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SNAME Transactions, Vol. 105, 1997, pp. 171-189

List Control SystemsFrank H. Sellars, Member, F lume Stabilization Systems, John P. Martin, Member, Flume StabilizationSystems, William L. Schultz, Visitor, Hyde Marine Systems

ABSTRACT

The ypesof ships hat normallyapply automatic ist control systems re reviewedand thereasonsor the applicationare summarized.Various ypesof list control systemshat arecommercially vailableare enumeratedn detail and the differences etween hesesystemsare explained. Design Considerations hat could define selection criteria are listed,including some otential problem areas.

NOMENCLATUREINTRODUCTIONLCS = list control systemHPU = hydraulic power unit

TDH = total dynamic headB = distance between list control tanksH, = port tank levelH, = starboard tank levelH, = head added by pumpc

= head loss due to friction= blower discharge head

AD = cross duct areaPa = atmospheric pressurePP = air pressure in port tankP, = air pressure in starboard tankg = blower discharge pressure= liquid volumetric flowQ, = air volumetric flow4 = list angleV, = fluid velocityV, = air velocity2

= fluid flow loss coefficient= air flow loss coefficienty, = liquid specific gravityYa = air specific gravity

Excess list of a ship during cargo handling operationscan be detrimental. Ferries and RO/RO vessels useloading ramps that are installed on either the bow, stern, orthe side of the ship. These ramps usually cannot sustainlists over 2 to 3 degrees without damage. Container shiploading and shipboard crane operations are delayed orinterrupted if list exceeds 5 degrees .Modern cargo liner opera tions depend on rapid loadingand off loading to maintain schedule. Automatic listcontrol systems have been developed to rapidly transferlarge quantities of water (2,000 to 110,000 gallons perminute) from side to side for this purpose. These listcontrol systems consist of pairs of wing ballast tanks andcrossover piping, a mechanical device to transfer water andan automatic electronic control system to sense list andtake the necessary corrective action. Systems that havebeen developed include centrifugal or axial flow pumpconfigurations as well as processes that use compressed airto move the water.The purpose of this paper is to review the factorsthat have led to the development of modem, high capacity

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that are in use and commerc ially available .Some aspects of list control system design criteriaare considered, including sizing o f tanks and cross overducts, selection of a prime mover and systemarrangement. Automatic control system requirements arediscussed.

experienced. In the manual mode, the flow direction iselected by hand and the flow rate may or may not bcontrolled electronically, depending on the technicacapabilities of the mechanical system being controlled.The manual mode will allow the ship to be pre-heeled tone side in anticipation of a large load being placed on thopposite side.LIST CONTROL SYSTEM APPLICATIONS

All ships have some form o f list control capabilityusing the ballast system. However, if the system is notautomated, considerable time and effort is required to lineup valves and start and stop pumps. In addition, ballastpumps have relatively low flow rates. Ships that oftenemploy automatic list control systems include containerships, roll-on/roll-off (RO/RO) cargo carriers, ferries,heavy lift ships and side casting dredges.MOVING A WEIGHT

RO/RO vessels deliver wheeled cargos of a variety ofsizes within tight schedules. They often use jumbo sternramps that are as wide as practicable (Argyriadis 1979).The torsional flexibility of the ramp limits the list that canbe accommodated when it is deployed to 2 or 3.Accordingly, list control systems (LCS) have beendesigned to keep list below this, or on an even keel,without interrupting cargo handling.Considering containerships, list control systems thatmaintain list within 2 will expedite placement ofcontainers in cells. Some operators will apply an initiallist of 1 outboard of the pier to accommodate deck cargo.

COUNTERBAL~WCINGWITH

LIST CONTROL TANKS

Heavy lift crane vessels use the LCS to keep list EFFECT OF CARGO LOADINGwithin crane operating limits (approximately 5). FIGURE 1DESCRIPTION, PRINCIPAL COMPONENTS ANDAPPLICABILITY OF LIST CONTROL SYSTEMS

The list control system moves water between sets ofwing ballast tanks to counter-balance an off-center load orthe transverse movement o f cargo. Figure 1 shows howloading cargo on a ship will cause list and how themovement of water between the list control tanks willcorrect this list.

The heart of the system is a reversible device whichis controlled by an electronic control system for thepurpose of moving water between wing tanks on the portand starboard sides. There are usually options for use ineither an automatic mode or a manual operating mode. Inthe automatic mode, the flow and its direction are selectedand controlled electronically to correct any list angle

An additional feature of some list control systems isthat the transverse metacentric height (GM), a measure othe stability of the ship, can be measured in port, prior tosailing. If a computer based electronic control system isavailable, the hydrostatic characteristics of the ship andthe capacity curves of the list control tanks can be loadedinto the memory of the electronic control system. Theforward and aft draft marks either can be enteredmanually or obtained via a link with a draft indicationsystem to determine the displacement. Water istransferred between the list control tanks until the shipassumes a pre-programmed angle. The computer candetermine the heeling moment from the tank capacitytables and the change in tank levels. Since thedisplacement and heel angle are known, the GM can becalculated with reasonable accuracy.

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Several types of list control systems have beendeveloped. Those that have been installed recentlyencompass systems that control an existing pump ofacceptable capacity as well as dedicated systems thatinclude axial flow pumps with either constant speed,variable speed, or variable pitch drives or that use aforced draft blower to transfer water by air pressure inplace of a pump. The type of LCS selected depends onthe vessel size and operational requirements. Electroniccontrol systems that use existing high capacity pumps(such as a ballast or fire pump) have been developed tominimize list. Such systems are generally used whenheeling moment correction requirements are relatively lowand ship operations do not require a short port stay.Independent heeling pumps with variable speed/pitchor blower systems are used when heeling moments arelarge enough to cause uncorrected list of over 1.5degrees. For longer cycle times and lower heel momentrequirements, but still requiring a higher capacity than canbe provided by an existing pump (such as occur onpassenger ferries or container feeder ships) a constantspeed axial flow or propeller pump system is oftenselected, since its cost is relatively low. For ships withshort turn around requirements and rapid cargo handlingrates, or that handle heavier cargos, such as RO/ROvessels that carry trains or cassettes, and military Sealiftships, either a variable speed/pitch propeller pump or acompressed air system is selected. Operations thatrequire rapid application of a large heel control momentsuch as heavy lift vessels with small allowable heelingangles, would use a compressed air system or a largecapacity variable speed/pitch pump.The system reaction time is the time required to reactto and correct a change in list. Variable speed/pitchpropeller pump systems and compressed air systems havereaction times as short as several seconds. Constantspeed, reversible propeller pump control systems takelonger to react.Selection Criteria for Heeling Tanks

All of these systems transfer ballast between twodesignated list control or heeling tanks. As the transversedistance between the centers of the tanks increases, theamount of water that must be transferred to obtain thenecessary heeling moment decreases. Therefore, thesetanks are usually narrow wing ballast tanks locateddirectly inboard of the shell in the parallel middle body ofthe hull. Such tanks generally are readily available intypical RO/RO or container ship arrangements. Theheight between the top and the bottom of the tank willhave an impact on the required power, and should be

minimized. In addition, the vertical distance between thebottom of the tank and the pump will influence the suctionpressure and the onset of cavitation, so it should bemaximized. These considerations lead to the conclusionthat long, shallow, upper wing tanks should be selected asthe heeling tanks, when available.The structure of the tanks should be checked to insuregood flow to the crossover duct. The vents or airexchange ducts should be adequately sized to ensure thatthe movement of large volumes o f water will not draw avacuum or increase the discharge head. At the start oflist control opera tions, each tank should be approx imately50% full to allow the maximum heel correction momentto be realized when the entire contents of one of the wingtanks is transferred to the other.List Control Using Available Pumps

