Maritime University of Szczecin
Akademia Morska w Szczecinie
2011, 25(97) pp. 21–27 2011, 25(97) s. 21–27
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 70-500 Szczecin, ul. Wały Chrobrego 1–2, e-mail: 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 Bala-stowych pozwala tak przeprowadzić operację, aby minimalizować negatywne skutki opróżniania zbiorników balastowych. Ich duża liczba i znaczna objętość wody balastowej może utrudniać przygotowanie optymalne-go planu wymiany. Zaproponowano wykorzystanie symulatora ładunkoweoptymalne-go do przewozu ładunków cie-kłych w przygotowaniu takiego planu dla wybranych gazowców.
Introduction
The ship in operation happens to sail under bal-last. 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
micro-organisms and bacteria in ballast waters between various regions of the world. When ballast water is discharged, these organisms often disturb the eco-logical balance in the natural environment of a re-gion. 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 Manage-ment Plan. Such plan should be approved by the administration and take into account guidelines set
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 man-agement as required by the Convention, detailed description of actions to be taken to implement the ballast water management requirements and sup-plemental ballast water management practices pro-vided 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 wa-ter, 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 contra-diction to the BWM provisions.
Sequential method of ballast water exchange
The ship that exchanges ballast waters in order to observe technical standards contained in regula-tions 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 me-tres in depth, taking into consideration the guide-lines 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 si-multaneous 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 ener-gy-consuming, the sequential method does not re-quire additional technical solutions in the existing ballast installations. Discharging and refilling of tanks, however, temporarily decreases ship’s stabil-ity and other safety-related properties. In the se-quential 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 influ-ence 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 bend-ing moments of the hull. The calculations are con-ducted in the process of discharge and refilling of subsequent tanks. Thus calculated values are com-pared with criterial values, and procedure is re-peated 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 proce-dure for each ship, regardless of its type and vary-ing ballast installation characteristics, is a simplifi-cation 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 con-dition should be possible after each step. The deci-sion 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
favourable, capacity of ballast water equipment has not decreased and the number of personnel in-volved 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 ac-count the assumed trim and draft requirements, avoid slamming, ensure that the propeller is sub-merged 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 safe-ty should be the basic criterion. Empsafe-tying 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 dan-gerous; 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 refil-ling,
inadequate operational parametric values during the exchange,
adverse weather conditions (wind, high seas). Of various operational threats, the most danger-ous 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 manoeuvr-ing 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 se-quence 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 empty-ing subsequent tanks and its influence on conse-quent 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 con-sideration the effect of wave action and ship mo-tions. Unfortunately, such studies have not been available yet. Shipowners developing Ballast Water Management Plans for ships in service do not ac-count 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 emer-gence 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,
determine the probability of ship safety risk and its duration during ballast exchange in specific weather conditions,
define possibilities of reducing the risks by alter-ing ship’s course and/or speed, or possibly the change in the sequence and number of simulta-neously 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 loca-tion) needed to ensure ship safety in a given op-erational situation.
Liquid cargo handling simulator
One of the modules of a Liquid Cargo Handling Simulator is the ballast module referred to as
Bal-last 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 &
Bal-last Handling, IMO 1.36 LNG Tanker Cargo & Ballast Handling, IMO 1.35 Chemical Tanker Car-go & Ballast Handling.
Two examples of ships with essential informa-tion on the simulainforma-tion of ballast installainforma-tion opera-tion 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] 67 900 62 700 Length overall Loa [m] 290 275 Length between perps Lbp [m] 275 260
Breadth B [m] 48.1 43.4
Moulded depth H [m] 27 26
Moulded draft T [m] 11.7 11.95 Volume of cargo tanks VH [m3] 135 000 130 000
Volume of ballast tanks VB [m3] 63 000 46 000
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 m3, BD2P –
3594 m3, BS4P – 2379 m3, BD5P – 4238 m3,
BS7P – 2706 m3, BD8P – 4262 m3, BS10P –
2462 m3, BD11P – 2108 m3;
Fig. 1. Visualisation of the ballast installation of an LNG-s carrier (spherical cargo tanks) [7]
• eight starboard tanks: BS1S – 2055 m3, BD2S –
3594 m3, BS4S – 2379 m3, BD5S – 4238 m3,
BS7S – 2706 m3, BD8S – 4262 m3, BS10S –
2462 m3, BD11S – 2108 m3;
• three central tanks: BC3 – 1500 m3, BC6 –
1500 m3, BC9 – 1500 m3;
• two forepeak tanks: BFP – 7782 m3, BFT –
1500 m3 (void space);
• afterpeak tank: BAP – 3110 m3;
• 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 m3/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 m3, B2P –
6182 m3, B3P – 6233 m3, B4P – 5518 m3;
• four starboard tanks: B1S – 4751 m3, B2S –
6182 m3, B3S – 6233 m3, B4S – 5518 m3;
• two forepeak tanks: BD – 1167 m3, BFT –
1780 m3 (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 iden-tical characteristics and provide charging pres-sure of ~2.8 bar at a flow of ~2,800 m3/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 point-ing at the image of the device and double clickpoint-ing 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, informa-tion 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,
Fig. 2. Visualisation of the ballast installation of an LNG-m carrier (membrane cargo tanks) [7]
degree of openness of throttling valves, level of liquid in a tank.
Among others, system failures may be as fol-lows:
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 sys-tem 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 TŚR [m],
draft aft TR [m],
draft forward TD [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 SFMAX,
value of max. bending moment occurring in ship’s hull BMMAX,
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 proce-dure 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 crite-rial 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 TD and draft aft TR).
For the preset sequence of tank emptying and re-filling 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 me-tacentric 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 (pres-sure),
others.
Besides, it is also possible to modify the estab-lished 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 per-formed:
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 im-portant 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 pre-sently used Ballast Water Management Plans. Be-sides, it should be mentioned that currently imple-mented 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 instal-lation brings about temporary solutions, not sup-ported 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 re-filled,
enable the establishment of optimal sequence of ballast water exchange,
enable preparing emergency scenarios with a failure of any element of the ballast installa-tion,
allow to present, in real time, changes in opera-tional parameters of the ship during the empty-ing and refillempty-ing of ballast tanks.
The development of such comprehensive analy-sis of the ballast water exchange by the sequential method will significantly facilitate a further analy-sis based on the BWM Plan, concerning the influ-ence of actual weather conditions and alteration of ship’s course and/or speed on the ship’s safety dur-ing that complicated process.
References
1. http://globallast.imo.org/
2. http://www.imo.org/Pages/home.aspx
3. IMO, Międzynarodowa Konwencja o kontroli i postępo-waniu ze statkowymi wodami balastowymi i osadami, 2004 (Konwencja BWM 2004). Wydanie PRS, 2006. 4. 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, www.sciencedirect.com
5. BIELAŃSKI J.: Considerations about the guidelines on safety of ballast water exchange at sea. Hydronav’99 – Maneuver-ing ’99, Joint 13th International Conference on
Hydrody-namics in Ship Design and 2nd International Symposium on
Ship Maneuvering, Gdańsk–Ostróda 1999.
6. IMO, International Code of Intact Stability, Edition 2009, London 2008.
7. http://www.transas.com/products/simulators/
Recenzent: dr hab. Leszek Smolarek prof. AM Akademia Morska w Gdyni
