A RISK MODEL FOR ASSESSING THE SHIP SINKABILITY IN UNCERTAIN CONDITIONS

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1 Mirosław Gerigk Gdańsk University of Technology A RISK MODEL FOR ASSESSING THE SHIP SINKABILITY IN UNCERTAIN CONDITIONS Abstract: The paper concerns the ship safety in daaged conditions. The existing ethods of safety assessent of ships in daaged conditions are devoted to design and are very difficult to apply in operational conditions. A ethod based on the risk and safety assessent of ships in daaged conditions is briefly presented in the paper. The level of risk is the easure of safety. The ethod applies the ship perforance approach together with the risk assessent. The risk analysis is based on application of the atrix type risk odel. The risk odel enables to take into account the uncertainties connected with the possible scenarios of an accident consisting of cobinations of the hazards, interediate events, additional events (releases) and consequences. Keywords: safety of ships in daaged conditions, ship salvage, risk assessent. INTRODUCTION The paper presents a few probles regarding a ethod of risk and safety assessent of ships in daaged conditions where the key issue concerns the risk odel. This odel should take into account the uncertainties connected with the possible scenarios of an accident. The scenarios consist of cobinations of hazards, interediate events, additional events and consequences. The research is connected with developent of a perforanceoriented risk-based ethod for assessent of safety of ships in daaged conditions []. The current ethods of assessent of safety of ships in daaged conditions are ainly based on the regulations included in the SOLAS convention (Chapter II-) [][3]. These ethods are ore directed towards solving the design probles than those connected with operation or salvage. It follows fro any reasons. First of all, these ethods are prescriptive in their character. Generally, these ethods are based on the probabilistic approach to safety but in soe cases the sei-probabilistic coponents are included. These ethods do not take into account all the possible scenarios of an accident. Application of soe of these ethods to certain types of ships e.g. car-carriers, ro-ro vessels or passenger ships ay lead to insufficient level of safety or provide unnecessary design or operational restrictions.

2 The following ethods rely on the regulations included in the SOLAS convention and ay be considered for application to the design, operational and salvage purposes. The first ethod is based on the holistic risk odel for the assessent of safety of ships in daaged conditions which can presented as follows [4][5]: R=P c x P c/fdpe x P c/fdpe/ns x P c/fdpe/ns/tts x C () where P c - probability of collision (hazard); P c/fdpe - probability of flooding having the ship hit fro given direction at data position with given extent conditional on collision; P c/fdpe/ns - probability of not surviving conditional on having flooding when the ship is hit fro given direction at data position with given extent conditional on collision; P c/fdpe/ns/tts probability of given tie to sink conditional on not surviving conditional on having flooding when the ship is hit fro given direction at data position with given extent conditional on collision; C consequences regarding the fatalities, property (cargo, ship) and/or environent. The second ethod is based on concepts of : casualty threshold, tie to capsize and return to port, where the basic ship safety objectives have been divided into three categories [6]: -category I - vessel reains upright and afloat and is able to return to port under own power (RTP Return To Port); -category II - vessel reains upright and afloat but unable to return to port under own power and is waiting for assistance (WFA Waiting For Assistance); -category III - vessel likely to capsize/sink and abandonent of the ship ay be necessary (AS Abandonent of the Ship). The third ethod is based on a concept of an absolute survivability where the Safe Return to Port (SRtP) attained subdivision index A SRtP should be calculated according to the ship residual stability characteristics [7]: A SRtP = 0,4 A SRtP,s + 0,4 A SRtP,p + 0, A SRtP,l () where the subdivision indices A SRtP,s, A SRtP,p and A SRtP,l regard the subdivision (s), partial (p) and light ship (l) loading conditions. The A SRtP,s, A SRtP,p and A SRtP,l indices should be calculated according to the following forula [7]: n A SRtP,lc = i= p i s SRtP,i (3) where lc loading index (lc = s, p, l); p i - probability that only the copartent or group of copartents under consideration ay be flooded, as defined in regulation 7- [][3]; s SRtP,i - probability of survival after flooding the copartent or group of copartents under consideration, in the final stage of flooding only, as defined in [7].

