November 30, 2018
Floating Production Storage and Offloading facilities (FPSOs) offshore Western Australia are exposed to persistent Southern Ocean swells which may induce onerous roll motions. The topside operational processes of FPSOs, and personnel safety, can both be adversely affected by excessive roll motions. Accurate prediction of roll motions is therefore essential for efficient FPSO design in this region.
In comparison to the other motions of a ship (heave and pitch etc.), the roll response is much more difficult to predict as non-linear, viscous effects generally provide a significant contribution to the available damping.
Optimisation of a new build FPSO for Australia had initially shown that the use of a large bilge radius was beneficial for ship motions and mooring design. However, the bilge radius used was outside the usual experience range for accurately predicting roll damping. A concern was raised that separation off the bilge radius would result in a reduced effectiveness of the bilge keel.
Dr Ian Milne (UWA) of the OFFshore ITRH investigated this problem in collaboration with Woodside’s naval architects, which involved the development and application of advanced computational fluid dynamics (CFD) models to simulate the flow around the hull and bilge keels for the various geometries of interest.
The CFD analyses provided critical new insights into the damping characteristics for the range of swell conditions likely to be experienced by the FPSO. In particular, the damping was found to be reduced by around 50 percent when compared to a conventional hull design, with the larger bilge radius resulting in weaker vortex shedding at the keel.
As a consequence of the smaller available damping, the roll motions for the large bilge radius hull would likely be excessive and hence significantly undermine the operational efficiency of the FPSO. This new work allowed a key risk for the project to be mitigated. The fundamental new insights on roll damping are also expected to assist the design of other FPSOs for years to come.
This collaborative work is a clear example of how the Offshore ITRH has provided high value outcomes for its research partners and had a significant impact on Australian operations.