AN EXPERT LOADING S YSTEM
FOR CHEMICAL AND PRODUCT CARRIERS
byL.Bardis*, G.GrigoropOulos*, S.Kokkotos** T.Loukakis, C. Spyropoulos* * G. Vouros* *
Department of Naval Architecture and Manne Engineering, National Technical Universit of \ihens,
9 Heroon Polvtccluuou sir., 15773 Zocrafos, Greece.
email rcoor' on.r.:ivo n:,:.izr
S. CSR Deokr:.e'.
Institute of Informaucs anu :e1uuiCat1Ons 15310 Agia Paraskevi Atukis. Crcece
c_mail : georgeviiLnrcps.ariadne-t.gr
Abstract
The Chcmical and Product Carricrs (CPC) are ships carrying at a singic voyage a variety of liquified chemical and oil product cargoes. Because these vessels
have a complicated layout of a large number of tanks and they transport
dangerous cargoes, currently. their loading is carried out using commercially
available computer codes, called loadmasters. The loadmasters estimate the ship
stresses and the hydrostatic characteristics of a vessel for a given loading
condition of the tanks. In the present paper an Expert Loading System (ELS) is
proposed for the near optimal or optimal loading of CPCs. The system designs a
loading condition meeting the captain's requirements and proposes a safe sequence for the loading of the vessel, taking into account the international regulations, the loading capacities of a given harbour and the vessel, the
allowable drafts of the vessel and her transverse stability characteristics. ELS is
fast and user friendly permitting its use even by computer illiterate users, accepts
user modifications and presents the results in an attractive way. Thus, port time
is reduced leading to significant money savings.
Keywords : Expert system, chemical product carriers, loading, optimization,
1 DTRODUCTION
The ship's master or the chief captain of a maritime company has to design a loading condition and follow a sequence of operations in order to safely load/unload a liquified
cargo carrier. The requirements are much more complicated in the case of Chemical
Product Carriers (CPC), i.e. ships with complicated layout, encompassing a large number of tanks and carrying at a single voyage a variety of liquified chemical cargoes. Among these cargoes, crude oil products as well as dangerous chemicals are included, depending on the materials used for the construction and finishing of the tanks and the quality of the installations for charging and discharging them. In order to assist the
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ship's masters in their work, some software applications are currently available, known as loadmasters, which, for a given ship, can estimate the hydrostatic characteristics and the stresses of the ship for a given allocation of cargoes in the tanks. However, time
and safety are essential factors for the efficient use
of loadmasters and advanced expertise is required from the user of the loadmaster. Therefore, an advisory system is needed to minimize the efford and maximize the efficiency of the ship crew. In this paper, an advisory Expert Loading System (ELS) is proposed for the near optimal or optimal loading of CPCs. The system designs an loading condition and devises a safesequence of operations for the safe
loading of the vessel, taking into account the
international regulations, the loading capacities of a given harbour and the vessel, the allowable drafts of the vessel and her transverse stability characteristics. ELS is user friendly permitting its use even by computer illiterate users, accepts user modifications and presents the results in an attractive way.
In the sequel, the design characteristics of the Expert Loading System are given, the structure of the user interface is outlined and the ELS kernel is described. Finally, the
advantages of the proposed system over the currently available loadmasters are
discussed and some guidelines for future improvement are proposed.
2 THE ARCHITECTURE OF ELS
The ELS has been developed on a UNIX workstation and is currently transferred on a portable PC in WINDOWS environment. An overview of the ELS is demonstrated in Figure 1.
The ELS Manager is the main module. Its task is to initialise some global variables, to
react to the user requirements and selections, and to activate
other modules. Furthermore, the Manager provides an explanation facility concerning the behaviour of the system during reasoning and action.The I,ference Engine consults the knowledge base and activates the appropriate
knowledge for problem solving. It implements the most basic control mechanism of the
ELS.
The Rule Base contains the BNF rules required for the Loading Planner and the Cargo Handling Unit to operate. It contains two sets of rules, one for the Loading Planner and one for the Cargo Handling Unit.
The Loading Planner plays a central role in the ELS. It contains the appropriate procedures for the construction of an allocation plan. These procedures are activated
by the corresponding rules in the Rule Base. The allocation plan specifies the
distribution of cargoes to the ship compartments, the allocation of ballast in the
(separate) ballast compartments, and the amount of consumable in the appropriate tanks, for each port of call.