3 . THE NEW METHOD OF RISK AND SAFETY ASSESSMENT OF SHIPS IN DAMAGED CONDITIONS The ethodology where the risk-based design and a foralized design ethodology were integrated together in the design process with the prevention/reduction of risk as a design objective, along with the standard design objectives was introduced by Vassalos and the others [5][][]. The proposed ethod is a kind of perforance-oriented risk-based procedure which enables the risk and safety assessent at the design stage, in operation or during the ship salvage. Within the ethod the holistic approach to safety assessent of ships is applied. It is based on application of a risk odel which includes all the possible scenarios of events during an accident. The ethod takes into account an influence of design and operational factors on safety and safety anageent related factors as well. The ethod is based on ipleentation of the syste integrated approach to safety, eleents of Foral Safety Assessent FSA, ship perforance-oriented approach and risk-based approach to safety []. For the ship perforance evaluation the statistics, investigations using the physical odels and nuerical siulation techniques can be applied. The ship perforance evaluation enables to deterine the interediate events, additional events (releases) and consequences and risk of each accident scenario. The ai is to achieve an adequate level of risk by reducing the risk if necessary. Providing a sufficient level of safety based on the risk assessent is the ain objective. It is either the design, operational or organizational objective. Then, safety is not a liitation any ore, existing in the regulations. It is just the objective. The easure of safety of a ship in daaged conditions is the level of risk. The proposed ethod ay be used at any ship s life circle, including safety assessent of the ship during a catastrophe. Soe eleents of the ethod can be used for safety assessent of different eans and systes of seaborne transportation. In the future the ethod can be useful in elaboration of a new ethodology of safety assessent of ships, which bases on the application of risk analysis. The ethod is based on the following steps: setting the objectives, hazard identification, scenarios developent, risk assessent, risk control (prevention, reduction), selection of designs (operational procedures) that eet the objectives. The structure of the ethod is presented in []. 3. THE RISK ANALYSIS USING THE MATRIX TYPE RISK MODEL The safety case considered in the paper regards the safety of a ship in daaged conditions when the ship skin is daaged due to the following hazards: collision, grounding, stranding or another reason. The risk and safety assessent for a ship in daaged conditions starts fro the odeling of the risk contribution tree. For each hazard a separate event tree should be odeled using the Event Tree Analysis ETA. Generally, sixteen safety functions have been used for each event tree fro the ship safety in daaged conditions point of view. These functions are as follows []: function 3

4 (avoiding the hazard); function (hull skin daage (flooding)); function 3 (position and extension of daage); function 4 (equalization of the ship heel at the preliinary stage of flooding); function 5 (loss of the ship stability at the preliinary stage of flooding); function 6 (loss of the ship stability during the interediate stages (and phases) of flooding; function 7 (loss of the ship stability at the final stage of flooding); function 8 (loss of the ship floatability at the final stage of flooding); function 9 (ship is waiting for assistance); function 0 (ship returns to port under own power); function (ship returns to port by taking in tow); function (ship is continuing the ission); function 3 (ustering and abandonent of the ship (evacuation)); function 4 (SAR action); function 5 (fire and/or explosion); and function 6 (eergency cargo unloading (pollution of the environent)). A position of each function can easily be found in Figure where the basic event tree for the safety assessent of ships in daaged conditions is represented. It is iportant to notice that each event tree ay have a dynaical character. The holistic approach to ship safety has been applied. According to this approach two ajor assuptions have been done. First that the syste failures can be either the hardware, software, organizational or huan failures. The second assuption was that the risk odel for assessent of safety of ships in daaged conditions should be the holistic risk odel. The risk associated with the different hazards and scenario developent was estiated according to the well known general forulae []: R i = P i x C i (4) where P i probability of occurrence of a given hazard; C i consequences following the occurrence of the data hazard and scenario developent, in ters of fatalities, injuries, property losses and daage to the environent. As it is entioned in () the risk odel ay have four different kinds of losses regarding the huan fatalities (HF), cargo and ship losses (CS), environent pollution (E) and financial losses ($) (C = C HF/C, C CS/C, C E/C, C $/C ). The risk analysis requires to calculate the conditional probabilities regarding the initial events ZI i, ajor events (hazards) ZG j, interediate events ZP k and final events ZK l which can be treated as consequences. The basic atheatical forulae are as follows []: P(ZI) = P(ZI i ) = [ ( ZI ),P( ZI ),,P( )] P for i= to n (5) ZIn for j= to. P ZG / ZI P(ZG j /ZI i )= P ZG / ZI P ZG / ZI ( ) P( ZG / ZI ) P( ZG / ZI ) ( ) P( ZG / ZI ) P( ZG / ZI ) ( ) P( ZG / ZI ) P( ZG / ZI ) n n n (6) 4

