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Transactions,

Vol. 105, 1997, pp. 171-189

List Control Systems

Frank H. Sellars, Member, Flume Stabilization Systems, John P. Martin, Member, Flume Stabilization Systems, William L. Schultz, Visitor, Hyde Marine Systems

ABSTRACT

Thetypesof shipsthat normallyapply automaticlist control systems reviewedand the are reasons the applicationare summarized.Varioustypesof list control systems are for that commercially availableare enumerated detail and the differences in betweenthesesystems are explained. Design Considerationsthat could define selection criteria are listed, including somepotentialproblem areas.

NOMENCLATURE INTRODUCTION LCS = list control system HPU = hydraulic power unit TDH = total dynamic head B = distance between list control tanks H, = port tank level H, = starboard tank level H, = head added by pump = head loss due to friction = blower discharge head c AD = cross duct area Pa = atmospheric pressure PP = air pressure in port tank P, = air pressure in starboard tank = blower discharge pressure g = liquid volumetric flow Q, = air volumetric flow 4 = list angle V, = fluid velocity V, = air velocity = fluid flow loss coefficient = air flow loss coefficient 2 y, = liquid specific gravity Ya = air specific gravity

Excess list of a ship during cargo handling operations can be detrimental. Ferries and RO/RO vessels use loading ramps that are installed on either the bow, stern, or the side of the ship. These ramps usually cannot sustain lists over 2 to 3 degrees without damage. Container ship loading and shipboard crane operations are delayed or interrupted if list exceeds 5 degrees. Modern cargo liner operations depend on rapid loading and off loading to maintain schedule. Automatic list control systems have been developed to rapidly transfer large quantities of water (2,000 to 110,000 gallons per minute) from side to side for this purpose. These list control systems consist of pairs of wing ballast tanks and crossover piping, a mechanical device to transfer water and an automatic electronic control system to sense list and take the necessary corrective action. Systems that have been developed include centrifugal or axial flow pump configurations as well as processes that use compressed air to move the water. The purpose of this paper is to review the factors that have led to the development of modem, high capacity

171

that are in use and commercially available . Some aspects of list control system design criteria are considered, including sizing of tanks and cross over ducts, selection of a prime mover and system arrangement. Automatic control system requirements are discussed.

experienced. In the manual mode, the flow direction is selected by hand and the flow rate may or may not be controlled electronically, depending on the technical capabilities of the mechanical system being controlled. The manual mode will allow the ship to be pre-heeled to one side in anticipation of a large load being placed on the opposite side.

LIST CONTROL SYSTEM APPLICATIONS

All ships have some form of list control capability using the ballast system. However, if the system is not automated, considerable time and effort is required to line up valves and start and stop pumps. In addition, ballast pumps have relatively low flow rates. Ships that often employ automatic list control systems include container ships, roll-on/roll-off (RO/RO) cargo carriers, ferries, heavy lift ships and side casting dredges. RO/RO vessels deliver wheeled cargos of a variety of sizes within tight schedules. They often use jumbo stern ramps that are as wide as practicable (Argyriadis 1979). The torsional flexibility of the ramp limits the list that can be accommodated when it is deployed to 2" or 3". Accordingly, list control systems (LCS) have been designed to keep list below this, or on an even keel, without interrupting cargo handling. Considering containerships, list control systems that maintain list within 2" will expedite placement of containers in cells. Some operators will apply an initial list of 1" outboard of the pier to accommodate deck cargo. Heavy lift crane vessels use the LCS to keep list within crane operating limits (approximately 5").

MOVING

A WEIGHT

COUNTERBAL~WCING WITH LIST CONTROL TANKS

EFFECT OF CARGO LOADING FIGURE 1

DESCRIPTION, PRINCIPAL COMPONENTS AND APPLICABILITY OF LIST CONTROL SYSTEMS

The list control system moves water between sets of wing ballast tanks to counter-balance an off-center load or the transverse movement of cargo. Figure 1 shows how loading cargo on a ship will cause list and how the movement of water between the list control tanks will correct this list. The heart of the system is a reversible device which is controlled by an electronic control system for the purpose of moving water between wing tanks on the port and starboard sides. There are usually options for use in either an automatic mode or a manual operating mode. In the automatic mode, the flow and its direction are selected and controlled electronically to correct any list angle

An additional feature of some list control systems is that the transverse metacentric height (GM), a measure of the stability of the ship, can be measured in port, prior to sailing. If a computer based electronic control system is available, the hydrostatic characteristics of the ship and the capacity curves of the list control tanks can be loaded into the memory of the electronic control system. The forward and aft draft marks either can be entered manually or obtained via a link with a draft indication system to determine the displacement. Water is transferred between the list control tanks until the ship assumes a pre-programmed angle. The computer can determine the heeling moment from the tank capacity tables and the change in tank levels. Since the displacement and heel angle are known, the GM can be calculated with reasonable accuracy.

172

Design Session

Several types of list control systems have been developed. Those that have been installed recently encompass systems that control an existing pump of acceptable capacity as well as dedicated systems that include axial flow pumps with either constant speed, variable speed, or variable pitch drives or that use a forced draft blower to transfer water by air pressure in place of a pump. The type of LCS selected depends on the vessel size and operational requirements. Electronic control systems that use existing high capacity pumps (such as a ballast or fire pump) have been developed to minimize list. Such systems are generally used when heeling moment correction requirements are relatively low and ship operations do not require a short port stay. Independent heeling pumps with variable speed/pitch or blower systems are used when heeling moments are large enough to cause uncorrected list of over 1.5 degrees. For longer cycle times and lower heel moment requirements, but still requiring a higher capacity than can be provided by an existing pump (such as occur on passenger ferries or container feeder ships) a constant speed axial flow or propeller pump system is often selected, since its cost is relatively low. For ships with short turn around requirements and rapid cargo handling rates, or that handle heavier cargos, such as RO/RO vessels that carry trains or cassettes, and military Sealift ships, either a variable speed/pitch propeller pump or a Operations that compressed air system is selected. require rapid application of a large heel control moment such as heavy lift vessels with small allowable heeling angles, would use a compressed air system or a large capacity variable speed/pitch pump. The system reaction time is the time required to react to and correct a change in list. Variable speed/pitch propeller pump systems and compressed air systems have reaction times as short as several seconds. Constant speed, reversible propeller pump control systems take longer to react.

minimized. In addition, the vertical distance between the bottom of the tank and the pump will influence the suction pressure and the onset of cavitation, so it should be maximized. These considerations lead to the conclusion that long, shallow, upper wing tanks should be selected as the heeling tanks, when available. The structure of the tanks should be checked to insure good flow to the crossover duct. The vents or air exchange ducts should be adequately sized to ensure that the movement of large volumes of water will not draw a vacuum or increase the discharge head. At the start of list control operations, each tank should be approximately 50% full to allow the maximum heel correction moment to be realized when the entire contents of one of the wing tanks is transferred to the other.

