Structural Reliability Applications in Risk-Based Inspection Plans and Their Sensitivities to Different Environmental Conditions

摘要

Structural system inspection practices for floating offshore facilities, such as FPSO’s and FSO’s, have for the most part followed the practice employed and developed via trading ships/maritime practice. Often, the influence of a particular repeated service/cargo loading regime, cargo corrosivity, or site environmental influence can play a major role for a particular asset. More specifically, there has been only cursory attempts made to first understand and then, where possible, take advantage of site-specific conditions and loading in the development of in-service inspection plans. In recent years there has been significant interest by the marine and offshore industry to apply structural reliability techniques to risk based inspection planning for marine vessels and floating production installations. In this regard, structural reliability based methods can assist in providing a framework for quantifying site-specific loading and degradation mechanisms (such as fatigue and corrosion), through a systematic consideration of the probabilistic uncertainty in each degradation mechanism. By applying structural reliability analysis and risk assessment techniques to inspection planning, the operator is given a tool by which he can justify the allocation of resources to those structural components with a higher risk profile, and at the same time potentially relax inspection activities for low risk components to optimize and target inspection efforts. In a companion paper [1], available structural reliability methods developed to date were summarized, and then applied to determine the inspection intervals based on site specific loading as applied to strength considerations of the hull girder as well as to stiffened and un-stiffened plate panels. By tracing the time-varying reliability index for these structural components, the risk-based inspection intervals can be determined. This methodology has recently been implemented to provide the foundation in a risk-based inspection (RBI) plan for a floating production unit offshore West Africa. The current paper will further consider the sensitivities resulting from the following two conditions for the strength reliability: (1) environmental load for different regions of the world, and (2) corrosion rate corresponds to offshore environment as well as storage conditions. Another two conditions are considered for fatigue reliability: (3) assumed initial crack size for a connection, and (4) crack growth parameter for either the air environment or marine environment that a connection may be subjected to. The sensitivity studies presented in this paper provide a quick reference to understand the RBI plans benefits via work scope optimization and cost reductions for floating production units operating in various regions of the world.