Deep and Shallow Water Marine Applications – An Overview

Some Dredging Equipments.

Abstract

1 to 5m3 is the normal size of the  bucketcapacities in  Grab dredgers.	300 to 400m3 per hour is the output by the grab  dredgers.

This article describes about the dredging process, their types and instruments used for dredging operations. Different types of dredging equipments that can be applicable in shallow as well as deep water environments are described here in detail. The types of dredgers, their dredging mechanisms and capacities are studied. Then it focuses on the dredging activities in Indian Context with special focus on the dredging activities in New Mangalore Port, 9nth major port in India, located in Karnataka.

Introduction

From the beginning of humans live on earth, they have been attracted by regions that are rich of water. People prefer to live near the water bodies because these regions are often the most fertile, offer the best possibilities to find drinking water and in most cases have a moderate and attractive climate. Last but not least, oceans, seas and waterways offer excellent opportunities for transport and for communication.

Waterborne transport is vital to domestic and international commerce. It offers the most economical, energy efficient and environmentally friendly transportation of all types of cargo. Technological developments and the need to improve cost effectiveness have resulted in larger, more efficient ships. This, in turn, has resulted in the need to enlarge or deepen many of our rivers and canals, in order to provide adequate access to ports and harbours.

Dredging plays an important role in both the construction and maintenance of the infrastructure facilities upon which our economic prosperity and social wellbeing depend. It provides sands and gravels for the filling of low-lying areas and for the construction of new land areas. Dredged aggregate is being used in concrete and as fill for reclamation projects also.

Dredging

Dredging is an excavation activity usually carried out partly or fully underwater, with the purpose of gathering up bottom sediments and disposing of them at a different location. This technique is often used to keep waterways navigable. It is also used as a way to replenish sand on some public beaches and for land reclamation.

Classification of Dredging

The dredging activity can be mainly classified as given below:

Capital Dredging- The dredging process when applied to the construction of new harbours, basins, entrance channels, trenches for foundation and pipelines, for deepening the harbour, etc. is classified as capital dredging.

Maintenance Dredging- The dredging process when carried out for removing the siltation in the existing harbours, rivers, channels and clearance of siltation in lakes, etc., the same is termed as maintenance dredging.

Purpose of Dredging Activity

Dredging operations can be employed for any one of the following purposes.

• Creation/Maintaining of artificial depths for ports and harbours.
• To replace unsuitable foundation materials with suitable materials.
• Construction of dams, sea walls, artificial islands etc.
• To provide fill materials for protection and replenishment of beaches.
• Mining of aggregates, sand, gravel and minerals from sea beds.

The Dredging Process

Capital dredging projects can be both extensive and expensive. Maintenance dredging is often a regular, perhaps annual ongoing, long-term activity. In either case, dredging process consists of the following four elements:

Pre-treatment- Pre-treatment is a process that consists of treating the ground surface before the excavation process. This is normally carried out by one of the methods – Mechanical and Chemical. Both the methods are applied normally for rock and hard surfaces. For normal soils, no pro-treatment is required and the material is disintegrated at the time of excavation.

Excavation- This process involves the dislodgement and removal of sediments and/or rocks from the bed of the water body. This should be facilitated by the dredging equipments (dredgers) by means of some particular dredging mechanisms – Mechanical, hydraulic or others.

Transport of excavated material- Transporting materials from the dredging area to the site of utilization, disposal or intermediate treatment, is generally achieved by one of the following methods:

• In self-contained hoppers of dredgers.
• In barges or pontoons.
• Pumping through pipelines and
• Using natural forces such as waves and currents.

Disposal of Dredged Material- In construction projects, dredging is driven by the demand for dredged material. In navigation and remediation dredging, the project is driven by the objective of removing the material from its original place. Management alternatives for dredged material can be grouped into the following three main categories:

• Sustainable relocation: Marine or fluvial sediment normally contribute to the sustainability of natural ecosystems. In environmental assessment therefore as a first option the relocation of a estuary in the natural environment should be considered.

