Overview
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 computed.
Theoretical basis
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 | 15-Parameter conformal 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:
Roll damping
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 test |
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 spectrum |
Validation
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.