List Control Using Available Pumps

Many container feeder ships, passenger ferries, or smaller RO/RO ships will use an electronic heel control system to operate an existing ballast or fire pump. When operating as part of the heel control system, the pump must be isolated from all other tanks except the heeling tanks. The main components of the system are: the electronic control system an "existing" centrifugal pump remotely operated valves level sensors A schematic of the system is shown in Figure 2. When operating as a list control system, the pump is always running, recirculating water through the tank with the highest level when no heel correction is required. When a preset "threshold" heel angle is exceeded, the valves in the suction and discharge lines are positioned to allow ballast transfer to offset the load. The transfer continues until another preset angle the "shut-off" angle, which is usually 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 and properly positions the valves to achieve either recirculation (no transfer) or the movement of ballast to An example of a valve correct the heel angle. arrangement that will meet these requirements is shown in Figure 2. This can generally be done with reasonably simple circuit boards and a computer based system is not necessary. The pump selected should be the pump that has the highest capacity, usually the ballast or the fire pump. The advantage of the system is economic since a dedicated pump is not necessary. However, capacity is usually limited when compared to a dedicated system.

Selection Criteria for Heeling Tanks

All of these systems transfer ballast between two designated list control or heeling tanks. As the transverse distance between the centers of the tanks increases, the amount of water that must be transferred to obtain the necessary heeling moment decreases. Therefore, these tanks are usually narrow wing ballast tanks located directly inboard of the shell in the parallel middle body of the hull. Such tanks generally are readily available in typical RO/RO or container ship arrangements. The height between the top and the bottom of the tank will have an impact on the required power, and should be

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._ '

CONTROL USING AVAILABLE FIGURE 2

PUMPS

The valves are sized to handle the maximum flow capacity and are remotely operated either pneumatically or These same valves can sometimes be electrically. arranged to fill and drain the tanks if the ballast pump is used. In order to avoid water hammer, it is important that the valves in the suction and discharge lines are opened and closed simultaneously. Level sensors are necessary to inform the control system when either of the tanks is full or empty and which tank has the highest level. High and low level alarms will not allow operations to continue in a direction The tank with the that will aggravate the situation. highest level will be selected for recirculation when transfer is not required.

a one or two stage design depending on the required discharge head. The electric motor is connected to the pump by a coupling and gear box. The gearbox is lubricated by an oil bath which is connected to a header tank which is equipped with a low oil level indication. Between oil seals and the ballast water, there is another sealing chamber connected to another header tank. The fluid in this sealing chamber protects the ballast water against oil contamination and lubricates and cools shaft seals. The heeling indicator is mounted in a cabinet suitable for separate bulkhead mounting. The list angle can be monitored on the heel indicator. Tank levels are neither measured nor used to control the system. Tank level switches are installed to stop the pump if either of the list control tanks is empty. The electromagnetic butterfly valve is installed in the crossover pipe and is operated by the control system. To avoid water hammer, the valve is opened when the pump is running and closed when the pump stops. The electronic control panel contains an analog control system. An electronic amplifier receives a signal proportional to heel from the inclinometer. The preamplifier is set to zero at zero list and any heel will offset the signal from the pre-amplifier. A starboard or port list is sensed by a zero comparator which is used to control the direction of rotation of the pump. A switch is provided to shut off the electronic control and permit manual control of the pump. Change to either automatic or manual operations can be done at any time in the operating cycle. Alarms will sound if the levels in either the heeling tanks or the lubrication and seal header tanks arc below minimums. In addition, if actual heel exceeds a preset maximum angle, an alarm is given. Use of a constant speed axial flow pump usually results in a slow reaction time, The pump must be started and the valve opened every time the heel angle differs from an even keel. In addition, there is a reduction in flow as head increases for constant speed operation.

Constant Speed Pump System Variable Speed/Pitch Pump Systems

A list control system using a constant speed reversible pump is shown in Figure 3. Main components include; the reversible propeller pump, a heel indicator and tank level switches, a butterfly valve, and an electronic control panel. The reversible propeller pump is driven through a gear box by an AC electric motor. The pump is reversed by reversing the motor direction of rotation. Available pump diameters range between about 10" to 16" and have A list control system utilizing a submersible, variable speed propeller pump is shown in Figure 4. A system using a constant speed, variable pitch propeller pump would be similar, with differences as noted below. The main components are: a propeller pump, a butterfly valve, an electronic control unit with heel sensor, and tank level transmitters..

174

Design Session

The pump shown in Figure 4 is a submersible unit installed in the crossover pipe. The tube housing the pump forms part of the crossover duct. Pump sizes range from 18" to 42" propeller diameter. The pump is driven by a hydraulic motor that is mounted in a pod within the housing. The direction of flow is fully reversible and speed is adjustable within the speed range. There is a fixed pitch, five bladed stainless steel propeller that, along with the pod and hydraulic motor, is supported by a strut. The tube, pod, strut and housing are fabricated of corrosion resistant materials. The pod and strut are filled after assembly with a water soluble grease. The placement of the pump and motor within the pipeline minimizes the space required and reduces the complexity of the installation. Hydraulic connections to the motor are run through the strut, They include two high pressure lines and a single, low pressure case drain. When the LCS is shut down or has been inactive for some time while the system is in operation, flow between the list control tanks is prevented by a wafer type butterfly valve installed in the crossover piping. The valve is assembled as a unit with a hydraulic actuator and position indicating switches. The hydraulic actuator allows remote operation from the electronic control unit. The position of the valve is either completely open or fully closed, which is confirmed by one of the position switches. The materials used in the valve and position switches are bronze and stainless steel to permit submersible operation. An accumulator can be added to the hydraulic system to shut the valve in the event of an electric or hydraulic power loss. The closing time of the valve is adjusted hydraulically or via the electronic control system to avoid any water hammer problems. The propeller pump and butterfly valve are driven by a hydraulic power unit (HPU) located near the pump and valve. This HPU is connected to and is controlled by the There is a main variable electronic control unit. displacement hydraulic pump to drive the propeller pump and a gear pump for the butterfly valve actuator, both mounted on a common shaft and driven by an AC electric motor. The direction and speed of rotation of the propeller pump is determined by a control plate in the main hydraulic pump. The position of this plate is set by signals from an electronic control unit. The open/close movement of the butterfly valve is controlled by a directional solenoid valve mounted on the HPU. Confirmation of the valve position is provided by magnetic position indicating switches mounted on the valve. 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 operating information and safeguards whenever the system is in use. The LCS is started and operated from this unit. Either automatic or manual operating modes are available. The electronic control unit also houses the list indication unit. This unit is an analog type electronic clinometer, which is accurate to within l/10 degree. The information sensed and transmitted by this clinometer is used by the control unit, along with information on the tank levels, to adjust the speed of the pump when the system is operating in the automatic mode. All vital operating, status and alarm information from other system components such as the HPU, the butterfly valve and the tank level transducers are fed into and displayed on the control unit to permit coordinated and controlled operation of the system. The control software is structured so as to appear as a single DOS program. However, its internal operation is structured to operate as several independent tasks for the 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 alarm conditions and executing the commanded control function. Each and all of these functions are arranged to operate independently and simultaneously. A continuous, instantaneous, accurate reading of the levels in each of the list control tanks is an important control feature. This is accomplished by submersible pressure transducers installed in the tanks inside pipes leading to the low point in the tank. This pipe is located far from the pump to avoid turbulence from this source. The information from these transmitters allows the control system to adjust the pump speed to obtain the desired flow rate while operating in either the automatic or manual modes. A similar system using a variable pitch propeller has also been developed. The pump is driven by a constant speed AC motor through a gear box, similar to the constant speed system described in the previous section. The same flow characteristics as the variable speed system can be obtained by varying the propeller pitch via the hydraulic system. These systems have an advantage over the constant speed systems detailed previously in that they do not have to close the valve when there are no flow requirements. The speed or the pitch can be adjusted to a "dead head" setting, maintaining any level difference between the heeling tanks without any flow. This difference allows the system to respond immediately to any heel correction requirements.

