offshore Structure Considering Corrosion and Fatigue
Authors: Li Y.S. , Kato D. , Ohtani Y. , Suehiro Y. , Hirayama H. 
Published: December 2007

In this paper, the applicability and effectiveness of an analysis methodology based on the probabilistic fracture mechanics are evaluated for the reliability assessment of offshore structures such as jackets, tubular space frames and others, considering corrosion and fatigue. An analysis program named PRAISE, which was widely used in the nuclear fields, is investigated. The appropriateness of the simulation models and analysis methods used in this program, such as the distribution models of the initial crack, the fatigue crack growth models, the solutions of the stress intensity factor, the failure criteria, the probabilistic simulation methods, are evaluated for the offshore structure. Furthermore, several analyses are carried out based on the experiment data about corrosion fatigue and fatigue for typical offshore structure materials, and the influence effects of fatigue and corrosion fatigue, the non-destructive inspection are examined.

Based upon these investigations and analyses, it is clear that the probabilistic analysis methodology can also be used for the offshore structures, by improving or including some functions and evaluation models.

Introduction

Generally, the offshore structures such as the platform for oil drilling often suffer from serious aging environment like corrosion and fatigue, and the maintenance is usually more difficult and expensive. In Japan, the early platforms have been used over 30 years. Therefore, the assessments such as structure integrity, remaining life and the suitability of inspection schedule have become more and more important.

Many studies about the safety assessment for offshore structural components have been carried out. For example, Kawahara1, Thorpe2 and Hsu3 proposed the methodologies for fatigue life prediction, Kawahara4, de Back5 and Thorpe6 gave many experiment data about crack growth rate for fatigue in air and corrosion fatigue in seawater.

By now, the safety assessments have been usually carried out by using deterministic methodologies. However, for offshore structures, there are many uncertain parameters such as load conditions of winds, waves and currents, the distributions of the initial crack, the crack detection probabilities of non-destructive inspections, the scattering of materials properties and so on. It is necessary to give a proper way to estimate these uncertainties.

Recently, Probabilistic Fracture Mechanics (PFM) has been developed for the reliability analysis and life extension assessment for important structural components in nuclear fields7–10, as a rational evaluation methodology. With this kind of methodology, the influence factor can be evaluated as a probabilistic variable following its own probabilistic distribution model. Many studies7–15 on reliability analysis, life extension assessment, and decision of inspection schedule based on risk by using this methodology have been carried out. Among these studies, a famous analysis code named PRAISE5–10 (Piping Reliability Analysis Including Seismic Event) has been developed in Lawrence Livermore National Laboratory (LLNL) of USA, and has been used widely in reliability analysis and life extension assessment for nuclear piping.

In this paper, the appropriateness of the PFM models used in PRAISE is evaluated for the offshore structure, and several analyses are carried out based on the experiment data about corrosion fatigue and fatigue for typical offshore structure materials, and the effectiveness of PFM is examined.

The Evaluation about the Applicability of the PRAISE Models for Offshore Structure

As described previously, the PFM analysis code PRAISE was originally developed for the nuclear piping. Thus, in this paper, the applicability of its methodology, simulation models are evaluated for offshore structure at first.