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AN INTELLIGENT CHEMICAL AND PRODUCT CARRIER
LOADMASTER
L. BARDIS, G. GRIGOROPOULOS, T. LOUKAKISt Deparznenz of Naval Architecture and Marine Engineering
National Technical University of A theris 9 Hero on Polyechniou st., 15773 Zografos, Greece.
and
S. KOKKOTOS, C. SPYROPOULOS. G. VOIJROS Institute of Informatics and Teleconimunicatioris
N. C.S. R. "Demokriros" 15310 Aghia PaPaskevi. Greece
CORRESPONDENCE ADDRESS: Dr. G.A.Vouros
Inst. of Informatics and Telecommunications NCSR Deniokzitos
Athia Paraskevi Attikis. Greece Tel: +30-1-6513310
Fax: +30-1-6532175
An Intelligent chemical andProductCorner Loathnas:er
AN INTELLIGENT CHEMICAL AND PRODUCT CARRIER
LOADMASTER
ABSTRACr
Chemical and Product Carriers (CPC-) are ships licensed to cars)' dangerous and sensitive products in liquid form. The production of cargo distribution and cargo handling plans for CPCs are very complicated and time consuming daily casks that are performed by experienced ship operators-with the aid of software applications called loadmasters. Loadmasters, ven a
cargo distribution, perform hydrostatic and stability calculations. This paper describes an
intelligent chemical and product carrier loadmaster that goes beyond current practice extending the capabilities of traditional loadmasters. The system designs cargo distribution plans and devises cargo handling plans for CPCs. satisfying user requrements. time and safety constraints. The paper presents in detail the overall architecture, the interface and the kinds of knowledge exploited by the system.
An Intelligent C'h ical and Product Carrier Loadrna.Tler 3
1.
Introduction
A Chemical and Product Carner (CPC) is a ship that carries chemicals, often dangerous, corrosive or sensitive. in liquid form. CPCs are ships with complicated layout, encompassing a large number of tanks, each with its own characteristics, such as heating equipment, special coating, etc. The allocation of cargoes to the ship compartments and the construction of a plan of cargo handling operations for the
actual loading/unloading of cargoes are very delicate, safety critical and time
consuming tasks. They are regulated by international rules and regulations, and must fulfil specific requirements and restrictions. Rules and regulations concern the floating condition of the ship. Requirements and constraints are imposed by the ship layout, the chemical reactivity between cargoes. as well as between cargoes and ship equipment. special charter requirements and general requirements such as economy and effective utilisation of time.
Ship operators are highly skilled persons with great experience in devising both
a cargo distribution plan and a plan of cargo handling operations. Their job is
assisted by software applications. called loadxnasters. These calculate the floating condition of the ship, when a specific cargo distribution isprovided. The complexity of the cargo distribution and cargo handling tasks and the expertise required to achieve safe and efficient solutions justify the need for an advisory expert system.
The present paper describes the Expert Loading System (ELS). ELS integrates an algorithmic module for calculating the floating condition of the slip, with a reasoning component. The system automatically designs a cargo distribution plan
An Intelligent Chemical and Product Carrier Loadinas:er 4
and specifies the operations needed for its realisation. ELS reduces the time needed
for performing these tasks on a CPC, indicates and eliminates the critical and
dangerous situations that may occur during these safety-critical processes [1,2,3].
Chapter 2 of this paper briefly presents the domain. Chapter 3 presents the
overall architecture of ELS. Chapters 4,
5 and 6 present the user interface.
algorithmic and expert system components of the system. Finally, chapter 7
discusses the general characteristics of ELS, its benefits and drawbacks, and ideas for further improvement.
2.