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CONSTANT SPEED PUMP SYSTEM FIGURE 3

STARTERPANEL

WATER TRANSFER BUlTERFLY V

PROPELLER PU

VARIABLE SPEED PUMP SYSTEM FIGURE 4

176

Design Session

Compressed Air System

A list control system using compressed air to move water between tanks is shown in Figure 5 . Principal components 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 pipe are similar to other systems. Tank level indicators are not included, however a tank level difference indication is provided. An electric motor driven forced draft blower supplies compressed air to the air-valve group. The valves are controlled by the electronic control unit and can rapidly pressurize one of the list control tanks and vent the other to create a differential pressure that moves water between the tanks. Flow rates of up to 110,000 GPM can be achieved, which corresponds to the output of a 42 inch diameter variable speed propeller pump at low discharge heads. . Tank water level measurements are not accomplished However, the pressure difference for this system. between tanks is sensed and used to give water level difference. Maximum water level sensors are provided to stop operation if the water level is too high. A butterfly valve is installed in the crossover pipe between tanks. It is not controlled by the electronic control unit and is open whenever the LCS is operated.

The forced draft blower runs continuously whenever the LCS is in operation and the electronic control unit operates the air valves. Manual and automatic operating modes are provided. In automatic operation, list angle measurements are used to select flow direction to correct list. The electronic control unit contains the roll sensing unit and list control algorithms. Auxiliary circuits are provided to distribute control signals ,and perform surveillance and alarm functions. This type of heel control system can also be used in the same tanks that are used for at sea stabilization of roll angles. Larger cross over ducts are required for this purpose. In addition, some of the same equipment can be used to control the movement of the stabilizer liquid.

DESIGN CONSIDERATIONS

List control system design considerations include selection of the size of the heeling tanks and the water transfer rate necessary to counter specified cargo handling requirements. System hydraulii: performance must then be developed and electronic controls optimized to satisfy these requirements. Design criteria for system definition, hydraulic performance, and programming the control system are summarized below. . The list correction requirements for each individual ship will depend on what is loaded, how it is loaded, and how fast it is being loaded. Some examples will illustrate the diversity of cargo and stowage considerations that have been factors in determining the specifications for a list control system. One of the most severe cases is the loading of railroad cars on a train ferry. The effect of the cars on the stem ramp and on the cars immediately behind them that are still on shore is such that the heel angles must be minimized. The maximum requirement develops when the outboard lane is being loaded and is quantified by the transverse location of the track, the average weight of the cars, and the loading speed. If the train is 10 cars long, each car weighs 45 tons and is 40 ft. long, the outboard track is 35 ft. off center, and the loading speed is 5 ft./second, the total moment required to keep the ship on an even keel is 18,000 ft. tons (45 tons/car x 10 cars X 35 ft.). The correction rate necessary is about 11,800 ft. 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 mounted crane may be required to lift a 40 ton container located on the quay about 75 ft. off the center line of the ship. Considering not only the weight of the container but also

COMPRESSED AIR SYSTEM FIGURE 5

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177

the boom of the crane and the lifting gear, the total transverse moment can be as high as 6,000 ft. tons, relatively high for a small ship. The slewing rate of the crane will determine the correction rate. (In many cases, this rate must be considered in line with practical limits on the speed of the movement of a pendant weight.) A practical rate would be in the range of 6,000 ft. tons/ minute, allowing the maximum cargo movement to be completed within one minute. The list control systems on the SEALIFT ships for the U.S. Military Sealift Command were sized to meet the highest of two requirements: moving an MlAl tank athwart ship from shell to shell in 1.5 minutes. moving a side-loading warping tug by crane from the center line to 25 ft. outboard within 2.5 minutes. Generally, the tug is controlling, due to the weight of the crane boom and lifting gear, and results in a moment of about 10,300 ft. tons and a rate of 4,120 ft. tons/minute To properly counteract these transverse moments, the list control system will have to transfer ballast between the designated heeling tanks. Size selection depends on two interrelated main factors: the transverse moment that can be developed and the time required to transfer the ballast. The maximum heeling moment generally can be developed by the tanks if they are half full and the entire contents of one tank is transferred to the other. Selecting long, narrow tanks directly inboard of the shell will maximize the distance between the transverse centers of the tanks and minimize required capacity. Considering the SEALIFT example given above, if the distance between tank centers on these PANAMAX ships is about 100 feet, the required capacity of each tank is a minimum of 206 tons (half capacity, 103 tons, will provide the required moment of 10,300 foot tons when moved the 100 ft. distance between centers). The pump must be capable of moving 41.2 tons/minute (4120/100) or about 10,800 GPM. If the tanks were wider, with the distance between the transverse centers decreased correspondingly, the capacity of the tanks and the pump would increase in proportion. The selection of the size of the cross-over connection between the heeling tanks is made on the same flow velocity basis as any pipeline. However, there may be some additional consideration to minimize losses in the connecting piping if an axial flow pump is selected, as the power requirements for this pump type are very sensitive to the total design head.

