Damage Survivability of a New Ro-Ro Ferry

J. O. de Kat* M. Kanerva** R. van 't Veer* I. Mikkonen**

*MARIN, The Netherlands **DELTAMARIN, Finland

ABSTRACT

This paper presents experimental investigations involving the dynamic behaviour and survivability of a damaged ro-ro passenger ferry. It concerns a modern design, which has an extensive lower hold for ro-ro cargo purposes. Two main damage scenarios have been considered: lower hold intact and damaged. Besides damage survivability in beam waves, the research focused on the ship dynamics as a result of transient flooding following a breach of the intact huil. The following parameters have been varied: KG, creation rate of damage opening, cross duet area, internal configuration and cargo deck permeability. Some conditions have been simulated numerically.

1 INTRODUCTION

Whereas much research has been devoted to damage stability of existing passenger ro-ro ferries in waves, relatively little is known about the dynamic behaviour associated with transient flooding and, likewise, about the behaviour of a ship with flooded lower hold. None of the current damage stability criteria account for transient flooding effects, which in certain conditions may endanger a ship as has been shown by Journée et al. [1] and by Vredeveldt and Uwland [2]. In addition, little is known about the damage survivability of a ferry when it is subjected to extensive flooding of the lower hold. To address these issues, model tests have been carried out with a new ro-ro ferry design.

The ro-ro passenger ferry market is expenencing constant pace of evolution. The key issues today are increased transport efficiency, high operational availability and, of course, safety. A typical product today is a ro-ro passenger ferry with 2500 lanemeters, 400 to 800 passengers as night version or 1500-1800 passengers as day version, speed from 22 knots up to 30 knots.

There are in total 1075 ro-ro passenger ferries in the world, of which 370 are below 110 m in length. But the major share of the ferries is between 120-180 meters, about 450 ferries. There is a big fleet renewal program ongoing for European ferry routes, at higher pace than ever before. Are we able to develop safer and more efficiënt vessel contigurations or are we just repeating existing concepts with minor adjustments? Many ro-ro ship accidents have occurred because of cargo shift in trailers causing trailers to move in heavy seas or because they haven been incorrectly loaded. Cargo decks should be designed for rational cargo handling even for short harbour calls and assuming extreme hurry due to eventual delays.

The ro-ro passenger ferries of tomorrow must be more efficiënt and safer to operate on the route and in harbour. The market situation is extremely interesting and at the same time confusing. Within short intervals SOLAS 90 came into force and was foliowed by Stockholm agreement for water on deck, and the new probabilistic rule approach is now under consideration. Cabotage traffic will be opened in EU and tax­free sales are already abolished. At the same time other safety issues are addressed. There is a lot of activity on the ro-ro passenger ferry market, perhaps more than ever before.

Shortly after the disaster of the 'Herald of Free Enterprise' it became obvious for the industry not to locate passenger cabins and other passenger spaces below the main ro-ro deck (freeboard deck) anymore. The volume below the main ro-ro deck is large and leaving it as void would be waste of valuable space. It could be used for cargo and/or for storage. However, the only efficiënt way to utilise this space is to use it for cargo, to apply the so-called lower hold configuration. This has become more or less an industry Standard either for trailers or for private cars. The hold is typically limited by longitudinal bulkheads inside the B/5 line and with deck above the B/10 line.

Up to this point SOLAS supports the lower hold configuration. But SOLAS and IMO especially fail completely after the bulkhead deck definitions to give even any guidelines of how to handle floodable length curves, and how to calculate actual damage stability. At the moment at least six different interpretations are available in Europe which have recently been applied for newbuildings. A265 is actually the only available official IMO method to handle damage stability for a longitudinally subdivided ship. This method is not at all feasible for modern vessels and thus seldom used. Some of the newbuildings are being calculated in accordance with the SOLAS 90 and disregarding completely the existence of lower hold. Damage statistics, however, show that 60% of collisions protrude beyond the B/5 line.