The CPC domain
CPCs transport liquid cargo grades to specific ports of call. The ports. the
amount of cargoes transferred as well as special cargo treatment required for
discharging or charging each cargo are specified in the charter contract. Ship
operators, with the assistance of loadmasters, allocate cargoes to specific ship
compartments and device a plan of cargo handling operations for the actual
loading/unloading of cargoes in each port of call. These CPC tasks present a
significant level of difficulty because of the sensitivity of the products involved, the complexity of the equipment and, more important. because of economy and safety factors. The constraints, rules and regulations, which must be fulfilled during CPC tasks, concern [1,3]:
An Intelligent Ozemiçal and Product Carrier Loadmaxter 5
1. The floating condition of the ship. It mcludes the following: Hydrostatic and strength values. These are
Draft. It can be fore, aft or
tnidshkp draft. Draft must be less than a
maximum draft valid at each port of call. Particularly, draft aft must be minimum in order to ensure adequate propeller immersion.
Trim. This is the difference between draft fore and draft aft. Trim should not
exceed one meter and in departure condition should be by stern to
minimise ship resistance. Trim optimiation results in reduced fuel
consumption and contributes to economy. This constraint can be violated
&n port.
Heel. It is defmed to be the amount of listing towards a side.
Maximum shearingforce. Shearing force tends to "slice" the ship in two. Maximum bending moment. Bending moments tend to bend and "break" the
ship.
Bending moments and shearing forces must not ..exceed strength limits at any point along the ship. This constraint may be relaxed at port but must be strongly satisfied at sea. The violation of this constraint n port should be
done only in extreme cases. Great bending moments may cause serious
damages to the ship hull.
Minimum !netacentric height (GM) and minimum areas under the GZ-8 curve. These constraints are specified by the SOLAS (Safety of Life at Sea) convention.
An Intelligent Chemical wd Product carrier Loadmas:er
2. Cargo compatibility corwaints. These include
Special cargo treatment. Many of the cargoes that a CPC carries are sensitive, corrosive or dangerous. These cargoes require special treatment, like heating during the whole voyage. Moreover, the operator must take care to allocate these cargoes to tanks with compatible coating and piping.
Compatibility between cargoes. Pairs of cargoes. which are dangerous when
mixed, or can be contaminated, are not allowed to be loaded in acoining
tanks.
Contamination requirements. A tank must be cleaned according to prescribed procedures before loading a new different cargo to it. All pipes, pumps and valves must be thoroughly cleaned too. This imposes practical limitations in
case a set
of pipes, which serves several tanks, must be used for
loading/unloading different cargoes in the same port.
3. Other safety and economy requirements. For instance.
A voidance of slack loading. A tank is slack when it is filled less than about 50% of its total volume. In this case the free surface of the cargo has .i large potential for movement, which in bad weather may shift the centre of gravity. Multiple loading and unloading. A CPC usually loads and unloads several cargoes in several ports. Thus, a tank may be discharged and then cl:iarged with a new cargo in the same port. The design of the cargo distribution and cargo handling plans must take into account this switching of cargoes in
intermediate ports
of call. Moreover, switching cargoes amonz ship
An Inieiligenz che,nical and Product carrier Loadmaster
Damage stability requirements. These are safety constraints that concern the final ship floating condition after one or more tanks lose their watertight
integrity. Damage stability calculations must be submitted to the port
authorities before the ship is allowed to leave.
Time limitations. Time spent in port is very expensive for the ship. There are port fees to be paid, and time is lost for the ship. For this reason the loading and unloading must be designed in a way that allows for the most efficient and fast use of the equipment. For example, switching between cargoes using the same set of pipes requires extensive cleaning operations. which are very time consuming. It is best to load or unload only one cargo per set of pipes. if possible, and clean the equipment while at sea.