Hydraulic Performance

List control system hydraulic operating conditions are complex when variable speed/pitch pumps are applied.. At the start of loading, the ship is on an even keel and the list control tanks are equalized. Considering a RO/RO cargo liner with a variable speed propeller pump as an example, loading starts on one side of the ship and the LCS automatically transfers water to the opposite side to minimize list. This can continue until all the water has been transferred to one side. During this phase of the operation, the pump is moving water against an increasing discharge head and RPM is increasing to maintain required flow. The second phase of loading then starts on the opposite side. The pump is reversed and water is transferred from the tank with a high level to the low level tank. In this case, there is an initial gravity flow that acts in combination with the pump. In some cases the control system may be required to reverse the pump direction to retard the gravity flow to obtain a controlled flow and prevent over correction of the list angle. The second phase can continue until the tank levels equalize and gravity flow is no longer present. After this the hydraulics are similar to the first phase. These operating cycles may be repeated numerous times during loading or unloading operations, depending on the size of the ship and the heeling tanks and the loading pattern. The hydraulic design of the system requires definition of a four quadrant set of pump head flow curves for use in designing the control system. The sign convention used in this paper is flow and RPM to starboard are positive. Positive head is defined as the difference between port and starboard tank levels (corresponding to a positive gravity flow). Accordingly, starboard (positive) pump flow corresponds to negative head and port (negative) pump flow corresponds to positive head. A set of four quadrant head flow curves developed from shipboard tests is shown in Figure 6. In the first quadrant head and flow are positive and the data represent combined gravity and pump flow to starboard. The second quadrant with positive flow and negative head is due to the pump only. The third quadrant has negative flow and head and corresponds to a combination of pump and gravity flow to port. The fourth quadrant has positive head with negative flow and represents pumping to port without gravity flow. In addition to developing the pump design and determining power requirements, tank vents must be designed for the high flows required by the system. The internal structure must have sufficient flow openings to allow internal air and water flow. For example,

178

Design Session

LIST CONTROL SYSTEM PUMP CURVES ?n---- -..__VARIA~~ESPEEDPUMP!_P_TESTDATA.. .-..----_

30 INCH DIAMETER PUMP

+

ORPM

-20 -30000 -20000

I -10000 0 FLOW (CPM) 10000 20000 3oom

FOUR QUADRANT PUMP CURVES FIGURE 6

I 20 -

GRAVITY FLOW TESTS AT 0 RPM SHIPBOARD SYSTEM TESTS I

A+----

GRAVITY FLOti I TESTS SHIPBOARD SYSTEM TESTS i/;---.i,

GRAVITY FLOW TESTS FIGURE 7

GRAVITY FLOW TESTS FIGURE 8

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179

transverse web frames at the bottom of tanks can obstruct flow to the pump suction. The system design must be such that water hammer This is usually accomplished by either is avoided. controlling the valve closing speed or the sequence of starting and stopping the pump and opening and closing the valve.

gravity head and gravity flow are reduced to the desired programmed flow. For combined pump and gravity flow it is assumed that the total flow is the sum of a gravity induced component and the pump flow at constant RPM and zero head. The gravity flow component is approximated by; GFLOW = A,*[2g(H,-HJK,]`"; (4)

Systems Using a Propeller Pump

Figure 4, shows the system schematic for a list control system with a propeller pump installed in the cross duct between the tanks. The tanks are vented to the atmosphere and the liquid system energy balance is given by; (pp-p&/y, +H,-H,+B*sin(+) +HA-HL = 0 ; (1) H, = K,*V,`/2g System head loss; The total dynamic head is defined as TDH =H,-Hs +B*sin(+) HA = -TDH -(B -~)l% ; + & , and (2) where GFLOW is in cubic feet per second and K, = system flow loss coefficient that includes losses due to relative flow over pump internals. Shipboard tests results have been used to evaluate the gravity head flow characteristics. A variable speed pump was used to establish an initial differential head. Flow was then reversed and a constant pump RPM was programmed. Gravity head-flow curves for zero RPM are shown in Figure 7. Test data for flow to port and starboard are shown together with a calculation of GFLOW in GPM for a loss coefficient K = 7. It should be noted that estimates of the loss coefficient without the pump are K = 5.4. Test results for gravity assisted flow to port at pump speeds of 400 and 500 RPM are shown in Figure 8. Gravity flow was calculated as the difference between the total flow in the test and previous test results at constant RPM and zero head. Test results for flow to port at zero RPM are also included on the Figure and slightly lower gravity flow was observed with the pump running.

Total dynamic head at zero list provides a basis for pump design when vent head loss is low. The energy balance for the air vent is given by; pa-pp = "ya (1 +I(,)V,,2/2g ; pa-ps = -"/a (1 +I$ )v,, 2/2g ; and the vent head loss is; Pp-Ps = - ?/a(I +K)*(v,, * + Ys2)/2g; (3) (flow into tank) (flow out of tank)

Axial Flow Pump Design

The design of propeller pumps is often based on model or full scale test data which cover variations of design parameters such as pitch ratio, blade area, number of blades and section shape. Pump diameter is selected on the basis of system head and flow requirements. Performance enhancement can be provided by adding stators in the crossover pipe and fitting a propeller hub. Pump design performance is checked by shop tests of completed units set up as close as possible to shipboard conditions. It should be noted that it may not be possible to simulate the highest levels of shipboard discharge heads in the shop.

A vent air velocity limit of 80 ft/sec is recommended. Assuming a 300 mm vent diameter, air density of .0764 lbs per cubic foot and a loss coefficient of 2, the air flow in each line is 28,180 GPM and the vent head loss is 0.75 ft This is less than 4 percent of the maximum gravity head for representative ballast tanks. It is noted that higher air flow velocities will quickly cause appreciable vent head loss.

Gravity Flow

In cases of water transfer from a tank with a high water level to the low level tank, a gravity flow can act in combination with the pump or blower. For automatic control, this will provide a high flow rate and reduce the time required to correct a list. For manual control programmed to maintain a constant flow rate, the pump may have to be reversed to slow down flow until the

Systems Using Compressed Air

Figure 5 shows the system schematic for a LCS using compressed air to move water. Consider the case where the blower is discharging air into the port tank and the starboard tank is vented to the atmosphere. There is no pump, and the liquid system energy balance becomes;

180

Design Session

(P,-P,) = "/I * TDH ;

(5)

Assume an isothermal perfect gas and continuity, then the specific gravity 7b of the compressed air out of the blower is; (6)

-/b = `ya * w h

The propeller efficiency varies between 50% and 80 % depending on the flow. Figures 9 and 10 show examples of propeller test data for the same fixed pitch pump at various RPM values. When a forced draft blower is used to transfer water; SHP = AHP/e, (14) eB = blower efficiency AHP = air horsepower The efficiency of axial flow blowers varies from 40% to 65% (Marine Engineering, 1992). The pump or blower drive system efficiency, es , must also be considered. For example, the efficiency of a hydraulic drive is in the range of 90 % . The required motor horsepower (MHP) is then determined by MHP = SHP/e,es (15) where: eM is the motor efficiency which, for an electric motor, varies form 85% at l/4 load to 93% at full load. where :

For air flow into the port tank; pp = & - x "(1 +I&)v,2/2gA For air flow out of the starboard tank; Ps = Pa + 19*(I+KpX2/2g G-9 (7)

The air pressure difference between the tanks is determined from equations (7) and (8) and combined with the water system energy balance, equation (5), to obtain the blower design discharge pressure.

