The motion of ocean-going ship with constant forward speed is considered in time domain. The double-body linearization is adopted for ship motion calculations. The finite element method (FEM) based on variational principle is applied to solve the linearized boundary value problem. Time integration of free-surface boundary condition is carried out by 4th order Adams-Bashforth-Moultn method. In order to remove reflected waves at the truncated boundary, numerical damping zone is utilized. At every few steps, five-point Chebyshev filtering scheme is applied to smooth out unwanted saw-tooth waves. To include body nonlinear effect, nonlinear restoring and Froude-Kylov forces are additionaly calculated by considering incident wave elevation and relative ship position. For the validation of developed code, responses of three different ship models, wigley III, series60 and s175 container, are compared with existing experimental results or other computations. Nonlinear effects on motion RAO and structural load are also observed in detail.
There are various levels of ship motion prediction methods from 2D strip method to fully nonlinear time domain method. Among them, 2D strip method and 3D panel method using the wave Green function in frequency domain are widely used by industry for practical purpose. The methods produce quite good prediction for seakeeping performance of a single hull when Froude number is moderate and wave steepness is low. However, in order to assess more accurate ship motion and wave-load in harsh environmental conditions, nonlinear time domain simulation is required. For unconventional ships such as FPSO, drillship, FSRU, ULCC, which experience various kinds of nonlinear loads, it is necessary to apply nonlinear time domain tools. The 3D time-domain methods can be categorized in two groups. One is the time-domain method using the transient Green function and convolution integral. Lapis and Beck (1985) solve forced motion of a ship with constant forward speed using panel method and impulse response function.
