M O D E L T E S T S A T M A R I N T E K W I T H D A M A G E D R O - R O V E S S E L S
Presented at WEGEMT Workshop on Damage StabiHty of Ships, Friday 20 October 1995,
at Department of Ocean Engineering,
Technical University of Denmark, Lyngby, Denmark
by
Vidar Aanesland
INTRODUCTION
The disasters of several passenger/RoRo vessels have set focus on the behaviour of damaged RoRo vessels and in particular the survivability in a harsh environment. A number of theoretical, numerical and experimental research projects have been initiated and the present work has been carried out as a part of the research program Safety of Passenger I RoRo Vessels". A set of model tests have been used partly to investigate the ship morions and capsize process as function of wave conditions and panly to vaHdate and verify a new simulation program developed by University of Strathclyde. During the experiments new model test procedures were defined and a number of questions regarding the experimental methods arose which wiU be focused upon in the following. Some comments will be given both to centrecasing vessels, sidecasing vessels as well as introducing transverse bulkheads on tiie Ro-Ro deck to limit tiie water ingress.
As new insight is obtained tiiroughout tiie present and other investigations, new approaches and various model test techniques may evolve. The present work is by no means a work witii final conclusions, but intended as a contribution to the present development of better understanding of tiie hydrodynamic mechanisms involved with damaged RoRo vessels and how to ensure tiiat new designs will meet higher standards witii respect to survivability.
SHIP M O D E L S
Two different models were used during the testing program. The first model has been tested extensively at tiie Danish Maritime Institute (DMI) on behalf of Department of Transpon tiirough ±tTX Ro-Ro Passenger Ferry Safety Studies. For a detailed description of tiie model ir is referred to tiie repons, / I / , after tiie disaster of Herald of Free Enterprise.
The second model, NOR.^, was parriculaily designed for the present project and was first tested at D M I in order to investigate water ingress on the Ro-Ro deck and in particular ± e in and out flow through tiie damage opening. Only tiie main particulars are given herein in Table 1, while details about the model can be found in tiie presentation from DMI.
Table 1. Main particulars
Main Particulars Model 1 NORA
Length 131.0 130.0
Breadth 26.0 25.5
D to main deck 7.8 8.5
Draught 6.1 5.75
Draught to main deck 12 200 tonnes 12 030 tonnes
Model scale 1:42.033 1:34.667
It is very important that the models are geometrically correct both with respect to the hull form as well as with respect to the flooded compartments and Ro-Ro deck inside the model. The morion characterisrics of the ship may change drasrically within seconds and hence it is very important that riiis change is due to the correct flooding. In particular, the arrangement on the Ro-Ro deck has to be modelled with die main dimensions well represented due to the shipment of water from side to side or from bow area to stem area. The wall thickness in the ship side should be as low as possible which is one of the reasons for using thin metal plates from the Ro-Ro deck and upwards. In order to investigate the water motion and be able to make good video recordings, much of tiie interior is made of transparent plastic.
Botii models have a midship damage condition where a 100 % SOLAS damage is covering botii die Ro-Ro deck and the compartment below. Hence, the damage freeboard is changed by die size of tiie damage length of tiie compartment below tiie water line. In tiie model tests tiie change of damage length is changed by placing pieces of plastic foam in the compartment
A weight elevator is installed on the models which enables the centre of gravity, KG, to be changed rapidly by bringing weight elements up and down in the centre plane of the model. However, tiiis means tiiat the radius of gyration is different for the various KG values.
I N S T R U M E N T A T I O N
The following channels have been measured:
- Wave elevarion in 2 reference points (1 fi.xed in the basin and 1 on tiie carriage) - Relative motion in 3 points (2 on damage side and 1 on intact side)
- Vertical and lateral acceleration in 1 point on intact side at position of relative motion probe - Vessel motion in 6 DOF
The OPTOPOS optical measuring system consists of 3 light emitting diodes mounted on the model. For tiie present model a modification of the standard system had to be made due to tiie necessity of heaving and lowering the model. Usually tiiree masts are installed on tiie model, one in tiie bow region, one midship and one in the stem region. The wires and cables must not come between tiie diodes and the cameras on shore. A special rig had to be designed where all three diodes were mounted and tiie rig itself was placed in tiie stem region of the model.
In addition 2 video cameras were instaUed on the model showing the water motion on the Ro-Ro deck, and during the tests an additional camera was recording from tiie side of tiie basin.
T E S T F A C I L I T Y
The experiments were carried out in tiie Ocean Basin at MARINTEK, Trondheim, witii dimensions 50 times 80 meters and a variable water deptii of 0 - 10 meters. A double flap wave generator is mounted on one of tiie shon sides in tiie basin, while a multi-flap wave generator system is mounted on one of tiie long sides. The waves are generated from eitiier one of the two wave generators or both simultaneously. Effectively, any wave direction on die ship may be generated. In order to obtain die longest time recordings, it is preferable to run die double flap wave maker which is also preferable when long-crested waves are to be used.
The Ocean Basin is equipped witii a carriage system which foUows the drifting model and it can easily be rescued when capsizing. The carriage also supports die cables from die measuring system and video cameras. For the main pan of tiie model tests, the model is freely drifting, while in a number of supplementary tests the model is supponed by a soft spring system. The purpose is to have a heading control system and enable the model to have a forward speed. The soft spring system was supported on die carriage which still was following the mean position of tiie model.
T E S T SET-UP AND T E S T P R O G R A M
The main objective has been to investigate die behaviour of die damaged models as function of significant wave height for given combinations of KG values and damaged freeboard. For each selected configuration, tiie wave height has been increased or decreased according to die previous tests in such a manner tiiat tiie limit between capsize and no-capsize is found. The no-capsize condition for die present tests are defined as surviving die given sea condition as Ions as die physical limitations of the Ocean Basin permits the model to drift. The drifting time is depend^ant on tiie wave heights due to die dependency on the mean wave drift forces. Higher wave heights will create Mgher drift forces and hence a higher drifting speed.
Sea condirinny^
Sea conditions have been calibrated for a Hs-range which covers tiie heights of interest, keeping tiie wave steepness constant, Hs / Ap = 0.04, where Hs is the significant wave height and Ap is die wave lengdi defined from die peak period of the spectnim. A JONSWAP spectnim has been used witii a Y - factor of 3.0 and wave height range is from 1 to 8 meters.
Three wave probes were mounted along the length of the basin during the wave calibration positioned at tiie centre of tiie tank and +/- 20 m towards and away from tiie wave generator. One additional reference wave probe, fixed at die basin side, was present during the complete test program.
Test program
The tests were performed pardy as a freely drifting model for the main test matrix and as tests with the model suspended in soft horizontal springs for the supplementary matrix.
Model test matri?^
Somewhat different condirions were specified for the two models because the various aspects of the capsize process were to be investigated. The parameters of interest were different combinations of KG values and damage lengths. From the possible combinations a set was selected partiy based on calculations performed prior to the experiments and partiy based on the results obtained as the tests went on. For each selected condition a set of wave heights were run selected from already calibrated waves described earlier.
In addition a nim±)er of model tests were carried out for a mean wave heading of 30 and 60 de^ees relative to the incoming wave system, and in combination with forward ship speed.
Test procedure
The procedure adopted for the tests was as follows:
1) Prepare the model for a new test 2) Start tiie video recordings
3) Start the wave generator and clock 4) Wait for 1 minute
5) Lower the model into tiie waves at a defined position in the basin 6) Release die cover
7) Follow tiie model witii tiie gondola with die heaving wire slack
8) Continue until the model is capsizing and has to be rescued or the model is reaching the beach
The initial conditions when starting the experiments have caused some concern when defining die test procedure. In any experiment it is important to have well defmed starting conditions so tiiat tiie tests are as repeatable as possible. In other words that two tests under die same conditions will have the same outcome. Any experiment will have to face tiiis problem but tiie difficulty in obtaining "equal" starting condirions differ greatiy depending on the type of model test. For experiments witii intact models the model test procedures have been developed over decades and is normally reasonably easy to define. The problems grow with the complexity of damaged models where water is supposed to enter and interact dynamically with tiie hull.
As seen from the above list, the same procedure has been used throughout the test program. Hence die ship will meet tiie same realisation of the sea in all the cases with a specified wave height.
Preparation of the model for a new test
After each test run the model is hoisted out of water by die heaving wires by which die model is connected to die carriage. The model is mounted in a crane system which consists of a tackle, a weighing-machine and a yoke by which it is possible to very fast check whedier any water has been entering die model and to return to starring point of a new test. I f any water has entered die model, it is removed by a vacuum cleaner system. Furdier, die model is checked visually and die cover to the damage is mounted.
Starting the video recordings, wave generator and the clock
The video cameras which are mounted on the model and die fixed camera onshore are staned by a signal from die control room. Hence die tape recorders are running when die wave generator and die clock are staned. The complete test sequence is dien available on tape.
Lowering the model into the water
The model is hanging above die water surface for about one minute in order for die sea condirion to develop. As die shoner waves travel more slowly than longer waves, it is common to wait for a certain time instance before starting the tests. In the present case the tests are staned as close as possible to die wave generators in order to get a long drifting distance and one minute is sufficienL
When die minute has gone, die model is lowered into the waves and after about 10 seconds die cover hiding die damage is removed. The model stans to take in water and sink to tiie level defined by die damage length.
