International Journal on Public Works, Ports & Waterways ......respectively the largest and third...

INTERNATIONAL ASSOCIATION OF DREDGING COMPANIES

International Journal on Public Works, Ports & Waterways Developments

Number 65 - December 1996

Terra et Aqua – Number 65 – December 1996

Terra et Aqua is published quarterly by the IADC, The International Association of Dredging Companies.The journal is available on request to individuals or organisations with a professional interest in thedevelopment of ports and waterways, and in particular, the associated dredging work.The name Terra et Aqua is a registered trademark.

EditorMarsha R. Cohen

Editorial Advisory CommitteeH. van Diepen, ChairmanJ. Boeter H. de Vlieger E.A.M. StraussH. Fiers C. Paris Solas P.G. RolandP.J.A. Hamburger

Editorial AddressTerra et AquaDuinweg 212585 JV The Hague, The NetherlandsTel. 31 (70) 352 3334Fax 31 (70) 351 2654

Please address inquiries to the editor.Articles in Terra et Aqua do not necessarily reflect the opinion of the IADC Board or of individual members.

© 1996 IADC, The NetherlandsAll rights reserved. Electronic storage, reprinting or abstracting of the contents is allowed for non-commercial purposes with permission of the publisher.

ISSN 0376-6411

Typesetting and printing by Opmeer Boekdruk Offset bv, The Hague, The Netherlands.

Front cover:Because of the complexity of dredging polluted silt, the dredge needs to be equipped with a computer-controlled dredge operator, which records, stores and displays all data, such as turbidity, water content, andsilt density (see page 19).

IADCDuinweg 212585 JV The Hague, The NetherlandsTel. 31 (70) 352 3334Fax 31 (70) 351 2654 International Association of Dredging Companies

2 Editorial

3 “Dilution”, A New Technology Used at the Pearl River Estuary PipelineInstallation Project

Noël Pille

Unable to excavate a conventional open trench because of dumping limitations, a newtechnique was developed to install the pipeline through “diluted” soil.

10 Management of Contaminated Mud in Hong Kong

Peter Whiteside, Kwok-choi Ng and Wai-ping Lee

Special pits dredged beneath the seabed and then capped with clean sand and mud areproving a solution to dredging contaminated waters in Hong Kong.

18 The Sweep Dredge: High Accuracy Dredging Trials Continue

Thomas Arts and Bert Kappe

More techniques for dredging thin layers in contaminated waters have been tested on theKetelmeer in 1996. A summary.

19 New Developments in Environmental Dredging: From Scoop to Sweep Dredge

Stefaan Vandycke

The winner of the 1996 IADC Award, this paper describes one of the recent dredgingtechniques tested on the Ketelmeer and now being used in the Port of Nieuwpoort,Belgium.

26 Books/Periodicals Reviewed

Reviews of a book, “Dredging, Remediation and Containment of ContaminatedSediments” and a computer programme “SEDTEC” on environmental subjects; as well two on offshore engineering-related works.

30 Seminars/Conferences/Events

Exhibitions and conferences in Vietnam and Indonesia in spring 1997; CEDA DredgingDay in the autumn; and several “Call for Papers”.


CO N T E N T S


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EDITORIAL

As 1997 approaches, the dredging industry is gearing up for the future.Research and development, especially in the treatment of contaminated materials,are going full speed ahead. As environmental issues have been raised, the dredg-ing industry has come to the fore in actively addressing these maritime pollutionproblems.

This is reflected in the articles found in this issue of Terra which present new,environmentally sound techniques: one, called “dilution” reduces the amount ofsoil needed to be dredged; the other introduces the scoop-sweep dredge for hand-ling contaminated soil; and the third explains below-seabed containment andcapping methods.

Also obvious as 1997 nears is that the Far East remains a hub of dredgingactivity. Not only are two of our articles based on activities in Hong Kong, but theFar East is also the venue for several important conferences and exhibitions.

The IADC has again chosen Singapore as the site of its “International Dredg-ing and Reclamation Seminar” to take place in February (application on page 32).In addition, the emergence of Vietnam has led to the decision to hold the “2ndAsian and Australasian Ports and Harbours Conference”, organised by EADA,and coordinated by CEDA, IADC, and Vinamarine, during the Maritime Viet-nam ’97 exhibition in April. The first Call for Papers has been issued (page 31).Clearly it is time to start thinking about the New Year and to start it right, join usat one of these upcoming events.

Marsha R. CohenEditor

”Dilution”, A New Technology Used at the Pearl River Estuary Pipeline Installation Project

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Noël Pille

“Dilution”, A New TechnologyUsed at the Pearl River EstuaryPipeline Installation Project

Abstract

This article explains a new technology, “dilution”, usedfor dredging and backfilling works during pipeline instal-lation at the Pearl River Estuary.

Soil information for Arco China’s Yacheng 13-1 GasPipeline Installation Project across the Pearl RiverEstuary in the People’s Republic of China challengedthe Contractors to develop new and innovativemethods for the installation of the pipeline across theUrmston Road Channel. Over this last section of the778 km pipeline from the Yacheng gas field offshorethe island of Hainan to Hong Kong, the pipeline had tobe installed on a firm and levelled seabed below a layerof soft to firm soil up to a depth of 20 m thick in someareas. This layer mainly consisted of previously dump-ed clay and silty marine deposits from many sources,including the new Chek Lap Kok Airport.

According to Hong Kong Authority requirements, only alimited amount of soil was allowed to be excavatedfrom the trench by conventional methods and disposedof at approved dumping locations. As this quantity wasfar less than the amount of material required to exca-vate a conventional open trench, a new dredging tech-nique was proposed by Jan De Nul N.V.’s EngineeringDepartment and further developed in close cooperationwith Arco China Inc. and Saipem-Emc J.V. The execu-tion method proposed was based on “diluting” thecohesive materials in subsequent layers to such areduced density and shear strength that the pipelinecould be installed through the diluted medium to thespecified design level at the bottom of the trench.Because of the dense and non-cohesive soils encoun-tered in the deeper areas, a combination of dilution andtraditional dredging methods were utilised to ultimatelyeffect the installation of the pipeline.

The dilution works at Urmston Road were executedduring September and October 1994 by the trailingsuction hopper dredgers J.F.J. De Nul (11,750 m3) andVasco da Gama (10,000 m3). These vessels were

respectively the largest and third largest trailing hopperdredgers in the world and had been modified for thenew technique. As a result of their size and capabilities,trench preparation could be executed over a minimumperiod of time at maximum efficiency.

After installation of the pipeline at Urmston Road, arock bund has been installed by means of a large self-propelled splitbarge, providing stability for the pipelineand protection against anchors.

Outside Hong Kong, at the location of the Jiuzhou andLinding shipping channels in the People’s Republic ofChina, a three-layer engineered backfill has beenapplied, executed by means of a side stone dumpingvessel. Works were executed by Jan De Nul N.V. forArco China under a subcontract for Saipem-Emc J.V.

Mr Pille graduated as a MechanicalEngineer from the University ofGhent, Belgium in 1975 after which hejoined the dredging department of Jan De Nul N.V. Since 1992 he hasbeen Manager of the Offshore WorksDepartment of its InternationalDredging Division. This department isinvolved with trenching and back-filling works for the installation ofoffshore pipelines, rock dumping andballasting of offshore structures, shoreapproach dredging and landfalls, andother offshore industry works.

Noël Pille

Figure 1. The Yacheng 13-1 Pipeline Installation Project from the Yacheng gas field offshore the island of Hainan to the Black Point Power Station at Hong Kong.

Introduction

Gas transportation from the Yacheng 13-1 gas fieldsoffshore the island of Hainan in the People’s Republicof China to the Black Point Power Station at Hong Kongrequired the installation of 778 km of 28“ pipeline in theSouth China Sea. Over the last 71 km, the pipeline hadto cross the Urmston Road, Linding and Jiuzhou ship-ping channels in the Pearl River Estuary (Figure 1). For the crossing at the Urmston Road Channel, thepipeline had to be installed on a firm and levelledseabed below a layer of soft materials of up to 20 m inthickness at certain locations. This required a new andinnovative method to be used as dredging by conven-tional methods and disposal at approved dumpinglocations was limited by Authority restrictions.After installation in this section, the pipeline had to beprotected by a rock bund providing 3.5 m cover abovethe pipeline and with top of backfill materials alwaysremaining -21.5 HKPD over the central section. Acrossthe Linding and Jiuzhou channels, a trench had to bepredredged in order to install the pipeline below mini-mum future draught requirements, and an engineeredbackfill had to be provided for sufficient stability of thepipeline and an adequate protection against shippinganchors.

NEW DREDGING TECHNOLOGY FOR SEABED

PREPARATION AT THE URMSTON ROAD

CROSSING BY MEANS OF “DILUTION”

The 28” pipeline was required to be installed acrossUrmston Road which lies within Hong Kong’s OuterDeep Bay dumping area on the border between theChinese and Hong Kong territorial waters.

The pit at Urmston Road was formed by dredgingmethods for reclamation purposes, extraction beingcompleted in 1991. The dredged seabed at that timewas alluvial, with the marine deposits having beenremoved prior to the extraction of sand. As such, thefirm seabed in the pit was known to be changing rapid-ly, due to the erratic shape of the earlier dredged bot-tom. Since 1991, a variety of materials had been dump-ed in the pit, largely comprising clayey and silty marinedeposits dredged by clam-shell and other methods(Figure 2).

The sources included materials from the new Chek LapKok airport works and other nearby and more distantprojects in Hong Kong, and from the People’s Republicof China. In the western half of the channel, the thick-ness of the layer of dumped mud and very soft to firmclay varied from 10 m to over 25 m. The density ofthese materials was up to 1.7 T/m3 with a shearstrength above 1 kPA.

This posed two major limitations on pipeline installa-tion. Firstly the highly irregular alluvium profile wasunacceptable from a pipeline stress point of view, andsecondly the varied soil properties could not be pen-etrated by the pipeline during installation in order toreach its specified design level. Because there waslimited soil information available across Urmston Roada detailed geotechnical and geophysical survey wasundertaken to finalise the pipeline installation scope ofwork. From the final report, the optimum profile wasanalysed, seabed preparation methodology developedand installation procedures prepared.

This seabed preparation method required the utilisationof two 10,000 m3 trailing suction hopper dredgers.


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the trench profile, dredging as a result of slope insta-bility had to be continued at some locations in order toachieve the target design level for pipeline installation.

By the combined result of these dilution and dredgingoperations, a mainly open trench was created intowhich the pipeline was ultimately installed. The as laidsurvey was performed in early October, directly uponcompletion of pipelay operations. It revealed that slopefailures had occurred between dredging and pipelayingin the deepest portion of the trench. However, a sub-sequent soil investigation revealed the soil to have ahigher sand content than before and now provided asuitable foundation for the pipeline.

Principles of the new dredging technique by means ofdilution(i) Diluting of the trench materials is executed in accor-dance with detailed procedures based on the workingprinciples of a trailing hopper dredger. The works areexecuted in successive layers of up to 2.0 m in thick-

Modification of one of the 10,000 m3 TSHD’s wasmade to allow high capacity water injection, via thesuction pipe, into the mud medium thus reducing its in-situ density.

To assess the effectiveness of the proposed dilutionoperations, extensive trials were undertaken. Thesetrials showed that sufficient reduction of soil shearstrength to allow penetration of a test pipe could not beachieved by controlled pumping of water at varyinglayer depths only. However, as already proposed in theinstallation procedures as an alternative method, itappeared that optimum effectiveness could be achiev-ed by dredging pre-determined box-cut layers with onesuction pipe and simultaneously discharging throughthe opposite suction pipe into the trench at the top ofthe nominated layer after adding water through one ofthe dredge pumps, thus reducing its in-situ shearstrength and achieving a returning target density of1.35 tons/m3.

Although the initial field trials of the dilution techniquewere proven to be successful, the presence of pocketsof consolidated clay and granular seabed materialnecessitated, particularly in the deeper sections, theuse of traditional dredging techniques. As part of thelighter components of the diluted soil was washeddownstream by the river currents and the sand settledto the bottom, dilution was no longer effective by thetime three quarters of the required depth was achiev-ed, and conventional dredging techniques then had tobe used to reach the design depth.

