Netherlands-based C-Job Naval Architects recently unveiled a new design of autonomous vessel to be used for coastal replenishment works in Dutch waters.
C-Job's Autonomous Low Energy Replenishment Dredger (ALERD) design was developed in response to a tender issued by the Netherlands' Directorate-General for Public Works and Water Management (Rijkswaterstaat) calling for a sustainable and cost-effective coastal protection solution The design improves upon an earlier autonomous vessel design optimised for underwater maintenance dredging in ports and harbours but with the operational profile now being changed to coastal replenishment.
Rolph Hijdra, C-Job Autonomous Shipping research lead engineer, said the ALERD is being developed to deliver sediment to coastlines to protect the land in a sustainable manner. As with the company's Autonomous Underwater Maintenance Dredger (AUMD) project, the hopper is designed to always be full of seawater or a mixture of water and sand/sediment, with discharge being executed through the bottom doors in the hull.
C-Job sought feedback from a number of local dredging companies to gain insights for the development of the new autonomous vessel. A lifecycle assessment showed that the overall lifetime environmental cost could be reduced by 83 per cent or more. Model testing was also carried out to assess the vessel's stability with different loads. Hijdra said that, together with simulations that varied the dimensions and power sources, the tests concluded that a vessel with a hopper volume of 2,500 cubic metres and powered by batteries would be the most cost-effective solution.
An operational profile analysis was performed using C-Job's in-house-developed algorithms based on foreshore replenishments carried out by conventional dredgers. These algorithms generated data relating to typical operational areas (water depths, sailing distances, etc.), the duration of dredging cycles, sailed transit speeds, and the extent to which works were carried out on a full-time basis. This will be used for additional studies looking at the energy that will be required for stability and buoyancy control.
C-Job said that constraints at this stage of the project did not allow for more in-depth analysis such as tank-testing or CFD analysis. However, the simulation model did enable users to change the main parameters and compare different designs and thus optimise the energy consumption required for each operational cycle.
To achieve zero emissions requires both low energy requirements and a sustainable electrical energy supply. C-Job then undertook a literature review of all the available options, and lithium-ion batteries and proton exchange membrane fuel cell systems emerged as what the company said are the most ideal alternatives currently available.
Among the studies reviewed were a number that included simulations analysing variations in factors including different hopper volumes, operating speeds, and different charging and bunker locations. These demonstrated that both power sources could deliver lower operational costs compared to the benchmark provided by the Rijkwaterstaat's Coastline Care Innovation (Innovaties in de Kustlijnzorg; IKZ) program. Therefore, the choice would depend on individual operational circumstances.
C-Job's participation in the IKZ project ceased at the end of 2021 since the company realised that the goal of having an ALERD operational by 2024 could not be met. However, the concept's potential was widely recognised, and the company is continuing its research and its promotion of the potential benefits of submersible dredgers.
C-Job believes this method of dredging will bring with it a range of as yet unknown challenges compared to traditional dredging. However, as there is growing clamour for sustainable solutions grows and the cost of clean energy systems falls, the momentum in this direction can only increase.