Environmental control and discharge operations

Among the tasks for which environmental control is possible in a dredging project, the management of discharged material stands out as the main focus of this work. When referring to the management of discharge in dredging tasks, the following three aspects should be considered:

1. Type of material to be dredged and discharged.
2. The volume of dredged and discharged material.
3. Discharge site for the dredged material.

The definitions of the above points are usually included in the scope of the EIA, which aims to ensure that dredging activities are carried out in an environmentally acceptable manner. In all cases, monitoring the mentioned aspects will involve a series of measurements and continuous monitoring to verify compliance as established in both the contract and the EIA.

Monitoring everything related to the type of material to be dredged/discharged will be primarily covered through strict processes of periodic sediment sampling and subsequent laboratory analysis. Regarding points related to the use of the discharge area, the range of possibilities for environmental control is broad and will depend on each project. This topic is addressed in the following section.

Monitoring the use of discharge areas

The defined and approved zones for the discharge of dredged material, as determined by the client, are critical elements when evaluating a project, especially for the contractor. The location and capacity of each site will have a direct influence on the production values of dredging equipment, determined by the distances to be covered and the number of zones to be used. Consequently, these variables will impact the overall costs of the project.

Considering the effect that the location of discharge zones has on costs, it is understandable that contractors seek to "save" on distances covered during their operations. Therefore, the control and supervision of dredging discharge by the client are fundamental for the proper development of the project and, above all, for compliance with environmental preservation aspects.

As mentioned, environmental monitoring of dredging activities has assumed a significant role and can be beneficial for all stakeholders. However, this does not mean that a cost-benefit analysis should not be considered, even when evaluating monitoring procedures to implement. Environmental monitoring can be a lengthy and expensive process. For larger projects, it may last for many years and require specific and high-value equipment. Therefore, not all monitoring methods apply to smaller projects.

A clear example of the above, with potential application in environmental monitoring, is the U.S. National Dredging Quality Management Program (DQM). This initiative by the US Army Corps of Engineers (USACE) aims to provide a standardised and automated system for remote monitoring, analysis and documentation of dredging projects using both government and contractor dredgers.

The DQM collects information from sensors installed on the vessel, calculates dredging activities and presents the information through standardised reports and graphical representations. The information obtained includes three elements: dredging cycle data; hopper load; and real-time dredger positioning.

While this system offers numerous benefits related to dredging operations and monitoring, especially in controlling the use of client- approved discharge zones, its implementation involves the need to acquire, install and maintain specific instrumentation, which can be costly. Additionally, its application outside the realm of USACE work could pose an additional complication, restricting its use.

However, for some dredging projects, depending on contractual obligations, daily production information from dredging contractors may not be available for environmental management purposes (Bell et al., 2022). This could prove to be another obstacle when defining monitoring tools by the client.

Having an efficient, simple and low-cost material discharge monitoring system would be highly beneficial for small projects, projects where specific control methods are not envisaged or situations in which not all information is readily available. From this premise arises the motivation for the present work and the idea of implementing the AIS system as a tool for environmental monitoring, leveraging the use of publicly available information.

Trailing suction hopper dredgers

This type of equipment falls within the category of "mechanical/hydraulic dredgers." Essentially, they are self- propelled vessels equipped with hoppers where dredged material is deposited and later expelled in the vicinity of the working area or transported horizontally to the discharge site. Dredging takes place while the dredger navigates at low speeds using suction pipes located on the sides of the vessel, which are lowered to make contact with the seabed. Material suction, a mixture of water and sediments, is achieved through centrifugal pumps. Figure 1 shows a trailing suction hopper dredger in operation.

These kind of dredgers are characterised by their great versatility due to the range of materials they can extract, the various discharge modalities and their ability to work in both protected and unprotected waters. These aspects give TSHDs the flexibility to be employed in various projects, including channel opening and maintenance dredging, beach nourishment and trench excavation, among others.

The most common operation of a TSHD can be understood as a series of simple dredging cycles, consisting of the following stages:

1. Loading the hopper – dredging stage.
2. Navigation to the discharge site (loaded).
3. Discharging the hopper at the discharge site.
4. Navigation to the dredging site (unloaded).

The dredger's productivity will depend on the type of material to be extracted, hopper capacity and the total duration of the dredging cycle. The distance to the discharge areas is a crucial factor in calculating efficiency, as it directly affects navigation times.

Automatic Identification System

The AIS is a data transmission system installed on ships and land-based stations, operating in the VHF marine band. The primary objective of this system is to enable the identification of vessels, meaning that ships communicate their position and other relevant information so that other vessels or land-based stations can be aware of it. This helps prevent collisions and assists the ship during its navigation. This tool contributes not only to navigation safety but also to traffic management efficiency.

