Damage stability tests of model representing Ro-Ro ferries performed at Danish Maritime Institute

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DAMAGE STABILITY TESTS OF MODELS REPRESENTING RO-RC) FERRIES

PERFORMED AT DMI

MICHAEL SCHINDLER, M.Sc. Danish Maritime Institute DK-2800 Lyngby, Denmark

ABSTRACT

This paper focuses on DMI's seven years experience in the field of model testing of damaged Ro-Ro ferry models exposed to rough seas. It outlines some of the design principles applied

during construction of DMI models and finally it summarizes some of the _{most important}

trends in model behaviour.

1. INTRODUCTION

Model testing of models representing damaged ships exposed to rough seas has become a very important tool for investigation of problems in the field of damage stability. The high degree

of complexity caused by strong non-linear dynamic effects related to wave motions, the

response of the damaged ship, and water ingress, makes these problems diffidult to simulate

by meats of mathematical modelling.

The same dynamic effects mean that the physical model testing makes heavy demands on the model construction and the test techniques. DMI has developed criteria for both of these.

DMI has a leading position with regard to danaage stability problems, drawing on more than 25 years of experience. In recent years, this position has been considerably strengthened due to the fact, that DMI has taken a very important part in damage stability projects initiated by the Department of Transport (UK) and a Nordic cooperation on the Safety of Passenger / Ro-Ro Vessels, later extended by a special test series for Nordic Shipowners.

-The models 'specially designed for these investigations showed a very high degree of

reliability, each spending several hundreds of hours in water. Our test technique, rigorously followed during every single test Series, produced reliable results with excellent repeatability.

2. TASK SPECIFICATIONS

DTP (UK) Investigation

The extensive model tests performed at DMI were divided into two phases. The main

objective of Phase I was to evaluate the importance of the four parameters: KG

Sea State

Damaged Freeboard Wave Orientation

in respect to the model's ability to survive a midship damage. The results were published in

/1/ and ftirther discussed in /2/.

The main objective of Phase II of this investigation was to examine the value of various

realistic suggestions for improving the damage stability of a Ro-Ro ferry in rough seas.

The devices systematically examined were mainly of the nature of reserve buoyancy or

watertight bulkheads.

The effects of internal devices such as:

Side Casings at 8/5 from the ship's side Side Casings at 2 m from the ship's side

Centre Casing

Transverse Full-Height Bulkheads on Ro-Ro Deck Transverse Half-Height Bulkheads on Ro-Ro Deck were examined for mid and foreship damage cases. The effects of external devices, such as:

Sponsons

Flare

Buoyancy Air Bags

were examined in case of midship damage, only.

In addition, and among other subjects, the effects of wind forces, size of the damage opening, transient moments, simulated permeability below and above the Ro-Ro deck and under-deck

space ventilation were examined. The results were published in /3/ and further discussed in /4/.

The Nordic Investigation

The main purpose of the investigation carried out by DMI (Task 2.2) is, by experiments with

a model of a damaged ferry exposed to rough seas, to provide data which will be used for validation and calibration of theoretical models of water ingress on the Ro-Ro deck. These

theoretical models are covered by Task 5.

The test matrix includes the following subdivision on the Ro-Ro deck: Open Deck, i.e. no subdivison on the Ro-Ro deck

Centre Casing

Side Casings at 3.5 m from the ship's side

all examined with and without full-height transverse bulkheads.

The parameters examined are essentially the same as during the Phase I of the DTp

Investigation, but extended by: Size of the Damage Opening.

Test Series for Nordic Shipowners

The objective of this study is by model tests to investigate if the reduced height _transverse

bulkheads on Ro-Ro deck would provide sufficient protection of a Ro-Ro ferry against capsize

in rough sea. Two different designs of these bulkheads are examined.

The tests are performed as survivability tests for fixed KG-values typical for existing ferries

of the same size as the model represents. The two parameters

Sea State

Damaged Freeboard

were examined in respect to the ability to survive a midship damage with the model equipped

with:

Centre Casing Side Casings

Both models were, from the beginning, dedicated to research programmes, which in some aspects means more demanding requirements for design and construction. Compared with typical "commercial" investigations, the number and range of parameters investigated in

research programmes are normally considerably higher. Related to the models, these are: Length of Damaged Compartments.

Damaged Freeboards

Positions of Damaged Zones Size of Damage Openings KG-Range

Various Arrangements on Ro-Ro Deck.

At the same time, all other demands like model weight, stiffness, and accuracy must not be reduced.

3. THE MODELS

The model used in the DTp investigation represents the lines of the existing British Ro-Ro Ferry "St. Nicolas".

"Nora", the model used in the Nordic Investigation incorporates

considerably higher freeboard than is typical for existing ferries and the lines correspond to the slightly transformed lines of a ferry now under construction for a Danishowner.

Main Dimensions

Not only are the models open from above, leaving the whole Ro-Ro deck area visible, but "St. Nicolas" _"Nora" Dimensions Full-Scale Model Full-Scale Model Scale -, 42.033 - 34.667 Length, Lpp

131.0 m

130.0 m

26.0 m

619 mm

25.5 m

_{6.1 m}

_{145 mm}

Depth to bulkhead deck

7.8 m

186 mm

241 mm

Height to top of superstructure

18.8 m

447 mm

17.5 m

Height to double bottom

1.6 m

38 mm

1.5 m

43 mm

DMI has put great efforts in making the majority of the space below the Ro-Ro deck visible

as well. This is important for inspection of this space in the event of leaks.

