Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling simulator

Transcription

1 Scientific Journals Maritime University of Szczecin Zeszyty Naukowe Akademia Morska w Szczecinie 2011, 25(97) pp , 25(97) s Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling simulator Wyznaczanie sekwencji wymiany wód balastowych gazowca LNG z wykorzystaniem symulatora ładunkowego statków do przewozu ładunków ciekłych Paweł Chorab Maritime University of Szczecin, Faculty of Navigation, Institut of Marine Navigation Akademia Morska w Szczecinie, Wydział Nawigacyjny, Instytut Nawigacji Morskiej Szczecin, ul. Wały Chrobrego 1 2, p.chorab@am.szczecin.pl Key words: ballast, ballast water exchange, LNG carrier, LNG simulator Abstract The sequential method of emptying and filling of ballast tanks at sea may pose extra threats for ship s safety. The prepared Ballast Water Management (BWM) Plan enables carrying out the operation so that negative effects of emptying each ballast tank are minimized. A large number of tanks and substantial volume of ballast water to be exchanged may create difficulties in preparing an optimized plan. The author proposes to use a liquid cargo handling simulator for the preparation of the BWM plan for selected LNG carriers. Słowa kluczowe: balast, wymiana wód balastowych, gazowiec LNG, symulator LNG Abstrakt W czasie opróżniania i napełniania zbiorników balastowych w morzu metodą sekwencyjną mogą pojawić się dodatkowe zagrożenia dotyczące bezpieczeństwa statku. Przygotowany wcześniej Plan Wymiany Wód Balastowych pozwala tak przeprowadzić operację, aby minimalizować negatywne skutki opróżniania zbiorników balastowych. Ich duża liczba i znaczna objętość wody balastowej może utrudniać przygotowanie optymalnego planu wymiany. Zaproponowano wykorzystanie symulatora ładunkowego do przewozu ładunków ciekłych w przygotowaniu takiego planu dla wybranych gazowców. Introduction The ship in operation happens to sail under ballast. Such situations occur when the ship has no cargo or is partly loaded, and the ballast water pumped into tanks is aimed at ensuring ship safety in terms of stability. As the vessel is discharged in the port of destination, it pumps in the amount of ballast water necessary for safe voyage. When new cargo is being loaded, ballast water is pumped out into the sea. In this way the quantity of seawater carried by ships under ballast annually amounts to as much as 10 billion tons [1, 2]. The exchange of ballast water between ports is connected with the transfer of living organisms, including microorganisms and bacteria in ballast waters between various regions of the world. When ballast water is discharged, these organisms often disturb the ecological balance in the natural environment of a region. To partly limit this problem the exchange of ballast waters in open ocean has been enforced. Obviously, such exchange should be performed in a manner avoiding any risks for ship safety. On 13 February 2004 the International Maritime Organization adopted the International Convention for the Control and Management of Ships Ballast Water and Sediments (BWM). Additionally, each ship should carry and use Ballast Water Management Plan. Such plan should be approved by the administration and take into account guidelines set Zeszyty Naukowe 25(97) 21

2 Paweł Chorab forth by the IMO. The plan should include, but not be limited to: detailed safety procedures for the ship and personnel connected with ballast water management as required by the Convention, detailed description of actions to be taken to implement the ballast water management requirements and supplemental ballast water management practices provided by the Convention. Each ship trading internationally should carry a Ballast Water Record Book. This record book is a document that should contain information on each discharge, exchange, or pumping in of ballast water, position of the operation, water salinity, initial and final volumes in the tanks, pumps used, area depth. This information constitutes evidence that the BWM is observed and can be controlled by competent authority. The Convention also provides how and where ballast waters should be exchanged. Besides, the Convention stipulates that relevant national regulations, even if in more detail address ballast water issues, they should not be in contradiction to the BWM provisions. Sequential method of ballast water exchange The ship that exchanges ballast waters in order to observe technical standards contained in regulations of the Convention should, whenever possible, do so in an area at