The Seakeeper hydrodynamic and
seakeeping analysis program is able to provide fast, reliable
calculation of vessel response and seakeeping characteristics
for many types of Maxsurf designs in a variety of sea states.
Seakeeper provides designers with
the tools necessary to quickly predict the seakeeping performance
of Maxsurf designs. It takes advantage of the power of modern
desktop computers to bring seakeeping prediction and analysis
tools within the reach of all naval architects and designers.
It is now possible, in a few seconds, to read in a Maxsurf design
and calculate the vessel's seakeeping characteristics. Seakeeper
is ideally suited to the comparison of the seakeeping characteristics
of design alternatives during initial design.
As with the rest of the programs
in the Maxsurf suite, the hull geometry required for the seakeeping
analysis is read directly from the trimmed Maxsurf NURB surface
model. This eliminates the need to prepare batch or offsets files.
In addition, all functions within Seakeeper are performed using
a graphical multi-window environment consistent with the other
programs in the Maxsurf suite.
Once the Maxsurf design file has
been loaded into Seakeeper, the user specifies the vessel speeds,
wave headings and spectra to be analysed; remote locations for
the calculation of relative and absolute motions, MSI (motion
sickness incidence), MII (motion induced interruptions), etc.
may also be specified. Other analysis parameters such as the number
sections, mapping terms, water density, etc. are also under user
control. Response amplitude operators (RAOs) are computed as well
as the added resistance, significant absolute and relative motions,
velocities and accelerations of the vessel in the specified sea
spectra for the different headings and speeds. Motion, velocity
and acceleration spectra at the centre of gravity and vertical
motions at the specified remote locations on the vessel are also
Seakeeper is a 3 degree-of-freedom
(heave, roll, pitch) vessel motion prediction program. It uses
strip theory to calculate the coupled heave and pitch response
of the vessel in deep water with arbitrary wave heading. The code
was originally developed by the Australian Maritime Engineering
Co-operative Research Centre (AMECRC) at Curtin University of
Technology in Western Australia. Formation Design Systems and
AMECRC carried out a joint research program to enhance the software
and make it available on the Windows platform. Seakeeper has now
been integrated with the rest of the Maxsurf range to provide
state of the art seakeeping performance prediction.
Conformal mapping methods are
used to calculate the vertical-plane section added mass and damping
for the vessel. Strip theory is then used to calculate the global
vessel added mass, damping and cross-coupling terms. The coupled
heave and pitch equations are solved to obtain the vertical plane
RAOs. The underlying strip theory formulation closely follows
that of Salvesen Tuck and Faltinsen (1970).
The roll response is calculated
independently using a user-specified damping coefficient and fixed
added roll inertia ratio.
It has been found that reasonably
accurate predictions of heave and pitch motions for catamarans
can be achieved by modelling a single demihull. It was found (Molland
et al 1995) that the interactions between the two demihulls
decreased rapidly as speed and demihull spacing were increased.
If a catamaran is modelled in this manner, the real roll stiffness
can be calculated by specifying the demihull separation.
The sectional hydrodynamic coefficients
for heave are calculated using conformal mapping techniques. Both
standard Lewis forms (3-parameter conformal maps) and higher-order
(up to 15 terms) mappings are available. The higher order mappings
are often better able to capture the detail of the hull sections
as may be seem below: actual hull sections are shown in green
and the conformal mapped sections in red.
|3-Parameter Lewis mapping
Visualisation of vessel response
Seakeeper provides a number of
visual ways of verifying the results that have been calculated.
This can be very useful when interpreting the results especially
when presented to non-naval architects. It also provides an alternative
way to obtain a feel for the vessel's response:
The roll parameters may be verified
by simulating a roll-decrement test. This heels the vessel to
a specified angle and then animates its progressive return to
equilibrium with the angle of heel decaying at each cycle.
|Simulation of roll-decrement
||Roll angle vs time graph
Response to Regular Waves
The vessel response may be simulated
in regular waves. This generates a repeating waveform that approaches
the vessel at the encounter frequency. The animation simulates
the vessel's motions as it passes through the wave field.
|Vessel response to regular waves
Response to Irregular Waves
The vessel response may
also be simulated in one of the spectra tested. This generates
a sequence of irregular waves consistent with the current wave
spectrum. This is a reasonable simulation of what a vessel might
be expected to experience in an actual sea state. The actual time
series data behind such a simulation can also be saved to a file
for further post-processing.
|Vessel response to wave
Seakeeper has been used extensively
for both commercial and research applications. The software has
been validated against a variety of data from various independent
sources: including model tests, full scale trials and other numerical
methods. Typical comparisons with results from towing tank experiments
are shown above. These show heave and pitch RAOs for a slender,
round bilge monohull in head-seas at Fn=0.5.