• Beneficial use: Beneficial use may be defined as “any use which does not regard the material as a waste”. A great variety of options are available, and the main types can be distinguished as -Beach Nourishment /Coastal protection, Used in land reclamation Projects, Used as a construction material and Used in different construction works.

• Disposal into Water: Dredged material is placed at designated sites in oceans, estuaries, rivers and lakes such that it is not isolated from the adjacent waters during placement. If in some cases the dredged material may be contaminated then it is disposed into isolated water so that it cannot move beyond the boundary of the disposal area.

Dredgers

A dredge is a device for scraping or sucking the seabed, used for dredging. A dredger is a ship or boat equipped with a dredging tool. The dredgers are mainly classified as follows based on the dredging mechanism adopted.

Mechanical Dredgers

Mechanical dredgers use mechanical power for excavation. The sea bed materials are moved or dislodged by a mechanical tool that may be a bucket, chisel or grab. The various types of dredgers under this category are:

Fig. 1 Grab Dredger

Grab Dredger- The grab dredger (Fig.1) is the most common among the mechanical dredgers. The dredger essentially consists of a crane fitted with the grab mounted on a pontoon or a self-propelled hopper barge. The crane is of a slewing type which lowers and hoists the grab into and out of water. The grab hopper dredger is normally provided with more than one grab crane.

It can be used in almost all types of soil. Its limitation is the low output. However, it is especially suitable for working in confined areas such as docks, alongside berths etc. The most popular one being used is a Clam shell grab. Grab dredgers are normally rated by their bucket capacities. Normal size of grabs varies from 1 to 5 m3. Some of the modern grabs have as high a capacity as 35 m3 an even the largest grab having a capacity of 200 m3. The output of the grab dredger will depend upon the number of cycles achieved and outputs of 300 to 400 m3 per hour.

Fig.2 Bucket Dredger

Bucket Dredger- A bucket dredger (Fig.2) is a stationary dredger that is equipped with a continuous chain of buckets, which are carried through a structure, the ladder. This ladder is mounted in a U-shaped pontoon. The drive of the bucket chain is on the upper side. The bucket dredger is anchored on fixing anchors. During dredging, the dredger swings round the bow anchor by taking in or paying out the winches on board. The buckets, which are filled on the underside, are emptied on the upper side by tipping their contents into a chute along which the dredged material can slide into the barges moored alongside. The chain is driven by the so called upper tumbler at top of ladder frame, which is connected either via a belt to the diesel or directly to an electro motor or hydro-motor.

The average size of the buckets is in the range of 50 to 1200 litres. Normal dredging depths vary from 5 to 18 m. Dredging depths up to 30 m can be achieved. The output varies normally in the range of 250 to 1000 cubic meters per hour.

Fig.3 Back-hoe Dipper Dredger

Back-hoe Dipper Dredger- This dredger is basically a back actor land based excavating machine mounted on a pontoon as shown in Fig.3. Back hoes are powered by line pull or direct hydraulic linkage. The outer arm of the back hoe has cutting edges and the teeth are fitted to increase the point pressure on the material to be dug.

To resist the high break-out force, the supporting pontoon is firmly held in place normally with three spuds. The bucket capacities vary from 1 to 13 m3 and are capable of dredging to a depth of 18 m and maximum dredging depth ranges from 25 to 30 m. The outputs are in the range of 100 to 400 cubic meters per hour.

Mechanical Rock Breaker- The rock breaker consists of a heavy pointed chisel mounted on a suitably designed pontoon. The chisel can be hoisted and dropped vertically on the rock to be broken. The chisel has a heavy cast steel point.

Modern version of the rock breakers have pneumatic or hydraulic hammer which breaks the rock with a frequency of 1.2 to 2 blows per second. With a normal 15 ton rock chisel and in average type of rock, outputs in the region of 10 cum per hour can be achieved. These broken rocks will be later removed by some other means of dredging. Fig 4.