Many container feeder ships, passenger ferries, orsmaller RO/RO ships will use an electronic heel controlsystem to operate an existing ballast or fire pump. Whenoperating as part of the heel control system, the pumpmust be isolated from all other tanks except the heelingtanks. The main components of the system are:- the electronic control system- an existing centrifugal pump- remotely operated valves- level sensorsA schematic of the system is shown in Figure 2. Whenoperating as a list control system, the pump is alwaysrunning, recirculating water through the tank with thehighest level when no heel correction is required. Whena preset threshold heel angle is exceeded, the valves inthe suction and discharge lines are positioned to allowballast transfer to offset the load. The transfer continuesuntil another preset angle the shut-off angle, which isusually much lower than the threshold angle, is reached.The electronic control system senses the list angle,compares it to the threshold and shut-off values andproperly positions the valves to achieve eitherrecirculation (no transfer) or the movement of ballast tocorrect the heel angle. An example of a valvearrangement that will meet these requirements is shown inFigure 2 . This can generally be done with reasonablysimple circuit boards and a computer based system is notnecessary.The pump selected should be the pump that has thehighest capacity, usually the ballast or the fire pump. Theadvantage of the system is economic since a dedicatedpump is not necessary. However, capacity is usuallylimited when compared to a dedicated system.

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._

a one or two stage design depending on the requiredischarge head.The electric motor is connected to the pump by coupling and gear box. The gearbox is lubricated by aoil bath which is connected to a header tank which equipped with a low oil level indication.Between oil seals and the ballast water, there another sealing chamber connected to another header tankThe fluid in this sealing chamber protects the ballast wateagainst oil contamination and lubricates and cools shaseals.The heeling indicator is mounted in a cabinet suitablfor separate bulkhead mounting. The list angle can bmonitored on the heel indicator. Tank levels are neithemeasured nor used to control the system. Tank leveswitches are installed to stop the pump if either of the liscontrol tanks is empty.The electromagnetic butterfly valve is installed in thcrossover pipe and is operated by the control system. Tavoid water hammer, the valve is opened when the pumis running and closed when the pump stops.CONTROL USING AVAILABLE PUMPSFIGURE 2

The valves are sized to handle the maximum flowcapacity and are remotely operated either pneumatically orelectrically. These same valves can sometimes bearranged to fill and drain the tanks if the ballast pump isused. In order to avoid water hammer, it is importantthat the valves in the suction and discharge lines areopened and closed simultaneously.Level sensors are necessary to inform the controlsystem when either of the tanks is full or empty andwhich tank has the highest level. High and low levelalarms will not allow operations to continue in a directionthat will aggravate the situation. The tank with thehighest level will be selected for recirculation whentransfer is not required.

The electronic control panel contains an analocontrol system. An electronic amplifier receives a signaproportional to heel from the inclinometer. The preamplifier is set to zero at zero list and any heel will offsethe signal from the pre-amplifier. A starboard or port lisis sensed by a zero comparator which is used to controthe direction of rotation of the pump. A switch provided to shut off the electronic control and permmanual control of the pump. Change to either automatior manual operations can be done at any time in thoperating cycle. Alarms will sound if the levels in eithethe heeling tanks or the lubrication and seal header tankarc below minimums. In addition, if actual heel exceeda preset maximum angle, an alarm is given.Use of a constant speed axial flow pump usuallresults in a slow reaction time, The pump must be starteand the valve opened every time the heel angle differsfrom an even keel. In addition, there is a reduction flow as head increases for constant speed operation.Constant Speed Pump System Variable Speed/Pitch Pump SystemsA list control system using a constant speed reversiblepump is shown in Figure 3. Main components include;- the reversible propeller pump,- a heel indicator and tank level switches,- a butterfly valve, andan electronic control panel.The reversible propeller pump is driven through agear box by an AC electric motor. The pump is reversedby reversing the motor direction of rotation. Availablepump diameters range between about 10 to 16 and have

A list control system utilizing a submersible, variablspeed propeller pump is shown in Figure 4 . A systeusing a constant speed, variable pitch propeller pumwould be similar, with differences as noted below. Thmain components are:- a propeller pump,- a butterfly valve,- an electronic control unit with heel sensor, an- tank level transmitters..

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The pump shown in Figure 4 is a submersible unitinstalled in the crossover pipe. The tube housing thepump forms part of the crossover duct. Pump sizes rangefrom 18 to 42 propeller diameter. The pump is drivenby a hydraulic motor that is mounted in a pod within thehousing. The direction of flow is fully reversible andspeed is adjustable within the speed range. There is afixed pitch, five bladed stainless steel propeller that, alongwith the pod and hydraulic motor, is supported by a strut.The tube, pod, strut and housing are fabricated ofcorrosion resistant materials. The pod and strut are filledafter assembly with a water soluble grease. Theplacement of the pump and motor within the pipelineminimizes the space required and reduces the complexityof the installation. Hydraulic connections to the motorare run through the strut, They include two high pressurelines and a single, low pressure case drain.When the LCS is shut down or has been inactive forsome time while the system is in operation, flow betweenthe list control tanks is prevented by a wafer typebutterfly valve installed in the crossover piping. Thevalve is assembled as a unit with a hydraulic actuator andposition indicating switches. The hydraulic actuatorallows remote operation from the electronic control unit.The position of the valve is either completely open orfully closed, which is confirmed by one of the positionswitches. The materials used in the valve and positionswitches are bronze and stainless steel to permitsubmersible operation. An accumulator can be added tothe hydraulic system to shut the valve in the event of anelectric or hydraulic power loss. The closing time of thevalve is adjusted hydraulically or via the electronic controlsystem to avoid any water hammer problems.The propeller pump and butterfly valve are driven bya hydraulic power unit (HPU) located near the pump andvalve. This HPU is connected to and is controlled by theelectronic control unit. There is a main variabledisplacement hydraulic pump to drive the propeller pumpand a gear pump for the butterfly valve actuator, bothmounted on a common shaft and driven by an AC electricmotor.The direction and speed of rotation of the propellerpump is determined by a control plate in the mainhydraulic pump. The position of this plate is set bysignals from an electronic control unit. The open/closemovement of the butterfly valve is controlled by adirectional solenoid valve mounted on the HPU.Confirmation of the valve position is provided bymagnetic position indicating switches mounted on thevalve.The electronic control unit, which is PC based,allows selection of the operating mode and provides

