Damage Stability of Ro-Ro Catamarans

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Oceanic Engineering International, VoL 2, No. 2, 1998, pp. 65-70

Accumulation of Water on

Deck

Martin Renilsonl and Trevor Manwarringl

Australian Maritime College, P.O. Box 986, Launceston, Tasmania, Australia, 7250

e-mail: MRenilson@mte.amc.edu.au T.Manwarring@mte.amc.edu.au

ABSTRACT

As a result of a number of capsizes of conventional Ro-Ro vessels, involving considerable loss of life, new requirements have been introduced which require the stability of all Ro-Ito vessels to be assessed with an assumed level of water on the Ro-Ro deck. The level of water used for this assessment is a function of residual freeboard and sea state in the area of operation, and is based on empirical data for monohull vessels.

Experiments have been conducted using simplified catamaran models to investigate the level of water that will accumulate on the Ro-Ro deck of a catamaran as a function of residual freeboard and sea state for two different demi-hull beam to overall beam ratios. The results from these experiments show that that the level of water accumulated on the Ro-Ro deck is much greater for both catamaran hull separations than for the monohull assumed in the new regulations.

As a result, a set of interim guidelines for determining the level of water to be assumed when assessing the damage stability of a Ro-Ro catamaran is proposed.

1. INTRODUCTION

Two recent major Ro-Ro passenger ship tragedies have highlighted the need for increased levels of Ro-Ro vessel

damage stability: the MV Herald ofFree Enterprise (6 March 1987) and the MV Estonia (28 September 1994). These are the most well known incidents, however there have been a total of

44 Ro-Ro vessel capsizes worldwide in the last 15 years

[Hutchinson et al. 1996].

Numerous studies have found that these vessels capsized due to accumulation of water on the Ro-Ro deck leading to rapid capsize even in calm water. As a result, regulations governing the stability of Ro-Ro vessels have been swiftly

introduced [Allen 1997]. These require that a vessel be safe with an assumed level of water on the Ro-Ro deck. The level

of water to be used was obtained from experience with

monohull Ro-Ro vessels and is a function of the sea state in which the vessel is operating and the residual freeboard after damage. No special criteria for the damage stability of Ro-Ro catamarans were provided.

In

order to

investigate the

level of water that

will accumulate on the Ro-Ro deck of a damaged catamaran, an experimental study was undertaken using two simplified

models with different hull separations.

Delft University of Technology

Ship Hydromachanics

Laboratory

Library

Mekelweg 2 - 2628 CD Delft The Netherlands

Phone: 31 15 786373 - Fax: 31 15 781836

All dimensions in metres Drawing not to scale

Figure 1. Body plans (full scale).

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Pinlcster, J. A., Incecilc, A., Collins, J. I., Fylling, I. J., Ikegami,

K., Maeda, H., Romeling, J., U., Sevastiani, L., Vassilev, P. 1993 The Ocean Engineering Conunittee, Final Report and

Recommendations, 20th International Towing Tank Conference, San Francisco.

Soylemez, M. 1996 A general method for calculating hydrodynamic forces. Ocean Engineering. 23(5) 423-445.

Soylemez, M. 1990 Motion response simulation of damaged

floating platforms. Doctoral. Thesis. University of Glasgow, Department of Naval Architecture and Ocean Engineering.

Yilmaz, 0. & Incecik, A. 1995 Dynamic response of moored semi-submersible platforms to non-collinear wave, wind,

and current loading. Proceedings of 5th International

Offshore and Polar Engineering Conference (ISOPE). The

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2. MODEL EXPERIMENTS

Model tests were conducted in the Ship Hydrodynamics Centre at the Australian Maritime College. Two simplified

prismatic catamaran models 1.65m long, representing typical

75m vessels with different demi-hull beam to overall beam

ratios, were tested unconstrained in irregular beam seas.

Each demi-hull comprised a

semi-circular underwater section with wall-sides above the design waterline as shown in figure 1. The two demi-hull beam to overall beam ratios were

chosen to represent the range of extreme 'wave piercer' to the narrowest conventional catamaran configurations. The vertical distance from the water surface to the underside of the tunnel was also representative of typical 75m vessels.

The principal particulars of the models and corresponding vessels are given in table I.

Both models were given a damage opening amidships with a longitudinal extent corresponding to 5.25m (3m plus 3% of the length of the ship) in accordance with SOLAS regulation 11-1/8.4.1. The damage was limited to above the Ro-Ro deck as

shown in figure 2.

To enable the quantity of water accumulated on the Ro-Ro deck to be measured, a simple trap door was fitted which could be closed. The water was then pumped into a bucket and

weighed at the end of the run.

To determine the effect of residual freeboard and significant wave height, tests were conducted for three full scale residual

Table 1. Principal particulars (undamaged condition).