least 200 nautical miles away from the nearest land, in waters of at least 200 metres in depth, taking into consideration the guidelines set forth by the IMO [3]. There are three basic methods of ballast water exchange: Sequential method: ballast tanks are emptied and then filled with replacement ballast water, one or more at a time, Flow-through method: ballast tanks are refilled with replacement ballast water that pushes out in-port or near-shore water, Dilution method: replacement ballast water is filled through the top of the ballast tank with simultaneous discharge from the bottom at the same flow rate and maintaining a constant level in the tank. The first of the methods described is the most commonly used in ships. The fastest and least energy-consuming, the sequential method does not require additional technical solutions in the existing ballast installations. Discharging and refilling of tanks, however, temporarily decreases ship s stability and other safety-related properties. In the sequential method, particular operations make up a specific sequence, an order in which discharge and refilling of each tank take place. The sequential method is used when the exchange of ballast is connected with the removal of a very large quantity of water while the ship is en route and refilling the tanks with replacement ballast water in the open ocean. This is a new procedure, different from the method of ballasting in the port, because at sea the ship is exposed to more risks, particularly the influence of wind and waves. Methods of establishing the sequence of ballast tank emptying and refilling The method of sequential discharges and refills is quite commonly used by ships, contrary to the flow-through method. The reason is that existing ballast installations are not adjusted to, inter alia, excessive pressures when replacement water is pumped in. In the sequential method each operation is part of the sequence of actions planned for an individual tank. While establishing the sequence of ballast water exchange, the following procedure is used. Ship s operational data are first determined: trim, drafts forward and aft, shear forces and bending moments of the hull. The calculations are conducted in the process of discharge and refilling of subsequent tanks. Thus calculated values are compared with criterial values, and procedure is repeated for each tank in turn. This manner of safety assessment refers only to ship s parameters in calm water. Besides, the application of the same procedure for each ship, regardless of its type and varying ballast installation characteristics, is a simplification and not fully satisfactory. According to the Convention [3], the sequence of ballast water exchange should be demonstrated at least for typical loading conditions taken from the approved Stability Information. The ballast water exchange sequence should be divided into steps, with the following data specified in each step: water volume in each tank, pumps used, approximate time of operation, longitudinal strength as a function of allowable values, stability information taking into account free liquid surfaces during discharge or refilling, draft values at fore and aft perpendiculars, other information. It is recommended that return to the initial condition should be possible after each step. The decision to continue an operation should be taken after making sure that the predicted ship s position does not differ from the actual one, weather forecast is 22 Scientific Journals 25(97)

3 Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling simulator favourable, capacity of ballast water equipment has not decreased and the number of personnel involved remains the same. If any of these factors is not as required, the ballast water exchange should be stopped or completely given up. Ship s listing, caused by unsymmetrical emptying and refilling of ballast tanks has to be taken into consideration so that each step takes place when the ship is in upright position (no list). The conducted operations have to be monitored in order not to generate lists during pumping. The steps have to take into account the assumed trim and draft requirements, avoid slamming, ensure that the propeller is submerged and that loss of vision from the bridge is minimal. It is very important to avoid vacuum during stripping or overpressure while refilling a tank. The exchange sequence may be different for various ship types and different loading states ship s safety should be the basic criterion. Emptying more than one ballast tanks on one side is avoided