Wandoo A – A Life Extension Case Study Implementing Reliability Methodologies

Abstract

The Wandoo Oil Field is located about 75km northwest of Karratha, Western Australia. Wandoo A (WNA) is a normally unmanned monopod platform installed in 1993 with a 15-year design life. Demonstrating fitness for service of this facility through the end of field life posed unique technical challenges given its features, including grouted connections, Tuned Liquid Damper (TLD) and the prospect of Wave-in-Deck (WID) in higher return period environmental events. This paper presents a life extension case study for this facility by exploring the overall life extension process and how that process is tailored to its unique structural aspects. In addition to evaluating the structure for metocean strength and fatigue loading, seismic and ship collision loading were also components of the assessment. Incorporating the current condition of the assets into the process, the probability of failure subject to the various environmental hazards was thoroughly studied to check conformance against industry acceptable limits using both design level and ultimate strength approaches. Extensive reliability assessment is performed with consideration given to factors of uncertainty including material strength, structural system capacity, structural system redundancy, WID and Wave-in-Jacket (WIJ) loading, and soil capacity. Analytical refinement was devoted to both capacity (structural and soil behavior) and demand (metocean/seismic events and return periods) considerations to increase confidence in the results. Probability Density Functions (PDFs) for metocean loading and for resistance of the structure and foundation are calculated in order to determine the collapse frequency (probability of failure calculation) based on reliability methodologies considering both First Order Reliability Methods and Monte Carlo simulations. Of particular focus for this work is supporting the ALARP (As Low As Reasonably Practicable) phase of the project based on the reliability outcomes. As with any ageing asset, there are components and systems that do not meet normal design allowable limits used for newer structures. This paper explores how these higher risk aspects of the facilities were studied so they may be addressed using an ALARP approach and how that process fits into the typical life extension process followed for WNA.