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Tankers used for offshore oil production and storage are kept in station by turret mooring systems, enabling the vessels to weathervane in the direction of the dominant environmental loads. These passive weathervaning systems have been observed in model experiments to be ineffective in swell-dominated long wave conditions. Over a range of wavelengths from 0.6 < λ/L < 2 (L – ship length), the vessel was observed to lose heading control in head sea condition, due to a pitchfork bifurcation that is initiated at a critical wavelength of 0.73L. A notable feature of poor heading stability is the existence of a stable equilibrium at a large heading angle (50–60°) with respect to the direction of oncoming waves. With lack of heading control, the ship motions, principally roll, can increase thus affecting onboard operations. Time domain analysis conducted with no added viscous damping shows reasonable agreement with experimental data for the final heading angle. Further numerical tests reported in a previous paper by the authors showed that small to moderate viscous damping in sway and yaw did not alter the final heading, while the role played by viscous damping in other modes (heave, roll and pitch) needed further investigation. This paper reports on a parametric study on the heading stability of a turret moored tanker using time domain tools. Viscous damping is systematically varied in different modes of motion and its effect on final heading equilibrium is assessed. It is shown that effects of pitch damping are stronger than heave or roll, and can eliminate heading instability altogether.

Tanker vessels used for offshore oil production and storage are kept at station by turret mooring systems, enabling the vessel to weathervane in the direction of dominant environmental loads. The disruption of heading equilibrium for a turret-moored tanker was predicted by experiments and numerical studies. A vessel was observed to lose control in head sea condition, wherein for wavelength from 0.73 < λ/L < 2 (L-ship length) the model drifted to a large angle of 45–60 degrees (Thiagarajan et al. 2013). Previous numerical analyses conducted by the authors identified that this heading drift reduced remarkably in the presence of wind. This finding is confirmed by an experimental study and reported in this paper. A geometrically scaled down version of a turret-moored FPSO at 1:120 scale of a prototype VLCC was tested at the Alfond W2 Wind & Wave Ocean Engineering facility of the University of Maine. This lab is a unique facility equipped with a high-performance wind machine over a multidirectional wave generator, and can create regular or random sea-states with wind speeds up to 7 m/s. The tests reported here were conducted with regular waves under two wind speeds (12 and 25 m/s full scale). It was observed that the presence of an initially bow wind can minimize the heading instability. The reason for this observation is described by analyzing the effect of the wind induced moments on the equilibrium condition. Free-decay tests were also conducted to investigate the contribution of the wind damping to the total damping. Measured results show that in the presence of wind, the damping values are higher than those estimated due to hydrodynamics only. It also has been discussed that this wind induced damping on FPSOs, can result in smaller heading angles. From this study, it is concluded that the wind can play a large role in the station-keeping dynamics of the moored-tankers.

This paper presents technical details for a unique newly constructed model testing facility for offshore renewable energy devices and other structures established through federal and state funding. The University of Maine (UMaine) has been an active contributor to research in the field of floating offshore wind turbine (FOWT) design and scale-model testing for the past 6 years. Due to a lack of appropriate test facilities in the United States, UMaine has led multiple 1:50 scale-model tests of FOWT platforms internationally, leading to the motivation to design and build a state-of-the-art test facility at UMaine which includes high-quality wind generation with waves and towing capabilities. In November of 2015, UMaine opened the Alfond Wind/Wave Ocean Engineering Laboratory (W2) at the Advanced Structures and Composites Center. This facility, shown in Figure 1, contains a 30m long x 9m wide x 0-4.5m variable floor depth test basin with a 16-paddle wave maker at one end and a parabolic wave attenuating beach at the other. This basin is unique in that it integrates a rotatable open-jet wind tunnel over the basin that is capable of simulating high-quality wind fields in excess of 10 m/s over a large test area. Since opening, the W2 has provided testing for various scale-model FOWT designs, oil and gas vessels such as a scale-model floating production storage and offloading (FPSO) vessel, and a large number of wave energy conversion (WEC) devices in support of the Department of Energy’s (DOE) Wave Energy Prize. In addition to scalemodel testing, the W2 facility supports a wide range of model construction equipment including a 2.0m x 4.0m x 0.1m tall 3D CNC waterjet, a 3m long x 1.5m wide x 1.4m tall 5-axis CNC router, and an additive manufacturing facility housing a 0.6m x 0.6m x 0.9m 3D printer. To expand the capability of W2, a towing system is currently being designed to operate in conjunction with the multi-directional wave maker, which is shown in Figure 5. This equipment will provide bi-directional towing for a variety of applications. In addition to standard resistance testing, the broad aspect ratio of the basin provides reduced blockage effects while the multi-directional wave maker allows for tow testing a large number of wave environments and headings. The moving floor enables intermediate to shallow water tow tank tests, which are important for capturing the wave kinematics applicable to coastal environments, while the relatively deep water depths support testing of large structures such as tidal turbines and tow-out operations for THE 30th AMERICAN TOWING TANK CONFERENCE WEST BETHESDA, MARYLAND, OCTOBER 2017 2 large offshore structures such as wind and wave floating energy platforms. To test the capabilities of this system, UMaine is constructing a 1:50-scale model of the David Taylor Model Basin (DTMB) 5415 to perform commissioning tests. The towing system is planned to be operational in 2018.

The exact calculation of the added resistance in waves is a seakeeping problem of high interest due to economic effect on ship exploitation. In this paper an open uniform B-spline based method is developed to calculate added resistance. Initially this method applied to calculate velocity potential and Kochin functions that are necessary for calculation of the added resistance by Kashiwagi’s formula. For this purpose the source strength and potential are distributed over body surface and described Open-uniform B-spline basic function. Computations are performed for different hull forms then results are validated by comparing them with practical results. The present method shows a good agreement in contrast to published results.