A safe way is to include the lower hold in all damage cases together with two side compartments as well as together with compartments behind and forward of the lower hold. If a vessel can fulfil the SOLAS 90 criteria for the above lower hold damages without extensive initial stability level it can be considered as a real damage safe vessel. Several examples have already shown that it is possible to construct a modern, efficiënt ro-ro passenger ship with a large lower hold and without utilising doubtful limitations for the damage definitions. Further work is, however, required for the damage stability regulations and interpretation. It is expected by the industry that the new probabilistic rules will give an equal situation for the industry.

The lanes on different ro-ro decks, typically 2 to 4 decks must be available even in very short harbour calls meaning efficiënt and wide ramp arrangement and wide enough lanes. The lower hold is a typical bottleneck. Inefficiënt lift or narrow single ramp may lead to the temptation to leave the hold empty and thus raising unnecessarily the vertical centre of gravity. Safe operation of the lower hold requires efficiënt cargo handling arrangement.

To evaluate the damage survivability of a new ship with lower hold, the vessel has been subjected to a number of parametric variations: • Damage scenario 1: worst SOLAS two-compartment damage, lower hold and

doublé bottom intact. • Damage scenario 2: same as scenario 1 with same SOLAS size opening in

outer huil, but with transverse damage extent exceeding SOLAS damage (transverse extent is 2 x B/5), lower hold flooded.

• Internal configuration: Presence of longitudinal and transverse bulkheads on main car deck.

• Permeability at main car deck level (presence of real cargo). KG.

The ship response to transient flooding conditions is considered as a function of: • Damage scenario 1 and 2. • KG variation. • Rate of creation of damage opening (slow and fast). • Influence of cross-flooding ducts. • Presence of waves.

Van 't Veer and De Kat [3] present detailed information on experimental and numerical investigations as regards transient and progressive flooding of typical ship compartments with different geometry variations. In addition to the transient flooding conditions, the ship has been tested using the model test procedures recommended by IMO Resolution 14 ("Stockholm Agreement").

2 RO-RO FERRY DESIGN

The ro-ro passenger ferry discussed in this paper is a typical modern design with large lower hold for trailers accessible with drive through principle, i.e. 3.3 m wide, free driving width, ramps at both ends of the hold. Figure 1 presents the principal arrangement. Side casings are arranged on the main deck, freeboard deck, to give additional stability range and to improve transient stability characteristics as described in this presentation. There are two pillar lines arranged on the main and upper deck to subdivide both decks into three sections, three lanes in the middle and two on both sides. Pillars are arranged to prevent cargo shift but they also provide a possibility of flood preventing arrangements if needed. All lanes are 3.2 m wide and ramps are arranged for aft and forward loading.

Figure 1 General arrangement of the vessel.

Table 1 contains the main characteristics of the intact ship as tested. The value of GM = 3.80 m corresponds to the operational design value. GM = 2.20 m represents a case with unrealistically low GM selected for research purposes (it is close to the minimum allowable GM according to the Stockholm calculation method with water on deck and lower hold damaged); the roll periods associated with the GM values are shown.

The huil form is shown isometrically in Figure 2. Figure 3 provides a view of the internal geometry (showing floodable spaces and cross duet arrangement). On either side of the damaged transverse bulkhead a cross duet is present: the cross-sectional area of the duet aft of the damage is 2.8 m2; this is 5.9 m2 for the forward duet. The

longitudinal and transverse bulkheads shown on the main deck were made removable for model testing purposes.

Table 1 Main ship particulars.

Lpp 170.0 m B 28.7 m H (to deck 6) 18.0 m H (to bulkhead deck) 9.0 m T 6.0 m A 19850 tonnes kxx 11.1 m kw 42.5 m GM 3.8 m T* 11.8s GM 2.2 m T* 15.5 s

Figure 2 Isometric view of the huil.

Figure 3 Layout of internal geometry (shown are main cargo deck with subdivisions,

lower cargo hold and wing compartments with cross ducts).