During the
last years, ship operators use specially designed softwareapplications, called loadniasters, which compute the hydrostatic and strength
values, given a cargo distribution. Operators provide the distribution of the cargoes
to the ship tanks manually. Loadmasters incorporate a database containing the
ship geometrical data, including hull and tank geometry: the distribution of all fixed weights along the ship. such as the ship hull and permanent stores: and typical consumable plans. i.e. fresh water, fuel oil. lubrication oil, for the departure, sailing, and arrival conditions. Ship operators inspect the output. If a condition, rule or regulation is not satisfied, operators apply corrective actions, such as redistribute cargoes, or allocate ballast to the ballast compartments, or even discard some cargo
An Intelligent Chemical and Product Carrier L.oad,nas:er
Even with the use of loadmasters, the design of a typical cargo distribution plan may require several hours. Furthermore, in order to realise the cargo distribution plan, a cargo handling p1n is required. In current practice, operators monitor the whole procedure. which may last 2 or 3 days, and foresee critical and dangerous
situations that arise during loading/unloading of cargoes and take precautions
against theni.
Some loadmasters specifically designed for a particular ship type, offer some
additional functionality. For example, a loadmaster for container-ships may
incorporate a container handling module (lashing arrangement, administration of data required by port authorities, etc.). A loadmaster for liquid cargo ships may include monitoring of tank levels by reading shipboard sensors installed in the ship tanks.
ELS integrates a CPC loadniaster with an expert system
It designs cargo distribution plans and devises cargo handling plans, that fulfil the rules, regulations and the special constraints that govern these tasks.3.
ELS Design Architecture
ELS is a highly modular software application [4]. Figure 1 displays the ELS architecture.
The User Interface of the ELS is a graphical menu-driven interface. This module accepts the user requests (button clicks as well as textual input) and passes them to the ELS Manager The User Interface accesses the relevant databases and tables and updates the screen. Data are displayed in both tabular and graphical forms.
An intelligent Chemical and Product Carrier Loadmaster 9
The ELS Manager is the main module of ELS. It operates as a request/service dispatcher, accepting service requests from the ELS modules and passing them to the relevant modules. When the requests are serviced, the results are passed, through the ELS Manager. back to the requesting module. For instance, the ELS Manager
forwards the Expert System requests for the ship floating condition to the
Algorithmic Module. When the calculations are performed, the results are
forwarded to the ELS Manager which passes them back to the Expert System.
The Algorithmic Module consists of a library of routines, which perform
standard hydrostatic and strength calculations. More specifically, the Algorithmic Module contains routines for the calculation of the ship floating condition for a
given cargo distribution plan. Furthermore, the Algorithmic Module contains
routines for damage stability calculations. These are calculations of the final ship
floating condition after one or more tanks lose their watertight integrity. The
calculations performed by this module are the set of calculations that conventional loadmasters perform. The Algorithmic Module, together with the User Interface, may be considered as a conventional loadmaster.
4p Intelligent Chemical and Product carrier Loadinaster 10 Product Database Potts Database
'esa
Re!ulatons Geometrtai Database ,' "Equipment Database Charter Table User) User Interface ELS Manaer Expert System Loadtng Handling) Planner I Uflltj Stabtbty ndStrenth' I ) Table ,-' Engine Caigo'\
! -Distnbution Table Loading Opcmtions Table A Rule Base Algoriclurne I Module Figure 1. ELSArchitectureThe Expert System is the component that distinguishes ELS from conventional loadniasters. It is responsible for designing a cargo distribution plan. and a plan of cargo handling operations for the actual loadinglunloa ding of cargoes at each port
of call. The Expert System takes into account all the regulations. rules and
constraints, as well as any requirements imposed by the user. User requirements concern loading .a specific cargo amount to a specific segregation or leaving a
An Intelligent Chemical and Product carrier Loadmaszer 11
specific segregation empty. The Expert System consists of the Inference Engine, the
Loading Planner and the Cargo Handling Unit. Moreover, during reasoning, it
consults the Rule Base.
The Inference Engine consults the Rule Base and activates the appropriate rules for problem solving. It implements the most basic control mechanism of the Expert System.