control of pump speed and direction as well as operatinginformation and safeguards whenever the system is in use.The LCS is started and operated from this unit. Eitherautomatic or manual operating modes are available.The electronic control unit a lso houses the listindication unit. This unit is an analog type electronicclinometer, which is accurate to within l/10 degree. Theinformation sensed and transmitted by this clinometer isused by the control unit, along with information on thetank levels, to adjust the speed of the pump when thesystem is operating in the automatic mode. All vitaloperating, status and alarm information from other systemcomponents such as the HPU, the butterfly valve and thetank level transducers are fed into and displayed on thecontrol unit to permit coordinated and controlled operationof the system.The control software is structured so as to appear asa single DOS program. However, its internal operationis structured to operate as several independent tasks forthe purpose of acquiring data from sensors and controls;updating data to displays, relays and the hydraulic system;presenting status information to the graphics display;receiving operator information; monitoring possible alarmconditions and executing the commanded control function.Each and all of these functions are arranged to operateindependently and simultaneously.A continuous, instantaneous, accurate reading of thelevels in each of the list control tanks is an importantcontrol feature. This is accomplished by submersiblepressure transducers installed in the tanks inside pipesleading to the low point in the tank. This pipe is locatedfar from the pump to avoid turbulence from this source.The information from these transmitters allows the controlsystem to adjust the pump speed to obtain the desired flowrate while operating in either the automatic or manualmodes.A similar system using a variable pitch propeller hasalso been developed. The pump is driven by a constantspeed AC motor through a gear box, similar to theconstant speed system described in the previous section.The same flow characteristics as the variable speed systemcan be obtained by varying the propeller pitch via thehydraulic system.These systems have an advantage over the constantspeed systems detailed previously in that they do not haveto close the valve when there are no flow requirements.The speed or the pitch can be adjusted to a dead headsetting, maintaining any level difference between theheeling tanks without any flow. This difference allowsthe system to respond immediately to any heel correctionrequirements.



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Compressed Air SystemA list control system using compressed air to movewater between tanks is shown in Figure 5 . Principalcomponents include;- an electronic control unit. (1,2)- an air valve group with pneumatic controls, (4)- a forced draft blower, (5)- a tank level difference indicator, (6)- a high water level sensor, (7), and- a butterfly valve, (8)The list control tanks and crossover p ipe are similarto other systems. Tank level indicators are not included,however a tank level difference indication is provided.An electric motor driven forced draft blower suppliescompressed air to the air-valve group. The valves arecontrolled by the electronic control unit and can rapidlypressurize one of the list control tanks and vent the otherto create a differential pressure that moves water betweenthe tanks. Flow rates of up to 110,000 GPM can beachieved, which corresponds to the output of a 42 inchdiameter variable speed propeller pump at low dischargeheads. .Tank water level measurements are not accomplishedfor this system. However, the pressure differencebetween tanks is sensed and used to give water leveldifference. Maximum water level sensors are provided tostop operation if the water level is too high.A butterfly valve is installed in the crossover pipebetween tanks. It is not controlled by the electroniccontrol unit and is open whenever the LCS is operated.

COMPRESSED AIR SYSTEMFIGURE 5

The forced draft blower runs continuouslywhenever the LCS is in operation and the electroniccontrol unit operates the air valves. Manual andautomatic operating modes are provided. In automaticoperation, list angle measurements are used to select flowdirection to correct list.The electronic control unit contains the roll sensingunit and list control algorithms. Auxiliary circuits areprovided to distribute control signals ,and performsurveillance and alarm functions.This type of heel control system can also be used inthe same tanks that are used for at sea stabilization of rollangles. Larger cross over ducts are required for thispurpose. In addition, some of the same equipment canbe used to control the movement of the stabilizer liquid.

DESIGN CONSIDERATIONS

List control system design considerations includeselection of the size of the heeling tanks and the watertransfer rate necessary to counter specified cargo handlingrequirements. System hydraulii: performance must thenbe developed and electronic controls optimized to satisfythese requirements. Design criteria for sys tem definition,hydraulic performance, and programming the controlsystem are summarized below. .The list correction requirements for each individualship will depend on what is loaded, how it is loaded, andhow fast it is being loaded. Some examples will illustratethe diversity of cargo and stowage considerations thathave been factors in determining the specifications for alist control system. One of the most severe cases is theloading of railroad cars on a train ferry. The effect of thecars on the stem ramp and on the cars immediately behindthem that are still on shore is such that the heel anglesmust be minimized. The maximum requirement developswhen the outboard lane is being loaded and is quantifiedby the transverse location of the track, the average weightof the cars, and the loading speed. If the train is 10 carslong, each car weighs 45 tons and is 40 ft. long, theoutboard track is 35 ft. off center, and the loading speedis 5 ft./second, the total moment required to keep the shipon an even keel is 18,000 ft. tons (45 tons/car x 10 carsX 35 ft.). The correction rate necessary is about 11,800ft. tons/ minute (45 tons/car divided by 40 ft./car X 35 ft.X 5 ft./second X 60 seconds/minute).A container feeder ship with a shipboard mountedcrane may be required to lift a 40 ton container located onthe quay about 75 ft. off the center line of the ship.Considering not only the weight of the container but also



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the boom of the crane and the lifting gear, the totaltransverse moment can be as high as 6,000 ft. tons,relatively high for a small ship. The slewing rate of thecrane will determine the correction rate. (In many cases,this rate must be considered in line with practical limitson the speed of the movement of a pendant weight.) Apractical rate would be in the range of 6,000 ft. tons/minute, allowing the maximum cargo movement to becompleted within one minute.The list control systems on the SEALIFT ships forthe U.S. Military Sealift Command were sized to meet thehighest of two requirements:- moving an MlAl tank athwart ship from shell toshell in 1.5 minutes.- moving a side-loading warping tug by crane from thecenter line to 25 ft. outboard within 2.5 minutes.Generally, the tug is controlling, due to the weight ofthe crane boom and lifting gear, and results in a momentof about 10,300 ft. tons and a rate of 4,120 ft.tons/minuteTo properly counteract these transverse moments, thelist control system will have to transfer ballast betweenthe designated heeling tanks. Size selection depends ontwo interrelated main factors: the transverse moment thatcan be developed and the time required to transfer theballast.The maximum heeling moment generally can bedeveloped by the tanks if they are half full and the entirecontents of one tank is transferred to the other. Selectinglong, narrow tanks directly inboard of the shell willmaximize the distance between the transverse centers ofthe tanks and minimize required capacity. Consideringthe SEALIFT example given above, if the distancebetween tank centers on these PANAMAX ships is about100 feet, the required capacity of each tank is a minimumof 206 tons (half capacity, 103 tons, will provide therequired moment of 10,300 foot tons when moved the 100ft. distance between centers). The pump must be capableof moving 41.2 tons/minute (4120/100) or about 10,800GPM. If the tanks were wider, with the distance betweenthe transverse centers decreased correspondingly, thecapacity of the tanks and the pump would increase inproportion.The selection of the size of the cross-over connectionbetween the heeling tanks is made on the same flowvelocity basis as any pipeline. However, there may besome additional consideration to minimize losses in theconnecting piping if an axial flow pump is selected, as thepower requirements for this pump type are very sensitiveto the total design head.