Fig.4 Rock Breaker

Hydraulic Dredgers

The hydraulic dredgers employ suction techniques in which the soil is drawn up by a centrifugal pump. These are used principally where material such as sand or mud is to be removed. It consists of a flexible pipe connected at one end to a powerful centrifugal pump. At the other, open end there is usually a device designed to break up the material to be dredged. The open end of the pipe is lowered to the bottom, where the material to be dredged is mixed with water, pumped up, and then discharged into hopper barges. There the heavy material settles, and the surplus water is allowed to overflow. The various types of hydraulic dredgers are:

Plain Suction Dredger

Fig. 5 Plain Suction Dredger

A plain suction (Fig.5) dredger is a stationary dredger that positions on one more wires, with at least one dredge pump, which is connected to the suction pipe and the delivery pipe. In its most simple form this type consists of a pontoon able to support a pump and suction pipe and to make the connection to the discharge pipe. More sophisticated vessels have separate suction and delivery pumps, water jets at the suction inlet and articulated suction pipes. Very long distances can be pumped by the addition of booster pumps in the line. Material may alternatively be loaded directly into barges moored alongside.

Good production can only achieved by this kind of dredgers either the soil is free running sand or the cut or breach height is sufficient at least 10m. The discharge of the soil sucked is done either by pipeline or by barges. Most suction dredgers are equipped with jet water pump(s) to assist either the breaching process or to improve the mixture forming process near the suction mouth.

The normal measures of size are the diameter of the discharge pipe, which can vary between 100 and 1000 mm, or the installed horsepower. Can dredge from depths up to 100 m. Production is very dependent upon the permeability of the material dredged and is best in clean sands.

Trailer Suction Dredger

Fig.6 Trailing Suction Hopper Dredger

The generally accepted definition of a trailer suction/hopper dredger (Fig.6) is a self-propelled, self loading and self-discharging sea going vessel with one or more flexible suction pipes equipped with special suction heads. As the name implies, the material is dislodged and sucked in, as the dredger moves forward with the help of a drag head attached to a suction pipe suspended on the side of the dredger. Different type of drag heads are in use depending upon the type of material to be dredged. The dredged material is sucked up by the centrifugal pump and the material is discharged into the hopper contained within the dredger. The dredger is suitable for all types of normal soils except for very fine sand and stiff clay.

The trailing suction dredger has developed into a powerful dredging tool with wide range of applications. Today trailing suction dredgers of 30,000 m3 capacity capable of dredging up to a depth of 35m and possessing a laden speed of 17 knots are in use.

Cutter Suction Dredger

Cutter Suction dredgers (Fig.7) are versatile and can deal with a wide variety of materials including clay, silt, sand, gravel and some grades of rock. The dredger mainly consists of a rotating cutter head which is mounted in front of the suction inlet which performs a rotary motion there by dislodging the soil and depositing in a position from where it can be drawn into the suction pipe. The dredger is anchored by means of spud about which it is swung by manipulating the anchor wire leading from the port and the star board sides.

The main limitation of the dredger is the sea condition and it can only work in light to moderate condition. Cutter suction dredgers are rated by their discharge pipe and vary in diameter from 150 to 1050 mm. The present day cutter suction dredgers have pipe dia. of 1200 mm and have pump capacity of about 5000 H.P. and total installed H.P. in the region of 21000 capable of pumping the material to a distances of about 6 km. The dredging depths are in the region of 20-25m. Typical output of the dredgers varies from 200,000 to 500,000 m3 / week.

Wheel Dredger

In the wheel dredger (Fig.8), the dredging wheel replaces the conventional cutter used in the cutter suction dredgers. The plane of rotation is at right angles to that of the cutter, providing smoother dredging process with higher output of solids. The dredging wheel combines the high controllable dredging process of a bucket dredger and an efficient hydraulic transport system of a cutter suction dredger. The dredging wheel essentially consists of two rings connected by the buckets. The bottomless buckets with sloping sides fitted with patented lips have solved the problem of adhesion of soil. It can be used virtually in all types of soils including sticky clay. The equipment is very well suited for alluvial mining.

