OCTOPUS Office 6 Basic

OCTOPUS Office 6 Basic contains the following features:
  • Creation of 2D and 3D hull models
  • Creation of 2D and 3D hydrodynamic databases
  • Creation of RAO's based on a 2D/3D hydrodynamic database and loading condition
  • Creation of loading conditions

Analysis sequence to calculate RAO's.

2D/3D Hull Modeller


2D Hull Modeller.

3D Hull Modeller.

OCTOPUS Office 6 contains two different tools to model ship hulls. Both methods are needed to create 2D hydrodynamic databases. In OCTOPUS Office 6 a set of standard hulls is available. It is also possible to define/shape other ship hulls. The 2D hull modeller is used to model 2D-hulls (seaway hull format). These hull models can also be used in the 3D hull modeller for meshing purposes. After defining a 2D and a 3D model it is possible to create a 2D hydrodynamic database.

The Hydrodynamic Database

In OCTOPUS Office 6, a hydrodynamic analysis starts with the calculation of a hydrodynamic database. This database does not depend on the loading condition.

The hydrodynamic database contains:
  • A definition of the geometry (3D).
  • Radiation pressure distributions for the six modes of motion.
  • Diffraction pressure distributions for all wave headings.
A 2D hydrodynamic database can be calculated using Octopus Office. Use is made of the 2d strip theory. The strip theory solves the three-dimensional problem of the hydro mechanical and exciting wave forces and moments on the ship by integrating the two-dimensional potential solutions over the ship's length. Interactions between the cross sections are ignored for the zero-speed case. So, each cross section of the ship is considered to be part of an infinitely long cylinder. The strip theory is a slender body theory, so one should expect less accurate predictions for ships with low length to breadth ratios. However, experiments showed that the strip theory appears to be remarkably effective for predicting the motions of ships with length to breadth ratios down to about 3.0, or even sometimes lower. For more complex structures Amarcon recommends a 3D hydrodynamic database.

Force RAO.
A 3D hydrodynamic database can be calculated using any 3rd-party 3D radiation/diffraction program. For ships with forward speed, DNV's 3D-radiation/diffraction program WASIM is used. Since WASIM is a time domain program, it would be necessary to model an auto-pilot to simulate a course-stable ship in waves and to solve the motion RAOs directly. For our purpose it is not necessary to model an auto-pilot because WASIM is only used to solve the radiation and diffraction problem in the time domain. The WASIM-results of these particular simulations are transformed to the frequency domain by Fourier techniques. After that the pressure RAOs are converted to a binary hydrodynamic database (bhdb). The final step is a reduction of the database to a compiled hydrodynamic database (chdb) by section-wise integration of the pressures stored on the bhdb-file. This results in longitudinal distributions of added mass, damping and excitation forces which can successively be used to rapidly evaluate any intermediate draft and trim without loss of accuracy.
Calculation of bhdb-file is not a part of OCTOPUS Office 6. Amarcon is able to deliver a 3D binary hydrodynamic database for ships and offshore structures (see consultancy).

Response Amplitude Operators (RAO's)

Having the hydrodynamic database available for a series of drafts, the hydrodynamic coefficients and wave excitation forces can now be computed for a particular loading condition. The following steps are carried out:


Motion RAO.
  • Calculation of the global mass parameters (total mass, CoG, radii of gyration, free surface moment). These parameters may be derived from the stability program (during design), or directly from a loading computer (during operation).
  • Calculation of the equilibrium position by solving the draft aft and forward using the mass parameters in combination with the 3D geometry description stored in the database.
  • Calculation of the added mass, damping and wave forces for the actual trim and draft, in which special care is taken for trimmed cases with respect to rotations and transformations.
Loading conditions can be created by using OCTOPUS Office 6 or imported from 3rd-party loading computers.

Viscous Roll Damping

Potential flow models need to be extended with viscous damping effects, otherwise roll motion will be over-estimated. A popular method is Ikeda's roll damping method, which includes the following non-potential damping contributions:
  • Frictional roll damping,
  • Eddy making roll damping,
  • Lift roll damping coefficient, and
  • Bilge keel roll damping.
Since the viscous roll damping coefficient itself is a function of the roll amplitude and frequency, it results in a roll transfer function which is nonlinear in the wave height. This implies that the linearized roll transfer function varies per sea state. To account for the nonlinear viscous damping behavior, the sea state dependent roll RAOs are solved by applying the principle of stochastic linearization.