The Cargo Handling Unit contains the appropriate procedures for the construction of a cargo handling plan. These procedures are activated by the corresponding rules in the Rule Base. The cargo handling plan of operations is performed on ship equipment and on the transfer lines provided from shore, to charge/discharge cargoes at each port of call. During this task, hydrostatic measures must be within the limits specified by the rules and regulations.
The Algoriihmic A'Iodzile is a library of routines executing standard hydrostatic and
strength calculations. This is a set of calculations that conventional loadmasters
perform. More specifically, it contains routines for the calculation of the ship floating conditions for a given allocation plan. Furthermore, there are routines calculating hull deflection for a given cargo distribution and the difference between air and water temperatures. Finally, the Algorithmic Module contains routines for damage stability
calculations. These are calculations of the final ship floating condition after one or
The User Interface contains all routines for data presentation and transfers to the Manager all the user requests. The user interface is based on the basic X window system library (Xlib), making ELS portable.
Sbthtv ,nd Scgh T.bI JhL,UtOfl T.blc P0.8 DB ,. Figure 1
The Geometrical Data Base contains all data, which are needed to define the ship hull geometry.
The Product Daia Base contains physical and chemical properties of liquid chemical
substances allowed to be transported by a particular ship. Of all physical properties, the
most important for ELS is the
density as a ftinction of temperature and chemical compatibility tables.The Eqziipment Data Base contains a description of the ship equipment related to cargo handling and storage. In the current ELS implementation it contains all data needed to describe the tank geometry, chemical compatibility of tank coating to all cargo grades allowed to be transported by the specific ship as well as characteristics of ship pumps used to charge-discharge liquid cargo.
The Charter Table contains a number of charter contracts. Each charter contract is a table indicating an ordered list of ports visited by the ship, and the type, amount, temperature, port of loading and unloading of individual liquid cargo items. Users activate one charter contract from the Charter Table, or define a new one through the user interface. The allocation plan and the cargo handling plan are calculated for the active charter contract.
The Stabi/ity and Strength Table contains hydrostatic data characterising the ship
floating condition. This table is updated by the Loading Planner and the Cargo
Handling Unit.
The Cargo Distribution Table is a table indicating the amount and type of cargo stored in each ship compartment for every voyage segment between subsequent ports of call. This table contains the results produced by the LoadingPlanner.
The Loading Operations Table contains the series of actions required to load or
unload compartments of the ship. This table includes the results produced by the Cargo Handling Unit. ELS Manager Agorithm.c MOdUC
-4.-C,cc; *00 1 T.blc Pljscr Handicng V Ecne Rc*oo d RL,00n. --Chncc TobLo-The Ports Data Base contains port data used by ELS. Such data include the maximum draft, number and capacity of shore transfer lines and the airdraft.
The Rules and Regulations is a list of maximum or minimum allowable values of hydrostatic or strength data, such as the maximum bending moment and the shear force distribution along the ship.
3 THE USER INTERFACE
The ELS system interacts with the user via a graphical menu-driven user interface. The
interface is designed so that it can be used by novice users with the
minimum of practice, and almost no computer experience.The main screen of ELS is depicted in Figure 2. It comprises the following parts: The H.vdrosianc A'feasiires' bar. This bar contains the values of the drafts of the ship, the heel, the bending moment and shearing force, the displacement as well as the
co-cr:es of the centre of gravity and the
metacentric height.77 A'Iessage bar. In this bar warnings and messages are displayed. Appropriate
:_S
may demand input from the user.The Graphical i'indou'. This window normally contains a graphical image of the ship's tanks. The contents of the tanks are displayed in colour according to the cargo's colour
coc e.
7*.. H'drostaiic Diagrams %mdow. This is a pop up window which is activated by selecting the appropriate menu item. It replaces the Graphical window.The Hydrostatic Diagrams window displays the bending moment, shearing force and statical stability (GZ-O) curves of the ship, along the ship's axis. Furthermore, the sailing and port maxima are displayed. The Hydrostatic Diagrams window is depicted in Figure 3.
Figure 2
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The Tank Contents tab/es. The first and second tables display the contents of the
cargo and ballast tanks. For each tank the name, the volume, the level, the ullage, the
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density and the temperature of the allocated cargo are displayed. The third table is the summary of the loaded cargo, consumable and ballast.
The main menu. The main ELS menu contains the following items: "Charter Contract",
"User Request", "Loading Condition", "Cargo Handling", "Save State", "Load State"
and "Sailing".
The "Charter Contract" item activates the Charter Contract window depicted in Figure
4. The user may change the
specified ports, the specified products, as well as thedetails of any port and product. The user can also change the hydrostatic measures' constraints specified in the middle of the window.
