Which Real-Time Block Placement Monitoring Solutions Exist for Breakwaters?

Monitoring block placement on a breakwater represents a significant technical and economic challenge. An incorrectly positioned block can compromise the structure's resistance to wave action. Correcting this issue after initial placement can prove very costly. There are a number of solutions currently available for the real-time monitoring and verification of underwater placement.

You will find below a summary of the main approaches.

Why is placement monitoring so important?

Single-layer concrete armour breakwaters (Accropode™, Xbloc®, Core-Loc™, etc.) are designed to follow precise interlocking rules, including density, block contact, orientation and out-of-profile position. These rules are difficult to verify with the naked eye, especially underwater. Furthermore, the later a non-conformity is detected, the more complicated the rework will be, particularly if the faulty block is already overtopped by several placed rows.

The main solutions available

1. Diver guidance

This is the most historically widespread placement assistance method. The diver is connected to the surface via an umbilical for breathing and audio communication, and visually guides the crane operator underwater. This solution is straightforward, requiring no advanced electronic or software technology. Placement is carried out effectively underwater when the diver is well trained in block placement and therefore aware of the placement rules.

It should be noted that the success of this method is contingent on underwater visibility; in conditions where visibility is poor or non-existent, the diver is unable to provide visual assistance to the operator. Furthermore, the presence of a person near the load during placement poses a significant safety risk. This is why some international maritime works companies no longer use divers on their block placement projects.

2. POSIBLOC or equivalent (TMC)

POSIBLOC is a guidance system based on an inertial sensor fitted directly to the block. This sensor transmits the block's position and orientation in real time to the operator via a 3D visualisation interface. The principle is straightforward: the sensor records the block's movements throughout the descent and placement phase, allowing the crane operator to adjust its position accordingly.

Its main appeal is its low CAPEX: it is one of the least expensive solutions in terms of initial investment among those available on the market. However, OPEX can quickly rise over time. The sensor is sensitive to impacts, and connection cable breakages can easily occur — this is unavoidable on a block placement site — generating replacement costs that are not always anticipated. Furthermore, the system's accuracy is subject to deterioration over time due to the inertial sensor's drift. Monitoring ceases immediately upon removal of the sensor post-placement, resulting in any subsequent movement or settlement going unnoticed.

3. The Echoscope (volumetric sonar)

The Echoscope is a volumetric sonar developed by Coda Octopus that generates a real-time 3D point cloud of the placement area, regardless of underwater visibility. In contrast to solutions that rely on a sensor attached to the block, this approach provides a comprehensive view of the block and its surroundings (underlayer, neighbouring blocks) during the placement process. An above-water point cloud generation solution is also available.

This solution is better suited to large projects with substantial budgets, as the sensor's investment cost is high. Real-time 3D point cloud visualisation facilitates effective understanding of the underwater situation and ensures accurate representation of the block's position underwater. However, the accuracy of the real-time point cloud is lower than that of a multibeam sonar, which complicates the analysis of placement criteria, such as contacts between blocks.

4. Real-time 3D monitoring — SEABIM (multi-sensor)

The solution developed by SEABIM uses 3D as useful data, derived from an indirect point cloud measurement combining an above-water LiDAR with an underwater scanning sonar. The software continuously detects and tracks the block's 3D position and orientation within the point cloud, then calculates the interlocking filters every second. 3D monitoring is maintained post-unhooking, enabling the detection of any post-placement movement or settlement.

CAPEX is higher than for an inertial system, as investment in a scanning sonar and a LiDAR is required. Please note that the scope of OPEX is confined to the software licence cost and standard sensor maintenance. The final modelling accuracy achieved is satisfactory, as it is based on a point cloud from a multibeam sonar. However, the refresh rate and point density of the underwater point cloud are lower than those of a real-time volumetric sonar.

Block positioning and detection system installed on the lifting crane's spreader beam during Xbloc® placement on the M24 NRL project in Réunion – RAZEL-BEC

View of the SEABIM Placement Companion placement assistance interface

Comparison table of solutions

In conclusion

There is no single solution that will work in every case. The choice depends on the size of the project, contractual requirements, on-site visibility conditions, and available budget. Each approach has its own strengths and limitations, so it is important to treat placement monitoring as a fully-fledged management tool, rather than a simple end-of-project inspection step.

Should you require further information regarding block placement assistance solutions, we would advise you to consult the resources offered by XBloc and Concrete Layer, which can guide you according to your project's specific needs.