Hydraulic PerformanceList control system hydraulic operating conditions complex when variable speed/pitch pumps are applieAt the start of loading, the ship is on an even keel and list control tanks are equalized. Considering a RO/cargo liner with a variable speed propeller pump asexample, loading starts on one side of the ship and LCS automatically transfers water to the opposite sideminimize list. This can continue until all the water been transferred to one side. During this phase of operation, the pump is moving water against an increasdischarge head and RPM is increasing to maintrequired flow.The second phase of loading then starts on opposite side. The pump is reversed and watertransferred from the tank with a high level to the level tank. In this case, there is an initial gravity fthat acts in combination with the pump. In some ca

the control system may be required to reverse the pudirection to retard the gravity flow to obtain a controllflow and prevent over correction of the list angle. second phase can continue until the tank levels equaand gravity flow is no longer present. After this hydraulics are similar to the first phase. These operatcycles may be repeated numerous times during loadingunloading operations, depending on the size of the sand the heeling tanks and the loading pattern.The hydraulic design of the system requires definitiof a four quadrant set of pump head flow curves for in designing the control system. The sign conventiused in this paper is flow and RPM to starboard positive. Positive head is defined as the differenbetween port and starboard tank levels (corresponding a positive gravity flow). Accordingly, starboard (positivpump flow corresponds to negative head and p(negative) pump flow corresponds to positive head.A set of four quadrant head flow curves developfrom shipboard tests is shown in Figure 6. In the fquadrant head and flow are positive and the data represcombined gravity and pump flow to starboard . Tsecond quadrant with positive flow and negative headue to the pump only. The third quadrant has negatflow and head and corresponds to a combination of puand gravity flow to port. The fourth quadrant has posithead with negative flow and represents pumping to pwithout gravity flow.In addition to developing the pump design determining power requirements, tank vents must designed for the high flows required by the system. Tinternal structure must have sufficient flow openingsallow internal air and water flow. For example,

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LIST CONTROL SYSTEM PUMP CURVES?n ---- -..__VARIA~~ESPEEDPUMP _P_TESTDATA.. .- .----_30 INCH DIAMETER PUMP

+ ORPM

-20 I-30000 -20000 -10000 0 10000 20000 3oom

FLOW (CPM)

FOUR QUADRANT PUMP CURVESFIGURE 6

I GRAVITY FLOW TESTS AT 0 RPMSHIPBOARD SYSTEM TESTS20 - I A+----

GRAVITY FLOW TESTSFIGURE 7

GRAVITY FLOtiSHIPBOARD SYST

I TESTSEM TESTS.i, i/;----

GRAVITY FLOW TESTSFIGURE 8



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transverse web frames at the bottom of tanks can obstruct gravity head and gravity flow are reduced to the desirflow to the pump suction. programmed flow.The system design must be such that water hammeris avoided. This is usually accomplished by eithercontrolling the valve closing speed or the sequence ofstarting and stopping the pump and opening and closingthe valve.

For combined pump and gravity flow it is assumthat the total flow is the sum of a gravity induccomponent and the pump flow at constant RPM and zehead. The gravity flow component is approximated bGFLOW = A,*[2g(H,-HJK,];Systems Using a Propeller Pump

Figure 4, shows the system schematic for a listcontrol system with a propeller pump installed in the crossduct between the tanks. The tanks are vented to theatmosphere and the liquid system energy balance is givenby;

where GFLOW is in cubic feet per second and K, system flow loss coefficient that includes losses duerelative flow over pump internals.

(pp-p&/y, +H,-H,+B*sin(+) +HA-HL = 0 ; (1)System head loss; H, = K,*V,/2gThe total dynamic head is defined as

TDH =H,-Hs +B*sin(+) + & , andHA = -TDH -(B -~)l% ; (2)Total dynamic head at zero list provides a basis for pumpdesign when vent head loss is low.The energy balance for the air vent is given by;

Shipboard tests results have been used to evaluate gravity head flow characteristics. A variable speed pumwas used to establish an initial differential head. Flowas then reversed and a constant pump RPM wprogrammed. Gravity head-flow curves for zero RPare shown in Figure 7. Test data for flow to port astarboard are shown together with a calculation GFLO W in GPM for a loss coefficient K = 7. It shobe noted that estimates of the loss coefficient without tpump are K = 5.4.

pa-pp = ya (1 +I(,)V,,2/2g ; (flow into tank)pa-ps = -/a (1 +I$ )v,, 2/2g ; (flow out of tank)and the vent head loss is;

Test results for gravity assisted flow to port at pumspeeds of 400 and 500 RPM are shown in Figure Gravity flow was calculated as the difference between ttotal flow in the test and previous test results at constaRPM and zero head. Test results for flow to port a t zeRPM are also included on the Figure and slightly lowgravity flow was observed with the pump running.Axial Flow Pump Design

Pp-Ps = - ?/a I +K)*(v,, * + Ys2)/2g; (3)A vent air velocity limit of 80 ft/sec is recommended.Assuming a 300 mm vent diameter, air density of .0764lbs per cubic foot and a loss coefficient of 2, the a ir flowin each line is 28,180 GPM and the vent head loss is0.75 ft This is less than 4 percent of the maximumgravity head for representative ballast tanks. It is notedthat higher air flow velocities will quickly causeappreciable vent head loss.

The design of propeller pumps is often based model or full scale test data which cover variations design parameters such as pitch ratio, blade area, numbof blades and section shape. Pump diameter is selecton the basis of system head and flow requirementsPerformance enhancement can be provided by addistators in the crossover pipe and fitting a propeller hu

Gravity Flow

Pump design performance is checked by shop testscompleted units set up as close as possible to shipboaconditions. It should be noted that it may not be possibto simulate the highest levels of shipboard discharge heain the shop.In cases of water transfer from a tank with a highwater level to the low level tank, a gravity flow can act incombination with the pump or blower. For automaticcontrol, this will provide a high flow rate and reduce thetime required to correct a list. For manual controlprogrammed to maintain a constant flow rate, the pumpmay have to be reversed to slow down flow until the

Systems Using Compressed AirFigure 5 shows the system schematic for a LCS usicompressed air to move water. Consider the case whethe blower is discharging air into the port tank and tstarboard tank is vented to the atmosphere. There is pump, and the liquid system energy balance becomes;

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(P,-P,) = /I * TDH ; (5)Assume an isothermal perfect gas and continuity, thenthe specific gravity 7b of the compressed air out of theblower is;

- b = ya * w h (6)For air flow into the port tank;

pp = & - x (1 +I&)v,2/2gA (7)For air flow out of the starboard tank;

Ps = Pa + 19*(I+KpX2/2g G-9The air pressure difference between the tanks isdetermined from equations (7) and (8) and combined withthe water system energy balance, equation (5), to obtainthe blower design discharge pressure.PB = yl*TDH +~a[ 1 + (~./W)(Va2/2g)(_1_+K,, (9)--2-I 1 - (YJP,)(V, /2g)(l + &>IThe blower total differential pressure is pa-p a andexpressing this in feet of salt water, h,,,:h, = TDH + (R/T)*[& -I- & ]IJ -&Iwhere;

(10)

x, =( %hwa2~2g)(l + Kdp)As =( -rJP,)(V,*/2g)(I + S,)The blower air horsepower ( for salt water) is defined as;AHP = h, Q, i516.91And since Q, =Q

AHP = WH P + (p, /y, )(X, + A) (Q / 516.91); (11)Cl-x, >

Power RequirementsThe basic system power required is the waterhorsepower (WHP), which is defined for salt water as;

WHP = GPM*TDH/3848 (12)where: GPM = ballast water flowTDH = total dynamic head (ft)For systems using a propeller pump, the requiredshaft horsepower is;

The propeller efficiency varies between 50% and80 % depending on the flow. Figures 9 and 10 showexamples o f propeller test data for the same fixed pitchpump at various RPM values.When a forced draft blower is used to transfer water;

where : SHP = AHP/e,eB = blower efficiencyAHP = air horsepower(14)

The efficiency of axial flow blowers varies from 40% to65% (Marine Engineering, 1992).The pump or blower drive system efficiency, es ,must also be considered. For example, the efficiency ofa hydraulic drive is in the range of 90 % .The required motor horsepower (MHP) is thendetermined by MHP = SHP/e,es (15)where: eM s the motor efficiency which, for anelectric motor, varies form 85% at l/4load to 93% at full load.

