Probabilistic analysis of flexible riser responses in storms

Dynamic flexible risers are an essential component which governs design of a floating production system, especially for offshore fields with harsh environment. Current design practices for flexible risers do not always capture the probabilistic nature of the offshore environment and its effects on the risers in a consistent manner. Unlike hulls and mooring systems of floating vessels, the probabilistic design methods for riser systems are somewhat lagging due to their complex structure, nonlinear mechanical properties and responses. As probabilistic response based design methods are increasingly in use for fixed and floating structures, inclusion of the riser systems into the framework of these methods is a logical extension. In this work, probabilistic prediction of extreme riser responses is investigated for a short and medium-term exposure periods, the latter being associated with a storm event. Investigation of several critical responses at several locations along a flexible riser supported by a weathervaning turret-moored vessel was conducted. Probability distributions of the response processes, of the extreme responses in stationary intervals and in a complete storm, their sensitivities and variability were investigated comprehensively. The contributing metocean effects were examined where possible, and the wave frequency (WF) and low frequency (LF) (slow drift) components of the riser responses were studied in isolation and then combined. This approach allowed for a breakdown of the effects on each critical response, with the overall objective of developing a comprehensive storm-based response based design process, which would be applicable to all riser responses. Initial steps included gauging the applicability of various typical distribution models to describe stochastic response processes as well as the extreme value distributions of isolated WF and coupled LF+WF responses. This was followed by the development of the maximum response distribution in a storm and its asymptotic formulation in a conditional format, with results indicating accurate representation of the numerical probability functions. Variability of flexible riser responses in two storms was investigated, including differences between the responses, effects of various metocean parameters and representation of the maximum riser response in a storm by the most contributing interval (sea state). Another focus of the work was assessment of the practicality of the application of the probabilistic design methods to industry projects. Initial efforts to generate the riser response data sufficient to predict long term responses at long return periods by time domain analysis revealed major 'big data' restrictions, most significantly the computational efficiency of time domain methods and contemporary hardware. This prompted the focus to be extended to both the time and frequency domain analyses, respective solver strengths and limitations. Observing these limitations, optimization of the riser response analysis was studied using various methods including frequency domain and hybrid time - frequency domain approaches. Further work is recommended to develop these conceptual processes, however initial efforts provided promising insights and results towards the development of response based design methods for flexible risers.