PB = yl*TDH

+~a[ 1 + (~./W)(Va2/2g)(_1_+K,, --2I 1 - (YJP,)(V, /2g)(l + &>I

(9)

Design of List Control Tank Vents

Undersized LCS tank vent lines can significantly reduce the water transfer rate achieved and jeopardize the system's ability to control list. A design procedure that uses the maximum air velocity in the vent line as a design criteria has been used to size vent lines. The pressure drop in the vent line must be checked to assure that the size selected does not produce a vacuum on the suction side during maximum expected flow (Crane Technical Paper 410). For commercial construction, a design limit air velocity of 80 ft/second is used, the limit used if ball valves are installed in the vent line is 50 ft/sec. .Using this approach, 12 inch diameter vents were selected for a LCS that had a maximum flow rate of 27,000 GPM. This vent size was selected on the basis of no ball valves in the line. If ball valves are installed in a vent line with flow velocities over 50 ft/sec, aerodynamic drag on the ball can case it to slam shut. This was observed during shipboard tests of a LCS and it was necessary to remove the balls to achieve proper operation. In systems that use air blowers, the air is transferred within the system rather than to the atmosphere. Vent lines necessary to meet the requirements of the regulatory bodies are installed below the minimum water level to avoid the possibility of pressurizing the tanks beyond the design criteria.

The blower total differential pressure is pa-p a and expressing this in feet of salt water, h,,,: h, = TDH + (R/T)*[& -I- & ] (10)

IJ -&I

where; x, =( %hwa2~2g)(l + Kdp) As =( -rJP,)(V,*/2g)(I + S,) The blower air horsepower ( for salt water) is defined as; AHP = h, Q, i516.91 And since Q, =Q AHP = WHP + (p, /y, )(X, + A) (Q / 516.91); Cl-x, > (11)

Power Requirements

The basic system power required is the water horsepower (WHP), which is defined for salt water as; WHP GPM TDH For systems using shaft horsepower is; where: = GPM*TDH/3848 (12) = ballast water flow = total dynamic head (ft) a propeller pump, the required

where:

SHP = WHP/e, ep = propeller efficiency

(13)

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COMPP LRISONOF SHIP AND SHOP TEST DATA PUMP STARBOARD AT 200 RPM r--.- ---.- ..-.-----..---.._-_--__ 1-O -~- __..

I

6000

8000 FLOW (GPMI

PUMP PROPELLER EFFICIENCY

FIGURE 9

COMPARISON OF SHIP AND SHOP TEST DATA

1.0 0.9 0.8

PUblP.$TARBOARD E --- -___~-

AT 500 RPM .

I

10000 FLOW (CPM)

I

20000

I

30000

PUMP PROPELLER EFFICIENCY FIGURE 10

182

Design Session

Water Hammer

Systems that use a remotely operated butterfly valve in the crossover pipe can experience water hammer if the valve is closed too rapidly. Valve closing in the presence of flow could occur, for example, during emergency shut down procedures. Valve operators are available that can close the valve as quickly as 0.70 seconds and water hammer has been experienced using this closing rate. When the valve closing period was increased to 3 seconds, water hammer did not occur. Care should be taken during the overall system design, either in the control system or in the selection of the valve actuator, to be sure a valve is not closed rapidly valve is being opened unless a corresponding simultaneously or pump start and stop sequence is timed..

g = gravitational acceleration VT = velocity at blade 0.7 radius The local cavitation number at 0.7R has been used to predict cavitation inception. For dead head conditions axial velocity is zero and at the 0.7 blade radius position; V, = 0.7nD D = propeller diameter n = propeller rotational speed, rps Figure 11 shows propeller dead head test data. Total dynamic head and cavitation number at 0.7 radius are shown as a function of propeller RPM. Cavitation and power limits are observed between 500 and 600 RPM.

Cavitation

List control systems that use a variable speed or variable pitch pump to transfer water can maintain a differential head between the list control tanks at zero flow with the valve open. This configuration can occur during either automatic or manual operation and is termed a dead head condition. Dead head increases as RPM increases and the maximum differential head that can be maintained by a variable speed pump is limited by propeller cavitation. Cavitation occurs when the local pressure on propeller pump blades falls below the water Cavitation will reduce the propeller vapor pressure. efficiency and more torque must be applied to maintain a desired head. At some point, the available horsepower limit is reached and the available dead head and RPM reach an upper limit. Cavitation limits on head can also affect constant speed pumps. The cavitation number is defined as the ratio of the difference between the static pressure and the water vapor pressure to the dynamic pressure (Principles of Naval Architecture, Vol II 1985).. When the cavitation number falls below 1, the total pressure on the blades is less than the vapor pressure. This definition of cavitation inception has been observed to provide good correlation for dead head test data limits. Cavitation No. = (p,-p,)/(O.SpV,2) where: p, = Pam = P, = h = P = it, +p& atmospheric pressure vapor pressure of water suction head mass density of water

DEAD HEAD mxn UEan SHOP TEST RESULTS

CAVITATION NUMBER FIGURE 11 Electronic Control System

The electronic control system discussed below is that used for a variable speed propeller pump system. It represents the state-of-the-art and is more extensive in terms of variables monitored and controlled than other Control of the LCS is systems presently available. provided by a microprocessor based real time computer system. This type of system, often called an embedded system, uses the computer to acquire data from tank level transducers, list angle sensor output, butterfly valve position sensor and other data for the hydraulic system which is monitored for safety and alarm purposes.

List Control Systems

183

The movement of ballast while changing either head or flow when using a variable speed pump requires real time computer control. Control is effected using data from tank water level transducers and a list angle sensor. A signal for control of pump speed is computed at intervals based upon transducer data and the mode of system operation. In manual control, constant ballast flow in the desired direction is provided. In automatic control, ballast is transferred in the direction required to restore zero list at a flow rate proportional to the angle of list. The propeller pump and associated butterfly valve are In addition to the control powered hydraulically. algorithm necessary to maintain required ballast flow, synchronous control of the butterfly valve is necessary to avoid water hammer. Further, the hydraulic system must be monitored to assure safe operation. The electronic components include a PC-like computer on a PC/104 standard module. The PC/104 components include the computer, a display control, a touch panel control, A/D and D/A converter modules with binary I/O and a PCMCIA memory card module to contain the control program and ship characteristics data. All together, the system is composed of the computer assembly, a graphics display with touch panel, and an electrically isolated interface to the transducers and ships machinery control system. The operator interacts with the system using the display, touch panel and panel switches. List control tank levels, list angle, ballast flow rate, pump RPM, and hydraulic system pressure, together with butterfly valve position and alarm indications, are presented both numerically and graphically on the display. With the addition of ship hydrostatic characteristics to the control software, the metacentric height, GM, can be obtained by applying a measured amount of ballast transfer to affect an ordered change in list angle. This feature is provided by many LCS control units as an aid to ship operators in determining stability prior to leaving port. Alarms provided by the electronic control system include hydraulic system temperature, pressure, and oil level, high and low list control tank levels and maximum list angle. The control system prevents components from operating in a direction that increases an alarm error. It allows operation in a direction that will cause an alarm to be cleared. As a result, no "override" or emergency modes are used to reset or reposition equipment after an alarm. Powering on and off is an important issue to the safe control of real time embedded systems. For both turn on and off situations, it is desirable that transient alarm

conditions be suppressed. To ensure that the system's components are properly secured before the computer is turned off, the electronic control system takes control of system power upon turn on, bypassing the system power on/off switch. It then monitors the switch in such a way that the operator's intentions are known. Should the operator request that the power be shut off while ballast is in motion, the butterfly valve is open, or the hydraulic power unit is in operation, the system first attempts to stop ballast flow, close the butterfly valve, stop the pump and shut down the HPU before disconnecting itself from power. In the worst case this process will take less than 5 seconds and the operator is seldom aware that this shutdown sequence check has occurred. By suppressing transient alarms during the startup/shutdown process, the annoyance of the alarm sound is avoided.

