Procedia Coputer Science Volue 51, 2015, Pages 2397 2405 ICCS 2015 International Conference On Coputational Science 3D siulation of ship otions to support the planning of rescue operations on daaged ships J.M. Varela 1*, J.M. Rodrigues 1 and C. Guedes Soares 1 1 Centre for Marine Technology and Ocean Engineering (CENTEC), Instituto Superior Técnico, Universidade de Lisboa, Lisbon, PORTUGAL Also Research Center for High Perforance Coputing, ITMO University, Saint-Petersburg, Russia *varela@centec.tecnico.ulisboa.pt, iguel.rodrigues@centec.tecnico.ulisboa.pt, c.guedes.soares@centec.tecnico.ulisboa.pt Abstract The paper describes a software syste to siulate the ship otions in a crisis situation. The scenario consists of a daaged ship subjected to wave excitation forces generated by a rando sea state. The siulation is displayed in an interactive Virtual Environent allowing the visualization of the ship otions. The nuerical siulation of the sea surface and ship otions requires intensive coputation to aintain the real-tie or even the fast-forward siulations, which are the only ones of interest for these situations. Dedicated tools to analyse the ship behaviour in tie are also described. The syste can be useful to evaluate the responses of the ship to the current sea state, naely the aplitude, variations and tendencies of ship otions, and help the planning and coordination of rescue operations. Keywords: Siulation syste, ship otions siulation, intensive coputation, real-tie siulation 1 Introduction The planning and anageent of rescue operations in the aritie environent is a vital task in post-accident situations. Typical rescue operations are firefighting, passenger evacuation fro ungoverned ships during fire or flooding situations, and the towing of daaged ships to ports. The first two are obviously ore urgent, however, the towing ay also be considered an operation that ust be perfored rapidly before the ship status or environental conditions degrade even ore. Due to the adverse nature of the aritie environent, these operations always involve a certain degree of risk both for the rescued and for the rescuer. Moreover, when there are huan lives in danger, the pressure for iediate rescuing increases the probability of new accidents. Fro Papanikolaou et al. (2015), it can be seen that any serious accidents still occur even when considering only the last 20 years. The ajority of accidents lie on the category of hull or achinery daage, which iplies the Selection and peer-review under responsibility of the Scientific Prograe Coittee of ICCS 2015 c The Authors. Published by Elsevier B.V. 2397 doi:10.1016/j.procs.2015.05.416
3D siulation syste for daaged ships risk of flooding and consequent towing (if the ship is not lost). Also, high overall frequency of accidents occur for ships carrying passengers, naely for cruise ships and therefore passenger evacuation is a top priority operation. When rescue operations are perfored in open waters, the sea state is a critical factor. For passenger evacuation, the aplitude of ship otions influences the oving capability of the passengers and ay deterine the use of aerial or aritie rescue equipent. In the IMO-MSC/Circ. 909 and IMO-MSC/Circ. 1033, it is stated that ship otions effects on passengers behaviour should be accounted for by a safety factor, however, their influence on the final value of this factor is not clear. The progressive flooding siulation provides an estiation of the reaining tie to perfor the rescue. For towing, it is iportant to evaluate the ship otions along the planned route. Therefore, knowing in advance how the daaged ship will behave in waves is very useful to plan the operations and coordinate eergency teas. Coputer siulations for these objectives are a well-recognized and efficient tool to analyse this type of scenarios. The increased power of odern siulation technologies allows the creation of coplex systes that cobine siulation odels of different areas such as seakeeping, anoeuvrability, progressive flooding or fire propagation into interactive eergency training siulators. A coputer syste to siulate and analyse the otions of daaged ships in well-defined sea states has been under developent (Varela and Guedes Soares 2007, 2014, 2015). Siulation-based odules are developed, ipleented and integrated into a syste which allows the 3D visualization of the ship otions in real-tie and in fast-forward siulations. The use of 3D visualization and Virtual Reality techniques in coputer siulations and expert systes to support eergency situations on-board ships dates back to the nineties with the research work in the Naval Research Laboratory (Tate, 1991; Tate et al., 1997). Soe research work has been published recently, regarding the developent of coputer siulation systes that use Virtual Reality techniques to support training and planning in eergency situations. In Baldauf et al. (2012), siulation-based odels have been integrated into training units and courses progras to create a siulation laboratory with cobined ship handling, safety and security test facilities. The siulator enables officers and crew to use safety equipent and available eergency systes while oving around inside the vessel. Varela et al. (2014) have integrated a progressive flooding algorith into a Virtual Environent, creating a Decision Support Tool for ship flooding eergency response. The Virtual Environent is used to setup, control and visualize the siulation properly. Briano and Caballini (2011) describe a siulator for training logistic operators in ports, e.g. crane operators and truck drivers. In their work, Virtual Reality siulation odels devoted to train Straddle Carrier, Quay Crane and Mobile Harbour Crane operators are ipleented and eergency situations such as fire or falling of containers are also considered in the siulation. In section 2, the syste design and software architecture is depicted, including the siulation odules, the workflow and dataflow of the siulation, and the ain functionalities of the syste. The next three sections describe the ain siulation odules, their underlying atheatical odels and the approach taken for the nuerical siulations. Finally, conclusions are taken in section 6. 