The Rule Base contains two sets of rules: one for the Loading Planner and one for the Cargo Handling Unit. Each rule is associated with a set of procedures which are associated with specific aspects of rules' execution. Procedures are stored in the coEresponding module to which the rule belongs.
The Loading Planner plays a central role in the ELS. This module. in
cooperation with the Inference Engine and the Rule Base, construct the cargo
distribution plan. The cargo distribution plan specifies the allocation of specific amount of cargo to the cargo tanks, ballast to the ballast tanks and the amount of consumables (oil, fuel, water) in the appropriate tanks. for each port of call.
The Cargo Handling Unit, in cooperation with the Ifiference Engine and the Rule Base. constructs the cargo handling plan for each port of call. The cargo handling pLan defmes the required operations on the ship's equipment (tanks, pipes, pumps, valves) as well as on the equipment provided from shore (transfer lines) in order to
achieve the cargo distribution plan for each port of call. During thi task the
hydrostatic and strength values must be within the limits specified by the rules and regulations.
The five ELS databases store data about products. ports of call around the
An Intelligent Chemical and Product Carrier Loadniasfer 12
equipment and layout. Some of the databases, like the product and port databases, can be changed by the user. Others. like the geometry database, are fixed.
The Product Database contains the physical and chemical properties of the liquid chemical substances allowed to be transported by the particular ship. The most important properties are the density of the cargo as a function of temperature, and the chemical compatibility. The latter defines pairs of cargoes that are not allowed to be loaded in adjoining tanks.
The Ports Database records data about the various ports of call, such as
maximum draft, number and capacity of shore transfer lines as well as the airdraft (maximum height of the ship over the water line).
The Rules and Regulations Database is a list of maximum and minimum values
for the hydrostatic and the strength measures, such as the maximum bending
moment, the maximum shearing force. the maximum draft, the maximum thm i the maximum heel.
The Geometrical Database contains the definition of ship's external surface in numerical format. This database is used for the calculation of the hydrostatic and strength values by the Algorithmic Module, as well as for drawing an image of the ship on screen.
The Equzoment Database contains a description of the ship equipment related to
cargo handling and storage. It records the geometry of the tanks in numerical
format, and other tank attributes, such as coating and heating equipment. These attributes define the range of cargoes that can be loaded in the specific tank. This database also records the characteristics and connectivity of the ship's pumps, pipes and valves.
An Intelligent Chemical and Product Carrier Loadmaster 13
The four ELS Tables are used by the ELS modules for recording the charter contract and the current status of the ship.
The Charter Table records the current charter contract. A list of old charter
contracts is also available.
The Stability and Strength Table contains the hydrostatic and strength data that characterise the current ship floating condition.
The Cargo Distribution Table indicates the amount and type of cargo stored in each ship tank for evezy voyage segment (part of the voyage between subsequent
ports of call). This table contains the cargo distribution plan produced by the
Loading Planner.
The Loading Operations Table contains the series of operations required to load or unload ship tanks at each port of call. These are the operations produced by the Cargo Handling Unit.
4.
The User Interface
The users interact with ELS via a gaphical menu-driven interface [5]. The ELS interface is designed so that it can be used by novice users with the minimum of computer experience and practice on ELS. Specifically, the user interface was designed to simplify use of the system. and then, to make the system more attractive to its end users. While developing ELS we kept in mind that end users care more about the easy and effective use of the system, rather than about the underlying
techniques that the system implements. Moreover, end users were actively
for theeffectiveness oftheuser intetface. It must bementioned that a large part of
our development effort was allocated to the User Ihterface.
Figure 2 pictures the mainELSscreen, which consists of five parts.
