This paper presents a case study on how a target safety level can be demonstrated by using Structural Reliability Analysis (SRA), which is especially beneficial for structural life extension assessment.

Life extension requires operator to document that the structure will continue performing safely past its original design life.

The most frequently asked question by the industry in the process of life extension is:

  1. Whether Design Fatigue Factor (DFF) can be reduced during life extension assessment

  2. While the most frequently asked question by the regulator in the process of life extension is:

  3. What is the current condition? When a particular component is non-inspectable, how do we know the condition and is it still safe to operate?

Both questions are related to structural reliability. Reliability analysis is to quantify uncertainties. The key criteria for life extension is to remain at the same level of safety as original design. When fatigue failure mode is of concern, the objective is not to maintain the same DFF, rather to maintain the same target safety.

This paper first discusses the background of DFF's and how it accounts for various uncertainties. Then the structural reliability methodology is introduced to explain the relationship between target safety and DFF. Probability of failure in terms of fatigue cracks grows over time. A few numerical examples are used to demonstrate how in-service inspection helps to reduce uncertainties and no findings of fatigue cracks reduces documented probability of failure and therefore increase target safety.

Further the paper addresses how to demonstrate that target safety is maintained for non-inspectable areas, such as TLP tendons, spar soft tank to truss, mooring piles, etc. A case study is used to illustrate a critical TLP tendon connection representing non-inspectable detail with high failure consequences. By collecting data such as fabrication and maintenance records, metocean and load measurements, more sophisticated fatigue analysis and structural reliability analysis can be used to demonstrate that same target safety level can be achieved with longer fatigue life. Historic data can be used to more accurately quantify the past fatigue damage, and introduction of monitoring at life extension will reduce uncertainties during continued operation. Effects on documented target safety level practically achievable with these data sets are quantified in the paper.

Overall, for both inspectable and non-inspectable structures, it is possible with proper structural reliability analysis to demonstrate that target safety level is maintained for many years past its original design life. Quality operational records and monitoring data can help, and careful selection of statistical parameters for SRA are critical in such assessments.

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