Internal turret moored floating body rotates naturally around a turret and consequently aligns itself along dominant load direction with significant yaw in this process. Since yawing is an interesting feature and an important design factor, it is necessary to carry out accurate prediction, especially in complex conditions such as multidirectional irregular waves. In this paper, the nonlinear yawing of the barge with internal turret mooring systems in bidirectional and bichromatic waves was investigated by a second-order time-domain method. The results indicate that large-amplitude, slowly varying offsets persisting in the second order yawing are evident. It is pointed out that second-order low frequency wave drift forces and incident wave heights have dominant influences on the dynamic responses of the internal turret moored barge.
Floating Production Storage and Offloading (FPSO) is deployed for oil and gas production in farther and deeper waters offshore. The FPSO is moored by a set of mooring lines connected to a freely rotating internal turret. The turret enables the vessel to rotate freely and consequently yaw to minimize the combined wind, current and wave-induced moment. The yawing of such vessels is one of factors of affecting motion behaviors, mooring loads and offloading operation.
The prediction of the yaw for the internal turret moored vessels is a complicated problem. Many researches have been done up to now. Bernitsas and Papoulias (1986) developed a mathematical model for the horizontal plane slow motions of single point moored ships and studied yaw motions and nonlinear stability. O'Donoghue and Linfoot (1992) performed model tests in a range of long-crested and shot-crested sea states and the highlighted effects of turret position and wave spreading on motions and line tensions. The stability in yaw of a turret- moored monohull in head sea regular waves was investigated by Liu et al. (1999), and large steady yaw offsets occurring at specific wave periods was indicated. Yadav et al. (2007) conducted the parametric study of an internal turret moored FPSO to evaluate the effects of turret position and hull length in yaw instability using the AQWA suite of hydrodynamic software. Recently, Cho (2012) and Cho et al. (2013) conducted model tests and stability analysis for an internal turret moored Floating Storage Regasification Unit (FSRU). Paton et al. (2005) observed high sway yaw coupled motions and the inefficiency of mathematical tools to predict such motions. Based on the review of these literatures, we found that more attentions about internal turret moored vessels have been payed to stability analysis and experiments in head sea regular wave or irregular waves. Little dynamic analysis has been done for such vessels in complex wave conditions.
