Producing hydrocarbons from deeper waters in various offshore environments requires operations and survival in complex seas made up of non-collinear winds, wind-driven seas and long period swells. Such bi-directional wave conditions appear in regions like west of Africa and Brazil (Campos Basin), where the 100-year condition comprises of extreme swells in addition to local waves of varying degrees of severity. A nascent interest of the offshore industry has been on studying the global performance of FPSO and FLNG systems in complex sea conditions in deep waters. This paper presents a study on the behavior of a turret moored Very Large Crude Carrier (VLCC) in typical bi-directional sea states at two experimental scales ranging from 1:60 to 1:120.It is still uncommon to find experimental capabilities for simulating and modeling bi-directional sea states. At the University of Maine's recently completed Alfond W2 wind wave facility, bi-dimensional sea-states are created by systematically combining two different wave systems from varying directions. The wave generator comprises of 16 independent paddles with the capability of generating regular and random uni-directional or bi-directional waves at angles up to +/- 60 degrees relative to the basin center line. A geometrically scaled down version of a VLCC which can be considered representative of an FPSO or an FLNG, at 1:120 scale was tested at the W2, whose hull and the mooring system were similar to a 1:60 scale moored-tanker hull tested at the National Research Council's Institute of Ocean Technology wave basin facility in Canada in 2006. The mooring system comprised of four spring-loaded lines attached to a forward turret.
Results show that statistics of the vessel global motions and mooring tension at the two scales compare very favorably. When the vessel is aligned with the oncoming swell, the influence of the wind-sea increases when its heading increases, and this is particularly noticeable on the horizontal plane motions (sway and yaw) and on the mooring line tension. The power spectral density of roll motion has a prominent peak at the swell period, which exceeds the peak at the natural frequency. Our previous experiments in uni-directional long period waves identified an instability in passive weathervaning systems of FPSOs, wherein for wavelengths from 0.6 < λ/L < 2 (L – ship length) the model drifted to a large angle of 45 – 67 deg with respect to head sea. This effect, noticed at both model scales, was subsequently verified by simulations and theoretical calculations. In the presence of bi-directional conditions, the wind sea appears to calm the instability, and the mean heading drift was reduced to 24 deg.