Seakeeping tests are performed in the HSVA large towing tank. The tank is equipped with two wave makers: At the end of the tank a double flap wave maker generates regular waves and irregular long crested seas according to any spectral shape. In the middle of the tank there is a 40 m long side wave generator consisting of 80 individually controlled flaps. Regular waves as well as long- and short-crested irregular seaways can be generated at wave headings from 20° to 160°. This enables testing of vessels also in beam and oblique seas with nonzero speeds. Combined action of both wave makers is possible to model wind waves and swell seas of different directions. Wave packets can be generated with both wave makers, allowing not only the determination of Response Amplitude Operators (RAO), but e.g. also freak waves can be embedded into sea spectra to model rare high waves.
Standard seakeeping tests are usually performed with free-running models in irregular seas at pre-set constant propeller revolutions, which mostly correspond to a given ship speed in calm water. Speed (i.e. propeller revolutions), sea states, and encounter angles can be varied in the test series. With the side wave generator model testing on a straight course in long- or short-crested seas of arbitrary directions is now possible. In addition the width of the tank allows testing of fairly large free-running models at encounter angles up to 30° from head or stern seas. These tests in quartering seas can be performed on zig-zag courses taking advantage of the long towing tank (300 m).
Tests with free running-models are performed with the purpose-built control and propulsion system for ship models SAS (Steuerungs- und Antriebssystem für Seegang). This system guides the model in seaway with its autopilot through the tank either heading- or position-controlled. With the SAS in operation the main and sub-carriages automatically follow the model in the towing tank, keeping it permanently in the range of the optical tracking system, which allows continuous measurement of the model motions and position in the tank. With the SAS deterministic testing and exact comparison of the results from different test runs has become possible.
Beside the automated control of the model position and heading the SAS-system can control up to four stabilizing fins, eight driving motors and eight rudders simultaneously. For the operation of the stabilizing fins at zero-speed on under way either an in-house control algorithm or a steering signal provided by the fin manufacturers can be used for the fin control. Thus comparison studies with different fin stabilizing systems of different manufactures can be carried out. Moreover the SAS-system is designed so that the in-house DP -system module can be connected in order to control up to eight thruster units.
In general, the following data is determined during standard seakeeping tests:
- pitch, roll, and heave motions
- accelerations at several ship positions
- relative motions
- mean attained ship speed
From the measured time records of motions and accelerations statistical values like the RMS or the significant and maximum values are determined by a statistical analysis. As well the occurrence of slamming, propeller emergence, and shipping of green water can be determined.
Self-propulsion tests in head waves are performed either with a laterally guided model or with a free-running model. During the tests the model runs against different regular waves or irregular long crested seas. Mean thrust, torque, and propeller revolutions as well as the ship motions and accelerations, if desired, are measured. In general, these tests result in a speed-power-prediction with the influence of wind forces for the full scale vessel, which e.g. gives information on the maximum achievable speed in seaways and the fuel consumption at operational speed.
For Tugs, Seismic Survey Vessels, etc. pull force tests are provided in head waves at different fixed towing speeds.
Wave added resistance tests are performed with towed models.
In head seas the model is guided below the towing carriage at a constant speed allowing free heave and pitch motions as well as limited surge motions. Beside the required towing force also motions and accelerations at chosen positions can be measured. Wave added resistance tests are usually performed in regular head waves for obtaining the RAO or in irregular long-crested sea states to get the resistance value in the given sea state.
Due to increasing interest in wave added resistance also in bow quartering seas and other directions the HSVA recently took a new towing arrangement into use, which allows the ship model as free motions in oblique seas as possible, but makes it simultaneously possible to measure the towing resistance. The ship model is towed with a vertical towing pole located in the middle of the ship. A vertical guiding pole at the bow controls the ship model direction. The ship motion components roll, pitch, and heave are completely free. The surge, sway, and yaw components are restrained with suitably soft springs allowing the cyclic motions of the model in seaway, but keeping it softly on its course and position.
For accurate measurement of wave added resistance the ship must be able to execute roll motions freely in all wave directions. For this it is important to have the roll axis of the ship model in the correct height above the baseline, also when the ship model is connected to the towing system. This is realized with two articulated force balances in the model.
The new towing arrangement allows not only the measurement of ship resistance in oblique seas, but also to study the efficiency of the propulsion in seaways of any wave directions.
Regular waves and wave package techniques (RAO)
With the Response Amplitude Operators (RAO) the linear system behaviour of a vessel in arbitrary sea state can be determined with statistical analysis. One possibility to derive the RAOs of a ship is to perform model tests in regular waves, and calculate the RAO directly from the test results. Alternatively, HSVA uses the transient wave package technique to determine the RAOs. The advantage of this technique is that a large range of wave frequencies can be realised within one single test of short duration: No influence from reflected waves has to be considered, nearly no statistical spreading of results takes place and very good resolution of the RAOs in the frequency domain is obtained.
Tests for offshore structures
Tests at zero speed for arbitrary wave encounter angles can be performed for offshore structures using a rotatable ring structure for easy and rapid change of headings. The offshore structure or vessel itself is softly moored to the ring and free to move in its 6 DoF.
Turret or spread mooring of offshore structures such as FPSO, FSO, or semi-submersible can be modelled with horizontal soft springs or with a truncated mooring system. Coupled dynamic motion of side-by-side or tandem connected multiple offshore structures can be modelled. Relative dynamic behaviour in waves as well as individual dynamic performance can be determined. Further, mooring assisted or full DP systems with tunnel or azimuth thrusters can be tested with the HSVA DP control hardware. For these tests an actual DP control system can be mounted to the HSVA control hardware in order to simulate the realistic DP system behaviour.
