Roll Damping Prediction for Vessels; Sheerlegs and Barges

Abstract

To guaranty safety of the vessel, operation and especially the people during offshore activities, a motion analysis is executed. The motion analysis determines the motions over the entire frequency spectrum. Motions are amplified when incoming wave frequencies come to the vicinity of the natural frequency of the vessel, hence a very interesting region concerning the safety of the operation. In the vicinity of the natural frequency the motions are highly dependent on the present damping, therefore an accurate prediction of the damping is desired. This thesis study aims to improve the roll damping operation of sheerlegs (barge-type vessels) Taklift 4 and Taklift 7 during a lifting opertation. This is achieved by implementing the viscous roll damping component and investigating the effect of a vertical center of gravity well above the waterline. A high VCG results in a coupled motion between roll and sway. The effect of a high vertical center of gravity on the potential and viscous roll damping is studied. Furtermore, the viscous roll damping is determined applying three different techniques. Finally all models are validated with data from pre-executed experiments. Theory developed by Ikeda, Tanaka and Himeno (IHT) is implemented to compute the viscous roll damping for ship shaped hulls and barge-type hulls. IHT theory is the ITTC recommended procedure for practical assesment of viscous roll damping in absence of experiments, as long as the hull dimensions and motion conditions are within the set limitations. Two numerical models are developed according to IHT theory, the IHT - Ship model and IHT - Barge model. The IHT - Ship model is verified and showed no discrepancies with the verifying document. However, the calculated viscous roll damping for the Taklift 4 is surprisingly small when compared to the potential roll damping. The estimation of the bilge radius and the lewis transformation to determine the velocity increment appeared to be the least accurate parameters causing an inaccurate viscous roll damping estimation. After the IHT - Barge model is developed, it is directly validated together with the ‘κ-formula’-model against the decay test data. Journee designed the κ-formula’-model, which contains the potential roll damping ratio and a viscous contribution multiplied with the square-root of the roll angle. The validation shows that the ‘κ-formula’-model consistently estimates the viscous roll damping in a more accurate manner for barges where 5≤B/T≤7.5. Unfortunately, the decay tests were limited with an initial roll angles up to five or six degrees. It is rather challenging to define a limitation concerning a roll angle for this approach. Therefore, the ‘κ-formula’-model can safely be used, but for large roll angles it is advised to maintain a conservative user approach. For barges where B/T=10, the IHT – Barge model is not appropriate and also the ‘κ-formula’-model is accurate for roll angles smaller than 2.5 degrees. It is suggested the free surface effects ‘hide’ the dependence of the viscous roll damping and the roll angle. The third technique to determine the viscous roll damping, is the so-called Magnuson approach. Magnuson applied an equivalent linearization procedure on the experimental data of the Noble Denton JIP to determine the viscous roll damping. To develop the Magnuson approach, experimental data providing motion response and environmental conditions is required. Such data is not available for the Taklift 4 and Taklift 7, hence results from pre-executed experiments are used, which among other things contains free decay test results and behavior in regular beam waves for rectangular scale models with similar dimensions as the Taklift 4 and Taklift 7. The main difference between the vessels studied in this thesis scope and pre-executed experiments, is the height the vertical center of gravity (VCG). A VCG above the waterline causes the response motion to change from 'pure' roll to a coupled roll-sway motion. Due to a phase difference between the coupled terms and b44 and b22 term, the B44p is decreasing for increasing VCG. The B44p coefficient becomes zero for a certain location of the VCG, which could not be explained physically in this thesis. The effect of the VCG on the b44v is positive. It is suggested the coupled roll-sway motion and secondary separation can account for this. An attempt is made to determine viscous effects of b24, b42 and b22. Very limited studies are executed on this topic, hence it is very difficult to determine the correctness of the computations. After all developed models were compared with each other, the Magnuson approach showed most promise. Therefore this approach is used to determine the viscous roll damping of the Taklift 4 and Taklift 7. Viscous damping ratios of 0.041 and 0.034 respectively, were computed for the most similar scale models, hence a viscous damping ratio of 0.0375 for both Taklift vessels is determined. It is left for future work to determine how the workability of the Taklift 4 and Taklift 7 is increased by the more accurate roll damping prediction.