COMPARISON OF AVAILABLE

SYSTEMS

The comparison of the various systems that are available can only be made on the ability of the systems to respond immediately to a circumstance that requires heel correction. The amount of heel correction that can be provided within a short period of time is part of this consideration. This quick response ability, which may not be necessary for all types of ships, depends not only on the control system but also on the capacity and type of mechanical system used to physically transfer ballast. Considering the types of systems that are commonly used today, a brief evaluation of the reaction times shows some significant differences. The most basic system, using an existing centrifugal pump with a system of automatically controlled valves to provide heel correction, as shown is Figure 2, also has the slowest reaction times. The fastest reaction time for this system can be obtained by having the pump running continuously during heel correction procedures. When no heel correction is necessary, the pump will be recirculating ballast through the tank with the highest level. The control system is generally set to recognize two list angle set points. The "threshold" angle is defined as the list at which heel correction will be initiated while the "shut-off" value is defined as the angle at which heel correction procedures will stop. When the "threshold" value is reached, the control system will configure the valves to transfer ballast between the appropriate tanks to decrease the angle until the heel angle reaches the "shutoff" value, when the valves will be reconfigured to resume circulation with no ballast transfer. The reaction time is dependent on the value assigned to the "threshold"

184

Design Session

angle, which must be large enough to avoid constantly opening and closing the valves. The time necessary to avoid possibility of water hammer while closing the valves must also be considered. However, since one valve on either the suction or discharge side of the pump is opening while the other is closing, this aspect is not as problematic as it is with axial flow pump systems. Generally, this type of system also has the lowest pump capacity, since the primary purpose of the pump is selected to fill ballast tanks in approximately 30 rather than 3 minutes or move water through a 3" fire hose rather than an 18" transfer pipe. Regulatory body requirements for fire pumps, for example, are less than 1,000 GPM. The system using a constant speed axial flow pump has some of the same operating limits as the system using an existing centrifugal pump. The system is inactive until the heel angle changes from 0" or some other value recognized by the control system, at which point the pump is started and the valve is opened. Pumping continues until an even keel is regained, when the pump is shut down and the valve is closed. Since this is an axial flow pump, the delivery can decrease as the discharge head increases, depending on the selected speed Gravity flow is not controlled. The and power. advantage this system has over the "existing pump" system is that the pump is specifically selected for list control and usually has higher capacities, in the range of 3,000 to 6,000 GPM. The remaining types of systems, the variable speed or variable pitch axial flow pump as well as the compressed air systems, all can maintain a head difference between the two heeling tanks without closing the valve. This is accomplished by adjusting either the propeller speed or pitch or the air pressure to maintain the head difference This allows the system to react without any flow. immediately to any change in heel angle. The ability to react will depend more on the sophistication of the control system rather than mechanical response characteristics. Figure 12 shows an example of the reaction time that can be obtained with a variable speed pump. The system is reacting to reverses in the slewing of a crane with a lift of about 100 tons. As the crane changes direction and the list angle passes through O", the direction of the pump changes almost immediately, as evidenced by the change in the levels in the heel control tanks. Given the capability of the mechanical components to provide an immediate response, the total reaction time will depend on the ability of the electronic control system to recognize the need for heel control, continually analyze the input information received from various sensors (e.g., the inclinometer and the tank level sensors) and issue the

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

REFERENCES

Argyriadis,D.A. et al "Design and Construction of Modem Roll-On/Roll-Off and Container Carriers."

Transactionsof SNAME, Vol 87, (1979)pp 313-350

SNAME, "Marine Blowers" ,( 1992) Engineering, Section 3

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

ACKNOWLEDGMENTS

Figure 3 is based on material provided by FRANK MOHN AS, Nesttun, Norway Figure 5 is based on material provided by INTERING GMBH, Hamburg, Germany

List Control Systems

185

TEST WITH 100 TON LOAD ON CRANE

24 22 ,. ..-#

I-

i

REMOVING WEIGHT FROM SHIP I.--. . I I I,

.W. I ( _.___ i--' -1, : ,

/ I

1'

h ' -i

:\: ' , , ,I ,' . I, , F' ; ,I.

j

-. \, .J

! I i ."%. \

-i i ., 1 . ! J .`.:

; .

\.,.*.e.*,-.

------0 .0--.-.-.-. I -I

/ /

4 2 3900 / I , I I I 4100 J I I

I

PORT LEVEL I \

I : -1

/ I rl 1

4000

4200 TIME [SEC)

4300

4400

4500

VARIABLE SPEED PUMP SYSTEM PERFORMANCE FIGURE 12

186

Design Session

Discussion

slowing down production because the cranes have to wait for the ship to remove excessive list. The answer probably lies somewhere in between. It seems that when evaluating how The description of the different list control systems is com- much list control is required a cost benefit comparison should prehensive and considers the main design aspects. Quick reac- be done of the list control system options available on the vestion and high flow rates of variable speed pump systems and sel versus the time and cost to properly sequence cargo operblower systems are important features for high performance ations in the terminal to reduce the listing moments. In today's list control. cost cutting world, when faced with the potentially high cost As to the power requirements of blower anti-heeling systems of a large capacity list control system there can be a tendency [section of the paper incorporating equations (12)-( 15)] the fol- to cut the cost and scope of the list control system and say lowing should be summarized: INTERING anti-heeling systems let's do a better job of planning at the terminal to resolve the according to Fig. 5 do not use axial flow blowers with an ef- issue since this is the solution without upfront capital expenficiency of 40 to 65% but since 1971 we use rotary positive ditures. However, some form of list control seems to be a nedisplacement blowers (roots-type rotary piston blowers) with cessity and the answer to the question of how big a system to a volumetric efficiency eBof 90% and a total efficiency of 8.5 install can probably be found in the cost benefit analysis. to 88% in anti-heeling system design conditions. In order to provide insight during the initial assessmentsof Design of list control tank vents of blower anti-healing sys- a potential project on the appropriate list control systemsto contems: Pressurizing of heeling tanks beyond the design criteria sider it would be interesting if the authors could provide some with a blower anti-heeling system is avoided by safety valves information on the relative costs of the systems described in at the blower units. Tank structure is designed to the pressure the paper. This would help the readers determine on an order preset at this valve, which will not be exceeded in operation. of magnitude basis which types of list control systemsare within The system is connected to atmosphere via the air valve group their budget and worth further study. Most of the other infor(No. 4 in Fig. 5). mation required for this type of preliminary assessmentis conOverflow prices ending below minimum water level are fit- tained in the paper. ted to protect the tanks against operator's failure to vent the tanks when filling or discharging. Steve Heskett, Visitor

