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OTO 2002/011

Efficient techniques for risk assessment of space frame platforms exposed to random wave and current loading : some thoughts on extension to cover effects of intermittency and directionality.

Offshore structures are subject to a wide variety of loads such as wind, wave and current) all of which exhibit a high degree of statistical uncertainty. The structural components, eg beams, columns, connections, piles, conductors and risers must be proportioned to resist the effects of these loads. Uncertainties in capacities or member strengths arise because of material and fabrication variability, and limitations in engineering theories to predict and conveniently interpret element and system response and capacity. As a result, a reliability-based approach is necessary to account for these uncertainties. This project aimed to extend the applicability of recently developed methods : a Factor Analysis Technique; a Dynamic Excess Kurtosis technique; and Najafian's Extreme Value probability model (NEV). The specific objectives were :- to incorporate the effect of current and load intermittency in the splash zone into the NEV probability modelling approach; and also to extend the techniques for efficient evaluation of kurtosis (a crucial parameter in the characterisation of a probability distribution) to cases when current , load intermittency and wave directionality are important. Describes a number of achievements: a robust implementation of the simulation technique, directed towards the reliable estimation of extreme values of response has been successfully
extended to account for current and load intermittency in the splash zone; the Factor Analysis technique has been extended to account for current, wave directionality and load intermittency in the splash zone. The results of this technique have been compared against those from time simulation technique to confirm both its accuracy and its much higher computational efficiency (about 25 times for long-crested seas and at least 100 times for directional seas). The effects of current and load intermittency on the statistical properties of both dynamic and quasi-static responses have been thoroughly investigated. Extension of the Dynamic Excess Kurtosis Technique and Najafian's Extreme Value probability model to the case of structures under the influence of load intermittency and current has been thoroughly investigated. Extensive investigation has shown that the Pierson-Holmes distribution can accurately account for the effect of current on the probability distribution of response. However, it cannot accurately model the effect of the load intermittency in the splash zone on the response probability distribution. A methodology for establishing an improved probability model for the response distribution arising from structures subject to load intermittency has been devised, the proposed methodology can also be used to determine the probabilistic properties of the dynamic response from those of the quasi-static response.

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Updated 2015-09-14