Roll Damping Analysis of In-Field FPSO Roll Response

Due to the resonance behavior of roll motions, roll damping is an important consideration for vessel motions and associated extreme and fatigue loading on the hull, topsides and risers of an FPSO. In many cases radiation damping is limited and passive damping devices such as bilge keels are installed to spur viscous eddies and hence limit the roll motions. This contributes nonlinear damping to an already complex problem. Designers often rely on model tests to assess this damping. Based on test results, empirical and semi-empirical estimation models have been developed for different ship types and are available in current literature, but examples of benchmark validation with real world data are limited. These benchmarks are often hindered by uncertainty in the observed weather conditions, vessel loading conditions and vessel heading with respect to the waves. This paper discusses these challenges and introduces a novel approach used to characterize the actual roll damping for an FPSO under real world conditions. The assumptions, methodology and results will be discussed in this paper. In this study, 5 years of hindcast weather data is examined along with FPSO heading and roll motion measurements. The roll damping characteristics of this FPSO was expected to change over the course of the measurements and the study documents the actual variation of roll damping under various conditions over this period.