Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
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Development of “Stability Recovery System”
that Maintains Ship Stability at the Time of Damage- Ensuring Cargo Loading Capacity and Easing
Restrictions on the Hull Form -KUNIAKI YAMATO*1

YOKI UEMURA*2

MASANORI ONZUKA*1

Due to the recent introduction of tighter damage stability regulations, requirements on theresidual stability under damaged conditions have become increasingly strict. In order to meet thetougher regulations, Mitsubishi Heavy Industries, Ltd. (MHI) has developed a stability recoverysystem to be installed on Roll-on/Roll-off (RoRo) ships, Pure Car & Truck Carriers (PCTCs) andRoPax ferries, to which the new regulations are applied. By equipping these ships with the stability
recovery system, in the event that the ship is damaged during the voyage, seawater accumulating in
the lower car section is drained into the void space at the ship bottom, quickly lowering the centerof gravity and restoring the stability lost as a result of the damage. This Technical Review provides
an overview of the stability recovery system, its effects, and the implementation thereof.

|1. IntroductionDue to the complete revision of the International Convention for the Safety of Life at Sea(hereinafter referred to as the SOLAS Convention) in Chapter II-1, the regulations on ship damage
stability which came into effect on January 2009 (hereinafter referred to as the damage stability
regulations) have been strengthened. To comply with the tougher regulations, MHI has developed
its unique and outstanding stability recovery system which reduces the risk of ship capsize at thetime of damage, and which has been patented in Japan*1 and overseas (Korea*2 and the U.S.A.*3)*1 Patent No.: 4814120*2 Patent No.: 1152059*3 Patent No.: 8087370

The stability recovery system is to be installed on three types of newly-built ships, to which
the damage stability regulations are applied, such as RoRo ships, PCTCs and Ropax ferries. Theseships have a large superstructure above the waterline in order to carry multiple cars. Their commonfeatures also include a slim shape below the waterline to achieve high-speed navigation. MHI
aimed to develop a low-cost and reliable system to be installed on these types of ships. This
Technical Review provides an overview of the stability recovery system, its effects, and examplesof the commercialized products thereof.

|2. Tightened damage stability regulations
Before the revisions, damage stability regulations based on the probabilistic concept wereapplied to cargo ships, including RoRo ships and PCTCs. In terms of passenger ships including
Ropax ferries, damage stability regulations based on the deterministic concept used to be applied.
In response to the recent revisions, the probabilistic damage stability regulations were also applied
to passenger ships, ensuring the consistency of the application of the rules.
The basic concept of the probabilistic concept is that the assessment is based on theprobability that only the watertight compartment or group of compartments of a ship underconsideration may be flooded, and the residual stability in an equilibrium condition after the*1

Ship & Ocean Engineering Division, Shipbuilding & Ocean Development*2

Manager, Ship & Ocean Engineering Division, Shipbuilding & Ocean Development

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)34

watertight compartments became flooded as the probability of survival after the flooding of thecompartment or group of compartments under consideration. Assuming various damage cases, theprobabilistic concept demonstrates that the Attained Subdivision Index, which is a value obtainedby adding the products of individual damage probabilities and survival probabilities, is greater thanthe Required Subdivision Index, which is obtained based on the formulae given to cargo ships andpassenger ships, respectively. In the deterministic concept, the assumed damage area ispredetermined, and the residual stability for every level of damage needs to satisfy therequirements. On the other hand, as the probabilistic concept assumes various damage areas, even
if the level of the residual stability after damage is low in some cases, it is only necessary to present
that the ultimate Attained Subdivision Index, based on all the damage conditions, is greater than theRequired Subdivision Index.
The new rules based on the probabilistic concept are basically the same as the conventional
damage stability regulations of the SOLAS Convention for cargo ships. However, the number ofdraft cases used in the calculation of the Attained Subdivision Index has been increased. Otherchanges include the revision of the probability of flooding, an increase in the Required Subdivision
Index, and so on, in order to tighten the conventional rules even for the same cargo ships. As forpassenger ships, in order to satisfy the requirements in the deterministic damage stabilityregulations, in addition to the probabilistic damage stability regulations, even in a worst-casescenario, in which a certain degree of damage has been done to the hull and the bow section, asufficient level of residual stability needs to be ensured.

|3. Overview of stability recovery system
The stability recovery system focuses on forcibly flooding seawater entering the hull into thevoid space near the ship bottom (downflooding), in the event that the ship is damaged during thevoyage, quickly lowering the center of gravity and restoring the stability lost as a result of thedamage.
An outline of the stability recovery system is illustrated in Figure 1. It consists of the void
space into which seawater is drained, seawater inlets with a watertight hatch, leading to the void
space provided on the car deck, and a stability monitoring and control system that monitors
flooding inside the hull and controls the open-close movement of the watertight hatch.