GmbH, Norderstedt, Germany

Dirk Hoflich, Visitor, INTERING

Eugene A. Van Rynbach,

Member

The authors are to be congratulated on writing a concise and informative paper describing the types of list control systems available today. It should be a handy reference for anyone involved in a new construction program or vessel modification program where list control is needed. It is informative in that it describes both the key components of a list control system and the issues involved in designing them. Sea-Land Service has list control systems on some of its ships, mostly of the type that utilizes existing pumps and ballast piping systems. However, systems that may have been adequate in the past may not be adequate in the future. Containerships are getting larger and terminals are being pushed to increase productivity, resulting in more cranes working a vessel at the same time. List control is becoming of increasing importance and can have a measurable cost impact on operations. As mentioned in the paper for some types of heavy cargoes and for rail cars, for example, list control is very much done by the vessel and is a necessity for a vessel in those trades. In container shipping, however, the answer to list problems is more complex and not always found on the ship. One of the main reasons for this is that the loading or discharging of a single container does not normally cause unacceptable list. It is the cumulative effect of loading many containers without watching the transverse balance of the ship that causes list. Therefore, well planned load sequencing can also be used to control list. The fact that there are two primary means of controlling list on containerships has the potential to lead to finger pointing between the terminal and the vessel if list and vessel capability to respond to list are not considered during cargo operations on the ship. The vessel could say their list control system, or if an automatic one is not fitted, the list control efforts by the crew cannot keep up with the changes in list brought about by the rapid cargo operations being done by the terminal without due consideration of the impact on list. The terminal could say the ship has an inadequate list control system and is

I wish to extend my congratulations to the authors for providing the shipbuilding industry with a comprehensive paper on the various types of list control systems that are on the market today. The paper gives marine engineers and shipowners a greater understanding of what may be achieved by each system, so that they can make a more educated determination of which system best suits their needs. There are a couple of comments I would like to make and receive the authors comments or concurrence: 1. This concerns statementsmade throughout the paper, that the tanks are initially filled to 50% of the tank capacity, and that maximum heeling moments are obtained by transferring the entire contents of one tank to the other. These statements have caused some confusion with production personnel and customer representatives during recent installations and testing of list control systems. The tanks being used as heeling tanks often are of a considerably larger capacity than is required to develop the necessary heeling moment, and in actuality the tanks are filled to 50% of, what I would call, the transferable capacity. The transferable capacity being the volume of water between the low level and high level pump cutout switches. The low level switch in some instances may be at a considerable height above the bottom of the tank, while the high level switch may be well below the top of the tank. Normally the water remains in the tanks after loading is completed. The distinction between 50% of tank capacity and 50% of transferable capacity must be made, since the heeling tank capacity figures are used to determine the ship's stability and ballast conditions prior to departure. 2. I would like to reemphasize that the location of the pump suction bellmouths within the heeling tanks must be given careful consideration. The high suction capacities of these pumps can create numerous cavitation and air pocket problems when the bellmouths are located in tight corners of heeling tanks. Large lumber holes on the top and bottom of tank web frames are a must, to ensure good tank ventilation and draining during pumping operations. The turbulences from pump suctions 187

List Control Systems

also create havoc with pressure transmitters for the level gaging systems. In most casesthe bellmouth doesn't need to be at the lowest point in the tank and can therefore be located away from structurally congested areasand well away from level gage pressure transducers which would normally be located at the lowest point in the tank. 3. The one criticism of the paper I have is that it didn't go into a great deal of detail about the compressed air type LCS. What are the advantages and disadvantages of a compressed air system as opposed to a variable speed/pitch pumping system? For example, the compressed air system would appear to have more piping and valves, but less pumping components. Also the compressed air system can be used for sea stabilization while underway. The hydraulic variable speed/pitch pumping system is susceptible to leakage. If the propeller pump is in a submerged location such as a double bottom ballast tank, these oil leaks can be of major concern to an environmentally conscious shipownerloperator. Horst Halden, Visitor, INTERING

Germany GmbH, Norderstedt,

As the worldwide leader and the only company experienced in blower- and pump-activated anti-heeling systems, INTERING thanks the authors for their efforts to bring this subject to the awareness of a broader marine audience. We fully support the authors' opinion about the importance of short svstem's reaction time and immediate comoensation of cargo &eights during loading/discharging to the benefit of the ship's harbor time. As an example: In the early 1970's, INTERING blower-activated anti-heeling systems led to harVariable Speed Pump System FLUME relate INTERING Blower-activatkd INTERlkG

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 Table A comparison between the FLUME and INTERING systems: 3. In the INTERING blower-activated system, tanks can be arranged in various places on various decks, while blowers are installed where space is available. 4. Blower systems in operation on the world's largest train ferries enable-without hydraulic shock-reverse of flow direction/start of complete flow in less than 1 set, with total power up to 600 kW. 5. Due to the INTERING concept "no moving parts in water," together with very little and easy maintenance and possible exchange of components, the blower-activated system is given preference where maximum reliability is requested. 6. Tank water, which is needed for anti-heeling operation, is double-used for roll stabilization in combined INTERING stabilizer and anti-heeling systems. 7. We appreciate that the competition follows INTERING's lead in us&g the anti-heeling system for stability measurement. Experience since 1973 led to extreme simplification and reliabilitv in third-svstem generation (1996) and to the same accuracy'as the results of tie shipyard;s inclining experiments. Extensive experiences are available on stability measurementsat

speed.

Additiona,

reference

Test in 10 Min-

JONER, J., HALDEN, H., "New TT-Ferries-Inclining utes," Schiff & Hafens July 1995.

Authors' Closure

System

I `Ihe authors

to axial flow blowers

with 40 - 6$ % efficiency.

oositive

INTWNG never used axial flow blowers, but ptarv blowers "roots tvpe" witb 88 - 80 X efficiency. 1

Control and Safev Safetv. Block of Water Flow

displacement

No Redundancy. in one Valve.

Arranaement Flexibilitv.

Redundancy. Air Valves for Control and Safety. Water Valve for Safety only.

Redundancy

1

One Pump per Tank.

NO cross-over Functions fw

Emergency. I

Reaction

1 to 5 Blowers in any Tank Combination. "Multiple Tasking".

Time < 1 see. far complete

I

?

