Stability Control on Ro-Ro Passenger Ships as the Main Factor of Ship’s Safety

: On Ro-Ro passenger ship the wide spectrum of captain’s responsibilities should be taken into consideration. One of the important responsibilities is the ship’s stability examination. The other measures as the ship’s condition, wind on ship with large windage area, rolling characteristics, severe seas etc., are important for ensuring the safe operating of ship, to minimize the risk to the ship, to the personnel and passengers on board, and to the environment. The international convention for the Safety of Life At Sea – (SOLAS 90) make into fact the continual development of safety standards in the 111 years since the sinking of the Titanic. Important enhancement stability, operational requirements and damage stability requirements were made as a consequence of several disasters at sea: “Torrey Canyon” in 1967, “Herald of Free Enterprise” in 1987 (183 dead), “Exon Valdez” in 1989, “Braer” in 1993, “Estonia” in 1994 (892 dead). In particular the dramatic loss of the Ro-Ro/Passenger vessels M/F “Herald of Free Enterprise” in 1987, and M/F “Estonia” in 1994, respectively, has resulted in the international regulation requiring enhanced damage stability requirements for this type of vessels, and in more stringent damage stability criteria adopted on a regional basis by Northern European countries (Stockholm Agreement, 1977).


Introduction
Ro-Ro / Passenger vessels have a supplementary damage stability regulation.These new regulations based on a wide range of related design parameters, such as the number, positioning and local optimization of transverse bulkheads, the presence and position of longitudinal bulkheads below the main vehicle deck, the presence of side casings, and the height of the main deck and double bottom.The effects of water on deck and of operational parameters as draught, Centre of gravity and trim.The current damage stability standard is that a Ro-Ro vessel should be able to sustain damage to any two adjacent compartments.

Damage Stability Criteria
In northern European countries, an increased standard of damage stability calculations is applied to existing Ro-Ro vessels, known as the Stockholm Agreement, which requires either fulfilment of the deterministic standards of SOLAS' 90 with an additional height of water on deck (maximum of 50 cm), or the demonstration, by means of model experiments, that the Ro-Ro vessel can survive the sea state in a damaged condition.

1.
Range of positive part of the GZ curve >10 DEG;

2.
The area under the righting lever curve ≥ 0.015 MRAD;

6.
Maximum GZ ≥ (heeling moment) / (dis-placement) + 0.04 m, taking into account the greatest of the following moments: • The wind pressure of 120 N/m², • The crowding of all passengers to-wards one side of the vessel, • The launching of a fully loaded davitlaunched survival crafts on one side.
The presented paper describes the results of practical use of the stability calculations and damage stability calculations for the Ro-Ro/Passenger vessel M/F "Polonia", serving in Southern Baltic.
The said vessel is shown in Figure 1.• Soft nosed and raked stem with bulbous bow, • Transom stern, • Two full length cargo decks of 2 670 m total loading lane length, including lifted car shelves, with 6 railway tracks of effective loading length 740 m on the main deck, • Machinery located aft, with 27.5 m of the engine room length, • Twin controllable pitch propeller propulsion-plant, • 4 x STORK-WARTSILA medium speed main engines of 15 840 KW total, • 3 bow thrusters of 1 600 KW each, • 1 stern thruster of 1 600KW, • Heeling compensation INTERING system, • Three accommodated decks above cargo hold, • Access to main deck via stern door and shore ramp, and via side doors on Port Side mid ships, and Port Side aft the vessel, using shore ramps, • Lifted car deck (shelves) on cargo deck, • Frame spacing of 625 mm.

Main Particulars
Main dimensions:

Calculation of Required Righting Lever
With respect to the criterion 6 of damage stability, the following heeling moments has been calculated for different vessel draughts -5,00 m, 5,50, and 6,20 m:

Moment Due to the Wind Pressure of 120 N/m²
The results of calculations the above moment due to the wind pressure of 120 N/m², has been presented in Table 2.   Taking into account the greater of the above heeling moments, the results of calculation of the righting arms are presented in Table 4.

Table 4. Summary of additional heeling levers
Lever due to the wind pressure: at draught = 5.0 m 0.088 m at draught = 5.5 m 0.081 m at draught = 6.2 m 0.074 m Lever due to crowding of passengers: at draught = 5.0 m 0.098 m at draught = 5.5 m 0.091 m at draught = 6.2 m 0.084 m Lever due to launching of survival crafts: at draught = 5.0 m 0.049m at draught = 5.5 m 0.048 m at draught = 6.2 m 0.046 m Source: M. Szymoński

Stability Calculations
In order to improve the ship safety during the sea voyage, the stability calculations are computerized, getting loading dependent issues, such as results of intact stability, longitudinal strength, and damage stability.Some stability calculations for the presented vessel are described below.