2.1 Damage Scenarios and Stability Data

For the design draft of T = 6.0 m the worst two-compartment damage SOLAS '90 damage was determined. The damage location associated with this condition is at the port side close to amidships. The length of the damage opening is 8.1 m. The doublé bottom is intact for this case, i.e., the damage opening extends vertically upwards as of the doublé bottom tank top. For damage scenario 1 the transverse extent is B/5 with isosceles triangular shape. For scenario 2 the damage opening is the same as for scenario 1, but the transverse extent is 2 x B/5; for this condition a rectangular opening was created in the side bulkhead of the lower hold. Because of an intact tank on the starboard side, the damage is asymmetrical and results in a steady heel to port.

Figure 4 illustrates the flooding extent for the different damage scenarios. Case 1A represents damage scenario 1 with subdivision on the bulkhead deck; case 1C is the same as 1A but with the car deck subdivision removed. Case 2A represents damage scenario 2 with subdivision on car deck, and 2B is 2A without subdivision.

Figure 5 shows the GZ curves for the damaged ship in different conditions, as described in Figure 4.

1A 1C

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Figure 4 Overview of different damage scenarios as used in the model tests.

The equilibrium floating position after damage I for initial GM = 2.2 m is as follows:

Damage scenario 1 Damage scenario 2 Draught 6.41 m 7.87 m Trim -0.21 m +1.56 m Heel 3.7 deg 3.2 deg Freeboard 1.61 m 0.34 m (in way of damage)

The equilibrium floating position after damage for initial GM = 3.8 m is as follows:

Damage scenario 1 Damage scenario 2 Draught 6.42 m 7.88 m Trim -0.23 m +1.53m Heel 1.3 deg 1.4 deg Freeboard 2.20 m 0.77 m

The minimum required GM calculated according to the Stockholm Agreement is 1.98 m for the damage scenario 1 (side damage) in case the bulkheads on the main car deck are present and 2.23 m in case bulkheads on the main car deck are removed.

For damage scenario 2 (lower hold damage) the minimum required GM calculated according to the Stockholm Agreement is 4.66 m. Here the bulkheads on the main car deck are assumed always damaged because the damage penetrates B/5.

(a) (b)

GZ EPHI

GZ EPHI

(c) (d)

GZ EPHI

Figure 5 Overview of GZ curves for different damage scenarios, see Figure 4 for

reference. (a) Initial GM = 3.8 m, B/5 damage, scenario 1 (b) Initial GM = 3.8 m, lower hold damage, damage scenario 2B (c) Initial GM = 2.2 m, B/5 damage, scenario 1 (d) Initial GM = 2.2 m, lower hold damage, damage scenario 2B

EXPERIMENTAL SET-UP

The model was fabricated using thin-walled (thickness 4 mm) fibreglass at scale 1 to 40. For bulkheads and decks in the internat floodable spaces a thickness of 3 mm was used. To simulate transient flooding following a side collision, the model was equipped with a sliding door mechanism that covered the damage opening. Using a winch it was possible to adjust the door speed, such that at the fastest speed it took

6.5 s (full scale) to create the complete opening. Forthe tests according to IMO Res. 14 the sliding door was removed in its entirety.

The modelled permeability of the fin stabiliser spaces on the (damaged) port and starboard side was 85%. For the main garage deck and lower hold a permeability of 100% was taken. In addition, for some tests scaled cars and trucks were positioned on the main garage deck to investigate the influence of their presence. The original design had two longitudinal bulkheads on the main deck, closed off fore and aft by transverse bulkheads, in order to comply with requirements from the Stockholm Agreement calculation procedure; for testing purposes these bulkheads were made removable.

Model instrumentation included the measurement of: surge, heave and sway; roll, pitch and yaw; relative motion at two locations near the damage opening; water elevation inside the vessel at 8 locations below main deck and at 11 locations on the main deck; wave elevation in the vicinity of the model and drift speed. The model was moored using a soft-spring system that did not interfere with the wave-induced motion behaviour. Testing took place in a MARIN basin with the following dimensions: 200 m x 15 m x 1 m.

For the tests according to IMO Res. 14, two irregular sea states were calibrated: (1) Hs = 4 m, Tp = 8 s, (2) Hs = 4 m, Tp = 12 s for five realisations of 30 min. duration. Also, for some selected cases test runs of 1.5 hour (full scale) duration were carried out to investigate long duration exposure to a given sea state.

