EXPERIENCES WITH CUTTER SUCTION DREDGE SIMULATOR TRAINING

 

R. E. Randall[1], P. S. deJong[2] and S. A. Miedema[3]

 

 

ABSTRACT

 

The cutter suction dredge is the most commonly used dredging equipment for dredging navigable waterways. The capital costs of these dredges are in the millions of US dollars and the dredging projects involve the removal of hundreds of thousand to just over a million cubic yards of dredged sediments. Production engineers usually accomplish estimating the production of dredges, and controlling the operation of the dredge is usually the responsibility of the dredge operator (leverman). In order to improve the performance and education of the production engineer and the leverman, cutter suction dredge simulators are available for training of these dredging personnel. Two dredge simulator short courses have been conduct since 1999 and the experience gained from these courses shows that they are valuable training tools. The simulators consist of dredging controls and instrumentation interfaced with a personal computer. Software has been developed that accurately models or simulates the dredge movement, the hydraulic transport, and the cutting of the sediments. Results of the dredge simulation exercise are recorded and used to critique the simulator exercise in order to demonstrate dredging fundamentals such as effects of cavitation, critical velocity, winch power, pump power, and maneuvering of the dredge. Short course participants indicate the simulator is realistic and that it is valuable for training new operators and production engineers.

 

Keywords: Dredge simulator, dredge training, cutter suction dredge, dredge training experience.

 

INTRODUCTION

 

The hydraulic cutter suction dredge is the most commonly used equipment for excavating and maintaining navigable waterways. The dredge size is defined by the diameter of the discharge line and range in size from 0.15 to 1.22 m (6 to 48 in). The most common size for a cutter suction dredge is 0.61 m (24 in). Small hydraulic cutter suction dredges are considered to range from 0.15 to 0.30 m (6 to 12 in). In the United States the cutter suction dredges accounted for 149.1 million cubic yards of the total 271.2 million cubic yards of the dredging volume during the fiscal years 1996-98.

 

Ladder & Ladder Pump

 

Spud

 

Pipeline

 

cutter

 

 

Figure 1. Cutter suction dredge Alaska (Courtesy of Great Lakes Dredge & Dock).

 

The cutter suction dredge consists of a large barge shape vessel that normally doesnt have propulsion equipment and is mobilized to the job site by other vessels. It has centrifugal pumps on board that are used to pump slurry (mixture of solids and water) to a disposal location. The dredge has a ladder that supports the cutter and cutter drive unit and the suction line leading to the suction side of the dredge pump. At the end of the suction line and cutter drive shaft is the cutterhead that is used to loosen and cut sediment that must be removed from the bottom of the waterway. The cutter may have special teeth for excavating the bottom. The excavated material is mixed with the surrounding water and drawn up the suction pipeline due to the low pressure created by the pump on the suction side. The dredged material may consist of clay, silts, sands and gravel and the slurry may be as much as 20% sediments by volume and the other 80% is the ambient fluid. In some cases, an additional pump may be located on the ladder when dredging depth is deep (greater than 9.1 m or 30 ft) in order improve production due to cavitation limitations.

 

On the discharge side of the main dredge pump, a long pipeline is used to transport the slurry to the disposal location that is typically a confined disposal site. The length of the pipeline may be as long as several miles. The power available and head developed by the pump or pumps must be enough to overcome the hydraulic friction losses occurring in the total pump and pipeline system. Additionally, the velocity in the pipeline on the suction and discharge side must be above the critical velocity required to suspend the sediments in the carrier fluid. This critical velocity depends primarily on sediment grain size, sediment specific gravity, and pipeline diameter. Additional pumps (called booster pumps) are sometimes needed to transport the dredged material through very long pipelines.

 

The dredged material lay on the bottom of the waterway and therefore, the dredge must move along the waterway (channel) to excavate the sediments. The dredge typically uses spuds, winches and anchors to move it. Spuds are large vertical cylinders that are located at the stern of the dredge. Hydraulic cutter suction dredges use two spuds arranged at a specified separation distance at the stern or with one at the stern and one in a carriage arrangement. Advancing the dredge is accomplished by alternately raising and lowering the two spuds at the appropriate positions and consequently the dredge walks up the channel or is advanced by the spud carriage.