Design of List Control Tank VentsUndersized LCS tank vent lines can significantlyreduce the water transfer rate achieved and jeopardize thesystem s ability to control list. A design procedure tha tuses the maximum air velocity in the vent line as a designcriteria has been used to size vent lines. The pressuredrop in the vent line must be checked to assure that thesize selected does not produce a vacuum on the suctionside during maximum expected flow (Crane TechnicalPaper 410). For commercial construction, a design limit

air velocity of 80 ft/second is used, the limit used if ballvalves are installed in the vent line is 50 ft/sec. .Usingthis approach, 12 inch diameter vents were selected for aLCS that had a maximum flow rate of 27,000 GPM.This vent size was selected on the basis of no ball valvesin the line.If ball valves are installed in a vent line with flowvelocities over 50 ft/sec, aerodynamic drag on the ballcan case it to slam shut. This was observed duringshipboard tests of a LCS and it was necessary to removethe balls to achieve proper operation.In systems that use air blowers, the air is transferredwithin the system rather than to the atmosphere. Ventlines necessary to meet the requirements of the regulatorybodies are installed below the minimum water level toavoid the possibility of pressurizing the tanks beyond thedesign criteria.

SHP = WHP/e, (13)where: ep = propeller efficiency



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COMPP RISONOF SHIP AND SHOP TEST DATA1-O PUMP STARBOARD AT 200 RPM--.- ---.- ..-.- ---..---.._-_--__ -~- __..I

6000 8000FLOW (GPMI

PUMP PROPELLER EFFICIENCYFIGURE 9

1.00.90.8

COMPARISON OF SHIP AND SHOP TEST DATAE --- -___~- PUblP.$TARBOARD AT 500 RPM.

I I I10000 20000 30000

FLOW (CPM)

182 Design Session

PUMP PROPELLER EFFICIENCYFIGURE 10


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Water HammerSystems that use a remotely operated butterfly valvein the crossover pipe can experience water hammer if thevalve is closed too rapidly. Valve closing in the presenceof flow could occur, for example, during emergency shutdown procedures.Valve operators are available that can close the valveas quickly as 0.70 seconds and water hammer has beenexperienced using this closing rate. When the valveclosing period was increased to 3 seconds, water hammerdid not occur.Care should be taken during the overall systemdesign, either in the control system or in the selection ofthe valve actuator, to be sure a valve is not closed rapidlyunless a corresponding valve is being openedsimultaneously or pump start and stop sequence is timed..

CavitationList control systems that use a variable speed orvariable pitch pump to transfer water can maintain adifferential head between the list control tanks at zeroflow with the valve open. This configuration can occur

during either automatic or manual operation and is termeda dead head condition. Dead head increases as RPMincreases and the maximum differential head that can bemaintained by a variable speed pump is limited bypropeller cavitation. Cavitation occurs when the localpressure on propeller pump blades falls below the watervapor pressure. Cavitation will reduce the propellerefficiency and more torque must be applied to maintain adesired head. At some point, the available horsepowerlimit is reached and the available dead head and RPMreach an upper limit. Cavitation limits on head can alsoaffect constant speed pumps.The cavitation number is defined as the ratio of thedifference between the static pressure and the water vaporpressure to the dynamic pressure (Principles of NavalArchitecture, Vol II 1985).. When the cavitation numberfalls below 1, the total pressure on the blades is less thanthe vapor pressure. This definition of cavitation inceptionhas been observed to provide good correlation for deadhead test data limits.

Cavitation No. = (p,-p,)/(O.SpV,2)where: p, = it, +p&Pam = atmospheric pressureP, = vapor pressure of waterh = suction head

P = mass density of water

g = gravitational accelerationVT = velocity at blade 0.7 radiusThe local cavitation number at 0.7R has beenused to predict cavitation inception. For dead headconditions axial velocity is zero and at the 0.7 bladeradius position;V, = 0.7nDD = propeller diametern = propeller rotational speed, rpsFigure 11 shows propeller dead head test data. Totaldynamic head and cavitation number at 0.7 radius areshown as a function of propeller RPM. Cavitation andpower limits are observed between 500 and 600 RPM.

mxn UEan SHOP TEST RESULTSEAD HEAD SHOP TEST RESULTS

CAVITATION NUMBERFIGURE 11Electronic Control System

The electronic control system discussed below is thatused for a variable speed propeller pump system. Itrepresents the state-of-the-art and is more extensive interms of variables monitored and controlled than othersystems presently available. Control of the LCS isprovided by a microprocessor based real time computersystem. This type of system, often called an embeddedsystem, uses the computer to acquire data from tank leveltransducers, list angle sensor output, butterfly valveposition sensor and other data for the hydraulic systemwhich is monitored for safety and alarm purposes.



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The movement of ballast while changing either heador flow when using a variable speed pump requires realtime computer control. Control is effected using datafrom tank water level transducers and a list angle sensor .A signal for control of pump speed is computed atintervals based upon transducer data and the mode ofsystem operation. In manual control, constant ballastflow in the desired direction is provided. In automaticcontrol, ballast is transferred in the direction required torestore zero list at a flow rate proportional to the angle oflist. The propeller pump and associa ted butterfly valve arepowered hydraulically. In addition to the controlalgorithm necessary to maintain required ballast flow,synchronous control of the butterfly valve is necessary toavoid water hammer. Further, the hydraulic system mustbe monitored to assure safe operation.The electronic components include a PC-likecomputer on a PC/104 standard module. The PC/104components include the computer, a display control, atouch panel control, A/D and D/A converter modules withbinary I/O and a PCMCIA memory card module tocontain the control program and ship characteristics data.All together, the system is composed of the computerassembly, a graphics display with touch panel, and anelectrically isolated interface to the transducers and shipsmachinery control system.The opera tor interacts with the system using thedisplay, touch panel and panel switches. List control tanklevels, list angle, ballast flow rate, pump RPM, andhydraulic system pressure, together with butterfly valveposition and alarm indications, are presented bothnumerically and graphically on the display.With the addition of ship hydrostatic characteristics tothe control software, the metacentric height, GM, can beobtained by applying a measured amount of ballasttransfer to affect an ordered change in list angle. Thisfeature is provided by many LCS control units as an aidto ship operators in determining stability prior to leavingport. Alarms provided by the electronic control sys teminclude hydraulic system tempera ture, pressure, and oillevel, high and low list control tank levels and maximumlist angle. The control system prevents components fromoperating in a direction that increases an alarm error. Itallows operation in a direction that will cause an alarm tobe cleared. As a result, no override or emergencymodes are used to reset or reposition equipment after analarm.Powering on and off is an important issue to the safecontrol of real time embedded systems. For both turn onand off situations, it is desirable that transient alarm

conditions be suppressed. To ensure that the systemcomponents are properly secured before the computer turned off, the electronic control system takes control system power upon turn on, bypassing the system powon/off switch. It then monitors the switch in such a wathat the operators intentions are known. Should toperator request that the power be shut off while ballais in motion, the butterfly valve is open, or the hydraulipower unit is in operation, the system first attempts stop ballast flow, close the butterfly valve, stop the pumand shut down the HPU before disconnecting itself fropower. In the worst case this process will take less tha5 seconds and the operator is seldom aware that thshutdown sequence check has occurred. By suppressintransient alarms during the startup/shutdown process, tannoyance of the alarm sound is avoided.