For tests of offshore structures, complex environments consisting of wind-seas, swell, wind, and current can be simulated simultaneously. Wind waves and swell are generated by the double-flap wave generator and side wave generator. Current and simplified wind action can be simulated by moving the model in longitudinal tank direction, and by applying a dynamic wind force at the centre of wind action in connection with the known wind coefficients.
Roll damping tests
The roll motions of ships are damped by non-linear restoring forces caused by wave reflection, various viscous effects and lift forces. Viscous damping effects are caused by friction forces acting on the ship hull and vortex separation occurring at the ship hull, bilge keels, and appendages. The roll damping strongly increases with increasing ship speed due to lift forces being generated by the appendages and the ship hull.
Most methods of theoretical seakeeping computations disregard viscous effects and as a consequence the roll damping is usually underestimated. For this reason damping in the computations is increased by using roll damping coefficients, which can be accurately determined based on model tests.
HSVA offers the following test procedures:
- Forced roll motion tests. Determination of non-dimensional roll damping coefficients by the harmonic excitation of the ship model by contra-rotating masses.
- Roll decay tests. Determination of the well-known p- and q-values.
Passive / active roll damping devices
There is a steady demand for active roll stabilising systems for zero speed, especially in the yacht and cruise industry.
The purpose of such systems is to reduce the roll motion of ships and therewith to enhance the comfort onboard. Several types of stabilising systems can be investigated at HSVA, such as:
- Active/passive roll damping tanks
- Zero Speed stabiliser fins
- Magnus rotors
The roll damping efficiency of an active roll stabilizing system is determined by comparing the roll motions of the ship without the device and with the device in passive and/or active modes of operation.
Tests for Open-Top Vessels
IMO-Resolutions require model tests for Open-Top Container Vessels in order to check the hourly amount of water ingress in the open holds against given threshold values. HSVA regularly performs such tests for Container and Multi-Purpose Vessels as well as for Heavy Lift Carriers.
Deterministic seakeeping model tests
Within several research projects HSVA has developed and established a computer controlled seakeeping model test procedure which is combined with deterministic wave generation techniques. For these tests remote-controlled models are used.
During the tests time histories of the ship position and orientation in six degrees of freedom as well as the ship’s global loads and local wave induced pressures on the ship’s hull (slamming) can be recorded.
Primarily, this model test technique is developed to study ship models in deterministic rough wave conditions in order to improve the understanding of the effects of such wave conditions on motions and structural loads on ships and to permit an accurate calibration and validation of seakeeping codes dealing with these extreme conditions. As well it is possible to investigate conditions in head and following seas leading to large roll angles or even to capsizing due to loss of stability at the wave crest, resonant excitation, parametric rolling or broaching.
Measurement of slamming forces and water impact
Large heave and pitch motions in heavy seas can cause the ship hull surface to impact with the water surface with a sufficiently high relative velocity to result in very high local pressures. The resulting slamming forces acting on the structure can cause vibrations and even damage. Measurement of the local forces or pressures helps in determining the critical loads. Force panels or arrays of pressure gauges are used.
Special problems often require non-standard test methods or measurements. For instance, following model tests and investigations are offered:
- Investigation of the survivability of a damaged ship (Stockholm Agreement / SOLAS)
- Investigation of water ingress and floodwater motion on damaged ships
- Capsizing tests
- Investigation of the water motion in swimming pools on ships
- Alternative assessment of the Weather Criterion by model tests according to MSC 1/Circular 1200
- Docking tests of landing crafts
- Launch and Recovery tests with daughter crafts (investigation of the stern ramp design)
- Investigation of wave induced loads and springing/whipping vibrations of a ship with a segmented model
- Tests on crew tenders landing on offshore wind energy towers
HSVA uses powerful computer programs for the determination of rigid body motions of vessels in a wider range of sea conditions. A linear strip theory method and a partly non-linear strip theory method are available for theoretical seakeeping investigations. Very other these methods are applied before actual model tests to find out most critical wave or speed conditions.
Linear strip theory method (STRIP)
At HSVA the linear strip theory program is used for routine calculations of ship motions. Calculations are done in the frequency domain. This program includes a special version for calculation of 6 degree of freedom motions and internal loads of twin hull ships. The calculation method accounts for the hydrodynamic interaction between the hulls. Active controlled stabilising fins and roll damping tanks can be considered. Empirical roll damping coefficients determined by model tests are included in order to significantly improve the roll motion prediction.
The calculation results in the determination of transfer functions (amplitude and phase) or different statistical values when the vessel is running in long or short crested seas. For instance, statistical values could be: Significant amplitudes of ship motions, relative motions and accelerations; mean added resistance due to the sea; probabilities of occurrence of slamming, and deck wetness and motion sickness values given as function of speed and relative course running in a certain sea state. Beside the determination of Operability Indices (OPI), long term predictions can be done, which deliver the probability of a successful operation over a longer time in a certain sea area.
Assessment of parametric rolling (ROLLSS)
The program ROLLSS is a very efficient and fast code for the simulation of parametric rolling. While the pitch, heave, sway and yaw motions are computed by a linear strip method, and the surge motion by a simple nonlinear approach, the roll motion is computed nonlinearly in time domain using the righting lever curves of static stability in waves. This approach is as simple as effective and makes it possible to investigate a wide range of combinations of wave periods and ship speeds to detect potentially dangerous zones.