 

Winches and wires on the port and starboard side of the dredge are used to swing the dredge back and forth across the channel bottom to bring the cutter in contact with the sediment that is to be removed. The swinging of the dredge is about the one spud that is driven into the sediment. The two spud walking dredge arrangement has a production efficiency of approximately 50 %, which means the dredge is removing sediment only half of the time. Dredges with the spud carriage arrangement are more efficient and usually can be removing sediment 75 % of the time. The winches must have enough power to move the cutter across the channel and the wires must be strong enough to prevent breaking and costly downtime for the dredge. The winch wires are normally attached to anchors placed in the channel and these must be moved as the dredge moves up the channel.

 

The operation of a cutter suction dredge just described is complicated and requires skilled dredge operators. These skilled operators (levermen) are essential for a successful and profitable dredging operation. In many cases, these skills are attained through on the job training and over many years under the tutelage of an experienced operator. The operator is assisted by crew who must move anchors, operate engines and pumps, and monitor pipelines. The dredging projects cost millions of dollars and the cutter suction dredge capital costs are in the millions of dollars. Consequently, it is prudent for dredging companies to invest in training their operators so that they have the best understanding of the hydraulic transport and dredge maneuvering principles.

 

Recently, Digital Automation and Control Systems (DACS 1994) and others (Cox et al 1995) have developed dredging simulators for the purpose of training dredge operators. Simulators have been used for a long time to train ship pilots and airplane pilots. The investment in dredge simulator training is expected to improve dredging efficiency and maximize profits for the dredging company and also reduce the costs of dredging projects. Miedema (1999) discussed the principles of developing dredge simulators that interface dredge controls with a personal computer that is the platform for a sophisticated hydraulic simulation of the entire dredging process including transport, cutting, and advancing the dredge.

 

The objective of this paper is to discuss the experiences obtained in two training courses sponsored by the Center for Dredging Studies and the Digital Automation and Control Systems (DACS). These courses were presented over a 1- day period in 1999 and a 2- day period in January 2000.

 

DESCRIPTION OF SIMULATORS

 

The cutter suction dredge simulator consists of two control consoles interfaced to a personal computer. The first DACS simulator used a 486/66 MHz personal computer using Windows 3.1. In 1999, the simulator was upgraded to Pentium II/133 MHz computer running in Windows 95. The software simulating the dredging operations is written in Visual Basic. Figure 2 shows the two simulators that were used during the January 2000 cutter suction dredge short course. The simulator on the left was borrowed from Great Lakes Dredge and Dock and the other simulator is the DACS system.

 

Great Lakes Dredge &Dock DACS/TAMU

Figure 2. Photograph of the cutter suction dredge simulators.

 

The simulator incorporates accepted hydraulic transport theories (Durand, 1953 or Wilson, 1997) and combines them with empirical data and a model for soil cutting (Miedema, 1995 & 1996). The user has the capability of specifying:

 

         Channel geometry

         Geotechnical data (grain size, sand/clay, etc.)

         Pipeline length and diameter

         Pumping power and pump curves

         Cutter drive and winch drive

         Dredge geometry

 

The simulator is designed to show operators and production engineers how to improve their skills and dredge performance through the use of instrumentation. All dredging processes including dredge motions (swing and advance), cutting process and slurry transport are simulated. The user can experiment with improving productivity without worrying about the consequences of plugging the line and suffering costly downtime. Using the simulator to replicate the dredging process, the operator can learn to achieve maximum production. In fact the actual dredge characteristics for the dredge that the operator is normally operating can be input.

 

The large computer screen shows a panel of gauges, a top view of the channel and dredge with swing wires, and a window that can be toggled between a side and rear view of the dredge. Pertinent information such as swing rate, production rate, total production, cutter depth, spud carrier position, and swing angle are illustrated at the bottom of the window. Error messages and time of dredging are shown at the bottom line of the window. The panel of gauges includes pump speed, pump power, cutter speed, cutter power, slurry specific gravity, slurry velocity, suction pressure, discharge pressure, port winch force, and starboard winch force.

 

 

Figure 3. Display on computer screen during simulator operations.