COMPARISON OF AVAILABLE SYSTEMS

The comparison of the various systems that aavailable can only be made on the ability of the systemto respond immediately to a circumstance that requireheel correction. The amount of heel correction that cabe provided within a short period of time is part of thconsideration. This quick response ability, which may nbe necessary for all types of ships, depends not only othe control system but also on the capacity and type mechanical system used to physically transfer ballasConsidering the types of systems that are commonly usetoday, a brief evaluation of the reaction times shows somsignificant differences.The most basic system, using an existing centrifugapump with a system of automatically controlled valves provide heel correction, as shown is Figure 2, also hthe slowest reaction times. The fastest reaction time fothis system can be obtained by having the pump runnincontinuously during heel correction procedures. When nheel correction is necessary, the pump will brecirculating ballast through the tank with the higheslevel. The control system is generally set to recogniztwo list angle set points. The threshold angle is defineas the list at which heel correction will be initiated whilthe shut-off value is defined as the angle at which hecorrection procedures will s top. When the thresholdvalue is reached, the control system will configure thvalves to transfer ballast between the appropriate tanks decrease the angle until the heel angle reaches the shuoff value, when the valves will be reconfigured resume circulation with no ballast transfer. The reactiotime is dependent on the value assigned to the threshold

184 Design Session


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angle, which must be large enough to avoid constantlyopening and closing the valves. The time necessary toavoid possibility of water hammer while closing thevalves must also be considered. However, since onevalve on either the suction or discharge side of the pumpis opening while the other is closing, this aspect is not asproblematic as it is with axial flow pump systems.Generally, this type of system also has the lowest pumpcapacity, since the primary purpose of the pump isselected to fill ballast tanks in approximately 30 ratherthan 3 minutes or move water through a 3 fire hoserather than an 18 transfer pipe. Regulatory bodyrequirements for fire pumps, for example, are less than1,000 GPM.The system using a constant speed axial flow pumphas some of the same operating limits as the system usingan existing centrifugal pump. The system is inactive untilthe heel angle changes from 0 or some other valuerecognized by the control system, at which point thepump is started and the valve is opened. Pumpingcontinues until an even keel is regained, when the pumpis shut down and the valve is closed. Since this is anaxial flow pump, the delivery can decrease as thedischarge head increases, depending on the selected speedand power. Gravity flow is not controlled. Theadvantage this system has over the existing pumpsystem is that the pump is specifically selected for listcontrol and usually has higher capacities, in the range of3,000 to 6,000 GPM.The remaining types of systems, the variable speed orvariable pitch axial flow pump as well as the compressedair systems, all can maintain a head difference betweenthe two heeling tanks without closing the valve. This isaccomplished by adjusting either the propeller speed orpitch or the air pressure to maintain the head differencewithout any flow. This allows the system to reactimmediately to any change in heel angle. The ability toreact will depend more on the sophistication of the controlsystem rather than mechanical response characteristics.Figure 12 shows an example of the reaction time thatcan be obtained with a variable speed pump. The systemis reacting to reverses in the slewing of a crane with a liftof about 100 tons. As the crane changes direction and thelist angle passes through O, the direction of the pumpchanges almost immediately, as evidenced by the changein the levels in the heel control tanks.Given the capability of the mechanical components toprovide an immedia te response, the total reaction timewill depend on the ability of the electronic control systemto recognize the need for heel control, continually analyzethe input information received from various sensors (e.g.,the inclinometer and the tank level sensors) and issue the

proper instructions to change the response characteristicof the prime mover (air pressure, propeller pitch, etc.).This often involves a feed back loop to determine if thedesired effect is being achieved and continuing correctionsto strive for the optimum response. The sophistication ofthe electronic control system can vary from supplier tosupplier and the performance of the same type of systemcan vary based on the experience and data available to theprogram writer.

REFERENCES

Argyriadis,D.A. et al Design and Construction ofModem Roll-On/Roll-Off and Container Carriers.Transactionsof SNAME, Vol 87, (1979)pp 313-350SNAME, MarineBlowers ,( 1992)

Engineering, Section 3

Crane Technical Paper 410, Flow of Fluids ThroughValves, Fittings, and Pipe Chicago (1960)SNAME, Principles of Naval Architecture, Vol II.(1988) pp 181-183.

ACKNOWLEDGMENTS

Figure 3 is based on material provided by FRANKMOHN AS, Nesttun, NorwayFigure 5 is based on material provided by INTERINGGMBH, Hamburg, Germany



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TEST WITH 100 TON LOAD ON CRANEREMOVING WEIGHT FROM SHIPI- i I I.--. I /I 124 I, ..W.I22 ( _.___-1 :,. ..-# i-- , ,

h :\: , j -. , ,I , . II, ,-i F ; \ , i .%.,I. .J \-i i ; ------, 1 . .0--.-.-.-. -0. . \.,.*.e.*,-.

J ..: I/ I I/J PORT LEVEL

-I: -1

4 / , /I II rl2 I I I \ II 13900 4000 4100 4200 4300 4400 4500

TIME [SEC)

VARIABLE SPEED PUMP SYSTEM PERFORMANCEFIGURE 12

186 Design Session


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Dirk Hoflich, Visitor, INTERING GmbH, Norderstedt,GermanyThe description of the different list control systems is com-prehensive and considers the main design aspects. Quick reac-tion and high flow rates of variable speed pump systems andblower systems are important features for high performancelist control.As to the power requirements of blower anti-heeling systems[section of the paper incorporating equations (12)-( 15)] the fol-lowing should be summarized: INTERING anti-heeling systemsaccording to Fig. 5 do not use axial flow blowers with an ef-ficiency of 40 to 65% but since 1971 we use rotary positivedisplacement blowers (roots-type rotary piston blowers) witha volumetric efficiency eBof 90% and a total efficiency of 8.5to 88% in anti-heeling system design conditions.Design of list control tank vents of blower anti-healing sys-tems: Pressurizing of heeling tanks beyond the design criteriawith a blower anti-heeling system is avoided by safety valvesat the blower units. Tank structure is designed to the pressurepreset at this valve, which will not be exceeded in operation.The system is connected to atmosphere via the air valve group(No. 4 in Fig. 5).Overflow prices ending below minimum water level are fit-ted to protect the tanks against operators failure to vent thetanks when filling or discharging.