as this creates a risk of capsizing. Two adjacent tanks must not be pumped out at the same time due to large shear forces and bending moments. The ballast exchange sequence is established in compliance with the regulations and restrictions in force. The sequential method for each ship is prepared in the form of a Ballast Water Management (BWM) Plan. The plan is worked out specifically for a vessel and approved by a classification society. Risks to ship safety during ballast water exchange From the viewpoint of ship stability-related safety the process of water exchange will be dangerous; in addition, risk will become greater in adverse weather conditions. The types of risk that occur during ballast water exchange at sea may vary for various ship types, as underlined in, inter alia, [4] and [5]. Analyses found in a number of publications indicate the major causes of risk: too long time of ballast water exchange, incorrect sequence of tank emptying and refilling, inadequate operational parametric values during the exchange, adverse weather conditions (wind, high seas). Of various operational threats, the most dangerous are considered to be: loss or significant deterioration of stability, increased ship motions, rolling in particular, emergence of the propeller at too low aft draft, which leads to worse propulsion and manoeuvring ability, bow emergence, which results in slamming and worsened visibility from the navigational bridge (blind sector ahead of the ship). If threats arising during ballast water exchange are not to decrease ship operating safety below an acceptable level, each step of the exchange sequence should comply with mandatory regulations and stability criteria [6]. These requirements may vary for various ship types and sizes. In practice, particularly in bad weather, some requirements are not met. The studies [4, 5] analyze the process of emptying subsequent tanks and its influence on consequent changes in ship parameters, but they do not evaluate the impact of weather conditions. In their conclusions, however, the authors draw attention to the need for more comprehensive analysis of ship safety during ballast exchange by taking into consideration the effect of wave action and ship motions. Unfortunately, such studies have not been available yet. Shipowners developing Ballast Water Management Plans for ships in service do not account for weather conditions either, and the safety limits for the exchange are set after a subjective evaluation of the ship s master. The following conclusions can be drawn from analyses of worldwide literature on procedures amd methods of ballast water exchange, research into ship safety related with such exchange and actual measurements conducted on ships in operation: there is insufficient research on stability-related safety of a ship exchanging ballast water, existing data do not account for the influence of real weather conditions on ship safety during ballast water exchange, no data are available on the evaluation of changes in ship motions during the emptying and refilling of ballast tanks, phenomena of slamming and propeller emergence during ballast exchange in waves have not been analyzed, no analysis has been made in reference to the extent to which alteration of course and/or speed will improve ship safety during the emptying and refilling of ballast tanks in bad weather. From critical analysis of the procedures used and the existing knowledge on ship safety during ballast water exchange and operational demands reported by shipowners, the following research problems can be formulated: determine the relations between ship s speed and course and weather parameters versus ship safety level during ballast water exchange, Zeszyty Naukowe 25(97) 23

4 Paweł Chorab determine the probability of ship safety risk and its duration during ballast exchange in specific weather conditions, define possibilities of reducing the risks by altering ship s course and/or speed, or possibly the change in the sequence and number of simultaneously emptied and refilled ballast tanks, search for an optimal sequence of emptying and refilling in given operational conditions (ship s speed and course, weather conditions), determine an optimal quantity of ballast water (including the number of tanks and their location) needed to ensure ship safety in a given operational situation. Liquid cargo handling simulator One of the modules of a Liquid Cargo Handling Simulator is the ballast module referred to as Ballast control system Line and valves. The inclusion of this module in the simulator