 

The dredging simulator uses input files to define the characteristics of the dredge, channel, and sediments. These file names are:

 


         Channel

         Clay

         Console

         Current

         Cutter

         Friction

         Gauges

         Ladder

         Pipeline

         Main pump

         Pontoon

         Sand

         Spud

         Wire



These files can be changed to specify a particular dredge and project specific condition. For example the characteristics of the dredge pump and performance curves can be input. The pipeline characteristics and fittings are defined in the pipeline file. An example of portions of the pipeline, channel and main pump input files are tabulated in Table 1.

 

A very useful capability of the simulator software is its ability to capture data during each simulator exercise, and these data are plotted to show the participant the result of all the data and actions during the exercise. An example of the output for production is illustrated in Figure 4 that shows slurry density, slurry velocity, swing speed, and production as a function of time and a data point is recorded for every time step in the simulation. The solid line in Figure 4 represents the average value of the recorded data over the exercise time period. The units SI or US presented are specified at the start of the simulation when it is started.

 

Table 1. Selected Portions of Simulator Input File (channel, pipeline, main pump).

Channel

Pipeline

Main Pump

Total width of top view and front view (m) - 150

Theory Used: 0 Durand, 1 Wilson fine, 3 Wilson coarse - 0

Power (kW) - 835

Water level of new channel (m) - 13

Pipe Section 0 Suction

Impeller diameter (m) 1.35

Width of old channel (m) 100

Pipe diameter (m) - 0.4

Revolutions (rpm) 450

Depth of old channel (m ) 10

Pipe length (m) - 26

Maximum Revolutions (rpm) 500

Slope of old channel (m) 30

Pipe roughness 0.0001

Data Points for Performance Curve

Width of new channel (m) 130

Pipe fittings

Flow m3/s Head kPa Eff NPSH m

Depth of New Channel (m) 15

Number of elbows

0.31 1150 0.55 80

Slope of new channel (m) 30

Number of swan necks

0.47 1060 0.75 80

Soil type (0=sand, 1=clay) - 0

Pipe Section 1

0.63 980 0.85 78

 

Pipe diameter 0.4

0.79 790 0.87 76

 

 

 

Figure 4. Example plots from simulator showing the data gathered by the simulator.

 

The actions of the participant using the simulator and error messages are recorded along with the time of the action or message (Table 2). Actions recorded include lowering the ladder, starting pumps, raising and lowering spuds, etc., and error messages such as main pump cavitating, cutter clogged, deposit in the pipeline, pipeline clogged, and no spud on the ground are displayed.

 

Table 2. A partial record of simulator user actions.

 

Date : 01-05-2000

Time : 14:57:37

Student : Name

Company : Name

ID : Student #1

Sessions: 1

 

0:00:00 Action! Start of session

0:00:00 Action! Start of simulation

0:00:00 Action! Cutter drive enabled

0:00:00 Message! Suction mouth above water, stop cutter drive

0:00:05 Action! Main pump enabled

0:00:05 Error! Suction mouth above water, stop main pump

0:00:25 Action! Starboard winch in dual operation

0:00:35 Message! Screen console input

0:00:37 Action! Starboard winch in single operation

0:00:37 Error! Suction mouth above water, stop main pump

0:00:48 Action! Port anchor moved by mouse X = 288.40 ft - Y = 135.27 ft

0:00:48 Error! Suction mouth above water, stop main pump

0:00:58 Action! Starboard anchor moved by mouse X = 30.13 ft - Y = 137.94 ft

0:01:18 Action! Starboard winch in dual operation

0:01:27 Error! Suction mouth above water, stop main pump

0:01:33 Action! Ladder lowered

0:01:33 Fatal error! Pipeline clogged

0:01:54 Action! Ladder hoisted

0:01:59 Action! Swing to starboard

0:02:10 Action! Ladder lowered

0:02:11 Action! Swing to starboard

0:02:19 Action! Swing to port

0:02:21 Action! Swing to port

0:02:35 Action! Free fall of the step spud

0:02:40 Action! Step spud lowered

0:02:40 Action! Work spud hoisted

0:02:43 Action! Swing to port

0:03:03 Error! Main pump cavitating, raise ladder

0:03:14 Action! Swing to port

0:03:21 Error! Main pump cavitating, raise ladder

0:03:25 Action! Swing to starboard

0:03:32 Action! Work spud lowered

0:03:33 Action! Free fall of the work spud

0:03:34 Action! Step spud hoisted

0:03:42 Action! Swing to port

0:04:00 Error! Main pump cavitating, raise ladder

0:04:01 Error! Main pump cavitating, raise ladder

0:04:07 Error! Main pump cavitating, raise ladder

0:04:17 Action! Swing to port

0:04:29 Error! Main pump cavitating, raise ladder

0:04:35 Warning! Cutter clogged

0:04:44 Action! Ladder hoisted

0:04:45 Action! Swing to port

 

 

 

 

COURSE ORGANIZATION

 

Several years ago (1995) the DACS cutter suction dredge simulator was placed at the Center for Dredging Studies at Texas A&M University, and a set of simulator exercises were developed by the Center with the intention of providing a training course for dredge operators and production engineers. In addition to the simulator exercises, relatively short presentations are given on fundamental dredge hydraulics, the sediment cutting process and a demonstration of the dredge simulator as shown in Table 3. The first course was presented in January 1999 using one simulator and experienced operators and production engineers were asked to participate to determine the utility of the simulator. The 1 1/2-day course was successful and the participants recommended that the course length be extended and offered as training for young operators and production engineers.

 

The second offering of the cutter suction dredge simulator short course was presented January 6-8, 2000. A course fee of $1200 was established to offset some of the course costs. The agenda for this short course is tabulated in Table 3. Eight participants arrive the evening before the first day and the cutter suction dredge simulator training manual (Randall and Albar, 1999) was given to the participants. The first day of the 2 1/2-day course consisted of presentations on dredge hydraulics and cutting of the bottom sediments and a demonstration of the two simulators. In the afternoon, all participants were given the opportunity to operate the simulator using simulator exercise number 7, which was a large spud carriage cutter suction dredge 0.76 m (30 in). Each participant was given 45 minutes to complete exercise 7. After all the participants completed the exercise, the data recorded for each participant was reviewed and explained. Exercise 7 had no limitations such as cavitation, long pipeline, power to the pump or cutter, or winch limitations, so the participants could become accustom to the simulator, control console and instrumentation information.

 

Table 3. Schedule for 2 1/2 Day Short Course

Time

Topic

 

First Day

8:00 - 8:30

Introduction

8:30 - 9:15

Dredge Hydraulics

9:15 - 10:00

Cutting of Bottom Sediments

10:00 - 10:30

Refreshment Break

10:30 - 10:45

Simulator Description & Operating Training Manual Review

10:45 - 11:30

Simulator Demonstration

11:30 - 12:30

Lunch

12:30 - 1:15

Simulator Exercise 7 for Participants 1 and 2

1:15 - 2:00

Simulator Exercise 7 for Participants 3 and 4

2:00 - 2:30

Refreshment Break

2:30 - 3:15

Simulator Exercise 7 for Participants 5 and 6

3:15 - 4:00

Simulator Exercise 7 for Participants 7 and 8

4:00 - 5:00

Review Simulator Exercise 7 Results

5:00

Adjourn

 

Second Day

8:00 - 8:15

Introduction to Day 2 Activities

8:15 - 9:15

Simulator Exercise 4, 5, & 6 for Participants 1 and 2

9:15 - 10:15

Simulator Exercise 4, 5, & 6 for Participants 3 and 4

10:15 - 10:30

Refreshment Break

10:30 - 11:30

Simulator Exercises 4, 5, & 6 for Participants 5 and 6

11:30 - 12:30

Simulator Exercises 4, 5, & 6 for Participants 7 and 8

12:30 - 1:30

Lunch

1:30 - 2:30

Review Simulator Exercises 4, 5 & 6 Results

2:30 - 3:30

Simulator Exercises 3 & 1 for Participants 1 and 2

3:30 - 4:00

Refreshment Break

4:00 - 5:00

Simulator Exercises 3 & 1 for Participants 3 and 4

5:00

Adjourn

 

Third Day

8:00 - 8:15

Introduction to Day 3 Activities

8:15 - 9:15

Simulator Exercises 3 &1 for Participants 5 & 6

9:15 - 10:15

Simulator Exercises 3 & 1 for Participants 7 and 8

10:15 -10:30

Refreshment Break

10:30 - 11:30

Review Simulator Exercise 3 & 1 Results

11:30 - 12:00

Short Course Critique & Certificate/CEU Presentations

12:00

Course Completion

 

On the second day, the exercises were for a 0.4 m (16 in) cutter suction dredge. Exercise 6 did not have any limitations so again the participants should be able to maximize the production for this simulated dredge. Exercise 5 had a 5000 m (16,400 ft) pipeline and this was to challenge the participants in controlling the slurry density and velocity within the limits of the pump to maintain production without plugging the pipeline. Participants were expected to have trouble maintaining the same production as achieved in exercise 6. In Exercise 4, the pump and winch power was reduced to demonstrate the effect on production. A review of the data recorded was again presented at the conclusion of these exercises for all participants.

 

Exercises 3 and 1 were started for four of the participants in the afternoon of the second day and completed by the remaining four students at the beginning of the last day, which was to be a half day. The pipeline length was reduced to 2000 m (6,560 ft), but otherwise it was the same as exercise 4. For exercise 1, the ladder pump was removed and the suction limitation (cavitation) was demonstrated. It was expected that the participants would experience a reduced production compared to Exercise 3. For all the Texas A&M exercises 1-6, the sand material was the same (d50=0.5 mm) and the parameters of water depth, cutter size, channel width, and spud separation remained constant.

 

SIMULATOR EXERCISES

 

There are seven built-in scenarios on exercises provided with the simulator software. The exercises are called TEXAM1 through TEXAM7. The TEXAM1 through 6 are a 0.4 m (16 in) cutter suction dredge with fixed spuds. TEXAM7 is a 0.75 m (30 in) dredge with a spud carriage. These exercise dredges are briefly described below:

 

         TEXAM1 This dredge has no ladder pump, only an inboard pump, and the discharge pipeline is 2000 m (6560 ft) in length. Both the suction and discharge pipes are 0.4 m (16 in) in diameter. Due to the relatively small suction pipe, this dredge will demonstrate cavitation readily at high slurry velocities and densities.

 

         TEXAM2 The cavitation problem is removed in this scenario by increasing the suction pipe diameter to 0. 5 m (20 in). In all other respects this is the same dredge as TEXAM1. This scenario is intended to demonstrate that greater production can be achieved by increasing the suction pipe diameter and thereby removing the suction limitation. However, production may still be limited in this scenario by deposition in the discharge line.

 

         TEXAM3 This is the same dredge as TEXAM1, but with a ladder pump installed. This scenario will demonstrate the improvement in production for a dredge when a ladder pump is added, all else being the same. With this dredge it is possible to operate at higher densities, but it may still be limited by deposition in the discharge line.

 

         TEXAM4 This dredge is the same as TEXAM3, but with a discharge pipeline of 5000 m (16,400 ft) in length. The discharge limitation will definitely be apparent in this scenario and will demonstrate the need for a more powerful pump.

 

         TEXAM5 The deposition problem is addressed in this scenario by replacing the pump in scenario four with a much more powerful one. All other aspects of this dredge are the same as TEXAM4, and the only limit on production is the power of the winches.

 

         TEXAM6 More powerful winches are added in this scenario, and the discharge pipeline is shortened back to 2000 m (6560 ft). It is otherwise similar to TEXAM5, and an increase in production will be apparent due to the stronger winches.

 

         TEXAM7 This is a spud carriage dredge with almost no limits. The operator will be able to learn the spud carriage operation and become familiar with the dredge simulator. The suction pipe diameter is 0.81m (32 in) and the discharge pipe diameter is 0.75 m (30 in). The discharge pipeline length is 5000 m (16,400 ft).

 

Exercises TEXAM1 through TEXAM6 uses these common parameters:

 

         Channel water depth = 17 m (55.1 ft)

         Soil is medium sand (d50= 0.5mm, d15 = 0.25mm, d85 = 0.75mm)

         Critical Velocity for 0.4 m (16 in) discharge pipe = 4.6 m/s (15 ft/s)

         Advance Angle (sin q = 1.1/6 = 0.18333, q = 10.6 degrees)

         More details data using in each scenario can be seen by clicking EDIT in the Simulation window

         More details regarding the dredge for scenario TEXAM1 through TEXAM6 are presented in Figure 3.

 

Scenario TEXAM7 uses these parameters:

 

         Channel water depth = 8 m (26.2 ft)

         Soil is medium sand (d50= 0.5mm, d15 = 0.25mm, d85 = 0.75mm)

         Critical Velocity for 0.75 m (30 in) discharge pipe = 6.5 m/s (21.3 ft/s)

         More details data using in each scenario can be seen by clicking EDIT in the Simulation window

         More details regarding the dredge for scenario TEXAM7 are presented in Figure 4.

 

These basic exercises are intended to demonstrate the effects of slurry velocity, slurry density, and dredge characteristics on production. After completing the lessons provided, a dredge operator or production engineer should have a basic understanding of these factors and should be ready to train for a specific project.

 

RESULTS OF EXERCISES

 

Seven exercises are available on the simulator, but only six of these were used in the January 2000 short course. Exercise number 7 is a large 0.75 m (30 in) cutter suction dredge that has a spud carriage and is equipped with a ladder pump. The main pump power and the winch power are more than adequate and the pipeline length is 5000 m (16,400 ft). The suction pipe is 0.81 m (32 in). It is believed that a large dredge with essentially no dredging limitations was the best for the participants to start there training and acclimate themselves to the simulator and the controls on the console. Since there were two simulators the class was divided in half and each participant completed all their exercises on the same simulator so they didn't have to readjust for different exercises. The software is identical on the two simulators, but the control consoles are different. The channel sediment is sand with a d50 of 0.5 mm and the channel water depth and width are 8 m (26.2 ft) and 100 m (328 ft), respectively. The cutter base diameter is 2 m (6.3 ft). All the participants were given about 45 minutes on the simulator with instruction to try to maximize the production without cavitating the pump or plugging the pipeline. Six of the participants plugged the pipeline after about 25 minutes on the simulator.

 

An example of the production data curves is illustrated for one of the participants in Figure 5. The results show the slurry velocity beginning to drop after 12 minutes into the exercise and at about 19 minutes the sediment began to settle in the pipe. Figure 5 shows the average specific gravity being pumped is about 1.15 and the average velocity is 14 ft/s. For sand with a d50 of 0.5 mm, the critical velocity for deposit in a 0.76 m (30 in) pipeline is approximately 23 ft/s. The velocity shown in Figure 5 is well below the critical velocity and consequently the sediment settled in the pipe, eventually plugging the pipe. This is evident when the velocity went to zero at about 20 minutes and the program gave a fatal error that the pipeline was plugged. This exercise also provides the participants the opportunity to experience working with a spud carriage.

 

Two simulators are operating simultaneously during the training and the computer screen from each simulator is projected on a screen so that everyone can observe the ongoing dredging. This information is useful to the instructors observing the participants and for the inactive participants to learn from those on the simulators. Figure 5 illustrates the information shown on the screen that included gauge readings, plan view of the dredge and channel, and a stern or side view of the dredge and channel. The operator could clearly see the location of the cutter and a green line marked the area that had been dredged. Information showing swing speed, production, cutter depth, swing angle, distance from the center of the channel and time were also shown.

 

Figure 5. Production data curves showing slurry density, slurry velocity, swing speed, and total production using exercise 7.

 

Once the participants are acclimated to the simulator, they are introduced to a smaller cutter suction dredge that has five exercises. The five exercises, which are numbered 1, 3, 4, 5, and 6, are compared in Table 4. The purpose of the different exercises is to expose the participants to increasingly more difficult conditions for dredging and to demonstrate hydraulic principles of available power, long pipelines, critical velocity, and cavitation limitations. It is intended to demonstrate the effects of these hydraulic limitations (cavitation, power for pump, power for winch, power for cutter, and critical velocity) on the production.