At these depths, in-situ trench materials were disposedof at agreed dumping grounds together with dilutedsoils which were loaded on top of the granular mate-rials until overflow level was reached. As slope stabilityhad been based on the presence of diluted material in


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Figure 2. Geotechnical profile at the Urmston Road crossing.

Figure 3. Principle of trench dilution (cross profile).

ness where boxcuts over the width of the draghead areremoved (Figure 3). In this respect, the trailing hopper dredger sails paralleltracks at a speed of around 1 knot at average 3.5 minterspacings whilst diluting each layer (Figure 4). In case of limited draghead penetration as a result ofmore cohesive soils, the height of the box has to bereduced.

(ii) The in-situ density and shear strength of the trenchmaterials are reduced by dredging each box measuring2.0 m x 3.5 m with one of the suction pipes. Before theentrance of the mixture in the suction pump, a quantityof water is added by controlled setting of one or moreinlet valves. The outgoing density is measured by theradioactive measuring devices at the outlet of the pump.

(iii) After passing the pump, the diluted mixture ispumped down to the trench by means of the othersuction pipe, which is, for this purpose, serving as afallpipe. The draghead of the suction pipe acts as diffu-serhead and is positioned at a depth approx. 2 m higherthan the nominated suction level.

Choice of equipment and required modifications The dilution works at the Urmston Road crossing wereexecuted by means of Jan De Nul’s trailing suctionhopper dredgers J.F.J. De Nul (11,750 m3) and VascoDa Gama (10,000 m3) (Figure 5). The following modifi-cations had been executed prior to start of the works: – Pumproom alteration for the outlet of the starboard

pump to be connected to the inlet of the port suctionpipe.

– Port suction pipe adapted for reverse flow.– Setting devices on water inlet valves for control of

outgoing mixture density.– Positioning and tracking displays and logging with

hard copies to be provided if required. – Draghead position systems installed.

Survey and monitoring Particular survey and testing methods have been devel-oped in order to enable monitoring of the works duringthe progress of dilution (Figure 6).(i) Density measurements were carried out in thediluted layers by means of the STEMA density probelaunched from the survey vessel.

(ii) The draghead position monitoring system installedon the dredgers provided online information of thedredge levels reached, and a longitudinal dragheadsurvey could be added to the daily report.

(iii) “Pipe dunking” tests provided information on theactual diluted levels reached. The pipe dunking wasexecuted by lowering a sealed air filled 12.2 m longsection of the concrete coated 28” gas pipe by means


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Figure 4. Principles of trench preparation by means of dilutionby using a larger trailing hopper dredger.

Figure 5. The J.F.J. De Nul, one of the largest trailing suctionhopper dredgers in the world, was modified for the newtechnique of dilution.

Figure 6. Digital record of cross-section of trench during thebeginning of the dilution works.

INSTALLATION OF THE ROCK BUND FOR

PIPELINE PROTECTION AT THE URMSTON

ROAD CROSSING

After completion of the dredging and dilution worksand successful pipeline installation at Urmston Road,the pipeline had to be protected by a rock bund provid-ing 3.5 m cover above the pipeline, and with top ofbackfill materials always remaining below -21.5 HKPDover the central section.

During the approx. 3 months execution period of thesebackfilling works, the trench profile remained remarka-bly stable. In view of the thickness and width of therock bund to be installed within the central part of theUrmston Road section, the works have been executedby means of Jan De Nul’s large self-propelled seagoingsplitbarge Verrazzano (Figure 7). The self-propelledsplitbarge positioned longitudinally above the pipeline atslack tide by means of its twin rudder propellors andbow thruster. The load of the vessel, approx. 2400 tonsof rock, was such that an average of three dumps wasnecessary in order to build up the theoretical profile.

In order to stabilise the pipeline as quickly as possible,the pipeline was pinned first by means of intermediatedumps of 50 m long every 100 m. Subsequent in-between dumps completed the first layer, and after-wards the second and the third layer were installed inaccordance with the dumping sequence of the pro-cedures. Defined “windows” were left open andcontinuously surveyed to monitor the position of thepipeline. These survey data were utilised by Saipem-Emc JV and Arco to determine and optimise the dump-ing sequence (Figure 8). Intermediate and final surveys were executed aftereach dump by means of bathymetric and seismicsurveys (Figure 9). Considering the method adopted, installation of rockarmour materials for this kind of dumping profile bymeans of a splitbarge proved to give acceptable resultsat lower costs than in the case of using a side dumping

of the dredger’s deckcrane whilst the vessel remainedon position during slack tide. The reading of the under-water weight of the pipe section (both ends closed)relative to the levels reached, provided informationabout to what anticipated levels the pipeline would sinkduring installation from the laybarge. After the initial phase, a scaled 1m long model of the28” coated pipe handled from the survey vessel hasbeen provided by Main Contractor in order to save timefor the dredgers.


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Figure 8. Cross-section of rockbund at Urmston Road.

Figure 7. Installation of rockbund at Urmston Road by meansof the 2000 m3 self-propelled splitbarge Verrazzano.

vessel. In total, over 350,000 tons of rock have beeninstalled by this method, providing protection for thepipeline against 27.5 tons stockless anchors over asection of 2180 m.

BACKFILLING AT THE JIUZHOU AND LINDING

SHIPPING CHANNELS

At the location of the Jiuzhou and Linding channels inthe territorial sea of the People’s Republic of China, aconventional design of a pre-dredged open trench withengineered backfill has been applied (Figure 10). Takinginto account the fine bottom material, and in accord-ance with filter rules, a three-layer backfill design hasbeen adopted providing a cover of 2 m above the top ofpipe (Figure 11).


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Figure 9. Seismic survey results of the as-dumped profiles.

Figure 10. Longitudinal backfilling profile at Linding channel.

Figure 11. Cross-section of Linding channel.

new and cost effective dredging and backfilling tech-niques have been developed.

Seabed preparation for installation of the pipeline onfirm seabed below cohesive and non-cohesive soils hasbeen applied by means of the newly developed “dilu-tion” principle executed with high capacity trailinghopper dredgers in combination with conventionaldredging techniques.

Detailed procedures have been prepared for executionof the works in accordance with these new principles.

Particular testing methods have been developed inorder to enable monitoring of the dilution and dredgingworks.

A rockbund has been installed within acceptable tole-rances by means of controlled bulk dumping by meansof a large self-propelled seagoing splitbarge.

A conventional three layer engineered backfill has beenapplied at the Jiuzhou and Linding Shipping Channels inthe People’s Republic of China.

As such, sufficient stability for the pipeline and protec-tion against a possible impact of 13.5 ton stocklessanchors have been provided for.

In order to provide accurate installation of all threelayers in these channels, the rock material has beenplaced by controlled sideward disposal by means of aside dumping vessel (Figure 12).

The side dumping vessel allows accurate placing ofpre-defined quantities of backfill material at pre-definedlocations. Up to 2000 tons of backfill materials areloaded on the four cargo decks. On arrival above thedumping location, the vessel positions itself parallelabove the pipeline, and installs each layer to the speci-fied thickness and width by means of controlled side-wards dumping.

During the dumping sequence, the vessel movessidewards in a controlled manner on its dynamic posi-tioning system using its twin rudder propellors and bowthruster. The dumping rate can be controlled by adjus-ting the speed of the hydraulic rams on the slide scra-pers. By means of a combination of the dumping rateand the speed of sidewards movement, any profile ofuniform or linear increasing or decreasing thickness canbe achieved. The rock was supplied by Pioneer Quar-ries Ltd. from local quarries located at Chung Hsin Chauand Little Spider Island in the People’s Republic ofChina.

Conclusions

As a result of particular circumstances and require-ments for the installation of the last 71 km of the 28”Yacheng 13-1 pipeline across the Pearl River Estuary,


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Figure 12. Installation of backfill materials by means of side-dumping vessel.

Peter Whiteside, Kwok-choi Ng and Wai-ping Lee

Management of ContaminatedMud in Hong Kong

Abstract

Marine sediments in Hong Kong, polluted by industrialand domestic wastes, have to be dredged for recla-mation foundations and navigational purposes. Usingconcentrations of seven metallic elements, the Environ-mental Protection Department categorises the sedi-ments as suitable for open sea disposal or as requiringcontained marine disposal. Since late 1992, special pitsdredged 15m below the seabed in a sheltered area of5m water depth have been used for the disposal of thecontaminated material. The pits are filled to within 3mof the seabed and then capped in three stages: 1m ofsand, 2m of clean mud, and finally, after the pit infill hasconsolidated, a further 1-2m of clean mud. The latter issoon recolonised by benthic fauna.

Approximately 10Mm3 of contaminated mud havealready been disposed of in a series of small pits. The total cost of the facility amounts to aboutUS$7/m3. Although most dredging of the contaminatedmud has been by grab, and most disposal by bottom-dumping barges, an increasing use of trailer dredgers isexpected. A 24-hour on-site management team directsand supervises the in-coming vessels and there is acomprehensive programme of environmental andecological monitoring covering sediment, water, biotaand ecotoxicology. No adverse trends have been identified. A larger,empty sand borrow pit, extending about 35m belowseabed, is now under study for future disposal. Thewhole disposal strategy has been reviewed using theLondon Convention’s 1995 Dredged Material Assess-ment Framework and has been found acceptable.

This paper is published with the permission of theDirector of Civil Engineering, Hong Kong Government.

Introduction

Although for some years now Hong Kong has beenimplementing controlling legislation to reduce waterpollution at source, the marine sediments close todeveloped areas of the territory are still contaminated

by past pollution. Over many decades, both domesticand industrially polluted wastes have found their wayvia foulwater sewers, stormwater drains and water-courses, into Hong Kong’s marine environment. Extensive field sampling and laboratory testing, particu-larly since 1991, have revealed large amounts of metal-lic pollutants associated with industrial processes suchas electro-plating, and most recent testing has addedorganic pollutants to the list.

Concentrations tend to be highest around submarineoutfall pipes where particulate pollutants have sedi-mented and where soluble pollutants have adhered tothe fine particles in the seabed sediment. Once on theseabed, pollutants have been mixed into the sedimentby ships’ anchors, commonly to depths of up to 3mand occasionally more. An estimated 30Mm3 of thiscontaminated mud will have required disposal between1991 and the year 2000 as part of Hong Kong’s pro-gramme of port, airport and urban developments(Figure 1).

Pollution control policy in Hong Kong is developed andimplemented by the Environmental Protection Depart-ment (EPD). In the late 1980s, when detailed planningstarted for the major programme of reclamations, EPDmoved to establish criteria for characterising levels ofcontamination of the seabed sediments which wouldhave to be dredged as part of the reclamations andassociated navigational improvements. Hong Kong’s“Dumping at Sea Ordinance”, originally stemming fromUK legislation, follows the London Convention (LC) inimplementing measures for the avoidance of marinepollution. Because China is also a signatory, this situ-ation will not change after the 1997 handover toChinese sovereignty.

Unlike many countries, such as the USA, Hong Kong’s“baseline” for the purposes of the LC is the highwatermark. All dredging activities, even those inland butwithin tidally affected freshwater drainage channels,are covered by the same legislation. In 1989, EPDadopted some interim guidelines for categorisation ofseverely contaminated dredged material in the north-west part of the territory.


10

En

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ent

In 1991, EPD replaced these guidelines with a set ofcriteria which have since been used to differentiatebetween dredged material suitable for open sea dis-posal and dredged material which is not (Table I). In common with a number of other countries, notablyin Europe, these criteria are based on levels of certainheavy metals. Sediments are classified as contami-nated if they fail the criteria for one or more metals.

In parallel with the introduction of the 1991 contami-nation criteria, the government decided that if highlycontaminated seabed mud had to be dredged then itwould be best disposed of by placing it in seabed pitsand capping with inert materials. A potentially suitable seabed pit already existed at thattime as a result of sand extraction for a new towndevelopment and so in April 1992, disposal experi-ments using uncontaminated mud were conducted inthis exhausted marine borrow pit to determine whetherit would be suitable for the disposal of contaminatedmud.