Figure 2. Main ELS screen
o The Menu Column is located, at the right part of the screen. The top of this
column is the. Exit Button, labelled ELS, that can be activated .at any tune, from any point in ELS. Below the Exit Button is the Application Area Indicator that displays
the cutrent application area. The indicator is a push button that
activates the main menu. Below the Application Area Lndicator is the Primary Menu Area. This area initially contains the main ELS menu. When the userselects an application area from the main menu, ELS replaces the main menu
with the area-specific operations menu At the bottom of the Menu Column is
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An Intelligent ck Load,nasrer 15
the Secondary Menu Area. This menu displays options, which are activated when an area- specific operation has been selected.
The Hydrostatic Values' Bar, at the top of the screen, displays the floating
condition of the ship.
The Message Bar, is located below the Hydrostatic Values Bar The Message Bar is a text fieldthat either displays helpful messages to the users or allow them to type whatever input ELS asks for.
The Graphical Window, at the centre of the screen,displays a gaphical image of the ship. the loaded cargoes are pictured in different colours, while the height of a cargo in a tank is proportional to the actual height in the real tank. The user may select to viCw a particular set of tanks (cargo. ballast, consumables. port, starboard) using the buttons inside this window. A label inside this window indicates the current port and time (arrival or departure). Users can change the port and time by seiecting the appropriate operation from the Menu Bar.
The Tables at the bottom of the screen isplay the cargo distribution. There exist
three such tables: The leftmost one displays the contents of the cargo or
consumable tanks, according to the selected view in the Graphical Wi.nclow. The middle one displays the contents of the ballast taiiks The rightmost one displays the amount of the currently allocated cargoes. The contents of the Tables depict the cargo distribution in the specific port, at the specific (arrival or departure) time.
There also exist four pop-up windows, which are activated with special user requests:
AnInzellzgent Chemical and Product Carrier Loadmasier 6
The Charter Contract Window, allows the user to enter. review and modify the charter contract. This window is pictured is figure 3.
The Hydrostatic Diagrwns Window, that replaces the Graphical Window, and displays the cunett values of the bending moment, shearing force and static stability (GZ-8) curves of the ship. along the ship's axis. The user can recall these diagrams at any tinie by selecting the appropriate menu choice. An instance of this window is pictured is figure 4.
The Help Window, displays the on-line help.
o The Explanation Window, displays the explanations generated by -the Expert
System.
Figure 3. G'hprzer Contract Window.
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Figure 4. Hydrostatic Diagrams Window.
5.
The Algorithmic Module
The Algorithmic Module is a library of routines perfôrniing all functions of a typical loadmaster. Such functions are:
Calculation of hydrostatic and stieugth parameters of the ship. Input comprises the fixed wei.hi distribution, the geometry of the ship, and the amount of cargo and ballast in each ship compartment.
Damage stability calculations, assuming loss of the ship watertight integrity. Input comprises the hydrostatic and strength values before the damage, and the damaged ship compartments. The calculation assumes that the whole damaged compartment has been replaced by sea water.
Calculation of influence coefficients. These calculations provide the change that should be done to the. amount allocated in a specified compartment. in order to change, in a specific way, the value of a stability or strength parameter. This function is particularly useful during the optimisation of the cargo distribution and the fme-tuning of the cargo handling plan.
An 1ntellient Chemical andProduct Carrier Loadm aster 18
6.
The ELS Expert System
The Expert System module of the ELS consists of the Loading Planner, the Cargo Handling Unit, the Inference Engine and the Rule Base. It is this part that makes ELS intelligent, in contrast to other loadmasters. The Inference Engine of the ELS uses the rules encoded in the Rule Base, and the procedures of the Loading Planner and Cargo Handling Unit to provide a near optimum cargo distribution plan and a sequence of cargo handling operations for each port of call.
6.1. Loading Planner and Cargo Handling Unit
The goal of the Loading Planner is to design a cargo distribution plan, given a charter contract. The Loading Planner takes into account the following restrictions:
..
Rules and regulations that govern the cargo distribution task. Specific charter requirements.Properties of the cargoes to be transported.
Compatibility between cargoes and cargo handling equipment such as cargo pipes. pumps, valves. etc.