Table 2. Vessel particulars for varying residual freeboard

freeboards:

1.5, 2 and 2.5m; and three significant wave

heights: 3, 4 and 5m. The JONSWAP Wave Spectrum for Irregular

Seas was used as defmed

in the Stockholm Agreement. Details of the corresponding vessel configurations for each case are given in table 2.

Dimensions in metres Drawing not to scale

Figure 2. Damage Opening.

Note that due to limitations with the ballasting system of the model it was not possible to maintain constant VCG and roll radius of gyration, k, as the displacement was increased. This is unlikely to have affected the results significantly as earlier work has shown that the water depth accumulated on the deck

is dependent only on freeboard and significant wave height

[Hutchinson et al. 1996]. In any case these parameters would both vary considerably with change in displacement on the full

scale vessel.

Oceanic Engineering International 66

Model N° 97-04 97-16 Full Scale

Scale 1:45.5 1:45.5

-LOA 1.65 m 1.65 m

75m

BOA 0.77m 0.44m

35 / 20 m

P (hull separation) 0.66m 0.33m

30 / 15 m

LIB (demi hull) Ratio 15 15 15

KG (undamaged) 0.134m 0.126m

-DR(Depth to Ro-Ro Deck) 0.155m 0.155m 7.05 m

Ro-Ro Deck Height 0.066m 0.066m 3 m

Hull Separation 30 m Hull Separation 15 m

Freeboard (f,.)

full scale

Displacement

VCG

k

Displacement

VCG

k

2.5m

3117.9t

8.65m

12.29

3117.9t

9.23m

5.92 2.0 m 3494.7 t 8.92 m 11.37 3494.7 t 9.42 m 6.37 1.5 m 4154.1 t

9.24m

11.37 4154.1 t

9.74m

5.46 75.00 r9.00

A

image Opeatirg

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3. RESULTS AND DISCUSSION

The maximum level of water accumulated on the Ro-Ro

deck is plotted as a function of significant wave height for the

two hull separations in figures 3 and 4. No values were obtained for the 15m hull separation condition at the 1.5m

residual freeboard as the model capsized for each of the three significant wave heights. All the results given are for the case where the damage opening was towards the oncoming waves. Some tests were also conducted with the damage opening away from the oncoming waves, however these always resulted in a reduced level of water on the deck.

It can be seen from figures 3 and 4 that the level of water,

which accumulated on the Ro-Ro deck, increased with an

3.00 2.50 0 1.50

2

1.00 0.50 0.00

Significant Wave Height Vs Water Depth

Model No 97-16

Damage Opening Toward Oncoming Waves

A 2.0 inResidual Freeboard

*2.5m Residual Freeboard Full Scale Hull Separation :15m

2.5 3 3.5 4 4.5 5 5.5 H1/3 (nri)

Figure 4. Level of water accumulation on the Ro-Ro deck. Full scale hull separation 15m.

Figure 3. Level of water accumulation on the Ro-Ro deck. Full scale hull separation 30m.

increase in significant wave height and with a decrease in residual freeboard. The effect of significant wave height is

much greater with the larger hull separation (figure 3) than the

smaller hull separation (figure 4).

The results indicate that for a residual freeboard of between 3m and 3.5m the accumulated water depth may reduce to zero.

The level of water accumulated is plotted against hull

separation for each of the three significant wave heights of 3.5, 4, and 5m in figures 5 to 13 respectively. The values assumed in the Stockholm Agreement for monohull Ro-Ro vessels are included at zero hull spacing for comparison.

The proceeding figures indicate that the measured water

depth for both hull separations is significantly greeter than that

set down by the Stockholm Agreement.

25

1.5m Residual Freeboard 15 30 Hull Separation P(m) Experiment o StockholM Agreement

Figure 5. Hull separation vs. water depth. Significant wave height=3.5m, residual freeboard=1.5m.

L.

2

_to 0.5 2.0 m Residual Freeboard Experiment 0Stockholm Agreement 15 30 45 Hull Separation P(m)

Figure 6. Hull separation vs. water depth. Significant wave

height=3.5iii residual freeboard,=2.0m. Significant Wave Height Vs Water Depth

ModelNo 97-04

3.00

Damage Opening Toward Oncoming Waves

2.50 2.00 A I-_0. _{1.5 m Residual Freeboard} CI 1.50 _A A 2.0 mResidual Freeboard 411 1.00 *2.5 mResidual Freeboard

Full Scale Hull Separation :30 m 0.50 0.00 2 3 4 5 6 H1/3m 2.5 1.5

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.c 0. 2.5 2

2

0.5 0 2.5 m Residual Freeboard 0 15 30 45 Hull Separation P(m) A Experiment A Stockholm Agreement

2.5

0.5

1.5 m Residual Freeboard

2.5 2 0.5 2.0 m ResidUal Freeboard Experiment 0 Stockholm Agreement 0 15 30 45 Hull Separation P(m)

2.5 2.5 m Residual Freeboard 0 0 15 30 Hull Separation P(m) A Experiment A Stockholm Agreement

