Rolling of FPSOs is a major concern, as excessive motions may result in several operational problems including process plant downtime. Moreover, it is well known that the loads induced by roll motions may govern the design of several structural elements of the unit, such as topside supports. In order to improve the hydrodynamic performance of new FPSOs, longer roll natural periods are targeted so that the resonance within the range of periods of incident waves is avoided. Nevertheless, such a measure does not completely mitigate the resonant motions and the unit may still respond at second order (at the difference between wave frequencies). Much work has already been devoted to the accurate estimation of second order roll motions of FPSOs. However, the state-of-the-art methods for structural assessments still rely on linear assumptions and are not able to account for second-order response. In this work, we present a novel approach to obtain a design load case for structural assessment of elements whose maxima stresses are governed by second order roll. The approach is based on the concept of response conditioned wave (RCW). As the process is non-Gaussian, it is not possible to derive the RCW analytically. Therefore, first order reliability method (FORM) is used to numerically obtain the most probable wave yielding the target roll-affected load value. It is shown that the resulting wave also leads to the most probable responses associated to the target roll motion, as an example first order heave motions, demonstrating the robustness of the method.

This content is only available via PDF.
You can access this article if you purchase or spend a download.