5 Minor daage Return to port under own power Minor daage Return to port by tow Pollution of environent Towing of daaged ship Ship sinking Loss of floatability Consequences: -fatalities, -property loss, -pollution of environent Ship sinking Consequences: -fatalities, -property loss, -pollution of environent Continuing the ission Return to port under own power Return to port by tow Waiting for assistance Evacuation Loss of floatability SAR activity Fire or explosion hazard Loss of stability Final stage of flooding Loss of stability Cargo and/or ballast unloading Interediate stage of flooding Loss of stability Preliinary stage of flooding Waiting for assistance Cargo leakage Cobinations of flooding: -wing copartents flooding -wing and internal copartents flooding -daage below the waterline -daage below and above the waterline Daage Minor daage ETA Collision Grounding Stranding Hazards FTA tree Fig.. A basic event tree for the salvage purposes 5

6 P ZP / ZG P(ZP k /ZG j )= P ZP / ZG P ZP / ZG for k= to. ( ) P( ZP / ZG ) P( ZP / ZG ) ( ) P( ZP / ZG ) P( ZP / ZG ) ( ) P( ZP / ZG ) P( ZP / ZG ) (7) for l= to. P ZK / ZP P(ZK l /ZP k )= P ZK / ZP P ZK / ZP ( ) P( ZK / ZP ) P( ZK / ZP ) ( ) P( ZK / ZP ) P( ZK / ZP ) ( ) P( ZK / ZP ) P( ZK / ZP ) (8) Because of the above atheatical odel used the entire risk odel is called as the atrix type risk odel. The risk odel enables to consider any possible scenarios of an accident using the event tree like presented in Figure. In the case when the additional events occur the probability of occurring the given consequences PoC(C i ) can be calculated according to the forulae presented in []. The typical additional events ay concern the water on deck, air cushions, cargo leakage, additional heeling occurrences, and passenger behavior. 4. APPLICATION OF THE METHOD AND RISK MODEL FOR THE SHIP SALVAGE The ship accidents at sea ay lead to the loss of life, loss of properties (ship and cargo) and pollution of environent. The odern approach to safety of seaborne transportation requires to apply the so-called life-circle approach by integrating the design for safety, safe operation and safe salvage ethods. Most of the salvage activities have been associated with using the rules of thub without a support based on the safety assessent process. The effective and safe ship salvage requires to develop the ethods, odels and tools for the assessent of risk and safety of ships in daaged conditions. The proposed ethod of risk and safety assessent of ships in daaged conditions together with the atrix type risk odel enables to assess a ship survivability at each stage of flooding including the preliinary, interediate and final stages of flooding. Such an assessent is necessary to predict any further deterioration of the daage condition. It ay be connected with further flooding of copartents and ship listing increase. This assessent also regards predicting how long it would take the ship to capsize or sink. If the ship survives flooding (a ship reains upright (or heeled) and afloat) and is unable to return to port under own power it is necessary to solve the probles associated with the towing when waiting for an assistance. The nuber of towing points and towing speed in 6