The AIS was approved by the International Maritime Organization (IMO) in 2002 and has been mandatory since December 2004 for ships subject to the SOLAS Convention that meet the following characteristics:

• Ships with a gross tonnage exceeding 500 GT.
• Ships on international voyages with a gross tonnage exceeding 300 GT.
• All passenger ships, regardless of size.

The AIS unit consists of a VHF radio transceiver capable of sending information about the station's identification, position, heading, speed and ship and cargo-related data, among others, to other vessels and land-based receivers. Once the unit is installed onboard and configured correctly, it transmits information continuously and automatically without the need for intervention by the ship's crew. Figure 2 provides a schematic representation of the AIS system.

The data generated by the AIS system can be stored for subsequent management and processing using software that allows a graphical representation of this information. This enables the performance of (offline) analyses, both static and dynamic, on the voyages of vessels equipped with an AIS unit onboard.

AIS for monitoring TSHD

Considering the operational methodology of TSHD dredgers, it is evident that the use of AIS-generated data allows for tracking the equipment during dredging tasks. This facilitates a detailed analysis of the operation and enables several verifications for the project. AIS also allows the monitoring of dredgers in each of the work cycle phases.

By obtaining a spatial and temporal detail of the operation, it will be possible to verify the use of the areas defined and approved by the contract, especially for the discharge of material. Additionally, and through a simplified methodology that will be explained in a later section, an estimate of the volumes dredged in each work cycle could be obtained.

A simplified method to estimate volumes with AIS

If AIS data generated by a TSHD is analysed during a dredging campaign, a quick and simple estimation of the extracted volumes can be performed as follows.

The starting point is the hopper capacity (m3) of the dredger used. This value is known as the nominal volume of the hopper (Vh) and is easy to know since it is a characteristic of each dredge, and is also the value used to classify TSHD equipment.

The next step will be to determine the capacity or effective volume of the hopper. This variable will be identified with the acronym Vhe (m3). This value represents the effective volume of material contained in the hopper in its fully loaded condition.

To obtain the Vhe, it is necessary to affect the value of the nominal capacity of the hopper (Vh) by a correction factor. This factor will be represented by the letter "F" and is called the utilisation factor. This value, which in the case study developed below has been adopted as 0.70, depends among other aspects on the:

• characteristics of the type of soil to be dredged; and
• work methodology used. Especially the overflow discharge time.

The hopper utilisation factor (F) is specific for each dredging campaign carried out and remains reasonably constant as long as the working conditions are not modified. These conditions include the type of soil, the dredging equipment used, the pumping power and the dredging methodology used (overflow time).

About the above, it should be noted that before the start of each campaign at a specific site, a planning and programming stage of dredging activities is carried out in the technical office. In this stage, the different factors that condition the operation during the tasks are evaluated, such as the type of soil, work and unloading areas, and characteristics of the equipment to be used, among others. This prior procedure seeks to guarantee the correct execution of a work routine, in which operations are carried out efficiently and systematically throughout the entire campaign. This concept of systematisation strengthens the premise of maintaining working conditions throughout the same dredging campaign.

When defining the utilisation factor, at least two methodologies are identified. On the one hand, it is possible to adopt a value based on work experiences and previous studies. That is, evaluate productions obtained by the equipment in previous campaigns at the work site or sites where the extracted materials and dredging conditions are similar. Previous studies and research can also be used as a reference. On the other hand, it is possible to define a value based on the results obtained and reported by the contractor for the first dredging cycles executed during the campaign to be studied.

Finally, the number of work cycles carried out during the period studied (Cd) must be determined, that is, the total number of trips made by the dredger with a full hopper between the extraction area and the unloading area. This data is one of the main results of the analysis of operations through AIS.

With the mentioned data, it will be possible to implement this simplified method that will yield estimated values of the volumes dredged during the campaign. To summarise:

• Nominal hopper capacity = Vh [m3]
• Hopper utilisation factor = F [%]
• Effective hopper capacity = Vhe = Vh x F [m3]
• Number of work cycles = Cd [-]
• Dredged volume = Vd = Vhe × Cd [m3]

It is important to mention that the calculated value corresponds to the volume of material extracted in the hopper, and due to soil characteristics, it is greater than the in-situ volume or the effective volume, which is generally calculated as the difference between two bathymetric surveys, one before and one after dredging.

Application case

To verify the validity of the described method, an analysis will be conducted using AIS data from dredging operations of a TSHD during a campaign carried out as part of the project for the maintenance of the access channels to the Port of Buenos Aires in Argentina.