Specific user requirements concerning the allocation of specific amount of
cargoes to specific compartments.
To achieve the target goal, the Loading Planner can perform the following
actions:.4,: Jnte1lien1 Chemical and Product carrier Loathnasler
Allocates a specific amount of cargo to a 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 segregations.
Ailocates ballast to specific ballast tanks to achieve a good and safe floating condition of the ship.
Submits requests to the Algonthniic Module to carry out additional stability and strength calculations to either test an allocation plan or to test whether it has achieved any progress towards eliriiirating critical situations.
The goal of the Cargo Handling Unit is to plan a sequence of cargo handling operations performed on the ship equipment and on the equipment provided from shore. in order to load ndJor unload the ship while in a port of call. To achieve the target goal, the Cargo Handling Unit performs the following actions:
Constructs a list of the cargoes that should be loaded or unloaded in a port of call and specifies the allocation of transfer lines to cargoes and segregations.
Defines the valve flow of the pumps for the loaded or unloaded tanks, in order to prevent critical and dangerous situations.
Requests for loading or unloading various ballast tanks in order to correct
An intelligent Chenzical and Product Carrier Load,naster 20
Submits requests to the Algoi-ithrnic Module to carry out ad itional stability and strength calculations to either test critical values dtn-ing intermediate stages, or to monitor the progress achieved towards eliminating critical situations.
The problems of allocating cargoes to the. ship tanks and planning the cargo handling operations have the following characteristics:
(I) The configuration of the problem solver changes as the amount of cargo and ballast changes in the cargo and ballast tanks.
The problem solver must respond dynamically to ever changing demands. For instance, it should change the rate of cargo or ballast loading or unloading, in order to prevent hydrostatic and strength value limit violations.
There is a tradeoff between economy and satisfaction of the floating condition constraints. For instance, in order for the Cargo Handling Unit to keep the trim or the strength values within limits, it should pause loading or unloading a number of ship tanks, for some time. This contradicts the economy goal since
the ship must reman a longer time in port. More important. the floating
condition constraints do contradict among themselves. For instance, the
prevention of a dangerous strength value may result in a violation of the trim limits. Furthermore, stability and strength constraints can prevent the Loading
Planner from loading 100% of the requested cargoes. In this case the
corresponding constraints may be relaxed and be partially satisfied in order to satisf' the economy goal.
An Intelligent Chemical and Product carrier Loadmaster 21
6.2. Knowledge Categorisation.
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 consists of the rules and regulations concerning the floating condition of the ship, the chemical compatibility between cargoes. as well as compatibilities between cargoes and ship equipment. and special requirements of the ship operator concerning the allocation and handlimz of cargoes. This knowledge is utifised by the heuristic and operational knowledge.
Heuristic kno;tledge is the knowledge that ship operators exploit during problem solving. This knowledge category includes:
An huelligent chemical and Product Carrier L.oadmas:er
knowledge about forming specific decisions in particular problematic situations. For instance, the rule
f there is a dangerous aft trim,
and the bending moment has an allowable value, then load a segregation positioned in an extreme fore posit ion
is a heuristic rule that forms the decision to load some cargoes in the ship bow in case the ship is dangerously overloaded aft and the bending moment limits are not violated.
knowledge about decision making in cases where constraints can be relaxed. For instance, such a rule is the following one:
no decision can be formed.
and this is not the final state of the ship. and the situation is critical but not dangerous, then do nothing
This rule directs ELS to wait for the next state, since the situation is
not dangerous yet. Thus, it allows for the possible improvement of the situation in subsequent states.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:
An mnzellieng Chemical and Produci Carrier Laadnas:er 23
If there is' progress in reducing trim,
and the bending moment has an allowable value, then the goal is' to reduce trim
Control knowledge Ipi-esents the problem solving state as well as the demands during any problem solving step. Such knowledge comprises the internal state of ELS and consists of the decisions formed by heuristic rules, the hydrostatic and sticugth measures provided by the Algorithmic Module, as well as the progress report, towards achieving a particular goal. The progress report and the hydrostatic measures are calculated after the execution of a specific action. The declarative
nature of control knowledge facilitates the system to respond dynamically to
changing demands, provides guidance for heuristic and operational knowledge execution, and allows for efficient and informative explanation of Expert System's behaviour.
Moreover, apart from control knowledge, the most basic control mechanism of the ELS Expert System is realised by the inference engine.
Operational knowledge comprises specific actions that should be performed in particular situatiOns. Actions help to achieve the desirable effects (e.g. to increase negative bending moment at a particular point on the ship axis) reported in the internal state of the ELS. In other words, control knowledge guides and justifies the execution of specific actions in particular situations.
Such an action, justified by the decision to "load a seresation positioned in an extreme fore position", is the following one:
An intelligent Chemical and Product Carner Loadnzaszer
If segregation Y is in an extremefore position. and is allowed to contain X tons of cargo Y. then load segregation Y with X tons of cargo C
6.3. Implementation of ELS Expert System
Control knowledge in ELS is represented by decision frames [6]. A decision
frame represents the internal ES state at a specific time point. Decision frames
record the hydrostatic and strength measures. the decision context, the violated
constraint, a judgement of the criticality of the situation, and a progress report
towards eliminating theviolated constraint. Figure 5 describes the generic format of the decision frame.
Knowledge about restrictions, as well as operational and heuristic knowledge are
implemented by Attribute Grammars. Attribute Grammars had been initially
proposed by D. Knuth as a formal specification tool for programming languages [7]. In [8.9.101 they have been proposed as a framework for knowledge engineering. We have chosen Attribute Grammars because of (a) the high level Language they provide, the Attribute Grammars language itself. (k) the increased problem solving abilities due to the incorporation of user defined semantic functions. (c) the need to convert the ELS PROLOG prototype to the C language. Attribute Grammars have been implemented in the C language. The existence of well defmed methodologies [8.9 10] for converting the PROLOG prototype to Attribute Grammars assisted the development of ELS using C.
An Intelligent Chemical and Product carrier Loadmaster 25
In ELS, the Attribute Grammars BNF rules are recorded in the Rule Base. Due
to the use of databases and tables, the Attribute Grammar requires only a few
attributes.
Attribute Grammar symbols
haveattached procedures.
These procedures are executed during the reasoning process. The procedures are contained in the Loading Planner and Cargo Handling Unit modules and check compliance to constraints, report the achieved progress, read and update the databases and tables,and request the calculation of the ship floating condition by the Algorithmic
Module.In particular, procedures can be categorised as follows:
Decision Frame
C onstràint: (Violated hydrostatic measure constraint.
chemical compatibility constraint)
C'riticality: (dangerous, critical, dont_care}
Progress: {yes.no}
Context: (fix_ballast, optimization, draft_allocation. load_total_amount
Current Status: (current values of hydrostatic measures) Previous Status: (the values of hydrostatic measures before the
last strategy instantiation)
Target Segregation Type: (fore. aft, middle, fore-middle, aft-middle)
Action mode: fload. unload)
An Intelligent Chemical and Product Carrier Loadmaner 26
.
Preconditions, that check whether a specific situation occurs. These procedurescheck for the violation of particular resthctions. They are utilised by heuristic
rules to recognise 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 databases and tables prior to the execution of a heuristic rule. For
instance, before the start of cargo allocation optimization, an initialisation
procedure is fired that changes the status of the ballast segregations to "empty", and assigns the value "optimization" to the "context" slot of the decision frame.
Postconditions, that check Whether a sequence of actions achieved the desirable
effect. The postcondition "progress" is a special one. because it is checked after the execution of every action. This postcondition determines whether the action achieved any progress towards eliminating the violated constraint recorded in
the constraint slot of the decision frame.