A mixture concentration of 70% has been reported. The normal dredgers are with output of 200-3600 m3 /hr. However specially built dredgers have drive powers up to 1100 kW and outputs up to 5000 cubic meters per hour.

Pneumatic Dredger

The dredger in this category is known as the ‘Pneuma System’. The Pneuma system (Fig.9) was developed by M/s. Pneuma International, Italy. The principal characteristic of the PNEUMA System plants, operated by compressed air, consists in the lack of rotating parts, or any type of mechanism, in contact with the mixture to be pumped. By gravity, the liquid passes through the external valves, which here it is pushed through an internal delivery valve, without contacting the mechanical parts placed on the surface.

The PNEUMA pumps, working due to the effect of the hydrostatic water column or of a vacuum system in case of shallow waters, can be considered a positive displacement piston pump, with compressed air taking place of a mechanical piston. The major advantages of the system are:

• Compactness and simplicity of operation.
• Low operating & maintenance cost.
• Capacity to dredge deeper depths.
• High degree of flexibility, and
• Produce mixtures with high density.

The system can be operated both from the land base and from the traditional barge and also can be used for long distance transportation. It can be used for dredging sand, clay, silt, gravel, etc. The system has been tried up to a depth of about 50 m. The concentration of solids in the dredge mixture ranges from 60 to 80% by volume.

Special Dredging/ Excavation Tools in Deep Water

Offshore Structures / Pipelines installation on uneven seabed may require seabed preparation work in order to control stresses and provide a sufficiently long lifetime. It is desirable to have an acceptable flat seabed for the pipeline or the subsea structure, and such pre-lay preparation of the seabed may require Excavation, Trenching, and Levelling etc.

SeaVator^TM Tool - AGR Subsea Ltd.

The SeaVator™ is a versatile and powerful mass flow excavation tool. It can be utilized in a variety of ways to both excavate and trench a range of pipeline sizes and soil conditions. SeaVator™ uses the principles of hydrodynamic flow and can be adapted to specific requirements through combination with specialized tools.

The package consists of a SeaVator™ unit and a purpose-built LARS (Launch and Recovery System) with dual guide wires specially designed to reduce tool motion in heavy seas. The system is capable of all general excavation tasks, pipeline and cable trenching and back filling. System equipment includes a hydraulic power pack, control cabin and umbilical reel. It is compact and easily installed on small vessels. Some of the parameters are: Total Weight- 3,500 Kg; Diameter-1,600mm; Height-1,700mm. And Excavation Rates being Sand 1000 cubic meters per hour; Silt-1000 cubic meters per hour; Gravel 1000 cubic meters per hour.

ClayCutter-X^TM – AGR Subsea Ltd.

The ClayCutter X™ represents the very best in state-of-the-art trenching and pipeline route preparation technology. It is not limited or compromised by water depth, soil stiffness or the angle of the excavation area. High pressure jet nozzles and water cannons combine to make ClayCutter X™ an outstanding product which can be easily deployed.

The ClayCutter X™ has a unique jetting system powered by high-pressure seawater from the surface, provided by AGR's 3,000 Hp trenching pumps. The tool has three operating modes which may be changed subsea with a suitable ROV. This flexibility allows operators to choose between jetting modes suited to general trench excavation, trench widening and very-hard-soils excavations. Various applications of ClayCutter-X^TM are:

• Pipeline route preparation in difficult seabed areas.
• Seabed excavation prior to platform removal.
• Deep trenching for ice scour protection.
• Seabed levelling for manifold installation.
• Mud line cellars and glory holes in ice-prone areas.
• Shore approach pre-trenching in competent soils and soft-rock areas. 

