Comparative Study of Motions and Drift Forces in Waves and Current

Station keeping analysis is an important activity in the early stages of any vessel/DP project that eventually determines the machinery and thruster configuration and thruster size selection. In order to obtain reliable results, it is crucial to apply engineering tools that realistically represent the flow physics and resulting hydrodynamic forces. Present computer tools are based on the assumption that wave drift- and current forces can be superimposed. However, there are also mutual interaction effects between waves, current and hulls that should be accounted for in the evaluation of the wave drift forces. In MULDIF, a 3D diffraction/radiation panel code developed by SINTEF Ocean within the framework of a JIP, this wave-current-body interaction is taken into consideration by a new potential flow numerical model. A case study with offshore vessels and general cargo ships of different main dimensions has been performed to assess the capabilities of MULDIF for station keeping purposes in wave and current environments. The first-order vessel motions as well as mean second-order drift forces for 0 kn forward speed without current have been calculated. Through an interface to SINTEF Ocean’s vessel response code VERES, MULDIF offers the possibility to include viscous roll damping due to hull friction, flow separation at bilge keels, lift effects as well as normal forces acting on bilge keels and hull pressure created by the presence of bilge keels. This reduces roll motions to a realistic extent as shown by the comparison of RAOs from MULDIF calculations and model tests. Roll reduction tank effects can currently only be considered through the external damping matrix. Model tests for the selected vessels have been performed in SINTEF’s Ocean Basin in a soft-mooring arrangement in different irregular sea states and headings in deep water. The models were equipped with two two-component force transducers, measuring the x- and y- components of the forces. The yaw moments have been calculated from the y-force measurements. In order to measure the vessel motions in six degrees of freedom, an optoelectronic position measuring system has been used. Selected cases illustrate the significant influence of wave-current interaction on motions and drift forces.