Numerical analysis of dynamic response and wave-load on multihull ship advancing in waves

Abstract

The motion response of a vessel traveling at forward velocity in a seaway is critical in assessing the vessel's operability, and determining the motion response during the preliminary design stage of every vessel ensures the safety of not only the passengers and crew, but also the cargo the ship is carrying. With advancement in ship design, vessels with orthogonal design characteristics, i.e., vessels with multi hulls, are being constructed and operated frequently now-a-days. As multihull vessels offer advantages like additional deck area and added transverse stability, they are becoming popular for sea travels and as such assessing the seakeeping characteristics of these vessels are of great importance. Which is why the aim of this research is to develop a robust numerical tool that is able to analyze the seakeeping characteristics of multihull vessels with acceptable engineering accuracy. The theory of ship motion has gone through radical advances through ages. Starting with strip theory, which was capable of analyzing ship motion at lower Froude number to 3D theories, which has the capability to incorporate the three-dimensional effects and can be used for a wider range of Froude number; there are multitude methods to apply for the development of a numerical code. The current research resorts to a widely used three-dimensional method, 3D Green Function method to develop the tool. The reason behind of choosing this method was the wide range of speed it can work in, the inherent capability of Green Function to satisfy a number of boundary condition and the fact that use of this method limits the meshing of the domain to only the wetted surface of the vessel. Previously, using the 3D Green Function method was challenging due to the lengthy computation time required; however, as computer processing power has increased exponentially, the Green Function method has become the obvious choice for ship motion analysis of multi-hull vessels. A number of vessels, both single hull and multi hull, were employed to assess the operability of the numerical tool developed through this research. A range of Wigley hulls were used to check the application on single hull vessels. For multihulls a number of catamarans, both mathematical model (Wigley Catamaran, Lewis Hull) and actual vessel (NPL catamaran, Delft 372 catamaran) were used to showcase the range of application. The analysis was run varying the demihull separation and varying the forward speed of the vessels. Additionally, trimaran vessel analyses were also run, to show that the application of the code was not confined to catamaran vessels only. The forward speed of the vessel was classified in three groups. Velocities less than Fn=0.20 was considered a low speed, for velocities 0.2<Fn<0.3 were considered a intermediate speed and velocities corresponding to Fn>0.3 was considered as high speed. Wave contours were also generated to assess the wet deck slamming phenomenon of multi hull vessels. The results show that the developed tool would be an excellent tool in providing preliminary design decisions for all types of vessels. However, there are discrepancies among the observed results and experimental results. The reason lies in the underlying theory of the method applied. As 3D Green Function method is based on potential flow theory, which assumes the fluid to be an ideal one whereas the experimental values are for real fluid. Still the numerical tool will be great addition in assessing the seakeeping characteristics of vessels.