Trailer dredger Geopotes IX which was employed onuncontaminated mud dredging for the new airportproject was used for the trial (Figure 2). Both simplebottom dumping and pumping of the mud back downthe suction pipe were tried but measurements indica-ted that the combination of 20m water depth and therelatively strong tidal currents resulted in sedimentlosses of about 6% when measured 300m down-stream (Binnie, 1993). This was regarded as unaccept-able and it was decided that an area of shallower waterwith relatively weak tidal currents was required.

Figure 1 shows the overall bathymetry in Hong Kong,and in particular, the areas of shallow seabed (<10m).Most of Hong Kong’s storm weather is associated withintense tropical cyclones called typhoons, cominggenerally from a southeasterly to southerly direction.The shallow area selected in 1992 (East Sha Chau;Figure 1) is not only sheltered from these storms butalso has relatively low tidal currents. Unlike the area of the trial dump, however, there wereno pre-existing pits at that time and so pits had to bespecially dredged for the disposal of the contaminatedmud. The East Sha Chau area (Brand et al., 1994) has awater depth of about 5m and is adjacent to HongKong’s new airport at Chek Lap Kok (Figure 3).

Management of Contaminated Mud in Hong Kong

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Table I. Hong Kong’s criteria for open sea disposal of dredged material.

Metal Cd Cr Cu Hg Ni Pb Zn

ppm dry wt <1.5 <80 <65 <1.0 <40 <75 <200

Note: Test results to be rounded off to the same significant figures as used in the table.

Peter Whiteside graduated from theUniversity of Glasgow, Scotland andthen worked as an engineering geolo-gist in Scotland until 1982, when hemoved to Hong Kong. After doingconsultancy work on deep exca-vations, slope stability and rock tun-nels, he joined the GeotechnicalEngineering Office of the Hong KongGovernment, where is is now theSecretary of the Fill ManagementCommittee.

Peter Whiteside

Kwok-choi Ng

Wai-ping Lee

Kwok-choi Ng received a PhD ingeology from the University ofAlberta in 1990. In between earningdegrees he worked in engineeringgeology, exploration geology, hydro-geology and geological research inHong Kong, Brunei, Cayman Islandsand USA. He joined the Hong KongGovernment in 1994, where he worksin the Fill Management Division ofthe Geotechnical Engineering Office.

Wai-ping Lee was the project engin-eer responsible for the managementof contaminated mud pits from 1993to 1995. He graduated from theUniversity of Hong Kong in 1987 andhas been involved in various maritimeprojects over the past seven years. He now works in the Research andDevelopment Section of the CivilEngineering Office of the Hong KongGovernment.

DESIGN OF THE DISPOSAL PITS

The pit and cap design centred around calculations ofthe potential for contaminated sediment in an uncap-ped pit to be remobilised by storm-induced bed shearstresses, and also the potential for the erosion of thecompleted cap of a pit. It was concluded that, if thehighest contaminated mud level was 9m below sealevel, the possibility of remobilisation of contaminatedsediment was acceptably low, and if the pit cap was atleast two metres thick the risk of complete erosion of acap was negligible. In addition, seismic boomer surveys

of the Holocene sediments in the area suggested thatthe maximum depth of natural scour of the seabedduring the last several thousand years was about 1m,and so this thickness of mud cap should not be erodedeven under extreme storm events not experienced forthe 150 years during which records have been kept inHong Kong. It was also important to preclude thepossibility of burrowing organisms reaching the con-taminated mud, which is commonly less than 0.5mthick.

The final cap design (Figure 4), took account of therequirements discussed above and also practical con-siderations mostly related to construction. The capcomprises a nominal 1m thick layer of sand whichsinks differentially into the surface of the contaminatedmud to densify the surface layer and, importantly,provides a valuable marker horizon for later coring ofthe completed caps. The second and most importantpart of the capping, is the nominal 2m thick layer ofclean (i.e. uncontaminated) mud. The third part of thecapping takes place about 1 year later, after the pit infillhas consolidated, and it involves placing a further layerof clean mud 1 to 2m thick to bring the upper surfaceof the cap up to the same level as the surroundingseabed. The resulting 3 to 4m thick layer of clean mudprovides a permanent barrier over the contaminatedmud and isolates it from any future contact with themarine environment.

The pits are dredged as deep as can readily be achiev-ed, which means in practice that they are dredged tothe base of the soft post-glacial marine deposits, com-monly about 15m below the seabed. Most of the pitdredging has been by grab dredger with pit sidesexcavated at about 1 in 6. After the first ContaminatedMud Pit (CMP I), the size of subsequent pits wasdecreased to provide an added measure of safety byreducing the surface area of contaminated mud thatmight at any time be exposed to turbulent water con-ditions which occur during some typhoons.

Since late 1992, a total of about 10Mm3 of contami-nated mud has been disposed of in the series ofCMPs. Figure 5 shows in graphical form the progres-sive formation and filling of different CMPs togetherwith the anticipated volumes requiring disposal up toabout the year 2002 when contaminated mud disposalis estimated to have reduced to less than 0.5Mm3 ayear.

Although most of the contaminated mud has beendredged from government projects and disposed of inCMPs operated by the government, private projectswith small volumes of contaminated mud needingdisposal are also permitted to use the governmentCMPs. These private projects pay a fee, currently set atHK$55.4 (about US$7), which is calculated to reflectthe actual cost of forming, operating, capping and


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Figure 1. Bathymetric map of Hong Kong showing recent andproposed reclamations and the location of the East Sha Chaucontaminated mud disposal area.

Figure 2. The Geopotes IX performed the disposalexperiments using uncontaminated mud.

The private company referred to above, preferred touse a trailer dredger to dredge the contaminated mudand in order to demonstrate the environmental accept-ability of using the trailer to place the mud in its CMP, itundertook a special monitored trial. This trial demon-strated losses no greater than from dumping grab-dredged mud and so the company was permitted toform, fill and cap its own pit. The trailer dredger used for the work, the Krankeloon,has a submersed pump on the suction pipe givingrelatively high hopper densities of about 1.4Mg/m3 andhas a split hull for dumping (Figure 6). The relativelyhigh hopper density was an important factor in redu-cing losses during placement of the mud. Its relativelyshallow draught also enabled it to operate in the EastSha Chau area. Because the capping mud was alsotrailer-dredged, there was no problem of a high densitycapping material sinking into a lower density contami-nated material.

The environmental benefits of dredging contaminatedmud by suction dredgers as opposed to dredging it byopen grabs have not gone unnoticed in Hong Kong.Although closed grabs are used, this is not yet standardpractice. Because of the need to dredge contaminatedmud in busy shipping fairways where stationary plant isnot feasible, more trailer-dredging will be used in thefuture.

monitoring the facility. In addition, one private projectwith a large volume of contaminated mud requiringdisposal has been permitted to form and fill its own pitat East Sha Chau following the same basic design (The Dock & Harbour Authority, 1996).

DREDGING OF THE CONTAMINATED MUD

Dredging MethodsThe mud disposed of in the government CMPs hasbeen dredged by grab and transported and placed by750 to 1,000m3 hopper barges. There have been threemain reasons for the use of grab dredging of the con-taminated mud:– Grabs have been better suited to the shallow, near-

shore and often constricted locations where most ofthe dredging has taken place.

– Hydraulically-dredged mud has a lower bulk densitythan grab-dredged mud and because the caps of thefirst few CMPs were formed using grab-dredgedmud it was undesirable for the capping mud to havea higher density than the underlying contaminatedmud.

– Most of the experience to date, and particularly theenvironmental monitoring data which showed noadverse impact, was based on the disposal of grab-dredged contaminated mud.


13

Figure 3. Layout and status of the contaminated mud disposal pits at East Sha Chau.

Contamination Levels in Dredged MaterialThe sampling and testing programme has shown thatsome of the mud deposited in the CMPs is actuallywithin the criteria for open-sea disposal – this is prob-ably due to two factors.

Firstly, fifty metres is the average separation of the siteinvestigation boreholes used to characterise the mud tobe dredged but the actual contamination levels can varyon a smaller scale. When broad areas are classified fordredging purposes a conservative approach is adoptedand this inevitably results in some uncontaminatedmud being classified as contaminated. Secondly, the grab-dredging process is not sufficientlydiscriminating to enable precise dredging of contami-nated layers and this results in dredging of both con-taminated and uncontaminated mud. Both thesefactors result in lower average concentrations of con-taminants in the barge.

To address these points, the cost-effectiveness of thepresent borehole spacing is being reviewed and tech-niques of environmental and more precise dredging arebeing studied. Because dilution is philosophically not anacceptable solution to disposal of contaminants, post-dredging reclassification for open sea disposal is notpermissible.

PLACING AND INVESTIGATING THE CAPS

With the exception of the private pit mentioned above,the formation of the caps up to early 1996 has usedbarges to place the sand and the mud, with the latterbeing grab-dredged. Each pit is divided into a grid of30m by 30m cells. A split-hopper barge load of1,000m3 of sand is first deposited as evenly as possibleover each grid cell by moving the partially open barge

gradually over the area. The nominal 2m thick mud capis then formed by placing two 1m thick layers of mudusing similar procedures as for the sand layer. Becauseof the greater ease of hydraulic placing of mud, thismethod will be employed for mud cap placementhenceforth and this will remove one of the objectionsto using trailer dredgers for placing of the contaminatedmud as discussed above.

Six-metre long vibrocore samples are regularly takenfrom completed caps to inspect the cap structure andto provide samples from the top two metres for chemi-cal testing. As may be expected, there are variations,particularly in the thickness of sand which is dependenton the construction technique and on the extent towhich the sand penetrated the contaminated mud.Nevertheless, chemical testing of the cap samples hasshown that the contaminants are being successfullycontained. In addition to ordinary bathymetric surveys duringconstruction, Chirp seismic profiling is also used toprovide a construction record (Evans & Woods, 1994;Selby & Foley, 1995). To help quality control, profilesare run before capping starts, again after the sand layerhas been placed, and then again after placing the mudlayer. The sand, which has different acoustic impedan-ce from the mud, shows up as a useful marker horizon.When calibrated with vibrocore data, the Chirp profilingis able to show details of the cap structure to a depthresolution of about ±200mm.

To provide an overall perspective of the East Sha Chauarea, the seabed in and around the CMPs is surveyedannually by 500kHz Side Scan Sonar and high resolu-tion 180kHz Swath Bathymetry. These surveys provideinformation on consolidation settlement of the infilledpits and on any erosion which might have been causedby tidal scour or storms.

ON-SITE MANAGEMENT

In July 1993, a 24-hour on-site management team wasestablished to provide close monitoring and control ofthe disposal operations. Located on a barge mooredoutside, but near the active disposal pit, this teamregisters incoming barges and supervises filling andcapping. In order to achieve as even a filling as pos-sible, incoming barges are progressively directed todifferent parts of each pit.In an attempt to limit dispersion of plumes formedduring dumping, a silt curtain has been deployedbetween two anchored barges so that incoming bargescan approach the disposal point from the upstreamside and then dump in what is effectively a confinedarea. This practice was instituted in 1993 at a timewhen there was great public concern for what wasthen a very new and unproven operation. Becausethere are now three years of detailed environmental


14

Figure 4. Schematic section through a contaminated mud pitshowing details of the capping layers.

mental groups and the general public. Concerns weregreatest in 1992 when disposal operations started butthey have progressively reduced since then as thesuccess of the operation has been demonstrated. Thelikely environmental impacts of the disposal arrange-ments at the East Sha Chau facility were assessed andare recorded in two reports finalised in March 1993(Premchitt & Evans, 1993, CES & Binnie, 1993). Bothreports were made public and were endorsed by thenon-government environmental advisory committee.

In 1995, the London Convention adopted the DredgedMaterial Assessment Framework (DMAF) which provi-des a more detailed and structured approach tomanagement and disposal of dredged material thanwas used in 1992 (IMO, 1995). In particular the DMAFoutlines procedures for material characterisation, dis-posal methods and disposal site selection. The EastSha Chau operations have now been subjected to theDMAF procedures, and it has been reaffirmed thatcontained marine disposal at East Sha Chau is thepreferred disposal arrangement (EVS, 1996).

