Dependence modelling of marine environmental variables and resilience assessment of offshore structures operating in harsh environments

Ramadhani, Adhitya Ryan (2023) Dependence modelling of marine environmental variables and resilience assessment of offshore structures operating in harsh environments. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

With increasing global energy demand, energy-related offshore activities continue to increase. As part of this expansion, exploration and operations in harsh ocean environments are becoming more common. Harsh environments are characterized by low temperatures, strong winds, high waves, and ice. These harsher conditions apply increased structural loads and may increase the probability of accidents. Such environmental variables are usually interdependent. Failure to consider these dependencies, when modelling harsh environmental loads, leads to less accurate predictions of design loads and consequently less accurate predictions of offshore structures’ capacity to withstand these loads. This work addresses the dependence issue between environmental variables and seeks to improve predictions of environmental loads and offshore structural capacity. The benefits of considering dependence structures using copula functions are assessed for bivariate and vine-copulas are assessed for multivariate cases of the environmental variables. In each case this is followed by applications to assess offshore structure resilience. Two types of dependence structures are studied: symmetric and asymmetric. Environmental loads are estimated using copula functions to see how significantly the correlation influences the estimation. The copula functions are then applied to assess structural capacity in terms of resilience. A bivariate application case uses copula functions to model two influencing factors that determine the velocity of an iceberg. Results show that the resilience of the offshore structure is mainly dependent on absorptive capacity. Multivariate models are then constructed using Vine Copulas, and a total environmental load is estimated. This multivariate copula model is applied to assess the resilience of an offshore structure subjected to multiple environmental loads. The study concludes that the accuracy of environmental load predictions can be improved using copula functions to model environmental factor dependencies. In addition, the concept of structural resilience provides a better means of considering the overall resistance of a structure subject to harsh and dependent multivariate environmental loads. Applications of the proposed methodologies in this thesis help to develop a robust approach to deal with uncertainties related to dependence structures between marine environmental variables. In addition, this thesis helps to develop safer offshore structures operating in harsh environment by estimating the structure’s capacity in term of resilience in the design stage.

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