Determination of the ballast water exchange sequence for ...

Zeszyty Naukowe 25(97) 21

Scientific Journals Zeszyty Naukowe 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: [email protected]

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 ładunkowego 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

Paweł Chorab

22 Scientific Journals 25(97)

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. 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 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,

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 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 simulation of ballast installation 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 m

3, BD5P – 4238 m

3,

BS7P – 2706 m3, BD8P – 4262 m

3, BS10P –

2462 m3, 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]



• eight starboard tanks: BS1S – 2055 m3, BD2S –

3594 m3, BS4S – 2379 m

3, BD5S – 4238 m

3,

BS7S – 2706 m3, BD8S – 4262 m

3, BS10S –

2462 m3, BD11S – 2108 m

3;

• three central tanks: BC3 – 1500 m3, BC6 –

1500 m3, BC9 – 1500 m

3;

• 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 m

3, B4P – 5518 m

3;

• four starboard tanks: B1S – 4751 m3, B2S –

6182 m3, B3S – 6233 m

3, B4S – 5518 m

3;

• 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 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, 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]

Rys. 2. Wizualizacja schematu instalacji balastowej gazowca „LNG-m” (sferyczne zbiorniki ładunkowe) [7]

Paweł Chorab


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 refilling 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

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