Environmental Monitoring ProgrammeEnvironmental monitoring of the East Sha Chau areacommenced in October 1992 prior to the disposal ofcontaminated mud, and since then has evolved anddeveloped into a very comprehensive programme ofphysical, chemical and ecological monitoring. Apartfrom continued monitoring while pits are being filled

and ecological monitoring to show that the operation isnot having any noticeable environmental impact, andbecause on-site observations indicate that the curtaingives little, if any, additional confinement, its use will bediscontinued for a trial period.

In early 1995, surveys were undertaken using thebackscatter intensity from an Acoustic Doppler CurrentProfiler to quantify the sediment losses during dumpingand it was estimated that where disposal takes placenear the edge of the pit at peak tidal flow, losses ofsuspended sediment outside the disposal pit rangedfrom 1.2% to 3.1%. During slack current conditions,losses were negligible (Dredging Research Limited,1995). Inaccurate dumping outside the designated disposalpits was an occasional difficulty when CMP I was inoperation, but this problem has been eliminated by theon-site supervision which started with CMP II. As anadditional control, all licensed barges carry a sealed,automatic self-monitoring device that registers thebarge position every five minutes and records thecoordinates and time of the actual disposal.

ENVIRONMENTAL ASPECTS

Environmental Impact AssessmentSince the start of the disposal operations environmen-tal concerns have been high among fishermen, environ-


15

Figure 5. Graph showing the rate of filling of contaminated mud pits (CMPs) and the estimated future disposal volume.

and capped, the environmental monitoring of the sitewill continue until at least two years after the com-pletion of the last disposal and capping operations atEast Sha Chau.

The environmental monitoring programme now coverssediment and water quality, aquatic biota and ecotoxi-cology. The monitoring, which is carried out indepen-dently of the disposal operations, is in two parts –cumulative impact monitoring and pit-specific com-pliance monitoring.

Cumulative monitoring is carried out to measure anylong-term effects on the overall disposal area and itcomprises regular monitoring of water quality, sedi-ment quality, abundance and diversity of benthicmacro-infauna, heavy metal loading in tissue of demer-sal biota (fish and invertebrates), and toxicity testing ofsediment.

The compliance monitoring programme involves obser-vation around each pit during its operational phase, andit includes regular testing of water and sediment quali-ty, and analysis of dredged material disposed of in thepit. A separate, three-month investigation of regionaldifferences in fish stocks and heavy metal levels intissue was undertaken from March to May 1995 duringwhich trawls of demersal biota (fish and invertebrates)were carried out at East Sha Chau and various refer-ence stations around Hong Kong.

A significant factor with the interpretation of all the EastSha Chau monitoring results is that the area is affectedby large seasonal and diurnal fluctuations in salinity andnatural suspended sediment load discharging from thePearl River, China’s third largest river. These estuarineconditions are also characterised by marked seasonalvariations in temperature.

Despite the difficulties of monitoring within this veryvariable background, the environmental monitoringprogramme has not identified any adverse trends in theparameters being monitored. It has been concludedthat the disposal procedures and capping methods areachieving the objectives of minimal losses duringdisposal and effective isolation of the contaminantsthereafter. The additional fisheries survey describedabove indicates that the taxonomic characteristics ofthe regional stations are similar to those at the East ShaChau stations and, more importantly, the statisticalcomparison between the East Sha Chau stations andthe regional stations has not indicated elevations intissue metal concentration of fish from East Sha Chau.

Recolonisation of Capped PitsThe benthic ecosystem of Hong Kong’s muddy seabedis dominated by generally small polychaete worms withoccasional crustaceans, molluscs and other fauna. Thepolychaetes are by far the most numerous and theirdominance as an early pioneering group typifies thenaturally dynamic seabed environment.

The seabed is seasonally remobilised by tropicalstorms, and in the East Sha Chau area is also subjectedto fluctuations in salinity between 4 ‰ and 28 ‰,temperature between 18 C and 33 C, and suspendedsediment levels between 10mg/l and 500mg/l as aresult of the discharge of the Pearl River. The softbottom benthic community which regularly has torecover from such severe physical changes, is there-fore adept at recolonising the substrate and re-estab-lishing itself.

Studies completed so far, have demonstrated, asexpected, that recolonisation of the soft mud used toform the CMP caps is taking place (EVS, 1996; SAIC,1994; Binnie, 1995). Full recolonisation will only bepossible after the final topping-up capping layers havebeen placed, and even for the first CMP this has onlyrecently happened. Nevertheless, it is expected thatafter about two years, the finished caps will be effec-tively indistinguishable from the surrounding naturalseabed. Although yet to be fully demonstrated, it istherefore expected that as the capping operations ineach pit are completed, the East Sha Chau area will bereturned to its original condition both in terms of sedi-ment type and ecosystem.

Conclusions: Future Disposal of Contaminated Mud

As shown in Figure 5, it is estimated that the CMP IIIseries of small pits will be full by mid-1997. In order tomeet disposal needs after that, a disused sand borrowpit (Figure 3) originally used for dredging of sand fill forconstruction of the new airport is now being studied foruse as CMP IV. Extending to about 35m below sea


16

Figure 6. The splithull trailer dredger Krankeloon provedenvironmentally acceptable for the work.

Evans, N.C. and Woods, N.W. (1994). “The Effects of Dredging and Dumping on the Marine

Environment in Hong Kong”. Proceedings of the Asia and

Australasian Port and Harbour Conference (AAPH ’94).

Kuala Lumpur, Malaysia. (Reprinted as Evans, N.C., “Effects

of Dredging and Dumping on the Marine Environment in

Hong Kong”, Terra et Aqua, No 57. IADC, The Hague,

The Netherlands. 1994. pp 15-25.)

EVS Environment Consultants.*“Review of Contaminated Mud Disposal Strategy and Status

Report on Contaminated Mud Disposal Facility at East Sha

Chau”. Report to the Geotechnical Engineering Office of the

Hong Kong Government. 1996.

International Maritime Organisation (IMO).“Dredged Material Assessment Framework (Resolution LC.52

(18))”. Adopted at the 18th Consultative Meeting of the

London Convention, International Maritime Organisation,

London. 4-8 December 1995.

Ooms, K., Woods, N.W. and Whiteside, P.G.D.“Marine Sand Dredging: Key to the Development of Hong

Kong”. Terra et Aqua, No 54. IADC, The Hague, The Nether-

lands. 1994. pp 7-16.

Premchitt, J. & Evans, N.C.*“Stability of Spoil and Cap Materials at East Sha Chau Con-

taminated Mud Disposal Area”. Special Project Report SPR

2/93. Geotechnical Engineering Office, Hong Kong. 1993. 20p,

plus 9 Figures and 5 tables.

SAIC.* “REMOTS Survey of Soft-Bottom Environments in Coastal

Waters of Hong Kong, May 5-June 6, 1994: Report No 5”.

Report to the Geotechnical Engineering Office of the Hong Kong

Government. 1994. 16p plus figures.

Selby, I. and Foley, M.“An application of chirp acoustic profiling: monitoring dumped

muds at seabed disposal sites in Hong Kong”. Journal of Marine

Environmental Engineering, Volume 1. 1995. pp 247-261.

The Dock & Harbour Authority. “Trailers used to remove contaminated mud in Hong Kong”.

The Dock & Harbour Authority. December/January 1996.

pp 186-187.

* These reports are public domain documents available for

consultation in the Civil Engineering Library, Civil Engineering

Building, 101 Princess Margaret Road, Homantin, Kowloon,

Hong Kong.

level, this pit is significantly deeper than the earlierCMPs because the sand extracted was alluvial sandbeneath the post-glacial soft marine mud.

However, by adopting the same general approach butstopping disposal at a lower level, possibly -12m, it isanticipated that about 22Mm3 of contaminated mudcan be disposed of in this pit, with a final cap thicknessof about 9m. The pit is actually in three sections, and asshown in Figure 5, it is envisaged that filling would bein three stages. The environmental impact assessmentand design for CMP IV are currently underway.

The local regulatory procedures, and in particular, the1992 sediment contamination criteria, are now beingreviewed and it is expected that an additional set ofcontamination criteria for very contaminated materialwill be developed. Organic contaminants and effects-based biological testing are expected to be included inthe revised management framework for disposal ofcontaminated mud. This review is expected to becompleted later in 1996 at which time non-marinedisposal options will also be considered for highlycontaminated mud.

References

Binnie Consultants Limited.* “Report on surveys and dumping experiments undertaken in

the redundant marine borrow pits off Black Point in Urmston

Road”. Report to the Geotechnical Engineering Office of the

Hong Kong Government. Volume I, 53p plus drawings.

Volume II, Drawings. 1993.

Binnie Consultants Limited.*“REMOTS and grab survey to assess benthic recolonisation

following backfilling at East Sha Chau (East) marine borrow pit

- June 1995”. Report to the Geotechnical Engineering Office of

the Hong Kong Government. 12p plus figures. 1995.

Brand, E.W., Massey, J.B. and Whiteside, P.G.D. “Environmental aspects of sand dredging and mud disposal in

Hong Kong”. Proceedings of the First International Congress on

Environmental Geotechnics. Edmonton, Canada. pp 1-10. 1994.

CES and Binnie.* “An Assessment of the Impact of Contaminated Dredged

Material Disposal at Contained Disposal Facilities with Refer-

ence to East Sha Chau - March 1993”. Report to the Environ-

mental Protection Department of the Hong Kong Government.

156p. 1993.

Dredging Research Limited.*“Measurements of Sediment Losses During Dumping from

Barges at the East Sha Chau Contaminated Mud Disposal Pits”.

Report to the Geotechnical Engineering Office of the Hong Kong

Government. 31p. August 1995.


17

Thomas Arts and Bert Kappe

The Sweep Dredge: High Accuracy Dredging Trials Continue

Introduction

To gather more knowledge on dredging thin layers inlarge lakes, dredging trials in the Ketelmeer, a highlycontaminated lake in the north of The Netherlands,were initiated by the Netherlands Ministry of Transport,Public Works and Water Management (Rijkswater-staat). The first two dredging trials were executed in1995 and a report of the preliminary results was pub-lished (see Terra et Aqua, number 61, December1995). The tested dredging techniques in 1995 werethe modified auger dredger, owned by HAM-VOW andthe environmental disc cutter from Boskalis. Two additional trials were conducted at Ketelmeerduring the summer of 1996. The new test involved thesweep dredger from Dredging International of Belgium(see following article, p. 19) and the modified bucketdredger from the dredging company de Boer.

SELECTION OF TECHNIQUES

The selection of techniques which were tested in theKetelmeer was based on a multi-criteria analysis. Thisresulted in a list of ten possible techniques; from thislist four techniques were chosen each representing adifferent approach for dredging thin layers of silt. Asummary of the principles of the selected techniques is:– Modified Auger Dredger: fully process controlled,

large width excavation head, positioning by anchorwires, cutting and transport to suction head withlarge auger, pipeline transport to disposal site.

– Environmental Disc Cutter: fully process controlled,cutter swing, positioning by spuds, pipeline transportto disposal site.

– Sweep Dredger: fully process controlled, cutterswing, no rotating mechnical parts in excavationhead, pipeline transport to disposal site.

– Bucket Dredger: traditional mechanical excavationtechnique, partly process controlled, transport todisposal site by means of barges.

ComparabilityThe following article on the sweep dredger containsthe results which are related to the location where thetests were performed. An example is for instance the

results of the measured turbidity. Certain questions arerelevant, such as, what are the erosion and sedimen-tation characteristics of the test area? what are theinfluences of currents, tides, waves, and depth of thetest area? what are the disturbances by external cau-ses like passing ships? and what method is used forthe determination of the turbidity?

Answering such questions is not always easy evenwhen there is a thorough description. For instance,what is the influence of the turbidity on the results, andhow do they compare with other results. Similarquestions can be asked about other criteria such asspillage and accuracy. The advantage of the trials in theKetelmeer are clear: by performing the trials in thesame location, a true comparison is possible betweenthe obtained results.

Dredging Technique Assessment CriteriaThe Ketelmeer trials were focussed on three mainenvironmentally-related criteria. The first is the accuracy,which was split into several parts to be able to distin-guish between dredging accuracy and positioning accu-racy. The second was the spillage, which can directlyinfluence the top layer quality of the dredged location.And third and last was the turbidity, because this is themain phenomenon for transporting contaminated parti-cles outside the dredging area. The three mentionedcriteria were closely monitored in relation to the achiev-ed production and transport concentration.

