Assessment of Seakeeping Performance and Wave-Induced Loads

MARINTEK operates state-of-the-art test facilities for measuring and evaluating a vessel’s performance with respect to seakeeping, wave-loads and structural integrity. We also develop and continuously improve our own computer tools for the same purposes. For cost-effective performance evaluation we combine physical testing with efficient, advanced, computer models to accurately assess every relevant response under any foreseeable conditions. This is done by using the computer models to identify conditions in which model tests are most useful. Test results are then used to validate and, if necessary, calibrate the computer models. Finally, the computer models are used to assess the vessel’s performance under all possible conditions.

A segmented model of a 280 m 40 knot slender pentamaran container ship in the Ocean Basin. 

Model testing

The physical model is typically built on a scale between 1:20 and 1:50. It can be used to measure both seakeeping performance as well as local and global wave-induced loads. The model may consist of three to six segments with flexible connections.

If the stiffness and mass distribution of the hull girder is available, we ensure that the natural periods of hull girder vibration are close to the full-scale values, which enables us to capture dynamic hull girder response (springing and whipping) more accurately. At the connections, bending moments, torsional moment and shear forces are measured.

A numerical model of the vessel is established using our VERES (VEssel RESponse) computer program. VERES has a computational module which is particularly intended for high-speed vessels. This module uses the so-called 2½ D method to find the velocity potential around the ship. For catamarans, hydrodynamic interaction between the hulls can be properly included. VERES also has a 2D module for analysis of conventional ships. In all modules rudders and motion-damping devices can be properly modelled, and autopilots can also be included, enabling the effects of autopilot settings to be assessed.

Seakeeping
A linear frequency-domain version of the 2½ D method is used for seakeeping calculations. This enables us to efficiently calculate seakeeping performance statistics for all relevant vessel speeds and wave conditions. Selected seakeeping criteria are entered into the postprocessor.

For operation in a specified area or on a specified route, the results can be combined with wave climate statistics (scatter diagrams) to produce data on the regularity of the vessel. This can be presented as the fraction (percentage) of the time when the vessel is fully operable (i.e. no criterion is violated).

Global hull girder loads
For assessment of hull girder loads, we recommend the use of nonlinear time-domain calculations in addition to the frequency-domain results. While the linear frequency-domain calculations enable us to cover all relevant conditions and identify the most critical ones, the nonlinear calculations provide more accurate results for the selected critical conditions. Nonlinear effects in the calculations include nonlinear hydrostatics and Froude-Krylov forces as well as slamming loads. In addition, the hull girder flexibility can be accounted for so that springing, and in particular whipping, responses are properly calculated.

Typical results from the calculations are design values for bending moments, shear forces and torsional moments at various positions along the hull girder. If data are available on the dimensions, materials and structural details of the cross-sections, we can estimate the fatigue damage per year, or the expected fatigue life of structural members for a given vessel operational profile.

Local slamming loads
The measured local slamming loads can also be supplemented by numerical calculations. This enables us to investigate slamming loads in other conditions, and the method calculates the temporal and spatial distribution of slamming pressure during extreme slamming scenarios. The calculated pressure can automatically be mapped onto a finite-element model of the structure for stress analysis and structural integrity assessment. The local slamming load calculations are performed using our Slam2D program.

Automatic control

MARINTEK has developed a real-time control system for our laboratories based on the latest technology in rapid control prototyping. The control software has been developed in a graphical environment and is compiled and automatically downloaded into the ship models. The following controllers/functions have been developed:

• Autopilots (course- and track-keeping)
• Ride control
• Motion damping
• Station-keeping and way-point tracking control
• Joystick control

The control software is first tested in a simulation environment, in which numerical ship models and sensor models are used to verify the control software. Actual hardware can subsequently be used in the simulator to create the correct user interface, and finally the control software is either downloaded to a model ship or into a full-scale application.

Visualization of results

MARINTEK has developed software that exports results from our computer programs and physical tests to state-of-the-art graphical visualization programs. This is very useful as a means of verifying the results of calculations and to demonstrate vessel behaviour to our clients, who also appreciate the possibility of presenting impressive computer visualizations to their own customers or sponsors.

Visualisation of vessel motions.

Contacts at MARINTEK: Ole Hermundstad / Dariusz Fathi