4 RESULTS: TRANSIENT FLOODING

When an intact ship is subjected to a sudden breach of the huil at the side, it can experience a large transient roll angle during the initial stages of flooding. The peak roll angle can be significantly larger than the final equilibrium heel angle depending on the value of the metacentric height. The first series of results shown concerns calm water conditions, with the presence of subdivision bulkheads on the main garage deck.

4.1 Influence of Damage Opening Rate

For damage scenario 1 with low GM (GM = 2.2 m), Figure 6 shows the roll response for two damage opening speeds corresponding to an opening time of 20 s (slow) and 6.5 s (fast).

ROLL deg

ROLL deg

10.00-,

0

10.00-1

o

— I — I — I —

200 SECONOS

100 300

Figure 6 Transient roll response following slow (upper graph) and fast damage

occurrence; GM = 2.2 m, lower hold intact (scenario 1A or 1C).

For these conditions the damage opening rate has some influence on the maximum roll peak: <|>max = 16.4 degrees for the fast damage, while (t>max = 13.0 degrees for the slowly created damage.

ROLL deg

10.00-,

O

ROLL deg

10.00-,

o

100 200

SECONDS 300

Figure 7 Transient roll response following slow (upper graph) and fast damage

occurrence; GM = 3.8 m, lower hold damaged.

For damage scenario 2 with GM = 3.80 m, Figure 7 shows the roll response for the same damage opening speeds as applied for scenario 1. In this case the lower hold is subject to flooding as well.

Figure 7 shows that the maximum roll peaks are limited to §max = 7.5 degrees and 5.2 degrees for the fast and slow damage, respectively. Inspection of the heave record shows that by the time the ship rolls toward its equilibrium angle after the transient roll peak, the ship has not yet reached its final draft; progressive flooding (into the lower hold) takes longer than the dynamic roll response.

From these observations it appears that damage created within a short time interval (shorter than the natural roll period of the intact ship) results in a somewhat larger transient roll peak than for the case where damage occurs more slowly (time interval exceeding the natural roll period). The maximum roll peak will decrease with increasing GM. Although the maximum roll peaks are larger than the final equilibrium angle, transient flooding in calm water does not endanger the ship even for the lowest GM considered (GM = 2.2 m).

4.2 Influence of Cross Ducts

To assess the influence of cross ducts on the cross flooding process and ship response, the aft cross duet was reduced in size by 75% (cross-sectional area is 25% of original area); the forward duet was not modified. Figure 8 shows the roll response for damage scenario 1 with GM = 2.2 m and fast damage.

10.00-.

ROLL deg o-

10.00-,

0-

I , , , , , , , 1 , 1 . r—i . 1 1 1 , r

0 100 200 300 SECONDS

Figure 8 Transient roll response for vessel with original cross ducts (upper graph) and

with reduced area of aft duet; GM = 2.2 m, lower hold intact.

ROLL deg

For these conditions the reduction in cross-sectional area of the aft duet leads to a slight increase in maximum roll peak: (t>max = 17.2 degrees compared with <t)max = 16.4 degrees for the open duet. Hence for these conditions the influence of cross duet size on maximum roll peak is marginal.

Another question of interest is to which extent cross flooding ducts are effective in transferring floodwater. A more detailed analysis of the case shown in Figure 6 provides relevant information; let us consider the case with slow damage opening rate, where we would expect the cross ducts to be relatively effective. Starting at the aft end of the opening, it takes 20 s to create the full damage opening on the port side.

ROLL deg

REL4 LOWPS m

5.00-, REL4 LOWSB

5.00-, REL6 LOWPS m

5.00-1

REL6 LOWSB

SECONDS

Figure 9 Measured transient roll response and water levels in (damaged) PS and intact SB compartments following slow damage occurrence; GM = 2.2 m, lower hold

intact.

Figure 9 shows the measured roll motion and the water elevation at the following locations of the damaged compartments underneath the main ro-ro deck (measured close to the cross duet openings): aft port and starboard side compartments (REL4) and forward port and starboard side compartments (REL6).