RESULTS OF THE SWEEP DREDGER IN THE

KETELMEER

In accordance with the other three trials, the sweepdredger was tested using several scenarios. A thinlayer (20 cm) and a relatively thick layer (up to 60 cm)were dredged. The emphasis was set on the removalof the actual contaminated layer (average thickness oflayer 40 cm). The major part of the acquired data is stillunder investigation, but the preliminary results showthat the design of the sweep dredger is suitable foraccurate removal of thin layers in the Ketelmeer. A slight increase in measured accuracy, spillage, and


18

continued on page 25

Stefaan Vandycke

New Developments inEnvironmental Dredging: From Scoop to Sweep Dredge

Mr J. Claessens, Flemish Region, Department of theEnvironment and Infrastructure, Maritime ScheldtDepartment, Antwerpen, Belgium for their contribu-tions to this paper.

Introduction

Access channels to locks are the major areas of siltationwithin the Scheldt estuary and, as such, are among thekey areas where maintenance dredging takes place.Until recently, this was successfully achieved with thehelp of sweep beams. This dredging method meets thebasic objectives in that it guarantees a safe navigationdepth and does so at the lowest possible cost.

THE SCOOP DREDGE ®

Recent environmental concerns, however, have meantthat the quantity of fine-grained contaminated materialin the estuary should be reduced, as well as thespreading of the contaminated fraction, throughout theestuary.A new policy for maintenance dredging is now requiredto meet the following conditions:– avoid, as much as possible, the resuspension of fine-

grained material once it has settled;– remove large volumes of fine-grained material from

the estuary;– store the excavated material within restricted boun-

daries, preferably isolated from the surroundings toavoid leakage and diffusion of contaminants;

– limit the volume of material on the disposal site; and– operate in an economically acceptable way.

The first alternative for the sweeping devices relies ontraditional equipment, such as the cutter suction dred-ge and/or the trailing suction hopper dredge. However, in respect of the new criteria (see Table I)each has both advantages and disadvantages.

With this in mind, it was decided to develop a new typeof dredge combining, as much as possible, the advan-

Abstract

The Flemish Community is continuously looking toimprove maintenance dredging works through carefulselection of dredging and disposal locations andthrough the use of more efficient dredging equipment.Most sediment pollution consist of fine grained sedi-ments. These sediments tend to settle in the entrancechannels to sealocks, in harbour areas or other water-ways with low currents. Therefore, special attentionhas to be paid to maintenance dredging procedures inthese areas, in order to reduce both the economic andecological impact of the removal of contaminated finegrained sediment.

To meet these environmental requirements and toachieve a cost-effective solution, a new type of dredgehas been developed in co-operation with the FlemishInstitute for Scientific-Technological Research Promo-tion in the Industry (IWT). This scoop dredge has beensuccessfully tested in the access channel to the KalloLock and in the adjacent harbour area. Based on thisexperience and on environmental dragheads of trailingsuction hopper dredges, further breakthrough in envi-ronmental dredging has been achieved. The sweepdredge combines the advantages of stationary dredgesand trailers. The sweep dredge is able to work in shal-low water and to remove very thin layers of pollutedfine-grained sediments at high concentrations. The principles of both dredges have been patentedworldwide.

This paper will discuss:– The criteria for dredging and removal of contami-

nated fine-grained sediments in an economic andecologically acceptable way.

– The development of the scoop dredge, the results ofthe dredging operations and the various test andmonitoring programmes.

– The development of the sweep dredge and theevaluation of its characteristics.

The author wishes to acknowledge Mr P. Standaert ofDredging International n.v., Zwijndrecht, Belgium and

New Developments in Environmental Dredging: From Scoop to Sweep Dredge

19

tages of both cutter and trailer types. It was decidedthat:– a stationary dredge is better suited to dredge on the

limited area of the access channel;– rehandling of the material (dredging into the hopper

and on-shore for reclamation) has to be avoided;– reclamation to a well-designed disposal or treatment

area has to be the basic sollution;– rotating devices are not to be used for fear of

creating turbidity;– the silt has to enter the dredge as far as is possible,

at in-situ density; and– accurate horizontal and vertical positioning during

dredging is essential.

These goals can be achieved by the combination ofcharacteristics taken from the draghead of a trailinghopper suction dredge (low turbidity, high density andwith no rotating device) and the movement and pump-ing characteristics of a cutter suction dredger (accuratehorizontal and vertical positioning; no-rehandling andreclamation through a closed pipeline).

To implement this, the stationary dredge Brabo wasconverted into a scoop dredge. The Brabo has a totalinstalled power of 6 865 hp and overall dimensions of88 x 15 x 4 m, with a draught of 2.65 m. It is equippedwith one ladder pump and two delivery pumps. Allmovements for dredging and walking are controlled bytwo walking spuds and two side anchors.

THE SCOOP-HEAD ®

A two-sided functional draghead was mounted to allowdredging in two opposing swing directions. This wasachieved in practice by the use of a turning blade thatscrapes the material from the water bottom into thesuction head of the dredge. At the end of the swing,the blade is turned in the opposite direction, the dredgewalks forward (between 1.5 and 2.5 m) and the drag-head continues scraping in the opposite direction. It isbecause of this functional scraping that the dragheadhas been named the “scoop-head” (Figure 1).

The outer casing prevents dilution with water and thecreation of turbidity in the surrounding water. On top ofthe scoop-head, an adjustable water inlet system hasbeen installed to prevent clogging and to ensure aregular feed of silt into the scoop dredge. Since thisdredge can work at depths varying from 3 to 28 m, theangle of the scoop-head to the ladder is adjustable withthe help of a support frame.

The presence of gas bubbles in the dredged materialprovokes cavitation of the ladder pump and reducesthe performance of the ladder and delivery pumps.Therefore, a specially designed degasification systemwas added to the basic scoop dredge which draws off


20

Upon acceptance of the IADC Award, Stefaan Vandycke (left)shakes hands with Mr Peter Hamburger, IADC SecretaryGeneral. Engineer H. Smitz, Chairman of the PaperCommittee of the International Harbour Congress looks on.

IADC Award 1996Presented during the 11th International Harbour CongressAntwerp, BelgiumJune 17-21, 1996

At the 11th International Harbour Congress organ-ised by the Royal Flemish Society of Engineers inAntwerp, Belgium in June 1996, Mr Stefaan Vandycke was presented the annualIADC Award by Secretary General of the IADC Mr Peter Hamburger. Mr Vandycke graduated fromthe University of Leuven in 1988, and has worked atPauwels Industrial, De Cloedt, Tai Ho J.V., and ispresently Superintendent Benelux at the head-quarters of Dredging International n.v., Zwijndrecht,Belgium. Each year at a selected conference the IADC grantsan award to a paper written by a young author. The Paper Committee of the conference is asked torecommend an author who must be under 35 years ofage and whose paper makes a significant contributionto the literature on dredging and related fields. Thepurpose of the award is “to stimulate the promotionof new ideas and encourage yourger men and womenin the dredging industry”. The IADC Award consistsof US$ 1,000, a certificate of recognition and publica-tion in Terra et Aqua.

Table I. Comparison of advantages and disadvantages of CSD and TSHD.

CRITERIA CUTTER SUCTION DREDGE TRAILING HOPPER

SUCTION DREDGE

RESUSPENSION IN DREDGING HIGH RESUSPENSION LOW RESUSPENSION AREA WHEN OVERFLOW IS OMITTED

EXCAVATION FROM THE GOOD GOODESTUARY

RESTRICTED STORAGE POSSIBLE POSSIBLE

ADDITION OF WATER FOR HIGH LOWTRANSPORT

COST REASONABLE HIGHER THAN A CUTTER

– monitor the performance of the dredging processand make suggestions for immediate and long-termimprovements to equipment and methodology; and

– check the environmental performance of the overallsystem.

During the project, 550 000 tons of dry material wereremoved, representing about 1.1 m m3 of in-situ mud,from the access channel. Productivity was 695 tons ofdry material per operational hour or at an average in-situdensity of 1.3 tons per cubic metre, 1 400 m3 an hourfor a pumping distance of 7 kms.

The performance of the dredging process was moni-tored by the continuous measurement of more thanten important parameters, such as concentration,velocity, swing speed, pulling force, pumping depth,and so on. From this monitoring programme, it wasconcluded that the dredging process can perform atalmost the same level as a normal draghead, but thatthe concentration decreases during anchor relocationactivities, during spud relocation, when the width of thecut is variable, and during the start up and slow downof the process.

The reduction of these idle times will be of majorimportance for the overall efficiency of the scoopdredge.

As far as an environmental performance is concerned,the efforts were focussed on the evaluation of theturbidity generated at the dredging site. During themonitoring programme, attention was paid both to theshort and medium term (48 hours) increase in turbidity(Figure 2).

During the control measurements, the backgroundturbidity was 22-24 mg/l at the surface, increasing to40 mg/l at the bottom. The influence of the dredgingactivities could not be measured at a distance of 50 m

gas and some silt just before the ladder pump. Thismixture is collected in a vacuum tank from where thesilt is pumped back into the suction pipe. A vacuum ismaintained in the vacuum tank by means of waterjetsand a set of venturi nozzles.

EVALUATION OF THE SCOOP DREDGE

A pilot project was set up from January to April 1993.During this time the new method was used for themaintenance of the whole access channel of the KalloLock on behalf of the Maritime Scheldt Department(Flemish Community).

Some 500 000 dry tons of silt had to be extracted withthe scoop dredge and hydraulically transported over adistance of three to seven kilometres through a closedpipeline to the diffuser pontoon Demer. This disposedthe silt in underwater pits in the Antwerp WaaslandHarbour without disturbing the dock water. Taking into account the fact that from the originalexcavation of the channel (completed in 1982) until1990, the normal maintenance dredging works werelimited to only the actual navigation channel, the char-acteristic of the silt to be excavated was very variable,from almost liquid mud in the navigation channel toalmost firm material in the lower layers at the edges ofthe channel where almost 15 m of mud had accumula-ted. As a consequence, the dredging cuts were ratherirregular.

In the centre of the area, the navigation channel had toremain open to shipping for most of the time, resultingin a great deal of idle dredge time whilst the floatingpipeline was opened and closed for navigational pur-poses.The pilot project was extensively monitored to:– determine the productivity and the quantities of silt

removed;


21

from the dredge, but in the immediate surroundings ofthe scoop-head (a distance of 5 to 10 m) a turbidityincrease of 2 to 5 mg/l was measured representingless than 10 per cent of the background turbidity.

Prior to a second maintenance programme in spring1994 a dredge pipeline of about 1 000 metres had beendrilled under the access channel by using directionaldrilling technology. This pipeline crossing avoids idletime due to stops for navigation. As expected, the


22

Figure 1. The scoop-head (general lay-out).

Figure 2. The scoop-head has no rotating cutting devices, so turbidity and addition of transport water is minimised.

LONGITUDINAL SECTION CROSS SECTION

CROSS SECTION BLADE RIGHT

5. turning blade

6-7. water inlet

8. fixed inner casing

9. fixed outer casing

10. flange

11. wearing ring

12. hinge joint

operating efficiency and performance improved consi-derably.

In this second programme another 400 000 tons of silt(800 000 m3) were removed.

To evaluate better the environmental performance ofthe scoop dredge a second test location was estab-lished in the docks itself. Indeed, the daily siltation of1.5 cm in the access channels made it impossible toevaluate the accuracy of the dredge.

These tests (April 1994) were attended to by the Dutch Hydraulics Laboratory of Delft and University ofLeuven. The results were very encouraging whenmeasured against the set criteria: – accuracy (vertical) better than 10 cm at 15-19 m

water depth;– mean turbidity increase less than 10 mg/l near the

dredge;– spillage was between 3-7 cm;– safety good, because no personal contact with

hydraulically pumped material;– no stops for debris.

During the next phase of silt removal in the accesschannel with the scoop dredge, further attention will bepaid to minimise the waterflow in the 7 km long dis-charge pipeline to reduce the pumped volume.