2 Syste design The syste was developed using the object-oriented prograing paradig in conjunction with a odular approach to the syste coponents. It is coposed by four ain odules: the Data Input, the Sea Surface, the Ship and the Graphics Engine odules. The Ship odule is then coposed by the Ship Motions and Flooding siulation odules as presented in Figure 1. 2398
3D siulation syste for daaged ships Figure 1 The syste is coposed by three siulation odules, one data input odule and a 3D Graphics Engine The sea state is given in the for of a directional wave spectru in frequency doain. Different approaches to estiate the sea state include ethods based on the stationary ship otions easureents (Pascoal and Guedes Soares, 2008), on the arine radar iaging of the sea surface (Nieto Borge and Guedes Soares 2000) or on wave-induced buoy displaceents (Jessen and Herbers, 2012). The Sea Surface odule is then responsible for coputing the free surface elevation according to the defined sea state. The other input is the ship daage. For this case, hull daages, naely holes that lead to the flooding of hull copartents are provided as well as the water level at copartents already flooded (Flood Condition in Figure 1). The Ship odule contains a set of pre-calculated transfer functions in the frequency doain for different ship speeds, headings, encounter frequencies, flooding situations. Given the wave phases and aplitudes in tie, the ship position is coputed in tie doain based on the transfer functions and on the current ship status. The Flooding odule adds the daage stability coponent into the syste. Fro the current ship status, hull daage and flood condition, the flooding algorith coputes a ship flooded status that is added as a new cargo condition to the calculations of the ship otions on the next cycle. In this case, the floodwater is treated as an added cargo into the ship copartents. The siulation workflow includes three ain phases: the scenario setup, the nuerical siulation and the analysis of the results, with consequent planning and anageent of rescue operations. In ore urgent situations, such as when passenger evacuation is required, the setup of the siulation scenario ust be done as soon as the alar is given. The inforation required is the sea state and the hull daages, naely holes that lead to the flooding of hull copartents. The Graphics API represents the ship status and the sea surface elevation given by the Sea Surface and Ship Motions odules respectively. 2399
3D siulation syste for daaged ships 3 Sea surface siulation The surface elevation at each location is given by the superposition of a large nuber of sinusoids with different aplitudes, frequencies and directions of propagation. This generates an irregular rando sea surface based on very sall steepness sinusoids fro which the induced ship otions can be coputed and non-linear ters can be neglected for cal and oderate sea states. The sea state is defined in the frequency doain by a directional spectru. The odule responsible for the generation of the sea state and siulation of the sea surface receives the spectral function directly fro the wave easureent devices. The discretization of the spectru is based on the ethod developed by Varela and Guedes Soares (2014), which ensures that the sea-state displayed by the real-tie nuerical siulations is very close to the sea state defined by the original spectru. A coordinate transforation is applied to the spectru function to convert it fro the frequencyangle coordinate space to the wave vector space. This is achieved by preserving integral equality between both spaces and according to the substitution rule (Fréchot, 2007). The directional spectru function in the wave nuber space in Cartesian coordinates is given by the following expression: E 1 g k, k S, x y (1) 2k k S is the spectru in the frequency-direction of propagation coordinate space. The discretization is uniforly applied in this coordinate space and depends of the siulation grid. The discretized coponents (wave systes) for a siulation grid with N M points are obtained fro the following expression: where, 2n 2 k, n, (2) L L x y where k n, is the wave vector of the coponent n, in the siulation grid, and n and are integers with bounds N 2 n N 2 and M 2 M 2. The wave vector defines the direction of propagation of the wave syste and the wave nuber of each coponent is given by the length of 2 the wave vector. The dispersion relation for deep waters, gk, is applied. Nuerical siulation of the sea surface in real tie with a relatively large nuber of coponents is still only possible with FFTs. The unifor discretization of the spectru in the wavenuber space allows the use of the IDFFT algorith to copute the su of the sinusoids at each grid point. The coplex FFT based representation of the wave height field at the siulation grid point p n, is given by the su of sinusoids with coplex aplitudes: ~ ikn, pn, h p t h k, t e (3), n, n, n ~ n, point of the siulation grid and h k t n,, where the vectors are evaluated at the aplitude Fourier coponents and are given by the following: i k t k H k, t a k e (4) are the height where ak is the aplitude of the wave syste derived fro the spectru function and k is the rando phase ter. Fro the statistical properties of the wave spectru, the following expression is derived for the wave aplitude: 2400
