Session: 02-12-01 Reliability Based Maintenance 1

Paper Number: 128159

128159 - Probabilistic Risk-Based Inspection Method for Offshore Jackets: Combined Fatigue Degradation and Collapse Under Extreme Environmental Overload 

The integrity of offshore structures is dependent on the structure’s system capacity and the applied loading through time. Inspections are used to establish the current condition of components comprising the structural system. Inspection plans can be developed through application of risk-based inspection methodologies as per the guidance in ISO 19901-9 [1]. Specifically, ‘no find’ inspection methods for structural members (such as flooded member detection, FMD) or detailed non-destructive inspection (NDT) such as ultrasonic inspection which can measure the size of defects, are employed to provide information on the weld condition. As ageing structures are life extended, semi-quantitative RBI methods can result in onerous and sometimes impractical levels of weld fatigue NDT inspections. There is a need to refine the inspection plan using a quantitative method. 

The present study outlines a Monte Carlo Simulation-based (MCS) probabilistic assessment methodology for inspection planning of offshore jacket structures. The method calculates the probability of collapse of a jacket structure when subjected to the combined effects of time-dependent fatigue degradation of structural elements such as severance leading to reduced jacket strength, and extreme metocean loading causing collapse. The outcomes of in-service weld inspections and online structural monitoring are incorporated into the probabilistic assessment by applying Bayesian updating and inspection probability of detection curves. 

The dates and scope of future inspection may be planned by calculating the probability of collapse at future time conditional on no-find inspection and monitoring outcomes. The inspections and monitoring plans are developed to ensure target acceptable probability of structural collapse or unacceptable widespread fatigue damage are maintained below acceptable quantified risk-based targets. 

The method enumerates sequences of fatigue failures using Monte-Carlo simulation and weld fatigue life probability distributions. Allowance is made for the redistribution of fatigue loading throughout the structure during the member fatigue failure sequences.    

Case study examples are presented describing the motivation for the use of this method as well as the updates to the inspection plans justified by the results. The approach outlined is a useful method for quantitatively planning inspections of late-life offshore jacket structures that can be applied by practicing engineers responsible for structural integrity management and/or life extension. 

References: 

1. International Standards Organisation, “Petroleum and natural gas industries – Specific requirements for offshore structures; Part 9: Structural integrity management”, ISO 19901-9:2019. 

Presenting Author: Mark Manzocchi Kent

Presenting Author Biography: Mr. Manzocchi has more than 30 years' experience in civil, structural and offshore engineering. Mark is a Chief Engineer and Kent's Global Subject Matter Expert on Structural Reliability, Metocean, and Code Compliance. He is a technical expert in the field of management of fixed and floating offshore structures, including metocean statistical analysis, probabilistic methods, and code calibration. In addition, he has comprehensive expertise in offshore structural analysis, assessment, design and integrity management. Mark holds a BSc in Civil Engineering and an MSc in Earthquake Engineering, both from Imperial College.

Authors:

Mark Manzocchi Kent
Bryan Horton Kent
Nicolas Rooms Kent
Jeremy Evans Kent
Oluwole Fajuyitan Kent
Alexander Macdonald Kent