Eugene A. Van Rynbach, MemberThe authors are to be congratulated on writing a conciseand informative paper describing the types of list control sys-tems available today. It should be a handy reference for any-one involved in a new construction program or vessel modifi-cation program where list control is needed. It is informativein that it describes both the key components of a list controlsystem and the issues involved in designing them.Sea-Land Service has list control systems on some of itsships, mostly of the type that utilizes existing pumps and bal-last piping systems. However, systems that may have been ad-equate in the past may not be adequate in the future. Contain-erships are getting larger and terminals are being pushed toincrease productivity, resulting in more cranes working a ves-sel at the same time. List control is becoming of increasingimportance and can have a measurable cost impact on opera-tions.As mentioned in the paper for some types of heavy cargoesand for rail cars, for example, list control is very much doneby the vessel and is a necessity for a vessel in those trades. Incontainer shipping, however, the answer to list problems is morecomplex and not always found on the ship. One of the mainreasons for this is that the loading or discharging of a singlecontainer does not normally cause unacceptable list. It is thecumulative effect of loading many containers without watch-ing the transverse balance of the ship that causes list. There-fore, well planned load sequencing can also be used to controllist. The fact that there are two primary means of controllinglist on containerships has the potential to lead to finger point-ing between the terminal and the vessel if list and vessel ca-pability to respond to list are not considered during cargo op-erations on the ship. The vessel could say their list controlsystem, or if an automatic one is not fitted, the list control ef-forts by the crew cannot keep up with the changes n list broughtabout by the rapid cargo operations being done by the termi-nal without due consideration of the impact on list. The terminalcould say the ship has an inadequate list control system and is

slowing down production because the cranes have to wait forthe ship to remove excessive list. The answer probably liessomewhere in between. It seems that when evaluating howmuch list control is required a cost benefit comparison shouldbe done of the list control system options available on the ves-sel versus the time and cost to properly sequence cargo oper-ations in the terminal to reduce the listing moments. In todayscost cutting world, when faced with the potentially high costof a large capacity list control system there can be a tendencyto cut the cost and scope of the list control system and saylets do a better job of planning at the terminal to resolve theissue since this is the solution without upfront capital expen-ditures. However, some form of list control seems o be a ne-cessity and the answer to the question of how big a system toinstall can probably be found in the cost benefit analysis.In order to provide insight during the initial assessmentsofa potential project on the appropriate list control systems o con-sider it would be interesting if the authors could provide someinformation on the relative costs of the systems described inthe paper. This would help the readers determine on an orderof magnitude basis which types of list control systemsare withintheir budget and worth further study. Most of the other infor-mation required for this type of preliminary assessment s con-tained in the paper.Steve Heskett, Visitor

I wish to extend my congratulations to the authors for pro-viding the shipbuilding industry with a comprehens ive paper onthe various types of list control systems that are on the markettoday. The paper gives marine engineers and shipowners agreater understanding of what may be achieved by each sys-tem, so that they can make a more educated determination ofwhich system best suits their needs.There are a couple of comments I would like to make andreceive the authors comments or concurrence:1. This concerns statements made throughout the paper, thatthe tanks are initially filled to 50% of the tank capacity, andthat maximum heeling moments are obtained by transferringthe entire contents of one tank to the other. These statementshave caused some confusion with production personnel and cus-tomer representatives during recent installations and testing oflist control systems.The tanks being used as heeling tanks often are of a con-siderably larger capacity than is required to develop the nec-essary heeling moment, and in actuality the tanks are filled to50% of, what I would call, the transferable capacity. The trans-ferable capacity being the volume of water between the lowlevel and high level pump cutout switches. The low level switchin some instances may be at a considerable height above thebottom of the tank, while the high level switch may be wellbelow the top of the tank.Normally the water remains in the tanks after loading is com-pleted. The distinction between 50% of tank capacity and 50%of transferable capacity must be made, since the heeling tankcapacity figures are used to determine the ships stability andballast conditions prior to departure.2. I would like to reemphasize that the location of the pumpsuction bellmouths within the heeling tanks must be given care-ful consideration. The high suction capacities of these pumpscan create numerous cavitation and air pocket problems whenthe bellmouths are located in tight corners of heeling tanks.Large lumber holes on the top and bottom of tank web framesare a must, to ensure good tank ventilation and draining dur-ing pumping operations. The turbulences from pump suctions

Discussion



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also create havoc with pressure transmitters for the level gag-ing sys tems. In most cases he bellmouth doesnt need to be atthe lowest point in the tank and can therefore be located awayfrom structurally congested areasand well away from level gagepressure transducers which would normally be located at thelowest point in the tank.3. The one criticism of the paper I have is that it didnt gointo a great deal of detail about the compressed air type LCS.What are the advantages and disadvantages of a compressedair system as opposed to a variable speed/pitch pumping sys-tem? For example, the compressed air system would appear tohave more piping and valves, but less pumping components.Also the compressed air system can be used for sea stabiliza-tion while underway. The hydraulic variable speed/pitch pump-ing system is susceptible to leakage. If the propeller pump isin a submerged location such as a double bottom ballast tank,these oil leaks can be of m ajor concern to an environmentallyconscious shipownerloperator.

bor time reduction of up to one hour for a cross channel ferry,resulting in an annual fuel saving potential up to 1700 tons.The following comments refer to the accompanying TableA comparison between the FLUME and INTERING systems3. In the INTERING blower-activated system, tanks can bearranged in various places on various decks, while blowers areinstalled where space is available.4. Blower systems in operation on the worlds largest trainferries enable-without hydraulic shock-reverse of flow di-rection/start of complete flow in less than 1 set, with total powerup to 600 kW.5. Due to the INTERING concept no moving parts in wa-ter, together with very little and easy maintenance and possi-ble exchange of components, the blower-activated system isgiven preference where maximum reliability is requested.6. Tank water, which is needed for anti-heeling operation,is double-used for roll stabilization in combined INTERINGstabilizer and anti-heeling systems.7. We appreciate that the competition follows INTERINGs

Horst Halden, Visitor, INTERING GmbH, Norderstedt,As the worldwide leader and the only company experiencedin blower- and pump-activated anti-heeling systems, INTER-

Germany

ING thanks the authors for their efforts to bring this subject tothe awareness of a broader marine audience.racyas the results of tie shipyard;s inclining experiments. Ex-lead in us&g the anti-heeling system for stability measurem entExperience since 1973 led to extreme simplification and relia-tensive experiences are available on stability measurementsatbilitv in third-svstem generation (1996) and to the same accu-speed.

We fully support the authors opinion about the importanceof short svstems reaction time and immediate comoensation Additiona, referenceof cargo &eights during loading/discharging to the benefit of JONER, J., HALDEN, H., New TT-Ferries-Inclining Test in 10 Min-the ships harbor time. As an example: In the early 1970s, utes, Schiff & Hafensuly 1995.INTERING blower-activated anti-heeling systems led to har-Variable Speed Pump System Blower-activatkd System

FLUME INTERlkGI Ihe authors relate INTERING to axial flow blowers with 40 - 6$ % efficiency.

INTWNG never used axial flow blowers, but ptarv oositive displacementblowers roots tvpe witb 88 - 80 X efficiency.1 Safetv. Block o f Water Flow

No Redundancy. Redundancy.Control and Safev in one Valve. Air Valves for Control and Safety.Water Valve for Safety only.1 Arranaement Flex ibilitv. Redundancy

One Pump per Tank. 1 to 5 Blowers in any Tank Com bination.NO cross-over Functions fw Emergency. Multiple Tasking.I Reaction Time

?I

< 1 see. far co mplete Flow Reverseup to SO0 kW. No shocks.i Movirm Parts. Reliabilitv. Warranty

in Water Moving Parts in Air only.?I

> 3OW Years accum. Operating Experience.3 Years Guarantee since 1962.Same Tanks for Roll Stabiliratiq

No I YesGM Accuracy

reasonable ? Within 1 % of official lndining Test Results.Type-approved by Intl Class Societies.Over 500 Years of accumulated Experience.Table A