7 the cal and rough weather conditions should be evaluated. During the towing the risk and safety assessent of the ship should be peranently conducted. Two different exaples of a ship perforance data (in the physical odel scale) regarding the ship roll function in tie doain and a concept of the ALARP risk evaluation criteria applied for assessent of safety of the ship in daaged condition are presented in Figure. Possible evacuation zone Non-tolerable risk level ALARP region Tolerable risk level Tie Possible evacuation zone Non-tolerable risk level ALARP region Tolerable risk level Tie Fig.. Two exaples of a ship perforance data (a ship roll in daaged conditions (in degrees) in tie doain (in seconds)) and ALARP risk acceptance criteria application 5. CONCLUSIONS Soe eleents of the ethod of risk and safety assessent of ships in daaged conditions together with the atrix type risk odel are presented in the paper. The proposed risk odel is uch ore coplicated than the odel given by Skjong et al. [4][5][][]. By using (5), 7

8 (6), (7) and (8) the proposed risk odel enables to estiate the risk level for all the possible scenarios of an accident when ship is in daaged conditions. The current research is associated with further developing the risk odels necessary for the ship perforance-oriented and risk-based assessent. Fro the practical point of view the research should bring a odel for the coputer siulation of the ship salvage process. Bibliography. Gerigk M.: Kopleksowa etoda oceny bezpieczeństwa statku w stanie uszkodzony z uwzględnienie analizy ryzyka. Monografie 0, Wydawnictwo Politechniki Gdańskiej, Gdańsk 00.. IMO (005): Report of the Maritie Safety Coittee on Its Eightieth Session. MSC 80/4/Add., London IMO (008): 4. Jasionowski A., Vassalos D.: Conceptualising Risk. Proceedings of the 9th International Conference on Stability of Ships and Ocean Vehicles STAB 006, Rio de Janeiro, 5-9 Septeber Skjong R., Vane E., Rusas S., Olufsen O.: Holistic and Risk Based Approach to Collision Daage Stability of Passenger Ships. Proceedings of the 9 th International Conference on Stability of Ships and Ocean Vehicles STAB 006, Rio de Janeiro, 5-9 Septeber Vassalos D.: Safe return to port. Seinar for the 50 th session of the IMO SLF Sub-Coittee, London, May IMO (009): Stability and Seakeeping Characteristics of Daaged Passenger Ships in a Seaway When Returning to Port by Own Power or Under Tow, A survey of residual stability argin, Subitted by Gerany. SLF 5/8/, London, 6 October Maritie Magazine (Nasze Morze, in Polish): What ay happen to the Baltic Sea (Co z ty Bałtykie, in Polish). Volue (4), Deceber Mohaghegh Z., Mosleh A.: Incorporating organizational factors into probabilistic risk assessent of coplex socio-technical systes: Principles and theoretical foundations. Journal of Safety Science, Vol. 47, 009, pp Shipbuilding Magazine (Budownictwo Okrętowe, in Polish): It can be a catastrophe (Może być strasznie, in Polish). Volue 3 (548), March SSRC (009): Vassalos D.: Passenger Ship Safety: Containing the Risk. Marine Technology, Vol. 43, No. 4, October 006, pp MODEL RYZYKA OCENY ZATAPIALNOŚCI STATKU W WARUNKACH NIEPEWNOŚCI Streszczenie: Artykuł dotyczy bezpieczeństwa statków w stanie uszkodzony. Obecne etody oceny bezpieczeństwa statków w stanie uszkodzony są ukierunkowane na projektowanie i trudno je zastosować w warunkach operacyjnych. Opisano etodę oceny ryzyka i bezpieczeństwa statku w stanie uszkodzony, w której iarą bezpieczeństwa jest pozio ryzyka. Zastosowano podejście oparte na ocenie zachowania statku i ocenie ryzyka wypadku. W analizie ryzyka wykorzystano acierzowy odel ryzyka, uożliwiający analizę scenariuszy wypadku przy uwzględnieniu niepewności związanych z wystąpienie zagrożeń, zdarzeń pośrednich, zdarzeń dodatkowych i konsekwencji wypadku. Słowa kluczowe: bezpieczeństwo statków w stanie uszkodzony, ratowanie statków, ocena ryzyka 8