Figure 1

Conceptual drawing of stability recovery system

As shown in Figure 2, the void space into which seawater is drained is located at the bottomof the ship for common use with other elements such as fin stabilizer rooms, duct keels (keels laidwith a duct structure), and ballast tanks with a load limit. The void space is ensured near the shipbottom by locating it in the section which also has other functions. Apart from downflooding
through the seawater inlets, as shown in Figure 3, air vent pipes are provided as necessary inside
the hull, in order to achieve quick and safe water injection and the lowering of the center of gravity
at the time of damage.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
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Figure 2 Downflooding through watertighthatchesFigure 3 Downflooding through air ventpipes

Furthermore, by equipping a ship with the stability recovery system, backup stability in thecase of damage is no longer necessary, avoiding the following flaws:
・ A decrease in propulsion performance and fuel efficiency, due to the widening of
the body for improvement of the initial stability・ A decrease in vehicle weight due to the loading of ballast water for reduction of
the center of gravity
・ A decrease in the number of cars on board and the blocking of the smooth flow of
traffic, due to smaller subdivisions of the lower cargo hold for reduction of secondary
flooding
・ A limitation to the number of cars on board for keeping the center of gravity down during
navigation|4. Implementation of the stability recovery system to actual ships
The ship with the stability recovery system deployed for the first time was a 170-meter-long
RoRo ship, which is capable of a speed of 23 kt with a load of about 170 trailer chassis and 100cars. Compared with the conventional type of the same grade, the energy saving ship featuresapproximately 10% less energy consumption. It was built at MHI’s Shimonoseki Shipyard(Shimonoseki, Yamaguchi Prefecture) and was delivered in March 2013. This ship is equippedwith the stability recovery system, which, in addition to ensuring available cargo capacity and thesmooth flow of traffic of the conventional type before the revision of rules, has achieved greater
energy-saving performance by allowing increased flexibility of the hull form.
Principal particulars of the new RoRo ship are listed in Table 1 shown below.
Table1

Principal particulars of RoRo shipwith stability recovery system

Flag
Class
Length (b.p.) ×Breadth × Depth (m)Draft(scan.) (m)Gross tonnage (International)
Service speed (kt)Main engine
Passenger capacity
Cargo capacity
Other equipment

JapanNK
158.0 × 27.0 × 23.27
6.70
Approx. 23,300
23.0
9S50ME-C8.2×1 set
13,920kW×127.0rpm
12 persons
172 Trailer chassis
100 Cars
Bow thruster × 1
Stern thruster × 2
Fin stabilizer × 1 set

The whole view of the new ship is presented in Figure 4, and a downflooding hatch provided
on the ship’s lower car deck is shown in Figure 5.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
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Figure 5 Downflooding hatch at
the lower car deck
Figure 4 RoRo ship equipped with stability recovery
system

|5. Conclusion
MHI has accumulated various technologies and experience based on considerable
achievements in building RoRo ships, PCTCs and Ropax ferries. The development of the newtechnology, the stability recovery system, has enabled MHI to design and build ships with a higher
level of safety, reliability, economic efficiency and environmental performance. MHI will strive toclearly understand customer needs and the trends of rules and regulations, which will growincreasingly diverse in the future, in order to enhance customer satisfaction and to develop newships with high added value. MHI will also make continuous marketing efforts by proactively
offering solutions based on our unique technologies underpinned by patents, such as the stability
recovery system.

References
1. Development of Stability Recovery System, KANRIN,No.43( 2012) pp.70
2. Shipbuilding and Marine Engineering in Japan 2013, The Japan Ship Exporters' Association
3. Masanori Onzuka, MHI’s New System Enhancing Damage Stability, INTERFERRY37th Annual
Conference 2012, Dubai, U.A.E.