Figure 10 shows the simulated roll response and fill levels for the same conditions as in Figure 9. The overall results are close to the measured data; in the simulations there are some oscillations inside the starboard side compartments during the flooding process, whereas in the model tests the flooding was more stationary. The simulation model has been described by De Kat [4] and by Van 't Veer and De Kat [3].

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50 100

tim* [f]

150 MO

Figure 10 Simulated transient roll response and water levels in (damaged) PS and intact SB compartments following slow damage occurrence; GM = 2.2 m, Iower hold

intact.

From Figure 9 we observe that as of the start of damage: it takes 22 s for the ship to reach its maximum transient roll peak to port; it takes 16 s for the aft PS compartment to fill completely; there is a delay of about 8 s for the forward PS compartment before it starts flooding, filling takes about 12 s; it takes 8 s for the first water quantities to flow through the aft cross duet and reach the aft SB compartment, but it takes 100 s before cross flooding is completed; it takes 8 s for the first water quantities to flow through the forward cross duet and reach the forward SB compartment, and it takes about 50 s before cross flooding is completed (the cross-sectional area is twice as large as for the aft duet).

GM and the heeling moment impulse exerted by the floodwater govern the time it takes for the ship to reach its maximum roll peak. The cross-flooding rate determines the time for the ship to reach its static equilibrium; in this case it takes about 140 s as of the start of damage to reach the final static heel angle. Cross flooding into the intact SB compartments is quasi-static: the oscillatory roll motions do not affect the flooding rate. After reaching the maximum transient roll peak, the ship rolls at its natural period (T = 24 s) about an exponential-like heel decay.

These observations suggest that during the first stages of water ingress cross ducts are not very effective in reducing maximum transient roll peaks, which can be significantly larger than the static equilibrium heel angle. Increasing the cross-sectional area of the ducts will reduce the cross-flooding time, but it is not possible to achieve complete equalisation within one roll period.

4.3 Transient Flooding in Regular Waves

A number of calm water transient tests were repeated in regular waves for damage scenario 1 and main deck subdivided. Two wave conditions were considered: (1) H = 4 m, T = 8 s, and (2) H = 4 m, T = 12 s. An example of measured roll response is shown in Figure 11, where the wave period is T = 12 s, high damage speed, and GM = 2.2 m. For this case the maximum roll peak is 16.5 degrees, which is essentially the same as is the case in calm water, see Figure 6.

For a given damage opening speed and wave condition, tests were also carried out with damage occurring at different points in time so as to assess the influence of the phasing of the wave with respect to the ship motion. It was found that the presence of the waves had no significant impact on the maximum transient roll peaks, regardless of damage speed and wave period. Also, the phasing of damage occurrence with respect to the wave had a negligible influence on the maximum roll peaks provided there is no ingress of water on deck.

10.00-

ROLL deg o-

r — i — > — i — i — i — . — i — • — i — i — . — i — , — i — i — i — i —

0 200 400 600 SECONDS

Figure 11 Transient roll response in regular wave (H = 4 m, T = 12 s); GM = 2.2 m.

Transient flooding tests have been performed also in irregular waves with a significant wave height of Hs = 4 m and peak period Tp = 8 s. It was found that the transient roll peak following damage occurrence was approximately the same as for the calm water and regular wave tests. For damage scenario 2 with lower hold flooded, it was found that the maximum transient roll peak was 11.5 degrees, i.e., significantly less than for the case with the lower hold intact.

5 RESULTS: DRIFTING IN IRREGULAR BEAM SEAS

5.1 Survivability in Maximum Sea State

The ship was tested according to the procedures specified in Annex 2 of IMO Circular Letter No. 1891 (Regional Agreement). For these tests the vessel was positioned in flooded condition with full damage opening beam to the oncoming waves. Some of the results are shown below. It was found that for the full load operating condition with GM = 3.8 m the ship was capable of surviving damage scenarios 1 and 2 in sea states with Hs = 4 m and Tp = 8 s and 12 s. In general the vessel behaviour was influenced more significantly by the short period waves with Tp = 8 s. In the most onerous case -- damage scenario 2 with lower hold flooded, no subdivision on main deck, Tp = 8 s sea state -- the ship was still capable of surviving safely with GM = 3.8 m. This did not apply to the case with low GM of 2.2 m, where for some conditions the ship was found to capsize with flooding of the lower hold. The discussion below will focus on the cases with low GM.

