Action 2.8: Report about appropriate dredging techniques - Coast-Best

Transcription

Progetto LIFE 08 ENV/IT 00426 COAST-BEST
Action 2.8: Report about appropriate dredging techniques
Envisan
7.2.7 Action 2.8 (Report about appropriate dredging techniques)
LIFE Project Number
LIFE08 ENV/IT 000426
LIFE+ PROJECT NAME or Acronym
CO-ordinated Approach for Sediment Treatment and BEneficial
reuse in Small harbours neTworks” COAST_BEST
Deliverable:
Report about appropriate dredging techniques (Action 2.8)
Mid-term report LIFE+ - Project LIFE08 ENV/IT 000426
Page 296 of 681
Life COAST-BEST ENV/IT 426
Envisan N.V. – Life COAST-BEST
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1/39
ACTION 2.8. Review of appropriate dredging techniques on the basis of
environmental and economic issues
TABLE
OF
CONTENTS
Table of Contents .................................................................................................................. 2
1
Evaluation of the ports to be dredged........................................................................ 3
1.1
1.2
1.3
1.4
1.5
Introduction ........................................................................................................... 3
Port of Rimini........................................................................................................ 3
Port of Bellaria ...................................................................................................... 4
Port of Cesenatico ................................................................................................. 5
Port of Porto Garibaldi .......................................................................................... 6
2
Evaluation of the preliminary characterization data ................................................. 8
3
Overview of dredging techniques ............................................................................. 9
3.1
3.2
3.3
3.4
3.5
4
Overview techniques ............................................................................................. 9
CSD – Cutter Suction Dredger.............................................................................. 9
TSHD – Trailer Suction Hopper Dredger ........................................................... 12
BHD - Backhoe Dredger ..................................................................................... 17
Evaluation of the performances........................................................................... 21
Selection of the working method for LIFE-Best Coast........................................... 23
4.1
4.2
4.3
4.4
4.5
4.6
5
Summary of data ................................................................................................. 23
Selection of the type of dredging techniques ...................................................... 24
Mitigation measures during dredging ................................................................. 25
Control during dredging ...................................................................................... 26
Survey and special equipment on board of the dredger: ..................................... 28
Monitoring, Measuring and management of the dredging operations: ............... 33
Economical evaluation ............................................................................................ 34
5.1
5.2
5.3
5.4
5.5
5.6
General – reasons to dredge ................................................................................ 34
Relation between economical / environmental and other issues......................... 34
Project specific economical impact..................................................................... 34
Overall Economical issues .................................................................................. 35
Economical issues concerning item 1 - Dredging............................................... 35
Conclusion........................................................................................................... 36
List of figures and tables ..................................................................................................... 37
List of Attachments ............................................................................................................. 37
List of references ................................................................................................................. 38
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1
Evaluation of the ports to be dredged
1.1 Introduction
The following ports have been evaluated (Port of Rimini, Port of Bellaria, Port of
Cesenatico, Port of Porto Garibaldi). The evaluation has been done on the basis of site
visits, areal pictures and the bathymetric charts. It follows below a short summary of the
different ports.
1.2 Port of Rimini
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Figure 1 – Areal view and marine chart Port of Rimini
The marina is bordered to the South with the harbour canal of Rimini and to the North with
the beach of S. Giuliano. The marina is well protected. The seabed varies between 2.40 to
4.50 meters below sea level.
1.3 Port of Bellaria
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Figure 2 - Areal view and marine chart Port of Bellaria
The Bellaria harbour is situated at the mouth of the River Uso; its entrance is protected by
two breakwaters of approximately 30 meters length. The seabed varies between 1.50 to
3.50 meters below sea level.
1.4 Port of Cesenatico
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Figure 3 - Areal view and marine chart Port of Cesenatico
In the harbour of Cesenatico there are a lot of fishing and pleasure boats and the seabed
varies between 2.0 to 3.0 meters below sea level.
1.5 Port of Porto Garibaldi
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Figure 4 - Areal view and marine chart Port of Porto Garibaldi
The Porto Garibaldi port is located on the final part of the channel Pallotta and is an
important fishing harbour. The entrance is protected by two breakwaters.
The seabed varies from 2.50 to 3.50 meters.
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2
Evaluation of the preliminary characterization data
The following preliminary general data are available:
Port
Porto Garibaldi
Cesenatico
Bellaria
Rimini
sand %
61-92
8-90
36-76 and 22-83
10 - 60
Pollution
Zn, TPH (PAH's), Zn, As, Cu
Zn, Cd
Zn, Cd, Cu
Table 1 - Data
The following extra parameters are needed for a further detailed technical evaluation:
•
•
•
dry matter
density
organic matter
The table below gives an overview of the dredging quantities during the last decade from
the different harbours.
Amount (m ) of dredged sediments disposed into the sea per year (1999 - 2008)
3
Harbour
1999
Bellaria
2000
2001
2002
2003
2005
2006
2007
12,936
15,046
5,882
14,794
29,500
14,550
10,200
11,248
24,800
21,150
5,000
4,100
24,040
1,400
Cesenatico
Rimini
Cattolica
Porto Garibaldi
18,270
2008
2,625
14,250
Total
102,908
36,048
49,330
Riccione
Cervia
2004
5,380
51,955
35,630
10,855
16,960
4,000
85,775
4,000
36,000
36,000
Table 2 - Quantities
The total quantities are approximately 340.000 m³ for a period of 10 years, resulting in an
average of 34.000 m³ pro year.
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3
Overview of dredging techniques
3.1 Overview techniques
Preliminary to the selection of an appropriate dredging technique on the base of
environmental and economic issues, it seems to us necessary to present an overview of the
different options. For this a brief description of the different techniques are summarized
below. Moreover we have made three videos to illustrate as good as possible the respective
working principles. These videos were presented during a recent presentation in Bologna.
In particular we have evaluated 3 types of dredging techniques:
Cutter Suction Dredger (CSD)
Trailer Suction Hopper Dredger (TSHD)
Backhoe/pontoon Dredger (BHD)
In what follows we summarise their working principles and analyze the strengths and
weaknesses of each of them, so that the reader can understand the choice we have made in
relation to the object of study.
3.2 CSD – Cutter Suction Dredger
The CSD is used mainly for capital dredging in harder soil, which has to be removed in
thick layers. The transport distance to the reclamation site should preferably be limited
(max 5 to 10 km) to allow for an economical pipeline transport. In the case of an
environmentally sensitive project, the dredging process must be controlled carefully. The
disloading and hydraulic transport process must be carefully optimized. To achieve this,
the optimum setting should be found by carefully varying cutting face height, step length,
cutter rotation speed, swing speed, pump engine power and pipeline resistance.
3.2.1
Working Principles of a CSD
The rotating cutter head will first cut out the materials to be dredged, in order to get them
in a suitable state for removal by hydraulic means. The loosened material then enters the
suction mouth, passes through the suction pipe and the pump (or pumps) and into the
delivery line. The Cutter Suction Dredge is operated by swinging around the central work
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spud using moorings leading from the lower end of the ladder to anchors. By pulling on
alternate sides the dredge clears an arc of cut, and then moves forward by pushing against
the work spud using the spud carriage.
Once the spud carriage has reached its end position (6 or 9 m) the auxiliary spud will be
lowered and the work spud raised, thus keeping the dredge in position. The main spud in
its spud carriage will then be brought back in its original start position, where after the
work spud will be lowered and the auxiliary spud raised in order to commence a new
cutting arch.
The side anchors are lifted and moved forward when the dredge has progressed far enough
and the force on the anchors is not sufficient anymore. The anchors are shifted using the
dredge’s own anchor booms system or with an auxiliary anchor handling vessel.
The control of the dredging process is maintained by means of the dredging computer and
the use of a Differential Global Positioning System (DGPS). The output of this positioning
system will be X and Y coordinates of the vessel. The Z coordinate is calculated by the
dredging computer.
Figure 5 - CSD
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The table below gives the working sequence of a CSD.
Table 3 – Sequence of a CSD
3.2.2
List of CSD advantages and disadvantages:
Good Accuracy of the excavated profile
Increase of suspended sediments especially with fine grained material
Dilution: due to the hydraulic character of the transport, water is added to the soil
for transportation purpose. Depending upon the type of soil, the amount of added
water varies.
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3.3 TSHD – Trailer Suction Hopper Dredger
The TSHD is often used for maintenance dredging projects or for deepening existing
channels. During such projects a limited thickness of softer material has to be removed,
and reclamation and/or disposal sites are available at variable distances. This type of
dredger is also used for the mining of sand and gravel offshore for reclamation projects
such as beach nourishment or the creation of artificial islands. Selection of the optimal
duration of the suction process and limiting overflow losses during dredging are the major
factors related to the environmental effects of this type of equipment.
3.3.1
Working Principle of a TSHD
A trailing suction hopper dredge is commonly used for dredging silty, sandy or gravely
soils or soft clayey soils. While all other types of dredgers rely on other tools for
transporting the dredged materials, a hopper dredge will store the dredged materials in its
own cargo hold, called the hopper. The dredged materials can thus be transported over long
distances. The TSHD is also able to unload its cargo by own means.
Dredging activities can therefore be divided in the following consecutive activities: loading
(dredging), sailing loaded, unloading and sailing back empty. A complete set of these four
activities is called a dredging cycle.
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Figure 6 – working principle TSHD
Sailing to the borrow area
The dredging cycle starts with the empty hopper dredge sailing to the offshore dredging
area guided by a navigation system. In this stage of the dredging cycle, the hopper dredge
is regarded as a normal cargo vessel.
Dredging
The dredging systems of a TSHD consist of one or two suction tubes, each driven by a
powerful centrifugal pump, called the sand pump. During the dredging, and in a process,
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which is quite similar to the domestic vacuum cleaning, the lower ends of the suction tubes
are trailing along on the seabed, while the sand pumps provide the suction power to lift the
materials from the seabed into the hopper.
Once the TSHD approaches the dredging area, the sailing speed is reduced and the suction
tubes will be hoisted over board and lowered to the seabed.
At the lower end of the suction tube, a special draghead is attached which is designed for
maximizing the dredging production during the loading phase. The suction power is
provided by the sandpump, which is normally installed in the pumproom in the engine
rooms of the dredge.
During the dredging, while the dragheads are on the seabed, the hopper dredge will
maintain a low trailing speed. Such trailing speed is depending on the nature of the
materials being dredged.
The materials thus lifted (dredged) from the seabed, will be pumped into the hopper as a
soil/water mixture. Care will be taken to minimise the water content in the mixture.
Specialised operators control the dredging process. The dredge master and the navigating
officer will, each one responsible for his area of control, co-operate closely. The
computerisation covers all possible parameters involved in the dredging: dredging
productions, engine and pump loads, draghead positions, hopper levels, etc…
Overflowing
It is economic to allow a certain degree of overflowing. This means that, while the soils in
the dredged soil/water mixture will settle in the hopper due to the gravity forces, the excess
water is discharged via an adjustable overflow system.
The overflow, which is built inside the hopper, consists of an in height adjustable funnel
mounted on top of a vertical cylinder which ends under the keel of the dredge. The excess
water is discharged under the dredge, which is the lowest level possible, thus minimising
the dispersion of fines into the surrounding waters.
Further, the design of the overflow is such that, by avoiding the entrapment of air in the
overflow water, a minimum of turbidity is created.
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In case where overflowing is contractually or environmentally prohibited, it is possible to
monitor the filling process precisely using the highly computerised dredging process
parameters. Sensors (so-called pingers) installed above the hopper will keep track of the
height of fluids inside the hopper. By comparing this to the height of the overflow funnels,
the filling process will be stopped when the fluids reach the funnel level.
Sailing to the discharge / dumping point
As soon as the TSHD is fully loaded, the suction tubes will be hoisted back onboard and
course will be set towards the area for unloading the hopper dredge. During this transit the
hopper dredge is sailing as a regular cargo vessel.
Discharging / dumping
There are several ways to discharge the hopper load.
a) Bottom dumping
The fastest way to unload the hopper is by discharging the load through the opened bottom
doors of the hopper.
When the hopper dredge has arrived on the spoil ground and the navigating officer is
confident that the hopper dredge is exactly on the area where the hopper load is to be
unloaded, the command will be given to open the bottom doors to dump the hopper load.
Figure 7 - TSHD
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Waterjets inside the hopper will ensure the hopper is completely empty and free of any
dredged soil prior to closing the bottomdoors.
A new dredging cycle can commence by sailing back to the dredging area.
b) Pumping ashore
Some TSHD are equipped with pumping ashore facilities. This enables them to pump the
hopper load via a combination of a floating pipeline and shore pipelines directly into a
reclamation area onshore. To this end a coupling system will be prepared consisting of a
flexible floating pipeline with at its seaside end a special bow connection piece. The other
end is connected to the shore pipeline.
The hopper dredge, upon arrival at the coupling area, will be connected via the bow
connection on board to this floating pipeline.
Now the jets in the hopper will fluidise the sand in the hopper. The sand pumps will pump
this fluidised mixture of sand and water through the pipelines to the reclamation or
disposal area.
For sections where the pipeline route has to cover large distances over water or where the
pipeline has to cross a surf zone or a shipping channel, a submerged pipeline, resting on the
seabed, will be chosen.
c) Reclaiming with a spray-pontoon
If the reclamation area is located under water and bottom-dumping the hopper load is not
possible; the unloading is often realised using a spray-pontoon. The spraypontoon is
connected to the hopper dredge using a similar pipeline system. This spraypontoon will,
during the discharging of the hopper load, be moved over prescribed tracks, to deposit the
load evenly over the required surface area.
At the discharge end, by adequately controlling the discharge process, care will be taken to
deposit the hopperload accurately within the set levels and horizontal boundaries.
When the hopper has been emptied, a new dredging cycle can commence by sailing back
to the dredging area.
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List of TSHD advantages and disadvantages:
3.3.2
The accuracy of the dredging depth is low compare with CSD, owing to the fact
that the position of the suction pipe is flexible and more difficult to control. A
vertical accuracy of approximately 15 to 25cm can be obtained provided
sophisticated and steering equipment is used. Normal accuracy is around 0.5 to 1 m
vertically and 3 to 6 m horizontally.
The actual dredging process creates less suspended sediment compared with CSD
as there is no rotating device in the draghead. Moreover, in the case of
environmental projects, such overflow can be limited (environmental valve, green
valve – reuse of the process water) or even prevented by stopping the dredging
process earlier. The cutting process is strictly horizontal. As such, the mixing of
soil layer can be controlled accurately.
Significant amounts of water are added during the suction process. With modern
monitoring and control equipment, this amount can be limited.