188 Design Session

Authors ClosureThe response that has been received reflectsthe reactions of ship operators, ship builders, andthe various manufacturers of list control systemswhich evidences the growing interest in these sys-tems. They are becoming a standard requirementfor many types of commercial newbuildings. Theopinions and viewpoints of the ship operators arecertainly welcome. In reply to Mr. Van Rynbach,the problems due to list angles when loadinglarger containerships can certainly be avoidedby paying close attention to the loading sequenceLoading one or two containers off center willnot cause difficulties, but a continuation of thistrend could cause loading delays, and the ensu-ing disagreements are well described. These in-cidences could be completely avoided with a highcapacity list control system, but the economic im-pacts cannot be ignored and should be the de-ciding factor. It is interesting that major problemscan be experienced not by the larger ships butby the smaller feeder ships that often are selfloading/unloading via onboard cranes. As con-tainers are added on deck and transverse stabil-ity decreases, he off center moment caused bylifting a container from the wharf can cause veryhigh list angles, in excess of 10 deg, and sometype of automatic compensation becomes a ne-cessity.Concerning the request for relative prices ofthe various system ypes, these are difficult to pro-vide as they are completely dependent on thepump capacity and related pipeline sizes. Someballpark estimatesare from 10 000 to 50 000 1997dollars for control systems that use an existing


19/19

pump, 50 000 to 100 000 for constant speed pump systems,150 000 to 200 000 for variable speed pump systems, and about300 000 and over for the air controlled system, depending onwhether it is combined with a roll stabilization system.We would like to thank Mr. Heskett for his observationsregarding transferable capacity as opposed to tank capacity. Inparticular, some conversions have the luxury of very large listcontrol tanks that will never utilize half the full capacity forlist control. In these instances, the initial tank levels are deter-mined by other considerations, such as trim or ballast require-ments. The control system software can be programmed to m eetthese needs. However, this is a high quality problem. In mostinstances, the heeling tanks are sized to m eet the specified heelcorrection requirements with little excess capacity.The reinforcement of the importance of free flow of both thefluid and the air within the heeling tanks is very welcome. Mostproblems encountered when commissioning these systems arerelated to a lack of attention to these details.The ability to use the same set of tanks to provide both rollstabilization and list control is a point often offered as an ad-vantage by the suppliers of air activated list control systems.A U-tube type roll stabilization tank can be utilized for listcontrol. However, this type of stabilizer tank has a single pe-riod response.The other type of passive anti-roll tank commonlyinstalled, a free surface type tank, has a response period thatcan be varied by changing the liquid level. This single responseperiod disadvantage of a U-tube can be overcome in a numberof ways, all of them expensive. The most common approachis to construct the tank so that it can respond to the shortestexpected natural roll period of the ship and use a control sys-tem to delay the response. With an air control system, a valvein the air cross over duct is opened and closed to hold the wa-ter on one side and lengthen the response time. This approachis often erroneously referred to as an active tank stabilizerbut is properly called a controlled passive stabilizer.The response period requirement for a U-tube is determinedby the width of the wing tanks and the height of the cross overduct (Field, 1975). The necessity of having a relatively clearcross over duct normally means hat there is a double inner bot-tom, as the normal structure in bottom tanks will not allow properflow. This consideration requires additional structure that is dif-ficult to install due to the confined spaces normally involved.The size requirements of a list control system are determinedby what is being loaded and the rapidity of the loading process.The size and configuration of a roll stabilization tank are de-termined by the stability conditions (righting moment and meta-centric height) of the ship ready for sea, after loading has beencompleted. Although, because hey look similar, it may initiallyseem that there would be some space savings advantage in us-ing the same tanks for both stabilization and list control pur-poses, this usually does not prove to be the case when consid-ering the design requirements. Trying to force the issue canresult in multiple tank arrangements, with some of the tanksused for one purpose and others for both. In some instances,tanks are built within tanks so that the proper response periodfor the stabilizer can be obtained without losing list controlcapacity (Halden, 1997).It is our opinion that the most economical approach is todesign the anti-roll tanks to stabilize roll at sea and the listcontrol tanks to minimize heel during the loading process. Thisusually results in two systems that are effective, simple to in-stall, and have the lowest capital and installation costs (seeSellars and Martin, 1992).In response to Mr. Hbjlich, we thank him for these impor-tant pieces of additional information and apologize for assum-ing the use of the wrong type of blowers. The efficiency wastaken from Marine Engineering, as referenced, and is indeed

for an axial flow blower. The use of rotary positive displace-ment blowers with a higher efficiency makes the power re-quirements for the air controlled system about equal to the pumpsystems. n addition, the safety valve on the blower units meansthat the tanks for an air controlled system can be designed tothe normal deep tank standards.It is difficult to respond to Mr. Halden as many of his re-marks are aimed at stressing the sales advantages of systemshe supplies and appear to be less than objective. For instance,we have no knowledge of the fuel savings enjoyed by the crosschannel ferry fitted with his system (our own experience inthis regard has shown that these claims are easy to calculatebut difficult to prove), the accumulated experience of his staff,nor the type of guarantee he offers, nor do we believe this isthe proper forum for such claims. Many of his points are sim-ilar to those answered above.As far as redundancy is concerned, the valve in the vari-able speed pump system is not used for control. The valve iseither open or shut and the speed of the pump is used for con-trol. If the hydraulic system shuts down for any reason, thereis a fail safe accumulator that will close the valve.The parts of a variable speed pump system that are in wa-ter are the pump with the hydraulic motor and the valve. Thematerials used are corrosion resistant: stainless steel and NI-BRAL. In fact, in many installations these items have been sub-merged in inner bottom tanks that are used for other purposesto save space.However, as mentioned by Mr. Heskett, hydraulicleakage could cause environmental concerns.As mentioned in the paper, the responseof the variable speedpump system is immediate. The response time for this system,and we assume the air controlled system, is limited not by thespeed of the mechanical components but by the gains pro-grammed into the control system software. Some reaction time,albeit short, is necessary to avoid overshoot and hunting.Hydraulic shock or water hammer is avoided when thevariable speed pump system is active as the valve is not closedduring normal operations. When the system is shut down, thecontrol system automatically adjusts the pump speed to obtainzero flow and the valve is pulsed to close in a programma-ble number of steps o avoid shock waves. If the valve is closedby the accumulator, the closing time is lengthened by flowcontrol devices.Contrary to Mr. Haldens belief, using the list control tanksto obtain a measure of the GM was first designed into a sys-tem supplied by FLUME in 1970 for use on two RO/RO shipsbuilt in Gdansk. The application of the engineering principalsis not difficult and the accuracy will depend on the accuracyin the level sensors n the tanks, the heel sensor in the controlunit, and the determination of the displacement. Errors in thesensors are less than 0.1%. If the displacement is obtained froma draft indicator, the accuracy can be very good when the ves-sel is not under way. However, if the draft marks are read fromdock side, there is considerable room for error.We would like to thank all of the discussers for their con-tributions to this paper and SNAME for providing a forum forthis presentation.Additional references

FIELD, S.B. et al, Comparative Effects of U-Tube and Free SurfaceType Passive Roll Stabilization Systems, Royal Institution of Naval Ar-chitects, April 1975.SELLARS, F.H. et al, Selection and Evaluation of Ship Roll Stabi-lization Systems, Marine Technology, Vol. 29, No. 2, April 1992, pp.M-101.HALDEN, H., Influence of Combine Stabilizer, Anti-heeling, and Sta-bility Test Systems on RO/RO S hip Design and Operation, LS-SD, 1997.

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