The Port of Buenos Aires, located in the homonymous province on the shores of the Río de La Plata, is the main container port in Argentina and one of the most important in the Latin American region. Managed by the Administración General de Puertos SE (AGPSE), it has a capacity of 1.5 million twenty-foot equivalent unit (TEUs) annually for total cargo and receives approximately 1,200 ships per year. This port accounts for over 60% of the country's container movement.

It is a multimodal port divided into two main sectors known as Puerto Norte (left in Figure 3) and Puerto Sur (right in Figure 3). This case study will focus on the access channels to the North sector.

Puerto Norte comprises 6 docks used for port operations and the service of deep-sea and cabotage vessels. This port sector accommodates five general cargo terminals with 23 berths and a quayside depth of 10.05 metres (m).

Access to the mentioned zone is achieved through two channels named Brown Channel, with a length of 4.7 kilometres (km) and Huergo Channel, with a length of 7.3 km. In total, there is an entrance of 12 km in length, 10.36 m (34 feet) in depth and a bottom width of 100 m. This access route connects with the Vía Navegable Troncal, which is a logistical corridor linking the main river terminals in Argentina. Figure 4 shows the mentioned channels.

The maintenance dredging of the access channels is one of the main projects tendered at the Port of Buenos Aires approximately every 3 or 4 years.

As of May 2024, the most recent dredging operations at the port were conducted by the company Compañía Sudamericana de Dragados, a member of the Jan De Nul Group. This company was responsible for the work in the channels after winning a public tender in 2019 for the opening dredging and 3 years of maintenance of the access waterways.

The activities covered in the contract from the aforementioned tender were primarily executed with TSHD, including the dredger Afonso de Albuquerque. This equipment will be used to study its operation and implement the proposed environmental monitoring methodology.

The dredger Afonso de Albuquerque, built in 2018, is a small-sized TSHD with a hopper capacity of 3,500 m3. It has been operational in Argentina since 2019, specifically in the Vía Navegable Troncal (the most important waterway in Argentina) and the channels of the Port of Buenos Aires.

The Albuquerque dredger specifications are:

• Length: 89.30 metres.
• Beam: 22.00 metres.
• Draught (loaded condition): 5.70 metres.
• Suction tube diameter: 800 mm.
• Maximum dredging depth: 27.60 metres.
• Discharge mechanism: bottom doors.

AIS data analysis

To analyse the operations of the TSHD Afonso de Albuquerque, verify the use of approved discharge areas and estimate the volumes extracted, AIS data generated by the dredger was collected during a 13-day campaign in September 2022 within the framework of the maintenance dredging in the Huergo and Brown channels. To present the results in this case, the analysis time was limited to five consecutive days between 8-12 September.

To visualise the available data, an AIS data processing subroutine of the software called IWRAP Mk2 (IALA Waterway Risk Assessment Programme) was used. This programme was developed by IALA, the International Association of Marine Aids to Navigation and Lighthouse Authorities, to conduct risk analyses. In this instance, the programme was solely used as a graphical viewer for AIS data.

Figures 6A and 6B display screenshots of the software during the playback of AIS data for the mentioned dates and the analysed sector of the channels. The dredge Afonso Albuquerque, highlighted in red, can be observed actively working.

Figure 7 displays a traffic density graph of all the dredger navigations during the five analysed days. This will be important when evaluating the use of the dumpsites.

Verification of the use of discharge zones

Tracking the movements of the dredger during its operation is a quick and straightforward way to verify compliance with the use of approved zones in the project. In the case of dredging the access channels to the Port of Buenos Aires, the discharge of material must be carried out within the zones defined for this purpose by the National Directorate of Waterways.

Figure 8 shows the areas approved for the discharge of material in green and the areas subject to discharge restrictions in red. The green areas are used for the discharge of material from different dredging works in the area. The sector where each dredge discharges will depend on the available depths and the draft of the ship. In the case of maintenance work on the access channels to the port of Buenos Aires, the areas in orange are usually used.

It is possible to see that the distance to be navigated by the equipment between the work sector and the dumpsite is significant, reaching an average value of 20 km.

Once the AIS analysis was completed, the verification of the use of the unloading areas was carried out. This was done by crossing the map of approved zones and the traffic density map of the dredger. The result is presented in Figure 9 where it is possible to see that the use was correct and therefore the Afonso de Albuquerque dredger respected what was established by the regulatory authority regarding the dredged materials placement.

While in this instance, the verification was done using a traffic density chart generated by AIS processing software based on data from the five analysed days, it is indeed possible to carry out this operation in real time. This allows for the immediate verification of compliance with approved zones during ongoing operations.