THE SWEEP DREDGE ®

The existing cutter dredge Vlaanderen XV has beenrebuild to become a sweep dredge. The most impor-tant change was the removal of the cutter and theinstallation of the sweep-head (Figure 3). Further thedredge was equipped with a purpose built silt degasifi-cation system.The Vlaanderen XV has a total installed power of2 550 kW and overall dimensions of 52 m x 11 m witha draught of 1.75 m. The maximum cutwidth equals80 m and the maximum swingspeed equals 50 m/min.The principal characteristics of the sweep-head can bededuced from Figure 3.A visor rather than a turning blade (scoop dredge) visormake it possible to operate in the two opposite swingdirections. The same visor makes it possible to adjustthe cutheight. The transport of the dredged material ishydraulic.


23

Figure 3. The sweep-head (general lay-out).

A buffer system is planned just behind the dredge. Thismakes it possible to slow down the suction (dredging)process without interrupting the reclaiming process. In this way the buffer system will prevent water fromhaving to be pumped to clean the dredge pipes beforestopping the dredge process.

FROM SCOOP TO SWEEP DREDGE

However the scoop dredge has its limitations. Sincethe height of the dredge head opening is not adjust-able, a great deal of water will be pumped when dredg-ing thin layers. In addition the scoop dredge Brabo hasa draught of 2.65 m and is thus more suited for applica-tions in deeper water.

To improve the performance of the scoop-head forclean-up dredging, attention should be paid to:– improving the removal of thin layers without exces-

sive water addition during dredging;– controlling the addition of transport water;– reducing the minimal dredging depths;– tuning in the dredged and pumped volumes;– optimising the process steering (even computerising

it);– visualising and storing of all process parameters;– implementing the latest technology developed in

recent years with trailing suction hopper dredges(Antigoon, Pearl River) in silt to cutter suction dredges.

For the development of the new scoop dredge, nowreferred to as the sweep dredge, the following con-ditions have been set out:– the concept of the dredge head is based on the

known current characteristics of a classic trailerhead;– the dredge head can be positioned with high accu-

racy, horizontally as well as vertically;– turbidity generation is minimal;– the dredge can work in low water depth;– spillage is minimal;– the thickness of the dredged layer is adjustable

between 20 and 60 cm;– a degasification system must be installed;– addition of transport water is adjustable;– the sweep-head is cutting mechanically avoiding

(hydraulic) erosion;– pump suction discharge is always adjustable;– the suction (dredging) process can be separated

from the reclaiming process.

Based on the above-mentioned requirements a classicstationary dredge equipped with a spud carrier anddredging by swinging has again been chosen. In thisway a controllable and steerable positioning of thedredge head is possible.

Furthermore it is possible to control and steer thedredged volumes (layer thickness and swing speed).

In November 1995, the new sweep dredgeVlaanderen XV started its first trials on the Brussels SeaCanal at Hingene (Figure 4). Maintenance dredgingworks are being carried out for N.V. Zeekanaal (FlemishCommunity).The test programme involves the evaluation of swing-speed, production, automatisation, and such, and thespecific environmental aspects concerning clean-updredging, such as turbidity generation, spillage, accuracyand selectivity.

The results were very good. From March 1996 throughMay 1996 the sweep dredge did the maintenancedredging works in the Port of Nieuwpoort. In June thesweep dredge moved to the Ketelmeer in The Nether-lands (Figure 5), where very elaborate tests wereexecuted for the Dutch Government (see page 18).Because of the good performance of the sweep dred-ge, the maintenance works in the Port of Nieuwpoortwere resumed with the same dredging equipment.

PROCESS STEERING

During the dredging of polluted silt one has to take intoaccount many parameters, e.g. in-situ silt levels, watercontent and silt density, generated turbidity, desiredconcentration, production and accuracy and so on. Inorder to optimise this process only a highly automatedprocess steering system can handle the operation. The dredge was for that reason equipped with acompletely in-house developed computer-controlleddredge operator. All data are stored and visualised on ascreen during the process (Figure 6).

During the dredging operation no manual intervention isneeded any more. For example, the opening of thesweep-head is automatically adapted as a function ofthe thickness of the silt layer which is measured on-lineduring dredging.


24

Figure 4. The sweep dredge during the test programme at Hingene.

Figure 5. Sweep dredge during trials on the Ketelmeer.

other traditional dredging equipment, such as cuttersand trailers. The sweep dredge is in fact the combina-tion of the latest in-house developed technology both inautomatisation, for instance, process control, and intrailing dredge heads for fine sediments.

References

Standaert, P., Claessens, J., Marain, J. and Smits, J. “The scoop dredge, a new concept for silt removal”.

CEDA Dredging Days 1993 - D2.

Conclusions

With the development of the scoop dredge and thesweep dredge, two important new tools have beencreated for the removal of (polluted) soft fine-grainedmaterials from waterways.

The scoop dredge is very well suited to the removal ofthick layers of silty sediments even at very great depths(4-25 m). The sweep dredge is suited for depths rang-ing from 3-14 m and can remove selectively in layersup to 20 cm.Both dredges do so with a minimum of environmentaldisturbance and at a competitive cost compared with


25

Figure 6 : Computer controlled dredging operations.

turbidity is partly compensated by higher productionfigures. As with the other trials the three weeks dredg-ing in the Ketelmeer proved not only to be valuable tothe initiator (Rijkswaterstaat), but also the dredgingcontractors gained vauable information which willenable them to optimise the tested dredging equip-ment for future projects.

Conclusion

The final report (in Dutch) describing the four comple-

ted tests is scheduled to be ready in the beginning of1997. An English language version can be made avail-able depending on the demand. A tremendous effortwas put into the trials by all the participants, especiallythe Survey department of the Directorate IJsselmeerand the crews of the dredgers. For further informationon the dredging trials queries can be directed to:Thomas Arts, Rijkswaterstaat USW, Postbus 2000,3502 LA Utrecht, The Netherlands, tel. +31 30 285 7889 and/or Bert Kappe, Rijkswater-staat RDIJ, Postbus 600, 8200 AP Lelystad, The Netherlands +31 320 297 480.

continued from page 18

The Sweep Dredge: High Accuracy Dredging Trials Continue

Charles W. Hummer, Jr.

Books/PeriodicalsReviewedDredging, Remediation, and Containment ofContaminated Sediments.ASTM Publication STP-1293. Philadelphia, PA, USA.1995. 352 pp. Hardcover, with illustrations and referen-ces. US$ 95.

R. Demars, Gregory N. Richardson, Raymond N. Yong and Ronald C. Chaney (Editors)

This publication contains twenty-two of the peer-reviewed technical papers presented at the symposiumof the same name held in Montreal, Quebec, Canadaon June 23-24 1994. The symposium was sponsoredby ASTM Committee D-18 on Soil and Rock, in co-operation with Environment Canada and the U.S. En-vironmental Protection Agency.

An excerpt from the introduction prepared by theeditors gives an excellent concise statement of thedifficulties and technological challenges facing the fieldof contaminated sediments which the symposiumaimed to address:

While the development of these [sediment qualitycriteria] was expected to have a positive effect onhuman health and well-being, they have significantlyslowed the process and increased the cost of dredgingclean sediments from the nation’s waterways. Thus there is a need to develop new standards relatedto dredging or to modify existing standards in an effortto make the dredging of clean sediments and theremediation process for contaminated sediments moreefficient. The primary objective of this symposium was to iden-tify test, methods, procedures and materials, used insupport of dredging, treatment and containment ofcontaminated sediments, that are in need of standard-isation.

The papers selected for presentation attempt to meetthis ambitious objective and are grouped into fivesubject areas:– Sediment Characterisation (four papers)

– Dredging Transport and Handling (four papers)– Restoration and Remediation (four papers)– Containment and Isolation (six papers)– Management Strategies (four papers)

The editors have also prepared and included an over-view which summarises the papers and is, in and ofitself, a fine concise statement of the state-of-the-artrelative to contaminated sediments and the subjectareas covered in the symposium. The conclusionsdeveloped as a part of this overview are particularlyvaluable and could well serve as a basis for an inter-national action plan.

The initial paper by Raymond N. Yong, “The Fate ofToxic Pollutants in Contaminated Sediments”, is fairlydetailed and complex in its entirety, but presents someuseful conceptual descriptions of the contaminationproblems. For those who wish to delve more deeplyinto the physical chemistry and more esoteric aspectsof contamination, the paper does a good job of presen-ting a view of the problem.

Some of the papers use case studies as a basis ofreporting successes, failures and findings related tospecific situations that ultimately serve as the databasefor developing generic solutions to the challenges.There is a good representation of worldwide experien-ces, particularly from Canada, the United States andJapan, and there are several papers from Europeanscientists.

Some of the case study-type papers stress the sameconcepts of the need for precision location of contami-nants in the field and the subsequent need for preci-sion dredging of these contaminated areas, the accu-rate positioning of equipment and often the need tohave some treatment of the effluent. Other papersdescribe experiences with various isolation techniquesand technologies. Of particular interest is a paper byRichardson et al., that discusses the contaminantdischarge pathways from confined disposal facilities.


26

developed to identify the best technologies for dealingwith contaminated sediment, the resulting software isa tool which also:– identifies suitable technologies for site-specific needs;– provides case studies for projects;– lists contacts for technology auditors; – lists project funding agencies; and– lists technology vendors/manufacturers worldwide.

Detailed and comprehensive information about 250removal and treatment technologies are presented, anumber of which have been successfully tested inCanadian and international commercial projects. Audit results and data from international programmes,including 29 Cleanup Fund demonstrations are includ-ed. The results show:– technology performance;– target contaminants and reduction values;– project descriptions with colour pictures and sche-

matic diagrams of technology.

The programme is set up so that the user can:– search for suitable technologies based on site con-

ditions, unit cost, efficiency and category.– find detailed data from case studies including

description and scale of project, technologies used,contaminants present, funding agencies and projectpartners; and

– locate auditors, developers and vendors with withinformation on country of origin, stage of develop-ment, regulatory approvals, and environmentalconcerns.

Typical users include all levels of government agencies;engineering and planning contractors; ports and har-bours; technology industries; environmental groups;acadamic institutions; researchers and consultants. The programme offers easy access to best tech-nologies and professionals in the field, access to information for those with specialised needs; andprovides a resource for vendors wishing to evaluatecompetition and create links.

System requirements are: IBM PC or compatible,Microsoft Windows Operating System (Versions 3.1 orWindows ’95), 8 MB of RAM, Hard disk with 20 MB offree space of 5 MB Hard disk space and CD Rom VGAor higher resolution video.

Reference is made to relevant published literature andupdates listing supplementary information and relatedsupport technologies will be available.

For further information please contact:Ian Orchard, Environment Canada4905 Dufferin Street,Downsview, Ontario, Canada M3H 5T4tel. +1 416 739 5879, fax: +1 416 739 4342email: [email protected]

Another paper of special note is entitled, “Strategiesfor Management of Contaminated Sediments”, byPalermo and Miller. The paper summarises some ofthe background, alternatives for disposal and remedia-tion, management strategies, plans for the future andalso has some useful conclusions. The framework forthe evaluation of dredged material alternatives, on asite-specific basis, is a tiered approach which includes: – no action, – contain in-place, – treat in-place, – remove and contain, and – remove and treat.

The concept of managing contaminated sediments as asolution, offers some optimism for the future withineconomic and environmental realities and has foundacceptance in the international fora.

For anyone involved in any aspect of the dredging ofcontaminated sediments, this compendium of sympo-sium papers is beneficial material to read and use as areference source in the future. The cost is reasonablewhen one considers the intrinsic value of the materialpresented. Obviously, there will be some papers thatwill find more utility than others, but as a package, thisis an excellent publication.

This book can be obtained from:ASTM Customer Service100 Barr Harbor DriveWest Conshohocken, Pennsylvania 19428-2959USA or,

ASTM European Office27-29 Knowl PieceWilbury Way, Hitchen, Herts SG4 OSXUnited Kingdom

SEDTEC.Environment Canada. Ontario, Canada. July 1996.IBM PC or compatible. Microsoft Windows OperatingSystem. C$ 150 plus shipping and handling.

Great Lakes 2000 Cleanup Fund

SEDTEC, Sediment Technology Directory, is a compu-ter software product listing worldwide technologies forthe removal and treatment of contaminated sediment.The user-friendly programme was developed by Envi-ronment Canada’s Great Lakes 2000 Cleanup Fund asa commercial initiative in response to an increasingnumber of requests for information from the privatesector, governments, academia and technology manu-facturers and vendors.

The directory is multipurpose, with powerful searchoptions compiled from information submitted by ven-dors and developers internationally. Though originally

Books/Periodicals Reviewed

27

of the concepts. But these are buttressed with goodpractical descriptions of applications and practise. newconcepts such as geotextiles, artificial seaweed, andother novel approaches such as gabions are presented.The use of process or flow diagrams for presentation ofthe various approaches was effective and made theconcepts more understandable even to one who is notfully familiar with the technologies involved.

In summary, the book effectively presents the subjectmatter and is a valuable reference work for either thedesign engineer or one who has a more programmaticpurpose in acquiring knowledge on the subject. Certainly, the book would be a must for the seriousmarine design or consultancy firms, if for no otherreason than it captures the state of the art in addition tothe theoretical aspects of the subject matter.

The book may be obtained from:A.A. Balkema Uitgevers bvPostbus 1675, NL-3000 BR Rotterdam,The Netherlands fax +31 10 413 5947 or,

A.A. Balkema PublishersOld Post RaodBrookfield, Vermont 05036 USAfax +1 802 276 3837

An Assessment of Techniques for Removing OffshoreStructuresNational Academy Press, Washington, DC. 1996.

Marine Board, Commission on Engineering and Technical Systems, National Research Council.

This report resulted from the Congressional GeneralAccounting Office review of the procedures employedor allowed by the Mineral Management Service (MMS)of the US government in the removal of offshore struc-tures that questioned some of the procedures. TheMMS then requested the National Academy to renderan objective review of its procedures. Specifically, theMarine Board was asked to review:– platform removal technology, including the costs of

alternative techniques;– appraise and examine innovative technologies and

techniques under development for the removal ofsuch structures;

– assess the occupational and environmental hazardsof explosive and alternative removal techniques, andto identify ways to mitigate the identified hazards.

The magnitude of the problem is described as there arenearly 3,000 platforms that populate the US federalouter continental shelf. Lease operators may removeplatforms when the costs of operating and maintainingstructures exceed revenues or when structures areobsolete or damaged. The options for disposal include

Offshore Breakwaters and Shore ControlA.A.Balkema, Rotterdam, The Netherlands. 1996. 560pp. Illustrated, index and references. NLG 225.

Krystian W. Pilarczyk and Ryszard B. Zeidler

This is a comprehensive technical engineering refer-ence on literally all aspects of the subject matter, thatis, offshore or detached breakwaters. The book isorganised in a logical sequence and the table of con-tents is detailed encough to be able to discern and referto discrete subject matter with ease. The book is divi-ded into ten chapters, three appendices and has anextensive list of references, useful addresses and adetailed index. The book is illustrated with cleardrawings and tables. The chapter headings are:1. Introduction2. Coastal Boundary Conditions3. Sediment Transport and Shore Profile Evolution4. Waves at Structures5. Functional and Geometric Design6. Structural Design7. Materials8. Construction Aspects9. Monitoring, Maintenance and Economics10. Worldwide Experience and Case Studies

The introductory chapter is well written and clearlyoutlines the presentation of the subject and the mainconcepts involved. From the initial introductory chapterfollows in good order the k owledge of site topography,waves, currents and other boundary conditions inChapter 2. Sediment transport patterns and quantitativecharacteristics are then necessary to determine thesand budget, erosion and/or accretion in Chapter 3.Chapter 4 deals with various wave transformationmodes at the structure and the control of both thestructure’s stability and the evolution of the beach filland the shore profile. Chapter 5 examines the far-fieldprocesses affecting the functinal design and the struc-ture’s layout and overall dimensions. Chapter 6 consi-ders the details of the structures components such asstability of stone layers in the breakwater or armoring.Chapters 7 through 9 progress to the consideration ofmaterials, construction technologies, maintenance,cost, monitoring and other aspects. Finally, examplesand lessons from various countries (Belgium, Bulgaria,Italy, Japan, Romania, Russia, Spain, the UK and theUS) give some practical applications and perspective tothe theories and concepts presented. Likewise, casestudies from Malaysia, the United Kingdom, the UnitedStates, Sri Lanka, Spain, Italy and The Netherlandsfurther add the realism of actual situations encounteredby the coastal engineers.

The book is well written and relatively easy to readwhen one considers the complex technical mattersdiscussed and presented in narrative and graphicalform. Theoretical design concepts are presented withsubstantial explanation of the physics and mathematics


28

– Explosives are an economical and reliable tool forremoving most structures, especially structureslocated in deep water.

– The requirement that structures be removed to adepth of a least 15 feet below the mudline is adisincentive to the development and use of non-explosive techniques and advance techniques usingsmaller explosive charges.

– The National Marine Fisheries Service ObserverProgram has significantly improved understanding ofthe effects of platform removals on sea turtles andmarine mammals. However, the effects of explosiveremovals on populations of fish that frequentlyreside near platforms are not well understood.

– The simplest means of blast mitigation are unlikelyto reduce significantly fish killed from explosiveremoval operations.

– Devices to scare fish away from platforms duringexplosive activity are not currently applicable for usein open ocean water.

– Limiting the number of near-simultaneous explo-sions to eight and limiting the weight of individualcharges to 40 pounds may have undesirable effects.

– Nonlethal effects of explosive removals on livingmarine resources on survival factors like productiveperformance or predator avoidance are not known. If species found near platforms represent the speci-fic year classes or are unique components of thereef ecosystem, the impact could be significant.

– Leaving platforms in place, partially removing them,toppling them in place, or using them for artificalreefs are options that are economically and environ-mentally attractive to many ocean users groups.Transport costs, concerns about liability and regulato-ry issues now limit their use.

The committee made a series of eleven recommenda-tions on the basis of the above conclusions. Five for theMMS, two for the NMFS, three for the offshore oil andgas industry and one specifically for the state agenciesin cooperation with the federal agencies and offshoreindustry.

In view of the implications of this issue on the domes-tic and international scene, this report is probably goodreading and reference material for the marine contrac-tor and consultancies.

complete removal with disposal ashore, placement inan approved ocean disposal site, conversion to a fishingreef, or removal for refurbishing and replacementelsewhere. The pace of platform removal continues toaccelerate with more than 100 removed from service ineach of the last several years (181 were removed in1993 alone). Explosives are routinely used in theremoval process.

As is the nature of Marine Board studies, they relied ona committee of experts on offshore civil engineering,geotechnical engineering, marine construction, under-water blast effects and mitigation, technical assess-ment, biology, ecology and management of livingmarine resources. The points of view of the offshore oiland gas industry and associated service industrieswere represented on the committee, as were theviews of scientists involved in research on the specificliving marine resources that may be affected by explo-sives used to remove offshore structures.

The report presents a synthesis of information gathe-red by the committee. It is intended to be used as aguide for the MMS form making decisions about regu-lations governing the removal of offshore structuresand about strategies for encouraging the use of tech-niques that will decrease damage to the environmentand to living marine resources.

– Chapter 1 is an overview of the status of platformsat the present time and the regulations governingremovals.

– Chapter 2 gives an assessment of alternative cuttingtechniques.

– Chapter 3 discusses technical considerations rele-vant to the selection of particular removal methods.

– Chapter 4 presents the environmental effects ofcurrent removal technologies.

– Chapter 5 is a comparative summary of the costsand benefits of alternative approaches to removals.

– Chapter 6 presents the major conclusions andrecommendations that follow from the findings ofthe investigation.

– Appendices contain additional background informa-tion, a list of individuals who made presentations tothe committee and biographies of committee mem-bers.

The matter of removal of offshore structures and speci-fically platforms has clear international implications. Anydomestic review by a major player in the field shouldbe of interest to the marine construction and oil indust-ry world wide. This report has a number of conclusionsthat interested parties should be aware of, including:– Regulations governing the removal of offshore

structures need to be sufficiently flexible to accom-modate the complex requirements of a wide varietyof structures, a spectrum of marine life and varioususers in the Gulf of Mexico.

Books/Periodicals Reviewed

29

Seminars/Conferences/EventsMaritime Vietnam ’97

International Exhibition & Convention CentreHo Chi Minh City, Vietnam

April 16-18 1997

Reflecting the emergence of Vietnam as an importantmarket and coupled with her becoming the seventhmember of ASEAN, Maritime Vietnam 97 is co-organ-ised by the Vietnam Chamber of Commerce and Indus-try (VCCI) and is supported by the Vietnam NationalMaritime Bureau. Following up on the success of the1996 event, investments are being made Vietnam inthe development of port facilities, deep seaports,export processing zones (EPZ), ship repair, dredging,ship and marine equipment and so on. The VietnamMaritime & Inland Shipping Exhibition incorporatesVietnam Port ’97. The 2nd Asian and Australasian Portsand Harbours Conference, organised by the EADA inassociation with CEDA, IADC and Vinamarine, will beheld during the Exhibition (see page 31).

For further information contact:RAI Exhibitions Singapore Pte Ltd1 Maritime Square, #09-01World Trade Centre, Singapore 099253tel. +65 272 2250, fax +65 272 6744

Amsterdam RAI, P.O. Box 777771070 MS Amsterdam, The Netherlandstel. +31 (20) 549 1212, fax +31 (20) 646 4469

Marine IndonesiaJakarta International Exhibition Centre

Kemayoran, Jakarta, IndonesiaApril 23-26 1997

The expansion and development of Indonesia’s marineindustry is now critical. With increases in containertraffic and recent deregulation to attract massive pri-vate investment the time is ripe for expansion of exis-ting port facilities and the development of port-relatedinfrastructure projects. For these reasons, MarineIndonesia will attract an international audience both asmajor exhibitors and as visitors.

For further information contact:PT Pamerindo Buana AbadiBank Bumi Daya Plaza, Unit 2102, 21st FloorJl Imam Bonjol 61, Jakarta 10310, Indonesiatel. +62 21 325560, fax +62 21 330406


30

CEDA Dredging DayRAI International Exhibition and Convention Centre

Amsterdam, The NetherlandsNovember 20 1997

This is the first announcement of the “CEDA DredgingDay”. Continuing a tradition started in 1980, it will takeplace during the Europort Exhibition ’97 week inAmsterdam.

This year’s theme is “New Developments in DredgingEquipment and Technology”. The emphasis will beupon innovative solutions which achieve higher effi-ciency, increased accuracy, cost-effectiveness andenvironmental compliance.

Presentations, delivered by highly qualified invitedspeakers, with long-standing international experience,will cover various aspects of dredging with specialemphasis on the improved results achieved throughthe use of innovative equipment and techniques. AnInternational Technical Paper Committee appointed byCEDA will invite authors to present and publish theirpapers. A panel discussion will follow the presenta-tions.

The Annual General Meeting of CEDA will be held afterthe conference, but the conference itself is open toanyone concerned with dredging.

Please address all enquiries to:Secretariat of CEDA, PO Box 3168,2601 DD Delft, The Netherlandstel. +31 15 278 3145, fax +31 15 278 7104email: [email protected]

nated sediments. Subjects will include, national policiesand strategies; site investigations; the fate of contami-nants; physico-chemical analysis; risks and qualitycriteria; ecological effects, management and control;treatment technologies (in situ and ex situ); disposal ofdredged material; beneficial uses of dredged material;and source control strategies.

Abstracts are strictly limited on one A4 and shouldinclude: the title of presentation; name(s) of author(s);full address of presenting author; and theme.Abstracts must be written in English and five copiesshould be sent to the Conference Secretariat as soonas possible.

Authors will be notified of the Committee’s decision byFebruary 1, 1997. The deadline for submission of thefull paper will be May 1, 1997.

For further information please contact the conferencesecretariat:Van Namen & WesterlakenCongress Organization ServicesP.O. Box 1558, 6501 BN Nijmegen, The Netherlandstel. +31 24 323 4471, fax +31 24 360 1159

Offshore Europe ’97Aberdeen Exhibition and Conference Centre

Aberdeen, ScotlandSeptember 9-12 1997

This Oil and Gas exhibition and conference is organisedby the Offshore Europe Partnership – a partnershipbetween Spearhead Exhibitions Ltd and the Society ofPetroleum Engineers. The theme of the conference is”Continuous Change – Learning from the 21st Centu-ry”. Technical papers and panel discussions are plan-ned on: Time to Market; Commissioning and Decom-missioning; Exploration: Deeper Water, DeeperTargets; Innovative Technology; Reservoir Manage-ment; Beyond CRINE.