3.4 BHD - Backhoe Dredger
The BHD is mainly used for the execution of relatively smaller dredging projects also in
the harder soil as the mechanical cutting forces, which can be applied, are considerable.
Recent developments in sophisticated monitoring and control equipment have improved
the accuracy of this dredger considerably.
3.4.1
Working principles of a BHD
General
The backhoe dredger is a common type of dredger, generally non-self propelled. The main
component is a hydraulic excavator, performing the dredging operation, mounted on a
pontoon.
The BHD mainly consists of a spud pontoon (a hull and spuds), a dredging excavator, an
onboard workshop and a bridge/living quarters.
The BHD is equipped with a computer system, used for on-line positioning and dredging
monitoring.
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As the BHD is generally non-self-propelled it will be assisted by a tugboat for repositioning during operations and towing during emergency situations. The tugboat needs
enough power to ensure safe handling during towing. The same tug will be used as a
supply boat to provide the BHD with the required consumables.
Figure 8 - CSD
General Working Principle
Figure 9 – working principle CSD
The BHD is equipped with three spuds: one spud is located in the centre of the pontoon at
the stern in a spud carriage system; this spud can be lifted and move along the centre line
of the pontoon (or the pontoon can be moved with respect to the spud fixed onto the sea
bottom); the two other spuds can only be lifted/lowered.
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The working method of the backhoe dredger is as such that the dredger is towed into
location by the assisting tug and is then fixed into position by its three spuds. Before
lowering the spuds, the exact position as shown on the DGPS positioning system is
checked in order to ensure that the spuds are lowered in the trench alignment. The dredger
will then move into the exact starting position by using the spud carrier and the bucket.
The dredger will excavate in steps 5 till 10 m length. When one step has been completed,
the dredger will release the front spuds from the sea bottom and raise them approximately
2 m above the seabed. The spud carrier then shifts the dredger backwards in the dredging
lane and then a new dredging cycle starts.
Repositioning of the Backhoe Dredger using the spuds is done as follows:
1. The spud in the spud carriage is lifted and moved to the front of the carriage.
2. On arrival of the spud at the end of the carriage the spud is lowered.
3. The two fixed spuds are lifted from the sea bottom while the crane bucket is
lowered onto the sea bottom.
4. The pontoon is then pushed against the spud in the carriage system backwards.
5. On confirmation of the correct alignment of the BHD the two fixed spuds are
lowered to the sea bottom and the excavation operations can start.
Dredging Control
For horizontal positioning the dredger uses Differential GPS systems in combination with
gyrocompasses, thus giving satisfactory accuracy.
For controlling the bucket position, the dredger is fitted with IHC digviewer / Seatools
Digmate systems or similar. These systems measure:
-
the angles for the boom, stick & bucket
-
the pontoon draught
-
the pontoon tilt
-
bearing
The operator can follow the excavation operation on video screens, one for horizontal
bucket position and the other for vertical bucket position. The system enables the dredge
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operator to follow the exact movements and the depth of the bucket, and facilitates digging
in a controlled manner to the designed limits.
In this system the required dredging levels and slope angles can be pre-set in the computer
so the operator can see the digging lines as well as the bucket position, in relation to the
pre-set limits, on his video screens.
Water level information is provided by a radio-linked tide gauge. The tide gauge is placed
in the water close to the dredging area. The dredger is equipped with a radiolinked receiver
to monitor the tide level during the dredging operation. The "digviewer system" receives
the actual tide level several times per minute and the dredging depth is automatically
updated.
The supervisor or the main operator on each shift keeps a log for noting events of
significance for the dredging operation, such as operation hours, breakdowns, repairs,
production rates, weather conditions, dredging area, dredging depth etc. The area, which
has been dredged during the last shift, is marked on specially designed dredging lay out
drawings.
The transportation of soil from the dredging areas to the dump area or quay wall is
executed by means of propelled split barges.
List of BHD advantages and disadvantages:
3.4.2
The accuracy is limited because the excavation bucket has to be repositioned at
every cycle. However, such monitoring system exists and accuracy of 10cm can be
obtained even if with reduced productivity.
Suspended sediments are released during the raising of the material in open buckets
as they move at a relatively speed through the water. In the case of fine grained
materials these sediments remain in suspension for a long period and the
accumulation can increase the turbidity at the dredging site above the natural
background levels. Close buckets that limit spill are available.
Thin layers can be excavated provided a good monitoring and control system is
available.
Dilution is highly reduced compared with CSD and TSHD.
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3.5 Evaluation of the performances
Based on the above mentioned information; a first evaluation of the performances can be
made. In relation to the Life project it will be more the environmental performances that
prevail.
However we have also made a summary of the technical (operational) performances.
For the time being, it is nearly impossible to make a detailed evaluation in relation to the
Life Coast Best project; as not enough parameters are currently available.
Based on our general knowledge of the dredging industry and the equipment available on
the market, we can only provide a general evaluation of the different parameters.
It is quite obvious that new and modern equipment are having in general better
environmental performances (noise, accuracy, etc..)
3.5.1
Production performances (depth / output rates):
In the table 4 an overview is given of the maximum production performances of the
different types of dredgers.
CSD
TSHD
BHD
Dredging Depth
(max meter) *
36 meters
155 meters
32 meters
Output rate
(m3/ hour) *
200 – 5000
Till 5000
50 - 1000
Table 4 – Production performances
(*) based on the actual state of the art “standard” equipment.
3.5.2
Environmental performances
A relation between the dredging techniques and the environmental performances is given
in the table below (table 5).
For the environmental performances the most imported parameters are covered; such as
accuracy, turbidity, mixing, spill, dilution, noise / sound.
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The table 5 below does not take into account the influence of special modifications /
adaptations and other tools used in the environmental dredging world.
CSD
TSHD
BHD
Accuracy
+
+
Turbidity
□/+
-/□
-/□
Mixing
□/+
+
Spill
□
□
+
Dilution
□
+
Noise sound
+
+
+
Table 5 – Environmental performances
+ : better then average
- : less then average
□ : average performance
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4
Selection of the working method for LIFE-Best Coast
4.1 Summary of data
Based on the information mentioned in chapter 1 and 2, all the ports are characterized by
the following aspects:
•
Presence of small pleasure sailing and/or fishing boats
•
Small areas / surfaces
•
Presence of buildings and housing in the neighbourhood of the dredging area
•
Organic and inorganic contamination
•
Strong variation of the seabed
•
Different distances to possible discharge area
There is a need for approximately 30.000 to 40.000 m³ to be dredged pro year. Moreover,
the dredging system must allow for sufficient flexibility due to the lack of space and at the
same time environmental dredging capability due to the contamination of the sediment.
Summarizing, the characteristics must be:
Dredge accurately (Accuracy +/- 10 cm);
Low turbidity during the dredging activity (environmental bucket)
Avoidance of spills of the dredged material;
In addition, the treatment tests that will be performed in this Life project will be primarily
based on separating the non-contaminated material from the contaminated one by a
physical separation of the sand from the finer material in order to reuse it (below you can
see a schematic picture):
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Figure 10 – treatment process
An important aspect to be considered during treatment is the management of the water
coming from the treatment of the dredged sediment. This factor may play an important role
in the whole economy of the process, particularly due to the foreseen continuous volumes
to be dredged, the distances from the location of the treatment area and the need to comply
with the regulations for water discharge into the sea after treatment.
4.2 Selection of the type of dredging techniques
Therefore, according to us, another main objective in the selection of the appropriate
dredging system, is to maximize the content of dry matter in the dredged material in order
to avoid extra cost in the management of the water dredged.
In this way it is possible to manage a lesser amount of water associated with the dredged
sediments, to limit the space needed for treatment, to limit the size and investment costs for
the necessary equipment for the water treatment and the costs associated with its running.
Taking into account the explanations given above (chapter 4.1), we will describe below in
detail the type of dredge that meets as much as possible these criteria, an environmental
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adopted BHD without auto propulsion. The main components are a hydraulic excavator
fitted with a environmental clamshell, as shown in figure 11, that will perform the dredging
operations, mounted on a spud pontoon.
Figure 11- Environmental closet bucket and CSD
The BHD is towed into location by the assisting tug and is then fixed into position by its
spuds.
4.3
Mitigation measures during dredging
In order to minimise any turbidity during the dredging activities the following measures
will be adopted:
position the bucket slowly on the bottom in such a way as to minimize the
resuspension of sediment at the bottom;
position the bucket correctly on the bottom in order to avoid over or under
dredging (precise removal of the contaminated sediments);
minimise the amount of water added during the excavation.
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4.4 Control during dredging
To control the positioning, the dredge must be equipped with Differential GPS systems,
which provide a satisfactory accuracy. In order to check the location of the environmental
bucket or clamshell it is necessary to equip the dredge with a system “digviewer” or
similar. This system allows the operator to follow the movements and the depth, and also
facilitates the execution of excavation in accordance with the limits defined.
Vertical control of the water level
4.4.1
The information on the level of water is supplied by a measuring instrument of the tides
connected via radio. The instrument is placed in the water near the area to be dredged. The
dredger is equipped with a radio receiver to monitor the level of the tides during the
dredging operations. The system “digviewer" will receive the level of sea in real time
several times per minute updating automatically the depth of dredging.
4.4.2
Horizontal control
The horizontal control is carried out through Differential GPS (DGPS). For this reason, the
dredger and the vessel used for the measurements will be equipped with a receiver DGPS,
while a receiver/transmitter differential is installed within or near the dredging area.
Figure 12 shows a typical configuration used on board of the dredger.
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Short
Range
antenna
Long
Range
antenna
Short
Range
antenna
Long
Range
antenna
GPS
antenna
Short
Range
antenna
GPS
antenna
Long
Range
antenna
Short
Range
antenna
GPS
antenna
NDR104
NDR104
RSRS4
23 85
2
RSRS2
48 32
5
IALA
beacon
receiver
PC1 PC2
DGPS receiver
Sercel NR203
Survey Monitor
R RS
S2 48
32 5
PC2
R RS
S2 48
32 5
PC1
R RS4
S2 8
32 5
R RS4
S2 8
32 5
PC1 PC2
R RS
S2 48
32 5
R RS2
S4 3
85 2
TIDE
receiver
DGPS receiver
Sercel NR109
PC1
PC2
DGPS receiver
DSNP Aquarius 5000
Steer Monitor
Atlas Deso 14
Echosounder
VGA Splitter
Atlas Deso 17
Echosounder
Atlas Deso 25
Echosounder
Odom 3200 MKII
Echosounder
Serial
interfacing
Survey PC
Moxa board
Survey Keyboard
Octans Gyro/heave/pitch/roll
Sensor
Survey
Trackball
TSS DMS 2 series
Dynamic Motion Sensors
Figure 12 - DGPS and control system
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4.5 Survey and special equipment on board of the dredger:
It is quite obvious that control the operation is fundamental for environmental dredging.
As mentioned in the previous chapter (4.4.); it will be necessary to have state of the art
special equipment on board of the dredger and to implement an adequate survey
methodology.
In order to execute an accurate dredging (x,y,z) the necessary precautions and special
equipment has to be installed on board of the dredger.
It is quite obvious that the operator on board of a dredger; does not “see” what is going on
under water. Therefore the necessary tools have to be provided; so that operator can “see”
what he is doing.
In figure 13 we see some typical computer screen configuration for a BHD operator, that
the operators in the activities assist.
Figure 13 – Special equipment on board of the dredger
So it will be necessary to install the necessary equipment; such as power box, PC panel,
GPS system; Microdigger, pitch and roll sensors on board of the dredger.
If the dredger is a BHD, the necessary boom and stick sensors have to be installed.
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Before the start of the dredging operations, all sensors and equipment have to be calibrated
in order to achieve a correct and well functioning dredging system.
•
Calibration of the Pitch and Roll sensor
•
Calibration of the Boom sensor
•
Calibration of the Stick sensor
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•
Calibration of the Dogbone
•
Bucket Angle
•
Bucket flat Angle
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The equipment described above, enables the operator of the dredger to “see” under water.
The tools at his disposition make it possible to execute the dredging operation with very
high accuracy.
Below some typical screenshots of the operators interface (Figure 14).
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Figure 14 – Screenshots of the operators interface
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4.6 Monitoring, Measuring and management of the dredging operations:
It is clear that not only the dredging technique is important; it is also the whole
management around the project that is important.
Today, environmental awareness is much higher then in the past.
It is not only the awareness that is important; it is also the implementation that is required.
So it is a whole concept of environmental management.
In this aspect the type of dredging technique is only a link in the chain.
The recent published information paper (June 2011) of CEDA is giving a good overview of
the following aspects:
•
Dredging projects and the enviornment
•
Environmental monitoring
•
Environmental management
•
Lessons learned from 15 years of dredging project experience.
A copy of this information paper is attached (attachment 1).
Final report - EN
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5
Economical evaluation
5.1 General – reasons to dredge
In general, there are different reasons to dredge, such as:
•
•
•
•
•
Improvement of harbour capacity
Improvement of inland waterways
Reuse of material
Infrastructure works
(Energy and mining)
It is clear that the reason to dredge the harbours mentioned in chapter 1; is to improve the
harbour capacity (draft of ships) and to improve the seabed quality.
5.2 Relation between economical / environmental and other issues
There is a strong synergy between economy and ecology. Also the legislation
(International, European, Italian and local) has an impact on the economical aspect.
However the economical impact of dredging is only a fraction if the sediments are
contaminated.
In general it varies between 10 to 25% of the overall budget, this is of course depending on
the level of contamination.
5.3 Project specific economical impact
There is a strong link between dredging activities and sediment treatment (capacity,
density, water content, and others).
The possibilities on land (land based storage and treatment area) having a consequence on
the economical aspect, such as:
•
•
•
•
Availability
Accessibility
Distance (road / pipeline...)
Opening hours
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5.4 Overall Economical issues
As mentioned before the economical impact of dredging and treatment of sediments can be
divided in different items; i.e.:
Item 1: Dredging
Item 2: Pre-treatment and treatment of sediments
Item 3: Reuse and disposal
These items can be divided in the following sub-items:
Item 1: Dredging:
Item 1.1. Mobilisation cost (plant and auxiliary equipment)
Item 1.2. Exploitation cost dredging spread
o Equipment
o Manpower
o Consumables
Item 1.3. Demobilisation cost (plant and auxiliary equipment)
Item 2: Pre treatment and treatment of sediments
Item 2.1. Mobilisation cost plant / infrastructure
Item 2.2. Exploitation cost
o Equipment
o Manpower
o Consumables
o Water treatment
Item 2.3. Demobilisation of plant
Item 3. Reuse and disposal
o Cost / benefit of re-use of material
o Disposal cost of contaminated materials
o Cost of water discharge
5.5 Economical issues concerning item 1 - Dredging
In the table below the reader can see the cost range for the dredging activity as described
above.
Description
Dredging
Mobilisation
Dredging in situ
Demobilisation
unit
unit
m³
unit
Table 6 – Economical aspects of dredging
Final report - EN
unit cost range
30.000€ - 80.000€
10€/m³ - 15€/m³
30.000€ - 80.000€
35/39
The costs related to the dredging activity are shown in table above and they are related to a
pontoon equipped with a hermetically sealed environmental bucket. In the case that the
treatment area is located far from the dredging area the additional costs for the transport
activity with hopper/carrier and the activities of unloading at the centre of treatment will
have to be added. We underline that the overall evaluation is a function of several
parameters as mentioned in chapter 5.3 and 5.4 and that at this stage of the project can not
be specified in detail.
5.6 Conclusion
In general we can conclude the following. Due to the relative small volumes to be dredged
(approx. 40.000 m3/year) And taking a dredging rate of approx. 500 m3/day only a limit
days of work are required (approx. 50 – 100 days/year or 2 to 5 months). It seems to us not
opportune to have a dredger idle on site for a longer period. So the dredger has to be
mobilised on a yearly base.
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List of figures and tables
Figure 1 – Areal view and marine chart Port of Rimini........................................................ 4
Figure 2 - Areal view and marine chart Port of Bellaria....................................................... 5
Figure 3 - Areal view and marine chart Port of Cesenatico .................................................. 6
Figure 4 - Areal view and marine chart Port of Porto Garibaldi........................................... 7
Figure 5 - CSD .................................................................................................................... 10
Figure 6 – working principle TSHD ................................................................................... 13
Figure 7 - TSHD.................................................................................................................. 15
Figure 8 - CSD .................................................................................................................... 18
Figure 9 – working principle CSD ...................................................................................... 18
Figure 10 – treatment process ............................................................................................. 24
Figure 11- Environmental closet bucket and CSD.............................................................. 25
Figure 12 - DGPS and control system................................................................................. 27
Figure 13 – Special equipment on board of the dredger ..................................................... 28
Figure 14 – Screenshots of the operators interface ............................................................. 32
Table 1 - Data........................................................................................................................ 8
Table 2 - Quantities ............................................................................................................... 8
Table 3 – Sequence of a CSD.............................................................................................. 11
Table 4 – Production performances .................................................................................... 21
Table 5 – Environmental performances .............................................................................. 22
Table 6 – Economical aspects of dredging.......................................................................... 35
List of Attachments
Attachment 1: 2011 Ceda information paper environmental control on dredging projects
Final report - EN
37/39
List of references
R. N. Bray “Environmental Aspects of Dredging”
SIP 3D “Proceedings of the International Seminar on Dredging, Dredging products and
sustainable development” – Tunisia 2010
www.european-dredging.eu
www.envisan.com
www.jandenul.com
Final report - EN
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Attachments:
Final report - EN
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A CEDA Information Paper - June 2011
Central Dredging Association
Environmental control
on dredging projects
Lessons learned from 15 years of turbidity monitoring
Marine and inland water-based infrastructure is a prerequisite
for the sustainable development of economic benefits and
public welfare. Dredging works are needed to realise and
maintain such infrastructure. It is widely recognised that such
projects create an impact on the environment and in recent
years awareness of these impacts has grown – on the client
side as well as among contractors.
The purpose of this paper is to share recent experience
gained from the development of marine infrastructure projects
in environmentally sensitive areas, with a particular focus on
the realisation phase. The main emphasis is on lessons learned
from 15 years of turbidity monitoring (see Figure 1) during
dredging and sediment placement operations.
Dredging projects and the environment
Environmental aspects play an important role during the
full cycle of project initiation, development, realisation
and operation. While detailing the project at hand, legal
frameworks such as the London Convention and the EU
Water Framework Directive pose strict environmental
controls. Any environmental effects – both adverse impacts
and benefits – are evaluated as part of the environmental
impact assessment, often in consultation with the legal
commissioner. This evaluation may result in revision
of the project design or the implementation of nature
compensation measures.
Contractors are responsible for minimising environmental
impacts during project realisation (process impacts).
Sometimes, the use of environment-friendly work methods
and robust environment management plans proves a decisive
factor in project tendering. Once contracted, environmental
monitoring schemes should be in place before construction
works begin. Given the costs associated with these schemes
and their strategic importance for environmental control,
Figure 1: Monitoring during dredging
development of these programmes should be integral to
project preparation.
Environmental monitoring
To ensure the care and protection of surrounding
ecosystems, while enabling the construction of marine
infrastructure, environmental monitoring has become
common on dredging projects since the Øresund Fixed
Link Project between Denmark and Sweden (1996). Major
marine infrastructure projects in, for example, Melbourne
1
Physical
Plume dispersion
Increased suspended solids
Sunlight
Increased light attenuation through water column
Decreased light in water column and on seabed
Light attenuation
Source
Biological
Source reduction
Reduced photosynthesis
Light
attenuation
Physiological and morphological changes in plants
Plume
dispersion
Seagrass
Seagrass response
Ecological consequences
(Australia), Rotterdam (Netherlands) or London (UK)
have called for extensive environmental monitoring. Such
large-scale programmes typically involve several types of
monitoring, each with a different objective:
• Surveillance monitoring or baseline monitoring: to assess
general project conditions and act as a reference for the
interpretation of dredging impacts. Monitoring involves flora,
fauna, hydrographic conditions, bed sediments and turbidity.
• Feedback monitoring or adaptive monitoring: to verify
pre-project environmental assessments (model predictions,
expert judgement) and to provide a base for possible
adaptation of the project design, planning and/or
work method.
• Compliance monitoring: to ensure compliance with the
environmental restrictions endorsed for the project at hand.
Each dredging project is unique and impacts vary
widely from one project to another, depending on local
hydrodynamic conditions (tide, waves), natural turbidity
levels, soil characteristics and dredging operations.
Environmental monitoring is needed to gain insights into
the actual relationship between impacts from dredging and
the response of sensitive ecosystems such as coral reefs and
sea grass. Such insights help to establish scientifically sound
environmental limit levels for dredging operations.
Most present-day monitoring programmes are based on
the assessment of turbidity levels, because the greater light
attenuation in the water column resulting from the increase
in suspended sediment concentrations is known to affect
marine life (see diagram above and Figure 2 overleaf).
Additional benefits of this parameter are its direct link to
dredging and placement operations and its relative ease of
measurement. Environmental restrictions typically involve
limits on sediment plume size at dredging and disposal sites.
Recently, environmental restrictions have focused on
other biological, chemical and/or physical parameters that
directly reflect ecosystem health at a particular project site.
However, limited capabilities for (operational) monitoring
of such complex processes and insufficient understanding of
the link to construction activities hamper widespread use of
these criteria in present-day dredging practice.
Environmental management
Management of environmental impacts has become a
standard component of marine infrastructure projects.
To avoid unforeseen delays and costs, environmental
monitoring should be integral to project planning. An
essential step is the compilation of an environmental
management plan (EMP) to provide full details on:
to the operational manageability of the environmental
monitoring programme.
Lessons learned from 15 years of dredging project experience
Environmental monitoring has taught us:
• Each project is unique. Nevertheless, with great care,
lessons learned from one project can be used for the next.
• Dredging-induced turbidity impacts should be evaluated
as an increase above background level, not as absolute
values. Environmental limit levels should be based on the
resilience of the local ecosystem, while accounting for
natural fluctuations in turbidity level.
• Monitoring programmes should be designed in an
adaptive manner, to allow for procedures to be reviewed
and, if appropriate, adjusted.
• Environmental monitoring should be an integral part
of project preparation and planning, to ensure effective
mitigation of possible environmental effects.
When made available to the outside world, environmental
monitoring data were also found to encourage stakeholder
involvement and to improve public awareness.
In this way, environmental monitoring is directly relevant to
the success of marine infrastructure projects and their appreciation by the general public.
Figure 2: Dredging plume dispersal
• Monitoring requirements: what is needed to assure
protection of the ecosystem? Includes a summary of
environmental restrictions, including specification of
monitoring parameters and limit levels.
• Monitoring approach: how to ensure compliance with
environmental standards? Includes an overview of
work methods, including specification of measurement
equipment, data sampling (frequency, location, depth),
data processing, data interpretation and dissemination
of results.
• Mitigating measures: what operational measures can be
taken in case of violation of environmental limits?
• Response procedures and responsibilities: what procedures
are in place if environmental warning or limit levels are
exceeded, and who is responsible for which action?
Often, no previous practical experience is available for
a specific site. Where this applies, it is recommended to
develop adaptive monitoring schemes, so that monitoring
efforts can be adjusted (reduced, refined or expanded) if
appropriate. It is important to realise that most ecosystems
respond to prolonged, rather than instantaneous, turbidity
impacts. For such cases, the use of time-averaged turbidity
measures (for instance, six- or 12-hour rolling average) to
assess impacts is justified. This in itself adds significantly
This document is presented by the Central Dredging
Association (CEDA). an independent, international,
easy-to-access platform for the exchange of knowledge
and experience on all aspects of dredging and marine
construction. CEDA publications are peer- reviewed by
internationally acknowledged experts and represent high
quality standards. Input for the document is obtained from
all professional groups within the CEDA membership which
represent a wide range of expertise, disciplines and nations.
CEDA publications provide impartial, state-of-the-art
information for academics, industry professionals, regulators,
decision-makers and stakeholders. This document, or part(s)
of it can be used freely by anyone, subject to reference made
to CEDA as the author. For more information please
refer to www.dredging.org
Radex Building
Rotterdamseweg 183c
2629 HD Delft, The Netherlands
T: +31 (0) 15 268 2575
E: [email protected]
www.dredging.org
3

Action 2.8: Report about appropriate dredging techniques - Coast-Best

Transcription

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