Controlling the model from the carriage
The model is freely drifting and taking the heading and drifting speed as given by the waves. By tiiis metiiod a full blackout is simulated and the model is completely free in 6 degrees of freedom. The hoisting wires will be slack during die drifting period but available as soon as die model stans to capsize. It is also possible to control die behaviour of die model from tiie online measuring system and give die staff on tiie carriage a signal when the model is about to capsize.
C O M M E N T S AND CONCLUSIONS
As experiments have been carried out with a model witii centrecasing and a model witii sidecasings it is interesting to note a significant improvement on die survivability for tiie latter configuration. It is then assumed tiiat the sidecasings have watertight subdivision. This is a consequencé of additional buoyancy placed at the ship sides as well as a reduction in flooded area on the Ro-Ro deck. The reduction in stability due to free surface effect is lower.
In the centrecasing condirion, die vessel was all the rime capsiring towards die waves. Even tiiough the Ro-Ro deck was open from side to side in the bow and stem region, tiie initial heel was towards
the damage and the water ingress only increased the heeling. In the sidecasing condition the niodel capsized to both sides depending on the initial heel and die specific wave trains that followed. In most cases the model capsized towards the side at which the model got ± e initial heeling. In some cases, however, the model started heeling towards the waves and suddenly got hit by some large waves which made the water flow to the opposite side of the deck. In such a case the model was not observed coming back towards die damage.
By introducing transverse bulkheads the survivability is significantly increased in botii the centrecasing and sidecasing design. The number and height of die transverse bulkheads needed for a given vessel will depend on the initial survivability without die transverse bulkheads. For lower significant wave heights half height bulkheads may be sufificient. In addition, if it is possible to lower and fasten ramps on top of these transverse bulkheads, it will further improve on the survivability. The violent water motion on the Ro-Ro deck, which can be compared to the sloshing in open water tanks, will be suppressed. At a certain significant wave height, however, the water morion inside the vessel will be so violent that only fiiU height nransverse bulkheads will be good enough. It should also be noted that in order to obtain a good effect of die ramps, any spacing between the ramps and the casings should be closed adjacent to the transvene bulkheads.
In the case of a centre casing design, it only needs ramps covering the half ship breadth provided diat die ramps are placed adjacent to the centre casing. Ramps at the ship sides do not seem to have any significant effect at all. In the sidecasing design, die ramps should be placed adjacent to the sidecasings. The positive damping effect due to die ramps can be compared to having tank tops in free surface tanks and water may not spill easily over die bulkheads.
Large openings in the centrecasing do not seem to have large negative effects on the survivability of the vessel In some cases the openings secure a shipping of water from tiie damaged side of the ship to the intact side and makes die vessel more stable.
Damage about midship seems to be the worst case when it comes to water ingress and danger of capsize. In tiiis case the vessel may stay on more or less even keel and the water may flow botii forwards or afiwards and the outflow through the damage is much less than die inflow.
A fi-eeiy drifting ship tends to tum the side to the waves. The model tests show that by changing the ship heading so that the waves are hitting die vessel with an angle of 60 degrees or less, relarive to die ship centre line, the vessel is able to take significantiy higher waves. I n practical terms it means that it is important to change the vessel heading in order to avoid beam sea waves. A ship with forward speed into the waves, however, can be even more dangerous that a freely drifting ship.
The damaged freeboard is one of the major parameters defining the survivability of a vessel. Hence, a freeboard of 0.5 m wiU be much better than 0.1 m. The wave heights which die vessel can sustain is pardy due to die freeboard itself because a smaller ponion of the waves will be able to reach the Ro-Ro deck, and partiy due to die fact that more water may be able to drain through the damage between the waves giving water ingress.
REFERENCES
OPEN DECK NORA D E C K A R E A = 3000m^ W A T E R ON V E H I C L E D E C K 5QC€Q -2000 S e a Slate : 4.5m TIME (sec) W A T E R ON V E H I C L E D E C K 8000 T I M E (sec)
K G : 9.0m Freeboard : 1.5m S e a State : 4.25m K G : 9.5m Freeboard : 1.5m
W A T E R ON V E H I C L E D E C K 6000 a 2000 5 1000 5 0 -JOOO S e a State : 4.75m TIME (sec) kGTÏÖ.5m~ W A T E R ON V E H I C L E D E C K TIME (sec)
Freeboard : 1.5m Sea State : 3.5m K G : 11.5m Freeboard : 1.5m
W A T E R ON V E H I C L E D E C K 3500 3000 2500 2000 1500 1000 500 0 -500 200 566- eóe-TIME (sec) S e a state : 3.0m W A T E R ON V E H I C L E D E C K 2500 2000 Ci c 1500 I JOOO
i
500 0 •500?5n
2ÈIH. TIME (sec)K G : 12.5m Freeboard : 1.5m S e a State : 2.75m K G : 13.0m Freeboard : 1.5m
