Abstract
Reliability of offshore platforms has become a very important issue in the Malaysian Oil and Gas Industry as, majority of the jacket platforms in Malaysian waters to date, have exceeded their design life. Reliability of a jacket platform can be assessed through reliability index and probability of failure. Conventional metocean consideration uses 100 year return period wave height associated with 100 year return period current velocity and wind speed. However, recent study shows that for Malaysian waters, the proposed metocean consideration should be 100 year return period wave height associated with 10 year return period current velocity and wind speed. Hence, this research investigated the effect of different metocean consideration, to system-based reliability of jacket platforms in Malaysian waters. Prior to that, the effect of different metocean consideration to the pushover analysis has also been studied. Besides, the significance of Pile Soil Interaction (PSI), wave direction and platform geometry were analyzed in a sensitivity study. Pushover analysis was performed on three jacket platforms representing three water regions in Malaysia to obtain Reserve Strength Ratio (RSR) as an indicator of the reliability of the jackets. Utilizing sensitivity study parameters mentioned above, seven different case studies were undertaken to study their significance on RSR. The RSR values of each case study were compared and incorporated as resistance model of reliability analysis. Besides, platform specific response model of each jacket has been generated using response surface technique which was later incorporated into the limit state function for reliability analysis. Reliability analysis using First Order Reliability Method (FORM) has been conducted in MATLAB to obtain the reliability index and probability of failure. Results from the reliability analysis were compared to analyze the effect of different metocean consideration. In this study, an updated and detailed methodology of system reliability analysis for offshore jacket platforms is presented. Relationship curves for the safety indices were generated as the outcome of this study. Probability of Failure is found to be inversely functional to RSR. The newly proposed metocean consideration eliminates the conservativeness in currently practiced metocean values. Parameters like metocean considerations and PSI greatly affect the RSR, hence affects Reliability index and Probability of Failure.
Highlights
Most of the offshore oil and gas facilities in Malaysia are fixed type platforms fondly known as Jackets
High oil price coupled with Enhanced Oil Recovery (EOR) technology, demands to extend the life of these platforms resulting in the platforms being subjected to higher loading due to required modifications/upgrading and work-over demands for which the platforms may not have been originally designed for Nichols et al (2006)
Case A and Case B were compared to each other to study the effect of Pile Soil Interaction on the value of Reserve Strength Ratio (RSR); by including PSI consideration in the pushover Fig. 9: The RSR results for Jacket 'C'
Summary
Current velocity and wind speed are the dominant factors in design and analysis of offshore jacket platforms. 100 year wave height associated with 100 year current and wind speed, is utilized for design and analysis. Pushover analysis is widely used in current offshore standards such as API, ISO and DNV to evaluate the ultimate capacity of the platform against the environmental loading (Golashani et al, 2011). Reserve Strength Ratio (RSR) is a measure of structure's ability to withstand loads in excess of those determined from platform design. This reserve strength can be used to maintain the platform in service beyond their intended service life.
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