Loading Conditions for Stability Calculations
The selected loading condition for stability calculations has been shown in Figure 3.The results of stability calculations are presented as the curve of righting arms in function of the heeling angle on Fig. 4 and also in Table 5.

Damage Stability Calculations
For the loading conditions described in section 4, two damage situations have been simulated.First of them concerns the flooding of dry spaces and water ballast tank in fore part of the double bottom and section below the main deck of the vessel: dry space (SP01) in double bottom, deep tank (WB02) and dry space (SP03).
The double bottom compartments layout is shown in Figure 5.The water ballast tanks are marked green colour, dry spaces are white, fuel and oil tanks are red and grey, and fresh water tanks are blue.
The results of calculations the hypothetical cases of damage some of double bottom compartments are presented in this paper.
The results have been obtained by use of the vessel's stability calculation software.

Case of Damage in Fore Part of the Vessel
In this case only 3 compartments in fore part of the vessel has been damaged, as per Fig. 6.The vessel in this state has a good stability and will float in equilibrium position.The damage stability complies with criteria of SOLAS'90 and Stockholm Agreement.The results of stability calculations for the above damage case, are presented in Figure 7.In this case the main parameters of ship's stability are: Metacentric heigh corrected GMc = 2.9 m.Righting arm GZ max = 0.54 m with no water on the train deck.
With water on the Ro-Ro deck the righting arm GZ max = 0.53 m.
The water level on Ro-Ro deck is 0.03 m.The Freeboard = 1.19 m.
When the water occurs on the Ro-Ro deck, the stability results correspond not only with criteria of SOLAS '90 but with Stockholm Agreement, too.The vessel's damage, presented in this point corresponds the situation of the stability loss when the 0.13 m of water accumulated on Ro -Ro deck.The damaged volumes of the vessel are located in the double bottom and below of the main deck of the midships, as it has been shown in Figure 8.
With no water on the Ro -Ro deck the vessel has a small stability margin, but she will float in equilibrium position.The stability is however not sufficient to comply with criteria of SOLAS'90, as presented in Figure 9.

Conclusion
Results of stability and damage stability calculations, presented in this paper are getting knowledge of practice in simplified stability information for the master.It's a very important element of safety management on the Ro -Ro Vessel.
Author of the paper, having the practice as the master of M/F "Polonia", is also experienced that absolute safety doesn't exist, and a large number of safety measures are difficult to execute.
The results of the above calculations are giving proof of the significance of simplified stability information for the master and tools for fast verification: if a vessel sinks or staying afloat.
The analyse of case, regarding to the damage of the fore spaces of the vessel, is testifying that the vessel is staying afloat, even if 40 tons of sea water is flooding the main deck.The stability calculations presented in this paper were performed by using the certified vessel's software for loading and stability calculations according to SOLAS 2009 andStockholm Agreement (1997).

Figure 2
Figure 2 The distribution of windage areas for draught of 5.00 meters 074 mMoment Due to Crowding of All Passengers Towards One Side of the Vessel as perTableThe above calculations were made for the case of launching of survival crafts in one side of the vessel.It should be noted that the vessel is supplied with two Marine Evacuation Systems (RDF Ltd) for both, Port and Starboard side, each consisting of dual track evacuation slide, embarkation (landing) platform and 14 liferafts (RDF Ltd), for 50 persons each.

Figure 4 .
Figure 4.The Curve of Righting Arms Source: M.Szymoński

Figure
Figure 6.Horizontal Section of Flooded Fore Part Compartments Source: M.Szymoński

Figure 7 .
Figure 7.The Results of Damage Stability Calculations for the Above Case Source: M.Szymoński

Figure 9 .
Figure 9.The Results Stability Calculations for the Case of Flooding the Midship's Compartments as per Figure 8 Source: M.Szymoński The next case corresponds to the sinking of the vessel due to the damage of four spaces in the midship's.The 195 tons of the sea water is coming to flood the main, open un-subdivided deck and results in the stability loss.

Table 5 . The Righting Arms in Function of Heel
Source: M.Szymoński