I

Flow Reverse up to SO0 kW. No shocks.

i ?

Movirm in Water

Parts.

Reliabilitv.

Warranty

Moving Parts in Air only. > 3OW Years accum. Operating Experience. 3 Years Guarantee since 1962. I

for Roll Stabiliratiq I GM Accuracy Yes

Same Tanks No

"reasonable" ?

Within 1 % of official lndining Test Results. Type-approved by Int'l Class Societies. Over 500 Years of accumulated Experience.

Table A 188

Design Session

The response that has been received reflects the reactions of ship operators, ship builders, and the various manufacturers of list control systems, which evidences the growing interest in these systems. They are becoming a standard requirement for many types of commercial newbuildings. The opinions and viewpoints of the ship operators are certainly welcome. In reply to Mr. Van Rynbach, the problems due to list angles when loading larger containerships can certainly be avoided by paying close attention to the loading sequence. Loading one or two containers off center will not cause difficulties, but a continuation of this trend could cause loading delays, and the ensuing disagreements are well described. These incidences could be completely avoided with a high capacity list control system, but the economic impacts cannot be ignored and should be the deciding factor. It is interesting that major problems can be experienced not by the larger ships but by the smaller feeder ships that often are self loading/unloading via onboard cranes. As containers are added on deck and transverse stability decreases,the off center moment caused by lifting a container from the wharf can cause very high list angles, in excess of 10 deg, and some type of automatic compensation becomes a necessity. Concerning the request for relative prices of the various systemtypes, theseare difficult to provide as they are completely dependent on the pump capacity and related pipeline sizes. Some ballpark estimatesare from 10 000 to 50 000 1997 dollars for control systems that use an existing

pump, 50 000 to 100 000 for constant speed pump systems, 150 000 to 200 000 for variable speedpump systems,and about 300 000 and over for the air controlled system, depending on whether it is combined with a roll stabilization system. We would like to thank Mr. Heskett for his observations regarding transferable capacity as opposed to tank capacity. In particular, some conversions have the luxury of very large list control tanks that will never utilize half the full capacity for list control. In these instances, the initial tank levels are determined by other considerations, such as trim or ballast requirements. The control system software can be programmed to meet these needs. However, this is a high quality problem. In most instances, the heeling tanks are sized to meet the specified heel correction requirements with little excess capacity. The reinforcement of the importance of free flow of both the fluid and the air within the heeling tanks is very welcome. Most problems encountered when commissioning these systems are related to a lack of attention to these details. The ability to use the same set of tanks to provide both roll stabilization and list control is a point often offered as an advantage by the suppliers of air activated list control systems. A U-tube type roll stabilization tank can be utilized for list control. However, this type of stabilizer tank has a single period response.The other type of passive anti-roll tank commonly installed, a free surface type tank, has a response period that can be varied by changing the liquid level. This single response period disadvantage of a U-tube can be overcome in a number of ways, all of them expensive. The most common approach is to construct the tank so that it can respond to the shortest expected natural roll period of the ship and use a control system to delay the response. With an air control system, a valve in the air cross over duct is opened and closed to hold the water on one side and lengthen the response time. This approach is often erroneously referred to as an "active" tank stabilizer but is properly called a "controlled passive" stabilizer. The response period requirement for a U-tube is determined by the width of the wing tanks and the height of the cross over duct (Field, 1975). The necessity of having a relatively clear cross over duct normally means that there is a double inner bottom, as the normal structure in bottom tanks will not allow proper flow. This consideration requires additional structure that is difficult to install due to the confined spaces normally involved. The size requirements of a list control system are determined by what is being loaded and the rapidity of the loading process. The size and configuration of a roll stabilization tank are determined by the stability conditions (righting moment and metacentric height) of the ship ready for sea, after loading has been completed. Although, becausethey look similar, it may initially seem that there would be some space savings advantage in using the same tanks for both stabilization and list control purposes, this usually does not prove to be the case when considering the design requirements. Trying to force the issue can result in multiple tank arrangements, with some of the tanks used for one purpose and others for both. In some instances, tanks are built within tanks so that the proper response period for the stabilizer can be obtained without losing list control capacity (Halden, 1997). It is our opinion that the most economical approach is to design the anti-roll tanks to stabilize roll at sea and the list control tanks to minimize heel during the loading process. This usually results in two systems that are effective, simple to install, and have the lowest capital and installation costs (see Sellars and Martin, 1992). In response to Mr. Hbjlich, we thank him for these important pieces of additional information and apologize for assuming the use of the wrong type of blowers. The efficiency was taken from "Marine Engineering," as referenced, and is indeed

for an axial flow blower. The use of rotary positive displacement blowers with a higher efficiency makes the power requirements for the air controlled system about equal to the pump systems. In addition, the safety valve on the blower units means that the tanks for an air controlled system can be designed to the normal deep tank standards. It is difficult to respond to Mr. Halden as many of his remarks are aimed at stressing the sales advantages of systems he supplies and appear to be less than objective. For instance, we have no knowledge of the fuel savings enjoyed by the cross channel ferry fitted with his system (our own experience in this regard has shown that these claims are easy to calculate but difficult to prove), the accumulated experience of his staff, nor the type of guarantee he offers, nor do we believe this is the proper forum for such claims. Many of his points are similar to those answered above. As far as redundancy is concerned, the valve in the variable speed pump system is not used for control. The valve is either open or shut and the speed of the pump is used for control. If the hydraulic system shuts down for any reason, there is a "fail safe" accumulator that will close the valve. The parts of a variable speed pump system that are in water are the pump with the hydraulic motor and the valve. The materials used are corrosion resistant: stainless steel and NIBRAL. In fact, in many installations these items have been submerged in inner bottom tanks that are used for other purposes to save space.However, as mentioned by Mr. Heskett, hydraulic leakage could cause environmental concerns. As mentioned in the paper, the responseof the variable speed pump system is immediate. The response time for this system, and we assume the air controlled system, is limited not by the speed of the mechanical components but by the gains programmed 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 the variable speed pump system is active as the valve is not closed during normal operations. When the system is shut down, the control system automatically adjusts the pump speed to obtain zero flow and the valve is "pulsed" to close in a programmable number of steps to avoid shock waves. If the valve is closed by the accumulator, the closing time is lengthened by flow control devices. Contrary to Mr. Halden's belief, using the list control tanks to obtain a measure of the GM was first designed into a system supplied by FLUME in 1970 for use on two RO/RO ships built in Gdansk. The application of the engineering principals is not difficult and the accuracy will depend on the accuracy in the level sensors in the tanks, the heel sensor in the control unit, and the determination of the displacement. Errors in the sensorsare less than 0.1%. If the displacement is obtained from a draft indicator, the accuracy can be very good when the vessel is not under way. However, if the draft marks are read from dock side, there is considerable room for error. We would like to thank all of the discussers for their contributions to this paper and SNAME for providing a forum for this presentation. Additional references

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

List Control Systems

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