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As already mentioned, the location of the marine borrow area is of the utmost importance for the evaluation of possible interventions. It can be concluded from the plan be-
low which intervention zones may be considered, taking into account the location of existing marine quarries.
Technical Information Sheet
DEEP DREDGING Opportunities for Jumbo-type trailing suction hoppers with deep-sea dredging installation (- 100 m): the practice of beach nourishment Abstract: Until the end of the 1980’s, the maximum dredging depth for the trailing suction hopper dredgers (TSHD’s) was directly linked to the maximum draught of these vessels. At about the mid 1990’s, a new generation of TSHD’s made their appearance. They were commonly called ‘Jumbo’, and had a capacity between 16.000 and more than 35.000 m³. The main sector of activity and operation of these vessels is the dredging and the transportation of large volumes of sand, used in vast reclamation projects - in particular in the Middle East and the Far East. Owing to the length of their hull, this new generation of dredgers has allowed to take very long suction pipes on board – hence making deep-sea dredging beyond 100 metres feasible. Distance of marine quarries to possible intervention sites
Conclusion Taking into account the increasing concern for respecting the environment, it is safe to predict that future tendencies for the realisation of beach nourishment projects by way of hydraulic reclamation of marine sand, will ultimately evolve towards the exploitation of borrow zones at big depth. A few projects have already been realised, and a lot of research is being done – in particular in the Mediterranean Sea.
AUTHORS Pierre Catteau Area Director Dredging and Environmental Marine Engineering (DEME) E-mail : Catteau@sidra.it Malik Boukebbous Proposals Manager Società Italiana Dragaggi S.p.A. E-mail : Boukebbous.malik@dredging.com
However, it is necessary that these projects have a sufficiently big dimension. They must fully benefit from the advantage of scale in order to guarantee an attractive operation. In order to realise projects of a sufficient dimension, it is advisable that coastal defence problems and related works would be managed, not by local authorities, but at a regional, interregional or even national level.
This Technical Information Sheet is based on the experience and “know-how” of Societá Italiana Dragaggi S.p.A. member of the DEME group and aims to give a technical contribution for the future beach nourishment projects. Società Italiana Dragaggi S.p.A. Via Carlo Zucchi, 25 - 00165 Roma Tel. 066604951 - Fax 0666049549 www.sidra.it - segreteria@sidra.it
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dredging, environmental and marine engineering N.V.
This new generation has led to the search and the exploitation of sand borrow areas at a depth of 100 metres and more, allowing the realisation of beach nourishment projects (coastal protection) while substantially limiting
the environmental marine impact. Indeed, the presence of marine vegetation almost always stops at depths of 35 to 40 metres. In addition, realising this kind of projects with Jumbo-type trailers allows to use borrow areas at a much bigger distance, i.e. up to 100 to 200 km between the borrow area and the beach nourishment area. Even then, dredging is done within acceptable economic limits. However, assigning Jumbo trailing suction hoppers can only be justified if total volumes involved match the depreciation for the costs of mobilisation and starting up the site. Indeed, this kind of dredging vessel would traditionally be assigned in areas such as the Far and the Middle East in the first place. In order to reach profitability, it is thus advisable that several beach nourishment projects would be clustered, for instance at a regional level, so that a project of several millions of m³ can be realised. Key words: Jumbo, deep-sea dredging, marine sand borrow areas.
Introduction The technological evolution in the field of trailing suction hopper dredger (TSHD) building has been driven by hydraulic engineering markets worldwide, and in particular by some specific needs in the dredging industry. On the other hand, the appearance of new plant and new technology has stimulated, and made possible new infrastructure projects, which were considered not to be feasible or profitable before the introduction of this new plant. It is this continuous movement between markets and
plant, which is the driving engine behind the recent strong technological evolution as well as the important investments that were recently made by the dredging industry. In particular, the appearance in the world fleet during the last decade of the past century of some ten dredgers that were commonly called ‘Jumbo’ (i.e. with a hopper capacity in excess of 16.000 m³), nowadays allows to dredge aggregates dredging at depths of more than 100 metres and gives a very concrete answer to some important problems in the field of coastal defence.
Mobilisation, preparation of the dredge, and scope of the project
Brief historical overview of the technical evolution of trailing suction hopper dredgers In the 1960’s, trailing suction hopper dredgers (TSHD) had a maximum capacity of some 4.000 m³. Almost all these dredges were only equipped with a discharge system at the bottom of the bin, allowing dumping of the dredged material. The principal objective of this equipment was to give an answer to the strong demand for new port infrastructures in Europe, as well as for deepening and maintenance of access channels to these ports. At that time indeed, the dredging depth was limited to 30 metres, which actually corresponded to the maximum draught of existing vessels on the market. One can safely claim that, by the 1960’s, dredging consisted of – more than anything else – deepening the bottom in order to allow merchant ships to transit.
TSHD Maas (capacity of the hopper unit: 3.170 m³; suction pipe: 750 mm)
In the 1970’s and the early 1980’s, at a time when the maximum hopper capacity of a dredger exceeded a volume of 10.000 m³, it became common practice to re-use dredged material and to create new land by hydraulic reclamation. Most dredgers built in that period were equipped with a system for rehandling the excavated material. In this way, disposing of the dredged material is no longer limited to the mere option of dumping. In 1980, dredging meant deepening, but pumping ashore of excavated material just as well. It should be noticed that nowadays reclamation has become far more important than dumping at sea. This evolution must of course be interpreted against the growing environmental concern for dumping areas at sea. For example in the Mediterranean, many ports have been forced to find suitable alternatives for the disposal at sea of the dredged materials from maintenance dredging of access channels and port basins. A lot of research has been done on the optimisation of systems for reclamation. Conventional pumps for discharging ashore were slowly replaced by pumps with a higher output and efficiency. These pumps have considerably improved the process of unloading and discharging ashore. This has happened by influencing the major parameters of the dredging process: increase of the flow rate, optimisation of the concentration and the manometric pressure, and minimising hydraulic losses and wear.
It is important to repeat that currently the principal market for Jumbo dredges is located in the Middle East and the Far East, where very important infrastructure projects are being carried out. Transforming a Jumbo dredge and adapting the vessel to deep-sea dredging requires an immobilisation of about one week to ten days, a period which is needed for installing an additional davit, the immerged pump, and additional sections of the suction pipe.
As a result, it is easy to understand that the fixed costs will severely penalise a beach nourishment project by way of deep-sea dredging, which would be limited to 200.000 or 300.000 m³. The fixed costs involve the mobilisation, the transformation and the demobilisation, meaning in the above example six weeks of fixed costs for one or two weeks of effective dredging. It can be concluded that sufficiently important interventions are needed for making deep-sea dredging economically viable.
Configuration of the coast The site preparation costs are also influenced by the configuration of the coastline, in particular the slope of the immerged beach. Jumbo-type dredgers have a loaded draught between 11 and 14 metres, meaning they require a sea bottom depth between 12.5 and 15.5 metres. The provision of floating or submerged lines for joining the coast is needed, in order to hydraulically transport the material. It is currently considered good practice to use only floating lines up to a distance of 500 metres. A typical length of a sinker line would be 1.500 metres, although
3.000 and even 4.000 metres is feasible. However, in the latter case, non negligible risks will occur and the mobilisation of a big number of auxiliary equipment has to be anticipated. As a consequence and taking into account the other characteristics of the project, it can be concluded that a beach replenishment project by way of deep-sea dredging will only be viable if the water level would match the draught of the vessel at a distance around 1500 to 2000 metres (very exceptionally 4000 metres).
Beach section to be replenished Jumbo-type dredges have a very important sand transport capacity, from 15,000 to even 25,000 m³ for the biggest. Depending on the kind of dredged material (which is mostly fine sand), replenishment output can reach values up to 10.000 m³ per hour.
As a result, the beach section to be replenished must be coherent with the Jumbo-dredge capacity, in order to allow profiling as the sand is pumped ashore. A section of about 300 m³ per linear metre of beach is considered a normal section for operating a Jumbo-type dredge.
Sailing distance Together with the important transport capacity of a Jumbo-dredge, its sailing speed of about 16 knots allows to consider sailing distances up to 150 and even 200 km.
Thanks to the assignment of a Jumbo-type dredge, a deepsea borrow zone can be used for a long section of the coast – covering a region or even beyond.
The table below presents a typical cycle of a Jumbo-type dredge on a deep-sea dredging run.
TSHD Antigoon (bin capacity 8.400 m³, suction pipe: 1.200 mm)
Typical cycle and fine sand production of a Jumbo-dredge
TSHD Pearl River (hopper capacity 24.146 m³, suction pipe: 2 x 1.200 mm): equipped for dredging beyond a depth of 100 metres
Dé s igna tion Specification
Distance 50km
Volume du puits Bin volume C hargem ent Loading N avigation Sailing C onnection Connection Pumping D échargemashore ent N avigation Sailing Total Total Production/cycle Production/cycle
Weekly production (measured in hopper) Production hebdomadaire (mesurée en puits)
(m_)
100km
200km
16,000
16,000
16,000
(m in)
180
180
180
(m in)
135
270
541
(m in)
20
20
20
(m in)
120
120
120
(m in)
125
249
499
(min)
580
839
1360
(m_/ hr) (m_/ week) Semaine)
1,655
1,144
240,000
165,912
706 102,353
Current applications of dredging at very large depths
Deep-sea dredging (100 metres and beyond) In order to increase the versatility of Jumbo-type dredgers and to open up new perspectives for the dredging industry, several companies that constitute the dredging business have in the past decade adapted the Jumbo-type dredgers, installing very long suction tubes that allow dredging at large depths of 100 metres and beyond. Little more than ten years ago the so-called Jumbo dredges (i.e. with a hopper capacity of more than 16.000 m³) made their first appearance on the market. They were the industry’s answer to strong demands for the transportation of marine sand in the Southeast Asian area (development of Singapore and Hong Kong in the first place). The construction of this kind of dredgers meant an important step for the dredging industry.
By comparison to their main function which is the transportation of huge volumes of sand over long distances, the traditional qualities and advantages of strong manoeuvrability and shallow draught have become relatively less important in the case of Jumbo-kind dredgers. Compared to the previous generation, the length overall (LOA) of this kind of dredgers has increased from 120 to 160 and even 200 metres and more. The maximum dredging depth is directly linked to the length of the vessel. Taking into account that the suction pipe must be taken on deck in a horizontal position during the sailing phase, it is quite logical indeed that the appearance of Jumbo type dredges has allowed to significantly increase the maximum dredging depth.
The first applications of dredging at large depth have taken place in the oil-and-gas industry, for excavations allowing the installation of sub-sea gas transport links (36’’ YungAn to Tung-Hsiao Offshore Pipeline Project, Taiwan). Other important applications of dredging at large depth are currently taking place in Korea, for the extension of the container terminal at the port of Pusan. A volume of 65 million m³ of material is being dredged at depths of some 100 metres. The distance between the borrow area and the port is about 100 kilometres. Finally, in Italy two important projects of beach nourishment are being executed in the regions of Latium and the Abruzzes, for which borrow areas are used at depths of resp. 100 and 95 metres.
Calculated pipeline profile without seabed correction Proposed dredgeline Original seabed
36’’ Yung-An to Tung-Hsiao Offshore Pipeline Project, Taiwan
Normal Configuration -30,00m / -50,00m m.s.l.
Extended Configuration for very deep dredging
> -130,00m m.s.l.
Comparison between a so-called ‘standard’ configuration and a configuration for dredging at large depths Comparison between a traditional dredger and a Jumbo-type dredger, capable of dredging at very large depths
The geometrical characteristics which can be observed on the above drawing, show the direct link between the length of a vessel and the maximum dredging depth. On the other side, dredging at big depth requires a particular configuration of the suction pipe in comparison with a traditional one: - Installation of a davit and an additional cardan; as such, the suction pipe is composed of three sections instead of two. This configuration is necessary since the weight of the
suction pipe itself would generate unacceptable stress in the structure of a traditional suction pipe, suspended on only two points. - Installation of a high-capacity submerged pump (3,5 to 6 kW according to the type of dredger). In order to generate a sufficient suction capacity (‘vacuum’) for dredging at depths of some 100 metres, it is absolutely necessary indeed to locate the pump at a depth of more than 30 metres – which excludes installation in the hull as is the case in a traditional configuration.
TSHD Pearl River with on-deck suction pipe allowing dredging up to a depth of 135 metres
Beach nourishment in Latium with material extracted from a depth of 100 metres
The current project in Latium, Italy, shows ideal characteristics for assigning a Jumbo type dredge equipped with a very long suction pipe which can win material at depths of 100 metres. - Total dredging volume: 2,500,000 m³ pumped ashore onto five different beaches, with a total coastal length of 12.8 km. This corresponds with a beach replenishment of 200 m³ per linear metre. - Sailing distance between the borrow zone and the beaches is between 80 and 120 kilometres. - Distance between the coast and the bathymetric – 12 m line is about 1000 metres. - Dredging depth: between 95 and 105 metres. - Type of dredged material: fine sand (220 microns) including a 25 to 30 per cent component of gravel, providing good resistance to erosion. Taking into account these two characteristics (fine sand and gravel), this kind of material is clearly considered of better quality than material that would be won in lesser depths. While the sand gives a good look for holiday makers, the gravel component provides a good resistance to the erosion. On the other hand, the beach nourishment project in the Abruzzes region offers less favourable characteristics (although not insurmountable) for the assignment of a Jumbo-type dredge with very long suction pipe.
- Total dredging volume: 500.000 m³ reclaimed onto five different beaches, with a total coastal length of 7 km. This corresponds with a beach replenishment of about 70 m³ per linear metre. - Sailing distance between the borrow zone and the beaches is between 80 and 140 kilometres. - Distance between the coast and the bathymetric – 12 m line is about 3.000 metres. - Dredging depth: between 90 and 100 metres. - Type of dredged material: fine sand (180 microns). In practical terms, the less favourable characteristics of the project in the Adriatic Sea result in a cost per m³ which is more than twice the cost of the Latium project. This is particularly influenced by the total volume of the replenishment operation, the configuration of the coast, and the distance between the beach and the borrow zone. However, it must be acknowledged that the Abruzzes beach nourishment project has been made possible and feasible by the dredging of the necessary material at large depth. Taking into account the many advantages of a dredging solution over a dry approach (in particular the limited impact on the environment, the speed of intervention, and the lower cost), the conditions of the Abruzzes project must be considered as very acceptable.
Advantages and constraints of dredging at large depth The possibility of having plant that is capable of winning sand at large depths (70, 100 metres and beyond) opens new perspectives for beach nourishment projects, as well as for providing material required for the development of large-scale port zones under construction. It is safe to believe that many projects of this kind will be seen in the coming years. In the past, geotechnical investigations for the search of sub-sea quarries were limited at a maximum depth between 30 and 50 metres – involving in fact only a very small part of the continental shelf. Very often, the exploitation of this kind of zones is delicate and may even inflict conflicts with local communities; by contrast, extracting marine sand at large depth (90 to 135 m) is much more favourable from an environmental point of view in the broad sense: 1. The presence of marine vegetation (type Posodonia) is found up to depths of minus 40 metres. This vegetation has a fundamental value for the preservation of water quality and for various marine species’ reproduction. Beyond the minus 40 m level, this marine vegetation disappears due to the diminishing incidence of light. Hence, the possibility of exploiting marine quarries at a medium depth is drastically reduced by the presence of this marine vegetation.
2. Increasing the depth of marine sand winning diminishes potential conflicts with local fishermen, without completely eliminating them though. This is especially the case with the smaller trawling vessels that operate close to the coast, the owners of which are often very much opposed to this kind of intervention.
The table below gives an overview of ‘Jumbo’-dredges, built in the past decade. Profondeur de Maximum dragage maximale dredging depth (m) (m)
A nnée de construction Year of construction (ou dernière (or last change) changement)
LLOA OA (m) (m)
Volume du puits(m³) (m_) Bin volume
AMST ER D AM
74
1996
159
16,830
GER AR D U S MER C AT OR
112
1997
152
18,000
VOLVO X T ER R AN OVA
102
1998
164
20,015
QU EEN OF T H E N ETH ER LAN D S
115
1998
173
23,347
N ILE R IVER
110
1999
144
17,000
QU EEN OF PEN T A OC EAN
60
1999
167
20,000
VASC O D A GAMA
135
2000
200
33,000
R OT TER D AM
69
2001
186
21,656
H AM 318
108
2001
177
23,697
KAISH U U
48
2002
157
16,500
JU AN SEBASTIAN D E ELC AN O
54
2002
150
16,500
WD FAIR WAY
120
1997/2003
232
35,508
GOR YO 6 H O
65
1985/2003
220
27,000
PR IN S D ER N ED ER LAN D EN
82
2004
156
15,961
PEAR L R IVER
135
1994/2005
182
24,146
Dredge
Drague
Finally, most dredges that are built these days, are equipped with a bulbous bow – which can be seen as a characteristic phenomenon for the increasing importance of the ‘transport’ component in the dredging process. Hydrodynamic improvements have been conceived just as well. They include the ‘Twin Gondola’ system which better integrates the propeller axes in the overall design of the vessel – op-
timising the hydrodynamic aspect of the dredger. Various periods, each one with its own characteristics, have allowed to successively optimise the different components of the hopper dredger cycle: the dredging proper, the transport, and finally the hydraulic discharge of material. The table below shows the evolution of typical parameters for dredges that were built in the past decades.
Illustration of the ‘Twin Gondola’ concept
Illustration of a bulbous bow
3. Winning of sand at large depth almost absolutely guarantees that the excavation will not influence the stability of nearby coasts. At these depths, the swell effect on the sea bottom is totally inexistent. By contrast at lower and medium depth, the swell would typically generate suspension effects and hence erosion. 4. Bigger depths of the extraction zones means that the intervention zones of dredging vessels can be moved away from the coast. This limits the visual impact and potential conflicts with tourist activities linked with the coastal environment. However, the exploitation of marine quarries at large depth also involves some constraints. In order to optimise the intervention and to remain within sound and acceptable budgetary limits, it is important not to ignore these constraints in the design phase of a project.
Typical characteristics of dredges per decade C apacité A nnée de moyenne Averagedes bin C onstruction Year of construction capacity puits (m_(m³) )
1970 - 1980
Profondeur de dragage Averagemoyenne dredg(m) (m) ing depth
S ystème de déchargement Discharge system Par le Through bottom fond
Système de Hydraulic discharge refoule ment
Vitesse de navigation Average cruising moyenne speed (noeuds) (knots)
Puissance moyenne au L ongueur pompes depower refoulement Average Average at the moyenne (m) length (m) hydraulic(kW) pumps (kW)
4 ,0 0 0
34
X
45%
1 1 .0
93
1 ,5 0 0
19 80 -19 90
6 ,5 0 0
39
X
80%
1 3 .9
116
2 ,8 0 0
1990 - 2006
1 2 ,0 0 0
64
X
82%
1 5 .0
130
5 ,3 0 0