The Influence of Hydrodynamic Assumptions on Ship Maneuvering Ghalib Taimuri, Tommi Mikkola, Jerzy Matusiak, Pentti Kujala & Spyros Hirdaris Aalto University, Mechanical Engineering (Marine Technology), Espoo,Finland ghalib.taimuri@aalto.fi 1 Introduction We present a preliminary assessment on the influence of the hydrodynamic assumptions associated with ship motions, for vessels maneuvering in calm waters. Three approaches are considered: (a) Model 1 idealising the maneuvering of a ship in 3-dof as per Brix (1993); (b) Model 2 representing the 6-dof hybrid time domain unified seakeeping / maneuvering model of Matusiak (2017); and (c) Model 3 - idealising combined seakeeping and maneuvering nonlinear characteristics in 6-dof by the time domain Green function method of McTaggart (2005). Simulations of turning circle and zig-zag manoeuvers are assessed and compared against available data for two tankers (KVLCC2 & Esso Osaka) and a container ship (DTC). It is concluded that in calm waters simplified models with hydrodynamic derivatives from either RANSE CFD or model tests can be used. However, implementation of well-validated CFD hydrodynamic coefficients may be more economic for the development of practical engineering tools, such as rapid assessment tools accounting for evasiveness in ship crashworthiness (Goerlandt et al. 2012). 2 Hydrodynamic models Over the years, maneuvering has been associated with open or restricted calm waters (i.e. in sheltered waters or in a harbor). For this reason, traditional maneuvering models assume that external actions relate with constant valued slow motion derivatives applicable at all frequencies of excitation with little or no account of ship dynamics (e.g. forward speed, heading angle, etc.). Seakeeping models assume vessel operations at a speed and heading in regular or random seas in the absence of control plane actions. They assess the influence of parasitic motions or responses of a rigid vessel to waves (e.g. Hirdaris et al., 2016). In combined seakeeping and maneuvering fluid actions that include the influence of hydrodynamic coefficients and wave environmental actions can account for operational scenarios that allow the vessel to respond in various degrees of freedom (e.g. Bailey et al., 2002 and Matusiak, 2017). From the viewpoint of operations and safety of a vessel, combining random seaway and control plane actions may be useful, especially for ships travelling with forward speed in close proximity to fixed or floating structures in severe open waters; or in shallow, restricted waters. In such cases it is possible to superimpose Froude-Krylov, diffraction, radiation and 2nd order mean forces computed from potential flow in maneuvering models that account for the influence of hull, propeller, rudder and drift forces. Hydrodynamic assumptions are from empirical formulae, CFD or model tests. Consequently, the position of the vessel during maneuvers and the associated ship speed data are transfered on seakeeping (Seo and Kim, 2011). This paper compares the following models for fine and full form vessels: • Model 1 idealises the maneuvering of a ship in calm waters by a 3-dof box-like model based on Brix (1993). The ship origin is in way of still water line and on the symmetry plane, at a distance ‘xG’(mid-ship) from the Centre of Gravity (CoG). The radiation part of the hull forces includes added mass coefficients as per Clarke et al. (1983) and Brix (1993). The mathematical equation is: [̇ − − 2 ] = + + + ℎ [̇ + + ̇ ] = + (1) ( + 2 ) . + [ ( . + )] = + where : ‘m’ is the mass of the ship [kg] and ‘IZ’ is the yaw moment of inertia; u, v and r, are surge, sway and yaw velocities respectively. Ship resistance, propulsion and rudder forces are represented with subscripts ‘ res’, ‘prop’ and ‘rud’ respectively. Subscript ‘hull’ denotes hull forces. • Model 2 presents the 6-dof hybrid time domain unified model of maneuvering in waves introduced by Matusiak (2017). The ship’s origin is on a line passing through CoG at still water plane and the vertical z-axis points downwards. The added mass and damping are calculated using strip theory as per Salvesen et al. (1970). Time dependent radiation forces are computed by convolution integral and hydrostatic forces are evaluated by a panel method. The mathematical equation is: