Hybrid solutions for cutter suction dredgers

Abstract

Cutter suction dredgers are a type of hydraulic dredgers often used in areas with hard soil where other types of dredging vessels would be ineffective. As a result of the wide range of type of soil that is being cut a lot of variation is seen in the power demand on the cutter suction dredger's diesel-electric power generation system. Consequently, the load on the main diesel engines rapidly changes. To smoothen power demand the addition of an energy storage system to the power generation system is investigated to reduce the load variation experienced by the main engines. Reduction of the load variation is also relevant for the application of dual fuel engines in cutter suction dredgers. For dual fuel engines exceeding of the loading limit poses a significant risk of the main engines switching from combustion of (liquefied) natural gas to combustion of diesel fuel to prevent misfiring or engine knocking. Better control of the engine load is expected to allow continuous combustion of liquefied natural gas during dredging operation.
From the analysis of the power demand measured during dredging operation it is concluded that energy storage systems capable of delivering high power and have relatively low energy storage capability are best suited for this application. Based on the required characteristics flywheel energy storage and supercapacitor energy storage are selected as suitable energy storage systems. For both energy storage systems a simulation model is created based on available literature. A benchmark simulation model of the driveline of a cutter suction dredger, including the power generation system and electrical network, is created and validated using measurement data. The power and energy storage capacity of the energy storage system are determined by exceeding of the loading capacity of the main engines defined by the engine manufacturer during several months of continuous dredging operation. Separate cases are investigated where a supercapacitor energy storage system and a flywheel energy storage system are added. Simulation results are compared to the results of the benchmark model. Furthermore, simulations are conducted where the main diesel engines are replaced by dual fuel engines running on liquefied natural gas with a methane number of 80. Results show that both energy storage systems are capable of increasing the percentage of load variation within the engine limit on the original driveline from 91% to at least 99%.
For dual fuel engines it is found that without an energy storage system 75% of the load variation is within the engine's loading limit, meaning switching to diesel fuel is inevitable during dredging operation. The addition of an energy storage system increases the percentage of load variation within the engine's loading limit to at least 98%. While the engine limit is still rarely exceeded, further analysis showed that the air excess ratio stays between the knock limit and misfire limit, meaning no switching to diesel fuel is required. As a result, continuous dredging operation using LNG as fuel is possible resulting in a reduction in fuel cost up to roughly 30%. During this research it is found that the effect of transient loads on engine wear could not be quantified. Further research into this subject is needed before conclusions can be made with regard to this subject. Also, no reduction in fuel consumption is seen as a result of the reduction of transient loading.