This paper presents a probabilistic framework for robustness assessment of the floating production system (FPS) under mooring failure, which impose great risk to progressive failure of the structure. The proposed framework addressed the difficulty in quantitatively evaluating the progressive failure of FPS concerning the uncertainties in various sea states and structural attributes. A case study of a Tension Leg Platform (TLP) under mooring failure was presented. The robustness of TLP was evaluated through progressive failure probability. Robustness analysis results showed that directions of environment load had a great influence in the robustness of structure. And progressive collapse process of TLP under mooring failure was figured as well.
A floating production system (FPS) is a type of offshore structures used in deepwater oil and gas field development. And it is typically moored to keep it in position by mooring lines anchored in seabed. When some certain moorings are damaged, it imposes great risks to progressive failure of the general system, such as the failure of other moorings, which would lead to a great economic and environmental loss. Robustness is an ability that a structure can prevent from such progressive failure, when it meets an initial failure. Most of the researches on assessment of offshore structure were only focusing on the probability of initial failure. robustness of structure is a further demand for the structure.
More and more institutes and researchers pay attention to the robustness of the offshore structure, but many of them just focused on the qualitative analysis. Most of quantitative study was about stationary structure, like onshore construction(Frangopol and Curley 1987; Biondini, Frangopol, and Restelli 2008), and discounted the dynamic response. But the dynamic response should be stressed for FPS. Moreover, the quantitative analysis often ignored the uncertainties of the environment and the structural system. There were some researches on assessing the structural robustness in probabilistic form. The structural robustness was evaluated with probability of progressive collapse, which was defined as a chain reaction (Ellingwood and Dusenberry 2010). Based on decision analysis theory, a robustness index was proposed as the proportion of direct risk to total risk (Baker, Schubert, and Faber 2008). And Lin and Zhu(Zhu et al. 2015; Lin et al. 2016) applied it in ocean engineering and put forward a method to evaluate the robustness of jacket platforms.