The "User Request" item enables the user to specifj special constraints concerning the allocation, as well as the actual loading/unloading of cargoes.
The "Loading Condition" item activates the
Loading Planner and the "Cargo
Handling" item the Cargo Handling Unit.
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options corresponding to the item of the main menu selected.
4 THE ELS KERNEL
The kernel of the ELS comprises the Loading Planner, the Cargo Handling Unit, the Rules Base and the Inference Engine. It is this part that justifies the predicate "Expert" of ELS. In fact, currently available loadrnasters include only the Algorithmic Module, a
User Interface and a number of tables providing data for the final
ship loadingcondition according to the loading plan prescribed
by the user. ELS uses the rules
encoded in the Rule Base, the Inference Engine and the procedures of the Loading Planner and Cargo Handling Unit to provide a near optimum load plan and a sequence of cargo handling operations for each port of call.
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Figure 4 4.1 Loading Planner and Cargo Handling Units.
The goal of the Loading Planner is to devise an allocation plan given a charter contract and a set of specific user requirements. The Loading Planner takes into account the following restrictions:
Rules and regulations by classification societies. Specific charter requirements.
Properties of the cargoes to be transported.
Compatibility between cargoes and cargo handling equipment such as cargo pipes, pumps, valves, stowage, etc.
The Loading Planner performs the following actions to achieve the target goal:
Allocates a specific amount of cargo to particular segregation. A segregation is a set of tanks served by common cargo pipes.
Exchanges the amount of cargoes allocated in a specific pair of segregation.
Allocates ballast to specific ballast compartments to achieve a good and safe float-ing condition of the ship.
Places requests to the Algorithmic Module to carry out additional stability and
strength calculations to (a) test an allocation plan and (b) test whether it has
achieved any progress towards eliminating critical situations.
The goal of the Cargo Handling Unit is to plan a sequence of cargo handling opera-tions performed on the ship equipment and on the equipment provided from shore in order to charge/discharge the ship at any port of call. To achieve the target goal, the Cargo Handling Unit performs the following actions:
Construct a list of the cargoes that should be charged-discharged at a port of call and speciFy how transfer lines will be allocated to them.
Adjust the valve flow of the pumps of the charged-discharged segregation in order to prevent critical and dangerous situations.
Charge-discharge ballast tanks in order to prevent critical and dangerous situations.
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Places requests to the Algorithmic Module to cany out
additional stability and strength calculations to (a) test critical values during intermediate stages and (b) test whether it has achieved any progress towards eliminating critical situations.The problems of allocating cargoes to the ship compartments and planning cargo handling operations have the following characteristics:
I. The configuration of the problem solver changes as the amount of cargoes and bal-last changes in the cargo and balbal-last compartments.
The problem solver must respond dynamically to changing demands. For instance, it should change the rate of charging-discharging cargoes, or ballast, in order to pre-vent a critical or dangerous situation.
There is a trade-off between economy and satisfaction of the floating condition constraints. For instance, in order the Cargo Handling Unit to keep the trim, or the strength, within the imposed limits, it should stop charging/discharging a number of ship compartments. This contradicts the economy goal since cargo handling would require additional time. Moreover, floating condition Constraints do contradict among themselves. For instance, preventing a dangerous strength may result in a violated trim. These trade-off's determine prioritisation of constraints and contribute to the efficiency of the system. Furthermore, stability and strength constraints may
prevent the Loading Planner from loading 100% of the cargoes amount. In this
case, the corresponding constraints should be either relaxed or partially satisfied in order to satisfy the economy goal.
4.2 Categories of knowledge.
The knowledge exploited by the Loading Planner and the Cargo Handling Unit of the ELS can be categorised as follows:
Knowledge about restrictions,
Heuristic knowledge about solving particular problems in the domain Control knowledge about problem solving states and goals of the system.
Operational Knowledge about specific actions that can be performed in specific situations to achieve particular effects.
Knowledge about restrictions comprises the rules and regulations concerning the floating condition of the ship, chemical compatibility between cargoes, as well as be-tween cargoes and ship equipment, and special requirements of ship operators concern-ing the allocation and handlconcern-ing of cargoes. This knowledge is stored in the appropriate data bases and is utilised by heuristic and operational knowledge.
Heuristic knowledge is knowledge that ship operators exploit during problem solving. This knowledge category includes:
knowledge about forming specific decisions in particular problematic situations. For instance, the rule
'if there is a dangerous aft trim,
and the bending moment has an allowable value, then load a segregation positioned in an extreme aft position'
is a heuristic rule which forms the decision "load a segregation positioned in an ex-treme aft position" in case the situation "a dangerous aft trim and the bending moment with an allowable value" occurs.
knowledge about decision making in cases where constraints can be relaxed. For instance, such a rule is the following one:
'if no decision can be formed,
and this is not the final state of the ship,
knowledge about resolving the trade-offs between requirements and constraints, as well as between constraints. Such trade-offs, as already mentioned, contribute to the efficiency of the system. Such a rule is the following one:
"If there s progress in reducing trim,
and the bending moment has an allowable value
then the goal is to reduce tnm"
Control knowledge represents the problem solving state and demands at any problem
solving step. The decisions formed by heuristic rules, the hydrostatic measures pro-vided by the Algorithmic Module, as well as the progress report, towards achieving a particular goal, form the internal state of the Expert Loading System. The progress report as well as the hydrostatic measures are calculated after the execution of a par-ticular action. The declarative nature of control knowledge facilitates the system to respond dynamically to changing demands and provide guidance for heuristic and op-erational knowledge execution.
Operailonal knowledge is knowledge about specific actions that should be performed to particular situations. Actions help to achieve the desirable effects (e.g. to increase negative bending moment at a particular point on the ship hull) realised by the irLernal state of the ELS. In other words, control knowledge recordings guide and justiv the execution of particular actions in particular situations.
Such an action, justified by the decision to "load a segregation positioned in an ex-treme aft position", is the following one:
"If segregation Y is in an extreme aft position, and is allowed to contain X tns of cargo Y, then load segregation V with X tns of cargo C
4.3 Representation of knowledge.
Knowledge about restrictions, as well as operational and heuristic knowledge are rep-resented by Attribute Grammars. Attribute Grammars had been initially proposed by D.Knuth as a formal specification tool for programming languages. In [5,6,7] they
have been proposed as a framework for knowledgeengineering.
In ELS, Attribute Grammars' BNF rules are contained in the Rule Base. Due to the use of data bases and tables, the Attribute Grammar uses only a few attributes. How-ever, Attribute Grammar symbols have attached procedures, that are executed during the reasoning process. These procedures are contained in the Loading Planner and Cargo Handling Unit modules and concern the calculation of the ship floating condi-tion, done by the Algorithmic Module, check compliance to constraints, report the progress achieved, read and update data bases and tables, and execute actions.
In particular, procedures can be categorised as follows:
Preconditions, that check whether a specific situation occurs. These procedures check for the violation of particular restrictions, are utilised by heuristic rules to recognise particular problematic situations, and by actions to check whether the particular conditions for action execution are met.
Initialisation procedures, that assign particular values to attributes and update the ELS data bases and tables prior to the execution of a heuristic rule. For instance, starting the optimisation of cargo allocation, an initialisation procedure that assigns Otns of ballast to the ballast segregations, and assigns to the 'context' slot of the de-cision frame the value 'optimisation', is executed.
Posiconditions, that check whether a decision achieved the desirable effect. These
are checked after
the execution of an action. A specific postcondition that is
checked after the execution of any action is the "progress" one. This checks
whether the action achieved any progress towards eliminating the violated con-straint recorded in the concon-straint' slot of the decision frame.
Side effect procedures, that update the ELS tables and data bases. Side effect pro-cedures perform the corresponding actions.
Control knowledge in ELS is represented by decision frames [4]. A decision frame represents the internal ELS state at a specific time point. In particular, a decision frame
records the hydrostatic measures, the decision context,
the violated constraint, a
judgement
of
the criticalityof
the situation, and a progress report towards eliminating the violated constraint. Figure 5 describes the generic format of the decision frame.Decision Fram
Cons:
Crttical Progress: Context: Current Status: Previous Status:Target Segregation Type:
Action mode:
(Violated hydrostatic measure constraint,
chemical compatibility constraint) (dangerous, critical, dont_care)
(yes.no}
(fix_baflast. optimisatlon. draft_allocation, load_total_amount)
(current values of hydrostatic measures}
(the values of hydrostatic measures before the last strategy instantation}
(fore, aft, middle, fore-mddle aft-middle}
(load, unload)
Figure 5
4.4 The Inference Engine of ELS.
The Inference Engine is based on original concepts published in [5,6,7] which have
been tailored to the needs of ELS. In particular, a parser for processing Attribute
Grammars, described in [5,6], has been modified so that it can handle the rules in the Rule Base, execute the initialisation and side effect procedures, and test the postcondi-tions and precondipostcondi-tions specified in the Load Planner and Cargo Handling modules. The parser uses a top down, left to right strategy for the selection of goals and the
execution of the appropriate procedures. Therefore, one can consider it to work in the
same way as PROLOG. However, it should be mentioned that it operates at !east ten
times faster than PROLOG does.
