Bilge keel damping from in-fi eld motion measurements


Chevron worked with MARIN on a novel approach to characterise the actual damping for an FSPO in real world conditions.


R


esults show that using damping from model tests means that roll decay in calm water is conservative


and that natural roll periods are less sensitive to the FPSO draught than often considered in the design phase. The details of the study will be presented in an OMAE paper this year, but Report takes an initial look at the approach and its applications for other fields.


Due to the resonance behaviour of roll motions, roll damping is an important consideration for vessel motions and the 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, 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.


Arjan Voogt, a.voogt@marin.nl 14 report


Bilge keel reduction In this study, MARIN examined five years of hindcast weather data, along with FPSO heading and roll motion measurements. The studied


FPSO is a VLCC sized converted tanker with an internal turret located in an approximate 1,000 metre water depth. The configuration of the bilge keel was modified resulting in an effective reduction of the projected bilge keel area of about 60% since its initial construction. The reduction was completed in a phased effort over a period of several years. Therefore, the bilge keel reduction process was ongoing for the duration of this study; so the results could capture the impact of a progressive reduction in roll damping as it was reduced.


The measured roll response is correlated to the hindcast weather conditions and contrary to the conclusions from the established norms, the FPSO heading appears to be primarily dominated by current, with wind playing a secondary role. Local sea and swell appear to have relatively little influence on vessel heading. Additionally, due to the lack of coherence between the current, wind, swell and local sea directions, the FPSO might be at higher risk of extreme beam swell conditions than was assumed during the design phase.


To compare the damping in the field with the damping estimated from model tests, a numerical model based on a roll Response Amplitude Operator (RAO) was set up. The natural period of roll was tuned to match the roll periods in the field. While the draught increases from 10 m to 16 m, the prevalent zero-up crossing period in the roll


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