5.2 Influence of Deck Cargo

For the damage scenario 2 case, tests were carried out with and without the presence of (floodable) ro-ro cargo fastened to the main deck. Figure 12 shows the motion response for the following conditions: scenario 2, GM = 2.2 m, transverse bulkheads removed from main deck, longitudinal bulkheads present, no cargo (100% permeability) on ro-ro decks, Hs = 4 m, Tp = 8 s. Figure 13 shows similar data for the same conditions as in Figure 12, but with the presence of realistic ro-ro cargo on the main deck.

The signals REL2 and REL4 denote the water height on the port side of the main ro-ro deck forward of the damage opening, where REL4 is located close to the position of the transverse forward bulkhead and REL2 is located midway between REL4 and the damage opening. When the lower hold is flooded, the ship trims slightly by the bow.

WAVE 2 m

ROLL deg

REL2 TOPPS m

2.00-.

REL4 TOPPS m ,J^#i4^w^^

1000 2000 3000 SECONDS

Figure 12 Ship with damage scenario 2 in beam waves (Hs = 4 m, Tp = 8 s), GM = 2.2 m,

only longitudinal bulkheads and no cargo present on main ro-ro deck.

WAVE 2 m

ROLL dog

REL2 TOPPS

m o

2.00-1

^ • M - 4 - ^ ^ U ^

REL4 TOPPS m Q.

Ltjikj^jiA^A^\^*t If kO^M^KJuaM)^1^

1000 2000 3000 SECONDS

Figure 13 Ship with damage scenario 2 in beam waves (Hs = 4 m, Tp = 8 s), GM = 2.2 m,

only longitudinal bulkheads and cargo present on main ro-ro deck.

A comparison between Figures 12 and 13 suggests that the presence of ro-ro cargo does not have a major influence on the motion behaviour of the ship. The water on the main deck shows smaller fluctuations for the case with cargo on deck, as illustrated by the REL2 and REL4 signals. The difference in resulting mean heel angle is about 1 degree.

Figure 14 shows the motion response for the following conditions: scenario 2, GM = 2.2 m, all transverse and longitudinal bulkheads removed from main deck, ro-ro cargo present, Hs = 4 m, Tp = 8 s. Figure 15 shows similar data for the same conditions as in Figure 14, but for damage scenario 1, i.e., lower hold not flooded. Figure 16 represents the same conditions as for Figure 14, but with GM increased to GM = 3.8m.

i — • — i — i — i — | — • — i — . — i — | — . — i — i — i — | — i — i — . — i — r

0 1000 2000 3000 4000 SECONDS

Figure 14 Ship with damage scenario 2 in beam waves (Hs = 4 m, Tp = 8 s), GM = 2.2 m,

no subdivisions on main ro-ro deck.

As shown in Figure 14, the ship capsizes after a long time period (about 4000 s) because of accumulation of water on the main deck. At the low GM of 2.2 m the ship does not capsize when the lower hold is not flooded, Figure 15; there is no water on the main deck of any significance. Increasing the GM to 3.8 m results in capsize avoidance, as shown in Figure 16.

ROLL deg

10.00-,

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2.00-,

REL2 TOPPS m o-

2.00-n

REL4 TOPPS m o-

— i — • — i — i — . — i — i — i — i — i — | — i

1000 2000 3000 SECONDS

4000

. Figure15 Ship with damage scenario 1 in beam waves (Hs = 4 m, Tp = 8 s), GM = 2.2 m,

no subdivisions on main ro-ro deck.