equipment puts its software in compliance with IMO model courses for tankers: IMO 1.35 LPG Tanker Cargo & Ballast Handling, IMO 1.36 LNG Tanker Cargo & Ballast Handling, IMO 1.35 Chemical Tanker Cargo & Ballast Handling. Two examples of ships with essential information on the simulation of ballast installation operation on LNG carriers are given in table 1. Table 1. Main particulars of selected LNG-s and LNG-m carriers [7] Tabela 1. Dane techniczne przykładowych gazowców typu LNG-s, LNG-m [7] LNG-s LNG-m Deadweight capacity DWT [t] Length overall Loa [m] Length between perps Lbp [m] Breadth B [m] Moulded depth H [m] Moulded draft T [m] Volume of cargo tanks VH [m 3 ] Volume of ballast tanks VB [m 3 ] LNG-m LNG carrier with membrane takns LNG-s LNG carrier with spherical tanks The LNG-s ballast installation consists of: eight portside tanks: BS1P 2055 m 3, BD2P 3594 m 3, BS4P 2379 m 3, BD5P 4238 m 3, BS7P 2706 m 3, BD8P 4262 m 3, BS10P 2462 m 3, BD11P 2108 m 3 ; Fig. 1. Visualisation of the ballast installation of an LNG-s carrier (spherical cargo tanks) [7] Rys. 1. Wizualizacja schematu instalacji balastowej gazowca LNG-s (sferyczne zbiorniki ładunkowe) [7] 24 Scientific Journals 25(97)

5 Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling simulator Fig. 2. Visualisation of the ballast installation of an LNG-m carrier (membrane cargo tanks) [7] Rys. 2. Wizualizacja schematu instalacji balastowej gazowca LNG-m (sferyczne zbiorniki ładunkowe) [7] eight starboard tanks: BS1S 2055 m 3, BD2S 3594 m 3, BS4S 2379 m 3, BD5S 4238 m 3, BS7S 2706 m 3, BD8S 4262 m 3, BS10S 2462 m 3, BD11S 2108 m 3 ; three central tanks: BC m 3, BC m 3, BC m 3 ; two forepeak tanks: BFP 7782 m 3, BFT 1500 m 3 (void space); afterpeak tank: BAP 3110 m 3 ; throttling valves for communication between ballast tanks and ballast lines, as shown in figure 1; centrifugal BP pumps; the pumps have identical characteristics and provide charging pressure of ~2.8 bar at a flow of ~2,800 m 3 /h); three sea chests Bch1, Bch2 with strainers; throttling valves on the charging line of each pump: Bv1, Bv2,Bv3; cut-off valves: BV4 BV19; non-return valve Bv20. The LNG-m ballast installation consists of: four portside tanks: B1P 4751 m 3, B2P 6182 m 3, B3P 6233 m 3, B4P 5518 m 3 ; four starboard tanks: B1S 4751 m 3, B2S 6182 m 3, B3S 6233 m 3, B4S 5518 m 3 ; two forepeak tanks: BD 1167 m 3, BFT 1780 m 3 (void space). ballast tanks communicate with ballast lines via throttling valves: throttling valves for filling tanks: Bv P, Bv S and Bv C, Bv6, Bv9, Bv22,Bv23, Bv24. centrifugal pumps BP. ; the pumps have identical characteristics and provide charging pressure of ~2.8 bar at a flow of ~2,800 m 3 /h); two sea chests Bch1, Bch2 with strainers; throttling valves on the charging line of each pump: Bv1,Bv2; cut-off valves: BV4 BV19; non-return valve Bv20. The devices are separately controlled by pointing at the image of the device and double clicking the left key of the mouse. This activates an extra window that allows to set working parameters of the device a pump or valve. Similarly, information may be obtained on a ballast tank e.g. the current volume of ballast water, percent of tank filling, level of filling etc. The operating indicators of the ballast installation show: operation of ballast pumps, cut-off valves open, Zeszyty Naukowe 25(97) 25

6 Paweł Chorab degree of openness of throttling valves, level of liquid in a tank. Among others, system failures may be as follows: strainer blockage at bottom valves, overheating of ballast pumps, freezing of ballast water, failures of valves and their opening-closing hydraulic system. Fig. 3. Examples of dialog windows of the ballast water system elements [7] Rys. 3. Przykładowe okna dialogowe elementów systemu instalacji balastowej. [7] Research problems applicability of the simulator in establishing the sequence of ballast water exchange The main function of the above ballast module is simulating the operation of the ballast installation on a given ship. Besides, the module can be used to determine the sequence of emptying and refilling ballast tanks. By simulating emptying / refilling of one or more ballast tanks the users can observe and analyze changes in these operational parameters: displacement D [t], mean draft T ŚR [m], draft aft T R [m], draft forward T D [m], trim t [m], initial metacentric height GM [m], righting lever GZ [m] free surface correction ΔGM [m], value of max. shear force occurring in ship s hull SF MAX, value of max. bending moment occurring in ship s hull BM MAX, value of blind