University of Twente Student Theses


Improved coupling Pharos-Diffrac : including spatial variations in wave conditions along a ship's hull in dynamic mooring analysis

Oude Vrielink, J. (2016) Improved coupling Pharos-Diffrac : including spatial variations in wave conditions along a ship's hull in dynamic mooring analysis.

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Abstract:A moored ship is subjected to different forces. When the conditions are too extreme, the lines can break and dangerous situations will occur. The time ships are not able to operate is defined as the down-time. During the down time of a ship, no cargo can be (un)loaded, therefore this time is expensive for port operators and shipping lines. In operational perspective, harbors are interested in the downtimes of their ships and in methods to reduce the down-times of ships in their harbors. On the other hand in design perspective, in the processes of designing ports these down-times are important to consider. To calculate mooring conditions, it is required to consider all relevant forces that act on a ship. Two forces have a dominant influence on the ship. Firstly, forces caused by waves (wave forces). These forces are applied to the ship’s hull. The second type of forces acting on the hull of the ship are the fluid reaction forces. These forces are caused by the motions of the ship itself. These types of forces can be split up into two parts: the added mass and the damping. The added mass is caused by the water around the ship being set in motion which will cause some extra moment of inertia. The damping forces are caused by the damping motions of the ship (Vis & Benit, 2016). For calculating mooring conditions (motions and line forces) three models have been used: A wave penetration model (PHAROS), a 3D ship diffraction model (DIFFRAC) and a dynamic mooring model (MOORINGS). A simplified but common methodology exists to calculate mooring conditions by using all three of the models, however this method brings uncertainties in output because the spatial variation of wave conditions along the hull of the ship is not taken into account. It is expected that the spatial variations of wave conditions along the hull of the ship have a significant impact on the mooring conditions. In certain conditions, the existing method only considers the wave conditions at one location on the hull of the ship. A method has been developed to consider the spatial variations of wave conditions along the hull of the ship. This has been done by coupling the wave potential model (PHAROS) to the hydrodynamic ship diffraction model (DIFFRAC) in a direct way. The aim of the research is to contribute to the development of a more realistic representation of mooring conditions in a dynamic mooring analysis. This is done by answering the question of: what is the most accurate way of connecting the models PHAROS and DIFFRAC for calculating the mooring conditions? This main question has been answered by performing a dynamic mooring analysis for the existing method and the newly developed coupling method. For determining the most accurate method, a third reference method has been included. This method calculates the mooring conditions in the most ideal way. This is achievable by excluding bottom depth variations in the analyzed situations. To get an indication of the working of the coupling, the wave forces have been calculated in DIFFRAC for the coupling method and the reference method. From a comparison, it became clear that the coupling method followed the trend of the ideal situation, however for higher frequencies the relative error became larger. The coupling method and the existing method have been compared with the MOORINGS output. From this output it became clear that the newly developed coupling method is the most accurate method for calculating the mooring conditions. However the effect of the improvement using the coupling is not significant. This can be clarified by the fact that calculations are based on wave conditions with a wave height of one meter. For more extreme situations it is expected that the results from the coupling method show significant improvements in comparison with the common method. It can be concluded that the new developed coupling theory has big potential to become the standard way of calculating mooring conditions in a dynamic mooring analysis. The current research is based on a situation with ideal conditions, and many simplifications have been made in this research. This decision has been made to get a pure comparison between the methods. However, for future research it is recommended to investigate whether the coupling will also generate the same improvements/results for more realistic situations.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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