3D siulation syste for daaged ships 2 k n, k 2 S kn, k dkndk 2S kn, k knk a kn k (6) where k k for unifor discretization of the directional spectru. n 4 Ship otions siulation In general, the nuerical seakeeping proble is solved following the procedure: 1) representation of the natural seaway as superiposition of any regular (haronic) waves; 2) the individual reactions of each ode of the floating body to these haronic waves are deterined; 3) all reactions are superiposed to get the behaviour of the body in waves. In this process it is iplicit that the body reacts to waves independently, which eans that the reactions will be sued linearly the sae way one sus various waves linearly, resulting in a linear dependence on the wave height. In reality, this is valid for sall aplitude (linear) waves, with height to length ratio equal or less of 1/20; higher aplitude waves ay also be considered taking into account these prepositions, however acknowledging its liitations. If these siplifications are not ade, the coputations becoe considerably ore expensive. So the nonlinear approach is ostly used only to solve particularly coplex probles related, for instance, to extree otions, such as capsizing investigations - when considering nonlinearities, the tie doain approach is the tool of choice. In general, siple ethods like the ones described in Lewis (1990) and Faltinsen (1993) suffice for obtaining global properties such as ship otions and accelerations. The ethod in Salvesen et al. (1970), coonly known as Strip Theory is also often used to copute the transfer functions for each condition, wave heading and frequency. The ethod of Santos and Guedes Soares (2008, 2009) is based on the strip theory and it accounts for the transient flooding and for the dynaics of the flooded ship. It is used to pre-copute the transfer functions for the nuerical siulations of the ship otions. A set of configurations is ade up constituting the database to be queried by the syste, regarding the real tie wave characteristics. Transfer functions are pre-coputed for specific ship speeds, headings and water levels in the flooded tanks. In the nuerical siulation, these values are evaluated on a per cycle basis. Motion aplitudes and phases are then coputed by linear interpolation between the transfer functions that are closer to the evaluated values. This results in a siulation where the transfer functions are updated continuously when the ship speed, heading or flood conditions changes. Transfer functions are coputed for the full range of encounter angles and wave frequencies defined in the wave spectru. Figure 2 represents a scheatic view of the transfer functions that ust be pre-coputed. Considering the exaple of Figure 2, the nuber of transfer functions, coputed and stored in the database, is given by the following expression: N 6 k t z p (7) where is the nuber of tanks that can be flooded, is the nuber of water levels considered for each tank, is the nuber of forward ship speeds, the encounter angles and the wave frequencies. In this case, the transfer functions are coputed for the 6 otions in the six degrees of freedo. Fro (7) it can be seen that if flooding situations of ore than one tank are considered, the nuber of transfer functions and consequently the size of the database increases substantially. Transfer functions are stored in linked lists ordered by increasing values of water levels, forward speeds and headings. In order to increase the perforance of the lookup procedure in the database during the real tie siulation, the syste stores the position in the list of the previous transfer functions that were used for the interpolations. As the ship speed, heading and water levels in tanks 2401
3D siulation syste for daaged ships suffer sall changes between two consecutive cycles, the transfer functions that will be used in the next cycle will ost probably be the sae, the next or the previous eleent in the list. Therefore, the transfer functions used in the previous cycle will be the starting point for looking up the new ones in the next cycle. Figure 2 Ship transfer functions are coputed for different flooding conditions, forward speeds, ship headings and wave frequencies. 5 Flooding siulation A progressive flooding algorith has been developed with a quasi-static approach, considering still water. The proble of a daaged vessel in waves is, thus, divided into to three fundaental probles: 1) deterination of the aount of water inside each flooded copartent; 2) calculation of the average position of the ship; 3) the behaviour of the ship, considering this quantity due to waves. The progressive flooding algorith is responsible for the first two ites. Wave induced flooding and outflow are not accounted, yet such an approach allows the forulation of a unified schee were tie doain results for flooding progression are coupled with frequency doain predictions of the behaviour of the vessel in waves. This otion will depend on the wave syste, but also on the flooding water additional ass and the inertial aspects resulting fro the oscillation of the free surface inside each flooded copartent. In this work, the first two are accounted for. A literature review and discussion on the reasoning that supports the validity of the aforeentioned approaches, relating to the flooding algorith, ay be found in Varela et al. (2014). Nevertheless, for the sake of copleteness, its ain characteristics are herein listed: 2402
