On the dynamic stability of a vessel during lifting operation at sea

In maritime industry, a sea-going vessel‚ÄövÑv¥s sustainability and operability are closely related to its stability and the intact stability rules have evolved from the righting-lever-based static criteria to the multi-tiered, performance-oriented, and dynamic-addressed second-generation stability criteria which assess a vessel‚ÄövÑv¥s dynamic stability at sea. While a normal ship‚ÄövÑv¥s dynamic stability is evaluated by the five stability failure modes, the intact stability of a vessel with particular operation at sea such as lifting onboard wind turbine installation vessels, floating cranes, and fishing vessels is yet to be addressed accordingly. This study aimed at addressing the vessel-specific stability with a suspended load in wind and waves. The focus is transverse stability and motion prediction is a key to achieve the aim. As such, four-degree-of-freedom (4-DoF) of a sample vessel (roll, pitch, heave, & sway) and 6-DoF of a suspended load have been considered in this study. The objectives were achieved through experimental and numerical investigation into the influential parameters on a sample ship‚ÄövÑv¥s transverse stability. The influential parameters were first identified through experimental model test and then the extent of their influence was studied. After a numerical model was developed, comprehensive verification study was carried out as well as validation against the Experimental Fluid Dynamics (EFD) result and good agreement achieved. The numerical model was later used for extensive test under various conditions in both shallow and deep water. The tests were conducted in beam wave/wind condition under lifting operation though which, the model‚ÄövÑv¥s transverse stability as well as the dynamic stability failure mode of excessive acceleration were evaluated and quantified. The results show that the URANS-based Computational Fluid Dynamics (CFD) solver is able to accurately capture the hydrodynamic behaviour of the model and the 4-DoF motion response has been justified. There are mainly three aspects of findings: stability with a suspended load, shallow water effects, and excessive acceleration stability failure mode. In the sense of intact stability, the influence of the suspended load is threefold: metacentric height (GM) variation, lever effect, and swinging pull. As from the result of scale-model tests, in calm water, GM variation by a mass of 0.66 (%˜ívÆ) results in a change of the roll natural period of about 4%. In addition, the suspended load alone at a position with the longest lever stretching out at side, heels the model up to 8 degrees corresponding to a reduction of the righting lever (GZ) value by about 40%. Another noticeable influence observed through URANS-based CFD simulation is when the connecting rope is long, it does not seem to increase or decrease the amplitude of the roll motion. Rather, it alters the behaviour of the roll motion ‚ÄövÑv¨ from regular sinusoidal-like roll to very irregular behaviour especially at higher wave excitation frequencies. A second point to make is when the water is shallow (since many lifting operations actually happens near shore such as cargo xxiii loading/unloading and near shore fishing), the model‚ÄövÑv¥s motion changed considerable. First, shallow water starts to have effects on roll, pitch, heave, and sway motions from a water-depth /draught ratio of about 7 as opposed to 4 suggested by ITTC (2005). Second, in contrast with that in deep water, the maximum variations of those motion amplitudes, are about 77%, 110%, 25%, and 500%, respectively. Third, double-peak-roll phenomenon appears to be both wave-frequency dependant and water-depth-related, however, at a water-depth/wavelength ratio of about 0.45 and above, the motions are not affected. Another aspect is the Excessive Acceleration (EA) stability failure mode. According to the results of current study, gravitational and heave components are both roll-angle-dependant that amount to roughly 75% and 10%. Although roll motion plays a major role in terms of Excessive Acceleration, the position where crew or passengers may present is also important. Interestingly, the direction of sway motion always follows roll motion and its influence on EA is more of wave-frequency-dependant. It is also found that in calm water, lifting speed does not affect EA but when the suspended load emerges from water to air, the magnitude of EA, roll motion, and connecting forces are all increased. In regular beam waves, the suspended load increases the magnitude of EA at all wave heights, and it is wave-frequency-dependant. In irregular beam waves, the exceedance probability of EA increases nonlinearly with wave heights, and it tends to be larger with a suspended load.