Abstracts are welcomed before January 13, 1997.

For further information contact:Val Johnston-Jones, Society of Petroleum Engineers4 Mandeville Place, London W1M 5LA, UKtel. +44 171 487 4250, fax +44 171 487 4229email: [email protected]

Exhibitors include manufacturers or vendors of hard-ware or providers of services relevant to the technicaland operating management of offshore oil and gasexploration and production companies, drilling contrac-tors, and offshore engineering contractors.

Call for Papers2nd Asian and Australasian Ports and HarboursConference

International Exhibition and Convention CentreHo Chi Minh City, Vietnam

April 16-18 1997

This is the first announcement of a conference inVietnam, organised by the Eastern Dredging Associa-tion (EADA) in association with the Central DredgingAssociation (CEDA), the International Association ofDredging Companies (IADC) and Vinamarine. The conference is being held in coordination with theexhibition Maritime Vietnam ’97.

The theme of the conference is “Developing andMaintaining Operational Ports and Harbours into the21st Century”. Suggested subjects for papers include,but are not limited to: – dredging operations in port and harbour areas;– handling of dredged material, including beneficial

uses;– studies and surveys, such as site investigations,

hydraulic studies and hydrographic surveying;– environmental considerations, such as reduction of

the impact of dredging and special equipment andstrategies;

– design and maintenance of marine structures.

Authors should submit the title and three copies of theabstract (max. 300 words or one A4) no later thanJanuary 6 1997. Authors selected will be advisedpromptly. The completed manuscript should bebetween 3000-5000 words and be submitted no laterthan March 7 1997.

Please send abstracts to:Mr J.F. DobsonSecretary EADA Technical Paper CommitteeGPO Box 1818, Brisbane, Q4001, Australia tel. +61 7 3258 4746, fax +61 7 3258 4704, or

Dr A. Csiti, Manager, CEDAPO Box 3168, 2601 DD Delft, The Netherlandstel. +31 15 278 3145, fax +31 15 278 7104

International Conference on Contaminated SedimentsCongress Centre “De Doelen”

Rotterdam, The NetherlandsSeptember 7-11 1997

This is an announcement and call for abstracts for theInternational Conference on Contaminated Sediments.ICCS is an excellent opportunity for governments, portauthorities, the industry and academia to present anddiscuss their problems with and solutions for contami-

Seminars/Conferences/Events

31


32

Day 1: Why Dredging?The Need for Dredging/Project Phasing

Day 2: What is Dredging?Dredging Equipment/Survey Systems

Day 3: How Dredging?Dredging Projects

Day 4: Preparation of DredgingContract

Day 5: Cost/Pricing and Contracts

Representatives of port authorities, companies, andindividuals interested in attending are requested tocomplete the preliminary registration form below assoon as possible and prior to December 31 1996, andreturn to:

IADC Secretariat, Duinweg 21,2585 JV The Hague, The Netherlandstel. 31 (0)70 352 3334, fax 31 (0)70 351 2654telex 31102 (dune nl)

Place: SingaporeDate: February 17-21, 1997Venue: Boulevard Hotel

In cooperation with the National University of Singapore(NUS) and the Applied Research Corporation (ARC),International Association of Dredging Companies ispleased to organise, for the third consecutive year, anintensive, one-week seminar on dredging and reclama-tion. Last January’s course met with such enthusiasticresponse, that IADC, building on this success, has decided to present this seminar again in 1997. Thecourse will be held at the Boulevard Hotel, Singapore. The costs are US$ 2750, which includes six nightsaccommodation at the Boulevard Hotel, breakfast andlunch daily, one special participants dinner, and a gene-ral insurance for the week.

The seminar includes workshops and a site visit to adredging project. Highlights of the programme are:

International Seminar onDredging and Reclamation

(please print)

Name ..........................................................................................................................................................................

Title ..........................................................................................................................................................................

Company ..........................................................................................................................................................................

Address ..........................................................................................................................................................................

..........................................................................................................................................................................

Tel. ................................................................................... Fax ...............................................................................

Please send this form and your deposit by cheque or credit card for US$ 500 in order to guarantee your place atthe seminar. Upon receipt of this form and your deposit your place in the seminar is confirmed. We will then sendyou further detailed information, final registration forms, and an invoice for the correct amount.

Without your deposit we cannot guarantee your place and accommodations at the seminar.

■■ A Cheque is enclosed.

■■ Please charge my credit card:

■■ American Express ■■ Eurocard/Master Card ■■ Diners Club

Account no.:

Expiry date:

Signature .............................................................................................................. Date ................................................

AfricaBoskalis Togo Sarl., Lomé, TogoBoskalis Westminster Cameroun Sarl., Douala, CamerounDredging International Services Nigeria Ltd., Lagos, NigeriaHAM Dredging (Nigeria) Ltd., Ikeja, NigeriaNigerian Dredging and Marine Ltd., Apapa, NigeriaWestminster Dredging Nigeria Ltd., Lagos, NigeriaZinkcon Nigeria Ltd., Lagos, Nigeria

The AmericasACZ Marine Contractors Ltd., Brampton, Ont., CanadaBeaver Dredging Company Ltd., Calgary, Alta., CanadaDragamex SA de CV, Coatzacoalcos, MexicoGulf Coast Trailing Company, New Orleans, LA, USAHAM Caribbean Office, Curaçao, NAStuyvesant Dredging Company, Metairie, LA, USAUscodi, Wilmington, DE, USA

AsiaBallast Nedam Malaysia Ltd., Kuala Lumpur, MalaysiaBallast Nedam Dredging, Hong Kong Branch, Hong KongBoskalis International BV., Hong KongBoskalis International Far East, SingaporeBoskalis Taiwan Ltd., Hualien, TaiwanDredging International N.V., Hong KongDredging International N.V., SingaporeFar East Dredging Ltd., Hong KongHAM Bangladesh Office, Dhaka, BangladeshHAM Hong Kong Branch, Wanchai, Hong KongHAM Singapore Branch, SingaporeHAM Taiwan Office, Taipei, TaiwanHAM Thai Ltd., Bangkok, ThailandJan De Nul Singapore Pte. Ltd., SingaporePT Penkonindo, Jakarta, IndonesiaTideway DI Sdn. Bhd., Selangor, MalaysiaVan Oord ACZ B.V., Dhaka, BangladeshVan Oord ACZ B.V., Hong KongVan Oord ACZ B.V., SingaporeVan Oord ACZ Overseas B.V., Karachi, PakistanVomsi India Ltd., New Delhi, IndiaZinkcon Marine Malaysia Sdn. Bhd., Kuala Lumpur, MalaysiaZinkcon Marine Singapore Pte. Ltd., Singapore

Middle EastBoskalis Westminster Al Rushaid Ltd., Dhahran, Saudi ArabiaBoskalis Westminster M.E. Ltd., Abu Dhabi, UAEDredging International N.V., Middle East, DubaiDredging International N.V., Tehran Branch, Tehran, IranGulf Cobla (Limited Liability Company), DubaiHAM Dredging Company, Abu Dhabi, UAEHAM Saudi Arabia Ltd., Jeddah, Saudi ArabiaJan De Nul Dredging, Abu Dhabi, UAEVan Oord ACZ Overseas BV., Abu Dhabi, UAE

AustraliaCondreco Pty. Ltd., Sydney, NSW, AustraliaDredeco Pty. Ltd., Bulimba, QUE., AustraliaJan De Nul Australia Pty. Ltd., Brisbane, QUE., AustraliaNew Zealand Dredging & General Works Ltd., WellingtonVan Oord ACZ B.V., Victoria, AustraliaWestHam Dredging Co. Pty. Ltd., Sydney, NSW, Australia

EuropeACZ Ingeniører & Entreprenører A/S, Copenhagen, DenmarkAnglo-Dutch Dredging Company Ltd., Beaconsfield,United Kingdom

A/S Jebsens ACZ, Bergen, NorwayAtlantique Dragage S.A., Nanterre, FranceBaggermaatschappij Boskalis B.V., Papendrecht, NetherlandsBaggermaatschappij Breejenbout B.V., Rotterdam, NetherlandsBallast Nassbaggergesellschaft, Hamburg, GermanyBallast Nedam Dredging, Zeist, NetherlandsBallast Nedam Dragage, Paris, FranceBoskalis Dolman B.V., Dordrecht, NetherlandsBoskalis International B.V., Papendrecht, NetherlandsBoskalis Oosterwijk B.V., Rotterdam, NetherlandsBoskalis Westminster Aannemers N.V., Antwerp, BelgiumBoskalis Westminster Dredging B.V., Papendrecht, NetherlandsBoskalis Westminster Dredging & Contracting Ltd., CyprusBoskalis Zinkcon B.V., Papendrecht, NetherlandsBrewaba Wasserbaugesellschaft Bremen mbH, Bremen, GermanyCEI, Bagger- en Grondwerken, Zele, BelgiumDelta G.m.b.H., Bremen, GermanyDraflumar SA., Neuville Les Dieppe, FranceDragados y Construcciones S.A., Madrid, SpainDravo S.A., Madrid, SpainDredging International N.V., Madrid, SpainDredging International N.V., Zwijndrecht, BelgiumDredging International Scandinavia NS, Copenhagen, DenmarkDredging International (UK), Ltd., Weybridge, United KingdomEnka-Boskalis, Istanbul, TurkeyEspadraga, Los Alcázares (Murcia), SpainHAM Dredging Danmark Aps, Korsør, DenmarkHAM Dredging Ltd., Camberley, United KingdomHAM, dredging and marine contractors, Capelle a/d IJssel,NetherlandsHAM-Van Oord Werkendam B.V., Werkendam, NetherlandsHeinrich Hirdes G.m.b.H., Hamburg, GermanyHolland Dredging Company, Hardinxveld, NetherlandsHolland Dredging Iberica S.L., Tarragona, SpainHolland Dredging Co. (Irl.) Ltd., Cork, IrelandHolland Dredging Co. (U.K.) Ltd., Farnham, United KingdomImpresa SIDER SpA., Rome, ItalyJan De Nul N.V., Aalst, BelgiumJan De Nul Dredging N.V., Aalst, BelgiumJan De Nul (U.K.) Ltd., Ascot, United KingdomNordsee Nassbagger- und Tiefbau GmbH, Wilhelmshaven,GermanyN.V. Baggerwerken Decloedt & Zoon, Brussels, BelgiumPhilipp Holzmann Aktiengesellschaft, Hamburg, GermanyS.A. Overseas Decloedt & Fils, Brussels, BelgiumSider-Almagià S.p.A., Rome, ItalySkanska Dredging AB, Gothenborg, SwedenSociedade Portuguesa de Dragagens Lda., Lisbon, PortugalSociedad Española de Dragados SA., Madrid, SpainSocietà Italiana Dragaggi SpA. “SIDRA”, Rome, ItalySociété de Dragage Holland (France) S.A., Bondues, FranceSociété de Dragage International “S.D.I.” S.A., Marly le Roi, FranceSodranord Sarl, Paris, FranceTideway B.V., Breda, NetherlandsVan Oord ACZ B.V., Gorinchem, NetherlandsVan Oord ACZ Ltd., Newbury, United KingdomVan Oord ACZ B.V., Zwijndrecht, BelgiumVolker Stevin Baggermaatschappij Nederland B.V.,Rotterdam,NetherlandsVolker Stevin Dredging B.V., Rotterdam, NetherlandsWasserbau ACZ GmbH, Bremen, GermanyWestminster Dredging Co. Ltd., Fareham, United KingdomZanen Verstoep B.V., Papendrecht, NetherlandsZinkcon Contractors Ltd., Fareham, United KingdomZinkcon Dekker B.V., Rotterdam, NetherlandsZinkcon Dekker Wasserbau GmbH, Bremen, Germany

Membership List IADC 1996Through their regional branches or through representatives, members of IADC operate directly at all locations worldwide.

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