WAVE 2 m

ROLL deg

10.00-,

^^^MHJti^^MlAUiiU'^'U^^

2.00-1

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I 1 1 1 1 1 1 1

2000 3000 SECONDS

1000 4000

Figure 16 Ship with damage scenario 2 in beam waves (Hs = 4 m, Tp = 8 s), GM = 3.8 m,

no subdivisions on main ro-ro deck.

6 EVALUATION OF LOWER HOLD CONCEPT

The lower hold concept for ro-ro passenger ferries was developed to improve the efficiënt use of available volume within the huil as cabins located below freeboard deck had to be avoided. However, rules or rule interpretations were not clear nor available for several issues, such as floodable lengths, damage extent, damage definitions, permeabilities, B/5 and B/10 definitions concerning recesses, e.g. for watertight doors and ramps, criteria to be applied for damage stability calculations, vent pipe locations, etc. The situation still remains the same even though it is already nearly 20 years since the first lower hold ferries were delivered. There are still at least seven different interpretations available from different European nations, all EU members, concerning floodable lengths and damage stability calculations for a ferry with lower hold. This is making the life difficult for shipping companies and for shipyards as well.

The ferry configuration presented in Figure 1 was originally designed to fulfil SOLAS 90 in all two compartment damages with and without the lower hold included in these damage cases. The lower hold and side casing arrangement gave also additional damage safety benefits that are not only theoretical but give margin against actual extensive damages.

The following most probable damage cases were studied: • Three side compartments together with lower hold, fulfilling SOLAS 90

without margin line. • Complete doublé bottom damage, fulfilling SOLAS 90 without margin line. • Collision damage extending over 9-10 compartments from bow including

lower hold and bulkhead deck, vessel surviving. • Maximum amount of water on deck being over three meters corresponding to

over 6000 tons, simulating an open bow door situation, fulfilling SOLAS 90. • Combined lower hold and two-side compartment damage plus

simultaneously water on deck, vessel surviving.

All the above damage cases could be met fulfilling SOLAS 90 final stage criteria, except for the margin line in some of the cases.

The lower hold damages with the longest possible hold actually show the best survivability and stability characteristics as there is no trim effect included, i.e. vessel just sinks deeper without excessive trim. The side casings on bulkhead deck are an essential part of the survivability and according to the model tests give a possibility to leave out all flood preventing doors and bulkheads on the main deck. This practically means that a lower hold configuration with side casings is a feasible arrangement for both damage safety and for efficiënt cargo handling.

The request from the market is to reach the maximum possible ro-ro deck capacity mostly within limited main dimensions. The lower hold capacity and especially length is very much depending on the machinery arrangement. Most of the ferries today have conventional diesel machinery directly geared with twin shaft lines. However, this arrangement looks automatically into forward location of the lower hold as the aft part is reserved for the machinery. Diesel electric machinery configuration give an opportunity to locate the diesel generators besides the lower hold, outside the B/5 longitudinal bulkheads. A further development is to apply pod propulsion, when the propulsion motors are removed outside of the vessel releasing space for the lower hold. Figure 17 presents engine room and lower hold arrangement for the three machinery options in a ro-ro passenger ferry.

Diesel-Mechanical

"Conventional" Diesel-Electric Lo>cr ho'd S6 X ol Lep. n

Pod Propulsion

Figure 17 Lower hold arrangements for different machinery space options.

The pod arrangement with the diesel generators located is separate water- and fire-tight compartments released the maximum volume for the lower hold within the given huil form. The hold is also located longitudinally symmetrically which means that in case of lower hold damages water in the hold is not causing excessive trim. Whatever damage position we may have the hold is always balancing the trim, e.g. aft engine room damages are typically critical for a ro-ro ferry due to big volume of the spaces combined with aft location. The engine room volume is now spread out into several compartments with more central position and if the damage is extended into the hold that has always a balancing effect.

Further development would be to utihse the possibility given by the new probabilistic damage stability rules. The B/5 bulkheads are no more a requirement as both longitudinal and transverse compartmentation is considered over several compartments. In front of the diesel generator rooms the lower hold could be widened closer to the huil releasing one additional lane into the hold.