3D siulation syste for daaged ships potential flow is assued Bernoulli equation used for flow calculations zero net flow based nonlinear equations used for accounting with full copartents free surface in flooded copartents reains horizontal exact calculation of pressures and forces, within an adaptive quad-tree eshing schee for considering interfaces, developed by Rodrigues and Guedes Soares (2014) quasi-static otion calculation RK4 solving schee for otion solution at each call Recent ipleentations of this algorith ay be found in Rodrigues et al. (2015), where a set of 90 probabilistically distributed daage configurations has been considered and applied to a shuttle tanker, so as to perfor the loads assessent of the daage structure. In Figure 3, the progressive flooding regarding one of these daage configurations is depicted. Worth of notice is the considerable leaking of the cargo tanks into the neighbouring ballast tanks, leading to a uch lesser final listing of the vessel. Figure 3 On the left the position of the ship with the levels on each copartent are shown; the eshing detail in way of the daage is shown on the right. The white polygons are the intersections of a box shaped box with the structure, defining the borders of the flooding openings. 6 Conclusions A coputer syste which siulates the ship otions of daaged ships in irregular waves has been presented. The ain purpose of the syste is to evaluate the ship otions on a crisis situation, naely to have an indication of the aplitude and acceleration values at various zones of the ship. In conjunction with other inforation, the results of the siulation can help rescue teas to better deterine the safer zones to evacuate passengers and the ost adequate equipent to use. The ethodology used for the sea surface siulation is based on the wave spectru estiated for the casualty scenario. Therefore, the realis of the siulation depends on the accuracy of the wave estiation. This is an iportant factor to have into consideration, because rescue teas that intend to use the syste, ust also have a reliable way of estiating the sea state. The siulation of the ship 2403
3D siulation syste for daaged ships otions is based on pre-calculated transfer functions, which are stored in a database and accessed in real tie during the siulation. This requires that the database ust exist a priori and be available to the rescue teas when the casualty occurs. Although with soe liitations, ainly related with the linear nature of the ethod used to copute the transfer functions, the strip theory ethod used in this work presents very good results. The odular approach adopted for the syste allows changing the coputation ethods in a siulation odule without affecting the others or the overall workflow of the syste, as long as interfaces are aintained. Therefore, new ethodologies for iproving the accuracy of the ship otions are already planned and will be ipleented in a near future. Acknowledgents The first author was funded by the Portuguese Foundation for Science and Technology (FCT - Fundação para a Ciência e Tecnologia) under its annual funding to the Centre for Marine Technology and Ocean Engineering (CENTEC).The second author was funded by the Portuguese Foundation for Science and Technology (FCT), under the grant nr. SFRH/BD/64242/2009. References Baldauf, M., Schröder-Hinrichs, J., Benedict, K., Tuschling, G. 2012. Siulation-Based Tea Training for Maritie Safety and Security, Journal of Maritie Research, 9(3): 3-10. Briano, E., Caballini, C. 2012. Siulation as a support tool for training logistic operators. Recent Researches in Engineering Education and Software Engineering, Rudas, Zahari, Sopian and Strouhal (Eds), WSEAS Press, Cabridge, UK, ISBN: 978-1-61804-070-1, pp. 188-193. Fréchot, J. 2007. Realistic Siulation of Ocean Surface Using Wave Spectra, Journal of Virtual Reality and broadcasting, 4(11). Faltinsen O.M., 1993. Sea loads on ships and offshore structures, Cabridge University Press International Maritie Organization (IMO), MSC Circular nº MSC/Circ. 909, Interi Guidelines for a Siplified Evacuation Analysis on Ro-Ro Passenger Ships, (4 June 1999). International Maritie Organization (IMO), MSC Circular nº MSC/Circ. 1033, Interi Guidelines for Evacuation Analysis For New and Existing Passenger Ships, (6 June 2002). Lewis, E.V., 1990. Principles of Naval Architecture. The Society of Naval Architects and Marine Engineers, Volue III. Nieto Borge, J. C. and Guedes Soares, C. 2000. Analysis of Directional Wave Fields Using X- Band Navigation Radar. Coastal Engineering. 40(4):375-391. Papanikolaou, A., Bitha, K., Eliopoulou, E., Ventikos, N. 2015. Statistical analysis of ship accidents that occurred in the period 1990-2012 and assessent of safety level of ship types. Maritie Technology and Engineering, Guedes Soares & Santos (Eds), Taylor & Francis Group, London, ISBN: 978-1-138-02727-5, pp. 227-233. Pascoal, R., Guedes Soares, C. 2008. Non-paraetric wave spectral estiation using vessel otions, Applied Ocean Research, 30(1): 46-53. Rodrigues, J.M., Guedes Soares, C. 2014. Exact pressure integrations on suberged bodies in waves using a quadtree adaptive esh algorith, International Journal for Nuerical Methods in Fluids, 76(10): 632-652. 2404
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