 

Table 4. Comparison of Cutter Suction Dredge Simulator Exercises Used in January 2000 Short Course

 

Characteristic

Exercise 1

Exercise 3

Exercise 4

Exercise 5

Exercise 6

Suction Pipe

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

Discharge Pipe

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

0.4 m (16 in)

Ladder Pump

No

Yes

Yes

Yes

Yes

Main Pump Power

Low

Low

Low

Maximum

Maximum

Winch Power

Low

Low

Low

Low

Maximum

Pipeline Length

2000 m

(6560 ft)

2000 m

(6560 ft)

5000 m (16400 ft)

5000 m (16400 ft)

2000 m

(6560 ft)

 

 

 

 

 

 

Common Characteristics

 

 

 

 

 

Sand grain size

d50 =0.5 mm

 

 

 

 

Water depth

16.8 m (55 ft)

 

 

 

 

Cutter size

1.2 m base

1.1 m height

1.0 m top

 

 

 

 

Channel Width

84 m (276 ft)

 

 

 

 

Spud Separation

6 m (19.7 ft)

 

 

 

 

 

Figure 6 shows a summary of the production in cubic yards/hr for eight participants while conducting the five exercises. As illustrated in Figure 6, participants 1, 4, 7 and 8 attained the best production over all the exercises. Exercise 6 was meant to have high production because there was maximum power, short pipeline, and a ladder pump. However, when the participants move to exercise 5, the production for most participants increased even though the pipeline increased from 2000 m to 5000 m (6560 to 16400 ft). In exercise 4, the power for the main pump was reduced and for most of the participants showed a decrease in the production. Exercise 3 has the same power as exercise 4, but the pipeline length was reduced to 2000 m (6560 ft). Seven out of the eight participants increased the production and this is attributed to the decreased line length. Exercise 1 removed the ladder pump and everything else remained the same as exercise 3. The effect of cavitation was clearly evident and resulted in the production by all the participants being reduced by 50 % or more. These results show the simulator exercises demonstrate the limitations on production caused by cavitating pumps, increase line length, and power to the pump. Winch and cutter power are also limitations, but new exercises need to be generated to demonstrate these limitations more clearly.

 

Figure 6. Comparison of production for each participant for the different exercises.

 

Exercises number 3 and 1 illustrated the effects of cavitation on dredge production. In exercise 3 the cutter suction dredge had a ladder pump and the short line (2000m, 6560 ft). The critical velocity for a 0.4 m (16 in) line transporting material with a d50 of 0.5 mm is approximately 15 ft/s. The data collected for exercise 3 (Figure 7) shows an average slurry density 1.2 with maximum specific gravity as high as 1.6. The average slurry velocity was 20 ft/s and the maximum swing velocity was 120 ft/min. During this exercise the average production is about 1000 cy/hr. The main pump suction pressure fluctuates between 30 in of Hg and -30 in Hg, which indicates the operator was attempting to dredge as high a specific gravity as possible regardless of the occasional main pump cavitating message. As a result, the production was maximized. The average slurry velocity during the exercise was approximately 19 ft/s, which is about 4 ft/s above the critical velocity.

 

Figure 7. Example exercise 3 production and main pump data using ladder pump and short pipeline.

In exercise 1, the ladder pump is removed and the operator tries to maximize the production. As shown in Figure 8 the operator is able to average a 1.2 specific gravity with peaks up to 1.8 specific gravity. The slurry velocity drops to an average of 16 ft/s that is just 1 ft/s above the critical velocity. The swing rate is reduced and the production is reduced to about 500 cy/hr that is a drop of one half. The main pump suction pressure is always negative and ranges between -6 and -30 in of Hg. The operator essentially is cavitating the pump through out the exercise as evidenced by the number of times the suction pressure approached and passed -30 in of Hg. A review of the error messages indicated that 65 main pump cavitating messages were received. Production would have reduced more if the operator tried to avoid cavitation by reducing the average specific gravity. These two exercises clearly show the benefit of the ladder pump in maximizing production and reducing wear and tear on the main pump due to excessive cavitation.