Spider^TM Dredging System – NEXANS

Spider^TM has been developed recently by Nexans, a Paris-based company. The Spider is a development based on merging the Capjet trenching system with the undercarriage of a Swiss timber vehicle in order to achieve maximum terrain capacity. It uses water jetting at up to 1,960kPa (284psi) pressure to fluidize the seabed followed by a suction system to remove large volumes of soil and can be controlled with 10-20cm precision, even at a depth of 1,000m. The movement of the vehicle is controlled by operators on a support vessel through a specific launch and recovery system (LARS). An umbilical interconnects the vessel to the Spider to provide necessary power and signals.

Dredging In Indian Context

India has a long coast line of about 7640 km. There are 12 major ports, six on the West Coast and six on the East Coast. In addition there are over 187 non major ports. The capital and maintenance dredging requirement of these ports involve huge expenditure. For efficient functioning of the ports, adequate depths are to be maintained at all times.

The demand for dredging continues to grow in India driven by greater focus on port infrastructure development, use of larger vessels, increased attention to coastal shipping and inland water transportation, changes in technology. This demand is set to accelerate further in the coming years. The Maritime Agenda 2010-2020 envisages increasing the draft in all major ports to a minimum of 14 metres (and in some ports to 17 metres). 

Dredging projects worth over Rs 200 billion have already been planned between 2010and 2020. However, the growing demand has not been complemented by a commensurate increase in the Indian dredger fleet. There are only a few dredging players in the country and most of them have their dredgers tied up with ongoing contracts. The dredging capacity in India needs to be significantly enhanced. There is also a need for improvement in dredging methods and for the introduction of new technology and equipment. The skills and knowledge of dredger operators need to be upgraded continuously. The frequency of the depth survey process needs to be standardized across all ports.

The capital dredging carried out during the 11th plan period (2007-12) is 278 million cum. The projected requirement of capital dredging during the 12th plan period (2012-17) is 640 million cum. The quantity of maintenance dredging carried out by the major ports, minor ports and fishing harbours during the ninth five year plan (2007-2012) was 291 million cum and the projected requirement during the tenth five year plan (2012-17) is 530 million cum. The maintenance dredging quantities vary from year to year.

Conclusion

Dredging technology is continuously developing and the demand for dredging is increasing every year, since the growth of nations largely depends upon the foreign trade and industrialization. More and more water transport infrastructures have to be developed to facilitate international trading and this will require huge amount of dredging activity. The following observations can be made:

• Dredging is one of the basic processes in developing water transport facilities.
• Latest technologies are incorporated and sophisticated instruments were developed to meet the challenges and to increase the efficiencies.
• Dredging technology will certainly see more advancements and novel technologies in the coming decades.

References

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2. Kamath M.M. 1985 - ‘Dredging - An Overview’ - Indian Ports, Volume XVIII. No.3.
3. Kamath M.M. 2001- ‘Capital Dredging at New Mangalore Port’ – Terra et Aqua, Number 82, March 2001, Pp. 8-19
4. Kare H., Eklund T. et. al. 2007 - ‘Ormen Lange Pipelines Installation and Seabed Preparation’- Offshore Technology Conference, U.S.A., 30 April–3 May 2007, Paper Ref. No. OTC 18967
5. Ministry of Shipping, GOI, 2011, ‘Report of Working Group for Port Sector for the Twelfth Five Year Plan, 2012-17.’
6. Ministry of Shipping, GOI, 2011, ‘Basic Port Statistics of India, 2010-11.’
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9. https://www.vanoord.com
10. https://www.agr.com
11. https://www.nexans.com
12. https:// www.pneuma.it

Authors:

Prof. Dwarakish G. S Department of Applied Mechanics & Hydraulics,  NITK, Surathkal, Mangalore, India

Mr. Mrinal S Murali  Department of Applied Mechanics & Hydraulics, NITK, Surathkal, Mangalore, India

Acknowledgements: This paper was submitted during the A3C–12: Awards, Convention & Consultants Colloquium, organized by Association of Consulting Civil Engineers (I), Mangalore Centre.