Side effect procedures. that update the ELS tables and databases. Side effect procedures perform the corresponding actions.
6.4. Inference Engine
The Inference Engine is based on original concepts published in [9.10,11].
However, it has been tailored to the needs of ELS. In particular. a parser for
processng Attribute Grammars, described in [8,9], has been modified so that it can
An Intelligent 'he,nica1 and Product Carrier Loadniaster 27
procedures, and test the postconditions and preconditions 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.
In particular. the ELS-taiored 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. X=Z) is made the current goal, the parser indexes the rule
X::=XiiXii...X.J...IXi...X
whose head is the current goal X and checks the preconditions of the rule. If the
precbnditions are true, then the initialisation procedures are evaluated and the
leftmost symbol X11 of the first alternative of the rule is made the current goal. This line of reasoning proceeds until a goal Y11 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 ate checked. If they are true, the side effect procedures are performed and success is reported in the parent node Y. The next symbol i2 of il in the rule with head Y is made the current goal and the parse tree expands to the right of Y. Finally, Y reports success to its parent node when an alternative has been successfully completed (all the symbols in the alternative have reported success) and its posiconditions 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, thenext alternative of its left sibling is processed. When no left sibling exists, (incase the
An intelligent Chemical and Product Carrier Loathno3rer 28
falsified goal was the first one of the cuxrent alternative of the parent node) the next alternative of the parent node is proced.
Figure 6. Reasoning Parse Tree
Figure 7 shows the interaction between the Inference Engine, the Rule Base. the Load Planner and the Cargo Handling Unit.
7.
Condusions
In the present paper the ELS system for designing cargo distribution and cargo
handling plans for Chemical and Product Cariers has been discussed. ELS
integrates conventional techniques with an expert system module that represents and exploits the knowledge of experienced ship operators. Thus. ELS differs from
conventional loadmasters in that
itis able to automatically design a cargo
distribution plan as well as a cargo handling plan, thus reducing the time needed for performing these tasks, and softening the critical and risky aspects faced during this safety critical process.
An Intelligent Chemical and Product Carrier Loadniasrer 29 Goal Rule No INFERENCE ENGINE (Rule No Alter No lndet) RULE BASE
Sha4oed boxes indicate the proc'iures that are ucd for the communication betwena the units.
Arros show the flow of data.
Rule No indicates the serial number of the currently active rule.
"Flag" indicates whether the prctonditions or postconditions of the currently active rule axe
met
"Goal" is thecurrentlyindeced Attribute Grammar symboL "Alter No" is the serial number of an alternative within a rule
"Index" is the serial number of an Attribute Grammar symbol within a specific alternative of
a rule.
Figure 7. Interaction between the Inference Engine, the Rule Base, the Loading Planner and the Cargo Handling Unit
ELS is designed as a set of cooperating modules, each highly speca1ised. The modules are controlled by the ELS Manager module, and communicate by passing
Goal Index
(Rule No Alter No
Goal)
LOADING PLANNER CARGO RANDLING LNIT
EvEff.
A A
Rule No Rule No Rule No Rule
flag flag
An Intelligent Chemical and Product Carrier Loadjnaster 30
messages and updating the common Databases and Tables. ELS has been designed during the EPSRIT II KBSSHIP project, and has been implemented in C and X Windows under UNIX. It provides user friendly operation via a Graphical User Intexface.
For the future we indent to extend ELS towards configurability. More
specifically we plan to design and implement a configuration manager, which will be able to customise the ELS databases, tables and rule base by analysing the diitised ship plans and processing the end user regulations. Moreover, we intend to built a generic framework for developing expert loading systems in an efficient way [1lJ. Thi will allow the customisation of ELS's knowledge to the particular needs and requirements of the shipping companies; as well as to the needs of its indiidua1 usex.
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An Intelligent Chemical and Product Carrier Loadmaster 31
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