Barge transportation of heavy objects
Master thesis
Permanent lenke
http://hdl.handle.net/11250/237779Utgivelsesdato
2010Metadata
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- Institutt for marin teknikk [3397]
Sammendrag
The topic for this master thesis is transportation of heavy platform modules on barges. The goal is to find the short-term extreme accelerations, or design accelerations, that are limiting for the feasibility of the transportation operation. Motion response analyses have been performed for six different combinations of barges and module weights as follows:
Barge Module
300 feet 1000 tonnes
300 feet 3000 tonnes
400 feet 1000 tonnes
400 feet 5000 tonnes
600 feet 5000 tonnes
600 feet 8000 tonnes
It has been observed that the roll acceleration is overestimated in non-viscous motion analyses of barge type vessels. Thus for the first case an additional analysis including viscous roll damping has been performed.
The software used to perform the analyses and the theory used has been reviewed. The following programs are described:Genie – modelling of the barge (hull, ballast tanks)HydroD (WADAM) – modelling of environment, hydrodynamic analysis by source techniquePostresp – combination and printing of motion characteristicsMatlab – statistical postprocessing, calculation of design accelerationsThe founding theories are linear potential wave theory and source technique. The viscous roll damping, which in reality is non-linear, was estimated in a linearised form using strip theory and empirical data. Based on the calculated motion characteristics, the design accelerations are estimated by short-term statistics, meaning the extreme accelerations are estimated in 3 hour seastates. This was performed for four different significant wave heights and all relevant wave periods.
The accelerations are calculated in the system centre of gravity, module centre of gravity and top and bottom corners of the module. The limiting criterion of a barge transportation is normally the forces the seafastening can withstand, thus the acceleration on the seafastening has been examined closer. Aker Solutions have roughly defined the seafastening capacity by setting absolute limits for the accelerations in the longitudinal, transverse and vertical direction.
Compared to the criteria for accelerations given by Aker Solutions, the results proved that the acceleration in the transverse direction is the limiting factor. Beam seas close to the roll eigenperiod gave the highest accelerations on the seafastening both in the transverse and vertical direction, suggesting that these accelerations are governed by the roll motion. When viscous roll damping was included, there was a significant reduction in the transverse acceleration, and a slightly smaller reduction in the vertical acceleration on the seafastening compared to the non-viscous case. However, the transverse acceleration was still the most critical with respect to the given criteria. There was little or no effect of viscous roll damping on longitudinal design accelerations.
Design accelerations for a 1000 tonne module were found for two different barges, a 300 feet barge and a 400 feet barge. The accelerations proved significantly smaller on the 400 feet barge, especially in the transverse direction. A 5000 tonne module was also tested on two different barges, a 400 feet barge and a 600 feet barge. As in the preceding comparison, the accelerations were significantly larger on the smaller barge. Thus, for a given load, switching to a larger barge can give a large reduction in the acceleration.
There are indications that the results achieved are conservative, including the case with viscous roll damping. However, this is only an indication. The viscous roll damping is difficult to estimate correctly and therefore a suggestion could be to investigate the validity of the viscous roll damping model through model tests.
A certain reduction in the roll acceleration was observed when the load was increased for the 300 feet barge, without giving an increase in the roll angle extremes. This indicates that for some cases a decrease in the metacentric height can, provided that the stability is sufficient, contribute to a decrease in the roll acceleration without increasing the extreme roll angle.
In high sea states, non-linear effects can become significant. The resulting effect on roll acceleration is unknown. Hence, it could be advantageous to investigate the extent of these effects using a non-linear calculation model.