CONCLUSIONS

An investigation has been carried out to assess the survivability of a ro-ro ferry under a variety of damage scenarios. It concerns a vessel with extensive lower hold, which was considered in both intact and damaged conditions.

Transient flooding and ship response have been investigated following damage to the intact huil, for which the following conclusions are drawn:

• A significant transient roll peak (to the damaged side) can result during the first stages of water ingress; this roll peak increases with decreasing GM.

• Under the conditions considered (including low GM) the vessel did not capsize during transient flooding.

• GM and the heeling moment impulse exerted by the floodwater govern the time it takes for the ship to reach its maximum roll peak. The cross-flooding rate determines the time for the ship to reach its static equilibrium, which is in the order of 150 secondsforthe ship considered.

• Cross flooding into the intact compartments is quasi-static: the oscillatory roll motions do not affect the flooding rate. After reaching the maximum transient roll peak, the ship rolls at its natural period about an exponential-like heel decay.

• The investigations suggest that during the first stages of water ingress cross ducts have a limited influence in reducing maximum transient roll peaks, which can be significantly largerthan the static equilibrium heel angle.

• When flooding into the lower hold takes place, the maximum roll peak is smaller compared with the case with lower hold intact.

• Waves have a negligible influence on the maximum transient roll angle. • Increasing the cross-sectional area of the ducts will reduce the cross-flooding

time, but it is practically impossible to achieve complete equalisation within one roll period.

For the ship drifting in damaged conditions in beam seas (Stockholm Agreement model test procedure), the following conclusions apply.

• For the damage scenario with lower hold intact the ship did not capsize in the worst conditions considered: at the lowest GM (2.2 m), no subdivision

bulkheads on main deck, and in the highest sea state, no significant deck flooding took place ensuring ship survival. With the lower hold flooded, the ship survived the highest sea state for the design GM of 3.8 m and without subdivision on the main deck. For the lowest unrealistic GM (2.2 m) considered and with the transverse subdivisions removed from the main deck, however, capsizing did occur after a period of about one hour exposure to the waves because of the accumulation of water on the car deck. The sea state with 4 m significant wave height and peak period of 8 s is the most onerous due to maximum relative motions at the damage opening, dominated by the heave motions. Relative motions at the damage opening are critical as regards the accumulations of water on the car deck. For this ship the heave motion governs the relative motions. Long duration testing, with a period of well in excess of half an hour full scale, may be necessary to ensure whether or not a ship will survive, especially when progressive flooding takes place through a complex geometry and a trend showing a progressively increasing mean heel can not be discemed. . For this ship the Stockholm Agreement calculation method resulted in the requirement for subdivision on the main ro-ro deck for the damage scenario with lower hold intact. The model tests, however, suggest that the ship (with low GM) will survive the most onerous sea state, so that in this case the model test results and calculation method are not equivalent, the calculation method resulting in more conservative requirements.

The feasibility of the lower hold concept for ro-ro passenger ferries has been proven in several recent newbuildings. Maximised lower hold with symmetrical position within the huil can be arranged with electric power plant machinery coupled with pod propulsion. Water on deck requirements can best be met without any flood preventing doors and bulkheads with side casing configuration on the main deck. Maximised size and drive through principle for loading and unioading guarantee easy and efficiënt operation.

REFERENCES

1. Journée, J.M.J., Vermeer, H. and Vredeveldt, A.W., "Systematic model experiments on flooding of two ro-ro vessels", Proceedings STAB'97 Conference, Vol. II, Varna, Sept. 1997

2. Vredeveldt, A.W. and Uwland, J.J., "Damage stability, A review of research in the Netherlands", TNO Report no. 97-CMC-R0291, Delft, July 1997

3. Van 't Veer, R. and De Kat, J.O., "Experimental and numencal investigation on progressive flooding and sloshing in complex compartment geometries", Proc. STAB 2000 Conference, Launceston, Feb. 2000

4. De Kat, J.O., "Dynamics of a ship with partially filled compartment", Proc. Second International Workshop on Stability and Operational Safety of Ships, Osaka, Nov. 1996.