Estimation of dredging volumes

Regarding the operations of the dredger between 8-12 September 2022, a total of 44 work cycles were recorded, resulting in between 8 and 9 cycles per day. In detail, the analysis revealed the following averages:

• Dredging time: 43 minutes.
• Loaded travel time: 56 minutes.
• Discharge time: 11 minutes.
• Empty travel time: 53 minutes.

It is noticeable that the distance to the discharge zone in this project is significant and strongly influences the number of work cycles that can be performed daily.

With the results obtained from the processing, especially the number of cycles performed, it is possible to apply the simplified methodology to estimate the volumes of dredged material (Vd) as indicated in a previous section.

The step before computing is to define a value for the hopper utilisation factor (F). For this, the following was taken into consideration:

• Soil type. According to AGPSE data, the material dredged in the Huergo and Brown channels corresponds to a mixture of silt, fine sand, and clay of a lean consistency, generally good for dredging.
• Characteristics of dredging cycles. The operation times resulting from the analysis of AIS data were evaluated, especially those intended for the dredging task itself.
• Information provided by the contractor to the AGPSE on the operations carried out during the analysed campaign. Although the analysis of five consecutive days is presented in this work, information on the operation is available for the entire 13-day campaign.
• Information on previous dredging work on the site and other works in areas near the Port of Buenos Aires.

In this case, a utilisation factor of 0,7 was adopted, and therefore, the effective capacity used in the calculation for the dredger Afonso de Albuquerque will be 2,450 m3 per cycle. The results obtained for the estimated dredged volumes are summarised in Box 1.

Next, to validate the method, Box 2 presents the comparison with the volumes extracted by the dredger during the analysed campaign. These data were obtained after being requested from AGPSE, which provided the corresponding daily operation reports. In these documents, the complete activity of the dredger is reported, including material extraction and discharge zones, dredging time and delays, the number of trips made and equipment production values.

It is possible to see in Box 2 that the results obtained are satisfactory. The volume estimation carried out using the proposed simplified method yields values whose order of magnitude is significantly representative in comparison with the values reported by the contractor to the AGPSE through the daily work reports. The differences found do not exceed 5%, except for one day which was around 9%, which is more than appropriate considering the simplicity of the method and the assumptions made to define the hopper utilisation factor.

Conclusions

Considering the content presented throughout this article, a series of final comments can be made as conclusions. Firstly, it is important to emphasise the significance of monitoring during dredging operations. While these tasks are usually the responsibility of the contractor and are contractually established, it is good practice for the client to have its control tools or methods.

When referring to environmental control in discharge zones, it essentially addresses three variables dealt with by the Environmental Impact Assessment (EIA) of each project: the type of material to be dredged and its behaviour; the location of the discharge sites; and the physical capacity to receive these.

This work has focused on the control of the last two mentioned variables. As mentioned, the definition and approval of these zones are based not only on operational or project profitability issues but also on environmental aspects and conflicts of interest can arise in this regard. Considering the influence that one single kilometre difference in the location of discharge zones can have on construction costs, it is understandable that contractors try to save on distances travelled. Therefore, control and supervision of dredging operations are fundamental in this aspect. The Automatic Identification System (AIS) emerges as a tool with great potential for monitoring dredging operations, especially those carried out by TSHD equipment due to the typical characteristics of their operation. Given the mandatory nature of this automatic identification system, established in 2004, obtaining AIS data from vessels to monitor should not be a problem.

Tracking the movements of dredgers via AIS in real time as well as after operations allows for simple and quick verification of compliance with approved work zones. Respecting approved discharge zones is crucial for the proper development of the project. On the other hand, the possibility of estimating dredged volumes with TSHD equipment through AIS data processing adds value to the use of this tool. Preliminary monitoring of the amount of material discharged in the areas approved for this purpose is another environmental aspect that can be covered with the proposed control methodology. The volume values estimated using this simplified method and their relationship with those measured by the contractor demonstrate the potential of the procedure, as long as the working conditions are maintained throughout the entire dredging campaign. Although this is an estimate, finding differences between what was calculated and what was reported by the dredger in charge could be an initial indicator to later request more precise controls, such as bathymetric surveys or direct measurements in the equipment hoppers.

Based on the above, it is correct to state that the use of AIS as an environmental control tool in dredging projects, especially when using TSHD equipment, has great potential. It is a simple, efficient and, above all, economical method since it does not involve the use of specific equipment as other control methods do such as the DQM. This last aspect should be emphasised since, as mentioned in this article, environmental monitoring can be beneficial for all stakeholders in a project, but this does not mean that a cost-benefit analysis should not be considered when analysing the monitoring procedures to be implemented.

The case study demonstrates that with the minimum necessary information, AIS can be an important environmental management alternative in a project, especially in smaller- scale projects or cases where daily operation information is not available.