2.5

-2

1.5 m Residual Freeboard

2.5

-2.0 m Residual Freeboard

0 15 30 45

Hull Separation P(m)

Experiment

Oceanic Engineering International 68

Experiment

<> Stockholm Agreement

15 30 45

Hull Separation P(m) 0 _{Hull Separation}15 30 45

P(m)

It 1.5

0

2

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2.5

2II

1.5

2,5 in Residual Freeboard

Experiment

2.50 11 2004. 1.50 J .0 1.00 0.50] co 0.00

Residual Freeboard Vs Water Depth 3.5 m Significant Wave Height

- Stockholm Agreement(monohull)

Experiment, P =30 m A Experiment, P = 15 m

_e_ Proposed Guidelines (catamaran)

1.50 2.00 2.50 3.00

Residual Freeboard (m)

Residual Freeboard Vs Water Depth

4.0 mSignificant Wave Height

1.00 1.50 2.00 2.50 3.00

Residual Freeboard Fr(m)

Figure 14. The effect

of

residual freeboardon accumulated

water depth. Significant wave height-3.5m.

Figure 15. The effect of residual freeboard on accumulated water depth. Significant wave height-3.5m.

As can be seen, the depth of water that accumulates on the Ro-Ro deck increases with hull separation at the 2m residual

freeboard, but decreases with hull separation at the 2.5m

residual freeboard. It is not possible to make this comparison at the 1.5m residual freeboard as the model with the 15m hull

separation capsized at each of the significant wave heights

tested at this residual freeboard.

The level of water accumulated on the Ro-Ro deck is plotted as a function of residual freeboard for a significant wave height

of 3.5 and 4m in figures 14 and 15, and compared, with the

values from the Stockholm Agreement. It can be seen, from

this figure, that the level of water accumulated on the Ro-Ro deck is much greater for both catamaran hull separations than it is for the monohull assumed in the Stockholm Agreement. Where the Stockholm Agreement states that no water on the

Ro-Ro deck should be used for assessment of stability, the model tests have indicated that a substantial levelwill exist for both the catamaran configurations.

As a result, a set of interim guidelines for determining the

level of water to be assumed when assessing the damage

stability of a Ro-Ro catamaran was proposed.

level of water to be assumed when assessing the damage

stability of a Ro-Ro catamaran is proposed. These guidelines

are included in figures 14 and 15, and are sumifiarised in

table 3. Experiments have been conducted using simplified catamaran models to investigate the level of water that will accumulate on the Ro-Ro deck of a catamaran as a function of

residual freeboard and sea state for two different demi-hull

beam to

overall beam ratios.

The results from these

experiments show that that the level of water accumulated on

the Ro-Ro deck is much greater for both catamaran hull

separations than for the monohull assumed in the Stockholm

Agreement.

Table 3. Proposed interim guidelines for determining the level

of water accumulated on the Ro-Ro deck of a catamaran for assessment of damage stability.

For the above proposed interim gUidelines, all intermediate values of both significant wave height and residual freeboard are to be determined by linear interpolation.

4. CONCLUDING REMARKS

Experiments have been conducted using simplified catamaran models to investigate the level of water that will accumulate on the Ro-Ro deck of a catamaran as a function of

residual freeboard and sea state for two different demi-hull

beam to

overall beam ratios.

The results from these

Residual Freeboard WaterLevel (m)1

(fr)(m) H113 =3.5m Hu;=- 14.0m 1 1.5 1.2 1.7 2.0 0.85 _.1.5 2.5 0.5 0.5 2.50 2.00 1.50 0. lel 1.00 0.50 0.00

li

-Stockholm

Agreement (rrionohull) Experiment, P =30m A Experiment, P =15m -9 Proposed Guidelines (catamaran) 0 15 30 45 Hull Separation P(m)

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rexperiments

show that that the level of water accumulated on the R.o-Ro deck is much greater for both catamaran hull

separations than for the monohull assumed in the Stockholm Agreement

level of water to be assumed when assesing the damage

stability of a Ro-Ro catamaran was proposed.

ACKNOWLEDGMENTS

The authors are indebted to their colleagues in the Ship

Hydrodynamics Centre at the Australian Maritime College for their assistance with the experiments

REFERENCES

Allen, T. 1997 Proceedings of the 1995 SOLAS Diplomatic COrtference on Ro-Ro Passenger Ferries. The Royal

Institution of Naval Architects Spring Meeting 1993.

Huth-in-son, B.L., Little, P. Molyneux, D., Noble, P.G. and

Tagg, R.D. 1996.

Safety Initiatives from The SNAME Ad Hoc Ro-Ro Safety

Panel, In Proceedings of Ro-Ro '96 Conference. Lubeck,

Germany. 21-23.

Stockholm Agreement Agreement concerning specific

stability requirements for Ro-Ro passenger ships

undertaking regular scheduled international voyages between or to or from designated ports in northwest Europe and the Baltic Sea. /MO Circulation Letter No 1891.