Manoeuvring tests are performed in HSVA's large towing tank using the Computerized Planar Motion Carriage (CPMC).
The CPMC has two different operating modes:
- The tracking mode is the more economic and efficient one to investigate rudder manoeuvres. In this mode the CPMC automatically tracks a model manoeuvring freely under the action of rudder and propeller manoeuvres. The surge, sway, yaw, roll and pitch motions are registered with high accuracy. As the CPMC is equipped with a hydraulic catching mechanism and manual intervention therefore is not required, HSVA's models can be large for better accuracy and less scale effects. The most frequent application consists in checking if a design's manoeuvring and course keeping behaviour complies with the IMO resolution MSC.137(76). For this, zigzag tests with different rudder and switching angles are performed. Additionally to the IMO standard zigzag tests 10°/10° and 20°/20° a complete series provides input data for the system identification algorithm in order to simulate numerically all other standard manoeuvres of interest for which the towing tank is too narrow.
- In the captive mode the ship model is guided by the CPMC along predetermined horizontal planar motions at given propeller rate of revolutions and rudder angle time series. The resulting hydrodynamic force time series are measured as well as propeller thrust and torque and the rudder forces if required. The roll component can be fixed as an adjustable heeling angle. The heave and pitch components in most cases are free to move, but they can be fixed as well. This mode is useful for two kinds of investigations:
- Rudder manoeuvres according to the IMO resolution MSC.137(76) as an alternative to the tracking mode.
- Dynamic Positioning investigations, also berthing, unberthing and turning under the influence of wind loads. This includes not only oblique towing and optionally turning, but also bollard pull tests to determine the characteristics and hull interaction of active control devices like rudders and propellers, azimuthing thrusters, podded drives, Kort nozzles and VSPs.
For manoeuvring in waves, which cannot be done with the CPMC, HSVA has a system called SAS (Steuerungs- und Antriebssystem). It can track the model by optical methods or by INS and can control up to eight steerable drives and also active fins.
Numerical simulation of rudder manoeuvres
Post-processing the measured time series with a system identification algorithm results in a numerical model, which allows the numerical simulation of the manoeuvring motions of the specific ship. This can be applied for several purposes:
- Standard manoeuvres, e. g. spiral tests, zigzag tests, turning circles and Williamson turns. This mainly serves to determine if the manoeuvring and course keeping behaviour complies with the IMO resolution MSC.137(76) and the explanatory notes in MSC/Circ.1053. If not, counter-measures like modifications of the appendage and rudder configuration, immediately manufactured during the tests, can in many cases solve the problem. These IMO standards apply to all ships of over 100 m in length between the perpendiculars and also shorter chemical tankers and gas carriers. They apply to the fully loaded condition, whereas full scale sea trials usually take place on ballast draught. In view of this there is the possibility to perform model tests and system identification on both draughts: The large draught for the IMO standards, and the ballast draught for the validation by comparing the system identification results with the full-scale sea trial results.
- Application by third parties, e.g. in simulators or bridge control systems, also from full scale tests.
The numerical model can consist
- either of hydrodynamic force and moment coefficients produced from captive tests
- or of acceleration coefficients produced from tracking mode tests.
For both methods, HSVA applies its long experience to skilfully select the relevant subset of coefficients and belonging functions to ensure a realistic and numerically stable behaviour of the simulation.
Dynamic Positioning, berthing, unberthing and turning
HSVA can perform dynamic positioning capability studies according to IMCA M 140. Wind loads are estimated from a catalogue of a large number of ship silhouettes and belonging wind load coefficients. Steady wave drift loads are calculated with a panel method. Steady hull current loads and the characteristics of the active control devices and their hull interaction are determined by captive model tests. For each environmental load direction, a special algorithm finds the combination of steering angles and revolution rates that yields minimum total power consumption. Thus, also largely redundant systems with many azimuthing thrusters can be simulated automatically without arbitrary human decisions. Various types of main propulsion systems can be simulated, also Kort Nozzles and VSPs. The required thrusts and powers are presented as polar diagrams (dynamic positioning capability plots). For DP2 and DP3 requirements, the analysis is repeated for the failure of individual devices or groups of devices.
As a similar steady equilibrium consideration, HSVA can simulate crabbing in wind to feature berthing and unberthing, and also steady turning, to find out what wind conditions require which tunnel thrusters for a safe harbour operation.
For a better understanding of why the propulsion system reacts to a specific environmental load direction with a specific distribution of thrusts and steering angles, HSVA can also provide an interactive HTML5 App for your tablet or browser.
HSVA also has hard- and software to perform DP in model tests in waves, featuring wind loads by cables. HSVA's control algorithm can be replaced by the customer's computer attached to HSVA's hardware.
Innovative CFD methods
HSVA also offers various CFD methods concerning manoeuvring tasks. A very promising development consists in improving an in-house developed Navier-Stokes solver to calculate the forces and moments on the hull in each point of time during a manoeuvring simulation. Although it will take some more time for the simultaneous prediction of ship motions and the viscous free surface flow around a manoeuvring ship to become true, first steps towards this have been taken at HSVA. Results for a ship in steady turn and in oblique motion agree well with experiments and yield useful information about forces, separation zones and vortex shedding.
HSVA's manoeuvring, seakeeping and offshore department has experience with the analysis of cables, mooring lines etc. attached to floating or submerged rigid bodies. Successful past projects concerned
- Submerged towed bodies: geometry and force of the cable, optimization of the fixation point for horizontal orientation.
- Moored systems: analysis of a multi-line mooring system under the influence of wind, current and waves, optimization of the mooring scheme.
- Offshore pipelaying, e.g. structural analysis of a stinger in waves.
- Drift trajectory of a ship hull out of control in wind, waves and current in the proximity of an island with risk of grounding.