sector ahead of ship s bow S [m], value of heeling angle φ [ ], others. Every ship should carry a Ballast Water Management Plan, containing the complete procedure of ballast water exchange described through subsequent steps of the exchange operation. It is mandatory to indicate a/m operational parameters values for each step and compare them with criterial values. There are factors, however, that may complicate the establishment of a correct sequence the order of emptying and refilling of ballast tanks: a large number of ballast tanks in a ship, substantial volume of ballast water, ballast pums unfit for continuous operation that may take more than a day, long time of emptying and refilling particular ballast tanks, difficulties in determining such sequence of operations that will not worsen one parameter at the expense of another parameter (e.g. relation between draft forward T D and draft aft T R ). For the preset sequence of tank emptying and refilling the ballast module offers the capability of monitoring of: changes in operational parameters (draft, list etc.) in real time, changes in stability parameters (e.g. initial metacentric height) in real time, stresses in the ship s hull (shear force, bending moment), values of the blind sector ahead of the ship s bow, technical parameters of ballast pumps (pressure), others. Besides, it is also possible to modify the established procedure of ballast water exchange; each change, e.g., of the sequence, may be simulated to obtain the results of such parametric changes. The balast module can be further exploited to prepare guidelines for the master of the ship where ballast water exchange operations will be performed: 26 Scientific Journals 25(97)

7 Determination of the ballast water exchange sequence for an LNG carrier using a liquid cargo handling simulator scenario with many steps, with small changes in operational parameters, suitable for use in severe weather conditions, scenario with few steps, causing relatively large changes in operational parameters, suitable for use in good weather, emergency scenarios, e.g. failure of one ballast pump, failure of any valve, most adverse and most dangerous emergency scenarios, where a failure occurs to the most important component of the ballast installation from ship s safety viewpoint, others. It should be underlined that simulation is in real time (there is an option of time compression), which enables constant preview of each of the above mentioned parameters, which is rare in presently used Ballast Water Management Plans. Besides, it should be mentioned that currently implemented plans comprise only one, not necesserily optimal solution for the subsequent emptying and refilling of tanks in the method herein described. A possible failure of any element of the ballast installation brings about temporary solutions, not supported by tips or instructions as such do not exist in plans developed to date. Conclusions The use of a Liquid Cargo Handling Simulator with its ballast module: Ballast control system Line and valves in the development of a ballast water exchange plan will: facilitate the process of determining the order in which ballast tanks should be emptied and refilled, enable the establishment of optimal sequence of ballast water exchange, enable preparing emergency scenarios with a failure of any element of the ballast installation, allow to present, in real time, changes in operational parameters of the ship during the emptying and refilling of ballast tanks. The development of such comprehensive analysis of the ballast water exchange by the sequential method will significantly facilitate a further analysis based on the BWM Plan, concerning the influence of actual weather conditions and alteration of ship s course and/or speed on the ship s safety during that complicated process. References IMO, Międzynarodowa Konwencja o kontroli i postępowaniu ze statkowymi wodami balastowymi i osadami, 2004 (Konwencja BWM 2004). Wydanie PRS, AKIYAMA A., UETSUHARA F., SAGISHIMA Y.: Ballast Water Exchange Procedures and their Problems. Transactions of the West-Japan Society of Naval Architects, 2000, 100, 41 53, 5. BIELAŃSKI J.: Considerations about the guidelines on safety of ballast water exchange at sea. Hydronav 99 Maneuvering 99, Joint 13 th International Conference on Hydrodynamics in Ship Design and 2 nd International Symposium on Ship Maneuvering, Gdańsk Ostróda IMO, International Code of Intact Stability, Edition 2009, London Recenzent: dr hab. Leszek Smolarek prof. AM Akademia Morska w Gdyni Zeszyty Naukowe 25(97) 27