Both model hulls are primarily made of GRP with the model sides of aluminium plates, while the subdivision above and below the Ro-Ro deck is made of transparent material.

For improving bending strength, stainless steel wires are cast into the GRP part of the hull.

In connection with the weight lift arrangement above the Ro-Ro deck (for varying KG), the models have solid longitudinal strength elements linked with the model sides by aluminium angle bars.

Upon completion of the construction work, check measurements of the models were carried

out.

4: TEST PROCEDURE

The tests were carried out in DMI's 240 m long towing tankç starting every time from a

position 20 m from the wavemaker. At this position, the damaged model was placed in calm

water. _

After the wavemaker was activated, the models were allowed to drift freely beam onto the oncoming waves, and data collection was started. All data collection lasted at least 60

minutes in ship scale, unless the models capsized. Safety lines were used to rescue the models in case of capsize. These light and flexible lines were slack during the measurements and did not effect the behaviour of the model. In this connection it is important, that all power supply

and data communication with the computers on board was through only few cables in the

umbilical cord attached to the models close to their natural roll centres.

Before succesive runs, the models were drained for water on the Ro-Ro deck and inspected

for water in dry compartments below the Ro-Ro deck. They were then again replaced at the test position, close to the wavemaker, and the tests were resumed.

5. MODEL BEHAVIOUR

Although the determination of exact survival / capsize points by means of survival / capsize

GM was not the objective of the Nordic Investigation, the observations Made during this investigation confirmed the main conclusions of the DTp Investigation. Furthermore, the results of the tests for the Nordic Shipowners are also in full agreement with the conclusions

readied in DTp Investigation.

The most important trends in terms of survival / capsize GM as observed during testing of

both models are stated below: Damaged Freeboards

survivability.

Miciship / Forward Damage

Assuming the same damaged freeboard the survivability of the forward damage is much better than it is for the miciship damage. Differences in lost buoyancy are not the only explanations.

A more important factor is the forward slope of the Ro-Ro deck. After the water floods the

Ro-Ro deck, the further increasing trim limits the amount of water on the Ro-Ro deck

improving safety.

Size of Damage Opening

The DTp Investigation concluded, that a smaller damage opening would provide less safety against capsize than would the standard Solas-74 size opening, although in the cases where

capsize occured, the rate of buoyancy / stability loss was considerably slower.

observations were confirmed during the Nordic Investigation. Ironically, the Nordic

Investigation indicates, that extension of the damage opening considerably above the Solas-74

size will also worsen the survivability of the model.

This is clearly an area for further

research.

Centre Casing / Side Casings / Open Deck

In general, the ability of the models equipped with centre casing to survive high waves is

much lower when compared to the models with side casings. There are two main reasons for this. The first is that the centre casing hinders the ingressing water in crossing over the centre line, thus depressing the damaged side. The second reason is, that the undamaged parts of the side casings represent considerably buoyancy.

When compared with the side casings, and at least with respect to a midship damage, the

results achieved during the DTp Investigation with the open deck, i.e. no buoyancy at all, were

even better.

The trends observed during the Nordic Investigation do not confirm this

observation, but they still strongly indicate the superiority of an open-deck when compared with the centre casing.

Transverse Bulkheads on Ro-Ro Deck

The full height transverse bulkheads on the Ro-Ro deck proved their effectiveness in

protecting against capsize. For the midship damage and centre casing combination, both models were tested in 5 m waves at GM-values as low as 2.2 in and never capsized. With side casings, "Nora" was tested down to the same GM-values and did not capsize either. In

terms of GM required to prevent capsize, "St. Nicolas" showed even better results.

In combination with the centre casing, transverse bulkheads of half height, when unprotected from above, provide only limited protection against capsize. Hanging car decks, although non-watertight, will provide some protection from water passing over the half-height bulkheads due to a damping of the sloshing effects. This effect on capsize has not been investigated and is an important subject for further research.

Equipped with side casings and bulkheads of a modified design, but same height, the tested model was much better protected against capsize.

100G_1(1_11

6. RECOMMENDATION

The observations of model behaviour made during the investigations described in this paper are of a general nature only. Any problem related to a specific Ro-Ro ferry in a specific

condition should be examined by model testing individually dedicated to the particular vessel.

CONCLUSION

DMI's understanding of the mechanism of capsize in rough seas

knowledge of the ability of various proposed external and internal devices for improving the chances of survival for a damaged ferry.

REFERENCES

,1/ _{"DMI 88116 - Ro-Ro Passenger Ferry, Safety Studies, Model Tests for F10 - Final}

Report of Phase 1", DMI 1990.

/2/ Paper No. 7: "Ro-Ro Passenger Ferries Safety Studies - Model Test of Typical Ferry", by K.F. Pucill and S. Velschou, Danish Maritime Institute, International Symposium on

the Safety of Ro-Ro Passenger Ships, The Royal Institution of Naval Architects and the UK Department of Transport, London 26 & 27 April, 1990.

/3/ "DMI 91153 - Ro-Ro Passenger Ferry Safety Studies, Phase 2, Model Test for F10 -Model 2 - Final Report", DMI October 1993.

/4/ Paper No. 5 "Ro-Ro Passenger Ferry Damage Stability Study - A Continuation of Model Tests" by M. Schindler and S. Velschou, Danish Maritime Institute, Symposium on

Ro-Ro Ship's Survivability, The Ro-Royal Institution of Naval Architects, London 25