z
ii
x,1 'ç...ç,, x21 x23.... X,k
Figure 6
In particular, the ELS-tailored Attribute Grammar parser works as follows:
At the beginning, the start symbol Z of the grammar is made the current goal. When
a specific Attribute Grammar
symbol X (e.g. XZ) is made the current goal, the
parser indexes the rule
X::=XiiX12...X1I..4Xi...X
whose head is the current goal X and checks the preconditions of the nile. When pre-conditions are true, then the initialisation procedures are evaluated and the leftmost symbol X1j of the first alternative of the rule is made the current goal. This line of rea-soning proceeds until a goal Yj with no descendants is reached. Therefore, as figure 6 shows, the parser builds a parse tree that is expanded in a depth-first fashion. Then, the postconditions of the last goal Y1 are checked. If they are true, the side effect procedures are performed and success is reported in the parent node Y. The next sym-bol Y12 of Yi in the rule with head Y, is made the current goal and the parse tree ex-pands to the right of Y. Finally, Y reports success to its parent node when all its
de-scendants have report success to it and its postconditions are true. Parsing succeeds
when all the immediate descendants of Z have reported success. However, if some
goal fails, due to false preconditions or postconditions, the next alternative of its left sibling is processed. When no left sibling exists, (in case the falsified alternative was the first one of the parent node) the next alternative of the parent node is processed.
5 CONCLUSIONS
In this work the software design of the ELS is described. The main advantages of the
ELS over the commercially available loadmasters are the production of a load plan and
a sequence of cargo
handling operations taking into account the hydrostatic andstrength calculations as well as the chemical compatibility between various cargoes and between cargoes and ship equipment. Since no special software tools have been used, ELS is a highly portable software. All that is needed for its installation is simply a re-compilation of the code.
Experience gained during the development of the prototype and advice provided by the ship operators revealed the directions of future development of the system. The Algo-rithmic Module should be enhanced to include treatment of the stranding problem and handling of the intermediate flooding stages when the ship is damaged. Furthermore, the construction of Geometrical and Equipment Data Bases is a cumbersome task in-volving manual preparation of a great number of data for each vessel, such as co-ordinates of selected points on the ship hull and tank boundaries obtained from draw-ings. A semi-automatic procedure should be developed to build both the aforemen-tioned data bases using a digitizer. In addition, a thorough analysis of the priorities the ship operators put on the same ship and affect seriously the load plan. In this respect, a framework should be devised for the fast customisation of the rule base to a specific ship and, on the other side, for the easy coding of the pre- and postconditions, the ini-tialisation condition and the side-effect procedures. Finally, the ongoing transfer of ELS on a PC-based machine under WINDOWS, will, certainly, improve even more the portability of the system.
6 REFERENCES
BARDIS L., T.A.LOUKAKIS, "The ELS Requirements Specification", Technical Note, 2163-TN-NTUA-004, NTUA, 1 Nov., 1989.
BARDIS L., T.A.LOUKAKJS, G.A.VOUROS, "An Expert Loading System for
chemical and product carriers", Stab '90, Italy-Napoli, 24-28 Sept, 1990.
BARDIS L., T. PANAYIOTOPOULOS, G.A. VOUROS, "The Expert Loading
System Design Specification", WP 13, Task 132, 2163-TN-NTUA-014, 1992.
COHEN P., J. DeLISIO, D.HART, "A Declarative Representation of Control
Knowledge", IEEE Transactions on Systems, Man and Cybernetics, Vol 19, No 3, pp 546-557, May/June 1989.
PAPAKONSTANTINOU G., J.KONTOS, "Knowledge Representation with At-tribute Grammars", The Computer Journal, Vol 29, No 3, pp24 1-245, 1986.
PAPAKONSTANT1NOU G., C.MORAITIS, T.PANAYIOTOPOULOS, "An at-tribute grammar interpreter as a knowledge engineering tool", Angewandte Informatik, 9/86, pp 382-388. 1986.
VOUROS G.A., C.D.SPYROPOULOS, "A methodology for conceptual represen-tation of knowledge using attribute grammars", Angewandte Informatik, 7/89, pp
287-293, 1989.
ACKNOWLEDGEMENT
The ELS has been developed under KBSSHIP ESPRIT Project P-2163, which was partially funded by EU.
