Probabilistic Inspection Planning of Jacket Structures
Authors: Moan Torgeir , Johannesen Johannes M. , Vårdal Ole Tom 
Published: October 2008

The aim of inspections is to reveal time dependent deteriorations and damages. The focus herein is on fatigue crack damages in North Sea structures.

A Probabilistic Inspection Analysis tool, PIA is presented. PIA is based on a probabilistic fracture mechanics model of fatigue crack growth as well as a simplified systems reliability approach. This approach makes it possible to systematically include the information achieved by inspection of the actual as well as other structures. PIA provides information about where and when to perform inspections to detect fatigue crack growth and thereby initiate repair actions that are found to be necessary.

The use of PIA has generally given a 70 to 80 percent reduction of the inspection programmes. Improved fatigue analysis and use of PIA has also increased the number of fatigue cracks that are detected by a factor fo 3 to 4.

Comparisons between observed fatigue crack growth and predictions by PIA have revealed shortcomings. Improvements of the methods and data used in PIA are discussed.

Traditionally, PIA has been used to recommend the frequency of EC/MPI inspection for welded structures. During the last years, PIA has also been applied to define inspection programmers based on flooded member detection (FMD).

Introduction

Adequate safety of offshore structures is achieved by applying ultimate (ULS) and fatigue (FLS) limit state criteria for components as well as accidental limit state criteria (ALS) for the global structure, Ref. 19. Fatigue is an important consideration for structures in areas with more or less continuous storm loading (such as the North Sea).

In addition to design measures, inspection, monitoring, and repair, if necessary, are important measures for maintaining safety, in particular with respect to fatigue, wear, and other deterioration phenomena. However, significant costs of underwater inspections make it important to properly prioritize and limit inspections. Moreover, in-service experiments with the actual as well as other structures should be utilized. Hence, at the design stage, design and inspection planning should be carried out to achieve an optimal balance of the different safety measures. In addition, the inspection programmer (methods, frequency, and repair strategy) needs to be sequentially updated, especially based on the specific inspection results that become available during operation. Obviously, more favorable performance than design predictions indicate is especially beneficial in connection with possible extension of the service life.

In practice, the additional information that becomes available during operation can be used to update the fatigue assessment, primarily in three alternative ways:Deterministic SN model: well suited for new design. However, inclusion of information from inspections is difficult since the Miner damage can not be directly measured. Also, if a joint is repaired to re-establish its initial state, using SN curves as for a new joint is questionable. The information on the development of damage vs. time is not utilized.Deterministic Fracture Mechanics (FM) model: the model facilitates quantities relating to damage that are directly measurable, e.g. cracks. The model may be calibrated to reproduce the crack growth history before failure of the joint considered. However, since the most common inspection result is no finding, the model will often not be applicable.