 

Summary, CONCLUSIONS, and recommendations

 

Two cutter suction dredge simulator short courses were conducted in 1999 and 2000. Five participants attended the first 1 1/2 day course and eight attended the second 2 1/2 day course. In the second course, two dredge simulators were used. In order to increase the time on the simulators, the number of simulators must be increased or the length of the course must be extended. The use of the computer projection equipment to display the simulator actions is very helpful to the participants not on a machine, and it encourages discussion among all the participants, which was an unanticipated benefit.

 

The simulator is a very realistic model for cutter suction dredges and has proven to be a reliable simulation of actual cutter suction dredging operations. Participants in two cutter suction dredge simulator short courses indicate that training on the simulator would benefit dredge leverman and production engineers.

 

The seven exercises that have been developed demonstrate hydraulic transport principles. The effect of winch and main pump power limits, length of pipeline, and ladder pump are demonstrated. Participants need time to adjust to the control and computer set-up. Most participants indicate they would like more time on the exercises and perhaps be able to repeat the exercises to improve their performance.

 

The first two short courses did not attempt to change the sediment or incorporate environmental conditions such as channel currents, or change channel geometry conditions. All of this could be accomplished, but it would increase the length of the course or require the addition of another simulator. Participants indicated they would like to try the different sediment conditions.

 

The critiques after each exercise are useful in showing each participant the results of their actions. The ability to capture the data and display it for the participant is considered valuable by the participants. The presentations on fundamentals of slurry transport and cutting of soils proved to be useful to the operators and production engineers. The participants recommend question and answer sessions. Most of the operators do not have a formal engineering education and the opportunity to have slurry transport fundamentals explained seemed to be well received. Explanations in lay terms was emphasized instead of engineering theories.

 

Critiques by the participants at the close of the short course indicated the course was well received. The length of the course was considered to be about right with a possible extension of one day for additional dredging conditions. Additional discussion on cutters and booster pumps was suggested. The simulator with the Windows 3.1 needed to be upgraded to Windows 95, and a faster computer with more memory will improve the handling of data.

Figure 8. Example exercise 1 production and main pump data showing effects of cavitation.

 

REFERENCES

 

Cox, C. M., Eygenraam, J. A., Granneman, C. C. O. N., and Njoo, M., A Training Simulator for Cutter Suction Dredgers: Bridging the Gap between Theory and Practice, Proceedings of World Dredging Congress, WODCON XIV, Amsterdam, The Netherlands, November, 1995.

Digital Automation and Control Systems (DACS), Hydraulic Dredging Simulator, Houston, Texas, 1994.

Durand, R., Basic Relationship of the Transportation of Solids in Pipes Experimental Research, Proceedings of the International Association for Hydraulic Research, University of Minnesota, 1953.

Miedema, S. A. Considerations in Building and using Dredge Simulators, Proceedings of the Western Dredging Association XIX Technical Conference and 31st Texas A&M Dredging Seminar, Louisville, KY, Center for Dredging Studies, Texas A&M University, College Station, TX, May 15-18, 1999.

Miedema, S. A. Modeling and Simulation of the Dynamic Behavior of a Pump/Pipeline System, Proceedings of the WEDA Technical Conference and Texas A&M Dredging Seminar, New Orleans, June 1996.

Miedema, S. A., Production Estimation Based on Cutting Theories for Cutting Water Saturated Sand, Proceedings of World Dredging Congress, WODCON XIV, Amsterdam, The Netherlands, November, 1995.

Randall, R. E. and Albar, A. Cutter Suction Dredge Simulator Training Manual, Center for Dredging Studies, Ocean Engineering Program, Civil Engineering Department, Texas A&M University, College Station, Texas, January 2000.

Wilson, K. C., Addie, G. R., Sellgren, A., and Clift, R., Slurry Transport Using Centrifugal Pumps, Blackie Academic & Professional: New York, Second Edition, 1997.

 



[1] Director, Center for Dredging Studies, Ocean Engineering Program, Civil Engineering Department, Texas A&M University, College Station, Texas, 77843-3136, 979-845-4568, r-randall@tamu.edu.

[2] Consultant, Digital Automation and Control Systems, Inc., P. O. Box 925456, Houston, Texas, 77292.

[3] Professor, Faculty of Mechanical Engineering and Marine Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands.